ReviewCritical overview of Nitinol surfaces and their modifications for medical applications
Introduction
Surface modification and coating of Nitinol (an acronym for NiTi Naval Ordnance Laboratory), a family of nearly equiatomic NiTi alloys with shape memory and superelastic properties, is a subject of numerous recent studies directed at improving the material’s corrosion resistance as well as its biocompatibility through elimination of Ni from the surface. This chemical element is known to be allergenic and toxic, though essential for the human body. Although it has been shown that the amount of Ni recovered in biological studies in vitro may be either very low from the beginning or drop to undetectable levels after a brief exposure to biological environments [1], [2], ‘the nickel case’ keeps reappearing. Thus, the recent results obtained on commercial ready-to-use orthodontic wires showed that the Ni release varied in a wide range from 0.2 to 7 μg cm−2 [3]. Moreover, it has been reported that the Ni release can actually significantly increase with time [4], [5], [6], [7], maintaining a high level up to 8 weeks and even for a few months [6], [7], indicating the need for better understanding of the material/surface interface. Based on the number of published papers on Nitinol surfaces, especially recently, one might conclude that this issue indeed deserves serious attention. Various techniques and protocols have been used for surface treatments; among them mechanical and electrochemical treatments, chemical etching, heat treatments, conventional and plasma ion immersion implantation, laser and electron-beam irradiation, design of bioactive surfaces, and a proper technique can easily be lost in that jungle of publications. Some of the procedures that were developed originally for pure Ti and their application to NiTi not only may not bring any improvement but, rather, can cause surface damage because of inevitable Ni involvement. Mechanisms of oxide formation on NiTi surfaces vary, depending on media, temperature and irradiation. The interpretations of the Ti and Ni elemental depth profiles in Nitinol differ because of the possibility of preferential sputtering of one of the alloy components that depends on the angles and energies of sputtering ion beams [8]. The absence of a precise description of atomic diffusion on the interfaces makes this situation even more complex.
An interesting development is that X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy have become routine methods, and most of the recently published studies on Nitinol have been done with an impressive list of techniques, including even energy dispersive X-ray spectroscopy (EDS) [3], [9], [10], [11], unsuitable for surface analysis. Despite a great number of new publications, some of which include detailed XPS surface studies, their analyses are very difficult especially when relevant to biological responses. Thus, instead of providing absolute values of elemental concentrations for Ni and Ti, various relative values are presented that do not give objective information on Nitinol surface chemistry. The important experimental details such as the Ar ion sputtering rates, the electron escape angles associated with a certain surface depth in XPS studies, the scanning rates in cyclic potential polarization, etc. are quite often missing. Because the selected scanning rates in potentiodynamic (PD) cyclic potential polarization are sometimes as much as 3–30 times higher [12], [13], [14] than those recommended by the American Society for Testing and Materials (ASTM) for testing medical devices [15], the results obtained might easily be overrated, especially with regard to localized corrosion resistance. Very little or no attention has been paid thus far to the selection of the original NiTi surfaces subjected to modification. As a result, the surfaces developed either did not perform in the way in which they were expected to, or their performance was overestimated because it was compared with the poorest bare Nitinol surfaces. Sometimes surface modification techniques are combined with procedures employed for the design of optimal shape memory and superelasticity. The consequence is that not only is the surface composition modified, but so also is the bulk of Nitinol; as a result, important parameters for medical applications such as shape recovery temperatures and mechanical properties are altered.
All the studies of surface modifications of Nitinol have been aimed at improving its corrosion behavior. It has been shown that localized corrosion resistance of bare Nitinol may vary significantly, depending on its surface state [7], [16], [17], [18]. In PD and potentiostatic (PS) potential polarization, Nitinol surfaces prepared appropriately do not break or pit up to 800 mV and even 1300 mV applied potentials. However, in scratch corrosion tests when surface damage is caused mechanically, the repassivation ability of Nitinol happens to be inferior to that of pure Ti, though comparable with the scratch healing ability of stainless steel. The pitting potentials of NiTi determined in scratch tests are low (from 150 to 300 mV) compared with PD and PS polarization, and this is the problem to be targeted in the development of Nitinol surface modifications.
The present review analyzes the current situation with Nitinol surface modifications and its progress during the past 7 years. Earlier publications on this subject have been covered in previous reviews [18], [19], and more detailed information on Nitinol biocompatibility is presented elsewhere [20]. The advantages and disadvantages of the surfaces developed and the limitations of the studies conducted are discussed, and the surface performances in Ni release, corrosion and biocompatibility are compared. For the reasons mentioned above, when analyzing the chemistry of bare Nitinol surfaces, the authors will often refer to their own papers on XPS studies, which were conducted at surface-sensitive angles and provided the actual elemental concentrations pertinent to biological responses.
Section snippets
Mechanically modified surfaces and biological responses
Traditional surface treatments for biomaterials include mechanical polishing (Mp), electropolishing (Ep), chemical etching (Ce) in acid solutions, heat treatments (Ht) under various conditions, sandblasting and short pinning. Because of its proprietary nature, information on Nitinol surface treatments typically is not disclosed. It will be demonstrated in the following sections that, in the absence of standard surface treatment protocols for Nitinol, mechanically ground or ‘polished-to-luster’
Conventional ion implantation and electron beam
Conventional ion implantation is a line-of-sight process in which ions are extracted from plasma, accelerated, and bombarded into a device. In the case of a non-planar device, manipulation is required to implant all its sides uniformly. This adds complexity, which is exacerbated by the need to provide adequate heat sinks to limit the rise in temperature during implantation [68].
A systematic study of the effects of the implantation of ions of oxygen (O), carbon (C), copper (Cu), zinc (Zn),
Sol–gel and hydrogen peroxide surface treatments
Attempts were made to form Ti-rich layers on NiTi through deposition of Ti from various solutions or through the chemical treatments developed originally for pure Ti [87], [88], [89], [90], [91], [92]. Thus, the sol–gel-derived NiTi surfaces explored in two studies [88], [92] exhibited a satisfactory corrosion behavior and were depleted of Ni. However, their performance was not better than that observed with bare Nitinol surfaces prepared appropriately. The examination of elemental depth
Nitinol surface under strain
Another important issue relevant to the design of Nitinol surfaces is the compatibility of new surfaces with the superelasticity of the material. The strains that the Nitinol superelastic implant devices are subjected to in the body may easily reach 3–4% during a cyclic performance within a superelastic plateau, and they can exceed the superelastic limit ⩾8% inside catheters during insertion [18]. As indicated in previous sections, bare Nitinol surfaces retained their integrity and corrosion
Conclusions and outlook
From the analysis presented in this review, it is clear that, using the developed approaches, Nitinol surface can be modified into various depths from nanometers to micrometers, and its coating thickness can be extended up to ∼30 μm. It is questionable, however, whether micrometer dimensions are desirable, especially for the thin profile cardiovascular implants devices where stent wall thickness, for instance, is being reduced to 30–50 μm to obtain better compatibility. Another complication to do
Acknowledgments
The Research Fund of K.U. Leuven is acknowledged for financial support. The authors also grateful to L. Meisner, D. Bogdanski, K. Cheung, and C. Heβing, who generously contributed the materials for this review as well as to G.K., who volunteered to edit this multidisciplinary ‘project’. We also acknowledge the useful discussions with G. Rondelli and his contribution to this review, as well as B. Harmon for his constant interest and encouragements. This manuscript was also authored by Iowa State
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