Wearable sensors are ubiquitous thanks to wireless technology that allows a person’s glucose concentrations, blood pressure, heart rate, and activity levels to be transmitted seamlessly from the sensor to the smartphone for easy access. more in-depth analysis.

Most wireless sensors today communicate via built-in Bluetooth chips which are themselves powered by small batteries. But those conventional chips and power sources will likely be too bulky for next-gen sensors, which take on smaller, thinner, and more flexible forms.

Now, engineers at MIT have designed a new type of wearable sensor that communicates wirelessly without the need for onboard chips or batteries. Their design, detailed today in the review Sciencepaves the way for chipless wireless sensors.

The team’s sensor design is a form of electronic skin, or “e-skin” -; a flexible, semi-conductive film that conforms to the skin like electronic tape. The core of the sensor is an ultra-thin, high-quality film of gallium nitride, a material known for its piezoelectric properties, meaning it can both produce an electrical signal in response to mechanical stress and mechanically vibrate in response. response to an electrical impulse. .

The researchers found that they could exploit the bidirectional piezoelectric properties of gallium nitride and use the material simultaneously for sensing and wireless communication.

In their new study, the team produced pure, single-crystal samples of gallium nitride, which they combined with a conductive layer of gold to amplify any incoming or outgoing electrical signal. They showed that the device was sensitive enough to vibrate in response to a person’s heartbeat, as well as the salt in their sweat, and that the vibrations of the material generated an electrical signal that could be read by a nearby receiver. . This way, the device was able to wirelessly transmit detection information, without the need for a chip or battery.

“Chips require a lot of power, but our device could make a system very light without having power-hungry chips,” says the study’s corresponding author, Jeehwan Kim, associate professor of mechanical engineering and science and engineering. of Materials, and Principal Investigator at the Electronics Research Laboratory.

“You can put it on your body like a bandage, and paired with a wireless meter on your cell phone, you can wirelessly monitor your pulse, sweat, and other biosignals.”

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

pure resonance

Jeehwan Kim’s group previously developed a technique, called remote epitaxy, which they used to rapidly grow and peel off high-quality, ultrathin semiconductors from graphene-coated wafers. Using this technique, they fabricated and explored various flexible and multifunctional electronic films.

In their new study, the engineers used the same technique to peel off ultrathin single-crystal films of gallium nitride, which in its pure, flawless form is a highly sensitive piezoelectric material.

The team sought to use a pure gallium nitride film as both a sensor and a wireless communicator of surface acoustic waves, which are essentially vibrations through the films. Patterns of these waves can indicate a person’s heart rate, or even more subtly, the presence of certain compounds on the skin, such as salt in sweat.

The researchers hypothesized that a gallium nitride-based sensor, stuck to the skin, would have its own inherent “resonant” vibration or frequency that the piezoelectric material would simultaneously convert into an electrical signal, the frequency of which could be recorded. by a wireless receiver. Any change in the condition of the skin, such as an accelerated heart rate, would affect the mechanical vibrations of the sensor and the electrical signal it automatically transmits to the receiver.

“If there’s a change in the pulse, or chemicals in the sweat, or even ultraviolet exposure to the skin, all of that activity can change the surface acoustic wave pattern on the gallium nitride film.” , notes Yeongin Kim. “And the sensitivity of our film is so high that it can detect these changes.”

Transmission of waves

To test their idea, the researchers produced a thin film of pure, high-quality gallium nitride and combined it with a layer of gold to amplify the electrical signal. They laid down the gold in the repeating dumbbell pattern -; a lattice-like configuration that gave some flexibility to the normally stiff metal. The gold gallium nitride, which they consider a sample of electronic skin, is just 250 nanometers thick -; about 100 times thinner than the width of a human hair.

They placed the new electronic skin on the volunteers’ wrists and necks and used a simple antenna, held nearby, to wirelessly record the frequency of the device without physically contacting the sensor itself. The device was able to detect and wirelessly transmit changes in the surface acoustic waves of gallium nitride on the volunteers’ skin based on 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 wirelessly detect and transmit changing sodium levels when a volunteer was holding a heating pad and began to sweat.

The researchers see their findings as a first step toward chipless wireless sensors, and they envision that the current device could be paired with other selective membranes to monitor other vital biomarkers.

“We showed sodium sensing, but if you change the sensing membrane, you can detect any target biomarker, such as glucose or cortisol related to stress levels,” explains the co-author and post- MIT PhD student Jun Min Suh. “It’s a pretty versatile platform.”

This research was supported by AMOREPACIFIC.


Massachusetts Institute of Technology

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