Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Kermen, Emre
Master of Applied Science - MASc
University of British Columbia
Orthotics have been utilized by clinicians for many years to treat foot-related abnormalities. With advancements in material sciences, the footwear industry started utilizing synthetic materials which have better and suitable properties. Clinicians, who prescribe foot insoles, need to have an extensive understanding of the properties and characteristics of insole materials, to make informed decisions to meet the patients' needs. This thesis showcases utilized techniques and systems to evaluate orthosis properties as well as current criteria to date. Researchers have utilized a variety of testing techniques to examine properties of insole materials including; bench testing, simulated in-shoe conditions, in-shoe testing, and finite element analysis. Even though, there is a great understanding of material properties with endless diverse composition and thicknesses of each material makes clinical recommendations on the choice of material an impossible task. As the footwear orthosis industry shifts the focus from material to design, some researchers explore various anisotropic materials to create a homogeneous insole that can support as well as relieve pressure on patient’s feet. The objective of this research entails design and evaluation of novel hydrogel material in the field of foot orthotics. To accomplish this, first the material and design criteria of insoles had to be identified. This was followed by design and fabrication of insole molds. One of the requirements of hydrogel synthesis was development of molds to shape the solution. Extended finite element analysis of the human foot model along with insole designs were conducted to validate the designs. Manufacturing prototypes phase begun with design and expansion of an exterior thermal cycling bath that could enclose the orthotic. The hydrogel samples that were synthesized within the bath were taken through material testing validating the application of the hydrogel in the footwear field.
New Approach to Foot Orthotics Materials: Hydrogel Insoles by Emre Kermen B.A.Sc., The University of British Columbia, 2018 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) September 2020 © Emre Kermen, 2020 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, a thesis/dissertation entitled: New Approach to Foot Orthotics Materials: Hydrogel Insoles submitted by Emre Kermen in partial fulfillment of the requirements for the degree of Master of Applied Science in Mechanical Engineering Examining Committee: Dr. Hadi Mohammadi, Faculty of Applied Science Supervisor Dr. Bahman Naser, Faculty of Applied Science Supervisory Committee Member Dr. Zheng Liu, Faculty of Applied Science Supervisory Committee Member Dr. Ayman Elnaggar, Faculty of Applied Science University Examiner iii Abstract Orthotics have been utilized by clinicians for many years to treat foot-related abnormalities. With advancements in material sciences, the footwear industry started utilizing synthetic materials which have better and suitable properties. Clinicians, who prescribe foot insoles, need to have an extensive understanding of the properties and characteristics of insole materials, to make informed decisions to meet the patients' needs. This thesis showcases utilized techniques and systems to evaluate orthosis properties as well as current criteria to date. Researchers have utilized a variety of testing techniques to examine properties of insole materials including; bench testing, simulated in-shoe conditions, in-shoe testing, and finite element analysis. Even though, there is a great understanding of material properties with endless diverse composition and thicknesses of each material makes clinical recommendations on the choice of material an impossible task. As the footwear orthosis industry shifts the focus from material to design, some researchers explore various anisotropic materials to create a homogeneous insole that can support as well as relieve pressure on patient’s feet. The objective of this research entails design and evaluation of novel hydrogel material in the field of foot orthotics. To accomplish this, first the material and design criteria of insoles had to be identified. This was followed by design and fabrication of insole molds. One of the requirements of hydrogel synthesis was development of molds to shape the solution. Extended finite element analysis of the human foot model along with insole designs were conducted to validate the designs. Manufacturing prototypes phase begun with design and expansion of an exterior thermal cycling bath that could enclose the orthotic. The hydrogel samples that were synthesized within the bath were taken through material testing validating the application of the hydrogel in the footwear field. iv Lay Summary Human body is supported from ground up. It is important to have proper alignment through the foot, as small foot problems might lead to bigger issues along the body. Foot orthotics, also known as insoles, are medical devices that are utilized for relieving pain that might be caused by foot abnormalities. Typically, abnormalities of the foot will cause the patient to change their gait affectively causing a change in foot anatomy, which leads to high pressure concentration zones. Currently the foot orthotics industry utilizes variety of foam like materials with various properties to achieve support and pressure distribution through the patients’ foot. This thesis will test application of novel hydrogel material developed at the Heart Valve Performance Laboratory (HVPL). Hydrogel is a synthetic solution that through thermal cycling solidifies. By adjusting the solution formulation, hydrogel can achieve multiple material properties, making it a perfect candidate for insole application. v Preface This thesis presents research conducted by the author, which was done under the supervision of Dr. Mohammadi at the Heart Valve Performance Laboratory. The research topic was provided by Dr. Mohammadi and the novel hydrogel material was granted to be utilized during the duration of this thesis. All research included in this thesis was conducted at the UBC Okanagan Campus. vi Table of Contents Abstract ......................................................................................................................................... iii Lay Summary ............................................................................................................................... iv Preface .............................................................................................................................................v Table of Contents ......................................................................................................................... vi List of Figures .................................................................................................................................x List of Abbreviations ...................................................................................................................xv Acknowledgements .................................................................................................................... xvi Dedication .................................................................................................................................. xvii Chapter 1: Introduction ................................................................................................................1 1.1 Purpose & Overview of This Thesis ............................................................................... 1 Chapter 2: Literature Review .......................................................................................................3 2.1 Material Criteria .............................................................................................................. 3 2.2 Design Criteria ................................................................................................................ 5 2.3 Testing Methodology ...................................................................................................... 6 2.3.1 Experimental Testing .................................................................................................. 6 2.3.2 Analytical Testing ....................................................................................................... 9 2.4 Material Properties ........................................................................................................ 14 2.4.1 Polyethylene (PE) Foams .......................................................................................... 14 2.4.1.1 Plastazote (Zotefoams, London, UK) ............................................................... 15 2.4.1.2 Pelite (OrtoPed ULC, QC, Canada) .................................................................. 18 2.4.2 Polyurethane Foams .................................................................................................. 19 2.4.2.1 Poron (Rogers Corporation, Arizona, USA) ..................................................... 19 vii 2.4.3 Rubber Foam ............................................................................................................. 25 2.4.3.1 Spenco (Spenco Medical Corporation, Texas, USA) ....................................... 25 2.4.3.2 Ethylene-vinyl acetate ....................................................................................... 27 2.5 Futuristic Designs ......................................................................................................... 29 2.5.1 3D Weft Knitted Spacer Fabric ................................................................................. 29 2.5.2 Polyvinyl Alcohol Cryogel (PVA-C)........................................................................ 30 Chapter 3: Insole Design & Fabrication ....................................................................................31 3.1 Initial Insole Design ...................................................................................................... 31 3.1.1 Arbitrary Insole Design............................................................................................. 32 3.1.2 Mold Design.............................................................................................................. 33 3.1.2.1 First Iteration ..................................................................................................... 33 3.1.2.2 Second Iteration ................................................................................................ 35 3.1.2.3 Third Iteration ................................................................................................... 36 3.2 Customized Insole Design ............................................................................................ 37 3.2.1.1 Digitization of Custom-Molded Insoles............................................................ 38 3.2.1.2 Mold Design...................................................................................................... 38 3.2.1.2.1 Aluminum Case Fabrication........................................................................ 40 3.2.1.2.2 First Generation ........................................................................................... 41 3.2.1.2.3 Second Generation ...................................................................................... 44 Chapter 4: Finite Element Modeling..........................................................................................49 4.1 Model Creation and Problem Statement ....................................................................... 49 4.2 Initial Model.................................................................................................................. 50 4.3 Foot Model .................................................................................................................... 54 viii 4.4 FE Base Model .............................................................................................................. 57 4.4.1 Stress Reduction........................................................................................................ 60 4.4.1.1 Donut Design at the Heel Region ..................................................................... 60 4.4.1.2 Donut Design at the Metatarsal Region ............................................................ 64 4.4.1.3 Softer Material at the Heel Region ................................................................... 66 4.4.1.4 Softer Material at the Metatarsal Region .......................................................... 68 4.4.2 FE Results ................................................................................................................. 69 Chapter 5: Prototyping................................................................................................................70 5.1 Hydrogel Manufacturing Process ................................................................................. 70 5.1.1 PVA-BC Mechanical Properties ............................................................................... 71 5.1.1.1 Compression Testing ........................................................................................ 71 5.1.1.2 Tensile Testing .................................................................................................. 73 5.1.2 Thermal Cycling System........................................................................................... 76 5.1.2.1 Exterior Bath Configuration & Prototypes ....................................................... 77 5.1.2.2 Final Prototype .................................................................................................. 80 Chapter 6: Conclusion & Future Work .....................................................................................81 6.1 Work Overview ............................................................................................................. 82 6.1.1 Insole Design & Fabrication ..................................................................................... 82 6.1.2 FE Modeling ............................................................................................................. 82 6.1.3 Prototyping ................................................................................................................ 82 6.2 Future Work .................................................................................................................. 83 6.2.1 Insole Design & Fabrication ..................................................................................... 83 6.2.2 FE Modeling ............................................................................................................. 84 ix 6.2.3 Prototyping ................................................................................................................ 84 Bibliography .................................................................................................................................86 Appendices ....................................................................................................................................95 Appendix A ............................................................................................................................... 95 A.1 Foot Models of the Indentation Simulations ............................................................. 96 A.2 Engineering Drawings of Ulceration within Insole .................................................. 97 A.3 Engineering Drawings of Donut Design at Heel Region .......................................... 98 A.4 Renderings and Engineering Drawings of Donut Design at Metatarsal Region ...... 98 A.5 Line Construction for von Mises Stresses at Ulceration Location ......................... 100 Appendix B ............................................................................................................................. 101 B.1 Exterior Bath Construction Procedures .................................................................. 101 x List of Figures Figure 2.1 A) Customized bench testing system to simulate human gait, utilized by Paton et al [5]. B) MobileMat™ (Tekscan Inc., USA) pressure sensing platform [10]. C) emed® (Novel GmbH, Germany) pressure sensing platform [11]. D) FScan™ (Tekscan Inc., USA). in-shoe pressure measurement system [12]. E) pedar® (Novel GmbH, Germany) in-shoe measurement system [13]. ..................................................................................................................................... 7 Figure 2.2 FE modeling of a foot from 2D magnetic resonance (MR) by Cheung et al. [28] images. A) Constructed 3D model of patient’s foot, identifying boundaries of foot skeleton and skin surface. B) (Left) FE meshed model of soft tissue. B) (Right) Meshed model of all 28 bones in the foot. ....................................................................................................................................... 9 Figure 2.3 FE modeling of the attachment points of the plantar fascia, long plantar, short plantar, and spring ligaments [28].............................................................................................................. 11 Figure 2.4 Procedures for creating an orthosis model from the patient's 3D surface foot model [28]. ............................................................................................................................................... 12 Figure 2.5 Meshed model of Matlab generated orthoses [28]. ..................................................... 13 Figure 2.6 A) FE simulation of the custom-mold interface. B) Comparison of FE analysis results: flat vs custom-molded arch-supporting insoles. [28] .................................................................... 14 Figure 3.1 Okaped insoles side, bottom and top views. Contours were utilized to digitalize insoles. .......................................................................................................................................... 31 Figure 3.2 Isometric and side views of arbitrary insole design. ................................................... 32 Figure 3.3 First iteration of mold printed with ABS on Stratasys Mojo 3D ................................. 33 Figure 3.4 Silicone models produced from first iteration mold printed on Stratasys Mojo 3D. Air pockets were a detrimental issue as a large one is apparent at the toe region. ............................. 34 xi Figure 3.5 Second iteration mold renderings. White part is the cavity portion whereas, red part is the cover for the mold. Red and white textures were applied to better visualize different components. .................................................................................................................................. 35 Figure 3.6 Third iteration of arbitrary mold design. In red is the top cover and in white is the cavity portion. Orientation chosen to display the push-fit feature. ............................................... 36 Figure 3.7 Third iteration mold result. .......................................................................................... 37 Figure 3.8 Assembled mold rendering. Molds numbered 1 & 2 make up the heel portion, 3 & 4; the midfoot section, and 5 is the toe piece. Red circle highlights the airhole designed to encourage air pockets to escape from the curing solution. ........................................................... 41 Figure 3.9 First iteration mold pieces printed on the Prusa MK3. Red line is the axis the pieces are brought together. ..................................................................................................................... 42 Figure 3.10 The two cured silicone models compared to the original insole sent by Wiivv. ....... 43 Figure 3.11 Isometric angles of mold pieces designed with a push-fit design. Diagonal mold pieces were displayed in each rendering for better viewing angle on the push-fit design. ........... 45 Figure 3.12 Push-fit test for the Prusa MK3 printer. All the measurements are in mm. Tolerances were displayed on the female parts. .............................................................................................. 46 Figure 3.13 Complete second iteration mold design. As seen in the figure, parts are attached together, displaying push-fit design. ............................................................................................. 47 Figure 3.14 Top view and Side view of cured silicone custom-molded orthotics ........................ 