Bionanohydroxyapatite/Poly(3-hydroxybutyrate-<i>co</i>-3-hydroxyvalerate) Composites with Improved Particle Dispersion and Superior Mechanical Properties

Abstract

The present work was inspired by a persistent limitation in the use of composite biomaterials for orthopedics, namely, the tendency of ceramic reinforcements to agglomerate in these composites due to interparticle van der Waals interaction. As a result, the composites possess poor mechanical properties that are unsuited for load-bearing applications. We propose using nanohydroxyapatite particles preadsorbed with heparin (nHA-HEP) to circumvent this issue. The key feature was the dual role that HEP would play in enhancing particle dispersion and biological response. Turbidity and zeta potential measurements revealed that the addition of HEP significantly improved the colloidal stability (23 days with minimal particle sedimentation) of nHA in water, dimethylformamide (DMF), and dimethylsulfoxide (DMSO) but had negligible effect in acetic acid. Anti-FXa activity results showed that 95% of the HEP adsorbed onto nHA retained its bioactivity in DMSO and water, while 51% was preserved in DMF and acetic acid. Composite fabrication was ultimately done using poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a biodegradable polymer, dissolved in DMF. A decrease in water contact angle of solvent cast films with increasing weight percent (wt %) of nHA-HEP/PHBV was observed as compared to nHA/PHBV films. Scanning electron micrographs illustrated that nHA/PHBV films had poor dispersion of particles within the matrix and that large agglomerates settled to the bottom of the films during casting. On the contrary, the nHA-HEP/PHBV matrices had effective dispersion of particles and enhanced tensile properties. We found that the tensile elastic modulus and strength of the films increased with increasing wt % of nHA-HEP. A value close to the human cortical bone was obtained at 30 wt % loading of nHA-HEP. In conclusion, understanding the relationship between process, morphology, and property led to the development of nHA-HEP/PHBV composites that show promise for load-bearing bone applications.