Surface engineering of biodegradable zinc alloys for biomedical implants: a critical assessment of degradation control, biofunctionality, durability and clinical translation

Global rise in orthopaedic disorders associated with an aging population has intensified demand for biodegradable metallic implants capable of eliminating the necessity for surgical implantation removal. Magnesium and iron, the two most studied bioabsorbable metals have limitations: Mg corrodes too quickly producing pockets of hydrogen gas whereas Fe corrodes too slowly to match tissue healing timelines. Zinc is a potential candidate because of its intermediate degradation rate, biological importance, and natural antimicrobial properties. However, pure zinc has insufficient mechanical strength and ductility to be used in load bearing applications and requires appropriate control of the degradation kinetics to prevent toxicity of Zn 2 + ions. In this review, the latest advances made in terms of alloying, thermomechanical processing, and surface treatment methods used in order to solve the problems mentioned above are summarized. Various methods such as physical methods (laser texturing, plasma treatment, mechanical structuring), chemical methods (acid/alkaline etching, surface alloying, hydrothermal treatment), and electrochemical methods (anodization, electrophoretic deposition, plasma electrolytic oxidation) are considered. The review also discusses the combination of all these techniques to create polymeric, bio-functional, and inorganic coatings which can control degradation rate, increase biocompatibility, provide antibacterial activity, and promote osseointegration. Key research gaps are identified, which include the difference in degradation rate and cytotoxic threshold, the lack of standardized testing procedures, long-term in-vivo and clinical data, and scaling up of the coatings. The future perspectives are described that may help in clinical translation of zinc-based bioabsorbable implants through alloy and additive manufacturing, stimuli-responsive coatings, and standardization. The durability of coated implants under dynamic physiological conditions is discussed, and quantitative translation targets are proposed.