48 Figure 4.1 Initial FE modeling results. The von Mises stresses have been plotted. Maximum stress can be monitored at 3.2 × 104 N/m2 (Pa). ....................................................................... 52 Figure 4.2 CAD modelling of 1 mm radius embedded within foam material. Image A displays side view, whereas image B is zoomed into the embedded sphere. ............................................. 53 xii Figure 4.3 Diabetic ulceration simulation achieved with a 1mm radius sphere embedded 3mm below the foam material surface. Maximum von Mises stressed observed 2.9 × 105 N/m2 (Pa)........................................................................................................................................................ 54 Figure 4.4 CAT scanned human foot. Top and side views have been displayed. ........................ 55 Figure 4.5 Preparation of the foot structure for FE analysis. Image A displays the 7 bones left behind that will be utilized. Image B showcases the trimmed model. Top and side views have been provided for each image. ...................................................................................................... 56 Figure 4.6 Each of the 7 bones were connected to one another via 2 mm wide rectangles. The connecting rectangles have been circled in red............................................................................. 57 Figure 4.7 Whole foot indentation. Foam material has been divided into two sections to display von Mises stresses at 1st metatarsal and the heel bone. The experienced stresses are 4.4 × 104 N/m2 Pa and 3.9 × 104 N/m2 (Pa), for the metatarsal and heel bone, respectively........................................................................................................................................................ 58 Figure 4.8 Ulceration stress concentration modeling of the whole foot. Foam material has been divided into two sections to display von Mises stresses at 1st metatarsal and the heel bone. The experienced stresses are 1.19 × 105 N/m2 Pa and 7.19 × 104 N/m2 (Pa), for the metatarsal and heel bone, respectively. .......................................................................................................... 59 Figure 4.9 Top and side views of the donut design at the heel location. Only the heel region has been demonstrated. The sphere seen embedded is the ulceration. ................................................ 61 Figure 4.10 FE analysis of donut design in heel region. The maximum experienced von Mises stress is 3.9 × 104 N/m2 Pa. ....................................................................................................... 62 Figure 4.11 A) Red line indicates the location of the von Mises stresses utilized to plot the B & C graphs shown below. B) Graph of benchmark heel ulceration. C) Graph of donut design. For xiii both graphs; x- and y-axes represent distance from the surface of the insole and von Mises stress, respectively. .................................................................................................................................. 63 Figure 4.12 FE analysis of donut design in heel region. The maximum experienced von Mises stress is 9 × 104 N/m2 Pa. .......................................................................................................... 64 Figure 4.13 Above: The results of benchmark for metatarsal ulceration. Below: The results of donut design. For both graphs; x- and y-axes represent distance from the surface of the insole and von Mises stress, respectively. ............................................................................................... 65 Figure 4.14 Top and side views of the softer material design. 10 mm around the ulceration location was embedded with a softer material. ............................................................................. 66 Figure 4.15 FE analysis of softer material in heel region. The maximum experienced von Mises stress is 3.1 × 104 N/m2 Pa. ....................................................................................................... 67 Figure 4.16 FE analysis of softer material in metatarsal region. The maximum experienced von Mises stress is 27 × 103 N/m2 Pa. ............................................................................................. 68 Figure 4.17 All the conducted FE simulation results are displayed. Top graph depicts the contact pressure in the insole, whereas the bottom graph is the pressures experienced in the ulceration location. ......................................................................................................................................... 69 Figure 5.1 Stress vs strain graph for compression testing of hydrogel (15% PVA, 0.85% BC, 6 cycles). .......................................................................................................................................... 73 Figure 5.2 Tensile Testing Apparatus. Left “CAD rendering”: Red highlights the pinching geometry. Right “Printed version”: Red highlights the drilled hole for rod insertion. ................. 74 Figure 5.3 Hydrogel point of failure (highlighted in red) from tensile testing. Highlighted in blue are the rods penetrated through the hydrogel strips. ..................................................................... 75 xiv Figure 5.4 Stress vs strain graph for tensile testing of hydrogel (15% PVA, 0.85% BC and 6 cycles). .......................................................................................................................................... 76 Figure 5.5 Final Configuration of the Anova cycling system and the exterior bath. .................... 78 Figure 5.6 Test prototype synthesized utilizing the exterior bath. ................................................ 79 Figure 5.7 Final prototype synthesized utilizing exterior bath. Formulation of hydrogel solution effected the viscosity, causing leakage. ........................................................................................ 80 xv List of Abbreviations HVPL The Heart Valve Performance Laboratory PVA Polyvinyl Alcohol BC Bacterial Cellulose ABS Acrylonitrile Butadiene Styrene PLA Polylactic Acid SLA Stereolithography FFF Fused Filament Fabrication 2D 2-dimensional 3D 3-dimensional FDM Fused Deposition Modeling STAR Survive and Thrive Applied Research FE Finite Element EVA Ethylene-vinyl acetate PE Polyethylene PU Polyurethane UTM Universal Testing Machine MR Magnetic Resonance xvi Acknowledgements First and foremost, I would like to express my sincere thanks to my supervisor Dr. Mohammadi, who gave me the opportunity to expand my knowledge in the field of Biomedical Engineering. Throughout my program he encouraged me to become a better engineer and without his mentorship I would not have been where I am today. I would like to present my special thanks to everyone in the Heart Valve Performance Laboratory (HVPL), who provided me with not only much needed mental breaks but alternate perceptives day-in and day-out. To all the technicians that provided insight during my studies. To my family, thank you for constant state of care and love. xvii Dedication To my family, friends & one and only ugly duckling 1 Chapter 1: Introduction Orthoses, also known as insoles, can be identified as shoe inserts that change foot's function. They have been prescribed by health professionals for over a century to treat biomechanical dysfunctions of the foot. Foot orthoses were first used as a treatment tool during the early twentieth century and continue to be the subject of research and technology [1]. In the early days, orthoses were typically utilized to redistribute plantar surface foot forces to relieve discomfort in pressure-sensitive areas of the foot, without any consideration for specific foot abnormalities that might have led to the pathological condition [2]. There are many factors that should be considered while selecting an appropriate material for a shoe insole. The ideal material should be able to distribute the load to areas adjacent to bony prominences. The selected material should achieve sufficient deformation while having some rigidity that limits the deformation is reached. This provides a general guideline for selecting material however, this classification might be misleading for the general public without a thorough knowledge of the properties of each of the materials. Depending on their thickness, layer composition, or form of the material used, most of the materials can range from being soft to rigid. 1.1 Purpose & Overview of This Thesis The purpose of this thesis is to create a procedural method of creating insoles with novel hydro-gel material. Furthermore, the effects of insole design will be displayed with the utilization of finite element (FE) analysis. This thesis includes design, manufacture, simulation and prototyping of novel hydro-gel insoles. This introduction chapter is followed by: • Chapter 2 “Literature Review”: Outlines current criteria within the creation of a functioning insole. In addition, a detailed literature review on common materials that are utilized for achieving specific properties have been discussed. 2 • Chapter 3 “Insole Design & Fabrication”: Describes the procedures taken to creating digital insole designs. Furthermore, features mold creations for these digital insoles and the silicone prototypes that have been made from those molds. • Chapter 4 “Finite Element Modeling”: Showcases how a digitized foot models and insoles can be utilized to gather information on the pressure experienced between two objects. Additionally, FE software can be utilized to evaluate effectiveness of insoles designs. • Chapter 5 “Prototyping”: Outlines work do to identify best suited material properties of hydro-gel material. Furthermore, details procedural steps taken to create an exterior bath for hydro-gel synthesizing process. • Chapter 6 “Conclusion & Future Work”: Describes the work that has been done up to date and authors recommendations for future work. 3 Chapter 2: Literature Review This chapter includes criteria that must be taken into consideration when designing and fabricating orthosis. In addition, methodologies utilized for evaluation of patient’s foot, as well as, a detailed review of the material properties of commonly utilized foams in footwear. 2.1 Material Criteria It is understood that mechanical factors play a major factor in the foot abnormalities, especially in diabetic patients, i.e. elevates the ulceration process in patients with the presence of nerve disease [3], [4]. To ensure a successful treatment of a patient’s foot, foot orthosis has to attain several performance objectives. • Shock absorption properties of a material are dependent on the density and resilience of said material. Low density may be preferred to preserve gait efficiency however, they are more likely to compress prematurely or ‘bottom out’, reducing shock attenuation [5]. On the contrary, high-density material will not compress, failing to have satisfactory shock absorption properties [5]. Similarly, the lower the resilience of material, the greater the shock attenuation capacity [6]. The most suitable material must showcase low resilience and high dampening, absorbing the most kinetic energy produced from the gait of the patient [5]. • Reducing shear forces is crucial for the plantar tissue of the foot [6]. High shear forces could cause displacement of one layer of tissue with respect to another within the foot, leading to the development of ulcerations, particularly imminent in the presence of diabetic patients. • Relieving areas of excessive plantar pressures are dependent on compressive stiffness and compression set of material. The stiffness of a material can be measured by applying 4 compressive or tensile stress and is defined by the resistance a material demonstrates to deformation. In the field of orthotic materials, compressive stress is more applicable as the stress transmitted through the material during walking or running reflects closer to compression. Low stiffness materials are utilized to ensure insole molds sufficiently to the contours of the bony prominences allowing for redistribution of pressure. The compression set is defined by the residual strain in a material after it has been applied a given compression and then allowed to recover for a time [7]. A cushioning material would still be expected to preserve more than 50% of its original thickness for it to have an acceptable degree of performance for an orthotics material [5]. • Providing proper support for the foot deformities. Foot deformities cause the patient to either consciously or subconsciously alter weight distribution through their feet. This is commonly experienced in diabetic patients with peripheral neuropathy [8]. As peripheral neuropathy disables nerve endings at the foot, deformities that cause peak pressures and change the anatomy of the foot go unnoticed. Thus, providing support is a key performance indicator for orthotics materials. Stiff materials can be utilized to achieve support and stability. • Bodyweight transfer through the foot during gait. For this objective to be achieved, all of the other criteria mentioned above have to be applied correctly by physicians and clinicians to best suit the patient in hand. In addition, the durability of the material combined is critical for the lifetime of the insole. Orthotics durability is determined by the number of cycles to loss of performance. Material testing within the field of insoles evaluates S-N curves where S represents cyclical stress and N is equal to the logarithmic scale of cycles to failure [5]. 5 2.2 Design Criteria In general, the purpose of orthotics is divided into two; functional or accommodative. The accommodative orthosis is designed primarily to alleviate any pain or foot deformities, whereas, functional orthotics are designed to provide support for a more flexible foot. It is important to mention whether an orthosis is functional or accommodative, albeit they tend to offer some degree of both properties. Both these functionalities play a critical role in mechanical forces experienced in the patient’s foot. In the footwear industry the terms insoles and orthosis commonly used interchangeably. As for design criteria, this paper will base its terminology on the definition that an orthosis is designed to change the biomechanics of the foot. Therefore, the following definitions are given for the terms used within the review: • Flat insoles are simply have been cut from a flat sheet of material. These orthoses do not frequently get prescribed to patients, as the pressure reduction capabilities of insoles described below are far superior. • Prefabricated insoles are mass-produced and accommodate users without any modifications. They are designed to either provide support or shock absorption for a specific part of the foot such as arch support or heel shock absorption. • Custom insole adds to the already existing prefabricated insole such that extra cushioning or alignment can be provided via i.e. metatarsal pad or heel lifts. • Custom fabricated insoles are made for a specific patient. A three-dimensional impression procedure captures a model of the patient’s feet, where the insole is then manufactured. These models are utilized for patients with more severe or complicated foot problems. 6 2.3 Testing Methodology As with every field of study, there are two main ways of testing in a hope of validating the expected results. Initially, the researchers can validate their claims via experimentation, this will allow for real-life results. Complications of experimentations include the process of setting up and validating of said testing, additionally testing samples and participants have to be arranged prior to the experiment. Moreover, numerous attempts, runs or trials have to be conducted as to further validation and limiting collinearity and controlling parameters can be challenging. On the contrary, simulation analysis can be conducted in an effort to further support claims made by researchers. Simulation analysis in the field of orthotics, typically, utilizes FE softwares. 2.3.1 Experimental Testing Researchers have developed various systems and apparatuses to better evaluate which materials are capable of performing the objectives listed above. Bench testing is referred to as the systems, more commonly known as Ultimate Testing Machines (UTM), that apply pressure or strain to a given material at a presupposed rate. Some researched custom designed and fabricated equipment for evaluating various properties of materials [9]. The main drawback of UTM is the incapacity of applying multiple constraints to the systems. When evaluating materials for the foot industry multiple characteristics have to be evaluated at the same time, i.e. resilience, compressive stiffness, shear forces, durability, etc. A custom fabricated UTM by Paton et al., designed to simulate human gait, can be visualized in Figure 2.1A. 7 Figure 2.1 A) Customized bench testing system to simulate human gait, utilized by Paton et al [5]. B) MobileMat™ (Tekscan Inc., USA) pressure sensing platform [10]. C) emed® (Novel GmbH, Germany) pressure sensing platform [11]. D) FScan™ (Tekscan Inc., USA). in-shoe pressure measurement system [12]. E) pedar® (Novel GmbH, Germany) in-shoe measurement system [13]. A) Reprinted from [5] with the permission from SAGE Publications Inc. Journals. B) Reprinted from [10] with the permission from Tekscan, Inc. C) Figure 2.1C has been removed due to copyright restrictions. It was an image of the emed® (Novel GmbH, Germany) pressure sensing platform. Original Source: [11]. D) Reprinted from [12] with the permission from Tekscan, Inc. E) Figure 2.1E has been removed due to copyright restrictions. It was an image of the pedar® (Novel GmbH, Germany) in-shoe measurement system Original Source: [13]. Evaluating materials more comprehensively is not possible via UTMs. Pressure sensing platform or in-shoe measurement technology developed by Tekscan Inc. (USA) and Novel GmbH (Germany) currently have been leading the field in testing equipment for assessing the suitability of an orthotic material. Both Tekscan Inc. and Novel GmbH have pressure sensing platforms 8 (MobileMat™ and emed®, respectively) and in-shoe measurement devices (FScan™ and pedar®, respectively) available. Pressure sensing platforms are a step closer to ideal conditions sought after by researchers. This technology allows patients to do foot function assessment, pressure mapping, and gait analysis alongside various other tests for researchers to investigate. Although pressure sensing platforms enable researchers to evaluate multiple characteristics of an insole material simultaneously, without making any assumptions to a patient’s gait, there are some shortcomings. For accurate data collection patients have to contact the platform without any shoes, thus eliminating the ability to investigate the effects of footwear or various designs of orthotics on the patient’s foot function. Additionally, the length of the platforms limits the researcher from obtaining multiple strides. Where this technology excels is the real-time analysis of a patient’s pressure measurement and foot function (Figure 2.1 B&C). Many researchers to date have utilized pressure sensing platforms to evaluate the effectiveness of various orthotics materials [14]–[16]. In-shoe pressure measurement technologies developed by FScan (Tekscan Inc., USA) and Pedar (Novel GmbH, Germany) provide data that can be collected as patients perform their daily activities. A thin pressure sensor gets implanted in patients’ footwear that allows researchers to measure various data such as total contact area, the force transmitted through the foot, and peak pressures. This eliminates the necessity to duplicate human gait in UTM systems to evaluate materials’ mechanical properties. Currently, this is the pinnacle of pressure measurement systems in the footwear industry. The versatility of this technology allows researchers to evaluate the effects of various materials in the patients’ foot function, as well as enabling rapid prototyping and testing of multiple insole geometries. 