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Wearable Technology Insights
Posted on September 4, 2019 by  & 

Wireless wearables powered by human skin

Smartwatches were big breakthroughs when they hit the market in the early 2000s. Even nascent versions got consumers excited with their ability to play games and access news feeds. Two decades later, that Fitbit® or Apple Watch® on your wrist can actually collect, store and transmit data. Wearable devices like these have amazing capacity within their tiny computers. But they have capacity limits, too. The culprit? The very thing that powers them - batteries. For more information see the IDTechEx report on Wearable Technology Forecasts 2019-2029.
Enter UMass Amherst researcher and Assistant Professor of Computer Science Sunghoon Ivan Lee. Inspired by research on stroke survivors that relied on bulky, battery-powered sensors, Lee's vision is to invent devices that not only operate without batteries but take the concept of battery-less wearables one step further. In his mind's eye, the next generation of wearables will transfer power between wireless sensors using a far more efficient conductor - human skin.
"There's been a lot of work with wireless power transfer," said Lee, "but we're the first to look at utilizing a person's skin. We're the first out with this type of research."
Lee's work is groundbreaking not just for its technological implications, but for the possibilities it opens up for personalized health monitoring. Collaborating with a team of three - Dr. Yeonsik Noh, Dr. Rui Wang and Dr. Jeremy Gummeson - Lee conducts his work at the UMass Amherst Institute for Applied Life Sciences (IALS) Center for Personalized Health Monitoring (CPHM). The project revolves around a novel concept of wirelessly transferring current through human skin to power battery-less wearable sensors. The self-powered sensors can be ultra-miniaturized and ergonomically designed for placement on small areas of the body, like a finger, an ear or even a tooth.
It's a technological innovation unreachable with conventional in-device batteries. Which is why Lee and his team believe their research can lay the groundwork to transform existing architectures and spawn a new generation of on-body sensors.
"We're working on a process that shrinks the size of devices so they can be placed on small parts of the body," Lee said. "And because you don't have to change batteries, there's a variety of ways in which wearable sensors can be improved and expanded."
It's the framework, in other words, for all sorts of applications - and one with the potential to revolutionize personalized health monitoring.
Lee's eureka moment arrived via a classic problem-solving process. "Our research was originally on understanding how stroke survivors use their limbs," Lee said. "If we could put a sensor on the finger, we could obtain clinically relevant information on impairment level." There was just one problem. The sensors were too bulky, and the batteries took up too much space. "More than half (of the sensor's space) is dedicated to batteries," Lee said. "Plus, batteries aren't flexible and can't be put in small parts of the body." So the team started looking for ways to make the sensors smaller, lighter, more pliable and more energy-efficient. They found their answer in the natural conductive properties of skin.
"We're using the human skin, which is composed of mostly water, as a conductor," Lee explained. "But human skin is one big chunk of conductive material, so there's no distinction between the signal wire (red) and the ground wire (black). So we're using the skin as a signal wire and air as the ground."
Since the concept of using a person's skin as a charging mechanism is new, experimentation is part of the process. And part of what makes this particular charging mechanism a better conductive material is its mass. "The sheer area of skin helps with conduction," Lee said.
So exactly how much more efficient is the transfer of power through human skin, you ask? That's what Lee and his team are exploring, thanks in part to the backing of a $40,000 two-year grant that Lee and Dr. Noh received for their preliminary research. Earlier this year, the pair won the UMass Amherst Office of Research Development's Armstrong Fund for Science competition for their proposal, "Enabling Battery-less Wearable Sensors via Intra-Body Power Transfer." The Armstrong Fund for Science is an annual award that supports some of the most promising research on the university's campus.
Lee's enthusiasm around the team's research is palpable. "We start by putting a battery-powered transmitter on the wrist and a battery-less receiver on the finger," he said. "As the power transfer happens, the electrode makes contact with the human skin, which emits and receives the signal."
Lee's team is also testing to see if the power rate changes for different gestures. As it does, the electrode that makes contact with the skin emits a signal. "Using something called capacitive coupling, we're testing for the distances between the electrode and human skin, using air as a virtual ground," Lee said.
Unlike a conventional sensor, Lee's technology allows the metal in the sensor to sit away from the skin - as much as a few millimeters above the skin's surface - which helps avoid motion artifacts. In addition to testing the distance between sensor and skin, they're also testing the size of a copper plate that floats in the air above their work area, which acts as a makeshift antenna for their virtual ground.
Future tests will examine the moisture in the skin. "We believe that sweat will conduct more electricity, but we haven't tested that yet," Lee said.
Right now, Lee's experiments are confined to finger placement. Where they go from there depends on what they learn. "There are safety requirements for different parts of the body," Lee said. "For the finger, we harvested about 500 microwatts of power, which can support low-end wearable electronics like a low-power smartwatch. We've learned we can collect, store and transfer data in a small computer chip. Size is a limitation at this point, but we hope to scale up."
Once they do, the next likely application may be an in-ear or in-tooth sensor. "Imagine inserting a battery-less sensor that's like a crown or implant," Lee said.
Lee is particularly excited about the prospects for that application. "When elderly people lose their teeth, it's important to measure the pressure or moisture level," he said. "We've been discussing the potential with Tufts Dental School."
The in-ear sensor presents intriguing possibilities, too. "We can monitor eye movements from inside the ear using something that looks like an earplug that's equipped with sensors," he said. "We could measure EEG, EOG, muscle signals and brain activities."
Over the next two years, Lee's team will continue their inquiry process, asking and seeking answers to questions like whether one transmitter can support multiple sensors - and, if so, what are the constraints. So far, they've relied on Bluetooth® to collect and transfer data. Someday, they hope to do it directly through the skin.
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