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E-tattoos are worn on the wrist, and are designed for continual blood pressure monitoring.
Graphene-based sensors are nearly invisible under the red markings, which are used to measure blood pressure. | Image: Courtesy of Dr. Roozbeh Jafari.
Blood pressure is one of the most important indicators of heart health, but it's tough to measure outside of a clinical setting frequently and reliably. For decades, cuff-based devices that constrict around the arm to give a reading have been the gold standard.
Researchers at Texas A&M University and The University of Texas at Austin are working to change that, enabling continuous monitoring that can provide a more comprehensive look at blood pressure than the single snapshot that comes with the cuffs.
The researchers developed an electronic tattoo that can be worn comfortably on the wrist for hours and deliver continuous blood pressure measurements at an accuracy level exceeding nearly all available options on the market today. This research has received $5 million in awards and funding from the National Institutes of Health (NIH).
"Taking infrequent blood pressure measurements has many limitations, and it does not provide insight into exactly how our bodies are functioning," said Dr. Roozbeh Jafari, professor of biomedical engineering, computer science and electrical engineering at Texas A&M and the co-leader of the project.
Blood pressure is the force of blood pushing against the walls of arteries. As blood flows to the arteries, they expand to let it through. Hypertension, or high blood pressure, happens when the force is too great or the arteries don't expand enough.
High blood pressure can lead to serious heart conditions if left untreated. It can be hard to capture with a traditional blood pressure check because it only measures that exact moment in time, a single data point.
"Blood pressure is the most important vital sign you can measure, but the methods to do it outside of the clinic passively, without a cuff, are very limited," said Dr. Deji Akinwande, a professor in the Department of Electrical and Computer Engineering at UT Austin and one of the co-leaders of the project, which is documented in a new paper published on June 20, 2022, in Nature Nanotechnology.
The continuous monitoring of the e-tattoo allows for blood pressure measurements in all kinds of situations: at times of high stress, while sleeping, exercising, etc. It can deliver hundreds and thousands of measurements more than any device thus far.
Mobile health monitoring has taken major leaps in recent years, primarily due to technology like smartwatches. These devices use metallic sensors that get readings based on LED light sources shined through the skin.
"All this data can help create a digital twin to model the human body, to predict and show how it might react and respond to treatments over time," Akinwande said.
However, leading smartwatches aren't yet ready for blood pressure monitoring. That's because the watches slide around on the wrist and may be far from arteries, making it hard to deliver accurate readings. And the light-based measurements can falter in people with darker skin tones and/or larger wrists.
The e-tattoo makes sense as a vehicle for mobile blood pressure monitoring because it resides in a sticky, stretchy material that is comfortable to wear and doesn’t slide around. The is a very thin layer of carbon, named graphene, similar to what we see in graphite or pencils, but the atoms are precisely placed next to each other to form one to a few atomic layers.  
"The sensor for the tattoo is weightless and unobtrusive; you place it there, you don't even see it and it doesn't move. You need the sensor to stay in the same place because if you happen to move it around, the measurements are going to be different," Jafari said.
The device takes its measurements by shooting an electrical current into the skin and then analyzing the body's response, which is known as bioimpedance. There is a correlation between bioimpedance and changes in blood pressure that has to do with blood volume changes. However, the correlation is not particularly obvious, so the team had to create a machine-learning model to analyze the connection to get accurate blood pressure readings.
In medicine, cuff-less blood pressure monitoring is the "holy grail," Jafari said, but there isn't a viable solution on the market yet. It's part of a larger push in medicine to use technology to untether patients from machines while collecting more data wherever they are, allowing them to go from room to room, clinic to clinic and still get personalized care.
“The term that we often use is called ‘medicine in the wild,’ which was coined by Dr. David Paydarfar, professor of medicine at Dell Medical School,” Jafari said.
The fundamental science on medical e-tattoos has advanced rapidly in the last few years, but the next step toward FDA-approved monitoring devices that people can use will take a lot of technological integration. That means working to integrate these sensors with smartphones, smart watches and other mobile devices that people can use to get routine, continuous readouts of data.
Other team members on the project are Dr. Dmitry Kireev and Neelotpala Kumar from electrical and computer engineering at UT Austin; Kaan Sel and Bassem Ibrahim from electrical and computer engineering at Texas A&M; and Dr. Ali Akbari from biomedical engineering at Texas A&M. The research was supported by grants from the Office of Naval Research, the National Science Foundation and NIH.
“Realizing unobtrusive and passive technologies for the frequent recording of blood pressure to address the worldwide problem of hypertension is among the most important global challenges in health care. I am pleased to see the progression of technology development following the initiation of the National Institute of Biomedical Imaging and Bioengineering (NIBIB)/NIH-India collaboration for the development of such technologies a decade ago,” said Dr. Roderic Pettigrew, first director of NIBIB, chief executive officer of Engineering Health and inaugural dean for Engineering Medicine at Texas A&M.