Extremely sensitive, thin, conformable tactile sensors will play a key role in the next generation of consumer wearable and clinical devices that measure blood pressure.
While there are many methods to measure blood pressure, all but the most invasive, intra-arterial techniques suffer from deficiencies that can lead to inaccurate or inconsistent results. Even the standard blood pressure cuff used by healthcare practitioners has limitations, such as signal quality, errors in human interpretation and calculation methods that rely on indirect or algorithmic interpretations.
The latest generation of thin, conformable tactile sensor arrays promises more precise, accurate measurement of pulse pressure waveform. These sensors will aid artery location and pressure measurement; and for consumer wearables will measure more than just heart rate.
Alternative automatic methods for measuring arterial pressure typically use an inflatable cuff to restrict flow, then measure pressure oscillations in the cuff to estimate systolic and diastolic pressure using proprietary algorithms. Such methods are often packaged as home use devices, but can have inaccuracies on the order of 10 mmHg and are particularly inaccurate on obese or those with conditions resulting in an irregular pulse.
Pulse oximeters monitor blood oxygen saturation and pulse rate, and can monitor blood pressure as well. These devices pass two wavelengths of light through the body to measure the changing absorbance information that is then used to infer blood pressure.
While these options have merits, neither approach meets the accuracy and repeatability standards of the lead organizations such as the Association for the Advancement of Medical Instrumentation (AMMI) and British Society of Hypertension.
Instead, a more direct measurement of the pulse waveform is gaining interest, one that enables ambulatory, non-invasive blood pressure measurement without cuffs by using advanced capacitive tactile sensing technology.
With capacitive tactile sensors, blood pressure can be measured through sophisticated arrays that map the pressure above the artery. These arrays can range from a number of discrete measurements to a large, dense array of hundreds of elements. The sensors, in direct proximity to the artery, deliver a detailed pulse waveform that is used to determine blood pressure and pulse information, including other parameters such as arterial hardening.
One reason that capacitive tactile pressure sensing is well suited for this task is that it can handle the extremely low pressures to be measured; blood pressure is measured in millimeters of mercury (mmHg), with 0.5 mmHg equaling roughly 0.01 psi.
In addition, tactile pressure sensors conform to the contours of the human body and other curved surfaces so they can be integrated into a variety of soft, flexible materials.
“The key advantages of these sensors are the sensitivity, the small form factor, the conformable materials enabling seamless integration into wearable devices and the tactile array configuration,” says Dr. Jae Son of Pressure Profile Systems (PPS). “The sensors are directly measuring pressure, not trying to infer it by optical or electrical properties.”
To build its tactile array sensors, PPS arranges the electrodes as orthogonal, overlapping strips. A distinct capacitor is formed at each point where the electrodes overlap. By selectively scanning a single row and column, the capacitance at that location, and thus the localized pressure, is measured. With this approach, a PPS tactile array can feature up to 8,192 integrated sensing elements while measuring pressures as low as 0.01 psi.
According to Dr. Son, another promising application for capacitive tactile sensors is arterial line placement, a common, but difficult, procedure performed in a clinical setting.
An arterial line is a thin catheter inserted into an artery. It is most commonly used in intensive care medicine and anesthesia to monitor blood pressure continuously and to obtain samples for blood gas analysis. This type of intra-arterial measurement is more accurate than noninvasive alternatives.
Locating the artery can prove difficult, even for trained clinicians. The mean diameter of the radial artery is only about 2.3 mm in adults. A weak pulse can make it even more difficult.
The device uses PPS tactile sensor arrays in a conformable material worn over the clinician’s index and middle fingers. The PPS sensor identifies the location of the pulse, and indicates the location using LEDs and a needle guide to facilitate needle insertion. The device also protects the clinician’s fingers against needle-stick injuries.
Today, the holy grail of blood pressure monitoring is for consumer wearables such as smartwatches and fitness bands. Currently, most are limited to measuring heart rate, but many are already developing next generation devices with wristbands capable of taking blood pressure, pulse and other key arterial measurements.