Nickel-titanium is a functional alloy currently used in various clinical applications, especially in vascular stents. There is an increased interest in the orthopaedic use of NiTi-based implants. The alloy enables the manufacture of applications of constant load, controllable motion, and minimal invasiveness. NiTi is considered biocompatible and it possesses mechanical properties that make it an especially good candidate for bone tissue surroundings. In our studies, we have investigated the effects of surface properties of NiTi on its biocompatibility. The martensitic phase was shown to have lower biocompatibility of material in comparison with austenitic NiTi. Cellular cytotoxicity increased and cell adhesion diminished on martensite phase. This was observed with both osteoblasts and osteoclasts. Our studies showed that the thickness of the oxide layer does not necessarily enhance the biocompatibility. The surface state of NiTi is strongly affected by thermal oxidation. Surface properties affect the initial adsorption of proteins and other macromolecules onto the biomaterial surface; this in turn impacts the following cellular responses, such as proliferation and differentiation, which are dictated by adhesion to the extracellular matrix components. Since adhesive force is connected to the interaction with the adsorbed molecules, it might be an important factor in the biocompatibility. Sol-gel derived titania-silica surface treatment was observed to increase the bone-implant coating of functional intramedullary NiTi nails. Sol-gel treatment together with the bending force increased the fixation of the implant (osseointegration). These studies indicate that the surface properties of NiTi are important for its biocompatibility.
The results of experimental studies performed during the past decade at Saint-Petersburg State University in collaboration with the Mechanics and Materials Science Research Centre at Ningbo University (China) and Lobachevsky State University of Nizhny Novgorod (Russia) with the aim of investigating the basic regularities of the high rate straining of NiTi shape memory alloys are reviewed. The studies were concerned with the mechanical behaviour of these materials at high rate compression and tension, and the effect of high rate straining on the basic functional properties (shape memory effect and two-way shape memory). Special attention was given to the application of dynamic fracture theory to NiTi shock loading and to methods for obtaining experimental findings concerning the theoretical parameters involved in the criteria for determining the transition of these materials from an elastic to an inelastic state while high rate straining was applied. The effect of the quasi-equilibrium structure of NiTi on martensitic transformations and the role of this structure in the formation of more complicated effects than shape memory and superelasticity were studied. The results obtained are used to elaborate a method for the improvement of the functional properties of NiTi and a procedure for reversing two-way shape memory induction.
The effect of metastable austenite straining on the basic functional properties of an NiTi alloy in quasi-equilibrium state was studied. Straining of austenite in a pre-martensitic state was shown to result in improvement of one-way and two-way shape memory effects (OWSME and TWSME). This improvement took place after straining in a narrow temperature range, and its manifestation at quasi-static and high-rate tensile tests was different: the increment of both characteristics was sufficiently large after quasi-static straining, but smaller after high-rate straining. Straining at temperatures that deviated significantly from the Ms temperature led to deterioration of OWSME and TWSME.
