Stretchy, soft, and as thin as a temporary tattoo, this new health sensor does something no other wearable device can. Peering into your veins, it tracks the speed of your flowing blood in real time. The study appears in Science Advances.

A team of engineers and medical researchers led by Chad Webb, a materials engineer at the University of Illinois in Urbana-Champaign, has just developed the first blood flow monitor "that can be used without requiring someone to come into a clinic and stay completely immobile while a test is performed," he says. As opposed to simply checking your heart-rate, measuring blood flow in your veins and arteries gives doctors and medical researchers a much deeper insight into everything from stress levels to the risk of vascular disease. Webb believes his new sensor could open an altogether untouched avenue of medical study: allowing researchers track study participants' blood flow "24/7, as people go about their daily lives," he says.

"I know! For a long time even I thought this seemed like something that just couldn't be true."

The mechanics behind the monitor are surprisingly simple. It's basically a tiny flake-like heat pad surrounded by a ring of heat sensors, all painted onto a silicone skin. When the monitor turns on, the thin heat pad is warmed to roughly 12 degrees Fahrenheit above your skin temperature. (The pad is so small this takes only 5 milliseconds.) After heating up, the ring of sensors track how long it takes your underlying vein to likewise warm up—giving a precise estimate as to the size of the vein underneath the sensor. Next, the sensors track how quickly the heat produced by the pad travels downstream in the vein. Together, those measurement produce an accurate estimate of blood flow. In multiple tests, Webb found the skin-like monitor was accurate even the wearer was exercising or otherwise moving around.

An infrared thermal image of the epidermal device as it monitors blood flow changes that occur in a vein on the forearm. The University of Illinois

Even though the pad heats up a tiny dot of your skin by a dozen degrees, the change is so delicate that even after 50 testers, "practically no one could tell if the device was on or off," says Webb. "I know—for a long time even I thought this seemed like something that just couldn't be true." The warming "is well below the skin's sensation threshold," he adds, and harmless.

Webb still has a few improvements he'd like to make. Although the sensor is thin, it relies on wires for both energy and data transmission. Webb would like to see a wireless version, and his colleagues have already started experimenting on possibilities like "adding tiny solar cells, or even piezoelectric energy harvesting," which would pull the energy the device needs from your body heat.

A 3-D rendering of the epidermal device. The University of Illinois

It's easy to imagine such a sensor having a future in consumer electronics, helping users better track their health ("blood flow monitoring is hugely important for people with diabetes," says Webb). But he sees the biggest promise in medical research.

"Vascular disease motivates a lot of research on blood flow, but the difficulties and limitations of collecting that data"—such as needing people to stay immobile in a lab—"really limit the kinds of questions you can ask," Webb says. "Collecting this data 24/7 with a wearable device is such a new concept that we don't really know what insights we could gain."

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