Electronic skins (e-skins) are designed to keep sensors in direct contact with the skin for long periods of time. They offer a tremendous opportunity to noninvasively collect information about human health, and as such, many devices of this sort have been developed in recent years. Designing an effective e-skin is not as simple as just sticking some sensors on the skin, however. In the course of normal, daily activities, sweat will accumulate under e-skins, which can cause sensors to malfunction, lead to irritation for the wearer, and even cause the device to detach from the skin.
Taking inspiration from both natural sweat pores and kirigami — the Japanese art of paper cutting — a team of MIT engineers and researchers in South Korea have developed a new type of e-skin. Their design is permeable to sweat, flexible, and able to comfortably and continuously maintain skin contact for long periods of time.
Initially, the team tested out a design with holes about the size of natural sweat pores (100 microns), and with the distance between holes being less than the diameter of a typical pore. This configuration was found to permeate sweat very efficiently, but the e-skin was brittle and not very stretchy.
Next, they tried cutting channels between the holes to create a dumbbell-shaped repeating pattern. This new design relieved strain on the device and allowed it to stretch and conform to the skin without breaking. The breathable, soft e-skin is reminiscent of actual human skin. Using this method, a prototype was developed that included sensors to monitor temperature, hydration, ultraviolet exposure, and mechanical strain, sandwiched between two sheets of e-skin. A sticky polymer adhesive was used to affix the device to the skin.
In a small-scale test of the e-skin, a volunteer wore the prototype device for a period of one week. Prior to applying the sensors, erythema was induced in the participant with an application of sodium lauryl sulfate. The recovery of the damaged skin was then monitored for the duration of the experiment. A control area was also treated with sodium lauryl sulfate, but was not monitored with a skin patch. The monitored area and the control area both exhibited an identical track of skin recovery. Moreover, the e-skin stayed in place for the full week without any malfunctions.
This new approach to e-skin design offers advantages over previous generation devices that were impractical for daily wear under normal conditions. As a scaffolding for sensors, it presents new opportunities for noninvasive, long-term monitoring of health. There is some work yet to be done, however. The softness that makes the e-skin comfortable also makes it susceptible to physical damage; the team is currently working on designs that will add further strength to the patches.