Effect of Heat Treatment on In Vitro Cytotoxicity of Ti-Nb-Zr Gum Metal Alloy

Strain-induced deformations and phase evolutions are two hidden factors that may influence cytocompatibility of Gum Metal alloys during processing for relevant implant applications. In the present research, changes in cell viability of a new Gum Metal Ti-Nb-Zr alloy in its cold-rolled state and after heat treatments (at 700, 850, and 900 °C) were investigated by a comprehensive study of microstructural phases and their role in deformation mechanisms as well as mechanical properties. In its cold-rolled state, the alloy showed a lamellar microstructure along with stress-induced <i>α</i>″ martensite and <i>ω</i> phases, as confirmed by optical microscopy (OM) and X-ray diffractometry (XRD) analysis. The instability in the <i>β</i> phase led to a strain-induced martensitic (SIM) transformation from <i>β</i> to <i>α</i>'/<i>α</i>″ phases, causing lower viability of MG-63 cells compared with commercially pure titanium. MG-63 cell viability was significantly higher (<i>p</i> < 0.0001) in the alloy heat-treated at 900 °C compared with those heat-treated at 700 and 850 °C. This can be directly attributed to the increased portion of the stable and dominant <i>β</i> phase. The stabilized <i>β</i> phase greatly improved the alloy's cellular response by reducing harmful phase interactions and maintaining mechanical compatibility with bone (admissible strain of 1.3%). Importantly, heat treatment at high temperatures (between 850 and 900 °C) effectively converted the stress-induced <i>α</i>″ and <i>ω</i> phases back into a stable <i>β</i> phase matrix as the dominant phase.