Biocompatibility Aspects of NiTi-Based Medical Implants

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.