A Sensor by Any Other Name Wouldn’t Be as Comfortable

With electronics of all sorts becoming smaller, thinner, and less expensive than ever before, they are making their way into more and more devices by the day. It is widely believed that one emerging application in particular — wearable health monitors — will have a major impact on all of our lives in the near future. But while the technology is sufficiently advanced to usher in this future, wearability still presents us with a number of problems. If a wearable device is uncomfortable or inconvenient, it will not get used, no matter how beneficial it might be.

Because it is so easily accessible, and loaded with such rich information about our health, sweat is an important analyte for wearable health monitors. Sweat sensors can give us information about kidney health, muscle performance, hydration levels, and much more — but only if the sensors are continually worn on the body. And that is where the problems begin. The best wearable sensors we have today are hydrophobic, meaning they repel the very substance they are designed to analyze. Accordingly, sweat sensors must be pressed very tightly against the body, often with the help of adhesives.

This is not only unpleasant for the wearer, but long term use can also lead to infections and other nasty conditions. Fortunately, we may be on the verge of a better solution. A group led by engineers at Waseda University has developed a wearable sweat sensor that is comfortable and can be loosely worn. This was made possible by borrowing some surface features from rose petals, which exhibit a unique wetting behavior, and patterning them onto their sensor.

Rose petals are known for a dual wetting property that seems almost paradoxical. When exposed to small amounts of water, they are hydrophilic, meaning droplets stick to their surface. But once water exceeds a certain level, they become hydrophobic, triggering a self-cleaning effect that causes water to roll off. By studying the microstructures of rose petals — tiny wrinkles, polygonal patterns, and spike-like protrusions — the team was able to replicate these textures in ion-selective membranes (ISMs) layered onto carbon nanotube substrates.

The structures were copied to create two new sensor types. One, modeled after the inner petals, showed especially high water retention, making it ideal for conditions where sweat production is variable, such as during exercise. The other, based on the outer petals, also demonstrated excellent stability and the same self-cleaning effect. Both designs overcame the long-standing hydrophobicity problem of conventional ISMs, which has limited their accuracy and comfort in real-world use.

The researchers then integrated the sensors into 3D-printed wearable devices with small microchannels designed to collect sweat. The sensors operated across a 2-millimeter gap from the skin, eliminating the need for adhesives or tight contact. In trials with runners, the devices accurately tracked sodium concentrations in sweat — an important indicator of hydration and electrolyte balance. Thanks to the rose-inspired self-cleaning property, the devices also managed sweat recirculation. It was shown that they could retain small amounts of fluid during dry periods and automatically clear excess when sweating increased, preventing erratic readings.

Looking ahead the team envisions these sensors not only making their way into consumer wearables like smartwatches, but also into prosthetics and exoskeletons. In such systems, real-time biochemical feedback could help users avoid overexertion and guide adaptive control.