Researchers at the University of Toronto have come up with a polymer coating that lets low surface tension liquids move over longer distances than currently possible, without losing any of the liquid. While still in the experimental phase, the coating has implications for microfluidics, where small quantities of liquids are transported within tiny channels, often less than a millimetre wide.
By reducing the quantity of sample and reagents required, the advance has potential to power lab-on-a-chip devices that offer faster and cheaper medical tests. It could eventually lead to diagnosing multiple conditions in minutes using only a drop or two of blood, says materials professor Kevin Golovin.
Current microfluidic devices have a key limitation: they can only effectively handle liquids with high surface tension, such as water. This property means that the liquid has a greater tendency to stick to itself than to the sides of the channel it is being transported through.
By contrast, low surface tension liquids, such as alcohols and other solvents, tend to stick to the sides of the channels, and can be transported for only about 10 millimetres before the droplet disintegrates.
“A lot of biomedical testing requires the use of solvents – commonly ethanol – which is a low surface tension liquid,” says Golovin. “For example, when you’re diagnosing liver or kidney disease, the biopsy is dispersed in ethanol.”
So Golovin and his team came up with a new coating that lets low surface tension liquids travel over distances of more than 150 millimetres without losing any of the liquid. This is about 15 times longer than currently possible.
They recently published their findings in Advanced Functional Materials. The coating is composed of two newly developed liquid-like polymers brushes, one of which is more liquid-repellent than the other.
The more repellent coating – a perfluoropolyether – acts as a background, surrounding the less repellent coating – silicone. This creates tiny channels along the surface. The channels allow for the liquids to move in a desired pattern without losing any fluid or requiring additional energy.
“Polymer brush coatings reduce the surface friction and allow the droplets to be transported passively,” says lead author Mohammad Soltani, a mechanical and industrial engineering PhD in Golovin’s lab. “Less friction means energy is not dissipated during liquid transport, which would cause it to get stuck. We then create a driving force by designing the channels with specific patterns.”
The team refined its work by making the channels wedged-shaped, taking cactus spikes as their inspiration. The spikes are thicker at the base than point, which allows liquid to move passively.
Golovin and his team are now working with microbiologist and biomedical engineers to test the technology in lab-on-a-chip devices. He would also like to see the coating used to make heat transfer and condensation more efficient.
“If you have a mist of fluid passing near a surface, it will condense on these channels and be transported away and you can collect it very efficiently,” says Golovin. “That would enhance heat exchangers, where you condense steam as part of the power generation process.”