Polymer Multilayers with pH-Triggered Release of Antibacterial Agents Svetlana Pavlukhina, † Yiming Lu, † Altida Patimetha, ‡ Matthew Libera, ‡ and Svetlana Sukhishvili* ,† Department of Chemistry, Chemical Biology and Biomedical Engineering and Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States Received August 19, 2010; Revised Manuscript Received October 9, 2010 We report on the layer-by-layer design principles of poly(methacrylic acid) (PMAA) ultrathin hydrogel coatings that release antimicrobial agents (AmAs) in response to pH variations. The studied AmAs include gentamicin and an antibacterial cationic peptide L5. Adipic acid dihydrazide (AADH) is a cross-linker which, relative to ethylenediamine (EDA), increases the hydrogel hydrophobicity and introduces centers for hydrogen bonding to AmAs. AmA retention in AADH-cross-linked hydrogels in high-salt solutions was enhanced while AmA release at low pH was suppressed. L5 retains its antibacterial activity toward planktonic Staphylococcus epidermidis after release from PMAA hydrogels in response to pH decreases in the surrounding medium due to bacterial growth. Staphylococcus epidermidis adhesion and colonization was almost completely inhibited by L5 loading of hydrogels. The AmA-releasing and AmA-retaining properties of these hydrogel coatings provide new opportunities to study the fundamental mechanisms of AmA- coating-bacteria interactions and develop a new class of clinically relevant antibacterial coatings for medical devices. Introduction The susceptibility of implantable biomedical devices to bacterial colonization and the resulting infection of surrounding tissue can catastrophically compromise device performance and lead to high treatment costs, significant patient discomfort, and increased morbidity. 1,2 Such biomaterial-associated infection is often linked to bacterial biofilms that consist of bacteria organized into communities with functional heterogeneity enclosed in a self- produced extracellular matrix. 3,4 When in the biofilm state, bacteria become orders of magnitude more resistant to antibiotics than bacteria in the planktonic state. 5-9 In many cases, the only solution is to remove the implanted medical device, resolve the infection, and pursue a revision surgery with a second implant. One promising approach to inhibit biofilm growth is the creation of functional antibacterial surface coatings. 10-13 Poly- mers in antibacterial coatings can either directly work as cationic agents with a bacteria-killing ability, 14 or they can serve as matrices for the delivery of other antibacterial molecules, such as silver ions or antibiotics. 15 There are several approaches to design antibacterial coatings, including the creation of surfaces that resist bacterial adhesion, 16-19 kill bacteria upon contact, 13,20 or leach antibacterial agents. 20,21 Antibacterial coatings that leach drugs can, however, suffer from the fact that the local drug concentration decreases with the elution time, and the coatings lose their antibacterial activity when the concentration of the antimicrobial agent falls below the minimum inhibitory con- centration (MIC). Therefore, nonleaching coatings designed to provide long-term protection against bacterial adhesion and colonization are also being explored. Functionalization of surfaces can involve adsorption of water- soluble polymers at a substrate to form a polymer monolayer, tethering of polymer chains to form brushes, 16,17,22 or the alternating deposition of monolayers of different polymer types to form multilayer films. 23 One attractive feature of the latter approach is the degree of flexibility with which to control the structure and properties of the resulting polyelectrolyte multi- layer (PEM) film. The film thickness, for example, can be controlled by varying the number of deposited layers, and the thickness of the individual self-assembled polymer layers can be controlled through the deposition conditions such as pH and ionic strength. 24,25 Layer-by-layer (LbL) self-assembled films can incorporate a variety of functional biomolecules, including proteins, enzymes, and polysaccharides, often without a sig- nificant decrease in the biomolecule activity. 26 Specifically important for the application of LbL technology to the modifica- tion of implant surfaces is the capability of depositing a conformal coating on a substrate of virtually any shape, as well as the ability to deposit films at any inner surface accessible to solvent, in a non-line-of-sight fashion. In addition, the LbL technique enables careful control of the film composition, stiffness, 27 and compliance, 28 all of which are critical parameters in designing antibacterial coatings. The potential of PEM films as antibacterial coatings has been explored by several groups. For example, antimicrobial agents (silver nitrate or cetrimide) were introduced within PEMs by codissolving with branched poly(ethyleneimine) (BPEI) for alternating deposition with poly(acrylic acid) (PAA) solutions. 29 Amphiphilic block copolymer micelles of poly(ethylene oxide)- block-poly(ε-caprolactone) were also used as a carrier of hydrophobic antibacterial drug triclosan. 30 The micelles were self-assembled within LbL films using the hydrogen bonding between the polyethylene oxide (PEO) micelle corona and PAA. By changing the environment to a physiologically relevant pH, the film was deconstructed to release the micelles, and the deconstruction rate could be controlled by thermal cross-linking of the film. 30 An amphiphilic linear-dendritic triblock copoly- mer of poly(amidoamine)/(poly(propylene glycol) bis(2-ami- nopropyl ether) (PPO) has also been used as a vehicle to enable the burst delivery of triclosan from surfaces. 31 Another strategy to endow LbL films with antimicrobial properties is the * To whom correspondence should be addressed. E-mail: ssukhish@ stevens.edu. † Department of Chemistry, Chemical Biology and Biomedical Engineering. ‡ Department of Chemical Engineering and Materials Science. Biomacromolecules 2010, 11, 3448–3456 3448 10.1021/bm100975w  2010 American Chemical Society Published on Web 10/28/2010