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

Abstract

There is an increasing interest in biodegradable metal implants made from magnesium (Mg), iron (Fe), zinc (Zn) and their alloys because they are well tolerated in vivo and have mechanical properties that approach those of non-degradable metals. In particular, Zn and its alloys show the potential to be the next generation of biodegradable materials for medical implants. However, Zn has not been as well-studied as Mg, especially for stent applications. Manufacturing stents by laser cutting has become an industry standard. Nevertheless, the use of this approach with Zn faces some challenges, such as generating thermal stress, dross sticking on the device, surface oxidation, and the need for expensive thin-walled Zn tubing and post-treatment. All of these challenges motivated us to employ photo-chemical etching for fabricating different designs of Zn (99.95% pure) stents. The stents were constructed with different strut patterns, made by photo-chemical etching, and mechanically tested to evaluate radial forces. Stents with rhombus design patterns showed a promising 0.167N/mm radial force, which was comparable to Mg-based stents. In vitro studies were conducted with uncoated Zn stents as control and Parylene C-coated Zn stents to determine corrosion rates. The Parylene C coating reduced the corrosion rate by 50% compared to uncoated stents. In vivo studies were carried out by implanting photo-chemically etched, uncoated Zn stent segments subcutaneously in a C57BL/6 mice model. Histological analyses provided favorable data about the surrounding tissue status, as well as nerve and blood vessel responses near the implant, providing insights into the in vivo degradation of the metal struts. All of these experiments confirmed that Zn has the potential for use in biodegradable stent applications.

Effect of Surface-Modification on In Vitro Corrosion of Biodegradable Magnesium-Based Helical Stent Fabricated by Photo-chemical Etching

“…By employing weight loss determination of corrosion, it was possible further to find that balloon expansion of the Parylene C coated stents resulted in a higher corrosion rate compared to unexpanded coated stents. A similar effect of Parylene C and balloon expansion on corrosion rates were described in our previous publication, using Zn as a material for making biodegradable stents by photo-chemical etching 17 . As we found in 16 , balloon expansion of stents may introduce defects and openings in the Parylene C coating, exposing more Mg to the solution.…”

“…However, all magnesium implants still face the challenges of fast corrosion and the production of hydrogen release needs to be addressed. Corrosion control can be achieved by altering the surface of the Mg biodegradable implants through polymer coatings, as demonstrated in our previous studies 17,19 . The coating that we explored previously, and continue to test in this report, is Parylene C, which is a Food Drug & Administration (FDA) approved biocompatible polymer.…”

“…Our group has designed, fabricated and tested a biodegradable metal stent with a helical shape. The results have been published by previously 16,17 . Biodegradable metallic helical stents have the potential to overcome some current issues with conventionally shaped stents, such as limited expandability and high manufacturing costs.…”

“…Details about stent fabrication using photochemical etching of biodegradable metal sheets have been published by our group [16][17][18][19][20]25 . The starting material was a rectangular sheet of AZ31 with dimensions of 250 mm × 250 mm and thickness of 250 μm, purchased from Goodfellow, USA (Oakdale, PA).…”

“…The sublimation process resulted in the formation of an activated polymeric Parylene C layer that uniformly coated the sample. The chemical process of sublimation, as well as the whole process of the coating formation, are described in our previous publication 17 . The preliminary step before coating the stents with Parylene C involved treatment with a silane primer solution, which served as a coupling agent.…”