9 2.3.2 Analytical Testing The application of FE softwares is vast; in the field of foot orthotics, it allows researchers to iteratively evaluate their designs. Setup for the simulation involves developing 2D scanned images of a patient’s foot into three-dimensional models. The user can apply various assumptions, constrictions, and loads on the imported 3D model for simulations to be conducted. Existing FE models of the foot were developed by various researchers under multiple assumptions such as linear material properties, linear boundary conditions, simplified or partial foot models, and neglecting friction or slip [17]–[19]. Depending on the focus of research, FE analysis allows researchers to focus on a certain part of the foot, enabling researchers to iteratively optimize insole designs [18], [20]–[27]. The degree of detail and precision can be adjusted via assumptions made; however, a more detailed study would be CPU demanding. Figure 2.2 FE modeling of a foot from 2D magnetic resonance (MR) by Cheung et al. [28] images. A) Constructed 3D model of patient’s foot, identifying boundaries of foot skeleton and skin surface. B) (Left) FE meshed model of soft tissue. B) (Right) Meshed model of all 28 bones in the foot. A) Reprinted from [29] with the permission from PERGAMON. B) Reprinted from [24] with the permission from Elsevier. 10 This process can be visualized in Figure 2.2 by Cheung et al. 2D images were gathered via magnetic resonance (MR) however, oftentimes computed tomography (CT) is utilized as well [30]. These 2D images are then formed into 3D models using various computer-aided design (CAD) softwares. Cheung et al. utilized MIMICS (Materialise, Leuven, Belgium) software to identify the boundaries of bone structure and skin surface. The group took the processed components into SolidWorks (SolidWorks Corporation, Massachusetts, USA) to form solid models for each bone and the skin tissue. The solid models were then imported and assembled in an FE software called ABAQUS (Dassault Systèmes Simulia Corp., Johnston, RI, USA). There are 28 bony premises in the FE model of the human foot, as shown in Figure 2.2C. Including the distal segments of the tibia and fibula and 26 foot bones: talus, calcaneus, cuboid, navicular, 3 cuneiforms, 5 metatarsals, and phalanges that consist of 14 components embedded in soft tissue. The group, instead of modeling connective tissue between the phalanges decided to connect the phalanges using a 2 mm thick solid element. Cheung et al. suggest these assumptions simulates the connection of cartilage and other connective tissue by cutting down on the processing time of the solution [24], [28], [31]. Additionally, a similar process was followed for the rest of the bones. Instead of modeling cartilage and connective tissue, interactions among the remaining 14 bones (excluding the phalanges) were defined as contacting elastic bodies to allow the simulation of relative bone movement [28]. 11 Figure 2.3 FE modeling of the attachment points of the plantar fascia, long plantar, short plantar, and spring ligaments [28]. B) Reprinted from [32] with the permission from SAGE Publications Inc. Journals. By modeling 72 ligaments, as well as the plantar fascia, were defined by connecting to the corresponding points on the bones. The attachment points of major ligamentous structures such as plantar fascia, long plantar, short plantar, and spring ligaments are depicted in Figure 2.3. Cheung et al. achieve the natural behavior of human foot via simplifying the FE model [28]. Most FE studies conducted regarding foot and ankle model for investigating insole designs idealizes all tissue, bones and connective tissue to be homogeneous, isotropic and linearly elastic [17], [18], [20], [22], [24], which was one of the other assumptions made by Cheung et al. The group utilized ABAQUS software, which has a variety of solid elements packages. Hexahedra elements (brick) can provide higher accuracy at a less computing time considering a complex geometry [28]. However, the limiting factor was the automatic-meshing algorithm of ABAQUS causing irregularly shaped meshed elements, 4-noded tetrahedral elements were used instead for 12 meshing the bone structure and encapsulating soft tissue. Truss elements were utilized to represent the ligamentous structures as ligament are assumed to only sustain tensile forces, no-compression option in ABAQUS was used to modify the elastic behavior of material eliminating compressive stresses on the ligamentous structures. As for interactions among the bony structures, the group assumed it to be frictionless as the lubricating nature of cartilage tissue and utilized the automated surface-to-surface contact algorithm in ABAQUS [24], [28]. A solid horizontal support was utilized to establish a foot-ground interface, the coefficient of friction between the interface being 0.6 [33]. Figure 2.4 Procedures for creating an orthosis model from the patient's 3D surface foot model [28]. A) Figure 2.4 has been removed due to copyright restrictions. It was an image of a diagram displaying steps taken from patient’s foot model to creating an appropriate orthosis. Original Source: [28]. The geometry for the orthosis was obtained from the barefoot shape of the patient. The 3D shape of the foot was digitized using a 3D laser scanner. An algorithm was then put into place in Matlab (Mathworks Inc.) to create surface models for the insole, which were then imported to SolidWorks for the creation of various thicknesses. This process is visualized in Figure 2.4. The insole model was then transferred into ABAQUS for FE meshing. Modeled insole was partitioned into 3 layers (insole, midsole, and outsole layers) as hexahedral elements were utilized to enhance the accuracy 13 of the FE analysis. The finalized meshed model can be seen in Figure 2.5. The frictional contact model was utilized to establish the contact simulation between the foot and the insole. Figure 2.5 Meshed model of Matlab generated orthoses [28]. A) Figure 2.5 has been removed due to copyright restrictions. It was an image of a meshed orthosis. Original Source: [28]. Two loading conditions were subjected to the model: balanced standing and midstance. Balanced standing simulation only considered the Achilles tendon loading while forces on the intrinsic and extrinsic muscles were neglected. Soft tissue contacting the ground was considered fixed, along with tibia and fibula segments in the model. Midstance simulation was obtained via the ground reaction and the active extrinsic muscle forces. The extrinsic muscles were speculated from muscle cross-sectional area and electromyography (EMG) data gathered by Perry et al. [34] utilizing a linear EMG-force assumption by Kim et al. [35]. Findings of Cheung et al. suggested pressure reduction can be achieved with a higher degree with custom-molded orthosis compared to the stiffness of orthotic material. Utilizing an arch-supporting foot orthosis as seen in Figure 2.6 distributes the weight of the patient much more effectively compared to flat insoles. 14 Figure 2.6 A) FE simulation of the custom-mold interface. B) Comparison of FE analysis results: flat vs custom-molded arch-supporting insoles. [28] A) Reprinted from [36] with the permission from Elsevier. 2.4 Material Properties This thesis will discuss and categorize the materials depending on the type. This is done to help the reader differentiate between the materials. Additionally, each material testing will be sectioned into possible four; depending on the testing methodology that has been utilized, as not every material has been subjected to the same category of testing. These sections are bench testing, simulated in-shoe conditions, in-shoe testing, and FE analysis. 2.4.1 Polyethylene (PE) Foams In a general study conducted where a variety of PE foams were put through mechanical and comfort-related standards for characterization tests to identify whether or not they would be applicable for insole manufacturing [37], [38]. Within the six PE foams tested, the authors concluded that as the foams in higher hardness range can be utilized in creating support and stability through the insole, whereas, the low hardness PE foams can be utilized in reducing plantar 15 pressure by conforming into the shape of the foot. Furthermore, they are perspiration resistant and able to absorb humidity inside the shoe. 2.4.1.1 Plastazote (Zotefoams, London, UK) Plastazote is a closed-cell product made from polyethylene. The material is cross-linked via either chemical linking or high-energy electrons. This kind of bonding allows the material to attain strength and toughness that would be required to perform in the shoe insole industry. Bench Testing Pratt et al. conducted a study that examined the shock absorption properties of Plastazote with four other materials [39]. The experiment included a ball bearing of 0.056 kg which was dropped from 1.02 meters on to each material. Plastazote may have been classified as having potential use in previous studies [40], [41] however, Pratt et al. discovers the inability of shock absorption of this material. The authors decided to carry out an extra test on old Plastazote, which was worn by one of the authors for 72 hours. In a more in-depth study, which assessed density, hardness, tensile strength, compression and compression set, by Rome et al. exhibited Plastazote having shock absorption properties [7]. Plastazote displayed less shock absorption properties compared to open-cell polyethylene foams but showed more shock absorption compared to closed-cell rubber material. Simulated In-shoe Testing Campbell et al. compressed two types of Plastazote for 10 minutes and 60 minutes with a maximum stress of 220 kPa [40]. Plastazote exhibited the greatest nonrecoverable strain of all materials, meaning the materials encountered a change of volume as cyclic loading continued. This would have to be accounted for, in the design of the insole. Additionally, Plastazote materials become increasingly stiff with higher stresses, which is a similar feature of skin and connective 16 tissue. Matching the material properties with the soft tissue-body support interfaces can reduce stress concentrations in the interface. Sanfilippo et al. examined the pressure reduction properties of Plastazote by taping the sheet of material, approximately 1/8 inch thick, on a force plate system [14]. The results were gathered after ten patients walked across the force plate a total of eight times. Plastazote was one of the materials that achieved the largest total plantar surface of the foot. The authors believe that this characteristic is used with the intent of reducing plantar pressures. Plastazote, by conforming to the shape of the foot, increases the area of the foot contacting the ground which allows for the material to absorb force, consequently resulting in greater pressure reduction. Curryer et al. exhibited the performance of Plastazote in reducing plantar shear forces [15]. The material was taped to a pressure measurement system. The results revealed that Plastazote was one of the least effective materials at reducing the resultant shear forces on the plantar surface of the foot. The authors mention that Plastazote, being a closed-cell, would have been expected to perform as well as Spenco, however, it did not. This may be explained by the nylon top covering Spenco has, and the role it plays in reducing shear forces. Furthermore, only Plastazote was able to reduce the braking forces to a greater degree than the propulsive forces. However, previous studies that utilized shear transducers have found that there was no clear distinction between braking and propulsive forces [42], [43]. Foto et al. for the first time evaluated combinations of two and three different materials [44]. The authors believed that materials used in orthosis should maintain a dynamic strain of 50% or better at 350kPa, thus they set-up in-vitro testing machinery to simulate four months of wear (100,000 cycles). Material combinations that had greater dynamic strain at a given pressure or ability to compress more was considered to be the more accommodative and effective at reducing plantar 17 pressures. Two different densities of Plastazote (#1 and #2) were utilized during this experiment, however, the authors fail to specify which Plastazote has a higher or lower density. The results exhibited that the material combination of Poron and Plastazote #2 had greater dynamic material deformation and lower dynamic compression set than the other combinations. Initially, Poron and Plastazote #2 combination were not as accommodating as the other materials but after 1,000 cycles it became and remained the most accommodating. Comparatively, Poron and Plastazote #1 combination was the authors’ third choice after rubber foam combinations. It is important to mention that when tested individually Plastazote #1 and #2 exhibited rapid wear and lower capacities to deform strain than Poron combinations. On the other hand, the results indicated the lowest shock absorption and the highest deformation when the two Plastazote materials were combined. In-shoe Testing Rogers et al. investigated the effectiveness of Poron insole and Poron/Plastazote insole combination before and after wear (50,000 steps) compared to no insole condition [45]. The results examined that Poron/Plastazote combination was more effective at reducing plantar pressure. In this combination, Plastazote enables the ability to redistribute plantar force over a wider area. On its own, Plastazote has poor elastic memory properties and is unable to return to its original thickness eventually “bottoming out” [46]. Bottoming out causes a reduction of force distribution and results in increased pressures. However, when combined with Poron the bottoming out effect found in Plastazote is slowed down. As previous research has stated Poron/Plastazote insoles continued to reduce peak pressure after they had been worn, compared with no insole condition [47]. 18 2.4.1.2 Pelite (OrtoPed ULC, QC, Canada) Pelite is best described as a mass of bubbles simultaneously composed of solid and gas phase [48]. The PE polymer is distributed within the walls of the bubbles and solidified where the bubbles intersect. The closed-cell configuration of this solid and gas phase restricts the gas from entering another cell. Bench Testing Sanders et al. extended the work of Campbell et al. by measuring the materials on a quantitative basis as opposed to the qualitative assessment procedure [9], [40], [41]. Two tests were conducted a 10 min and 60 min long, utilizing a diameter of 11.1 mm while employing a custom-designed and constructed compression testing device capable of delivering 220 kPa at a frequency of 1 Hz [9]. Soft Pelite and Medium Pelite demonstrated the greatest decrease in specimen thickness at 4.6 and 4.53 percent, respectively, of original thickness, indicating plastic deformation. Additionally, over the 60 min loading test, Soft Pelite experienced the maximum encountered compression set at 3990 kPa. After a week of testing, Soft Pelite had a mean decrease in thickness of 2.9 percent [9]. The results combined by Sanders et al. matched with the founding of Campbell et al. Soft and Medium Pelite experienced the greatest nonrecoverable strain out of all the materials [9]. This has to be taken into consideration in the design, the material changes in volume due to continuous cycling loading. Simulated In-shoe Testing In Sanfilippo et al.’s study conducted to examine pressure reduction properties of five commonly used insole materials, including Pelite, Plastazote, and Spenco, the group employed a force plate system as the materials were cut in the size of the plate with a thickness of 1/8 inch [14]. In total 480 trials were conducted with ten subjects, results indicated all the tested materials, including 19 Pelite, reduced peak pressures, as well as increasing the plantar surface of the foot to the ground. Even though there was peak pressure reduction as well as an increase in the total plantar surface area, Pelite was the poorest performing material overall. 2.4.2 Polyurethane Foams Polyurethane (PU) foams are considered the most important tranche of the polymeric foams [49]. World foam production is dominated by PU foams [50], [51]. PU foams are responsible for 50 percent of global PU consumption [52]. PUF is a thermosetting, non-heat-moldable foam that is manufactured either as an open or closed-cell foam. 2.4.2.1 Poron (Rogers Corporation, Arizona, USA) Poron is an open-cell PUF that has been developed by Rogers Company of Connecticut utilizing a patented process. It is engineered from a combination of polyether and polyester resins and it's uniform structure allows Poron to have excellent compression set resistance. Bench Testing In an in-vitro experiment conducted, by Campbell et al., to measure if the 31 selected materials were capable of absorbing high compressive pressures applied by a Universal Testing Machine. The authors designed the compressive force to apply rapid deformation on the materials to eliminate the materials that would bottom out. In the selected 31 materials authors included three different types of Poron; Poron-20125, Poron-‘Sport’ and Poron-17125. The results of the experiment showed that Poron is capable of achieving pressure distribution [41]. Sanders et al. conducted a study where Poron experienced minimal unrecovered strain meaning, the material maintained its thickness and volume after releasing from each cycle of loading [9]. Poron’s stress and strain curve examined a moderate-to-high initial slope followed by a plateau and then a steeper slope. The stress and strain curve exhibit the material does not become stiffer 20 as stress is increased but instead, it compresses significantly after the initial loading cycles and then becomes stiffer at higher stress levels. The same group conducted an additional experiment where the coefficient of friction (COF) for the skin and sock interfaces with the material [9]. Results placed Poron to be in the lowest COF group. The authors indicated that materials that had lower compressive stiffness also had a lower COF. However, this link was abandoned when surface characteristics of the materials were considered. Poron having a very smooth surface resulted in having a low COF. Similarly, Paton et al. exhibited four types of Poron: 92 (6mm), 94 (6mm), 96 (6mm), 4000 (6mm and 3 mm), to examine resilience, stiffness, coefficient of friction (COF), durability and compression fatigue features of the materials [5]. Resilience was tested by dropping a 25-mm ball bearing from a height of 1 meter on to the prepared material and was calculated as the rebound height as a percentage of the original rebound height without any material. Three of the Poron samples (92, 94, 96) displayed low resilience which was less than 10 percent. Additionally, Poron 4000 specifically, sustained no visual thickness loss after the repetitive loading test. The author’s concluded that 96 and 4000 (6mm thick) types of Poron demonstrated the most desirable dampening characteristics. Faulí et al. expanded on their study previously done by Andrés et al., in 2006, with updated standards for the characterization of material tests [37], [38]. The study exhibited six polyurethane foams from which three of them were Poron. The results revealed PUR foams in general (including Poron), are most suitable for cushioning applications. Showing the ability to absorb high energy makes them have low resilience and to absorb humidity generated inside the shoe makes them perspiration resistant. Additionally, high compression set resistance and high compression fatigue resistance enhances the durability of the insole made from PUR foams. 21 Simulated In-shoe Testing Foto et al. examined the pressure reduction effect of combining two and three different materials [44]. The authors prepared in-vitro testing machinery to simulate four months of wear (100,000 cycles) on to the combined materials. The results exhibited that the material combination of Poron and Plastazote #2 excelled at reducing pressures after 1,000 cycles. However, other Poron combinations like Poron+Microcel+Plastazote and Poron+Plastazoe #1 did not achieve the same results. The authors noted the importance of choosing the right material combinations, as well as the right density of that particular materials to be combined to achieve specific mechanical properties. Campbell et al. expanded their initial study of 31 materials from testing materials purely by compression fatigue to simulating human gait through repetitive and sustained loading, and heat [40]. Since all the different testing conditions were conducted separately authors looked for materials that had passed in all testing conditions including the first study ([41]). In all the testing conditions Poron family of materials (‘Sport’, 20125, and 17125) performed superior to the other materials tested: only exhibited less than 20 percent loss of original thickness proceeding sustained and repetitive loading. In-shoe Testing Rogers et al. evaluated the effectiveness of Poron and Poron/Plastazote at reducing plantar pressures under the forefoot area compared to wearing no insoles [45]. The authors measured pressure-time integral and peak pressure under the forefoot before and after 50,000 steps compared to no insole testing condition. Data were collected utilizing the FScan in-shoe pressure measurement system. 