Porous Boron Carbide Matrices: Effects of Surface Polishing on Initial Cytocompatibility in Human Fibroblast Culture—An In Vitro Study

Background/Objectives: Porous boron carbide (B4C) is considered a low-immunogenic candidate material for bone-contacting implants due to the possibility of fabricating constructs with controlled porosity. As an initial screening step, this in vitro study aimed to evaluate the cytocompatibility of porous B4C matrices with different degrees of surface polishing in human fibroblast culture, and to assess how surface treatment modulates cell–material interactions. Methods: Porous B4C samples manufactured using a free sintering method were divided into four groups: unpolished, and polished with 400, 1000, or 3000 grit abrasives. Surface topography was characterized by atomic force microscopy and scanning electron microscopy. Cytocompatibility was assessed using an extract test and by culturing human fibroblasts (FEC 16-2 line) on the samples for 7 days; the number of attached cells and the proportions of viable and dead cells were then evaluated using fluorescence staining with acridine orange and ethidium bromide. Results: In the extract test, no cytotoxic effect was observed for any surface. In the surface experiments, the 400 grit surface showed the most favorable profile, with a significantly higher number of attached cells and a significantly higher proportion of viable cells compared with the 3000 grit surface. Correspondingly, the proportion of dead cells was significantly lower on the 400 grit surface. The proportion of viable cells exceeded 80% in all B4C groups. Quantitative SEM analysis revealed that the 3000 grit surface had a significantly smaller median pore size compared with the unpolished surface and a significantly higher pore density compared with the 400 grit surface. Conclusions: Porous B4C ceramics demonstrated initial cytocompatibility in a fibroblast screening model: no cytotoxic effect in the extract test, and viable cell proportions above 80% across all surfaces. Surface polishing modulated the fibroblast response, with the 400 grit surface showing a significantly higher number of attached cells and a significantly higher proportion of viable cells than the 3000 grit surface. However, the measured AFM roughness parameters did not differ significantly among the polished groups. Instead, quantitative SEM analysis revealed significant differences in pore size and pore density, which may contribute to the observed biological response. Given the small sample size, the use of non-osteogenic cells, and the absence of mechanical characterisation (elastic modulus and compressive strength), these results are exploratory and do not establish the suitability of porous B4C for bone tissue engineering or support any stand-alone load-bearing application. Further investigation using osteogenic cells, surface chemistry analysis, and in vivo models is warranted.