The present paper reviews aspects related to the biocompatibility of NiTi shape memory alloys used for medical applications. These smart metallic materials, which are characterised by outstanding mechanical properties, have been gaining increasing importance over the last two decades in many minimal invasive surgery and diagnostic applications, as well as for other uses, such as in orthodontic appliances. Due to the presence of high amounts of Ni, the cytotoxicity of such alloys is under scrutiny. In this review paper we analyse work published on the biocompatibility of NiTi alloys, considering aspects related to: (1) corrosion properties and the different methods used to test them, as well as specimen surface states; (2) biocompatibility tests in vitro and in vivo; (3) the release of Ni ions. It is shown that NiTi shape memory alloys are generally characterised by good corrosion properties, in most cases superior to those of conventional stainless steel or Co-Cr-Mo-based biomedical materials. The majority of biocompatibility studies suggest that these alloys have low cytotoxicity (both in vitro and in vivo) as well as low genotoxicity. The release of Ni ions depends on the surface state and the surface chemistry. Smooth surfaces with well-controlled structures and chemistries of the outermost protective TiO2 layer lead to negligible release of Ni ions, with concentrations below the normal human daily intake.

BiFeO 3 thin films were fabricated on ͑111͒Pt/Ti/SiO 2 /Si substrates via Bi-acetateand Fe-acetylacetonate-based chemical solution deposition and spin-coating techniques. The processing parameters were optimized in order to obtain films with high resistivity. The optical properties ͑refractive indices and extinction coefficients͒ were measured by means of ellipsometry ͑HeNe laser, = 632.8 Å͒. Microstructure characterization was made by means of atomic force microscopy, grazing incidence x-ray diffractometry ͑XRD͒, and texture analysis. Additionally, powders prepared from a stoichiometric precursor were investigated by means of thermogravimetric and differential thermal analyses and XRD. It is demonstrated that the formation of perovskite-type BiFeO 3 is accompanied by the appearance of bismuth oxide at low temperatures which then transforms into Bi 36 Fe 2 O 57. For the films it was found that annealing in oxygen leads to higher indices of refraction, lower roughness, and smaller grain size. Complete crystallization of the films was achieved at a substantially lower temperature compared to that of the powders. A ͑100͒ ͑pseudocubic͒ out-of-plane preferred orientation was revealed for specimens annealed in air and oxygen. It is supposed that the crystal lattice of the thin film is close to cubic possibly due to stress development at the substrate/film interface. The electrical properties of the films were measured at room temperature by impedance analysis. The piezoelectric properties were determined using a laser vibrometer. Room temperature resistances measured at 1 kHz for metal-film-metal configurations for the specimens annealed in air and O 2 were 14 ⍀ and 1.35 k⍀, respectively. This is explained in terms of the high sensitivity of the oxidation state ͑ϩ2 or ϩ3͒ of iron ions to oxygen stoichiometry in the specimens. Further electrical characterization of the specimen annealed in O 2 revealed very low frequency dispersion of the dielectric constant. A dielectric loss of 1% or less was detected in a wide range of frequency. The films annealed in oxygen showed piezoelectric activity with a value of the piezoelectric coefficient d 33 of 12 pm/V. A relatively weak ferroelectricity ͑remnant polarization 2P r of approximately 1 C/cm 2 ͒ was detected for the specimens annealed in oxygen.

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