Coating medical supplies with an antimicrobial material is one approach that bioengineers are using to combat the increasing spread of multidrug-resistant bacteria. Multidrug-resistant Staphylococcus aureus (MRSA) and related pathogens, for example, can lengthen hospital stay and even cause death. A research team at the A*STAR Institute of Bioengineering and Nanotechnology in Singapore has now developed a highly effective antimicrobial coating based on polymers. The coating can be applied to medical equipment, such as catheters, explains Yi-Yan Yang, who led the research. The coating was inspired by a well-known family of antimicrobial materials called cationic polymers. On contact, these materials kill microbes by attaching to, infiltrating and ultimately rupturing their cell walls. When these polymers are modified to form a coating, however, their antimicrobial activity is usually compromised. They also tend to accumulate a layer of dead microorganisms on their surface. “This can trigger an immune response and inflammation in the patient, and may also block the antimicrobial function of the coating,” Yang explains. To overcome these limitations, the team developed their polymer-based hydrogel coating to have antifouling as well as potent antimicrobial properties. They made the coating by combining a ‘block’ of poly(ethylene glycol) (PEG)—which is known for its fouling resistance—with a polycarbonate. They then made the polycarbonate block functional by adding two components: cationic groups to capture passing pathogens; and water-repellent hydrophobic units to puncture their lipid-rich cell membranes and kill the cell. The team showed that their gel coating was highly effective at killing a range of multidrug-resistant bacteria and fungi and preventing pathogens from growing on surfaces. A simple rinse with a buffer solution was sufficient to remove the dead cells, confirming the coating’s antifouling capabilities. The team also confirmed that the coating is harmless to red blood cells and does not irritate the skin. Furthermore, the researchers showed that the hydrogel could be added to the surface of a standard hospital catheter, preventing microbial growth. As the coating can be formed under mild, physiological conditions, the hydrogels can also be used as a wound dressing, Yang notes. “For example, hydrogel dressings could form after spraying the gel precursor solution onto wounds,” she says. The research team’s next step will be to investigate wound healing using these gels in animal studies. “At the same time, we will also seek industry partners to help commercialize these hydrogels, especially for medical device coating applications,” she says.

Michael Kessler, associate professor of materials science and engineering and Biopolymers & Biocomposites Research Team member, has teamed up with Byron Brehm-Stecher, associate professor of food science and human nutrition, and Richard Larock, distinguished professor emeritus of chemistry, to develop soybean oil-based coatings that have antibacterial properties. Although there are antibacterial coatings that are commercially available, they are not biobased and they are fabricated by adding antimicrobial chemicals into the coatings. The research team used soybean oil from the local grocery store to create polymers that are biorenewable and prepared in an environmentally friendly manner without the use of volatile organic compounds. Then they incorporated cationic charges into the polymer structure to make the coatings antibacterial. Cationic compounds have the ability to bind to bacteria and other microbes and disrupt their structure, leading to cell injury and/or death. "Cationic molecules, such as antimicrobial peptides, are widespread throughout nature, protecting all sorts of animals against bacterial infections. So these coatings are a great example of art imitating life," Brehm-Stecher said. After Kessler's team produced the coatings, Brehm-Stecher's lab worked to test the coatings' antibacterial performance and to characterize their activities against the bacterial pathogens Listeria monocytogenes and Salmonella Typhimurium. "Together, L. monocytogenes andSalmonella spp. are responsible for 47% of foodborne disease-related deaths traced to known agents in the United States," Brehm-Stecher said, explaining the choice of these pathogens. They found that the coatings exhibited strong antibacterial properties against these pathogens. These coatings could have applications in many fields where the killing of pathogens or prevention of surface colonization is essential. "These coatings could potentially be used to create an active surface that kills microbes on contact," said Brehm-Stecher. "Control of microbes in a food processing plant or in a hospital operating room is easier if they can be killed before they have a chance to grow and establish themselves in the environment." The same benefits might be realized for food packaging, but further work is needed to examine whether any components of the coatings might be able to migrate into the food, which would be undesirable. The researchers will continue to investigate the performance of these and other coatings against multi-drug-resistant bacteria, fungi and viruses. They will also begin to study how long the antibacterial property will last.