Hypertension (high blood pressure) is the number one risk factor for premature death worldwide, affecting 70 million American adults (one out of three). Day-to-day blood pressure measurement is the best way to monitor and mitigate the risks of hypertension.
While there are many methods for performing blood pressure measurement, all but the most invasive, intraarterial techniques suffer from deficiencies that can lead to inaccurate or inconsistent results. Even the standard blood pressure cuff used by healthcare practitioners has significant limitations. These limitations essentially are derived from several issues: 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. Advancements will be realized in both a clinical setting, for applications such as improved artery location and pressure measurement, and in a consumer setting for next-generation wearables such as fitness bands and smart watches that will measure more than just heart rate.
Blood Pressure Monitoring
Blood pressure is defined as the pressure exerted by circulating blood upon the walls of blood vessels during a cardiac cycle. Blood pressure is usually expressed in terms of the systolic (maximum) pressure over diastolic (minimum) pressure and is measured in millimeters of mercury (mmHg).
For noninvasive, intermittent measurement of blood pressure, the gold standard is the blood pressure cuff, or sphygmomanometer. Using this device, a trained healthcare provider listens with a stethoscope for the Korotkoff (tapping) sounds as the cuff is gradually deflated to determine the systolic and diastolic pressure.
Although broadly accepted and widely used, studies have shown that manual blood pressure measurement can include errors as large as 10 mmHg for systolic and diastolic pressures. In particular, the procedure is sensitive to a clinician's hearing acuity and overall diligence while preforming the procedure.
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 patients or those with conditions resulting in an irregular pulse.
Pulse oximeters, which have been traditionally used to monitor blood oxygen saturation and pulse rate, are now being used to 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 both 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, noninvasive blood pressure measurement without cuffs by utilizing advanced capacitive tactile sensing technology.
Capacitive Tactile Sensors for Blood Pressure
Using capacitive tactile sensors, blood pressure can be measured using sophisticated arrays that map the pressure above the artery. This can range from a few discrete measurements to a large, dense array of hundreds of elements. These sensors, in direct proximity to the artery, deliver a detailed pulse waveform that is then used to determine blood pressure and pulse information, including other parameters such as arterial hardening.
One reason that capacitive tactile pressure sensing is so well suited for this task is that it can handle the extremely low pressures that need to be measured; blood pressure is so slight that it is measured in millimeters of mercury (mmHg), with 0.5 mmHg equaling roughly 0.01 psi. To conform to the contours of the human body and other curved surfaces, tactile pressure sensors are designed to 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. The sensors are directly measuring pressure, not trying to infer it by optical or electrical properties.
Pressure Profile Systems’ (PPS) sensor technology is used in medical devices such as the SureTouch Breast Exam, an FDA-cleared, painless, radiation-free, screening clinical breast examination that provides immediate results and is more sensitive than a clinician's touch. The technology is also used in the Manoscan, a 36-sensor catheter system that collects information about esophageal performance in much greater detail than conventional manometry.
To build its tactile array sensors, the company 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 tactile array can feature up to 8,192 integrated sensing elements while measuring pressures as low as 0.01 psi.