When bacteria start building cities, we’re in trouble. The normally free-floating cells can gather in large numbers and secrete a slimy matrix that they live within. These communities are called biofilms, and they grow wherever there is a surface to support them. Hospital catheters are prime real estate, but they’ll settle on everything from plumbing to oil refineries to ship hulls.
Within a biofilm, bacteria are extraordinarily durable. Antibacterial chemicals have a tough time reaching them within their slimy fortress. Even if they do, there’s always a batch of dormant cells that can persist through a chemical onslaught and restart the community. They’re involved in the majority of persistent hospital infections, and it’s easy to see why. You could bleach a biofilm for an hour and still fail to kill it. They’ve survived in pipes that are flushed with toxic chemicals for a week.
Since killing biofilms is a Sisyphean task, some scientists are trying to prevent them from forming at all. They’ve tried textured surfaces, chemical coats, and antibiotic-releasing layers. But Joanna Aizenberg has developed a new solution that goes well beyond what the competitors can do. Inspired by the flesh-eating pitcher plant, she created a material so slippery that biofilms simply cannot form upon it.
The material is called Slippery Liquid-Infused Porous Surfaces or, more aptly, SLIPS. It’s more slippery than any other man-made substance, and I wrote about it last year when Aizenberg first announced it:
It makes a duck’s back look like a sponge. It is “omniphobic” – it repels everything. All manner of liquids, from water to blood to crude oil, roll straight off it. Ice cannot form on it. It even heals itself when damaged.
The SLIPS consists of thousands of stacks, each a thousand times thinner than a human hair. These hold a liquid lubricant in place, and the liquid forms a smooth flat layer at the top of the stacks. That’s what makes the structure so slippery. The same set-up is found in the pitcher plant, on the rim of its pitcher-like leaves. When insects walk upon it, they lose their footing and fall into the pool of digestive fluids below. Inspired by this structure, forged over millions of years of evolution, Aizenberg created a synthetic version. Here’s how it performed:
Drops of water, blood and crude oil sit on the SLIPS as spheres. If the SLIPS are gently angled, the drops roll off, leaving nothing behind. Ice won’t form on the slips either – the second the crystals come together, they slide off. Nor can insects get a grip – an ant, climbing after a dollop of jam, slips off just as it would on the rim of a pitcher plant (with the jam quickly following).
SLIPS are around ten times as slippery as the next best synthetic [materials]. They are smoother, they work under high pressures, and they can be made transparent. They can also heal themselves. When Wong damaged the solid structure, the liquid part simply refills the affected area within less than a second. Best of all, they’re easy to make.
At the time, Aizenberg suggested a wide range of applications, from graffiti-proof walls to frictionless pipelines to ice-resistant windshields. She also suggested that medical devices would be hard to contaminate if they were covered in SLIPS, and her team have now proven that point.
Her student Alexander Epstein and postdoc Tak-Sing Wong showed that Pseudomonas aeruginosa, an opportunistic bacterium that causes many hospital-based infections, finds it hard to grow on SLIPS. Its cells just can’t get any purchase. Faced with a smooth liquid surface, they cannot anchor themselves with their usual toolbox of threads and proteins. They can’t swim through the liquid either. The bacteria have no problem forming biofilms on other slippery surfaces, such as Teflon. On SLIPS, they stay as free-floating, isolated cells. Just tilt the surfaces, and the bacteria slide away, leaving nothing behind.
Epstein and Wong also tested SLIPS in a flow chamber, which mimicked the constant pulse of liquid you’d get in a catheter or a plumbing pipe. Biofilms are known to grow under such conditions, and they happily did so upon a layer of PTFE – the same substance that the SLIPS stacks are made from. But once the PTFE was structured in microscopic stacks and covered in the lubricant, it reduced the growth of three bacteria species (P.aeruginosa, Escherichia coli, and Staphylococcus aureus) by 96 to 99.6 percent after a week.
That’s 35 times more than the next best option: a surface treated with polyethylene glycol (PEG). In one study, a titanium surface coated with PEG could reduce the growth of biofilms by 86 percent after 5 hours. Beyond that, the bacterial cities started rising. By contrast, the SLIPS kept bacteria almost totally at bay for a week. Aizenberg describes this result as “a step change beyond state-of-the-art technology”. No other synthetic surface can do the same, at least not without being incredibly toxic.
Best of all, the team once again demonstrated how stable the SLIPS can be. The solid and liquid parts are chosen so that the liquid doesn’t wash away or evaporate from the stacks over time. It tolerates aggressive flows of liquid. It can withstand a week of submersion in water that’s 10 times saltier than sea water, or in extreme acids or alkalis. It shrugs off as much ultraviolet radiation as you’d get after a full year under in the southwestern US sun.
Aizenberg describes her study as a “proof of concept”. She now wants to understand how exactly the SLIPS are preventing the biofilms from forming. And, as I mentioned last year, the team are trying to tweak the materials so that they are even more durable than they currently are. Still, it’s hard not to get excited about the results as they stand. The SLIPS could provide an easy and effective way of preventing everything from medical devices to ship hulls from becoming urban centres for bacteria.
Reference: Epstein, Wong, Belisle, Boggs & Aizenberg. 2012. Liquid-infused structured surfaces with exceptional anti-biofouling performance. PNAS http://dx.doi.org/10.1073/pnas.1201973109