Ion-doped hydroxyapatite: An impasse or the road to follow?

Doping hydroxyapatite (HAp) with foreign ions has been increasing in popularity as a chemical approach to augment the existing and impart new properties to it. However, it is uncertain whether this approach can elicit properties strong enough to make HAp competitive for commercial applications in biomedicine and elsewhere. In particular, material properties, such as grain size, morphology, surface charge, porosity, topology and others may prove to be more effective in controlling properties of HAp than the dopant choice and concentration. This study tackles this question through a meta-analysis of the dependence of selected materials properties on dopant concentrations reported across the literature, showing that doping is a convenient way of controlling some, but not all properties in HAp and should be implemented ideally in combination with other structural and compositional factors of influence. This meta-analysis is coupled to a bibliographic analysis of the evolution of trends in terms of the type, frequency and concurrency of dopants in HAp, covering the entire Periodic Table, all 72 elements incorporated into HAp so far and the 1990 - 2019 period split into successive lustra. The study demonstrates a continuously ascending trend in popularity of certain elements, such as Sr, Ce and Se, and a continuously declining trend of others in the past two decades, including F, Y, I, Ti and most lanthanides. For the fourth period transition metals a trend of constancy was observed and for yet other elements, including Ag and Mg, a period of ascent was succeeded by a steady decline, the onset of which coincided in the case of Ag with the peak in the airing of the concerns over its cytotoxicity by the scientific community. Reasons for these trends with respect to most elements are explained and connected with trends governing broader realms of materials science. It is shown that the choice of ions as dopants in HAp is largely driven by broader trends in materials science, rendering the doped HAp a mirror to view these existing and past trends in. Theoretically, by following the historic progression of these trends, a gaze in this mirror could assist in extrapolation of future trends, which normally come at the cost extinguishing some of the existing ones. This immersion in more general developments in materials science opened the path toward drawing connections between the bibliographically assessed geopolitical trends in doped HAp and in broader aspects of the scientific community, including the growing gap between the rich and the poor pervading it. The analysis reported here is conceptually novel and could be applied to numerous other materials.