Blood pressure is a key vital sign that provides information about our cardiovascular health. But a doctor usually measures blood pressure only a few times a year, and these measurements may not always be accurate (maybe, for example, you’re nervous in the doctor’s office). A wearable device that could monitor blood pressure continuously would provide more informative readings, allowing doctors to observe trends and detect consequent changes in blood pressure, ultimately improving heart health.
Nearly a decade ago, in collaboration with the Indian government, NIBIB spearheaded a international effort to develop a handcuff-free wearable system. After years of research, scientists are now perfecting such a device. Made from graphene, one of the world’s thinnest materials, the device sits on the bottom of the wrist and can measure blood pressure with accuracy comparable to that of a standard blood pressure cuff. While the technology is still in its early stages, researchers anticipate that the monitor will be usable 24/7. The device was recently described in Nanotechnology from nature.
“The blood pressure monitor was developed in the late 19th century and the core technology has not undergone significant changes,” explained Tiffani Lash, Ph.D., program director of NIBIB’s Division of Health Informatics Technologies. “Unlike traditional blood pressure cuffs, this device could enable continuous and discreet monitoring, which could pave the way for 24-hour remote blood pressure assessment. As cardiovascular disease is the leading cause of mortality worldwide, such a device has the ability to mitigate potentially preventable deaths.”
At first glance, the blood pressure monitor looks like a temporary tattoo: the graphene and the tiny components inside look like small transparent stickers that stick directly to the skin. The application is even similar: place the device, which is attached to a thin strip of paper, on the skin, moisten the paper with water and carefully remove it, leaving the blood pressure monitor behind.
Similar “electronic tattoos” have been investigated before, but they typically employ metal films that don’t fit snugly against the skin, explained study author Deji Akinwande, Ph.D., a professor at the University of Texas at Austin. “With previous devices, there is a lot of slippage between the metal film and the skin, which creates motion artifacts that can mask the actual signal we are trying to capture,” he said. “Because graphene is so thin and flexible, it does not suffer from motion artifacts and we can capture accurate measurements over an extended period of time.”
Tattoos do not measure blood pressure directly, as traditional monitors do. Instead, tattoos are based on bioimpedance, a measure of how the body resists an electrical current. The tattoos contain small electrodes that apply a small current to an artery in the wrist and then detect how the artery affects that current. These bioimpedance measurements can be converted into blood pressure readings using machine learning.
The researchers evaluated their bioimpedance tattoos on seven healthy individuals. On average, each participant was monitored continuously for approximately four hours, resulting in approximately 2000 measurements per participant. Participants performed various exercises to intentionally increase their blood pressure, such as completing handgrip exercises, pedaling on a stationary bike, or performing the Valsalva maneuver (when one tries to forcefully exhale when their mouth is closed and their nose is pinched). During these experiments, participants also wore a medical-grade blood pressure monitor to provide baseline blood pressure measurements, and the researchers found that their bioimpedance tattoos could determine participants’ blood pressure with high accuracy.
Next, the researchers evaluated their system’s ability to calculate blood pressure readings in circumstances beyond those evaluated in their initial experiments. Participants were invited to do additional activities, such as walking or strenuous exercise (such as an extensive, sweat-inducing outdoor walk or push-ups). After these activities, the researchers assessed blood pressure readings (which are taken when participants are stationary). While the bioimpedance tattoos could withstand exposure to light, heat, water or sweat, the accuracy of their blood pressure decreased slightly during post-exercise monitoring. Researchers will continue to incorporate additional data, including measurements of exercise-induced higher blood pressure, into their machine learning algorithms, which should improve their accuracy.
The researchers also envision further optimization of their device so that blood pressure data can be transmitted wirelessly. From now on, tattoos require connection to external equipment through fine gold wiring, which allows a blood pressure reading.
“This first-generation device is still in its early stages, but the promise of this technology is that it could literally be used at all times and provide a wealth of biophysical data,” said study author Roozbeh Jafari, Ph.D., a professor at Texas A&M University and senior author of the NIBIB grant. “Blood pressure and hypertension are linked to serious and life-threatening health conditions, and the ability to monitor this vital sign continuously could inform proactive interventions that could improve public health.”
and how investigation indicates that blood pressure levels have increased during the pandemic, it may be increasingly important to find ways to continuously control blood pressure, especially among people with hypertension.
This study was funded in part by a NIBIB grant (R01EB028106). Other funding sources include the Office of Naval Research (grant N00014-18-1-2706) and the National Science Foundation (grant 1738293).
Study reference: Kireev, D., Sel, K., Ibrahim, B. et al. Continuous cuffless blood pressure monitoring using graphene bioimpedance tattoos. Nat. Nanotechnology. 17864–870 (2022). https://doi.org/10.1038/s41565-022-01145-w
