Shape memory alloys, and in particular NiTi alloys, are characterized by two unique behaviors, thermally or mechanically activated: the shape memory effect and pseudo-elastic effect. These behaviors, due to the peculiar crystallographic structure of the alloys, assure the recovery of the original shape even after large deformations and the maintenance of a constant applied force in correspondence of significant displacements. These properties, joined with good corrosion and bending resistance, biological and magnetic resonance compatibility, explain the large diffusion, in the last 20 years, of SMA in the production of biomedical devices, in particular for mini-invasive techniques. In this paper a detailed review of the main applications of NiTi alloys in dental, orthopedics, vascular, neurological, and surgical fields is presented. In particular for each device the main characteristics and the advantages of using SMA are discussed. Moreover, the paper underlines the opportunities and the room for new ideas able to enlarge the range of SMA applications. However, it is fundamental to remember that the complexity of the material and application requires a strict collaboration between clinicians, engineers, physicists and chemists for defining accurately the problem, finding the best solution in terms of device design and accordingly optimizing the NiTi alloy properties. 1. Introduction Nowadays, shape memory alloys (SMA), and in particular nickel-titanium alloys (NiTi), are commonly used in biomedical applications (see among others [1–7]). The main attractive features of this class of materials are the capabilities of: (1) recovering the original shape after large deformations induced by mechanical load (pseudoelasticity) and (2) maintaining a deformed shape up to heat induced recovery of the original shape (shape memory effect). The explanation of these peculiar behaviors can be found in the crystallography and thermodynamics of SMA (see among others [8–11] and references therein). Indeed, SMAs are characterized by two solid phases: the austenitic phase ( ), which is stable at high temperatures ( austenite finish transformation temperature) and with high symmetry, and the martensitic phase, which is stable at low temperatures ( martensite finish transformation temperature, with ) and with low symmetry. In particular, the martensite can exist in two configurations: (i) the stress-free martensite, characterized by a twinned multivariant ( ) crystallographic structure, which minimizes the misfit with the surroundings (austenitic phase), hence not associated
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