Based on the chemical composition, synthetic CaPs for biomedical applications and their major properties are summarized and listed in Table 1. The biomedical performance of CaPs is closely related to their solubility, which may play an important role in controlling the cytotoxicity 29 and inflammatory response 30. CaPs have different solubility, and the comparative extent of dissolution is: MCPM > MCPA > DCPD > DCPA > CDHA > OCP > ?-TCP > TTCP > ?-TCP > HA 24. The dissolution of BCPs depends on the ?-TCP/ HA ratio: the higher the ratio, the greater the dissolution extent 31. The solubility coefficient, stability, and phase transformation of these CaPs also heavily depend on environmental conditions, as shown in Table 1.
Most CaPs highlight their biocompatibilities in vitro and in vivo, as shown in Table 2. Their in vitro cytocompatibility have been demonstrated with several cell lines, including fibroblast cells, pre-osteoblastic cells, bone marrow cells, and mesenchymal stem cells derived from mice, rabbits, and humans, as detailed in Table 2. It has been shown that cell behaviors might be different in vitro from in vivo, so it is vital to understand the interaction of the implants with tissues in a living system. Different animal models are chosen for different purposes, e.g. rat or mice for subcutaneous examinations, rabbit for surface interaction studies with femoral bone, and large animal models such as dog, sheep, and goats to verify the practicability of the implants in a more realistic clinical situation 32, and the chosen implanting position is usually the femur, tibia, or mandible bones 33. CaPs are generally known to be osteoconductive, but not osteoinductive, in the absence of supplements 34, 35. However, several CaPs have demonstrated the osteoinductive ability to drive osteoblastic differentiation via cell-ECM interaction 35. The osteoinductive properties depend on several architectural features of the CaPs, such as surface geometry, topography, chemistry and charge, porosity, and pore size.