Polyelectrolyte multilayers with intrinsic antimicrobial functionality: the importance of mobile polycations.

Cationic contact-killing is an important strategy for creating antimicrobial surfaces that prevent viable bacteria attachment. Recent studies have shown that highly swollen, compliant surfaces resist bacterial attachment and a sufficient density of mobile cationic charge can effectively disrupt bacterial cell membranes. Polyelectrolyte multilayers (PEMs), a popular coating system for surface modification, have been used to kill bacteria through the incorporation of contact-killing or leaching biocides. In this work, we show that manipulation of multilayer assembly and postassembly conditions (e.g., pH) to expose mobile cationic charge can create antimicrobial PEMs without the addition of specific biocidal species. As a model system, we explored PEMs comprising poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (SPS) assembled at high pH and subsequently immersed in low pH solutions. This system undergoes a reversible pH-dependent swelling transition, and we demonstrate that antimicrobial functionality at physiological pH conditions can be turned on and off with suitable pH treatment. In both airborne and waterborne bacteria assays, the viability of two strains of Gram positive Staphylococcus epidermidis (S. epidermidis), one biofilm forming and one nonbiofilm forming, and two strains of Gram negative Escherichia coli (E. coli) was effectively reduced on SPS/PAH multilayers displaying accessible cationic charge. To generalize our results, the pH assembly conditions of PEMs comprising poly(acrylic acid) (PAA) and PAH were also modified to introduce antibacterial capabilities.