Surface Wettability Is a Key Feature in the Mechano-Bactericidal Activity of Nanopillars

Nanopillar-textured surfaces are of growing interest because of their ability to kill bacteria through physical damage without relying on antimicrobial chemicals. Although research on antibacterial nanopillars has progressed significantly in recent years, the effect of nanopillar hydrophobicity on bactericidal activity remains elusive. In this study, we investigated the mechano-bactericidal efficacy of etched silicon nanopillars against Pseudomonas aeruginosa at nanopillar hydrophobicities from superhydrophilic to superhydrophobic. Assessing cell viability and bacterial morphology in immersed wet conditions, we observed negligible bactericidal activity; however, air/liquid interface displacement during water evaporation established a bactericidal effect that strongly depends on substrate hydrophobicity. Specifically, bactericidal activity was highest on superhydrophilic surfaces but abated with increasing hydrophobicity, diminishing at substrate contact angles larger than 90°. Calculation of the surface tension and Laplace pressure forces during water evaporation for each substrate subsequently highlighted that the total capillary force, as an external driving force responsible for bacterial deformation, is significantly weaker on hydrophobic substrates. These findings suggest that superhydrophilic nanopillared surfaces are a superior choice for mechano-bactericidal activity, whereas superhydrophobic surfaces, although not bactericidal, may have antibiofouling properties through their self-cleaning effect. These findings provide new insights into the design and application of nanopillared surfaces as functional antibacterial materials.