Controllable biodegradation and enhanced osseointegration of ZrO2-nanofilm coated Zn-Li alloy: In vitro and in vivo studies.

Zinc and its alloys have emerged as a new research direction of biodegradable metals (BMs) due to the significant physiological functions of Zn2+ ions in human body. However, low inhibitory concentration threshold value to cause cytotoxicity by Zn2+ ions during in vitro study and delayed osseointegration in vivo are two key flaws for the bulk Zn-based BMs. To combat these issues, we constructed a barrier layer of ZrO2 nanofilm on the surface of Zn-0.1(wt.%) Li alloy via atomic layer deposition (ALD). A decreased release of Zn2+ ions accompanied with accelerated release of Li+ ions was observed on account of galvanic coupling between the coating compositions and Zn-0.1Li alloy substrate. Cytocompatibility assay reflected that ZrO2 nanofilm coated Zn-0.1Li alloy exhibited improved cell adhesion and viability. Histological analysis also demonstrated better in vivo osseointegration for the ZrO2 nanofilm coated Zn-0.1Li alloy. Hence, the present study elucidated that the ALD of ZrO2 nanofilm on Zn-based BMs can effectively promote osseointegration and control their biodegradation behavior. STATEMENT OF SIGNIFICANCE: Zn-Li binary alloy was reported recently to be the promising biodegradable metals with ultimate tensile strength over 500 MPa, yet the low inhibitory concentration threshold value to cause cytotoxicity by Zn2+ ions is the obstacle needed to be overcome. As a pilot study, a systematic investigation on the ZrO2 nanofilm coated Zn-Li alloy, prepared by atomic layer deposition (ALD) technique, was conducted in the present study, which involved in the formation process, in vitro and in vivo degradation behavior as well as biocompatibility evaluation. We found a controllable corrosion rate and better in vivo osseointegration can be achieved by ZrO2 nanofilm coating on Zn-Li alloy, which provides new insight into the surface modification on biodegradable Zn alloys for usage within bone.

Challenges in the use of zinc and its alloys as biodegradable metals: Perspective from biomechanical compatibility.

To date, more than fifty articles have been published on the feasibility studies of zinc and its alloys as biodegradable metals. These preliminary in vitro and in vivo studies showed acceptable biodegradability and reasonable biocompatibility in bone and blood microenvironments for the experimental Zn-based biodegradable metals and, for some alloy systems, superior mechanical performance over Mg-based biodegradable metals. For instance, the Zn-Li alloys exhibited higher UTS (UTS), and the Zn-Mn alloys exhibited higher elongation (more than 100%). On the one hand, similar to Mg-based biodegradable metals, insufficient strength and ductility, as well as relatively low fatigue strength, may lead to premature failure of medical devices. On the other hand, owing to the low melting point of the element Zn, several new uncertainties with regard to the mechanical properties of biomedical zinc alloys, including low creep resistance, high susceptibility to natural aging, and static recrystallization (SRX), may lead to device failure during storage at room temperature and usage at body temperature. This paper comprehensively reviews studies on these mechanical aspects of industrial Zn and Zn alloys in the last century and biomedical Zn and Zn alloys in this century. The challenges for the future design of biomedical zinc alloys as biodegradable metals to guarantee 100% mechanical compatibility are pointed out, and this will guide the mechanical property design of Zn-based biodegradable metals. STATEMENT OF SIGNIFICANCE: Previous studies on mechanical properties of industrial Zn and Zn alloys in the last century and biomedical Zn and Zn alloys in this century are comprehensively reviewed herein. The challenges for the future design of zinc-based biodegradable materials considering mechanical compatibility are pointed out. Common considerations such as strength, ductility, and fatigue behaviors are covered together with special attention on several new uncertainties including low creep resistance, high susceptibility to natural aging, and static recrystallization (SRX). These new uncertainties, which are not significantly observed in Mg-based and Fe-based materials, are largely due to the low melting point of the element Zn and may lead to device failure during storage at room temperature and clinical usage at body temperature. Future studies are urgently needed on these topics.

Evolution of the degradation mechanism of pure zinc stent in the one-year study of rabbit abdominal aorta model.

In the present study, pure zinc stents were implanted into the abdominal aorta of rabbits for 12 months. Multiscale analysis including micro-CT, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and histological stainings was performed to reveal the fundamental degradation mechanism of the pure zinc stent and its biocompatibility. The pure zinc stent was able to maintain mechanical integrity for 6 months and degraded 41.75 ± 29.72% of stent volume after 12 months implantation. No severe inflammation, platelet aggregation, thrombosis formation or obvious intimal hyperplasia was observed at all time points after implantation. The degradation of the zinc stent played a beneficial role in the artery remodeling and healing process. The evolution of the degradation mechanism of pure zinc stents with time was revealed as follows: Before endothelialization, dynamic blood flow dominated the degradation of pure zinc stent, creating a uniform corrosion mode; After endothelialization, the degradation of pure zinc stent depended on the diffusion of water molecules, hydrophilic solutes and ions which led to localized corrosion. Zinc phosphate generated in blood flow transformed into zinc oxide and small amounts of calcium phosphate during the conversion of degradation microenvironment. The favorable physiological degradation behavior makes zinc a promising candidate for future stent applications.