Artificial sweat pores allow electronic skin to breathe during exercise
Thin, wearable films packed with discreet sensors promise to change the game when it comes to health monitoring, with the potential to track everything from vitamin C levels, to blood sugar, to signs of cardiovascular trouble. A new design for these so-called electronic skins addresses one of their key shortcomings so far, by taking inspiration from the sweat pores in human skin to prevent the accumulation of moisture that can compromise their performance.
Led by mechanical engineers at MIT, the scientists set out to solve one of the problems plaguing electronic skins so far. Many of these are made of polymer-based materials that aren't all that breathable, while some use materials like woven fibers that let air through but not moisture. This can mean the electronic skin simply peels off after a short while, or the sensors packed inside malfunction or are unable to pick up a clear signal.
“Sweat can accumulate between the e-skin and your skin, which could cause skin damage and sensor malfunctioning,” says Jeehwan Kim, associate professor of mechanical engineering at MIT. “So we tried to address these two problems at the same time, by allowing sweat to permeate through electronic skin.”
To do this, the team turned to actual human skin for inspiration. Through their investigations, the team identified a few helpful characteristics, such as the average diameter of each pore being around 100 microns, and that the pores are distributed randomly across the skin. They then used simulations to explore how artificial pores could be laid over the top without blocking the function of the natural ones.
“Our simple idea is, if we provide artificial sweat ducts in electronic skin and make highly-permeable paths for the sweat, we may achieve long-term monitorability,” says MIT postdoc Hanwool Yeon.
Through some experimentation, the team landed on a design for an electronic skin that appears to offer this type of functionality. It consists of flexible semiconductor films stacked together to form ultra-thin sensors, with each layer bearing tiny pores spaced very closely together to allow for the passage of sweat. To increase the strength and flexibility of the film, the team cut tiny dumbbell-shaped channels in between the pores, a pattern inspired by the paper art form kirigami.
“If you wrap a piece of paper over a ball, it’s not conformable,” Kim says. “But if you cut a kirigami pattern in the paper, it could conform. So we thought, why not connect the holes with a cut, to have kirigami-like conformability on the skin? At the same time we can permeate sweat.”
The team's electronic skin was packed with sensors to monitor temperature, hydration, UV exposure and pulse, and successfully did so when continuously stuck to a participant's wrist and forehead over a week. The skin gathered reliable measurements even throughout sweaty activities such as treadmill running for 30 minutes and consumption of a spicy meal.
“With this conformable, breathable skin patch, there won’t be any sweat accumulation, wrong information, or detachment from the skin,” says Kim. “We can provide wearable sensors that can do constant long-term monitoring.”
Despite these promising early results, the team still sees plenty of room for improvement. They'd like to make the skin stronger and more durable so it can survive everyday activities like showering without needing to be protected by tape, for example.
The research was published in the journal Science Advances.