New approaches in evaluating metallic candidates for bioabsorbable stents

Abstract

A series of unconventional approaches has been developed at Michigan Technological University, which is able to screen candidate materials for use in bioabsorbable (or bioresorbable) stents by reducing the scale of necessary animal studies and the complexity of biocorrosion analyses. Using a novel in vivo approach, materials formed into a simplified wire geometry were implanted into the wall of the abdominal aorta of rodents for several weeks or months to measure the extent of in vivo degradation, quantify mechanical strength over time, characterize the resulting products, and assess biocompatibility. An in vitro method was developed to identify bioabsorbable candidate materials, reproduce the corrosion products formed in vivo, and predict the degradation rate of stent materials. To accomplish this goal, wires were encapsulated in an extracellular matrix and corroded in cell culture media in vitro. Encapsulation of the wires in vitro was necessary in order to mimic in vivo stent encapsulation within a neo-intima. Alternatively, accelerated in vitro corrosion for materials with very low corrosion rates was accomplished by exposing fibrin-coated wires to a steady flow of cell culture media. After in vivo and in vitro tests, wires were subjected to tensile testing to quantify the rate of material degradation and loss of mechanical strength.

Zinc Exhibits Ideal Physiological Corrosion Behavior for Bioabsorbable Stents

“…It is important to note that the research community has not yet reached a consensus on some criteria presented in this table. For example, there are at least two different ideal grain‐size ranges proposed in the literature,11, 12 and there is still disagreement on the subject of aluminum as an allowable alloying addition 13, 14…”

“…Research spanning the last decade has focused on iron18–20 and magnesium8, 21 and their alloys as bioabsorbable stent materials. Over the past two years, the authors have assessed the in vivo corrosion behavior of iron and magnesium using a specialized, rat‐based method of evaluation 13, 22. These analyses, in combination with characterization of samples corroded in vitro,23 have proven that iron is an unsuitable material for coronary stent application.…”

Rates ofin vivo(arterial) andin vitrobiocorrosion for pure magnesium

“…In previous work by the authors, it was noted that a large volume of flowing media appeared to modify only the kinetics of degradation, while the degradation mechanism remained unaffected. 9 Similarly, inclusion of an extracellular coating (fibrin) modulated the degradation rate of magnesium, presumably by modulation of mass transport adjacent to the wire surface. However, the fibrin coating did not result in discernable differences in corrosion progression.…”

“…The physiological corrosion of magnesium and its alloys has been a key area of study in biomedical materials engineering in the last decade, with one long-term objective being widespread use of bioabsorbable endovascular (i.e., coronary) stents. [1][2][3][4] Investigative methods for the corrosion of bioabsorbable metallic devices are wide-ranging, using environments ranging from pseudophysiological salt solutions, 5,6 to simulated body fluids 7,8 or cell culture media, 9,10 to murine [11][12][13] and porcine [14][15][16][17][18] animal models. The wide array of corrosion environments and techniques used have resulted in variability in corrosion rates and mechanisms.…”

Editorial

“…The fi rst article in this issue of Emerging Materials Research focuses on new approaches in evaluating metallic candidates for bioabsorbable stents. 3 This article, by Patrick K. Bowen, Jaroslaw Drelich, Robert E. Buxbaum, Rupak M. Rajachar and Jeremy Goldman of Michigan Technological University (MTU), focuses on the series of unconventional approaches that have been developed (at MTU), which are able to screen candidate materials for use in bioabsorbable (or bioresorbable) stents by reducing the scale of necessary animal studies and the complexity of biocorrosion analyzes. Using a novel in vivo approach, materials formed into simplifi ed wire geometry were implanted into the wall of the abdominal aorta of rodents for several weeks or months to measure the extent of in vivo degradation, quantify mechanical strength over time, characterize the resulting products and assess biocompatibility.…”