In a significant development, Massachusetts Institute of Technology (MIT) engineers have developed a new category of wireless wearable skin-like sensors for health monitoring that doesn't require batteries or an internal processor.

The team's sensor design is a form of electronic skin, or "e-skin" — a flexible, semiconducting film that conforms to the skin like electronic Scotch tape, according to a press release published by MIT.

"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," said Yeongin Kim, study's first author, and a former MIT postdoc scholar.

"And the sensitivity of our film is so high that it can detect these changes," added Kim, who is now an assistant professor at the University of Cincinnati.

Wireless technology makes it possible for a person's blood pressure, heart rate, glucose levels, and activity levels to be easily communicated from the sensor to the smartphone for additional analysis, making wearable sensors incredibly common.

The majority of wireless sensors currently communicate using embedded Bluetooth chips, which are powered by tiny batteries.

Gallium nitride, a substance recognized for its piezoelectric capabilities, which allow it to both emit an electrical signal in reaction to mechanical strain and vibrate mechanically in response to an electrical impulse, forms the sensor's core in the form of an ultrathin, high-quality film.

The researchers discovered that they could employ gallium nitride's two-way piezoelectric characteristics for sensing and wireless transmission at the same time.

The scientists created pure, single-crystalline samples of gallium nitride, which they combined with a gold conducting layer to enhance any incoming or outgoing electrical signal.

They showed that the substance could vibrate in response to a person's heartbeat and the salt in their sweat and that the vibrations produced an electrical signal that could be interpreted by a nearby receiver. This allowed the device to communicate sensor data without a chip or battery wirelessly.

"Chips require a lot of power, but our device could make a system very light without having any chips that are power-hungry," said the study's co-author, Jeehwan Kim, an associate professor of mechanical engineering.

"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," added Kim, who is also a principal investigator at MIT's Research Laboratory of Electronics.

A highly sensitive piezoelectric material in its pure, defect-free state was peeled away from ultrathin single-crystalline sheets in the latest work by engineers using Kim's old method.

A piezoelectric material would concurrently turn the inherent, "resonant" vibration or frequency of a gallium nitride-based sensor that was bonded to the skin into an electrical signal, whose frequency a wireless receiver could detect.

The electrical signal that the sensor automatically provides to the receiver would alter if the circumstances of the skin changed, such as an accelerated heart rate.

The scientists created a tiny coating of high-quality, pure gallium nitride. They considered gallium nitride and gold an example of electronic skin because it is only 250 nanometers thick, or roughly 100 times thinner than the breadth of a human hair.

They strapped the novel e-skin to the wrists and necks of volunteers and used a small antenna held close by to wirelessly record the sensor's frequency without actually touching the sensor itself.

The device could wirelessly detect and transmit variations in the surface acoustic waves of the gallium nitride on the skin of participants that were associated with their heart rates.

According to the researchers, the findings represent the first step toward chip-free wireless sensors. They also believe that the current technology might be used in conjunction with other selective membranes to track additional important indicators.

"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," said Jun Min Suh, co-author of the study and an MIT postdoc.

"It's quite a versatile platform."

The research was first published in the journal Science.

Recent advances in flexible and stretchable electronics have led to a surge of electronic skin (e-skin)–based health monitoring platforms. Conventional wireless e-skins rely on rigid integrated circuit chips that compromise the overall flexibility and consume considerable power. Chip-less wireless e-skins based on inductor-capacitor resonators are limited to mechanical sensors with low sensitivities. We report a chip-less wireless e-skin based on surface acoustic wave sensors made of freestanding ultrathin single-crystalline piezoelectric gallium nitride membranes. Surface acoustic wave–based e-skin offers highly sensitive, low-power, and long-term sensing of strain, ultraviolet light, and ion concentrations in sweat. We demonstrate weeklong monitoring of pulse. These results present routes to inexpensive and versatile low-power, high-sensitivity platforms for wireless health monitoring devices.