Flexible and Tough Superelastic Co–Cr Alloys for Biomedical Applications

Abstract

present, the three dominant metallic alloys used as biomaterials are Co-Cr alloys, stainless steels, and Ti alloys, and Mg alloys provide a biodegradable option. [1][2][3] There are basically three qualities required of a metallic biomaterial: good biocompatibility, high corrosion resistance, and good wear resistance. Recently, the need for a low Young's modulus similar to that of human bones has been highlighted. A large difference in the Young's modulus of the implant and the bone has been shown to result in the loosening of implants and bone atrophy due to the "stress shielding effect." [2,4] At 190-240 GPa, the Young's modulus of the Co-Cr alloys and stainless steels commonly used at present is much higher than that of the 10-30 GPa of human bones. [2,5] Many kinds of β-type Ti alloys with a body-centered-cubic (BCC) structure and a relatively low Young's modulus have been developed. [1] The Young's modulus of these polycrystalline alloys is typically in the range of 50-80 GPa. [1,2] The Young's modulus of a single-crystal Ti-Nb-Ta-Zr has been reported to be 35 GPa in the <001> direction, [6] and a low apparent Young's modulus of 20 GPa has been shown for some Ti-Nb-based alloys when stress-induced martensitic transformation is involved. [7] While these alloys are appropriate for use as the stems and the acetabular cups of totalThe demand for biomaterials has been increasing along with the increase in the population of elderly people worldwide. The mechanical properties and high wear resistance of metallic biomaterials make them well-suited for use as substitutes or as support for damaged hard tissues. However, unless these biomaterials also have a low Young's modulus similar to that of human bones, bone atrophy inevitably occurs. Because a low Young's modulus is typically associated with poor wear resistance, it is difficult to realize a low Young's modulus and high wear resistance simultaneously. Also, the superelastic property of shape-memory alloys makes them suitable for biomedical applications, like vascular stents and guide wires. However, due to the low recoverable strain of conventional biocompatible shape-memory alloys, the demand for a new alloy system is high. The novel body-centered-cubic cobalt-chromium-based alloys in this work provide a solution to both of these problems. The Young's modulus of <001>-oriented single-crystal cobalt-chromium-based alloys is 10-30 GPa, which is similar to that of human bone, and they also demonstrate high wear and corrosion resistance. They also exhibit superelasticity with a huge recoverable strain up to 17.0%. For these reasons, the novel cobalt-chromium-based alloys can be promising candidates for biomedical applications.

Smart Biomaterials in Biomedical Applications: Current Advances and Possible Future Directions

“…[60] Research on stent design has focused not only on implementing SMAs, but also on studies that indicate that SMPs can be developed to assume the exact utility of CoCr self-expanding cardiovascular implants. [61] Another implementation of SMAs and SMPs is heart valves. Continuing with leaflets, NiTi ones are being developed as scaffolds to provide an extracellular matrix, permitting cell adhesion.…”

“…[ 60 ] Research on stent design has focused not only on implementing SMAs, but also on studies that indicate that SMPs can be developed to assume the exact utility of CoCr self‐expanding cardiovascular implants. [ 61 ]…”

A polymer-like ultrahigh-strength metal alloy

“…Over the past decades, signi cant efforts have been made to seek metal alloys with simultaneously high strength and low modulus [4][5][6][7][8][9][12][13][14][15][16][17] ; but an alloy showing both steel-like high yield strength (σ y >1 GPa) and polymer-like low modulus (E ~10 GPa) still remains unattainable. So far, several alloys based on shape memory alloys (SMA) 5,16,17 are reported to reveal 1 GPa-class high strength and a moderately low modulus E ~30 GPa, and a conventional Mg-Sc strain glass alloy 7 has recently been shown to possess a lower modulus E ~20 GPa but with a lower strength σ y ~0.3 GPa.…”

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Futuristic technologies like morphing aircrafts and superstrong arti cial muscles are hinged on metal alloys being as strong as an ultrahigh-strength steel (with a high yield strength σ y >1 GPa) yet as exible as a polymer (with an ultralow elastic modulus E ~10 GPa) 1-3 . However, achieving such "strong yet exible" alloys has proven challenging [4][5][6][7][8][9] . The di culty lies in an inevitable trade-off between strength and exibility 5,8,10 , which precludes a high-strength alloy from being of polymer-like ultralow modulus.Here we report a Ti-50.8 at.% Ni strain glass alloy showing an unprecedented combination of an ultrahigh yield strength σ y ~1.8 GPa with a polymer-like ultralow elastic modulus E ~10.5 GPa, together with a superlarge rubber-like J-shaped elastic strain of ~8%.

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Crystallization kinetics, microstructure evolution, and mechanical responses of Cr-Co alloys

“…Chromium-Cobalt (Cr-Co) alloys are extensively used in dental, knee, and hip arthroplasty applications because of their unique combination of high strength, wear and corrosion resistance, and excellent fatigue resistance [1]. To prepare Cr-Co alloy specimens for fabricating bioprostheses, additive manufacturing has been considered as a promising and desirable production method to replace traditional techniques including casting and milling [2].…”