In traditional Chinese medicine, a doctor assesses different facets of a patient’s health by taking pulses at the patient’s wrists. In our modern world, we’ve got fitness trackers, smart watches, and similar wearable health devices doing just that—continuously monitoring our pulse to derive an array of biometric insights. Thanks to continued innovation and integration, the biosensors inside these devices are becoming increasingly accurate while consuming less power—ideal characteristics for wearable health applications.
Optical sensors represent the most common type of biosensor. The versatility of optical sensing makes it suitable for a wide range of applications. As light, whether coherent or non-coherent, passes through matter, it interacts with the matter, becoming absorbed, reflected, scattered, dispersed, or otherwise altered. After the light pulses have passed through living tissue, we can study the magnitude, shape, or spectrum of these pulses to derive information about the analytes in the media.
As an example of how optical sensing works, let’s consider optical heart-rate monitoring. Photoplethysmography (PPG) is an optical measurement of the volumetric change of blood in tissue because of the cardiac cycle. 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. PPG uses a light to interrogate the piece of tissue, and light received through the tissue corresponds with the variation of the blood volume.
From an optical design standpoint, heart-rate monitoring devices typically utilize 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. Other challenges to address in these designs are the signal-to-noise ratio (SNR), ambient light cancellation, power consumption, and motion compensation. For the most accurate results, heart-rate monitoring algorithms typically need an SNR of greater than 10dB.
Advanced PPG ICs have sophisticated algorithms and signal processing techniques that overcome the challenges that can hamper the ability to generate accurate biometric readings. I’ll talk more about optical sensing technologies during my session at the upcoming Sensors Expo & Conference. Come by for a listen from 1:30-2:20pm on Wednesday, June 27, in the MEMS & Sensors track at the McEnery Convention Center in San Jose, California.
Meantime, I encourage you to imagine the possibilities for healthcare beyond wearable health monitors to the goal of personal wellbeing. After all, sensors are tucked inside more of the everyday devices in our homes, and sensor data is truly useful when the information derived is actionable. What if, for example, we paired biometric data with information gleaned from our surroundings? This knowledge could impact where and when we exercise, what kinds of adjustments we need to make to get a better night’s sleep, and so on. There are, indeed, some promising new capabilities ahead in the world of healthcare.
Be sure to attend Ian Chen’s session, “Emerging Healthcare Applications for Optical Biosensing” at Sensors Expo & Conference in San Jose, CA on Wednesday, June 27, 2018 at 1:30 PM. Attendees will receive an overview on using reflectometry for a pulse plethysmograph (PPG) waveform and details about the physical and physiological principals at work. They will also learn about current capabilities of wearable biosensors and the future direction of optical biosensing applications.
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