The Prospects of Zinc as a Structural Material for Biodegradable Implants—A Review Paper

Abstract

Abstract:In the last decade, iron and magnesium, both pure and alloyed, have been extensively studied as potential biodegradable metals for medical applications. However, broad experience with these material systems has uncovered critical limitations in terms of their suitability for clinical applications. Recently, zinc and zinc-based alloys have been proposed as new additions to the list of degradable metals and as promising alternatives to magnesium and iron. The main byproduct of zinc metal corrosion, Zn 2+ , is highly regulated within physiological systems and plays a critical role in numerous fundamental cellular processes. Zn 2+ released from an implant may suppress harmful smooth muscle cells and restenosis in arteries, while stimulating beneficial osteogenesis in bone. An important limitation of pure zinc as a potential biodegradable structural support, however, lies in its low strength (σ UTS~3 0 MPa) and plasticity (ε < 0.25%) that are insufficient for most medical device applications. Developing high strength and ductility zinc with sufficient hardness, while retaining its biocompatibility, is one of the main goals of metallurgical engineering. This paper will review and compare the biocompatibility, corrosion behavior and mechanical properties of pure zinc, as well as currently researched zinc alloys.

In Vitro and In Vivo Testing of Zinc as a Biodegradable Material for Stents Fabricated by Photo-Chemical Etching

“…Generally, corrosion occurs via cathodic and anodic reactions in a near neutral (pH around 7.4) physiological environment. Metals like Mg produce oxides, hydroxides, and hydrogen gas as byproducts during their corrosion [4]. Zn also undergoes similar reaction processes, as stated below, but, unlike with Mg, this process does not produce hydrogen gas.…”

“…Metallic Zn appears to be one of the few physiologically acceptable metals that has not yet been actively considered for the production of bioabsorbable stents. Zn and its alloys exhibit a wide range of ultimate tensile strengths, varying from 87 to 399 MPa, and elongation to failure values from 0.9% tõ 170% [4,5], so several alloys fit some of the criteria outlined in Table S1. It has been reported, in the same reference, that Zn is more ductile than Mg.…”

“…Another advantage of Zn is its corrosion characteristics. Among the three biodegradable metals, Fe has the lowest corrosion rate, which is too slow for stent applications, and its corrosion products accumulate and are difficult for the body to remove [4]. Mg has the highest corrosion rate, which could negatively affect tissues by releasing too much hydrogen and, because of this, the mechanical integrity of the stents can be compromised prior to the time needed for tissue repair.…”

“…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].…”

“…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]. One of the earliest in vivo studies was reported in 2013, in which pure Zn wires were implanted in the walls of the abdominal aortas of adult rats and left in for up to 6 months with no toxicity or negative tissue effects [3].…”