SmartPhone Blood Pressure

Smartphone based blood pressure measurement

This research was published in Science Translational Medicine in March 2018. This article is in the top 5% of all research outputs scored by Altmetric. A startup company, Digitouch, has the exclusive license to the IP. (Also, we developed a standalone iPhone application for blood pressure measurement. Check here for details: Link)

Introduction

This is a smartphone-encasing with custom sensors that allows blood pressure (BP) measurement from your fingertip. This device allows cuff-less BP measurement and displays the result on the smartphone screen when the user presses her fingertip against the device. High BP is a major risk factor for stroke and heart disease and presently, cuff- based devices are used as BP monitors. Since people may not have the means or interest in using these bulky devices, hypertension awareness and control rates are low. This inspired us to design a cuff-less smartphone accessory to measure user’s BP during their daily life. Cuff-less BP monitoring using a smartphone accessory is a viable solution for hypertension management, not just because of the processor capabilities of the phone, but also due to its widespread use and portability.

How it works ?

This device is a smartphone accessory that measures BP using the same principle as an automatic cuff device, in minimal time (~30 sec), and improved convenience. In an automatic cuff BP monitor, your upper arm is squeezed with an inflatable cuff to increase the external pressure over the artery under the cuff. In this smartphone device, the user presses her fingertip against the phone to increase the external pressure of the underlying artery without a cuff. Similar to a cuff BP monitor, the proposed device has a dedicated light and pressure sensor to measure arterial oscillations and applied pressure, respectively. After a successful measurement, the smartphone application implements an algorithm to compute BP using the aforementioned signals. If the signal quality is deemed inadequate, the application asks the user to try again rather than giving an erroneous BP measurement.

Design Process

This was a 3-year project that went through a series of prototype iterations that finally evolved into the present smartphone form factor as discussed above. Our first prototype, which was a proof-of-concept design, was a breadboard assembly with transducers, an IR light sensor and a pressure sensor array, to measure arterial oscillations and the applied finger pressure for oscillometric blood pressure measurement. Signals from the transducer were recorded using an NI DAQ and all analyses were done offline in Matlab. We learned that the fingertip, at about the level of the base of the finger nail, can be an effective position to measure BP accurately. We built a better prototype, which was physically more robust, to systematically investigate our findings on finger location. Using this modified device we collected more data from human subjects and compared the measured BP to those reported from other FDA approved BP monitoring devices. Results were positive, and this motivated us to design the next version of the device which had a form factor of a smartphone encasing. The encasing had a sensor unit to measure signals, a rechargeable power supply for all the embedded electronics, a micro-controller to digitize analog data, and Bluetooth circuit to transfer data from the sensor-unit to the application running on the phone. Unfortunately, first version of the design was neither user-friendly nor robust and hence, we iterated a few design parameters like material type and component layouts to arrive at the present version of proposed device.

Other devices to measure Blood Pressure

Several non-invasive methods are available for monitoring BP. They all require a cuff to measure BP. Cuffs are not readily available and therefore do not permit ubiquitous BP monitoring. That is, people in low resource settings may not have any access to cuff-based devices; others must go out of their way to use these devices; and even people who own a device cannot carry it with them wherever they go. Other cuff-less devices are under investigation. However, most of these devices need regular calibrations with cuff BP measurements. The proposed smartphone based BP monitoring device, on the other hand, is a user-friendly, calibration-free, and cuff-less device that works on a time-honored principle of BP measurement.

Results

We prospectively tested the smartphone based device on multiple users, and compared the accuracy of our measurements to other FDA approved BP monitoring device. Our paper discusses the results in detail.