Blood pressure measurement hasn’t changed much since the invention of the inflatable cuff-based sphygmomanometer in 1881. People can use the device to give readings a few times a day, but that’s not enough to give a holistic view of cardiovascular health.
New electronic tattoos made of graphene continuously read blood pressure for days. The ultrathin, light sensors could allow monitoring of a patient’s blood pressure while they go about their daily activities.
“Blood pressure is a really important vital sign,” says Roozbeh Jafari, an electrical and computer engineering professor at Texas A&M University. “Stress can kill. Stress leads to changes in blood pressure, but we have no way of measuring or understanding it. So if there’s a technology to measure blood pressure continuously, it’s a game changer.”
People with cardiovascular disease and hypertension have to monitor blood pressure routinely. To do that unobtrusively, researchers have developed cuffless methods in recent years. Some rely on wearable acoustic sensors that measure changes in ultrasound signals traveling through tissue, or pressure sensors that can detect expansion of blood vessels in the wrist. These are bulky and can move around, distorting measured signals.
Light-based sensors, such as the ones in fitness bands and smartwatches, can measure heart rate accurately from capillaries in the skin, but the light does not penetrate deep enough to detect the blood pressure wave.
The new blood pressure sensor uses graphene electronic tattoo sensors invented in 2017 by electrical and computer engineering professor Deji Akinwande at the University of Texas at Austin. Jafari and Akinwande collaborated on the new graphene blood pressure sensors that they reported in in Nature Nanotechnology.
To make the tattoos, the researchers grow one or two layers of graphene on copper foil, coat it with an ultrathin 200-nanometer-thick layer of acrylic, and transfer it to commercial temporary tattoo paper. Graphene’s atomic-level thickness means the patches conform to skin really well, Akinwande says, so they stick without the need for adhesives and stay in place. Plus, the wearer doesn’t feel them.
In a prototype version of the continuous blood-pressure monitoring system, segments of copper foil are taped to a wrist. The image labels identify the person’s radial and ulnar artery, as well as the locations for “injection” (introduction) of a low-level, alternating current signal. University of Texas at Austin/Texas A&M University/Nature Nanotechnology
To determine blood pressure, the researchers use the graphene sensors to measure bioimpedance—the electrical impedance to the flow of current through tissue. They attach two rows of six graphene tattoos each on a participant’s wrist. Each row is placed right above one of the two main arteries in the wrist. The outermost patches in each row send small electrical currents into the arm, and the inner four patches measure voltage changes, which helps calculate impedance.
Impedance is reflective of blood volume change in the arteries, Jafari says, but it does not directly translate to blood pressure. So the team built a machine-learning algorithm to accurately estimate blood pressure based on several different parameters including blood volume change, the transit time of the pressure pulse between two sites, the time difference between the arrival of pulses at the two arteries, and the arteries’ elastic properties.
As a demonstration, the researchers put the graphene sensors on seven human participants, and measured blood pressure while the volunteers performed active exercises. To determine accuracy, they compared the readings with periodic readings taken using the gold-standard blood pressure cuff technique.
Some participants went on a strenuous walk outside in 38 °C and others performed push-ups. The tattoos did not degrade after exposure to light and heat or contact with water or sweat. The researchers could monitor blood pressure continuously for five hours. Longer monitoring is easily possible, Jafari says, since the tattoos last for seven days.
This first-generation sensor is wired. Voltage data goes from the tattoos to the readout electronics via metal wires. But Akinwande says they plan to develop a wireless sensor next. “It would require us to design a chip that would send data wirelessly,” he says.
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