Blood-oxygen saturation (SpO2), an indicator of oxygen levels in capillary blood, provides a wealth of insights into our well-being. Coupled with other vital signs, SpO2 offers clues into areas including heart health, sleep patterns, and respiratory function. Normal levels of blood oxygen fall in the 95% to 100% range, while a measurement of 90% or less warrants a consultation with a doctor.
In the past, we'd have to visit a medical facility to have our SpO2 checked. Thanks to wearable devices with biosensors, we can now continuously monitor this and other vital signs. Continuous monitoring, in turn, alerts us—and medical professionals—to patterns and potential conditions that may otherwise go undetected.
Pulse oximetry is the technique used to measure blood-oxygen levels noninvasively. It is based on the modulation of transmitted light by the absorption of pulsatile arterial blood as well as different absorption characteristics of oxygenated hemoglobin and deoxygenated hemoglobin. When pulsatile arterial blood absorbs and modulates incident light passing through the tissue, it forms the photoplethysmographic (PPG) signal. PPG represents an optical measurement of the volumetric change of blood in tissue stemming from the cardiac cycle. Typically, LEDs provide the light source on the transmit path and photodiodes are on the receive path to collect the light that refracts and reflects off of the blood flow. Algorithms turn this data into valuable insights by taking the raw PPG signal and converting it into an SpO2 number.
Measuring SpO2 on the wrist, however, is much more challenging than doing so on the finger due to the low blood perfusion in the wrist area. The system-level design effort is quite challenging. The design must consider everything from spacing of the LEDs and photodiodes to biological factors such as skin tone and the presence of tattoos to signal-to-noise requirements. With wrist-based pulse oximetry, the photodiode and the LED are typically placed on the same side, with the photodiode collecting light reflected from various depths under the skin. Measuring SpO2 usually requires two LEDs with different wavelengths. To achieve the best PPG signal, the LED illumination wavelength should be near the absorption peaks of oxygenated hemoglobin. Developing the sophisticated algorithms that are needed to turn the data into actionable insights is also a difficult task requiring specialized expertise.
Complete Reference Design for Wrist-Based SpO2 Solutions
To streamline the development of accurate, wrist-based health-monitoring wearables, Maxim has unveiled the market's first complete wrist-based, system-level design for solutions that continuously monitor SpO2 as well as heart rate and heart-rate variability (HRV). MAXREFDES103 can reduce development costs and shave up to six months off the development cycle by eliminating some key barriers to entry into this market: opto-mechanical system design and development of high-performance, validated algorithms. With the combination of SpO2 and HRV, engineers can now deliver meaningful insights for the fitness and wellness markets, including applications such as sleep quality, sleep apnea detection, stress, calories burned, muscle oxygen (VO2), recovery time, and other new use cases.
MAXREFDES103 makes it fast and easy to evaluate hardware and embedded firmware for heart-rate and SpO2 monitoring on a wrist-based platform.
The reference design is available in a wrist-worn wearable form factor and delivers FDA-grade SpO2 algorithms (ARMS of <3.5%), sensors, optical design hardware, software design files, and, more importantly, access to raw data. Thanks to its algorithms, it can cover corner cases involving very low perfusion (perfusion index as low as 0.05% vs. 0.1% for competition). MAXREFDES103 features the MAX32664C biometric sensor hub with embedded algorithm for heart rate and SpO2. The MAX32664C processes PPG signals from the MAX86141 analog front-end (AFE) sensor. Algorithm output and raw data can be streamed through Bluetooth to an Android app or a PC GUI for demonstration, evaluation, and customized development. Design files, firmware, and software are available from the MAXREFDES103 page.
You'll have a chance to see a demo of the MAXREFDES103 at the upcoming embedded world 2020 Exhibition & Conference. Maxim will be at booth #4A-6-6 with a full slate of activities. In addition to our MAXREFDES103 demo, we'll also have demos of other healthcare as well as analog, automotive, industrial, and security solutions. Several of our technical experts will participate in speaker sessions, too.
With MAXREFDES103, you've got a path for fast prototyping and development of your health monitoring wearable solution.
Guidelines for SpO2 Measurement Using the Maxim MAX32664 Sensor Hub ›