Antimicrobial modifications of biomaterial surfaces have been explored extensively as a means to prevent infection associated with tissue-contacting biomedical devices. As an alternative to traditional antibiotic elution-based delivery methods, which can promote the development of antimicrobial resistance, we have been exploring an approach where polyanionic microgels are deposited on biomaterials surfaces and cationic antimicrobials are sequestered within them for many weeks by complexation. Nevertheless, when challenged by bacteria the antimicrobial can be released and provide for local microbial killing, a process that appears to be driven by the high negative charge and hydrophobicity associated with the bacterial cell envelope . For this so-called self-defensive behavior  to be successful, the complexation strength must be such that the antimicrobial can remain sequestered for extended periods of time in physiological medium that contains various salt species, all of which are able to interfere with the electrostatic complexation. We have previously shown that poly(acrylic acid) (PAA) can sequester colistin, an FDA-approved antibiotic with +5 net charge, in low ionic strength buffer for weeks. Increasing the ionic strength, however, screens the colistin-PAA complexation interaction and enables rapid burst release of the colistin. In contrast, we have identified particular antimicrobial peptides that remain stably sequestered in PAA microgels exposed to buffers with physiological ionic strength for extended periods of time. To better understand the key antimicrobial characteristics that influence complexation, we have been using model peptoids whose size, charge, hydrophobicity, and aromaticity can all be systematically varied. Peptoids are N
-substituted glycines whose side chains are appended to the nitrogen atom rather than to the Î±-carbon of an amino acid. Much of our initial work has concentrated on peptoid TM1 (Mw
= 1819.36 Da, +5 net charge), a helical 12mer which is an effective antimicrobial. We complex it with PAA microgels synthesized by membrane emulsification and UV photopolymerization. We use in situ
optical microscopy to measure changes in the diameter of microgels immobilized on glass substrates as a means to follow peptoid loading, sequestration, and release behavior. We find that the average microgel diameter decreases by a factor of about 40% when exposed to TM1-containing low-ionic strength phosphate buffer. The diameter increases minimally over one week when the ionic strength is increased to 0.14 M, indicating that the peptoid remains sequestered. Using a PetriFilm assay we furthermore show that TM1-loaded microgel-modified glass substrates resist colonization by staphylococcal bacteria. Varying from the basic TM1 motif, we have studied a series of other peptoids, including TM22 which shares the same net charge but replaces aromatic N
-S-phenylethyl moieties found in TM1 with N
-butyl groups. We find stable complexation even when the net charge is lowered provided that aromaticity remains, clearly indicating that Ï-cation, Ï-anion, and/or Ï-Ï interactions are influencing the complexation strength in a manner less sensitive to salt screening than the electrostatic interactions. More generally, these findings can help develop design principles to guide the choice of microgel chemistry and possible complexed antimicrobials available to create self-defensive surfaces able to simultaneously promote healing and inhibit infection.
 J. Liang, H. Wang, and M. Libera, Biomaterial surfaces self-defensive against bacteria by contact transfer of antimicrobials. Biomaterials, 2019. 204(June): p. 25-35.
 X. Xiao, W. Zhao, J. Liang, K. Sauer, and M. Libera, Self-Defensive Antimicrobial Biomaterial Surfaces. Colloids and Surfaces B: Biointerfaces, 2020. Available online 21 April, 2020 (https://doi.org/10.1016/j.colsurfb.2020.110989).