Role of Phase Stress in Variations of Cell Behavior on NiTi

Phase stresses that arise from volume mismatch between austenite and martensite crystal lattices were studied at different stages of direct and reverse martensitic transformations in NiTi alloy for medical application. The absolute stress values of tensile and compressive stresses that operated in austenite and martensite respectively were shown to be Gaussian functions of phase composition: they increased from zero in the initial single-phase state, achieved maximum at the equilibrium between the initial and new phases and decreased to zero in the ultimate phase. Phase stress distribution on the surface of simulated two-phase sample and related distinctions in osteoblast-like cells’ behavior were analyzed. Negative effect of compressive stress on cell viability and probable mechanism of cell apoptosis are discussed. Recommendations concerning the choice of permissible size discrepancy between organ and implant as well as NiTi alloy composition to avoid/minimize negative consequences of two-phase state after implant installation are suggested.