50,000 steps were registered using a pedometer. The results correspond with the previous studies, indicating that Poron insoles were able to reduce peak pressure by distributing 22 plantar forces over a larger area via increasing the surface contact area of the foot [14], [44]. Even after being worn, the Poron insole continued to reduce peak pressure compared to no insole however, there was a significant increase in peak pressure (before mean = 130.70kPa, after mean = 152.36kPa). Even though previous studies have supported the preservation of peak pressure reduction, no study was found to support the increase in peak pressure [46], [53]. The authors justify this from the micro-architectural damages or changes that might have occurred since the first set of data collection took place in February and second in April. Poron/Plastazote combination also provided peak pressure reduction before (mean = 134.20kPa) being worn and after (mean = 139.19kPa) compared to no insole condition. While both insoles were effective at reducing peak pressure, Poron/Plastazote combination was more effective than Poron insole after 50,000 steps. Poron/Plastazote insole visibly molds to the shape of the subject’s feet, this allows the load to be redistributed through the compression of the material instead of breaking down the plantar skin of the subject [15]. Tong et al. investigated the pressure reduction abilities of different padding and insole materials utilized by podiatry clinics [54]. Plantar pressures were measured for five volunteers, using FScan in-shoe pressure system, initially in barefoot condition (BF), and following with four pairs of 6.4 mm thick insoles: Slow Recovery Poron® (SRP), Poron® (P), Poron®+Plastazote (firm) (PPF) and Poron®+Plastazote (soft) (PPS). Additionally, the authors prepared 7 mm semi-compressed felt (SCF) padding with a circular cut-out locating at the first metatarsal joint to simulate an active ulceration site. The padding is a very common short-term clinical practice for the podiatrist to utilize for offloading an area of the foot. However, this padding is not suitable for pressure reduction after one week thus a new SCF padding has to be prescribed [55], [56]. The results concluded PPF combination was significantly more effective at reducing pressure compared to 23 other materials. Naturally, Poron® with a softer Asker index would be more accommodating to dissipate pressure however, Plastazote is more resilient, durable and it conforms to the shape of the foot maintaining pressure reduction further [45]. Therefore, the combination of Poron® and firm Plastazote insoles allows the initial impact to be absorbed by Poron® whilst Plastazote takes the shape of the foot, increasing the surface area over time. This material was also equally effective at reducing the focused area of pressure where the neuropathic ulcerations would develop. Birke et al. examined the effects of seven various hardness of Poron materials on the pressure reduction abilities [53]. Nineteen volunteers with diabetes and a history of plantar foot ulceration participated in this study, but at the time they were free of ulcerations. The authors appointed the pedar® system, which is an in-shoe pressure measurement system. Sensors were placed and secured against the plantar surface of the foot with a nylon stocking, which was then fitted over orthoses inside the shoe. Participants performed nine walking trails at their own pace for a distance of 40 feet. Pressure measurements were recorded for five steps in the middle of each walking trails of the following nine trails: no orthotics in a standard shoe, Poron conditions one to seven, and the patient's custom-molded orthotics and shoe. The results revealed, mean peak pressure was lower when the patients were walking with Poron conditions 3, 4, 5 (Shore ‘O’ hardness of 22, 27, 32, respectively). The softer and firmer formulations of Poron were less effective but were still more effective than no orthosis condition. Sheets of quarter-inch thickness were utilized for this study and authors state that anything thinner than ¼ inch may not relieve pressure because of bottoming-out effect [19]. Even though flat sheets of Poron reduced foot pressure, they were not as effective as the patients’ custom-molded orthoses, which reduced mean peak pressure by 55%. Generally, molded orthotics are more effective at reducing pressures than non-molded orthotics [57]. 24 Burns et al. investigated the effects of common orthotics on a 59-year-old male patient who has been living with 40 years with type 1 diabetes mellitus [58]. The patient has been suffering from foot pains for five years. Upon medical assessment, the authors revealed that the patient exhibited peripheral neuropathy and severe peripheral arterial disease. Although the patient had health complications, his foot structure was considered to be normal by Foot Posture Index [59]. The study evaluated the orthotics in comfort and effectiveness criteria. The authors allowed the patient to have trial walks in eight common orthotics materials used in the industry at the time, four of which were Poron. Pedar system was utilized to detect the plantar pressures captured at the forefoot, midfoot, and rearfoot regions. This decided which orthotic was most effective at reducing pressures. The comfort level of these orthotics was assessed during the trail walks of the 20-m walkway, with a visual analog scale [60]; 0 mm being not comfortable at all, 150 mm is the most comfortable. Only Poron material combinations tested in this study achieved plantar surface reduction, where slow-rebound cellular urethane and soft cellular urethane (6 mm Poron Performance/6 mm Poron Medical) reduced the peak pressures by 23%. There were, however, two orthotics that achieved greater pressure reduction; one was a combination of 3 mm Spenco with 6 mm Poron Medical which attained 26-percent, and contoured multi-density foot orthoses named Step 2 Evolution by Thanner GmbH, a company based in Germany, achieved a pressure reduction of 55%. The second portion of the study appointed the most comfortable (and most effective pressure reducing orthoses in an ABAC alternating treatment design over an eight-week period (A= no orthoses, B = most effective orthoses, C= most comfortable orthoses, for 2 weeks duration). The patient stated that despite being the most comfortable initially the slow-rebound cellular urethane and soft cellular urethane combination orthoses caused foot pain by the end of the day. 25 Finite Element Analysis In an FE study conducted by Goske et al. Poron Cushioning was tested to validate its force dampening properties [17]. A model of the heel geometry of a healthy adult male was obtained from a magnetic resonance image. The same adult male was then brought into to measure the force produced from patients heel strike, in a standing position, with and without materials placed, and during gait. The main novel technique this study had tested for compared to previous work ([18], [19]) was the analysis of multiple design variables such as contact area of insole and thickness of insole. With the total contact thinner insoles, simulations exhibited higher peak pressure with the softest material tested (Poron Cushioning). 2.9 percent increase in peak pressure behavior can be attributed to bottoming out of the material. Poron Cushioning was compressed to 59 percent of its original thickness at maximum heel strike load. 2.4.3 Rubber Foam Rubber foam is considered to be under the umbrella of elastomers. In the 1930s, Dupont invented synthetic rubber neoprene. Neoprene is utilized extensively industry-wide in the manufacture of foot orthotics. It is usually on the undersurface of a rigid or a semi-rigid plastic shell. Main uses include; balancing or posting materials, and also utilized for providing some degree of cushioning and shock absorption. 2.4.3.1 Spenco (Spenco Medical Corporation, Texas, USA) Spenco is a neoprene closed-cell with entrapped nitrogen bubbles and a polyamide (nylon) top cover. They are dedicated to absorbing vertical forces, torque and shear forces. This can be useful for preventing blisters in athletes and ulcerations in diabetic patients. Spenco achieves these benefits by reducing the increased plantar foot pressures which cause damage to the soft tissue on the patient's foot. 26 Bench Testing In a very early study conducted by Pratt et al. in 1986, the shock absorption of five materials was measured by the impact of a ball bearing of 0.056 kg which was released from 1.02 meters onto each material [39]. Results exhibited that Spenco was not the most suitable material when the shock absorption aspect was required for an insole. The authors added if there were a compression set experiment conducted for Spenco, it would not last as long as other materials might. Sanders et al.’s study disproved the hypothesis proposed by Pratt. When put through the 10-minute compression testing Spenco experienced a significant decrease in materials thickness which is an indication of plastic deformation [9]. However, after the 60-minute compression testing was finalized Spenco was able to maintain its thickness, volume, and energy-absorbing capabilities. When measuring COF for the material’s shear properties, at a low shear force of 2 N, Spenco demonstrated the lowest coefficient compared to the other materials. This was linked due to the materials though rubber-like consistency and a membrane of woven covered surface. Simulated In-shoe Testing Curryer et al. measured the effect of Spenco on plantar pressures by taping the materials on to a force plate measurement system [15]. Most studies focus on peak pressures however it is essential to the amount of time the force is applied. This study exhibits the force-time graph as well as the amount of time the forces are loaded. Spenco was one of the three materials that were able to reduce peak vertical forces, even though the reduction was not significant. The group concluded that Spenco was the most suitable for orthotics out of the other foam material. However, the gel materials tested in the study displayed superior shear force reduction than Spenco. Knowing the bottoming out effect that gel materials are susceptible to; the group hypothesized a combination of gel with Spenco could have a greater reduction of plantar pressures. 27 Sanfilippo et al. utilized the EMED-SF force plate system to measure plantar pressure reduction capabilities of Spenco among other materials [14]. The materials were taped along each end to prevent stretching and sliding while no reduction in shear forces. Results demonstrated that using insole materials reduced force, peak pressures and increased the plantar surface of the foot to the ground. Spenco among two other materials achieved the largest total plantar surface of the foot. The same results were concluded with McPoil et al.’s study, which used the force plate system, as they exhibited the greatest increase in contact area with Spenco [16]. McPoil et al. differentiated their study by dividing the plantar surface of the foot into three sections: forefoot, midfoot, and rearfoot. Spenco dramatically increased the total plantar surface area of the foot, via conforming to the shape of the foot, the most out of the other materials tested in this study. Additionally, Spenco was the most effective to reduce maximum plantar pressures experienced in the rearfoot region. This reduction was significant compared to the other materials. 2.4.3.2 Ethylene-vinyl acetate Ethylene-vinyl acetate (EVA) foam is one of the more common materials utilized in orthotics manufacturing for shock absorption, a density range between 300 kg/m3 and 400 kg/m3 is ideal for insole molding in commercial manufacturing [61]. By changing density range it is possible to control the elasticity and resilience of the material [62]. Bench Testing EVA foams that obtain a low hardness are the most suitable materials for adaptation [37], [38]. Faulí et al. and Andrés et al. reached the consensus that EVA materials have low compression set resistance and low compression fatigue resistance which allows the materials to deform significantly. EVA foams within low hardness range can be useful in reducing plantar pressure by conforming to the shape of the foot and provide comfort by removing humidity generated inside 28 the shoe. EVA materials that have high hardness values can be utilized as filling materials as they are capable of providing stability and support to the insole; their high compression set resistance and high compression fatigue resistance enhance the durability of the insole. Simulated In-shoe Testing Dixon et al. in 2003, expanded on the study done by House et al. [63]: they introduced new prefabricated insoles, created various running simulations (100, 1,000, 10,000, 40,000, 60,000, 100,000 cycles) and measured the effects of plantar forces in terms of bio-kinematics of lower limb chain. The results were consistent with the findings of previous studies, where insole C (polyurethane foam with EVA heel cup) displayed the lowest peak plantar force, peak loading rate, and stiffness. Additionally, insole C presented the highest impact absorption with more than 30% reduction compared to no insole conditions. House et al. examined the effects of three different, commercially available, prefabricated insoles have on the plantar pressures, in military combat boots of army recruits [64]. Each brand new, prefabricated insole was exposed to 40,000 cycles of the maximum load of 500kPa to simulate 100-130 km of running. Two of the three insoles were able to reduce peak pressures; all polyurethane foam insole of 6 mm thickness (insole A) and polyurethane foam insole of 6 mm thickness with high-density EVA heel cup (insole C). In the preliminary tests, insole C showed higher stiffness than insole A however when measuring peak deceleration rates, insole C managed a 50 percent reduction compared to the no insole conditions. After completion of the in-vivo experiments, results between these two insoles unveiled insole C found to be the most successful at reducing peak pressures. 29 2.5 Futuristic Designs After years of research on different types of foam like materials, as well as their mechanical characteristics, industry have shifted the focus on design and shape of orthotics over the last decade. With the development and easy accessibility of FE analysis softwares, researchers shifted their scope on the unique design of insoles to better reduce plantar pressures experienced on a patient's feet. This technique is much applicable for custom orthotics manufacturers, as every patient suffers a unique variety of peak plantar pressures and developing an engineering solution to custom fit orthotics for patients creates a more effective platform. However, there are still some researchers pushing the limits of insole materials as their developing and testing hybrid materials. 2.5.1 3D Weft Knitted Spacer Fabric A study conducted by Hamedi et al. focuses on the offloading capabilities of a weft 3D spacer fabric for people with diabetes [65]. The authors’ main objective was to identify the configuration at which the spacer fabric can absorb as much energy as possible to keep the relevant transmitted stress under a certain limit. They evaluated the effects of spacer material, diameter and slope at which spacers were positioned. Shape memory alloys (SMA) were selected due to their superelastic properties, enhancing the cushioning properties of the 3D knitted spacer fabrics. For investigating the effect of SMA materials, 30 samples were divided into six groups; the first group used steel wire, the second group utilized polyamide and the other four sample groups used SMA wires, with various diameters, as spacer monofilaments. The group assigned INSTRON 5566 machine to conduct quasi-static compression tests, the specimens were compressed up to 80% of their initial thickness, at a rate of 12mm/min [65], to avoid excessive compression of outer layers. Results of the testing exhibited better-cushioning behavior for the samples with SMA wires as spacer yarn compared to the steel and polyamide spacers. In addition, for the same type of material, 30 energy absorption increased as the diameter of the spacer yarn increased. When two different spacer slopes were examined, it was found that the SMA sample with higher inclined spacers was more resistant to compression and had higher energy absorption capacity. To summarize the authors' findings, 3D weft knitted fabrics with coarse SMA spacers in high-inclined patterns are the most suitable option for cushioning pads. 2.5.2 Polyvinyl Alcohol Cryogel (PVA-C) PVA is a hydrophilic polymer with various mechanical properties desired for biomedical applications. It can be transformed into solid hydrogel by physical cross-linking, using a low-temperature thermal cycling process, eliminating any toxic residuals that might be present in other solidification methods like; ℽ-irradiation, electron beam or use of chemical agents [66]. A solution of suspended bacterial cellulose (BC) is added to PVA to create a 15% PVA with a 0.5% BC solution by weight fraction [67]. This hybrid solution is then transferred into desired molds to be thermally cycled between 20°C and -20°C at a rate of 0.1°C/min [68]. It is vital to note; longer the samples were exposed to the freeze-thaw process the group found an increase in the stiffness of the hydrogel samples [66]–[68]. The sixth cycle being maximum obtained stiffness as there was no observed change in the stiffness of the material post six cycles. Mohammadi created rectangular samples of 25mm by 5mm to perform tensile tests, utilizing the servo-hydraulic material testing system (INSTRON 8872) [66]. The samples were secured with tissue grips with ~10mm distance, and tests were conducted at a pulling rate of 1mm/s to a maximum 60 percent tensile strain [66]. The results revealed hydrogel samples (after four cycles) showed the same stiffness characteristic in circumferential and radial directions as porcine heart tissue. Even though the research on PVA based hydrogels does not extend into the insole industry, the anisotropic nature of the material is bound to have an impact. 31 Chapter 3: Insole Design & Fabrication This chapter details procedures taken to design and fabricate insoles. The main purpose of this chapter is to enlighten the reader of how the insoles were designed, and what steps were taken to fabricate. Insole design will encompass from design procedures to mold creation and fabrication. The fabrication process will only entail silicone models, as to creating a hydrogel model will be covered extensively in Chapter 5: Prototyping. 3.1 Initial Insole Design The initial insole design was inspired by the existing insoles that the HVPL had acquired from previous projects. SolidWorks™ was selected as the CAD software for its availability, easy but effective interface. Organic contours that exist in the human foot reflects on to insoles. This makes it incredibly difficult for free hand modeling. Thus, previously acquired insoles were utilized to attain these natural contours. The images of the insole were taken from specific angles to showcase the contours from each viewing point. As seen in Figure 3.1; contours observed from top, bottom and side views allowed for the natural shape of the insole to be digitized on SolidWorks. Figure 3.1 Okaped insoles side, bottom and top views. Contours were utilized to digitalize insoles. 32 3.1.1 Arbitrary Insole Design It is important to note that a custom molded insole should be specific to one patient, as every patient has different foot anatomy. There are also mass-produced orthotics that are made to fit certain conditions patients might have. These are the orthotics that can be found in drug stores around the nation. The orthotics that were previously acquired by HVPL were samples manufactured by Okaped (company based in the Okanagan). Whether the samples were manufactured for a certain person or general use, the idea behind creating a digitized insole is to validate manufacturability of hydrogel insoles. Figure 3.2 Isometric and side views of arbitrary insole design. Digitized insole was created using ‘surface’ features on SolidWorks. This feature allowed to design and manipulate natural contours found on human feet. When all the natural contours were satisfactory, the surfaces were stitched together and a solid was created using various features on the CAD software. Finalized model of the insole was then taken into a finite element (FE) software to test effectiveness of the design which will be discussed in Chapter 4: Finite Element Modeling in detail. 