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Biomimetic synthesis of poly(propylene-fumarate)-calcium phosphate composites for tissue engineering Hakimi Mehr, Dorna

Abstract

A novel in-situ co-precipitation process for the synthesis of poly(propylene-fumarate)-calcium phosphate composites was developed. In this process the calcium phosphate phase nucleates and grows in the presence of poly(propylene-fumarate) (PPF), in a novel two-solvent system including tetrahydrofuran (THF) and water. It was found that the presence of the organic solvent (THF) does not affect the phase evolution of the calcium phosphate. Both in the presence and absence of THF crystalline dicalcium phosphate dihydrate (DCPD, brushite) and poorly crystalline hydroxyapatite (HAp) form, and transform to crystalline HAp after 24 hours of synthesis time. Contrary to the organic solvent, PPF has a significant influence on the calcium phosphate phase that forms in its presence. It is found that PPF provides a template for the formation of the calcium phosphate phase through a coordination bond between the calcium ion and the carbonyl group of the polymer. As a result of this templating, hydroxyapatite can form in a significantly shorter period of time (~1 hr) compared to the system where PPF is not present (24 hrs). The nature of the calcium phosphate phase that forms in the presence of PPF depends on the molecular weight and concentration of PPF. High concentration of PPF in the composite (e.g. 80%) stabilizes an amorphous calcium phosphate (ACP) phase and hinders its transformation to crystalline apatite, while low concentration of PPF (e.g. 5%) promotes the formation of crystalline apatite. Higher molecular weight PPF (Mw = 4500) is found to be more efficient in stabilizing the amorphous phase compared to lower molecular weight PPF (Mw = 1800). While high molecular weight PPF stabilizes ACP, low molecular weight PPF promotes its conversion to crystalline apatite. TEM observations revealed that flake-like hydroxyapatite crystals form in the absence of PPF while spherical ACP particles form in a composite containing 80% PPF. The ACP nano-particles (50-100 nm in diameter) are homogeneously distributed within the PPF matrix. The PPF-calcium phosphate composite can crosslink into a 3D structure by UV irradiation.

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