Carnivorous Plant Inspires Bacteria-Proof Liquid Coating
Caption: Scanning electron micrograph of a biofilm. Scientists recently grew a biofilm where they could tweak bacterial populations to any proportion.
Slippery liquid films can keep germs from forming life-threatening coatings on medical devices, according to researchers who say their idea was inspired by carnivorous plants.
Bacteria naturally collect into fortresses known as biofilms. These slimes are the cause of persistent infections; they afflict about 10 percent of all hospital patients in the United States and kill nearly 100,000 people annually by releasing harmful toxins or clogging vital catheters, according to the researchers.
"By creating a liquid-infused structured surface, we deprive bacteria of the static interface they need to get a grip and grow together into biofilms," Alexander Epstein, a member of the Harvard University team that conducted the research, said in a statement.
The problems created by biofilms aren't just medical. They also coat ship hulls ?giving larger forms of life a way to latch onto the hulls and increasing the amount of fuel seafaring vessels burn by up to 40 percent ?and contaminate everything from oil refineries to air-conditioning networks.
As much as one might want to remove biofilms, they can prove extraordinarily resilient. For instance, biofilms can survive in bottles of iodine for up to 15 months, and an hour's exposure to chlorine bleach still leaves live cells.
Instead of attempting to treat biofilms after they have infested a surface, researchers are investigating ways to prevent them from developing in the first place. These strategies typically involve solid coatings that either release antibacterial compounds or are too slippery for bacteria to grab onto. The problem is these coatings generally fail to last. Ones with antibacterial compounds wear off over time or can get masked by microbial secretions, and any defects in a solid slippery coating can give bacteria the foothold they need to grow.
"Bacterial cells are so robust, they are capable of recolonizing pretty much any solid surface we can come up with within hours, no matter how we try to deter them," researcher Joanna Aizenberg, a materials chemist at Harvard University, told InnovationNewsDaily.
Instead of solid coatings, researchers are developing slippery nontoxic liquid coatings that give bacteria no place to anchor onto. These coatings are being called SLIPs, for "slippery liquid-infused porous surfaces."
In multiple-day-long experiments involving three clinically dangerous bacterial species, SLIPs prevented biofilm formation 96 percent to 99.6 percent of the time. The findings were detailed online July 30 in the journal Proceedings of the National Academy of Sciences.
"This is an absolutely revolutionary change," Aizenberg said. "We tested our surfaces for at least a week, and we know they can continue to repel biofilms for much longer times."
The scientists got the idea for their invention by studying the slippery coatings on the Nepenthes pitcher plant, a carnivorous plant found in Indonesia and elsewhere in the tropics.
The researchers tested their coatings by submerging the treated surfaces into liquids, including concentrated brine.
First they covered each surface with a porous solid Teflon membrane. Then they covered the porous membrane with their lubricating fluid, which was chemically attracted to Teflon but would not mix with the liquid the surface was immersed in.
The lubricant fluid was not toxic to the bacteria, but it kept them from getting a firm grip.
"In essence, we turned a once-bacteria-friendly solid surface into a liquid one," researcher Tak-Sing Wong said in a statement. "As a result, biofilms cannot cling to the material, and even if they do form, they easily 'slip' off under mild flow conditions."
"Biofilms have been amazing at outsmarting us, and even when we can attack them, we often make the situation worse with toxins or chemicals," Aizenberg said. "With some very cool, nature-inspired design tricks, we are excited about the possibility that biofilms may have finally met their match."
The liquid surfaces proved stable even in concentrated brine, in both extremely acid and alkaline solutions, and under intense ultraviolet light.
The researchers also flowed bacterial-laden fluids over the treated surfaces at relatively mild rates of about 1 centimeter per second, the kind of flow rates one might find in the body. The surfaces are now being tested at the much faster, turbulent flows that ship hulls might face, "to protect ships against marine biofouling," Aizenberg said.
"SLIPs is a much broader platform than just medical devices," she added. "We have data SLIPs can prevent adhesion of pretty much anything — say, ice or oil."