A Critical Review on Metallic Glasses as Structural Materials for Cardiovascular Stent Applications

Abstract

Functional and mechanical properties of novel biomaterials must be carefully evaluated to guarantee long-term biocompatibility and structural integrity of implantable medical devices. Owing to the combination of metallic bonding and amorphous structure, metallic glasses (MGs) exhibit extraordinary properties superior to conventional crystalline metallic alloys, placing them at the frontier of biomaterials research. MGs have potential to improve corrosion resistance, biocompatibility, strength, and longevity of biomedical implants, and hence are promising materials for cardiovascular stent applications. Nevertheless, while functional properties and biocompatibility of MGs have been widely investigated and validated, a solid understanding of their mechanical performance during different stages in stent applications is still scarce. In this review, we provide a brief, yet comprehensive account on the general aspects of MGs regarding their formation, processing, structure, mechanical, and chemical properties. More specifically, we focus on the additive manufacturing (AM) of MGs, their outstanding high strength and resilience, and their fatigue properties. The interconnection between processing, structure and mechanical behaviour of MGs is highlighted. We further review the main categories of cardiovascular stents, the required mechanical properties of each category, and the conventional materials have been using to address these requirements. Then, we bridge between the mechanical requirements of stents, structural properties of MGs, and the corresponding stent design caveats. In particular, we discuss our recent findings on the feasibility of using MGs in self-expandable stents where our results show that a metallic glass based aortic stent can be crimped without mechanical failure. We further justify the safe deployment of this stent in human descending aorta. It is our intent with this review to inspire biodevice developers toward the realization of MG-based stents.

Nanoglass‐based balloon expandable stents

“…Recently, metallic glasses (MGs) have received considerable attention for biomedical device applications owing to their structural and functional properties, many of them superior to those of conventional metallic alloys (e.g., SS 316L). MGs display high strength, hardness, resilience, corrosion resistance, and biocompatibility . MGs are obtained by rapid quenching from the melts of alloys, which prohibits formation of a crystalline solid as in conventional alloys, and leads to an amorphous (glassy) solid structure .…”

“…A drawback of MGs is their lack of tensile ductility due to severe localization of plastic deformation in narrow bands (so called shear bands) upon their yield. It has been recently shown that by considering certain design criteria, the superior resilience of MGs could be exploited for self‐expandable stent applications which do not need plastic deformation during the stent crimping and deployment process . Nevertheless, the lack of ductility in MGs prohibits them to be used in balloon expandable stent applications where plastic (permanent) deformation is crucial for their deployment process.…”

“…NGs have been synthesized by consolidation of ultrafine MG powders generated by inert gas condensation . The glass–glass interfaces, which are induced in the microstructure of NGs during their synthesis, significantly improve their ductility by preventing the generation and propagation of dominant shear bands, the failure mechanism of MGs at ambient and body temperatures . As a result, both the plasticity and failure limit of MGs are enhanced .…”

“…MGs display high strength, hardness, resilience, 36 corrosion resistance, and biocompatibility. [37][38][39][40] MGs are obtained by rapid quenching from the melts of alloys, which prohibits formation of a crystalline solid as in conventional alloys, and leads to an amorphous (glassy) solid structure. 41 MGs with amorphous structures have higher elastic limits, yield strengths, and resilience than their crystalline counterparts.…”

“…It has been recently shown that by considering certain design criteria, the superior resilience of MGs could be exploited for self-expandable stent applications which do not need plastic deformation during the stent crimping and deployment process. 37,47,48 Nevertheless, the lack of ductility in MGs prohibits them to be used in balloon expandable stent applications where plastic (permanent) deformation is crucial for their deployment process. To overcome the lack of ductility in MGs, a new class of MGs, so called nanoglasses (NGs), has been proposed.…”