Biofabricated tissue model for determining biocompatibility of metallic coatings

Metallic biomaterials are extensively used in orthopedics and dentistry, either as implants or coatings. In both cases, metal ions come into contact with surrounding tissues causing a particular cell response. Here, we present a biofabricated in vitro tissue model, consisting of a hydrogel reinforced with a melt electrowritten mesh, to study the effects of bound and released metal ions on surrounding cells embedded in a hydrogel matrix. We evaluate the biocompatibility, bioactivity, and antibacterial properties of these metal coatings. Our approach involves integrating physical vapour deposition coating technology with 3D bioprinting methods. To produce tissue models, melt electrowritten (MEW) meshes composed of polycaprolactone (PCL) were printed and integrated into cell-laden methacrylated galatin (GelMa). The mouse embryonic fibroblast cell line (NIH3T3) was used. GelMa concentration and printing parameters for MEW were adjusted and mechanical analysis of the models was performed to find the optimal material composition. Optimized models were placed on the glass slide surfaces coated with typically non-toxic metals, i.e. titanium (Ti), tantalum (Ta), zirconium (Zr), silver (Ag), tungsten (W), and niobium (Nb). Except for W, all other coatings were stable in a physiological wet environment, as studied by SEM. The viability of the cells at different distances from the coated surface was analyzed. Antibacterial tests against pathogens Staphylococcus aureus and Escherichia coli were used to assess the models' resistance, important for infection control. While Ag coatings showed toxicity, Nb, Ta, Ti, and Zr coatings promoted fibroblast growth, with the highest cell viability after 14 days of culture revealed for Ta and Nb. The strongest antimicrobial effect against E. coli and S. aureus was observed for Ag and W, while Ta exhibited antibacterial activity only against S. aureus. From a broader perspective, our work offers an effective 3D in vitro model for an in-depth characterization of the biocompatibility of metals and metal coatings.