In a world of smart watches and fitness trackers that continuously monitor heart rhythm, skin temperature, quality of sleep, and more, blood pressure stands out as a key vital sign that’s absent from their suite of data. Indeed, the way blood pressure is measured hasn’t changed much in more than a century: Use an inflatable rubber cuff to squeeze an artery until the blood flow is cut off, then gradually release the pressure until blood can push its way through on part, then all, of the heartbeat cycle. For blood-pressure measurements to be made compatible with low-power wearable electronics that can go anywhere, they need to be collected in a fundamentally different way that doesn’t rely on direct mechanical forces.
Toward that end, Roozbeh Jafari (Texas A&M University), Deji Akinwande (the University of Texas at Austin), and their colleagues have developed a blood-pressure sensor that’s based on a temporary tattoo made of graphene protected by an ultrathin polymer film. The tattoo—the shiny patches shown in the photo—is about as lightweight as it can possibly be. Nevertheless, it’s robust enough to remain on the skin for several days and even survive light washing.
The sensor measures blood pressure through bioimpedance—essentially the tissue’s resistance to an alternating electrical current. Blood is a fluid rich in ions, so it conducts electricity well. When a blood pulse passes through the artery under the tattoo, the impedance drops.
But there’s not a simple, direct relationship between the measured impedance and blood pressure. The blood-pressure measurement needs to be teased out from subtle features of the shape of the impedance curve over the pulse cycle. In other words, it’s a task for machine learning.
The graphene tattoo is ideal for the needs of machine-learning algorithms. Unlike conventional metal electrodes, which introduce measurement error when they shift position on the skin as the adhesive holding them in place deforms, graphene clings to the skin directly, so it sticks in exactly the same position until it gets sloughed off with a layer of dead skin cells. Because of that immobility, the sensor can collect a large, stable set of training data on test subjects as they perform activities designed to raise and lower their blood pressure. Once trained, it yields accurate blood-pressure measurements for hours to days thereafter.
The researchers’ eventual goal is to develop the sensor into a plug-and-play device that can accurately measure blood pressure on a new individual without the need for any additional training and that has all the necessary computing power packaged on a wearable device like a smart watch. They also hope to expand blood-pressure measurements to different parts of the body—such as the arteries in the neck that supply blood to the brain—that are too dangerous or impractical to probe with the century-old artery-squeezing technology. (D. Kireev et al., Nat. Nanotechnol., 2022, doi:10.1038/s41565-022-01145-w.)