The objectives of this study were to prepare composites of tyrosine-derived polycarbonate with different calcium phosphates (TyrPC/CaP) and (1) correlate their physico-chemical properties, including in vitro bioactivity (manifested by the formation of an apatitic layer on the composite surface) to in vivo bone regeneration in a rabbit calvaria critical size defect (CSD) model, and (2) investigate changes in CaP properties due to incorporating these into a scaffold architecture. We used dicalcium phosphate dihydrate (DCPD), CaHPO4.2H2O; octacalcium phosphate (OCP), Ca8H2(PO4)6.5H2O; beta-tricalcium phosphate (β-TCP), Ca3(PO4)2; and synthetic bone mineral (SBM), (Ca,Mg)10(PO4,CO3)6(OH,F)2.
Tyrosine-derived polymers were synthesized by condensation reaction of triphosgene and tyrosine-derived diphenol. OCP, DCPD, and SBM were synthesized in-house, while β-TCP was obtained from a manufacturer. Analytical tools to characterize both CaPs and composites included SEM, micro-computed tomography (microCT), x-ray diffraction, and inductively coupled plasma. Dissolution rates in acidic buffer and in vitro bioactivity in fetal bovine serum were also determined. In vivo bone regeneration after 6 weeks was assessed via microCT using a 15mm CSD in the calvaria of New Zealand White rabbits.
(1) Porous (85%), bimodal (200-400mm macropores, and <20mm micropores) interconnecting architecture in all composites; (2) dissolution rates of the composites decreased in the order: TyrPC/SBM > TyrPC/DCPD > TyrPC/ β-TCP > TyrPC/OCP; (3) all composites formed an apatite layer after immersion in FBS indicating in vitro bioactivity; (4) TyrPC/OCP composite scaffolds outperformed the three other scaffold types.
TyrPC/CaP composites tested have potential as scaffolds for tissue engineering in bone regeneration. All composites evaluated in this study showed in vitro bioactivity, while the TyrPC/OCP had the highest amount of tissue regeneration. This may be attributed to the dissolution rate of this particular composite, which provided the appropriate calcium and phosphorus release rate for the cascade of biological processes to occur.
Keywords: Calcium phosphates, Implants, Polymers, Regeneration and Tissue engineering