A variety of purposes require prolonged antimicrobial activity or repulsion of microorganisms for which conventional antibacterial administration is less likely to be effective. The use of materials to deliver antibiotics rather than conventional drug delivery methods requires more invasive methods but can provide localized, prolonged, and stimulus-dependent antibacterial activity. Various forms such as hydrogels, films, coatings, scaffolds, implants, and nano-based forms such as nanoparticles are being used to design antibacterial strategies. Medical devices such as catheters and intravenous lines can be sources of microbial infection which can potentially be prevented with antimicrobial materials; in addition, the formation of biofilms can impede their functions, making antimicrobial or antibiofilm materials necessary for their continued function. Similarly, implants for bone may be necessary to induce bone regeneration but can also act as sources of infection which impedes their effectiveness. Surfaces that are touched by many people can act as vectors of infection; antimicrobial coatings on such surfaces can reduce the transmission of microbes. Antimicrobial films can be useful in preventing food spoilage and reducing food waste and food-borne illness. Fabrics with antimicrobial coatings can reduce disease spread and the energy costs and need for cleaning. The forms of materials are important for their activities. Some commonly used examples are: Hydrogels: They are moldable and injectable materials, and their low density and degradability make them useful for drug delivery and wound healing. Their solvent accessibility also makes them effective at stimuli-sensitive materials. As with many of the materials noted, hydrogels are not inherently antibacterial and require antibiotics or other antimicrobial components to exert antibacterial activity. The surface area of hydrogels can allow them to act in place while being exposed to cells or bodily fluids, which allows either diffusible antibiotics or gel-bound antibiotic agents such as antimicrobial peptides to be used. The hydrogel material can also protect the antibiotic agents against degradation, allowing them to be more effective at the same dosage or to be equally effective at a lower dosage. Hydrogels are impermanent, and their low densities and solvent accessibility make them useful as stimulus-responsive materials. Antimicrobial hydrogels may respond to acidity, either reversibly (through conformational shifts) or irreversibly (by chemical reactions such as hydrazone cleavage). They can also be degraded by enzymes such as hyaluronidase which are specific to pathogenic bacteria or by toxins secreted by bacteria, enabling selective antimicrobial activity.