An e-skin film developed at MIT could be the first step toward chip-free, wireless sensors. [Image courtesy of MIT]
MIT engineers have developed a new wireless, wearable sensor without semiconductors or batteries in what they say is a first step toward chip-free wireless sensors.
This electronic skin — or e-skin — is a flexible, semiconducting film that MIT describes as a sort of electronic Scotch tape.
The device has an ultrathin gallium nitride film that can respond to mechanical strain with an electrical signal and vibrate in response to an electrical impulse. The researchers made pure, single-crystalline samples of gallium nitride and paired it with a conductive layer of gold to boost the income and outgoing electrical signals, MIT said.
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That allows the e-skin to not only detect vital signs like the vibration of a heartbeat, but to use the device’s vibration in response to generate an electrical signal and transmit it to a nearby receiver.
“Chips require a lot of power, but our device could make a system very light without having any chips that are power-hungry,” Jeehwan Kim, the study’s corresponding author, said in a news release. “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 is an associate professor of mechanical engineering and of materials science and engineering at MIT and a principal investigator in the Research Laboratory of Electronics.
Kim and the other researchers had previously developed a way to quickly grow and peel away ultrathin semiconductors from wafers coated with graphene. The technique is called remote epitaxy, and it has allowed them to make and experiment with e-skin films.
They used the remote epitaxy technique in their latest study, and found that even slight changes in the skin’s condition from an elevated heart rate would affect the sensor’s vibration and the signal it puts out.
“If there is any change in the pulse, or chemicals in sweat, or even ultraviolet exposure to skin, all of this activity can change the pattern of surface acoustic waves on the gallium nitride film. And the sensitivity of our film is so high that it can detect these changes,” said co-author Yeongin Kim, an MIT postdoc who is now an assistant professor at the University of Cincinnati.
MIT described how the researchers tested their e-skin concept:
“The researchers produced a thin film of pure, high-quality gallium nitride and paired it with a layer of gold to boost the electrical signal. They deposited the gold in the pattern of repeating dumbbells — a lattice-like configuration that imparted some flexibility to the normally rigid metal. The gallium nitride and gold, which they consider to be a sample of electronic skin, measures just 250 nanometers thick — about 100 times thinner than the width of a human hair.
“They placed the new e-skin on volunteers’ wrists and necks, and used a simple antenna, held nearby, to wirelessly register 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 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.”
The researchers said the current e-skin device could be used to monitor other vital biomarkers.
“We showed sodium sensing, but if you change the sensing membrane, you could detect any target biomarker, such as glucose or cortisol related to stress levels,” co-author and MIT postdoc Jun Min Suh said. “It’s quite a versatile platform.”