The morphological, structural and chemical transformations during the biomineralization process associated with the apatite formation from bovine-derived hydroxyapatite (BHAp) and BHAp/Nagelschmidtite composite obtained from the mixture of BHAp and Vitryxx® bioactive glass are here presented. The study is focused on the structural characteristics of apatite formation and maturation process taking place at the biomaterial surface. The use of a biological HAp is of interest aiming to mimic the natural process of bone mineral matrix formation in the crystallization mechanism. This process was characterized by transmission electron microscopy (TEM), where the initial stage was identified as the formation of a hydrated amorphous matrix followed by the appearance of randomly oriented nanocrystals that later coalesce and grow with preferred orientation to c-axis. This behavior is a biomimetic process based on the alignment of natural bone apatite along collagen fibrils. The understanding of natural processes of mineral matrix formation at micro and nanoscale levels on bioactive materials is of great interest to address the effect of size, structure, and composition of apatite nanocrystals on the biologic phenomena involved in osseointegration.

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A mechanism for the formation and crystallization processes of bone-like apatite grown on non-stoichiometric silicocarnotite (SC) is here proposed. Single-phase SC powders and ceramics were obtained from fixed mixtures of hydroxyapatite and bioactive glass 45S5. The bioactive behavior of SC was assessed by immersion in Hank´s solution at different times.Afterward, a systematic theoretical-experimental study of the structural properties at the micro and nanoscale using TEM was performed and correlated with SEM, EDX, XRD, and Raman techniques to determine the apatite mineralization process from the SC phase. The initial stage of apatite formation from SC was identified as the hydration and further polymerization of silanol groups, resulting in a silica-based hydrogel, which plays a critical role in the ionic exchange. As a result of the adsorption of ionic species from the medium into the silica-based hydrogel, the precipitation of crystalline apatitic structures starts through the emergence of newly formed SC nanocrystals, which act as a template for the crystallization process of a substituted apatite with SC-like structure. Then, due to the polymorphism between SC and HAp structures, the apatite layer retains the SC periodic arrangement following an epitaxial-like growth mechanism. Identification of the apatite layer formation mechanism is critical to understand its physical and chemical properties, which controls the long-term dissolution/precipitation rate of bioactive materials and their performance in the biological environment.