Have you every worn a pair of rubber gloves to complete a task on a hot summer day? The gloves may stay dry on the outside, but in a short time the insides will be sopping wet because your hands sweat in the heat. And that liquid has no place to go because it’s contained by the gloves. So what does this have to do with wearable health tech? Smart patches are playing an increasing role in remote patient monitoring and wireless sensing of vital signs and other biometrics. But when the patient sweats, this can disrupt the sensors or even cause the adhesive to fail and the patch falls off.
Researchers at MIT worked with partners in South Korea to create a “sweat-proof” electronic skin. They already had developed remarkably thin film sensors that can be used to detect metrics such as temperature, hydration, and movement. One problem with these flexible films, however, is that they were fragile and did not stretch well. And they blocked sweat, which could lead to skin irritation and adhesive failure.
Human skin has a random arrangement of pores that are about 100 microns across. The research team soon discovered that simply punching tiny holes in the sensor film didn’t work, because they would not necessarily align with the pores, which meant that they would not let sweat pass through efficiently. Eventually they settled on a pattern of interlocking, dumbbell-shaped holes. When combined in multiple layers, there were sufficient channels for the sweat to pass through. As an added benefit, this pattern of holes also improved the ability of the films to stretch and conform to skin in spite of movements.
Early experiments had a test subject wear the sensors on the wrist and forehead continuously for a week. The sensors performed reliably, even when the subject sweated profusely during a treadmill workout and while eating a spicy meal.
The porous sensor films are still fragile and easily damaged by friction. The researchers are working to find ways to make the sensors more resilient.