Millions of bone graft procedures are performed annually for repairing bone defects. Synthetic calcium phosphate-based (CaP) materials are commonly used as bone graft substitutes. Their chemical similarity to bone guarantees biocompatibility while macroporosity of CaP materials enables ingrowth of natural bone tissue and firm integration into the bone. Since the mechanical performance of the synthetic materials is generally inferior to natural bone, gradual resorption of CaP implants and simultaneous replacement by natural bone is desirable. Hydroxyapatite (HA), as the thermodynamically most stable CaP phase in the human body, is almost non-resorbable. Therefore, many CaP implants are designed as biphasic calcium phosphate (BCP) composites of HA and the more soluble tricalcium phosphate (similar to the mineral whitlockite). In the presence of Mg ions, the formation of whitlockite is favoured over hydroxyapatite. During degradation of BCP implants, whitlockite-incorporated Mg ions are released and stimulate the formation of natural bone. Furthermore, Sr ions are known to stimulate bone formation and retard bone resorption at the same time. Hence, resorbable Mg- and Sr-containing implants may counteract bone loss and fracture related to osteoporosis. Additionally, the antibacterial effect of Ag ions could be employed to avoid wound infection during surgical graft procedures.
Aiming at the development of novel antiosteoporotic and antimicrobial bone implant materials, macroporous biominerals, such as skeletal materials of coral (aragonite, CaCO3) and sea urchins (Mg-calcite, Ca0.9Mg0.1CaCO3), were pseudomorphically replaced by CaP while the natural porosity was preserved. During hydrothermal mineral replacement, Sr ions, in phosphate solutions temporarily stabilized by EDTA, were incorporated into the HA phase of the resulting material. This approach was unsuccessful for Mg incorporation. Superficial replacement of CaP materials in AgNO3 solution produced Ag phosphate coatings, endowing them with antibiotic properties.