Surface modifications of Nitinol for biomedical applications

Cathodic electrophoretic deposition (EPD) has been utilized for the fabrication of composite films for the surface modification of NiTi shape memory alloys (Nitinol). In the proposed method, chitosan (CH) was used as a matrix for the incorporation of other functional materials, such as heparin, hydroxyapatite and bioglass. Chitosan–heparin films were deposited from solutions of non-stoichiometric chitosan–heparin complexes. It was found that the addition of anionic heparin to the solutions of cationic chitosan resulted in a significant increase in the cathodic deposition rate. The thickness of the films prepared by this method varied in the range of 0.1–3 μm. The ability of the chitosan–heparin films to bind antithrombin, as measured by binding of 125I-radiolabeled antithrombin, was much greater than that of pure chitosan films. Composite chitosan–hydroxyapatite films, with thickness of 1–30 μm, were obtained as monolayers or laminates, containing chitosan–hydroxyapatite layers, separated by layers of pure chitosan. The hydroxyapatite nanoparticles showed preferred orientation in the chitosan matrix with the c-axis parallel to the substrate surface. The films showed corrosion protection of the Nitinol substrates in Ringer's physiological solutions. The feasibility of the fabrication of composite films containing hydroxyapatite and bioglass in the chitosan matrix has been demonstrated. The method offers the advantages of room temperature processing. The deposition mechanisms and possible applications of the films are discussed.