‘Smart skin’ takes reliable vitals during sweat-inducing activities

The researchers patterned the sensor-embedded sticky patch with artificial sweat ducts, similar to pores in human skin, that the researchers etched through the material’s ultrathin layers. The pores perforate the patch in a kirigami-like pattern, similar to that of the Japanese paper-cutting art. The design also ensures that sweat can escape through the patch, preventing skin irritation and damage to embedded sensors.

The kirigami design also helps the patch conform to human skin as it stretches and bends. This flexibility, paired with the material’s ability to withstand sweat, enables it to monitor a person’s health over long periods of time, which has not been possible with previous “e-skin” designs, the team said.

“With this conformable, breathable skin patch, there won’t be any sweat accumulation, wrong information, or detachment from the skin,” said Jeehwan Kim, associate professor of mechanical engineering at MIT. “We can provide wearable sensors that can do constant long-term monitoring.”

Kim’s research team specialise in fabricating flexible semiconductor films, and have pioneered a technique called remote epitaxy, which involves growing ultrathin, high-quality semiconductor films on wafers at high temperature and selectively peeling away the films, which they can then combine and stack to form sensors far thinner and more flexible than conventional wafer-based designs.

Recently, their work drew the attention of the cosmetics company Amorepacific in South Korea, which was interested in developing thin wearable tape to monitor changes in skin. The company struck up a collaboration with Kim to use the group’s flexible semiconducting films in something worn over long periods of time.

For design inspiration, the researchers looked to human sweat pores. They found that the diameter of the average pore measures about 100 microns and that pores are randomly distributed throughout skin. They ran some initial simulations to see how they might overlay and arrange artificial pores, in a way that would not block actual pores in human skin.

“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 monitor ability,” lead author and MIT postdoc Hanwool Yeon explained.

The team started with a periodic pattern of holes, each about the size of an actual sweat pore, and found that if pores were spaced close together, at a distance smaller than an average pore’s diameter, the pattern would efficiently permeate sweat. But they also found that if this simple hole pattern were etched through a thin film, the film was not very stretchable, and it broke easily when applied to skin.

The researchers found they could increase the strength and flexibility of the hole pattern by cutting thin channels between each hole, creating a pattern of repeating dumbbells, rather than simple holes, that relaxed strain, rather than concentrating it in one place. This pattern, when etched into a material, created a stretchable, kirigami-like effect.

“If you wrap a piece of paper over a ball, it’s not conformable,” Kim explained. “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.”

Following this rationale, the team fabricated an electronic skin from multiple functional layers, each of which they etched with dumbbell-patterned pores. The skin’s layers comprise an ultrathin semiconductor-patterned array of sensors to monitor temperature, hydration, ultraviolet exposure, and mechanical strain. They sandwiched this sensor array between two thin protective films, all of which overlays a sticky polymer adhesive.

The researchers tested the e-skin by sticking it to a volunteer’s wrist and forehead (pictured above). The volunteer wore the tape continuously for over a week. Throughout this period, the new e-skin reliably measured his temperature, hydration levels, UV exposure, and pulse, even during sweat-inducing activities, such as running on a treadmill for 30 minutes and consuming a spicy meal.

The team’s design also conformed to the skin, sticking to the volunteer’s forehead as the team asked him to frown repeatedly while sweating profusely, compared with other e-skin designs that lacked sweat permeability and easily detached from the skin.

Kim plans to improve the design’s strength and durability. While the tape is both permeable to sweat and highly conformable, thanks to its kirigami patterning, it’s this same patterning, paired with the tape’s ultrathin form, that makes it quite fragile to friction. As a result, volunteers had to wear a casing around the tape to protect it during activities such as showering.

“Because the e-skin is very soft, it can be physically damaged,” Yeon said. “We aim to improve the resilience of the electronic skin.”