33 3.1.2 Mold Design Mold making process is essential for a successful prototype of hydrogel insole, as the hydrogel material is a product of synthesized hydrogel solution. The process of which the hydrogel material is synthesized will be covered in detail at Chapter 5: Prototyping. A mold had to be created to obtain prototypes of the insole. For each iteration of the mold design, the Stratasys Mojo 3D printer was employed. The Stratasys Mojo 3D utilizes a fused deposition modeling (FDM) technology, while using acrylonitrile butadiene styrene (ABS) thermoplastic material as its filament. The advantage of using the Stratasys Mojo 3D, is its ability to create detailed and complex structures with utilizing a soluble support filament. 3.1.2.1 First Iteration First mold was designed for a scaled down model of the arbitrary insole. This was done to ensure the design process was proven successful before developing full-sized insoles. Initial iteration of this mold was created simply to satisfy basic needs of creating a prototype. The mold seen Figure 3.3 was printed at a 0.5 scale. The purpose of scaling down the prototype was to ensure proper curing of the silicone material. Two things that are crucial for the success of this mold: simplicity of the solution pour and curing process, and to proficiently remove the cured material out of the mold. Figure 3.3 First iteration of mold printed with ABS on Stratasys Mojo 3D 34 Silicone material utilized to test the effectiveness of the mold was a two-part silicone rubber from Smooth-On. The mixture ratio recommended by Smooth-On was a 1A:1B by weight or volume. Part A would be poured into a container to be stirred thoroughly. The same amount of part B would be added into part A to be stirred thoroughly before being poured into the molds. The solution would then have to be allowed to cure for ~12 hours. Curing process was depended on the mold itself, if the mold was exposed to air it would take less time. The result of this mold can be seen in Figure 3.4. Silicone model was easy to remove from the mold. This is a promising technique, especially for the removal of silicone. The air pockets seen from Figure 3.4 is a potential issue. Additionally, the mold must be sealed completely when utilizing this design for hydrogel material. The synthesizing process requires the mold to be completely submerged and these issues will be addressed in the second iterations. Figure 3.4 Silicone models produced from first iteration mold printed on Stratasys Mojo 3D. Air pockets were a detrimental issue as a large one is apparent at the toe region. 35 3.1.2.2 Second Iteration As discussed in the previous section, second iteration will focus on creating a mold that will be covered, as well as air pockets that might be captured inside the solution. As seen in Figure 3.5, this mold consists of two parts; the cavity (white) and the top cover (red). The top cover has two air holes that are 1.5 mm in diameter, this was implemented to provide an escape for the air pockets. Additionally, four larger holes were designed into the perimeter of the mold. These 4.1 mm in diameter holes were included to fit M4 bolts. This was done to seal the mold, not allowing any solution to pass through the two parts. Figure 3.5 Second iteration mold renderings. White part is the cavity portion whereas, red part is the cover for the mold. Red and white textures were applied to better visualize different components. During the design process, to be more conservative with filament usage, the excess material was shaven down closer to the insole itself. This caused the mold to be unstable. When silicone poured into the mold and all the bolts were tighten, solution started dripping out. The seal provided by the 36 bolts were not satisfactory. The dripping was uncontrollable and therefore, no silicone model was created. 3.1.2.3 Third Iteration This iteration of the mold design will focus on the issue encountered in the previous section. The source of the solution dripping out of the mold was caused by gaps between the cavity and the top cover. Instead of adding more bolts to the mold to create better sealing, a push-fit design was adopted. A lip, 0.5 mm wide by 1 mm height, was protruded out of the cavity mold. This protrusion was 1 mm away from the cavity, allowing the user to overfill the mold slightly. With the implemented airholes, the excess solution can be squeezed out, eliminating any air pockets that might be captured in the solution. Figure 3.6 Third iteration of arbitrary mold design. In red is the top cover and in white is the cavity portion. Orientation chosen to display the push-fit feature. 37 The silicone was prepared and poured into the mold as explained in previous subsection 3.1.2.1. Additionally, M4 sized bolts were tightened on the mold to ensure no leakage was present. After minimum of ~12 hours of wait period, the cured silicone mold was carefully removed. The excess material was cut-off, and the result can be seen in Figure 3.7. This result confirmed the principle of producing a hydrogel insole, while utilizing 3D printed parts as molds. Figure 3.7 Third iteration mold result. 3.2 Customized Insole Design Upon partnership of HVPL and Wiivv Inc. for an Engage Grant, custom orthotics were acquired for the purpose of implementing the novel hydrogel into the Wiivv Inc. design. Wiivv Inc. is a footwear company located in Vancouver, British Columbia that specializes in manufacturing custom-made insoles and sandals. Customization for each customer begins with their smart device. Wiivv ultimately utilizes their customers’ mobile devices along with scan agnostics and computer vision technology, as a 3D scanner to get an impression of their foot. Addition of adaptive manufacturing allows Wiivv to produce custom-molded footwear for customers from the comfort of their homes. Wiivv custom-made orthotics are composed of PU foam and 3D printed hard shells. PU foam is utilized for its pressure distribution properties, whereas, 3D printed shells are used for support. The objective of this collaboration is to create an insole out of hydrogel material that can be 38 implemented into the Wiivv design. Accomplishing this required creating a digitized version of the orthotics. Once the custom-molded orthotics were received from Wiivv, the insole must be 3D scanned and imported to a CAD software for mold creation. 3.2.1.1 Digitization of Custom-Molded Insoles Digitization was conducted with the assistance of Survive and Thrive Applied Research (STAR) facilities in UBC Okanagan. VIUscan laser based handheld 3D scanner by Creaform was utilized to capture the insole. The custom insole was pinned into a small wooden pedestal, during the 3D scanning process. This allowed the orthosis to stay stationary while capturing the object from multiple angles. Three detection markers were placed on to the insole, allowing for VIUscan to identify the objects surface. The VIUscan has a resolution of 100µm, which results high detailed CAD files. High resolution within this 3D scanner, provides detailed digitized results. The scanned insole file obtained more than 100,000 triangles defining the object. From past experiences, SolidWorks importing function struggles to load files that contain more than 10,000 triangles. Thus, a software for working with triangle meshes called Meshmixer was acquired. The main goal for this software was to decimate the file, combine the smaller triangles that make up the model into bigger ones, and to decrease the file size. Decreasing the file size resulted in the file losing the level of detail that VIUscan provided. However, for the purpose of creating a working prototype, the level of detail in the decimated file was satisfactory. 3.2.1.2 Mold Design As with most 3D items, the imported file had proven to be more complex than the designed insole in section 3.1.1. The CAD file had no flat surfaces which forced a different solution to the design progress. The mold was designed to be divided into five pieces. If the mold is stacked from heel 39 up, the bottom two pieces would create the heel, middle two; the midfoot region, and the toe piece, which would sit on top of the midfoot, would create toe region. It was planned that the four bottom pieces would be assembled inside a case and filled with the solution, whereas the toe section of the mold would be filled separated and then swiftly placed on top of the rest of the mold, completing the assembly. It is important to note; a case around the mold had to be manufactured to hold all the mold pieces in position. The importance of a case will be explained in the section below (section 0). When assembled, the case would be tightened to hold the mold together and eliminate any leakage of solution that might ooze through. The size of the insole necessitated the prototyping process to be housed outside of HVPL. Print bed for Stratasys Mojo 3D was small (12.5 x 12.5 x 12.5 cm) and if utilized to print, would require a complex assembly to hold each piece together. Thus, the Prusa MK3 available on campus were utilized. All the pieces, except the toe mold, were planned to be printed on Prusa MK3. Prusa MK3’s utilizes FDM technology. Polylactic acid (PLA) filaments were chosen for mold prints for the pricing and the durability of the material. This was done to not only cut the costs down, but Prusa MK3 printers have a larger printer bed (25 x 21 x 20 cm) compared to the Stratasys Mojo 3D available in the HVPL. This allowed the mold pieces to be larger, decreasing contact surfaces and ultimately minimizing spaces for solution to escape. As for the toe mold piece, because of the complex structure and precise nature of the airhole, it was chosen to be printed on the Stratasys Mojo 3D. This allowed the toe mold piece to keep its complexity. Utilization of the soluble filament in the Stratasys Mojo 3D printer enables the part to be printed precisely while preserving the complexity. 40 3.2.1.2.1 Aluminum Case Fabrication A case had to be manufactured to enclose the mold design, which holds the solution inside. There are two main purposes of this case; firstly, to create an airtight system to block off liquid interference as the case will be fully submerged in the cooling liquid, but more importantly, to allow maximum heat conduction throughout the mold. As the hydrogel synthesizing process will be explained in section 5.1: “ Hydrogel Manufacturing Process”, the temperature fluctuation bares imminent importance to the cross-linking process of hydrogel material. Two options were available to manufacture such a device: metal fabrication or FDM 3D printing. Depending on the printing settings 3D printer are capable of printing at a 100% in-fill, meaning each layer would be a solid plastic. This is far more preferred for thermal conductivity properties than having air gaps between each layer, as air has 0.026 𝑊/(𝑚 𝐾) at 25°C [69] whereas ABS is 0.173 𝑊/(𝑚 𝐾) [70] and PLA is 0.13 𝑊/(𝑚 𝐾) [71]. Comparatively, metals are far superior thermal conductors. However, design and fabrication of metal is time consuming and requires machining tools. Fortunately, the Machine Shop located on UBCO campus is readily accessible. Thus, this case was determined to be manufactured out of metal instead of being 3D printed. Six pieces of parts were designed on SolidWorks, then tolerances between all the parts were consulted with technicians. The case panels were cut from a sheet of aluminum into six individual pieces utilizing an abrasive waterjet cutter and finished with milling machines with the assistance of technicians. Following that, each panel had to be drilled in specific locations, to create bolting points. Each location was planned ahead of time to ensure all six panels would fasten together tightly, minimizing liquid leakage. Center drill-bit and a 7/32 inch drill-bit was utilized on a milling machine with a digital readout to identify each location precisely. The final step of the manufacturing process was the tapping operation to ensure tight assembly between each panel. 41 3.2.1.2.2 First Generation As briefly discussed in section “Mold Design”, the mold will be compartmentalized into five pieces. The rendering of these five pieces can be viewed in Figure 3.8. Four of those pieces were designed to make up most of the mold, whereas the toe piece will be assembled last in the hopes push out any air pockets that might have formed during the process. The mold was designed around an axis that dissected through. This axis created two continuous parts of the digitized insole. These two parts were then halved for the ease of cured material removal. However, these four pieces were cut short, approximately at the metatarsal region. The fifth piece was designed sit on top of the two sides. The main purpose of the fifth mold piece was to allow for an opening in the mold for an easy pour of the solution when the rest of the pieces were assembled. Figure 3.8 Assembled mold rendering. Molds numbered 1 & 2 make up the heel portion, 3 & 4; the midfoot section, and 5 is the toe piece. Red circle highlights the airhole designed to encourage air pockets to escape from the curing solution. After completion of the 3D print, the printed parts were laid out, as seen in Figure 3.8, for easy visualization. The interference between three interlacing molds were tested. Four of the pieces 42 were inserted into the aluminum case. This allowed for silicone solution to be poured into the mold. When the bottom four molds were filled, with the assistance of a syringe, the top mold was filled and seated on top of the rest of the mold. The aluminum case was tightened to minimize the possibility of seepage through the various mold pieces. Figure 3.9 First iteration mold pieces printed on the Prusa MK3. Red line is the axis the pieces are brought together. The same procedures for silicone preparation were followed in section 3.1.2.1. The only difference was the curing time. As this was a full-sized mold and the silicone solution was not exposed to air, the mold was allowed to cure for ~24 hours. After the ~24 hours had passed the mold was carefully disassembled, making sure the cured silicone did not tear during the process. Upon opening of the aluminum case, it was apparent there was a lot of leakage between the mold pieces. A lot of silicone 43 had pooled at the bottom of the aluminum case. When the mold was removed, the leakage problem was further emphasized. As seen from Figure 3.10, after the failed attempt, mold was filled for the second time and it was cured in another orientation. In both cases the leakage between the mold pieces caused only a portion of it to cure. Figure 3.10 The two cured silicone models compared to the original insole sent by Wiivv. 44 Results of this mold forced a calibration on the design aspect of this mold. Functionality wise, the mold itself held well. The five components of the mold made it easy for the cured silicone to be removed, and filling of the mold was easy. Only aspect that needed improvement is the connection between the bottom four pieces. A gasket design and a push-fit design will be considered. 3.2.1.2.3 Second Generation After the silicone test of the first-generation mold, better interference between the separate mold pieces were required. A gasket solution was considered; tailor-made gasket would be created from a silicone like material, which would be placed in a previously designed groove that was implemented into the mold. In theory it works, however, from the other projects that have utilized this option demonstrated practical issues. Instead of complicating the design by adding additional material, the new mold was designed to push-fit. A push-fit design would allow for the mold pieces to be pushed together for easy assembly. The functionality of this design was confirmed in the half-sized mold iterations, in section 3.1.2.3. Applying a push-fit design into the mold introduces barriers that the solution would have to penetrate. The push-fit mold will consist of a male and a female part. The male part will obtain a lip that protrudes from the surface where female mold would be seated. The designed molds can be viewed in Figure 3.11. It was important to note that the top piece had not been modified at all. A modification for leakage deemed unnecessary, considering no previous leakage between the interfaces when tested in First Generation. 45 Figure 3.11 Isometric angles of mold pieces designed with a push-fit design. Diagonal mold pieces were displayed in each rendering for better viewing angle on the push-fit design. The main concern with this new design was the fit issue between the male and female parts. Since these parts will be printed on the Prusa MK3, the molds must be custom made for those printer capabilities. The MK3 has a nozzle diameter of 0.4 mm. By designing the lip on male molds that protrude 1.6 mm, this allows for 4 layers of solid PLA acting as a barrier between the male and female molds. Additionally, the tolerance between male and female molds must be as tight as possible. This was achieved via trial and error process. 1 x 1 x 0.5 cm cubes with grooves on top were printed in varying widths. This was combined with a male 1 x 1 x 0.5 cm cube embedded with a fixed 1.6 mm lip protruding through the center. The printed cubes can be seen in Figure 3.12. After gradually decreasing the tolerance between the two cubes best fit was found to be at a tolerance of 0.01 mm between the pieces. Thus, it was decided all the push-fit tolerances on the mold would be printed at 0.01 mm. 46 Figure 3.12 Push-fit test for the Prusa MK3 printer. All the measurements are in mm. Tolerances were displayed on the female parts. When the mold pieces were printed, it was obvious the molds were not push-fit as planned. This was believed to be caused by multiple reasons. Firstly, in the fit test the corner tolerance was not tested, whereas corners had to be used. Second reason might have been the orientation of the prints. However, after sanding down some of the edges on the male parts, the molds were able to push-fit together which can be view in Figure 3.12. Silicone solution was prepared as explained in section 3.1.2.1. The bottom 4 molds were assembled inside the aluminum case, which were then filled with silicone. The remaining silicone was filled inside a 200-ml syringe as to accurately fill the toe mold. Once the toe mold was filled, it was assembled with the rest of the mold inside the aluminum case. 47 Figure 3.13 Complete second iteration mold design. As seen in the figure, parts are attached together, displaying push-fit design. After ~24 hours of curing time the mold was disassembled, and the silicone orthosis was carefully removed. As seen in Figure 3.14 the second generation of molds were successful. There was still some leakage of solution however, these leaks were contained by protruded lips designed into the system. The excess solution was carefully trimmed and displayed in Figure 3.14. This method was deemed successful and will be utilized in Chapter 5: Prototyping, when fabricating hydrogel orthosis. 48 Figure 3.14 Top view and Side view of cured silicone custom-molded orthotics 49 Chapter 4: Finite Element Modeling This chapter details procedures taken to evaluate effectiveness of various insole designs. The evaluation will be conducted via FE softwares and for this thesis only Comsol was utilized. The purpose of FE softwares was to predict effects of insole geometry to the pressure distribution. Geometries were first designed and finalized in CAD software before imported to be conducted FE analysis. The purpose of FE software is to reduce the pressure experienced in patients’ foot, first a benchmark simulation had to be created. Following that, a problematic simulation, which in this case high-stress concentrations, had to be designed. Lastly, simulation of designed insoles conducted, and their functionality were assessed. 