UC San Diego

Marine CaCO₃ skeletons have tailored architectures created by nature, which give them structural support and other functions. For example, seashells have dense lamellar structures, while coral, cuttlebone and sea urchin spines have interconnected porous structures. In our experiments, seashells, coral and cuttlebone were hydrothermally converted to hydroxyapatite (HAP), and sea urchin spines were converted to Mg-substituted tricalcium phosphate [beta]-TCMP, while maintaining their original structures. Partially converted shell samples have mechanical strength, which is close to that of compact human bone. After implantation of converted shell and spine samples in rat femoral defects for 6 weeks, there was newly formed bone growth up to and around the implants. Some new bone was found to migrate through the pores of converted spine samples and grow inward. These results show good bioactivity and osteoconductivity of the implants, indicating the converted shell and spine samples can be used as bone defect fillers. Calcium phosphate powders were prepared through different synthesis methods. Micro- size HAP rods were synthesized by hydrothermal method through a nucleation-growth mechanism. On the other hand, HAP particles, which have good crystallinity, were prepared by wet precipitation with further hydrothermal treatment. [beta]-TCP or [beta]-TCMP powders were prepared by a two-step process: wet precipitation of apatitic tricalcium phosphate ('precursor') and calcination of the precursor at 800⁰C for 3 hours. [beta]-TCMP or [beta]-TCP powders were also prepared by solid-state reactions from CaHPO₄ and CaCO₃ with/without MgO. Biphasic calcium phosphate, which is mixture of HAP and [beta]-TCP, can be prepared though mechanical mixing of HAP and [beta]-TCP powders synthesized as above. Dense [beta]-TCP and [beta]- TCMP ceramics can be produced by pressing green bodies at 100MPa and further sintering above 1100⁰C for 2 hours. [beta]-TCMP ceramics ̃99.4% relative dense were prepared by this method. Dense [beta]-TCP ceramics have average strength up to 540MPa. Macroporous [beta]-TCMP ceramics were produced with sucrose as the porogen following a two- step pressing method. Porous [beta]-TCMP ceramics were also prepared by replication of polyurethane sponge. [beta]-TCMP ceramics with porous structures in the center surrounded by dense structures were created. The outside dense structures give the scaffold mechanical strength, while the central porous structures enable cells migration and vascular infiltration, and finally in-growth of new bone into the scaffold