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Two Health Sensors Unite in One Powerful Gadget

The tattoo-like skin patch tracks blood pressure and heart rate while monitoring alcohol and caffeine levels

2 min read
wearable glucose monitor
Photo: UC San Diego

What’s not to love about a good flexible health sensor? Someday technology based on such bendable electronic tech might well replace some of those chunky wearables in the marketplace today with sleek, golden skin patches.

Now, a team at the UC San Diego Center for Wearable Sensors has created a stretchy skin patch that combines electrochemical sensors for alcohol, caffeine, glucose, and lactate with an ultrasound-based sensor that monitors blood pressure deep inside the body. Described in the journal Nature Biomedical Engineering, it’s the first wearable device that tracks heart signals and biochemical levels at the same time, the authors said.

“Conventionally, those two types of signals are monitored separately by different devices,” said study co-author Sheng Xu, a UCSD nanoengineer. “By bridging the gap between those two, we can get a more comprehensive view of what’s going on in the human body.”

The stick-on patch could help doctors remotely monitor patients during the COVID-19 pandemic as people want to avoid in-person visits. It could also enable those at home to track high blood pressure, diabetes, or other medical conditions without discomfort, says co-author Joseph Wang, director of the UCSD Center. 

And it’s not just for healthcare, he added: The team expects the patch to be used by sports and military groups to monitor athletes and soldiers, and is working with PepsiCo to use it to measure the nutritional effects of food products.

For over a decade, Wang’s lab has been developing a set of wireless, flexible sensors that use printed electrodes to sense chemicals on the skin. One such sensor detects lactate, alcohol, and caffeine—after sweat stimulation. Another measures glucose in fluid just under the skin by passing a mild electrical current over the skin to extract the fluid.

In 2018, Xu’s team constructed a bendable patch that uses an array of electronic “islands” connected by spring-shaped wires to produce ultrasound waves. Those ultrasounds can detect and record the dynamics of a pulsing blood vessel as deep as four centimeters below the skin, thereby non-invasively measuring blood pressure and heart rate. Despite being an important vital sign, blood pressure has long been missing in consumer wearables because it requires measuring forces far under the skin, and most consumer health sensors detect only surface signals.

“We came up with a joint proposal to marry the two completely different techniques on a single tattoo,” said Wang. The biggest challenge in combining the disparate sensors was eliminating crosstalk that could introduce noise or artifacts into the signals, said Xu.

Once they had a working prototype, the collaborators tested the device on a handful of volunteers in the lab who wore the patch while doing various tasks, including exercising, eating, and drinking. 

Each measurement from the patch was verified by a gold-standard monitoring device, such as a blood pressure cuff, glucometer, or breathalyzer. The team also twisted and bent the patch while on the skin, and it maintained accurate readings with no crosstalk between the sensors, said Wang.

The prototype version described in the paper is linked to a computer and power source by flexible cables, but the group is working to eliminate those cables and lower the energy demands of the patch, said Xu. When the team is ready to scale-up the manufacturing of the patch, Xu and Wang expect it will be low-cost enough to dispose of after use. “Stay tuned,” Xu said.

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