Sweat-analyzing sensor copies the cactus to gather more liquid
When it comes to detecting biomarker chemicals in a person's body, sweat-analyzing sensors offer a less painful alternative to blood sampling. A new wearable sensor takes a unique approach to collecting that sweat, by mimicking cactus needles.
We've already seen a number of skin-adhered sensors that analyze their wearer's sweat, detecting biomarkers associated with things like blood glucose levels, stress levels, and certain diseases. Unless the person is unusually hot or physically active, though, collecting sufficient amounts of sweat is often challenging.
Hoping to address that problem, scientists at South Korea's Pohang University of Science and Technology (POSTECH) looked to the cactus plant.
In the arid environments in which they live, cacti need to take in water wherever they can find it – this includes the droplets of water vapor that condense on the tips of their needles. Via a phenomenon known as Laplace pressure, those droplets are drawn inwards along the needles until they reach the plant's skin, where they're absorbed.
Laplace pressure is defined as "the pressure difference between the inside and the outside of a curved surface that forms the boundary between a gas region and a liquid region." On a cactus needle – which is narrow at the tip and broadens out toward the base – the increasing contact area creates this effect in water droplets, essentially rolling them in.
Instead of needles, POSTECH's flat disc-shaped sensor features multiple sweat-collection channels that radiate outward from a central reservoir like the spokes of a wheel.
Each channel is made of a hydrophobic (water-repelling) material, with a triangular wedge of hydrophilic (water-attracting) material embedded within it. Because the narrow end of that wedge is at the outside end of the channel – and the broad end is inside by the reservoir – it's able to use Laplace pressure to pull in even trace amounts of sweat.
In lab tests, the sensor was shown to be much more effective than existing microfluidic sweat-collection systems, regardless of the inclination of the device.
"Difficulties in collecting sweat has hindered its use in wearable healthcare devices," says Prof. Kilwon Cho. "This newly developed patch solves that issue by quickly collecting sweat and facilitating its use in various wearable healthcare devices, including blood sugar monitoring."
A paper on the research was recently published in the journal Advanced Materials.