Chip and Battery-Free Electronic Skin Measures Heartbeat

There’s been a lot written over the last few years on the attributes of gallium nitride in the power industry, but one aspect of the material I had never read about before is that it is has piezoelectric properties, meaning that it can turn mechanical strain to electricity, and electricity to mechanical vibration. Researchers at MIT have taken advantage of both these features to create electronic skin that can operate wirelessly, and more importantly, can operate without any ICs or batteries. The researchers harnessed those two piezoelectric properties simultaneously for sensing and wireless communication.

The electronic skin is intended for use in wearable sensors. Like a smart watch now, the electronic skin will be able to measure attributes such as glucose concentration, blood pressure, heart rate and activity levels. While devices, like smart watches, that perform those functions now use batteries and communicate through a wireless chip, an electronic skin product without those components can be made smaller, thinner and more flexible. The MIT engineers’ design, published in the journal Science, offers a way to build chip-free wireless sensors that can communicate with a device like a smartphone.

The team’s sensor design is a a flexible, semiconducting film that can adhere to the user’s skin. An ultrathin, single-crystalline sample of gallium nitride is paired with a conducting layer of gold to boost electrical signals. The device is sensitive enough to vibrate in response to a person’s heartbeat, as well as the salt in their sweat and generate an electric signal from those vibrations.

“Our device could make a system very light without having any power-hungry chips,” said the study’s corresponding author, Jeehwan Kim, an associate professor of mechanical engineering and of materials science and engineering, and a principal investigator in the Research Laboratory of Electronics. “You could put it on your body like a bandage, and paired with a wireless reader on your cellphone, you could wirelessly monitor your pulse, sweat, and other biological signals.”

Kim’s group previously developed a technique, called remote epitaxy, that can quickly grow and peel away ultrathin, high-quality semiconductors from wafers coated with graphene. For the electronic skin, the engineers used the technique to peel away ultrathin single-crystalline films of gallium nitride. The team used the gallium nitride film as both a sensor and a wireless communicator of surface acoustic waves. The patterns of these waves can indicate a person’s heart rate, or the presence of certain compounds on the skin.

The researchers hypothesized that the sensor would have its own inherent resonant vibration that the piezoelectric material would simultaneously convert into an electrical signal, the frequency of which a wireless receiver could register. Any change to the skin’s conditions, such as from an accelerated heart rate, would affect the sensor’s mechanical vibrations, and the electrical signal that it automatically transmits to the receiver. To test that idea, the researchers placed the electronic skin on volunteers’ wrists and necks, and used a simple, nearby antenna to wirelessly measure the device’s frequency without physically contacting the sensor itself. The device was able to sense and wirelessly transmit changes in the surface acoustic waves of the gallium nitride on volunteers’ skin related to their heart rate. The gallium nitride and gold layer measured only 250 nm thick.

The team also paired the device with a thin ion-sensing membrane — a material that selectively attracts a target ion, and in this case, sodium. With this enhancement, the device could sense and wireless transmit changing sodium levels as a volunteer held onto a heat pad and began to sweat.

Kim’s co-authors include first author and former MIT postdoc Yeongin Kim, who is now an assistant professor at the University of Cincinnati; co-corresponding author Jiyeon Han of the Korean cosmetics company AMOREPACIFIC, which helped motivate the current work; members of the Kim Research Group at MIT; and other collaborators at the University of Virginia, Washington University in St. Louis, and multiple institutions across South Korea.

The research was supported by AMOREPACIFIC.