Advanced bioceramic composite for bone tissue engineering: Design principles and structure-bioactivity relationship

The synthesis of a new resorbable porous bioactive silica-calcium phosphate composite (SCPC) that can be used as a tissue-engineering scaffold for bone regeneration is described. The effects of chemical composition and thermal treatment on crystallization and the mechanism of phase transformation in SCPC were evaluated. In the silica-rich samples, beta-rhenanite (beta-NaCaPO4,) and alpha-cristobalite (SiO2) were the dominant phases after treatment at 800degreesC. On the other hand, in the calcium phosphate-rich samples, calcium pyrophosphate (Ca2P2O7) was formed in addition to beta-rhenanite and alpha-cristobalaite. X-ray diffraction analyses showed a shift in the 2theta value of the main peak(s) of all phases indicating the formation of solid solutions. Phase transformation reactions were accompanied by a loss of water molecules that contributed to the formation of pores in the size range 10-300 mum. All SCPC samples adsorbed a significantly higher quantity of serum protein than bioactive glass (p < 0.0001). In addition, the silica-rich SCPC adsorbed a significantly higher amount of serum protein than the calcium phosphate-rich samples (p < 0.003). While the crystallization of amorphous silica into L-quartz significantly inhibited serum protein adsorption, the transformation of L-quartz into alpha-cristobalite solid solution (ss) significantly enhanced protein adsorption. On the other hand, in conjunction with the transformation of brushite (CaHPO4) into pyro- and tri-calcium phosphates, there was a significant decrease in protein adsorption. However, as pyro- and tricalcium phosphates transformed into beta-rhenanite, by thermal treatment, protein adsorption increased markedly. Critical-size bone defects grafted with silica-rich SCPC were filled with new bone and contained minimal residues of the graft material. Bone defects grafted with bioactive glass enhanced new bone formation, however, with very limited resorption. The enhanced resorption of SCPC in vivo correlates well with the higher rate of silica dissolution from SCPC than bioactive glass. The facilitated Si dissolution was associated with rapid bone regeneration in defects grafted with SCPC. The enhanced bioactivity properties of the SCPC are due to its chemical composition, modified crystalline structure, and high porosity. The new SCPC may be used for a wide variety of applications in the field of bone reconstruction including tissue-engineering scaffolds for cell and drug delivery. (C) 2004 Wiley Periodicals, Inc.