Most biometric wearables today have moved to using an optical methodology called photoplethysmography (PPG), which shines light into the body and measures the light that bounces back – here is what you need to know about Optical Heart Rate Monitoring (OHRM).
PPG is very difficult to properly and accurately measure, but it is probably the best technique that will be closest to the accuracy of the chest strap, a defacto standard in medical heart monitoring. A good high-quality PPG signal is absolutely critical to proper heart rate accuracy measurements as well as many other biometrics parameters that are important to monitor, such as breathing rate, cardiac efficiency, blood pressure, and other important bodily parameters. It is relatively easy to get heart rate for a stationary person, but you will not be able to accurately get to the more advanced metrics of a person on the move without some real creative algorithms and a very accurate measurement technique impervious to motion.
PPG has traditionally been used to measure blood flow changes and translate these changes to pulse rate. Existing products using PPG have been great for hospitals but far too large and uncomfortable for the average person to effectively monitor their vital signs. Size, ergonomics, weight and accuracy are especially important for measuring blood flow during exercise or in outdoor activity—Oh right—and there’s the need for accuracy and repeatability.
I had spoken previously in a conference call to Dr. Steven LeBoeuf, co-founder and President of Valencell, Inc., to understand this PPG technique. LeBoeuf, before Valencell, led the optoelectronic biosensor program at GE Global Research, where he managed the development and productization of biosensor systems and developed cutting-edge nanosensor technology. Before joining GE, LeBoeuf developed optoelectronic solid-state materials and devices while researching at North Carolina State University. LeBoeuf holds a Ph.D. in Electrical Engineering from N.C. State and a B.S. in Mathematics and Electrical Engineering from Louisiana Tech University.
I finally met him at Valencell’s booth at CES in Las Vegas this year (See Figures 1 and 5). LeBoeuf is a bright and innovative engineer and extremely passionate about his technology that can revolutionize the Health and Fitness market. We spoke for quite some time at his booth at CES amongst gymnastic fitness people, in various forms of strenuous exercise, were being continuously monitored (See Figures 2, 3 and 4) using Valencell’s patented PerformTek biometric signal extraction method.
Similar to the traditional PPG approach, PerformTek sensor technology measures weak blood flow signals by shining light at the skin with an optical emitter and sensing the scattered light with a photodetector. The profound difference is that the PerformTek signal extraction method takes it a step further, employing motion-tolerant PPG, which actively removes noise signals associated with motion artifacts (such as skin motion and footsteps) and environmental exposure (such as sunlight) from the photodetector signal using a method invented and patented by Valencell's R&D team. This technique results in a cleaned-up signal that contains more accurate information about blood flow.
Because blood flow modulates with heart rate and respiration rate, PerformTek-powered algorithms can accurately extract heart rate, RR-interval, respiration rate, and other blood flow parameters from the photodetector signal even during intense exercise.
The PerformTek is more accurate and consistent at estimating calories burned when compared with a pedometer because the algorithm for calories burned leverages both the optical and accelerometer signals, whereas pedometers are limited to the accelerometer signal only. It is a fact that you burn most of your calories by your body working hard in the process of keeping you alive, whether you're moving your feet or not. Pedometers can only measure footsteps, but since PerformTek-powered products can also measure what's going on inside your body (not just footsteps), products using this technique are much better at estimating true calories burned.
In order to achieve an accurate, reliable and scalable PPG signal, a design needs good Opto-mechanics and proper Signal extraction algorithms:
- Multiple form factors
- Multiple emitters
- Gross displacement
Signal Extraction Algorithms
- Power management
- Scalable biometric roadmap
And finally, Optical spacing distances are critical for accurate monitoring as well. As emitter/detector spacing increases, so does accuracy increase.
Valencell’s technology enables users to have wearables that have the capability to continuously and accurately measure low level weak blood flow (PPG) signals even while strenuous activity is being performed. In order to accurately extract, in real-time, the true PPG signal, a state-of-the-art technique is needed to actively remove optical noise due to skin and body motion, and even sunlight. This clean extracted can now enable more advanced biometrics to be accurately measured such as blood oxygen levels, blood pressure, heart rate variability, respiration rate, VO2 and other parameters as well for advanced health and fitness assessment. See Figure 9.
Armbands and wristbands
Armbands and wristbands work well with two-dimensional optics and green/yellow light sources work best.
Earbuds work well with three-dimensional optics and Infrared (IR) light sources are critical.
LeBoeuf stated that he actually began with the goal of “one sensor, one spot on the body” concept. He told me that the ear is the best place on the human body to measure all that is important because of its unique vascular structure to detect heart rate (HR) and respiration rate. Also, the tympanic membrane radiates body heat so that we are able to get accurate body temperature here.
He went on that the human head area is the best place to measure physical activity as determined by lab tests. Since everyone wears audio headsets and earbuds nowadays while, walking, exercising and jogging, if you put good audio, ergonomics and biometrics together then you have a winning combination for a pleasant experience combined with accurate health and fitness monitoring. Valencell’s algorithm can accurately track blood pressure in the ear as well.
Valencell specializes in integrated in-ear wireless hardware solutions for the rapidly expanding mobile fitness and health market. The company has recently licensed their PerformTek technology to BioConnected for the development of its HR-SENSE all-in-one advanced Bluetooth wireless biometric training earphones. These enable an end-user to monitor multiple personal biometrics such as: heart rate, distance, pace, speed, cadence, total steps, energy expenditure, net-fat burned, current time, time elapsed and elevation.
To gauge user activity level and to generate activity context, the PerformTek earbud sensor module leverages the accelerometer that is built in to the sensor module. The PerformTek signal extraction method applies a proprietary algorithm to the accelerometer signal to measure running, cycling and activity cadence and to estimate speed and distance traveled. An additional proprietary algorithm estimates VO2 and calories burned by processing both the accelerometer signal and blood flow signal.
A unique feature of the BioConnected HR-SENSE is the ability to assess and monitor Heart Fitness Age – a critical factor in determining overall body fitness. In addition to the biometric measurement capability and the highly intuitive BioConnected App, the BioConnected HR-SENSE Bluetooth wireless earphones deliver music and allow phone calls to be made and received during exercise sessions.
I expect to see many more such innovations come from this technology in the future.
What do you think about this technique and application? Can you imagine or conceive any other uses for this solution?