Bioceramic scaffolds for bone tissue engineering

Synthesis of a porous bioactive 3-D scaffold that exhibits controlled resorbability and possesses mechanical properties compatible to host bone is a challenging task in orthopedic reconstructive surgeries. In the present study, silica-calcium phosphate nanocomposites (SCPC) were engineered with different chemical compositions and porosity contents (32-69%). X-ray computed tomography and mercury porosimetry revealed that SCPC scaffolds were characterized by a uniform distribution of interconnected pores in a wide size-range (3 nm--650 mum). Nanoporosity in the scaffolds originated from dehydration and particulate sintering, where as microporosity was controlled by the porogen content. The ranges of compressive strength and modulus of elasticity of SCPC containing 32-56% porosity were 1.5-50 MPa and 0.14-2.1 GPa respectively, which matched the corresponding values for trabecular bone. The compressive strength of dense SCPC was dependent on the Si-content, and acquired values (93-285 MPa) comparable to that of cortical bone. Inductively coupled plasma analysis demonstrated that Si-rich SCPC exhibited superior dissolution kinetics compared to bioactive glass (BG) and hydroxyapatite (HA-200). Moreover, FTIR analysis revealed that Si-rich SCPC rapidly developed a surface apatite layer after 2 h of immersion in simulated body fluid, while a similar layer was detected on BG after 8 days of immersion. Quantitative real-time PCR results showed that neonatal rat calvarial osteoblasts attached to Si-rich SCPC expressed significantly higher levels of osteocalcin and osteopontin mRNA compared to that by cells attached to HA or polystyrene (TOPS). This suggested a role of Si in stimulating the differentiation and mineralization of osteoblast precursor cells. The effects of dissolved phosphorus on cell function were also investigated by incubating SCPC-, HA- and TOPS-cell systems in culture media supplemented with 3 mM beta-glycerophosphate {lcub}MEM (+){rcub}. The dissolved P-content in MEM (+) increased significantly upon incubation with SCPC owing to the material dissolution products. Osteoblast phenotypic expression on SCPC was significantly decreased after four days of incubation in MEM (+), indicating that sustained exposure to elevated P-levels in the media can down-regulate osteoblast function. Our results show that silica-calcium phosphate nanocomposites can provide a 3D porous, bioactive and resorbable template for cell delivery applications in tissue engineering. Moreover, the bioactive matrix and controlled dissolution of SCPC provide a natural stimulus for bone cell differentiation in vitro, and could obviate the need for exogenous phosphate supplementation.; Keywords. Silica-calcium phosphate nanocomposite, bioceramic scaffolds, porosity, osteoblast gene expression, bone tissue engineering...