## 1 Introduction
\[\text{Anodic reaction}:\text{ Zn}\rightarrow\text{Zn}^{2+}+2\text{e}^{-} \tag{1}\]  
\[\text{Cathodic reaction}:\text{ O}_{2}+2\text{H}_{2}\text{O}+4\text{e}^{-} \rightarrow 4\text{OH}^{-} \tag{2}\]  
\[\text{Zn}\text{(OH)}_{2}\text{ formation}:\text{ Zn}^{2+}+2\text{OH}^{-} \rightarrow\text{Zn}\text{(OH)}_{2} \tag{3}\]  
\[\text{ZnO formation}:\text{ Zn}\text{(OH)}_{2}\rightarrow\text{ZnO}+\text{H}_{2} \text{O} \tag{4}\]  
Under physiological conditions, where pH is controlled, electrons generated from the anodic reaction (1) would be consumed during the reduction process at the cathodic reaction (2). The other reactions (3) and (4) are likely to occur on the surface of the Zn implant. Again, note that this occurs without releasing any hydrogen gas, which is unlike Mg implants, where controlling hydrogen gas release is an important consideration [3; 8; 9]. Due to the presence of chloride ions in normal physiological solutions, Zn(OH)2 and ZnO would aggressively react with chloride ions forming more soluble chloride salts such as Zn(OH)2.ZnCl2. This process of dissolving the corrosion products Zn(OH)2 and ZnO exposes bare Zn metal again to physiological solutions, which promotes continuous cycles of anodic and cathodic reactions. With this progressive exposure, particles of Zn(OH)2 and ZnO would be separated from the Zn matrix and either precipitate or become suspended in the physiological medium [8; 9]. Corrosion of Zn releases ions that are important physiologically, which offers additional support for this metal to be explored as biodegradable material for stent applications. Zn is present in all organs, tissues, fluids and body secretions, with 86% of its mass residing in skeletal muscle and bone, 6% in the skin, 5% in the liver, 1.5% in the brain, and the remaining distributed amongst the other tissues [10; 11]. At the cellular level, 30-40% of the Zn existing is located in the nucleus. Zn is known as an essential element for basic metabolic processes and the recommended daily dietary intake of Zn ranges from 15 mg day\({}^{-1}\) for adult males to 2 mg day\({}^{-1}\) for infants [3; 12]. Due to its physiological importance, mammals have developed sophisticated mechanisms to remove Zn from dietary constituents, absorb Zn, regulate its concentration in body fluids, transport it safely to all tissues, particularly to sites where its presence is required, and safely excrete excess amounts through the kidneys [11]. While Zn and its alloys appear to be efficacious in terms of corrosion rate and mechanical properties, major processing challenges remain before they can be manufactured into a tubular stent preform [3]. Fabricating stents by laser cutting became an industry standard, but this expensive approach introduces certain issues. Our patented alternative is based on photo-chemical etching to transfer a pattern of the stent struts onto a Mg sheet [13]. This type of Mg stent has been characterized in vitro and the results are published elsewhere [14; 15; 16; 17; 18; 19]. In this study, we demonstrate that photo-chemical etching can be used to create stent materials out of pure Zn and that these materials are well tolerated by tissues in an initial in vivo study. Another advantage of Zn for stent applications is that this metal, unlike Mg, is opaque to x-rays. Vascular stents are typically implanted by a catheterization procedure using x-ray fluoroscopy to guide the stent through the vasculature and position it in the target artery. Thus, the stent and/or stent deployment system must be sufficiently radiopaque (not transparent to X-rays) for visualization during fluoroscopy. Unlike Mg and its alloys, our studies demonstrated good radiopacity of the Zn stents fabricated in this work, as shown in Figure S1 (Supplementary Information). Previous studies have demonstrated that metallic Zn shows biocompatibility when implanted in arteries in preclinical studies, greatly highlighting its promise as a base material for bioabsorbable stents [3; 4; 5; 20].