4.1 Model Creation and Problem Statement There might be various reasons why a patient might be required to wear custom molded orthosis. These reasons might be chronic, requiring the patient to wear orthosis constantly or circumstantial, for a given period. Simulation of a chronic condition would rather be patient specific and would be quite complex to model accurately depending on the condition, whereas circumstantial orthosis wear would differ from patient to patient. A widely common case, diabetic ulcerations, was chosen as a common condition to simulate on FE. Diabetic ulcerations are caused by unnoticed structural changes to the patients’ feet that eventually creating high pressure concentration zones that leads up to necrotic tissue and ulcers. Years of unregulated insulin levels in the body could cause neuropathy. Neuropathy is a nervous system disorder that is caused by damaged nerves. Peripheral neuropathy is the specific condition where commonly the nerve ending in a patient’s feet and legs are damaged, causing a loss of sensation. On the other hand, the excess glucose in the blood stream, attaching to the collagen fibers in tissue causes glycation, stiffening of tissue. This combined with possible injures to the feet and lack of 50 sensation of the patient’s feet, causes increased pressure zones. As every sign goes unnoticed, the pressure concentrations eventually cause the tissue to break down, resulting in ulcerations. If the patient is not still not observant of their feet, ulceration could lead to necrotic tissue. Necrotic tissue is when the cells within die abruptly before their life cycle, due to a disease, injury or lack of blood supply. When a patient reaches this point the necrotic tissue must be removed, depending on when the patient realized the necrosis an amputation might be necessary. This process can be halted via relieving the patient’s feet off the high-pressure zones. This is where a custom-molded insole plays a crucial role. Every patient’s foot anatomy varies, as same with the diameter of the ulceration or high-pressure zone areas. Thus, a custom-molded patient specific orthosis is a must. Utilization of FE softwares allows how effective the insole would be. Evaluating a custom-molded orthosis requires a patient. For the purpose of this thesis two common areas of high-pressure concentration zones that have commonly been experienced among diabetic patients will be considered: heel and first metatarsal regions. These regions on the foot will be the focus of the FE simulations. 4.2 Initial Model Initial model for FE simulation was constructed to validate the working principles of the software as well as the design. 2-dimensional (2D) axis symmetric model was chosen from the COMSOL interface to simplifying the problem. This method allows only half of a symmetric model to be simulated, thus shortening the simulation time. The initial model had to include an object (a bone) that came in contact with another object, in this case foam like material or insole. First metatarsal ulceration was selected for this initial model. A rectangle (25 x 20 mm) was constructed in COMSOL, where the long edge was coincident with the symmetric axis. To create a similar geometry to the metatarsal bone, one of the edges that was 51 not sitting on the symmetric axis was fillet with a radius of 20 mm. When this shape rotated around the symmetric axis would resemble distal portion of the first metatarsal bone. As for the insole, a rectangle with 20 mm thickness and 60 mm in length was modelled 5 mm below the metatarsal bone. Thickness was modelled larger than a typical insole, however for initial simulation it was found to be satisfactory. The contact of two objects were simulated via applying displacement function to the metatarsal bone. The long bottom edge of the foam material was fixed. Two different material properties were needed, for each object, to perform the simulation. The bone and foam materials Young’s modulus and Poisson’s ratio was chosen to be 7.3 GPa, 0.3 [25] and 0.3 MPa, 0.4 [24], respectively. As no force was applied to create contact within two objects the density of materials was not required for simulation. Linear material properties were assumed. In Figure 4.1 the results of the simulation can be examined. The maximum von Mises stress is examined right below the surface of the foam material. 52 Figure 4.1 Initial FE modeling results. The von Mises stresses have been plotted. Maximum stress can be monitored at 𝟑. 𝟐 × 𝟏𝟎𝟒 𝐍/𝒎𝟐 (𝐏𝐚). Now that the indentation simulation was acceptable, a base line for ulceration simulations needed to be modelled. This was achieved with a 1 mm in radius sphere modelled at 3 mm below the surface of foam material, which can be visualized in Figure 4.2. Material properties for this sphere embedded inside the foam was chosen to be same as the metatarsal bone. In diabetic patients with ulcerations, it has been deducted that skin tissue at the location of the ulcer initially begins to deteriorate inside the tissue itself, working itself towards the surface [5], [72]. Thus, implementation of a sphere just below the foam material’s surface is deemed as an acceptable representation of diabetic ulcerations. 53 Figure 4.2 CAD modelling of 1 mm radius embedded within foam material. Image A displays side view, whereas image B is zoomed into the embedded sphere. Afore mentioned in this section, the FE models of the foam and metatarsal foam were 2D axis symmetric models. Thus, CAD modelling of the foam material embedded with the sphere was cylindrical. The purpose of these simulations and CAD modeling was to create an area that is representative of the metatarsal region of the foot. In the CAD modeling, the foam was given transparent properties to aid the reader visualizing the embedded sphere. An engineering drawing of this model can be found in Appendix: A.2. Identical material properties, as well as, physics were applied to the ulceration indentation model. After addition of the sphere the simulation was run. The results exhibited a maximum von Mises stress of 2.9 × 105 N/𝑚2 (Pa), which can be seen in Figure 4.3. This is a successful representation 54 of stress concentration in diabetic patients. Same method will be utilized to simulate the stress concentration in human foot models in the upcoming sections. Figure 4.3 Diabetic ulceration simulation achieved with a 1mm radius sphere embedded 3mm below the foam material surface. Maximum von Mises stressed observed 𝟐. 𝟗 × 𝟏𝟎𝟓 𝐍/𝒎𝟐 (𝐏𝐚). 4.3 Foot Model As the successful simulation of initial models, a more realistic model had to be constructed. This required a foot model. A meshed model of human foot was obtained from the HVPL, which was constructed from computerized axial tomography (CAT) scan. The meshed solid can be view in Figure 4.4. As seen from Figure 4.4, 26 individual bones make up the foot structure. Since the FE simulation will be focusing on the pressure reduction in 1st metatarsal, heel and big toe regions rest of the bony structures must be separated from this model. 55 Figure 4.4 CAT scanned human foot. Top and side views have been displayed. The FE modeling will take place in 2D to eliminate complexity of the system while capturing the regions in question. In Figure 4.5A, the 7 bones that will be utilized for the FE analysis. These bones make up the medial portion of the human foot. The medial portion of the human foot was all that required to simulate the previously mentioned pressure concentration zones. The zones in question have been circled in red in Figure 4.5A. mentioned pressure concentration zones. 56 Figure 4.5 Preparation of the foot structure for FE analysis. Image A displays the 7 bones left behind that will be utilized. Image B showcases the trimmed model. Top and side views have been provided for each image. Utilization of this 3D model required some modification, as only 2D FE analysis would take place. As seen in Figure 4.5B, two planes that were 5mm away were constructed and utilized to slice the model. However, the absence of ligaments and soft connective tissue that would allow the bones 57 to be held together while daily activities are not present in the CAD file. Absence of these tissue in the FE software results in unrealistic behavior. Thus, 2 mm in width membrane were constructed between each bone to ensure the model would stay together during the simulation. Figure 4.6 Each of the 7 bones were connected to one another via 2 mm wide rectangles. The connecting rectangles have been circled in red. 4.4 FE Base Model The finalized model was imported to COMSOL. Same procedures were followed in section 4.2. One difference from the ‘Initial Model’ section was the model set-up. Since whole foot model was able to be gathered, it would not be correct to assume axis symmetric model, instead 2D model was selected. Bone material properties as well as foam material properties were carried over. For the structural integrity of the model, the connection rods were assigned bone material properties. This was done to ensure the model would unanimously be displaced and contact the foam material. The contact boundaries were selected to ensure continuous indentation of the two objects, in heel 58 and first metatarsal regions. The contact between the bone structure and the foam was achieved by displacing the foot model 5 mm into the foam. Figure 4.7 Whole foot indentation. Foam material has been divided into two sections to display von Mises stresses at 1st metatarsal and the heel bone. The experienced stresses are 𝟒. 𝟒 × 𝟏𝟎𝟒 𝑵/𝒎𝟐 (𝐏𝐚) and 𝟑. 𝟗 × 𝟏𝟎𝟒 𝐍/𝒎𝟐 (𝐏𝐚), for the metatarsal and heel bone, respectively. 3 mm displacement into the foam material was prescribed to every domain that constructed the foot. As seen in Figure 4.7, the indentation simulation was successful. 3.9 × 104 N/𝑚2 (Pa) were experienced in the region of heel bone which, in line with the initial modeling that was conducted. Whereas the metatarsal pressure concentration was slightly more than what was encountered at the heel location. This might have been caused by the nearest contact point of the metatarsal bone being slightly closer than the heel. This caused metatarsal to penetrated deeper into the foam material. This was considered a successful benchmark for pressure distribution of a health human foot. 59 Figure 4.8 Ulceration stress concentration modeling of the whole foot. Foam material has been divided into two sections to display von Mises stresses at 1st metatarsal and the heel bone. The experienced stresses are 𝟏. 𝟏𝟗 × 𝟏𝟎𝟓 𝑵/𝒎𝟐 (𝐏𝐚) and 𝟕. 𝟏𝟗 × 𝟏𝟎𝟒 𝐍/𝒎𝟐 (𝐏𝐚), for the metatarsal and heel bone, respectively. Following foot indentation, ulceration simulations were modelled. A 1 mm in radius circles were embedded 3 mm into the foam material, identical to the CAD model in Figure 4.2. The embedded 60 circles were given bone material properties, similar to the ‘Initial Model’ section. The simulation results can be seen in Figure 4.8. Additional images can be viewed in Appendix: A.1 The maximum von Mises stress encountered at the heel was 7.19 × 104 N/𝑚2 (Pa) and the stress experienced in metatarsal reached 1.19 × 105N𝑚2(Pa). These results correlated with the results obtained in literatures on this topic [24], [25], [28]. 4.4.1 Stress Reduction Implementation of various designs into insole have one sole purpose: eliminating the stress concentration experienced in that region. Alleviation of stress concentrations is currently the best option available for diabetic patients with ulcerations. This can be achieved by two main routes; design and material properties. An optimized design will allow for distributing the stress via increasing the contact area between the two objects or directing the stress to different parts of the objects. Thus, a donut design was implemented into both regions of the foot. Additionally, the stiffness of the material around the ulceration was reduced to elevate the stress concentration zones. 4.4.1.1 Donut Design at the Heel Region A donut shape was designed on top of the insole to accomplish alleviation of stress concentration. This can be visualized in Figure 4.9. As seen from the figure the insole has been given transparent properties to help reader visualize the location of the donut design to the ulceration location. Engineering drawings for this design can be further evaluated at Appendix: A.3. The width of the donut shape was determined through the width of the heel bone getting in contact with the insole. The thickness of heel bone determined the distance between the donut peaks. 61 Figure 4.9 Top and side views of the donut design at the heel location. Only the heel region has been demonstrated. The sphere seen embedded is the ulceration. The simulation was run in the similar fashion as the previous sections. The donut design was given foam like material properties, as with the rest of the foam. Whereas, the ulceration sphere and rest of the foot structure were given bone material properties. As seen from Figure 4.10, the donut model was successfully simulated. Initially, the results indicate that instead of one large stress concentration, the donut design have distributed it into two smaller stress concentration locations, which can clearly be visualized from Figure 4.10. The maximum experienced von Mises stress at the heel location was reduced greatly to 3.9 × 104 𝑁/𝑚2 (Pa) from the ulceration benchmark simulation. Additionally, the pressure observed at the location of the ulceration was dramatically reduced. 62 Figure 4.10 FE analysis of donut design in heel region. The maximum experienced von Mises stress is 𝟑. 𝟗 × 𝟏𝟎𝟒 𝑵/𝒎𝟐 (𝐏𝐚). A vertical line was constructed inside the insole which dissected the ulceration sphere, this can be visualized in Figure 4.11A. This line utilized to determine the depth of maximum pressures encountered within the insole. In Figure 4.11B, the graph displayed, depicts the stresses through the constructed line. X-axis represents distance from the surface of insole (in mm), whereas y-axis is the von Mises stresses. Figure 4.11B is the values gathered from the same line constructed in the benchmark heel ulceration simulation, whereas Figure 4.11C is the donut design simulation. These graphs clearly indicate that donut design had drastically decreased the stresses experienced at the ulceration from 5 × 105 𝑁/𝑚2 (Pa) to as low as 2500 𝑁/𝑚2 (Pa). Additionally, the maximum stress experienced was displaced to the surface of the insole rather than the ulceration location. 63 Figure 4.11 A) Red line indicates the location of the von Mises stresses utilized to plot the B & C graphs shown below. B) Graph of benchmark heel ulceration. C) Graph of donut design. For both graphs; x- and y-axes represent distance from the surface of the insole and von Mises stress, respectively. 64 4.4.1.2 Donut Design at the Metatarsal Region Following the success of donut design in the heel region same principles were applied for the metatarsal region. As seen from Figure 4.12, similar initial deductions can be made that were made in previous section. It is apparent in this figure the donut geometry has divided the stress concentration into smaller regions away from the ulceration location. The maximum stress experienced in the insole is 9 × 104 𝑁/𝑚2 (Pa), whereas without the donut design the pressure was 1.19 × 105 𝑁/𝑚2 (Pa). Figure 4.12 FE analysis of donut design in heel region. The maximum experienced von Mises stress is 𝟗 × 𝟏𝟎𝟒 𝑵/𝒎𝟐 (𝐏𝐚). In addition to reduction of maximum von Mises stresses, the stresses at the ulceration location has been reduced as well. The graphs below presents the von Mises stresses experienced through the defined line. This defined line was constructed in the same fashion as the previous section, 65 dissecting the ulceration. An image of dissecting line can be examined at Appendix: A.5. Top graph in Figure 4.13, indicates a maximum pressure of 5.5 × 104 𝑁/𝑚2 (Pa) at around 3.5 mm below the surface of the insole, the ulcer location. Comparatively the maximum stress of donut design is experienced at the ulcer location is 3.8 × 104𝑁𝑚2(Pa). Figure 4.13 Above: The results of benchmark for metatarsal ulceration. Below: The results of donut design. For both graphs; x- and y-axes represent distance from the surface of the insole and von Mises stress, respectively. 66 4.4.1.3 Softer Material at the Heel Region A softer material model was implemented around the ulceration location. This was done to reduce the stress concentration via lowering the elastic modulus of the insole material. A 10 mm in diameter cylinder was embedded around the location of ulceration. This can be visualized in Figure 4.14. Young’s modulus for this softer material was lowered to one-third of the rest of the insole material, at 0.1 MPa. The rest of the model was kept identical to previous indentation simulations. Figure 4.14 Top and side views of the softer material design. 10 mm around the ulceration location was embedded with a softer material. 67 As seen from Figure 4.15, the resulting simulation suggests, the application of softer material properties with the insole reduced the stress concentration zones greater than donut design. The maximum stress experienced in the insole is 3.1 × 104 𝑁/𝑚2 (Pa). Compared to the reduction achieved by the donut design (5 × 104 𝑁/𝑚2 (Pa)), the softer material model was able to reduce the maximum von Mises stress by 26%. However, when examined the ulceration location, the softer material was able to achieve a stress reduction of 68% at 1.6 × 104 𝑁/𝑚2 (Pa). This is where the donut design edged out the Figure 4.15 FE analysis of softer material in heel region. The maximum experienced von Mises stress is 𝟑. 𝟏 × 𝟏𝟎𝟒 𝑵/𝒎𝟐 (𝐏𝐚). 68 4.4.1.4 Softer Material at the Metatarsal Region The same design and principles were applied for the metatarsal region following the success of the application of softer material model in the heel region. As seen from Figure 4.16, the resulting indentation correlates closes to the indentation in the previous section. The maximum stress experienced in the insole is 2.7 × 104 𝑁/𝑚2 (Pa). This was a 55% increase in the maximum von Mises stresses reduction in the metatarsal region compared to the stress reduction experienced with the donut design. When examined the ulceration location, the maximum von Mises stress experienced was 1.7 × 104 𝑁/𝑚2 (Pa), which was lower than the donut design implemented at the metatarsal region. Figure 4.16 FE analysis of softer material in metatarsal region. The maximum experienced von Mises stress is 𝟐𝟕 × 𝟏𝟎𝟑 𝑵/𝒎𝟐 (𝐏𝐚). 69 4.4.2 FE Results Both donut design and softer material models reduced the contact pressures. However, when compared, it was apparent that overall the softer material model had a bigger impact on reducing high stress concentration zones caused via the ulceration, as seen in Figure 4.17. Observing the contact pressures within the ulceration location revealed the donut design at the heel location was more efficient in reducing and redirecting the pressures compared to the softer material model. On the other hand, the softer material model achieved a greater pressure reduction at the metatarsal region. It can be deducted that implication of both softer material model and donut design would be beneficial in reduction pressure concentration zones in diabetic patients. Figure 4.17 All the conducted FE simulation results are displayed. Top graph depicts the contact pressure in the insole, whereas the bottom graph is the pressures experienced in the ulceration location. 70 Chapter 5: Prototyping The prototyping chapter of this thesis will divide the work done to accomplish synthesizing process into two sections. Firstly, the material synthesizing procedure will be discussed. This procedure is a novel procedure developed in the HVPL. The material properties of the hydro-gel as well as the synthesizing process had already been established. In the HVPL main use of hydrogel material is creating synthetic heart valves and arteries, which was accomplished via a system built in the HVPL. Second portion of this chapter will be an in-depth description of the expansion process of the system to accommodate an orthotics mold. 5.1 Hydrogel Manufacturing Process Hydrogel, also known as cryogel, is low temperature freezing and thawing cycled polyvinyl alcohol (PVA) which is reinforced by bacterial cellulose (BC) natural nano fibers [67]. For hydrogel to be synthesized, it needs to be cycled between 20° and - 20° Celsius at a 0.1°/min rate. This transformation is accomplished via physical cross-linking, which eliminates any toxic residuals that might have been present in other solidification methods such as, ℽ-irradiation, electron beam or use of chemical agents [66]. This process allows the BC inside the solution to align which effectively allowing the hydrogel to gain its mechanical properties. The cryogel synthesized is PVA, 99+ % hydrolyzed, which has a molecular weight of 146,000-186,000 [67]. 