Hydroxyapatite materials-synthesis routes, mechanical behavior, theoretical insights, and artificial intelligence models: a review

Over the years, hydroxyapatite (HAp) has been a well-researched biomaterial because of its bioactive and biocompatible properties with remarkable applications for bone tissue engineering. The robust structure of HAp allows for a host of applications in biomedicine. HAp is enriched in calcium and phosphate, can be sourced from synthetic or natural precursors with significant characteristics notable of biomaterials, and can be produced by facile protocols for clinical use. Nonetheless, HAp prepared from natural or synthetic sources are different due to the conditions of processing. One of the factors in this direction and for the high performance of bioceramics in biomedicine is a robust mechanical strength that prevents failure of the HAp scaffolds. Stemming from these, and of particular interest, is the porosity of the HAp-derived scaffolds that plays a major role in the mechanical properties in vitro and in vivo. Many reports have it that there are reduced mechanical properties vis-à-vis the inherent high porosity of the scaffolds, and these must be balanced in line with the degradation rate of the scaffolds. Gradients in pore sizes and crack propagation tendencies are important to lead to new production methods with the potential to generate scaffolds with morphological and mechanical properties designed to meet bone repair needs. Nowadays, validating mechanical and materials engineering properties with the aid of atomistic simulations using density functional theory (DFT) and artificial intelligence (AI), and the complement of experimental studies, is gradually becoming an important research domain within the scientific community. The importance of these theoretical and AI methods can be ascribed to the comprehension of the non-linear relationship between some measured properties using experimental datasets. Hence, this review explores a re-cap and the state of knowledge regarding sustainable natural sources of HAp, data on mechanical property measurements, the link between porosity and mechanical properties of HAp-derived materials for bone tissue engineering, a relatively new method for characterizing the mechanical behavior of HAp, computational trends in biomaterials research, and recent trends on the biomedical applicability of HAp.