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Polymer coating lets liquids go with the flow


New advances in fields including microfluidics could be possible with a new coating that allows liquids to move across surfaces without fluid loss.

microfluidics
New polymer coatings, developed by Professor Kevin Golovin and his team at the University of Toronto, show the precision with which liquids can move across surfaces (Image: Mohammad Soltani / University of Toronto Engineering)

This new coating – created in the DREAM (Durable Repellent Engineered Advanced Materials) Laboratory, led by University of Toronto Engineering Professor Kevin Golovin – was inspired by the natural world.

“Nature has already developed strategies to transport liquids across surfaces in order to survive,” says Mohammad Soltani, researcher in the DREAM Laboratory and lead author of a new paper recently published in Advanced Functional Materials.

“We were inspired by the structural model of natural materials such as cactus leaves or spider silk. Our new technology can directionally transport not only water droplets, but also low surface tension liquids that easily spread on most surfaces.”

The innovation is claimed to have important implications for microfluidics. By reducing the quantity of sample and reagents required, and automating protocols for working with them, microfluidics can power lab-on-a-chip devices that offer fast, inexpensive medical tests. Proponents hope this could lead to diagnosing multiple conditions in minutes using one or two drops of blood.

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Current microfluidics can only effectively handle liquids with high surface tension, such as water. This property – cohesion – means that the liquid has a greater tendency to stick to itself than to the sides of the channel it is being transported through.

High surface tension liquids form discrete droplets that can be moved around independently. Cohesion can even be exploited to pull the liquid droplets along the channel through a capillary action.

By contrast, low surface tension liquids, such as alcohols and other solvents, tend to stick to the sides of the channels, and can currently be transported for only about 10mm before the droplet disintegrates. Capillary action no longer applies, so this transport requires an external force to move the droplets.

The new coating enables low surface tension liquids to be transported over distances of over 150mm without losing any of the liquid, about 15 times longer than currently possible.

According to the DREAM team, the technology uses two newly developed polymer coatings, one of which is more hydrophobic than the other. Both are composed of liquid-like polymer brushes. The more repellent coating acts as a background, surrounding the less repellent coating and creating tiny channels along the surface. The channels allow for the liquids to move in a desired pattern or direction without losing any of the liquid during transport or requiring additional energy input.

“Polymer brush coatings reduce the fluid friction and allow the droplets to be transported passively,” said Soltani said in a statement. “Less friction means more energy is available to transport the liquid. We then create a driving force by designing the channels with specific patterns.”

The ability to transport low surface tension liquids without loss will allow for advancements in lab-on-a-chip devices. Using these unique coatings, researchers can transport liquids over a longer range, move multiple liquids at the same time along a precise pathway and even merge and split droplets without losing any volume or experiencing cross-contamination.

This technology will also help limit waste in research labs. With no residue left behind on the surface of the device and therefore no risk of cross-contamination, researchers can reuse the same devices.

“We’re looking at using this technology for microfluidics bioassays, as this is an area where every drop of liquid is precious,” says Golovin. “Our findings also have great potential to advance point-of-care diagnostics, such as liver or kidney disease, where biological marker screening is often performed in non-aqueous media.”



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