0.877 wt% of BC was suspended in distilled water. Acetobacter xylimum bacterium was utilized for producing suspension, which took place in shake flasks by a fermentation process [66]. Later, the PVA solution was transferred to the BC solution to make a hybrid solution [67]. The consistency of this solution is depended on the strength of hydrogel biomaterial needed after cycling. It is vital to note; longer the samples were exposed to the freeze-thaw process the group 71 found an increase in the stiffness of the hydrogel samples [66]–[68]. The PVA-BC solution is then transferred into an aluminum mold which will be placed into a thermal unit. This thermal unit acts as a heated/refrigerated circulator. 5.1.1 PVA-BC Mechanical Properties There are multiple factors that could affect the mechanical properties of PVA-BC hydrogel. These factors include mass fraction of PVA solution, mass fraction of BC solution and thermal cycles. Many variations of these factors have been tested before where most of these factors have been singled out. The results of these studies suggest that in a 10% PVA solution increasing the mass fraction of BC solution, maximum was tested at 0.85 wt %, results in an increased stiffness [73] and as the solution goes through more cycles, six being the maximum, it increases in stiffness [66], [68], [73]. Same principles apply with the PVA content, as the more mass fraction of PVA solution increases the elastic modulus of the hydrogel [73]. 5.1.1.1 Compression Testing For compression testing of hydrogel material solutions of 15% PVA with 0.85 wt% BC were prepared. This solution was utilized to fill an aluminum mold of 30 x 25 mm2. Mold was placed in the thermal cycling device and was put through 6 cycles. This was done to achieve maximum stiffness for hydrogel material. Following the synthesizing process hydrogel was brought to the Composite Research Network Laboratory at UBC Okanagan Campus, where Instron 5969 UTM was utilized for the compression testing test. The machinery was programmed to apply 1 N/s force on to the material. The hydrogel block was placed inside the machine and compressed to 25% of original height. From the gathered data points true stress and true strain values were calculated. 72 The equations below were utilized to produce the graph seen in Figure 5.1 below. Where 𝜎𝐸𝑛𝑔 & 𝜀𝐸𝑛𝑔 is the engineering stress and engineering strain, and 𝜎𝑇𝑟𝑢𝑒 & 𝜀𝑇𝑟𝑢𝑒 are the true stress and true strain respectively. 𝐸 corresponds to the materials elastic modulus. 𝐹 is the force applied to the samples and 𝐴 is the cross-sectional area. 𝑙0 is the initial length of samples and ∆𝑙 is the change in length of the samples as the force was applied. 𝜎𝐸𝑛𝑔 = 𝐸𝜀𝐸𝑛𝑔 (1) 𝜎𝐸𝑛𝑔 = 𝐹𝐴 (2) 𝐸𝜀𝐸𝑛𝑔 = 𝐸∆𝑙𝑙0 (3) 𝜎𝑇𝑟𝑢𝑒 = 𝐸𝜀𝑇𝑟𝑢𝑒 (4) 𝜎𝑇𝑟𝑢𝑒 = 𝜎𝐸𝑛𝑔(1 + 𝜀𝐸𝑛𝑔) (5) 𝜀𝑇𝑟𝑢𝑒 = ln(1 + 𝜀𝐸𝑛𝑔) (6) 73 Figure 5.1 Stress vs strain graph for compression testing of hydrogel (15% PVA, 0.85% BC, 6 cycles). 5.1.1.2 Tensile Testing Similar to the compression testing the materials were tested in Composite Research Network Laboratory at UBC Okanagan Campus and the Instron 5969 was utilized for the tensile test. However, a gripping tool had to be designed and fabricated as the slippery nature of hydrogel material. Tensile attachment apparatus, in Figure 5.2, was designed and printed utilizing the Prusa MK3 FDM printer. The groove highlighted in red, was designed to pinch the hydrogel material when bolted to an identical attachment. In total, four attachments were utilized, two pinching from either end. 00.020.040.060.080.10.120 0.05 0.1 0.15 0.2 0.25Stress (MPa)Strain25% Compression74 Figure 5.2 Tensile Testing Apparatus. Left “CAD rendering”: Red highlights the pinching geometry. Right “Printed version”: Red highlights the drilled hole for rod insertion. When attached to the Instron machinery, it was observed the pinching mechanism was not enough to hold the hydrogel during testing. Thus, holes were creating into the apparatus, seen in right image in Figure 5.2, highlighted in red. The hydrogel material would first be secured tightly between the two attachments, then a rod would puncture through both the attachments and hydrogel. 75 Figure 5.3 Hydrogel point of failure (highlighted in red) from tensile testing. Highlighted in blue are the rods penetrated through the hydrogel strips. Hydrogel solution with same properties (15% PVA, 0.85wt% BC) were filled in an aluminum case with dimension of 60 x 50 x 5 mm. After 6 cycles the aluminum mold were removed, and the hydrogel was cut into 6 strips of 10 x 5 mm2. The machinery was programmed to apply 1 N/s tension to the hydrogel material. The testing was continued until each hydrogel strip was ripped. During the tensile testing it became apparent, insertion of the rod through the hydrogel material disrupted the tensile testing. Hydrogel would tear from the puncture point. This problem could have been diverted via custom designed hydrogel molds. However, one out of 5 strips of hydrogel were torn as intended (Figure 5.3). Resulting stress-strain curve was constructed in Figure 5.4. The resulting elastic modulus was calculated to be 2.2 𝑀𝑃𝑎 which was found to be in line with the literature conducted on this material [66], [73]. 76 Figure 5.4 Stress vs strain graph for tensile testing of hydrogel (15% PVA, 0.85% BC and 6 cycles). 5.1.2 Thermal Cycling System Currently, at the HVPL, Anova refrigerated circulators are employed to complete this cycling for hydrogel to be formed. The Anova units are designed internal and external bath temperature control applications. The system itself is comprised of an LCD controlled thermostat (header) and a refrigerated bath mini tower (base). The header contains a thermostat, a pump and is capable of controlling high-temperature cutoff, low liquid level warning, pump speed, heating and cooling rates via the LCD interface. The base is a fluid reservoir bath tank accessible by a top lid. The model utilized in the HVPL is A-40L which the header is capable of cycling between -40°C to 200°C and the base is 17/14/27 inches big and has a volume of 16 liters. Whereas the opening to the bath is 11.5/6/7 inches [74]. At the HVPL cooling process is controlled via an Arduino board -0.100.10.20.30.40.50.60.70 0.1 0.2 0.3 0.4 0.5 0.6Stress (MPa)StrainTensile Testing77 that has already been programmed. The Arduino board is connected to the header, which takes over the cycling process. The process goes through six cycles of 20℃ to -20℃ and back to 20℃. Three criteria were set, necessary for the external bath to function; large enough for area for the aluminum case to be submerged, waterproof as the external bath has to be filled with cooling liquid, and thermally isolated as the cycling process fluctuates between -20°C and 20°C. Thus, a Styrofoam box was purchased to custom fit the needs of the external bath. 5.1.2.1 Exterior Bath Configuration & Prototypes The final configuration of the exterior bath included the Anova header and base, Coleman cooler, a reservoir with the cooler and a venturi pump. Detailed steps and procedures taken to get to this configuration can be viewed in Appendix B.1: Exterior Bath Construction Procedures. As seen from Figure 5.5, the configuration is composed of two main parts; Anova thermal cycling device, located below the cooler, and exterior bath, which sits on the table. The exterior bath contains a reservoir that holds the cooling liquid within, which can be viewed bottom image of Figure 5.5. The venturi pump, which can be seen connecting the tubing from the Anova device to the cooler, was designed to dissect the inflow two; one leading into the reservoir, the other back into the Anova base. The problem encountered with this design is that even though the venturi divides the flow into two and returns one back into the base, the reservoir is filled faster than the suction provided via the venturi pump. This eventually will cause the reservoir to overflow causing a “Low Liquid Alert” and halting the process. Thus, the red circles highlighted in Figure 5.5 are the binder clip that was utilized to decrease the volumetric flow rate that is directed into the reservoir. 78 Adding the binder clip had resolved this issue, slowing down the rate at which the reservoir was filled up. This caused the suction out of the reservoir to be faster. At low temperatures this caused a problem, since suction was greater than the pump rate, suction from the reservoir only would occur at the surface of the liquid. Consequently, air would be mixed in with the liquid that was suctioned into the base causing foam formations, which in severe cases the foam would froth aggressively and spill out of the Anova base. Figure 5.5 Final Configuration of the Anova cycling system and the exterior bath. The system was started thermal cycling to observe the foam build-up. Additionally, the temperature difference between the base and the exterior bath had to be tested to evaluate the 79 effectiveness of the bath. Furthermore, a 3 x 3 x 2.5 cm aluminum mold was filled with hydrogel solution (15% PVA, 0.85wt% BC) and placed inside the reservoir before the cycling was initiated. At the bottom of the first cycle (at -20℃), the foam formation was apparent though minimum. When the Anova header displayed -20℃, a digital thermometer was placed into the reservoir, near the outlet location. The thermometer recorded -19.3℃, a difference of 0.7℃. Considering the location of the thermometer, the exterior bath was functioning as intended. Figure 5.6 Test prototype synthesized utilizing the exterior bath. When the cycling process had finalized and aluminum case was stripped, it was apparent the cycling process was successful. The synthesized hydrogel can be seen in Figure 5.6. This confirmed the functionality of the system. 80 5.1.2.2 Final Prototype The orthotics mold was filled to be synthesized. The only hydrogel solution available in the HVPL was a 5% PVA and 0.3wt% BC. This solution was not preferred, as discussed in ‘PVA-BC Material Properties’. The lack of PVA-BC in the solution would cause the hydrogel material to have weak mechanical bounding, resulting in low stiffness. Additionally, because of the lack of ingredients the solution lacked the viscosity. Even though this was not the preferred solution to be utilized in foot orthosis, the thermal cycling process had to be tested. As the solution was filled into the mold, it was obvious the lack of viscosity of the solution was causing a leakage. Figure 5.7 Final prototype synthesized utilizing exterior bath. Formulation of hydrogel solution effected the viscosity, causing leakage. At the end of the cycling process for the orthotics mold, the resulting hydrogel orthotics can be seen in Figure 5.7. As mentioned before viscosity of the hydrogel solution caused some of it to leak out of the mold into the gap between the mold and the aluminum case. Furthermore, the mechanical properties of the synthesized hydrogel less than ideal. All things considered, the cycling process was successful, and the same process could be repeated with a hydrogel solution with sought after content. 81 Chapter 6: Conclusion & Future Work Orthotics are designed for people with foot abnormalities. As every patient differs from the next, there is not a concise formula that can be applied to all patients to prevent issues that arise with foot abnormalities. The process for prescribing shoe insoles includes; health professionals typically utilizing a form of three-dimensional impression taking procedure to model patient’s foot. This will allow the professionals to determine the areas of excessive plantar pressure while providing stabilization and support for the joints in the foot. However, with all the different formulations and many thicknesses of each material and the wide range of materials available, it would not be possible to pinpoint which materials are best prescribed for specific cases. Simulated in-shoe and bench testing methodologies are limited in the sense that not all the factors contributing to the effectiveness of a material can be tested, whereas in-shoe measurement systems can evaluate these attributes simultaneously. With the development of FE modeling softwares, researcher have shifted their focus on the design and shape of footwear orthosis rather than material as FE modeling softwares are more accessible than in-shoe measurement systems. These types of softwares also allow researchers to test rapidly without the need of patients for trials. It is important to mention the same insole materials that have been used for the past 20 years are still being used to date. As there is no solidified formulation for the material production or thickness being implemented into insoles for patients, researchers could clinically recommend one material over the other. This thesis showcased various futuristic designs as the group believes there is a gap in the industry. Most recommended insoles are a combination of various materials to provide stress distribution as well as support throughout the patient’s feet. With the development of anisotropic materials, insoles can be tailor-made for different patient requirements, while having 82 a homogenous foot orthotic structure. Still, many more studies within the scope of the footwear industry must be conducted to prove the durability and viability of these materials. 6.1 Work Overview Application of hydrogel into footwear industry is a completely uncharted area, the work done had to showcase feasibility of hydrogel insoles, which is detailed in the sections below. 6.1.1 Insole Design & Fabrication The nature of hydrogel synthesizing required a mold creation thus, a methodological way of mold creation had to be established. Within the “Chapter 3: Insole Design & Fabrication”, an arbitrary insole was designed, molded and fabricated. This proved the capability of FDM printing for mold making process. When the collaboration between HVPL and Wiivv started, custom-molded orthotics were 3D scanned. Later, that file was utilized to make an orthosis mold for hydrogel synthesizing. For convenient filling and removal of the mold, various designs were fabricated and tested. This resulted in the push-fit design. The push-fit mold design allowed the solution to be poured, with minimum leakage. This was proven with the silicone model that was fabricated with this design. 6.1.2 FE Modeling FE analysis were conducted to evaluate effectiveness of insole designs. Foot models from meshed files were converted into 3D solids. On FE software benchmarks were constructed. These benchmarks were consisted of health human foot indentation and diabetic foot indentation. The diabetic foot model provided a platform where the outcome of insole designs was measurable. 6.1.3 Prototyping This section involved validating hydrogel material’s material properties. There are various formulations to which hydrogel material can be constructed with. Research was conducted to 83 analyze which formulation would be applicable. Literature suggests hydrogel solution with 15% PVA and 0.85% BC cycled 6 times in custom made freeze-thaw bath would result in maximum stiffness. Thus, two sets of testing were conducted on the Intrson UTM. This was done to ensure the elastic modulus of the material was similar with the foam like materials utilized in the industry currently. The compressive and tensile tests proved the material properties suggested in the literature. Following that an exterior bath was constructed. The orthotics mold was not able to fit into the existing thermal cycling device. The exterior bath allowed for a larger mold to be fitted while keeping the temperature difference between the base and the bath at less than 1℃. 6.2 Future Work This thesis was a preliminary study into the methodologically manufacturability of novel hydrogel-based insoles. Future recommendations for the functionality, testing and manufacturability are discussed below. 6.2.1 Insole Design & Fabrication Future development of various insole designs would be recommended to be kept in the approximately same size that as the aluminum case. Reason being, the manufacturing of aluminum case was time consuming. Aluminum case is essential for synthesizing of hydrogel; thus, it cannot be completely removed from the design process. Additionally, the interface between the molds can be further improved. Even though, the model was perfectly functional for viscous solutions such as silicone, a better interface between the mold pieces would allow for less viscous solutions to be encapsulated, without leakage. 84 6.2.2 FE Modeling Comsol software was utilized for simulation of contact behavior between the foot model and insole material. It was satisfactory to visualize the effects of different insole designs on diabetic ulceration. However, it was found that Comsol’s algorithmic solver was not the best suited for contact behavior simulations. Even at 2D level Comsol would struggle. My recommendations for the next generation of FE modeling would be to utilize ABAQUS software instead. In addition, application of a stronger software will allow next generations to be conducted in 3D, alongside with more realistic material properties can be applied to the model. The foot model should be improved. A soft tissue layer alongside with bone structure is recommended for future work. A further step would be addition of connective tissue and ligaments to the system. Identifying these elements would result in a much more realistic FE simulation. Utilizing in-shoe pressure measurement sensors is recommend, as it can capture the pressure profile of patients’ gait. The pressure profiles can be directly added to the FE analysis. Thus, making the insole designs and iterations more reliable. 6.2.3 Prototyping For future work, hydrogel material properties have to be further tested to validate durability of the material under various conditions. Cycling load, compression set, indentation testing are valuable candidates for validations of a material in the field of foot orthotics. The bath for synthesizing process, even though functions as desired, can be improved for future generations. The main concern of the whole configuration was the excessive foam build up caused by the cooling liquid would leak out of the Anova base, halting the thermal cycling process. Essentially the causation of this problem boiled down to the liquid mixing inside the reservoir. This can be completely avoided with addition of copper tubing instead of clear tubing. Copper 85 tubing can be utilized to create coils within the reservoir, eliminating any foam build up. 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[74] ANOVA, “Temperature Control Instruments Water baths, Ciruculators, Chillers.” 2014, [Online]. Available: http://www.riverssolutions.com/uploads/5/3/5/4/53545675/anova_ebrochure.pdf. 95 Appendices Appendix A Appendix A includes additional images from FE modeling chapter. 96 A.1 Foot Models of the Indentation Simulations Figure A. 1 Full foot models of the simulations displayed in Figure 4.8. 97 A.2 Engineering Drawings of Ulceration within Insole Figure A. 2 Engineering drawing for ulceration model. Units are in mm 98 A.3 Engineering Drawings of Donut Design at Heel Region Figure A. 3 Engineering Drawing for the donut design. A.4 Renderings and Engineering Drawings of Donut Design at Metatarsal Region Figure A. 4 Top and side views of donut design at the metatarsal location. 99 Figure A. 5 Engineering drawings of donut design at the metatarsal location 100 A.5 Line Construction for von Mises Stresses at Ulceration Location Figure A. 6 Line dissecting the ulceration at the metatarsal region. Utilized to construct graphs in Figure 4.13. 101 Appendix B Appendix B includes detailed procedures taken to design and construct a functioning exterior bath for prototyping purposes. B.1 Exterior Bath Construction Procedures It is important to mention data sheet of the Anova head the HVPL acquired, is rated for pumping 30L/min at 5psi, given this information an external pump deemed unnecessary [74]. More crucial aspect of the design was to converse the temperature of the cooling liquid. Thus, a Styrofoam box was purchased to be utilized as an exterior bath. Styrofoam Box Design The Anova header has a built-in outlet and inlet holes at the rear. At first it was not apparent as to how the pump was functioning. Whether it created positive pressure in the base and pumped out the cooling liquid or negative pressure causing a suction from the inlet. This was quickly determined with an experiment. Two clear tubes were attached to either end of the header and the tubes were led into an exterior bucket with cooling liquid. Theory behind this experiment is when the header is turned out it would be easy to determine whether pump is creating positive or negative pressure within the base. When the header turned on the cooling liquid started rushing into the bucket and there was no return flow back into the inlet. This result determined that exterior bath must be elevated above the Anova base and header, allowing for cooling liquid to return to the base without an exterior pump. The height adjustment of the Anova base was enforced not only cause the buying a pump would be an extra expense, but additionally finding a pump that can function at -20°C for maximum eight hours continuously was not easy. Some pumps, like peristaltic pumps, could have been utilized for this purpose however, those of which that were rated for -20°C were very expensive and industrial. 