June 14, 2018
|By: Ian Chen
Executive Director, Sensors, Industrial & Healthcare Business Unit; Maxim Integrated
Biosensors inside smartwatches, fitness trackers, and similar wearables are becoming increasingly accurate in monitoring various health parameters. At the same time, they're also consuming less power, which is perfect for the small form factors of these applications.
The most common type of biosensor is the optical sensor, whose versatility makes it suitable for a wide range of applications. Coherent or non-coherent light interacts with the matter that it passes through; it is absorbed, reflected, scattered, dispersed, or otherwise altered. Once the light pulses have passed through living tissue, experts can study the magnitude, shape, or spectrum of these pulses to derive information about the analytes in the media.
Optical heart-rate monitoring provides a great example of how optical sensing works. As blood flows through your body, the cardiovascular pulse wave that moves from the heart and propagates through your body periodically distends the arteries and arterioles in the subcutaneous tissue. Photoplethysmography (PPG) provides an optical measurement of the volumetric change of blood in tissue as a result of the cardiac cycle. The process uses a light to interrogate the piece of tissue. The light received through the tissue corresponds with the variation of the blood volume.
Smartwatches may just be the start of a wave of innovative healthcare wearables that take advantage of increasingly accurate optical sensors for monitoring parameters such as heart rate.
Heart-rate monitoring devices typically use CAD models of tissue and the optical heart-rate monitor integrated together in optical design software. The rays of light are traced from each LED and detected at the photodiode. Differences in the optical properties of skin will influence the magnitude and quality of the PPG signal detected. Signal-to-noise ratio (SNR), ambient light cancellation, power consumption, and motion compensation are other challenges to address in these applications. Heart-rate monitoring algorithms typically require an SNR that is higher than 10dB in order to generate the most accurate results.
I'll talk more about optical sensing technologies—including the latest in advanced PPG ICs and the opportunities for healthcare—during my session at the Sensors Expo & Conference. My talk takes place from 1:30-2:20pm on Wednesday, June 27, in the MEMS & Sensors track at the McEnery Convention Center in San Jose, California. Maxim will also showcase a variety of demos at Booth 1035:
Also, Kionix, a global MEMS inertial sensor manufacturer, plans to showcase our wearable wristband solution in their booth at the conference. This solution is based on the MAX86140 optical pulse oximeter and heart-rate sensor. The evaluation kit for the MAX86140 features a Kionix accelerometer.
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