Dynamic structural evolution in biogenic hydroxyapatite plates during hydrothermal treatment and sintering

Biogenic hydroxyapatite (BHA) has been extensively studied in porous configurations; however, dense geometries such as flat plates remain largely unexplored despite their potential for structural biomedical applications. In this work, the physicochemical and mechanical evolution of dense BHA plates derived from bovine cortical bone was systematically studied as a function of successive hydrothermal treatments and follow by sintering. Infrared spectroscopy revealed a progressive removal of the organic matrix, resulting in a transition from an organic-dominated to a mineral-dominated and ultimately fully ceramic system, while preserving the characteristic phosphate and carbonate groups of hydroxyapatite, as shown in the IR spectra. Atomic force microscopy revealed a non-monotonic evolution of surface topography, with roughness increasing during intermediate stages, reaching a maximum at HT2 ( R a ≈ 513 nm and R q ≈ 768 nm), followed by a reduction after further treatment and sintering due to densification and crystallite coalescence. Mechanical testing indicated a progressive decrease in stiffness and strength with increasing hydrothermal treatment, directly associated with the loss of the organic phase, while the HT2 condition exhibited high variability linked to structural heterogeneity and defect sensitivity. The sintered sample showed brittle behavior with low deformation capacity. X-ray diffraction and Rietveld analysis confirmed structural stability of the hexagonal hydroxyapatite phase with no significant crystallite growth ( τ ≈ 26 − 30 nm). These results demonstrate a dynamic, nonlinear structural evolution governed by the interplay between organic removal, mineral reorganization, and thermal densification, leading to distinct organic, transitional, mineral, and ceramic regimes.