Elsevier

Biomaterials

Volume 24, Issue 21, September 2003, Pages 3689-3696
Biomaterials

Calcium phosphate coating of nickel–titanium shape-memory alloys. Coating procedure and adherence of leukocytes and platelets

https://doi.org/10.1016/S0142-9612(03)00241-2Get rights and content

Abstract

Nickel–titanium shape-memory alloys (NiTi-SMA) were coated with calcium phosphate by dipping in oversaturated calcium phosphate solution. The layer thickness (typically 5–20 μm) can be varied by choice of the immersion time. The porous nature of the layer of microcrystals makes it mechanically stable enough to withstand both the shape-memory transition upon cooling and heating and also strong bending of the material (superelastic effect). This layer may improve the biocompatibility of NiTi-SMA, particulary for osteosynthetic devices by creating a more physiological surface and by restricting a potential nickel release. The adherence of human leukocytes (peripheral blood mononuclear cells and polymorphonuclear neutrophil granulocytes) and platelets to the calcium phosphate layer was analyzed in vitro. In comparison to non-coated NiTi-SMA, leukocytes and platelets showed a significantly increased adhesion to the coated NiTi-SMA.

Introduction

Shape-memory alloys (SMA) on the basis of nickel–titanium alloys (chemical formula NiTi; Nitinol®) possess interesting properties that have stimulated great interest in clinical medicine, namely, the shape-memory effect and the superelasticity (see Refs. [1], [2] for comprehensive treatments of the subject). In clinical use are stents for various applications [3], [4], [5], orthodontic wires [6] and staples for foot surgery (by Barouk et al. [2]). Other applications like osteosynthetic devices [7] and bone substitution materials [8] have also been suggested [2].

These excellent mechanical properties are certainly of great potential use in clinical medicine, but the high nickel content of the material has triggered studies on the biocompatibility of this material. Of course, the problems of acute nickel toxicity [9] and of allergic reaction [10] have to be addressed for all the above devices. It was shown that NiTi readily passivates with a surface layer of TiO2 (like pure titanium) so that the nickel release is confined to the first few days if such materials are immersed in vitro [11], [12], [13]. Many positive results in vitro and in vivo have been reported [2], [3], [7], [8], [11], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24] (see a recent publication where we have summarized the current literature on the biocompatibility of NiTi [25]).

However, some open questions remain about possible release of nickel from nickel–titanium alloys. For instance, cytokine synthesis was induced in vitro in monocytes and microvascular endothelium cells [26] and in peripheral blood mononuclear cells (PBMC) [27], corrosion products of NiTi gave adverse effects on smooth muscle cells [4], a local accumulation of nickel was observed [28], fluoride was found to enhance the corrosion rate of NiTi [29], and the surface finish appears to play a significant role for nickel ion release [30]. By in vitro cell culture experiments using functionally graded objects with a composition from Ni over NiTi to Ti, we could show that a nickel content up to a 50:50 molar ratio of nickel to titanium does not adversely affect the cellular adherence, proliferation, and morphology of osteoblast-like osteosarcoma cells (SAOS-2, MG-63), primary human osteoblasts (HOB), and murine fibroblasts (3T3), in contrast to NiTi alloys containing more than 50 mol% nickel [25].

In general, from reviewing the literature it can be concluded that nickel–titanium alloys are useful for many medical devices but, as they contain so much nickel, there remain reservations about the long-term performance of implants. To extend the potential for osteosynthetic devices made of nickel–titanium shape-memory alloys (NiTi-SMA), we have coated the material with calcium phosphate

  • (a)

    to further improve the biocompatibility in bone contact, following the strategies developed for femoral and dental endoprostheses, and

  • (b)

    to further suppress the release of nickel that occurs initially within the first days and possibly after mechanical load application at the implantation site.

Because the classical plasma-spray technique involves a thermal stress that may influence the microstructural characteristics responsible for the shape-memory effect, we used the dip-coating method introduced by Kokubo, van Blitterswijk, de Groot and others [31], [32], [33], [34]. In a comparative study by Wheeler et al., plasma-spraying and dip-coating methods gave comparable in vivo results upon implantation of titanium in rabbits [35].

Section snippets

Coating procedure

For comparison, three kinds of materials were coated by the same procedure: pure titanium (Goodfellow), Ti6Al4V (Goodfellow) and superelastic NiTi (Memory Metalle; “medical grade”; characterization by differential scanning calorimetry: Ap=6–37°C; Mp=6–29°C, depending on the batch). Sheets of these materials were cut into rectangular pieces (typical dimensions: 10×10×0.3 mm3) and cleaned with acetone (10 min), ethanol (10 min) and distilled water (rinsing). The plates were then boiled for 60 min in

Results and discussion

The surface of a coated NiTi plate is shown in Fig. 1. The platelet-like morphology of the crystals is typical for solution-deposited calcium phosphate films [37]. By IR spectroscopy and X-ray powder diffraction, we found that the layer consists mainly of octacalcium phosphate with some content of hydroxyapatite. A side view reveals the porous nature of the film with interdigitated platelets (Fig. 1b). A second immersion after drying results in additional crystal growth within the first layer

Conclusions

A layer of calcium phosphate was precipitated on NiTi SMAs. The layer shows a sufficient mechanical stability to withstand the shape-memory effect and superelastic bending. Even if the topmost layer is removed, there remains a thin surface layer that induces calcium phosphate crystallization (a self-healing process) in contact with oversaturated calcium phosphate solutions (similar to blood or saliva). The shape-memory properties of the NiTi are not altered by the coating procedure. The gentle

Acknowledgments

We are grateful to the Deutsche Forschungsgemeinschaft (SFB 459: Shape-memory Technique) for generous funding of our work. M.E. thanks the Fonds der Chemischen Industrie for financial support. This work is a part of D.B.'s Ph.D. thesis.

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