http://research.vuse.vanderbilt.edu/srdesign/2009/group8/Papers/2008_Shabalovskaya_Critical%20Overview%20of%20Nitinol.pdf

Critical overview of Nitinol surfaces and their modifications for medical applications.

Shape memory properties provide a very attractive insight into materials science, opening unexplored horizons and giving access to unconventional functions in every material class (metals, polymers, and ceramics). In this regard, the biomedical field, forever in search of materials that display unconventional properties able to satisfy the severe specifications required by their implantation, is now showing great interest in shape memory materials, whose mechanical properties make them extremely attractive for many biomedical applications. However, their biocompatibility, particularly for long-term and permanent applications, has not yet been fully established and is therefore the object of controversy. On the other hand, shape memory polymers (SMPs) show promise, although thus far, their biomedical applications have been limited to the exploration. This paper will first review the most common biomedical applications of shape memory alloys and SMPs and address their critical biocompatibility concerns. Finally, some engineering implications of their use as biomaterials will be examined.

Shape Memory Materials for Biomedical Applications

Metals, in addition to ceramics and polymers, are important class of materials considered for replacement of non-functional parts in the body. Stainless steel 316, titanium and titanium alloys, Co-Cr, and nitinol shape memory alloys are the most frequently used metallic materials. These alloys are prone to corrosion in various extents. This review briefly discusses the important biomaterials, their properties, and the physiological environment to which these materials are exposed. Corrosion performance of currently used metallic materials has been assessed and threat to the biocompatibility from corrosion products/metal ions is discussed. The possible preventive measures to improve corrosion resistance by surface modification and to increase the bioactivity of the metallic surfaces have also been discussed. Importance of the formation of oxide layers on the metal surface, another aspect of corrosion process, has been correlated with the host response. The gap areas and future direction of research are also outlined in the paper.

Corrosion degradation and prevention by surface modification of biometallic materials

Shape memory alloys provide new insights for the design of biomaterials in bioengineering for the design of artificial organs and advanced surgical instruments, since they have specific characteristics and unusual properties. This article will examine (a) the four properties of shape memory alloys, (b) medical applications with high potential for improving the present and future quality of life, and (c) concerns regarding the biocom-patibility properties of nickel-titanium alloys. In particular, the long-term challenges of using shape memory alloys will be discussed, regarding corrosion and potential leakage of elements and ions that could be toxic to cells, tissues and organs.

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Shape memory alloys: Properties and biomedical applications

A porous article of a nickel-titanium alloy, said article having a porosity of 8 to 90 % and said porosity being defined by a network of interconnected passageways extending throughout said member, said network exhibiting a permeability of fluid material effective to permit complete migration of the fluid material throughout said network, and said member being elastically deformable; the porous article is useful as a biomedical implant and in other non-medical applications including the working element of cryogenic surgical instruments

Porous nickel-titanium alloy article

In spite of the generally good biocompatibility of TiNi revealed in numerous in vivo studies, medical applications of these shape memory alloys as implants are hindered due to the lack of knowledge on the nature of their biocompatibility. Better material characterization is necessary for understanding the chemical and physical properties which determine biocompatibility. The purpose of the present investigation is to use angle‐resolved x‐ray photoelectron spectroscopy to identify the differences in surface chemical state and composition between the layers formed on the surface of TiNi alloys after mechanical polishing, chemical etching, autoclaving in water and steam, and exposure to H2O2 or air. The surface chemistry drastically depends on the preparation method; the Ni surface concentration can be varied in the range 0–30 at. %. In natural conditions (air, water, and steam) TiNi surfaces revealed a tendency to be covered by TixNiyO2x oxides (where x varies in the range of 7–20 and y varies in the range ...

S. A. Shabalovskaya

Papers

Surface spectroscopic characterization of TiNi nearly equiatomic shape memory alloys for implants