INVESTIGATION OF BIODEGRADABLE IRON-MANGANESE ALLOYS WITH VARIOUS POROSITY
Bioresorbable iron-manganese (Fe-Mn) alloys are considered as a new class of biomaterials for the development of orthopedic fixation devices due to their promising mechanical properties, comparable to the human cortical bone, and the ability to degrade in the physiological environment and release small quantities of metallic ions/particles that are absorbable by the host. The greatest challenge for developing an ideal resorbable Fe-Mn alloy is to increase the degradation rate of the alloy without compromising the alloy biocompatibility, that is, causing zero or minimal local and systemic toxicity to the tissue. Another challenge is to improve osteo-integration through inducing a cascade of events leading to tissue ingrowth.
The incorporation of porosity into the Fe-Mn alloys aimed to increase the corrosion rate and to provide the three-dimensional structure for cellular activity and nutrient transport. The Fe-30wt.%Mn alloys with 0-, 5-, 10-, and 60-volume percent porosity were produced through the space holder technique in powder metallurgy. The space-holder material, ammonium bicarbonate (NH4HCO3), was sieved to a particle size ranging 355~500 µm. The microstructures and mechanical properties of the alloys, as well as the influence of the degree of porosity on the alloy corrosion rates comparing to the concentrations of the degraded metal ions were investigated. Although the Fe-30Mn alloys containing 60-vol% porosity exhibited the lowest average ultimate compressive strength of 381 MPa among the tested groups, they were still mechanically stronger than a typical human wet compact bone. Furthermore, the alloys had the highest average corrosion rate of 0.98 ± 0.20 mm/year, compared to 0.13 ± 0.07 mm/year for the non-porous Fe-30Mn alloys. Nevertheless, the extract from the 60%-pore group had a cytotoxicity effect to the bone marrow stem cells (BMSCs) at an average normalized cell viability of 58%, which was below the standard viability of 70%, considered as cytotoxic in the indirect cytotoxicity study. The cytotoxicity study also corresponded to the highest level of transition metal ions Mn2+ released into the media for the 60%-pore group at an average ion released rate of 7 mg/day, compared to the other groups presenting similar Mn2+ released rates about 4 mg/day after 1 day of incubation. The extreme case of the 60%-pore group demonstrated the tradeoff between the corrosion rates and biocompatibility. On the other hand, the 10%-pore group showed an average ultimate compressive strength of 737 MPa comparable to the stainless steel 316L, an average corrosion rate of 0.260 ± 0.09 mm/year, which was 2-fold higher than the non-porous group, and an average cell viability of 86% close to the non-porous group. It is promising based on the above results, however, the osteo-integration of the 10%-pore group in terms of cell-to-cell and alloy-to-cell interactions was not ideal.