Nanoengineered Superhydrophobic Surfaces of Aluminum with Extremely Low Bacterial Adhesivity

Bacterial adhesion and biofilm formation on surfaces are troublesome in many industrial processes. Here, nanoporous and nanopillared aluminum surfaces were engineered by anodizing and postetching processes and made hydrophilic (using the inherent oxide layer) or hydrophobic (applying a Teflon coating) with the aim of discouraging bacterial adhesion. Adhesion of Staphylococcus aureus ATCC 12600 (Gram-positive, spherically shaped) and Escherichia coli K-12 (Gram-negative, rod-shaped) was evaluated to the nanoengineered surfaces under both static and flow conditions (fluid shear rate of 37 s^-1). Compared to a nonstructured electropolished flat surface, the nanostructured surfaces significantly reduced the number of adhering colony forming units (CFUs) for both species, as measured using agar plating. For the hydrophilic surfaces, this was attributed to a decreased contact area, reducing bacterial adhesion forces on nanoporous and nanopillared surfaces to 4 and 2 nN, respectively, from 8 nN on flat surfaces. Reductions in the numbers of adhering CFUs were more marked on hydrophobic surfaces under flow, amounting to more than 99.9% and 99.4% for S. aureus and E. coli on nanopillared surfaces, respectively. Scanning electron microscopy revealed a few bacteria found on the hydrophobic nanopillared surfaces adhered predominantly to defective or damaged areas, whereas the intact area preserving the original nanopillared morphology was virtually devoid of adhering bacteria. The greater decrease in bacterial adhesion to hydrophobic nanopillared surfaces than to hydrophilic or nanoporous ones is attributed to effective air entrapment in the three-dimensional pillar morphology, rendering them superhydrophobic and slippery, in addition to providing a minimized contact area for bacteria to adhere to.