102 Additionally, an exterior pump would require its own micro controller as the pump is required to be turned on when the Anova header’s pump is on and turned off automatically when the pump is off. Elevating the exterior bath eliminates complexity that would require for the exterior pump. Achieving Best Possible Thermal Distribution Inlet and outlet locations on the exterior bath deemed critical as the cooling achieved in the bath will only be controlled via the flow into the exterior bath from the inlet. To achieve maximum circulation in the bath, as well as, maximum temperature distribution, inlet and outlet feeds were placed on adjacent walls of the exterior bath. This placement of inlet and outlet flows can be seen in top view in Figure B. 1, reason behind this placement is to create as much turbulent flow as possible, allowing the cooling liquid to be distributed among the Styrofoam bath. As visualized in the side view of Figure B. 1, inlet flow is positioned above the outlet flow to increase temperature distribution and create circulation. Inlet flow and outlet flow on top view can be identified via the flow arrows, whereas on side view inflow is still identified via the flow arrow and outflow is directed into the page. 103 Figure B. 1 Flow diagram of the Styrofoam exterior bath shown with arrows, whereas inlet and outlet are show with lines (top view: inlet and outlet, side view: inlet) and circle (side view: outlet). Construction and Assembly of Styrofoam Bath After the inlet and outlet locations were decided and marked on the exterior bath, a step-drill bit was utilized with a power drill to create gradual holes. Step-drill bit was employed as a precautionary measure to get a tight fitting between the piping and holes. As the step-drill bit took away material from the exterior bath clear tubing that will used to redirect the cooling liquid was tested to ensure there was a tight interface between the Styrofoam and tubing. When the interface was satisfactory, the tubing had to be fixed and sealed properly. Cooling liquid would be reaching -20℃ thus, the sealant had to be effective at those temperatures. Considering the criteria RTV silicone adhesive offered by Permatex was utilized. This sealant is generally utilized for water proofing and is resistant to water and oil. It is also able to function in the temperature range between -60℃ to 204℃. The tubing was placed into the already drilled Styrofoam box and then the silicone adhesive was applied in both the interior and the exterior side of the Styrofoam box. The adhesive was applied generously on both sides to ensure no liquid 104 leakage. 24 hours were allowed for the for adhesive to cure completely and testings were conducted. Initial Testing Exterior cooler was placed on workbench whereas the base and the header were placed on the ground. The configuration of this setup can be seen in Figure B. 3. The inlet tubing for the exterior bath (cannot be seen in Figure B. 3, on the posterior side of the box), was attached to the outlet of Anova header (shown with B in Figure B. 3), as the outlet from the exterior bath (A n Figure B. 3) was attached to the inlet of the header (A in Figure B. 3). A precautionary bucket was placed underneath the bath to encapsulate and leaks that might happen throughout the refrigeration cycle. Figure B. 2 Configuration of Styrofoam exterior bath and Anova refrigerated circulator When the cycling was turned on it was apparent that the Anova header was pumping at a higher rate than the return of the fluid back into the base, causing a “Low Liquid Alert” consequently shutting off the pump and making the system freeze till it was reset. This was an issue as the 105 refrigeration cycling would barely start before the alert halts cycling process. Upon further inspection the header it was evident that the pump rate could be controlled. Preselected pump rate was “Auto”, the other setting included: “Low”, “Medium” and “High”. Even the pump rate set at “Low” the inflow rate into the exterior bath exceeds the outflow rate of the liquid leaving the bath. Upon this discovery, it was clear an outsourced pump had to provide suction back into the base. After the failed test, the exterior bath was not drained completely, and search for a pump had begun. At the end of the day, it was clear only pumps, that fit the criteria of functioning more than six hours continuously and rated for -20℃, were industrial and expensive machinery. 24 hours into the search for a pump, it was observed that left over cooling liquid inside the exterior Styrofoam bath had started seeping through the bath’s walls. As seen from Figure B. 3, after 24 hours the seepage is prominent to the eye. In Figure B. 3 section B, it is clearly seen the seepage is going through multiple points of the wall, and in section C of the image the amount of liquid caught by the precautionary container is clearly visible. These developments forced to reevaluate the viability of this system. Exterior bath had to be constructed out of something more durable while thermal insulation was kept at maximum. 106 Figure B. 4 Seepage of cooling liquid from the Styrofoam bath after 24 hours. Cooler Box Design Coleman Xtreme® 5 Marine Cooler was selected for the thermal insulation rated to conserve ice up to 3 days at an outside temperature of 32℃ and inner dimensions allow for the aluminum case to fit as well as allows for a larger mold to be fitted, if needed. The thickness of the cooler walls was sought after as they were 5/8 inches thick and uniform. This allowed for easy penetration through the wall and would accommodate for easy installation of inlet and outlet flow points. Upon purchase of this exterior bath, a new way of inlet and outlet flow sealing was sought after. Permatex silicone adhesive created a solid seal for the Styrofoam box however this seal is a permanent one whereas more versatile attachment would improve the long-term performance of the exterior cooler. Stainless steel weldless bulkhead (½” NPT | Flat Gasket 2 ½”) attachments 107 provided the sought-after versatility. Two bulkheads were purchased, for both inlet and outlet, along with two brass fittings (½” ID hose barb | ½” Male NPT) to attach the tubing into the bulkheads. The bulkheads are included with silicone flat gaskets in either end to achieve proper sealing, the threading on either end allow for female tubing attachments to be inserted. They are 2 ½ inches in length, are designed for pre-drilled hole of 7/8 inches in size and maximum wall thickness between 1/4 and 5/8 inches. Achieving Best Possible Thermal Distribution For the cooler design, because of the shape of the box both inlet and outlet flow locations were selected on the same wall. This is because the width of the cooler had the most uniform walls for maximizing sealing via the silicone gaskets mentioned in the section ‘Cooler Box Design’. Additionally, thinnest wall thickness is observed on width of the cooler, as a result the two flow points were positioned on top of one and other. Since the tubing directing the cooling liquid from the base into the exterior bath is not thermally insulated, stacked inlet-outlet position (seen in Figure B. 4) was preferred to minimize the heat lost through the tubing. Figure B. 5 Flow diagram for cooler exterior bath 108 Construction and Assembly of Cooler Bath The locations of the inflow-outflow holes were identified and marked before drilling. The drilling process started similar to Styrofoam bath, using a step drill bit. Step drill bit was utilized initially capable of creating ½ inch diameter holes, and a 7/8-inch drill bit was employed to allow for the bulkheads. Power drill was set to reverse spin (counterclockwise) when utilizing 7/8” drill bit. This allowed more control over the power drill and the bit, as when drilling into hard plastic (much like the outer shell of cooler) clockwise rotation causes the flutes of drill bit to latch on to the plastic making the power tool to be drawn into the material. When the holes for bulkhead attachments were satisfactory, to avoid leakage into the insulating layer of the cooler, one-sided aluminum tapes were placed through the hole, in an over lapping pattern to minimize amount of leakage. Before attaching brass fittings to the bulkheads, the ½” male NPT portion was taped with Teflon. Teflon tape was applied same direction as the threading on the fitting, thus creating a watertight system. For the last step of the construction of the exterior bath, bulkheads were tightly attached to the cooler and then the brass fitting were attached to the end of the bulkheads. Initial Testing To test the water resistance of the attachments, the clear tubing was connected and from inlet water was filled into the cooler. The exit tubing was elevated above inlet, thus allowing for water to accumulate inside. When the water levels were reached up to the inlet location the system was observed for five minutes to ensure no leakages were present. When the water resistance of the system was proven, it was placed inside the precautionary plastic bucket incase cycling temperatures disrupt the seals. It is important to note that an additional amount of cooling liquid will be added into the exterior bath. The cooling liquid is filled just below the outlet location, this allows the cooling liquid to return into the base when the 109 cycling starts. The cooler was also set up in a way where a slight decline towards the outlet flow. The idea behind this was to increase the return time of the liquid via increasing the potential water column height. Tilt applied to the cooler allowed the pump to run in “Auto” mode without resulting in “Low Liquid Level” alert and eventually halting the system. Further into the initial test some characteristics were observed. As the temperatures starts to decline, the surface energy of the cooling liquid is decreased significantly. The nature of the exterior bath and the synthesizing process encourages the cooling liquid to be circulated to have maximum thermal distribution. Currently, as the cooling liquid is fed into the exterior bath, it is mixed with air causing foams, especially at lower temperatures. One full cycle (+20℃ to -20℃ back up to +20℃) was completed, however when the system went back down to below zero degrees for the second cycle, the foam build-up caused an overflow of the cooling liquid eventually giving a “Low Liquid Level” alert. As the header allowed for various of pump rates, the cycling was started at “Low” setting. Although, the pump rate was sufficient at higher temperatures, as the liquid temperature starts dropping below zero degrees, the pump rate starts to decrease significantly. When the rate is set to “Low” there isn’t enough power going through the pump to be able circulate the cooling liquid through the exterior bath. Thus, running the system at anything other than “Auto” pump rate is not viable. Another issue that surfaced was caused via the tilt to the exterior bath. The tilt causes the aluminum case to be partially submerged under the cooling liquid, as this is not ideal, improvements to the current system must address these issues as well. 110 Reservoir Addition This implementation to the system was critical for the purpose of the material synthesizing quality. An additional reservoir inside the exterior bath always allows liquid to be held inside the reservoir, effectively granting the mold to be submerged in the cooling liquid. Reservoir utilization in theory will allow for better prototyping as thermal distribution from the cooling liquid will be maximized as more of the mold will be submerged inside the cooling liquid. Exact measurement of the cooler interior and the aluminum case were taken to outsource a tuple ware container that would fit in the cooler, while the mold can be seated within. After the purchase and installation of the reservoir, current configuration of the system will be on a tilt as well as the cooler itself, much like in the previous section. Even though this means the mold won’t be fully submerged inside the reservoir it will be covering majority of the aluminum case compared to the previous solution. Initial Testing Initial observations of this configuration proved the addition of the reservoir allows majority of the mold to be submerged. When the cycling initiated it was apparent the liquid flow was more gradual than previous set up. A more gradual flow allowed the cooling liquid to trickle down to the outlet location, whereas without a reservoir the inlet flow would hit against the opposing cooler wall, then mix in with the returning liquid. This observation proved to be true when the system was initiated. With the addition of the reservoir the gradual fluid flow seems to have diminished the mixing of air and cooling liquid. An hour into the initial testing, temperatures around -10℃ (bottom of first cycle), the foam creation was at the minimum that had observed compared to the previous tests. After the first cycling had successfully concluded, there were minimum foam build up. This was evaluated as a 111 successful addition to the exterior bath and was left to finish the cycling process. However, after another hour, the “Low Liquid Alert” halted the process. Foam build up caused the liquid to pour out of the base. This was uncharacteristic, in theory, the foam caused by low surface energy of the cooling liquid should have been equalized via the temperature rise, eliminating most of the foams formed. Cooling liquid properties effects the amount of air mixing in with the liquid causing foam, and in the system HVPL has been running windshield wiper fluid rated for -40℃. This liquid could have been replaced by ethylene glycol, however, due to its hazardous properties was not sought after. Venturi Pump At the time of this, the HVPL had recently purchased a high end stereolithography (SLA) 3D printer. This printer is capable of manufacturing industry quality products, printing at a resolution of 25-300 µm. This grants HVPL to manufacture mechanical heart valves in high resolution to be tested in-vitro studies. The advantage of this printer for this thesis is to be able to create complex structures, whereas an FDM printers would not be capable. Complex structure opens the possibility of creating an apparatus that can act as a pump. Venturi effects works on principle of pressure field differences. A venturi tube creates a constriction in a pipe that varies the fluid’s flow characteristics travelling through a tube. The throat in increases the flow velocity consequentially causing a pressure drop. This relationship is described in the ideal, inviscid, incompressible form of the Bernoulli equation: 𝑝1 − 𝑝2 =𝜌2(𝑣22 − 𝑣12) Where 𝑝 is pressure, 𝜌 is the fluid density and 𝑣 is velocity. Equation highlights, in the event of a pressure drop, the velocity increases and vice versa. 112 Figure B. 6 Venturi Tube. Cross-sectional area of (1) is greater than (2), which effects velocity of the fluid at location (1) to be lower than at (2) and the static pressure in the first measuring tube (1) to be higher than in the second (2). In a typical venturi pump, compressed air is supplied to create the pressure difference. As compressed air has a much higher velocity the pressure gradient is greater, thus creating greater suction. In this system compressed air would mix in with the cooling liquid causing more foam build up. Instead of compressed air, the cooling liquid will be utilized to create the pressure gradient. 113 Figure B. 7 SolidWorks design and rendering of Venturi Pump. Considering these principles, a venturi pump was design and rendered on SolidWorks. As labeled in Figure B. 6, the cooling liquid will be pumped out of the header into the venturi pump, where the flow will be split into two. One will go into the cooler inlet which will feed into the reservoir. The other connects into the cooler outlet flow. If examined closely tubing connecting into the cooler outlet flow has a smaller cross-sectional area. As explained previously by decreasing the cross-sectional area, velocity is increased consequently pressure is decreased. Theoretically, pressure drop at the venturi nozzle will create suction from cooler outlet which will be connected into the reservoir. 114 Figure B. 8 Tubing attachments designed and rendered. Side and cross-sectional views are shown. After the venturi pump was finalized, attachments for tubes had to be designed. As a precautionary measure, tube attachments were previously purchased and verified that they were correct size for the tubes acquired. The dimensions of these attachments were taken and were utilized to recreate in digital form as seen in Figure B. 7. Modelled tube attachments were assembled on to the venturi pump. Four attachments were assembled on the pump using the 115 “Mate” feature on SolidWorks. When the assembly was complete the file was saved as a part. This was done to ensure all the parts’ presence in the 3D print file. Figure B. 9 Side and cross-sectional views of final form of the venturi pump. Initial Testing The curing process of the 3D parts from the Form 3 includes a 10-minute bath in isopropyl alcohol to ensure removal of all the excess liquid resin and a 15-minute exposure to ultraviolet (UV) lights to confirm the rigidity of the printed parts. Once the curing process was complete, the venturi pump was tested in the laboratory sink with water. Tap was directly connected to the “header outlet” (Figure B. 6), while one end of a tube was connected to the “cooler outlet” (Figure B. 6) attachment and the other was submerged in a beaker filled with water. As the tap was turned off the liquid from the beaker started to drain, proving the venturi pump was functional. 116 When the part was ready to be installed it was apparent there was not enough clearance between the cooler and the reservoir for the tubing to fit. The clearance was measured, and appropriate parts were designed and printed. A 270° and a “U” fitting were designed, modelled and printed. Additionally, to have a stable fastening of the inlet flow tubing into the reservoir, an attachment was designed. These designs can be seen in Figure B. 9. In image (C) of Figure B. 9, the lip was designed to attach on to the reservoir. The circular cut-outs were designed to dissect the flow in two directions for enhanced thermal distribution. Figure B. 10 SolidWorks models of A) 270° fitting B) "U" fitting C) Reservoir flow fastener Construction and Assembly of Venturi Pump When the curing process was complete, the attachments were positioned rigid and ready to be utilized in the cycling system. In (A) of Figure B. 10, all the attachments can be seen next to each other. This figure was taken in the hopes of visualizing the resolution and capability to print complex designs on the Form 3 SLA printer. Image (B) in the same figure gives a side view of how the attachments will be set in place on the reservoir. There will be two tubes attached to the “U” fitting, one will be extended into the reservoir whereas the other will attach to the 270° fitting. The other end of 270° fitting is attached to the “cooler outlet” location in the venturi pump. The final configuration of the reservoir inside the cooler can be seen in image (C) of 117 Figure B. 10. Finally, in image (D) of Figure B. 10 it can be observed that the part printed to dissect the flow into two directions to create thermal distribution did not function as intending. Instead most of the flow is sprayed outside the reservoir causing even more mixing of air with the cooling liquid. Figure B. 11 270°, "U", and reservoir fittings printed, assembled, and tested in the exterior bath. To address that, a simpler design was constructed as seen in Figure B. 11. The tubing attachments were mated onto either end of the piping. This allows user to determine inflow location inside the reservoir by simply adding extra tubing. Additionally, angle of liquid entering the reservoir was quite extreme causing most of the liquid to be sprayed out, which was fixed in the revised version. 118 Figure B. 12 Revised version of reservoir flow fastener.
Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
New Approach to foot orthotics materials : hydrogel insoles
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