The problem with designing a blood pressure monitoring wearable is down to the method of monitoring blood flow. This requires a cuff-like device which squeezes the arm. This means any wearable, based on current methods, would need to squeeze the wrist of the wearer every few minutes. This is unlikely to prove popular with users, especially once the novelty wears off.
Blood pressure is the pressure of circulating blood on the walls of blood vessels. The measurement is expressed in terms of the systolic (maximum during one heart beat) pressure over diastolic (minimum in between two heart beats) pressure. The units of measurement are as millimeters of mercury above the surrounding atmospheric pressure. For example, 140/90mmHg, would represent a high blood pressure or hypertension. If a person has high blood pressure, this higher pressure puts extra strain on their heart and blood vessels. Over time, this extra strain increases your risk of a heart attack or stroke.
An alternative measure of blood pressure and one suited to a wearable device is to record subtle pressure changes at the surface of the skin. An appropriate place to record this is just above one of the main wrist arteries (the radial artery). Taking readings here would allow blood pressure to be monitored without cutting off blood circulation.
To construct a wearable device to do this requires research and modelling, and this is what scientists are working on. To enable this NIST Physical Measurement Laboratory researchers have made a working physical model that can be used to test wearable devices in a laboratory. This takes the form of an artificial arm, replicating the mechanical properties of blood pulsing through an artery surrounded by a substance designed to mimic human tissue.
Speaking with Controlled Environments magazine, lead researcher Zeeshan Ahmed explains more about the project: “One of the things we want to understand is, if a sensor is sitting up here on top of the silicone, what is it really seeing?”
He adds: “Is it just seeing the primary wave from a pulse of fluid going through? Is it seeing a lot of reflection waves, when the primary wave bounces off the metal plate? How does the pressure change over the time it takes for each pulse of water to pass through the artificial artery?”
The sensors developed take the form of ultra-thin optical fibers. So far the results are promising and also reproducible. In terms of accuracy, the sensors are able to detect pressures from 170 millimeters of mercury down to 60 mmHg. This means a wearable sensor is very close to commercialization.
In related news, Digital Journal recently reported on a new medical study which showed how the shape and flow of blood through the heart can acts as model to predict the likelihood of a person developing a stroke.
