Cytotoxicity Assessment and Indentation Size Effect of Low-Cost Experimental Implant Alloys

The development of affordable lightweight alloys with a combination of high mechanical strength and excellent biocompatibility is crucial for next-generation biomedical implants, particularly for middle- and low-income classes. This study investigates the relationship between processing, microscale mechanical behavior, and cytotoxicity of experimental titanium alloys (Ti-3Fe, Ti-4.5Al-1 V-3Fe, and Ti-6Al-1 V-3Fe) and a low-density stainless-steel (LDSS) alloy (Fe-20Mn-7Al-1C-3Cr-3Cu-3Mo), produced via casting and sintering. Their performance was benchmarked against those of commercial Ti-6Al-4 V and 316L stainless steel. Micro- and macro-indentation tests were conducted to evaluate the indentation size effect (ISE), and the Nix-Gao model was applied to quantify strain-gradient plasticity through the estimation of statistically stored dislocation (SSD) and geometrically necessary dislocation (GND) densities. In parallel, in vitro cytotoxicity was evaluated using NIH-3T3 fibroblast cells following the ISO 10993-5 guidelines. The experimental titanium alloys exhibited pronounced ISE behavior and the highest GND densities, indicating enhanced resistance to plastic deformation at the microscale compared to the LDSS alloys. Cytotoxicity results showed excellent biocompatibility for Ti-6Al-4 V and 316L and good compatibility for the experimental Ti-Fe alloys, with cell viability exceeding 70% at 100% extract concentration. In contrast, the LDSS alloys showed the lowest cell viability, with both the as-cast and sintered LDSS having a cell viability of less than 70%, thereby necessitating a detailed corrosion performance evaluation and further optimization. This performance was correlated with high Mn dissolution detected in the extract medium after the cytotoxicity assessment. The results demonstrate that experimental titanium alloys provide a promising pathway toward lightweight biomedical implant materials that combine microscale strengthening with acceptable biocompatibility.