Unfortunately, blood pressure (BP) measurements currently require the use of a cuff that temporarily stops blood flow. A wearable BP “watch” using today’s technology would squeeze the wrist every few minutes, making it impractical to use. A better method might gauge subtle pressure changes at the surface of the skin above one of the main wrist arteries — the radial artery — without regularly cutting off circulation. But before this new technology can be developed, there is a need to understand what the pressure inside a blood vessel looks like on the surface of the skin. This requires a physical model that can be used to test wearable devices in a laboratory.
A blood pressure wrist “phantom” was developed that is essentially an artificial arm that mimics the mechanical properties of blood pulsing through an artery surrounded by human tissue. The phantom provides precise measurements of the force on the blood vessel wall, and the force on the soft tissue and the skin.
The blood pressure phantom consists of a slab of soft silicone, which substitutes for human tissue, sitting on top of a metal plate, which substitutes for bone. A pliable tube runs through the silicone to mimic an artery, through which fluid flows via a mechanical heart pump.
The materials match the properties of skin, soft tissue, bone, and artery walls. But unlike actual live human tissue, the phantom can easily accommodate sensors running through it, measuring the pressure changes that occur each time water is pumped through the tube.
The sensors currently under test are thin optical fibers called Bragg gratings, designed to block a specific frequency, or color, of light from passing through them. When the pressure changes inside the Bragg grating, so does the color of light that is blocked. Researchers can use this change in color to identify the pressure that was applied to the fiber. The final phantom likely will incorporate about a half-dozen of the Bragg sensors running through the silicone and over the top, as well as inside and outside the artificial artery.
Preliminary tests are being conducted to gauge the performance of the sensors using a prototype without the mock artery. Instead of pumping water through a tube, pressure is applied to the silicone by crushing it with weights; for example, to mimic a BP of 140/60, masses of about 1 to 1.8 kilograms are used, equivalent to approximately 2 to 4 pounds. The sensors are able to detect pressures from 170 millimeters of mercury (mmHg, equivalent to about 22.5 kilopascals, kPa, or about 3¼ pounds per square inch, psi) down to 60 mmHg (about 8 kPa, or a little more than 1 psi) with a resolution of 2 mmHg (about 250 Pa, or less than 0.04 psi). In terms of weight, this means that they are measuring masses of about 1 kg with a resolution of just 20 grams.
The results also are reproducible. Each time the silicone is crushed, it springs back to its original form, so that the results are the same no matter how many times the experiment is run.