Multidimensional Characterization of Surface Properties and Microbial Biofilm Formation in Commercial Flowable Resin-Based Dental Composites

The interaction between restorative materials and microbial biofilms plays an important role in restoration longevity and the development of secondary caries. This study aimed to perform a multidimensional characterization of four commercially available flowable resin-based dental composites by evaluating their surface roughness, Vickers microhardness, surface morphology, and microbial biofilm formation under standardized in vitro conditions. Filtek™ Bulk Fill Flowable Restorative (3M), Tetric EvoFlow (Ivoclar Vivadent), BRILLIANT Flow (Coltene), and G-ænial™ Universal Injectable (GC) were investigated. Surface roughness was determined by contact profilometry, microhardness was assessed using the Vickers method, and microstructural characteristics were analyzed by scanning electron microscopy (SEM). Microbial biofilm biomass formed by Gram-positive bacteria, Gram-negative bacteria, and Candida albicans, including both reference strains and clinical isolates, was quantified using a crystal violet assay to compare material-dependent microbial colonization. G-ænial™ Universal Injectable exhibited the lowest roughness values, whereas BRILLIANT Flow showed the highest microhardness. SEM analysis revealed a more homogeneous surface morphology for Filtek™ Bulk Fill Flowable Restorative and G-ænial™ Universal Injectable, while Tetric EvoFlow and BRILLIANT Flow displayed increased topographical heterogeneity. Although modest differences in biofilm biomass accumulation were observed among the investigated composites (4.37–12.21% relative biomass reduction compared with the control), no material demonstrated a distinct advantage under the experimental conditions. The integrated evaluation of surface roughness, microhardness, surface morphology, and microbial biofilm formation provides a comparative characterization of commercially available flowable resin-based composites and contributes to a better understanding of material–biofilm interactions. These findings may support the future development of multifunctional restorative biomaterials with improved biofilm-modulating properties.