How to Get Accurate In-Ear Heart-Rate and SpO2 Monitoring

October 29, 2019

Jim Harrison  By: Jim Harrison
 Guest Blogger, Lincoln Technology Communications 


Sensor use is growing by leaps and bounds. Sensors are found in almost every industry, with especially high usage in consumer electronics, cars, smart homes, and medical applications. Today's smartphones incorporate at least five sensors. New vehicles now have multiple optical sensors for safety and parking, plus seven different types of sensors on a typical engine alone.

The total global sensor market was valued at $139B in 2017, and is projected to reach $287B by 2025, growing at a CAGR of 9.5%. The medical sensors market alone was valued at around $11.7B in 2018, according to Zion Market Research, and is expected to reach approximately $19.8B by 2025, at a CAGR of about 7.80%. There are wearable electroencephalography sensor technologies for brain-to-computer interfacing. There are also implanted sensors. An example is an image sensor for the visually impaired. And there are many research efforts in this area. One of many examples is wireless, battery-free, biodegradable blood-flow sensor developed by Stanford University engineers and medical researchers.

Health-monitoring sensors are perhaps the fastest growing area. Some say that the adoption of sensors for new and wonderful wearable medical devices is being driven by our aging population. But, everyone will benefit from these new technologies, and the younger crowd is quickly jumping on board. New and improved sensor technologies are being designed into smart watches and many medical wearable devices. They are also going into non-portable monitors that are used in the home, as well as apparatus used in medical facilities.

Designing sensor-based wearables that deliver the high accuracy needed for them to be considered valuable tools in healthcare is no simple feat. There are many considerations to address, from the placement of the monitoring devices to blood perfusion levels. For more details on the challenges of obtaining the accurate heart-rate and blood-oxygen saturation (SpO2) measurements that provide insights into health and well-being, read the application note, "Track Your Heart Rate While You Listen to Your Favorite Track."

Optical Module for In-Ear Applications

SpO2 is an estimate of the amount of oxygen in the blood. A blood-oxygen test is used to check how well a person's lungs are working. Oxygen level in the blood is also the most reliable indicator of a drug overdose. Pulse oximetry is of particular value with newborns, where the babies do not thrive with inadequate oxygenation, but too much oxygen and fluctuations in oxygen concentration can lead to vision impairment or blindness. Also, hospitals are using accurate SpO2 measurement during anesthesia and postanesthesia care.

Pulse oximeters work by alternately flashing a red LED (600-750nm wavelength) and an infrared LED (850-1000nm wavelength) into a body part. A photodiode sensor is then used to measure the absorption of the red light for oxygenated hemoglobin (HbO2) and of the infrared light for deoxygenated hemoglobin (Hb). Hb absorbs more (and reflects less) red light. HbO2 absorbs more infrared light.

Blood-oxygen saturation can be determined by comparing the values of Hb and HbO2. Transmissive oximetry measures the light transmitted through tissue (usually a finger) and reflectance oximetry (as used by this device) measures the light reflected by tissue (often the wrist or the ear).

The MAXM86161 is a very low-power, single-supply, highly integrated optical module for heart rate and SpO2 measurement. Optimized for in-ear applications, the IC comes in a tiny 0.169 x 0.114 inch windowed package with red, IR, and green LEDs controlled by three low-noise 8-bit DACs, and a high-efficiency PIN photo-diode connected to a transimpedance amplifier and 19-bit A/D converter. Heart rate is measured with the single green LED.

The transimpedance amplifier of the MAXM86161 converts the few micro amps of current from the photodiode to a few millivolt signal. This signal then goes through a high-pass filter to reduce background-light interference, and then to the A/D.

Block Diagram of the MAXM86161Figure 1. Block Diagram of the MAXM86161.

The IC operates from a 3.0V to 5.5V VLED single supply over -40° to 85°C. It supports a standard I2C interface and fully autonomous operation and has a 128-word built-in FIFO. The MAXM86161EVSYS evaluation kit is available for $150.

Watch this video for an introduction to the MAXM86161.

50% Less Power for Temperature Measurement

Engineers designing the next generation of wearable health and fitness applications need high accuracy along with very low power for applications such as wrist-band wearables. The wrist is not an ideal location for clinical monitoring of body temperature, but gives a good relative and daily trend indication. Accuracy is still very important here.

In hospitals, having an accurate, integrated temperature sensor means no more sticking a thermometer in the patient's mouth once an hour. Now there is a continuous graph reading of multiple body temperatures at the nurses' station with better accuracy as well as a more comfortable experience for the patient.

Temperature changes for an extended period may indicate a common fever caused by infection, while a sudden drop in body temperature in a post-surgery patient may be an indication of a life-threatening condition. Hence, the ability to quickly and accurately measure body temperature is very important to physicians. Recent research has indicated that localized variations in skin temperature may be a marker for disease, including malignant tumors. Multiple temperature monitors on the body can be an important diagnostic tool.

A design using the MAX30208 digital temperature sensor IC can reduce the power required for temperature measurement by 50%. This chip is one of those 'genius' devices with very high accuracy (±0.1°C), operation from a single supply (1.7V to 3.6V), low cost, and current draw of just 60µA during measurement and 0.5µA in shutdown. What more can one ask for?

Block Diagram of the MAX30208Figure 2.Block Diagram of the MAX30208.

The IC allows for multiple devices to be addressed on the single bus. It has a I2C serial interface with a 32-word FIFO buffer for temperature data and comes in a very small 2mm x 2mm x 0.75mm, 10-pin thin LGA package. A unique ROM ID allows the device to be NIST traceable. In addition to the FIFO, the memory mapped registers contain high and low alarm trigger registers and a sensor setup register.

The MAX30208 10-pin thin LGA packageFigure 3. The MAX30208 10-pin thin LGA package.

Instead of using a thermal pad on the underside of the package, temperature is measured at the top of the package, as far away as possible from its pins, minimizing the potential for parasitic self-heating. The MAX30208 is available now. The MAX30208EVSYS# evaluation kit is available for $56.

Watch this video for an introduction of the MAX30208.

Conclusion

Engineers designing medical devices have a leg up now thanks to advanced sensor technologies. With these technologies, designers are delivering a new wave of health-monitoring wearables that can enable a shift toward more proactive, preventive care.

To learn more, read the application note, "Guidelines for the Opto-Mechanical Integration of Heart-Rate Monitors in Wearable Earbud Devices."