MEDICAL & PHARMACEUTICAL FEATURE
MONITORING BLOOD PRESSURE: the advanced tactile sensor revolution
Tactile sensors will play a key role in the next generation of consumer-wearable, clinical devices that measure blood pressure, says Pressure Profile Systems
H
ypertension, or high blood pressure, is the number one risk
factor for premature death worldwide, affecting one in four adults in the UK. Blood pressure measurement is one of the best ways of monitoring and mitigating the risks of hypertension. While there are many methods
for performing blood pressure measurement, 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 significant limitations. These limitations 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 realised 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, one of the most important measurements in medicine, 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 millimetres of mercury (mmHg). For non-invasive, 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. The procedure is particularly sensitive to physician hearing acuity as well as their overall diligence while performing 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 in the order of 10 mmHg and are particularly inaccurate on obese people 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 leading organisations such as the Association for the Advancement of Medical Instrumentation (AAMI) 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 utilising 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 number of 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
Capacitive tactile pressure sensing can handle the extremely low blood pressures that need to be measured
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. 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,” 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.” Sensor technology from PPS is
already used in medical devices such as the SureTouch Breast Exam, a Screening Clinical Breast Examination that provides immediate results and is more sensitive than a clinician’s touch, and the Manoscan, a 36-sensor catheter
INSTRUMENTATION | DECEMBER/JANUARY 2017
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