Microcracking and porosity in calcium phosphates and the implications for bone tissue engineering

Calcium phosphates including hydroxyapatite (HA) have been widely studied as bone scaffold materials. However their mechanical properties are highly variable and may be a function of thermal expansion anisotropy (TEA) induced stresses and microcracking. There is confusion concerning the mechanisms and microscopic identification of microcracks in HA. This study presents clear evidence of microcracking from micrographs of as-sintered HA surfaces which avoids the complications of identifying TEA-induced microcracks on fracture surfaces. Additionally, the existing literature of TEA-induced microcracking is briefly reviewed and pertinent papers involving likely microcracking in HA are analyzed. The recent realization in the biomedical literature notes that microcracks are of critical importance in remodeling and repair of damaged bone tissue. Hence, the similarities between microcracking in HA used for scaffolds in bone tissue engineering and that in the normal bone repair process is of importance in designing HA scaffolds with improved mechanical properties and biocompatibility.