Toward bioactive calcium carbonate and hydroxyapatite-based coatings via plasma electrolytic oxidation – A review of challenges and current state-of-the-art

Abstract

Bioactive implant coatings on metallic substrates are pivotal in modern biomedical engineering, offering both temporary and long-term solutions for repairing hard calcified tissue defects. Coatings combining calcium carbonate (CC) and hydroxyapatite (HA) are particularly effective in mimicking natural calcified tissues, embodying the principles of fourth-generation biomaterials. Existing techniques largely constrain the development of fully functional CC/HA coatings. Concurrently, plasma electrolytic oxidation (PEO), one of the most prospective approaches for metallic surface chemical modification, remains underexplored in the context of carbonates. This paper comprehensively reviewed the challenges and limitations associated with the parallel incorporation of CC and HA into PEO coatings, focusing on the additives' aggregate state, physicochemical properties, synthesis methods, and PEO process parameters. Additionally, an optimum strategy for creating Ti-based CC/HA coatings was identified via deductive approach and causal analysis. Standard PEO baths comprising only soluble components were recognized as inapplicable, whereas those representing biphasic particle suspensions exhibited undeniable advantages for the stated purpose. The ultimately preferred bath recipe consisted of CC particle suspension synthesized via carbonation route and further subjected to competing dissolution-precipitation and chemisorption reactions in dibasic sodium phosphate electrolyte. In sum, the current study provides a reliable conceptual basis toward practical implementation of CC/HA PEO coatings.