Optical Sensors are the Key to Health-Monitoring Wearables—And Size is Critical

July 11, 2020 by Jake Hertz

Optical sensors have made it big in the medical device industry. And with wearables taking on increasing roles for health monitoring, size is paramount.

Since the 1840s, studies on fiber optics—and optical sensors in particular—have opened doors for sensing many properties, like light intensity, vibration, temperature, pressure, strain, liquid level, pH, chemical analysis, concentration, density, and many more.


Simplified optical sensor for object detection

Simplified optical sensor for object detection. Image used courtesy of Z. Kappassov et al.

Given this extreme versatility, it’s only natural that optical sensors have made their way into many different fields—from communications to petrochemical analysis. But this technology is particularly useful in the medical industry, helping researchers better study viruses, toxins, tumor biomarkers, tumor cells, antibodies, and drugs

What about this device, along with its place in a larger biosensor system, makes it so practical in such healthcare scenarios? 


Optical Biosensors

According to researchers Dambrosky et al., an optical biosensor refers to a small device that includes both a sensor for biorecognition and an optical transducer system. This device generates a signal that is proportionate to a measured substance's concentration. 

One field that has particularly benefited from the adoption of optical sensor technology is that of wearable health and fitness. 

Optical biosensors have been embraced by the healthcare industry because they offer many advantages over conventional analytical techniques.

For instance, this technology has appeared in applications including glucose sensing, laminate cure analysis, protein analysis, dosage form analysis, and many more. 


Example of optical sensor detecting antibodies

Example of optical sensor detecting antibodies. Image used courtesy of Dambrosky et al. 


Using these sensors, medical device manufacturers are able to offer real-time, label-free detection of biological and chemical substances. Label-free detection entails that a "detected signal is generated directly by the interaction of the analyzed material with the transducer." 

Furthermore, designers benefit from the high sensitivity, small size, and low price offered by optical technologies.


A Recent Example of an Optical Sensor

This week, Maxim Integrated announced the MAXM96146, which they claim is the world's thinnest optical sensor solution for health and fitness products. This device can illustrate some of the general concepts of optical biosensors in a concrete way. 

This solution, like many comparable products, is targeted toward the fitness wearable market, coming preinstalled with algorithms for activity classification, heart rate monitoring, and SpO2 monitoring. Maxim calls it a “drop-in” solution that can save up to six months to market for designers and manufacturers. 


Patient using a smartwatch including Maxim's new optical sensor for biometric monitoring

Patient using a smartwatch including Maxim's new optical sensor for biometric monitoring. Image (modified) used courtesy of Maxim Integrated

The company also claims that a complete system can be achieved using the MAXM86146 along with the simple addition of an LED and an accelerometer.


What Is an Optical Sensor's Place in a Medical Device? 

The new solution, the MAXM86416, combines Maxim’s optical biosensing analog front end, an Arm MCU, and two high sensitivity photodiodes. Aiming to build a comprehensive solution, Maxim designed the chip to include an SPI interface, two independent 19-bit ADCs, and a proprietary ambient light-cancellation circuit. 

For battery-powered wearables, low power is crucial to the design. From the datasheet, it seems this goal was achieved with the MCU sinking 4.2 μA in deep sleep, and the front end consuming 10 μA at 25sps. The majority of power seems to be used in driving the IR LEDs, requiring a typical 132 mA. The solution also includes special power management blocks meant to optimize battery life. 


Block diagram of MAXM86416

Block diagram of MAXM86416. Image used courtesy of Maxim Integrated


Amongst all these features, the real highlight of the solution is its size, measuring at 4.5 mm x 4.1 mm x 0.88 mm in a 38-pin OLGA package. Maxim claims this is 45 percent thinner than a discrete approach.

Size is an especially critical factor in medical devices, especially in smartwatches, which are increasingly designed with biometric monitors.  


For Healthcare and Beyond

Wearables in the field of personal health and fitness are undoubtedly growing in popularity. An increase in demand calls for better and more versatile optical biosensors—like those that are smaller and more power-efficient. 

Have you worked with optical sensors for applications beyond healthcare? Share your experiences in the comments below.