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.
Arterial Line Locators
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 much more accurate than noninvasive alternatives.
Arterial lines are typically inserted by respiratory therapists, and sometimes by physicians, phlebotomists, anesthesiologist assistants, and nurse anesthetists. The catheter is usually inserted into the radial artery in the wrist, but can also be inserted in other arteries in the body.
Overall, arterial line insertion is considered safe, with a rate of major complications below 1 percent. However, insertion of an arterial catheter is an invasive procedure and complications can occur. 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. To insert the arterial line, the clinician typically uses the left hand to detect and feel for the pulsating artery, then inserts the needle and extracts a blood sample using the right hand.
To hit the artery, the clinician may need to insert the needle multiple times, which can be painful and uncomfortable to the patient. Because the artery is innervated, the patient sometimes jumps or twitches when the needle is inserted leading to clinicians’ needle-stick injuries. This is, unfortunately, relatively common and can be a transmission path for blood-borne infections such as Hepatitis and HIV.
In fact, an accidental needle-stick of a physician in the United Kingdom ultimately became the stimulus for a grant from the Scottish government to seek a safer, more reliable method of inserting arterial catheters. Through a partnership with the University of Strathclyde in Glasgow, Scotland, the company set out to develop a device to simplify arterial localization and make it safer.
For this particular project, the goal was to design a low-cost, portable sensing system capable of locating an artery accurately to within 0.5 mm. The device needed to enable a one-shot procedure by relatively untrained staff that was significantly safer than existing methods.
The device uses tactile sensor arrays in a conformable material that is worn over the clinician's index and middle fingers. The tactile 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.
OSHA regulations require all compliance officers in hospitals to perform annual assessment of new personal protective equipment. Not only does this device provide a more effective means to perform artery punctures, the newly developed device offers an ideal solution for eliminating needle-stick injuries.
Blood Pressure and Consumable Wearables: Smart Watches and Fitness Bands
Today, the holy grail of blood pressure monitoring is to be integrated into consumer wearables such as smart watches 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.
Seoul, Korea-based Kairos Watches, for example, has developed several products designed to deliver high-tech elements such as text messages, push alerts, and apps to those that otherwise still want to own and wear a traditional high-end Swiss analog watch. The options include a transparent display that fits over the lens of an analog watch, as well as a Bluetooth-enabled wristband with integrated display and touch sensors, called the T-Band.
More advanced models of the T-band include a nine-axis gyroscope, accelerometer, compass, optical sensors, and a galvanic skin sensor that detects skin temperature and sweat. The company is currently working to integrate capacitive tactile pressure sensors into the slim form factor of the T-band for daily monitoring of pulse and blood pressure.
Chinese Pulse Medicine
In addition to being embedded in watch-like monitoring devices, tactile pressure sensors for pulse pressure measurement also lend themselves to technologically enhancing traditional Chinese medicine.
Chinese pulse diagnosis has been used for thousands of years as one of the primary diagnostic tools in Chinese medicine. Even with the advent of x-rays and ultrasounds, the practice continues to be paramount in the recognition of disease patterns for treatment with Chinese medicine.
Pulse diagnosis is used to isolate the malfunctioning organ or system that is causing the symptoms to identify the cause. This is observed in the pulse, because as the function of the organ changes, it will also produce changes in the artery due to causes such as inflammation, volume of blood, or quality of blood.
Traditionally, Chinese practitioners use three fingers to take a person's pulse, and factors such as pulse velocity and width are considered. Reliable diagnosis requires an experienced practitioner to differentiate the nuances of pressure. This is a particularly challenging and often subjective procedure that can lead to misdiagnosis by less-experienced physicians. PPS is currently working with medical device manufacturers to develop clinical systems that would help make Chinese pulse medicine more quantitative. Many Chinese companies are also using sensors to develop applications specifically for Chinese pulse wave analysis — not only to assess health, but also to monitor the effects of herbal treatments.
Capacitive tactile pressure sensing technology is enabling a new wave of healthcare products based on noninvasive arterial pressure measurement. This new technology can measure pressure in a way that is different from traditional methods. It will revolutionize how easily people can take blood pressure.
This article was written by Jae S. Son, PhD, founder and CEO of Pressure Profile Systems, Los Angeles, CA. For more information, Click Here . A video demonstrating capacitive sensing for single point and distributed areas is available at here .