Wearable Sensors Monitor Biometrics Underwater—and in Deep Tissue

August 29, 2021 by Ikimi .O

Health wearables are one of the most in-demand consumer electronics. Here's a roundup of a few recent sensor-level innovations.

This week, Fitbit announced its most recent iteration, the Fitbit Charge 5, which includes monitoring for heart rate, sleep, and fitness (among other metrics). With Fitbit and similar health wearables continually releasing new features, it's worthwhile to examine the circuit-level innovations driving these upgrades. 


Fitbit Charge 5

Fitbit Charge 5. Image used courtesy of Google

Several recent studies—including one on hand-drawn, water-resistant sensors and another on ultrasonic tissue monitoring—may shed some insights on the future of health wearables. Further, a recent patent from Apple for "hydration monitoring" in the Apple Watch indicates how some of these advances have already made their way into the industry. 


Submersible Sensors Take the Plunge

Researchers at Soongsil University, Korea, recently proposed a simple and cost-effective method for fabricating a water-resistant wearable sensor. The researchers used a hand-drawing method to sketch the sensor’s circuit—with no assistance from design software.

Most hand-drawn sensors convert deformation from force to electronic signals using piezoelectric, capacitive, or piezoresistive principles. In this study, the researchers relied on the piezoresistive principle using three layers: two electrode layers separated by a thin, low-conductive one. Variations in resistance allowed the system to send electronic signals to the sensors.

The resulting pressure sensor was built on a flexible pyralux film, a wet-tissue sensing layer coated with carbon nanotube inks (CNTs), and thin adhesive layers. These materials enabled the sensor to work underwater.


 Pyralux-integrated sensor working underwater

 Pyralux-integrated sensor working underwater. Image used courtesy of Taylor & Francis


The researchers claim their pyralux-integrated sensor is highly sensitive and thin (~0.26 mm), making it suitable for various applications, including human-machine learning. By integrating applied machine learning, the researchers were able to achieve up to 94 percent accuracy for underwater touch action. This research, they say, could create opportunities for electronics and complex motion recognition for divers. 


Ultrasonic Phased Arrays for Deep-tissue Monitoring

Recently, a study published in Nature investigated whether ultrasonic waves could be used to monitor deep-tissue functions for inpatient and outpatient settings. The researchers hoped that this technique would have high penetration depth and spatial resolution—requirements that the existing thermal, terahertz, optical, chemical, and radiofrequency sensing mechanisms lack. 


Ultrasonic phased arrays

Ultrasonic phased arrays penetrate tissue far deeper than other instruments. Image used courtesy of Nature Biomedical Engineering

The researchers noted that an ultrasonic wave's unfocused single-element transducer can only penetrate three to four centimeters, making it unreliable for deep-tissue monitoring. But when a phased-array technique was used, the energy was enhanced to improve penetration. The resulting beam was easily steered to improve the spatial resolution.

The researchers also incorporated elastic capabilities into the device, allowing natural integration and closeness with the human skin for long-term monitoring.


Diagram of an ultrasonic phased array patch

Diagram of an ultrasonic phased array patch and the layers of tissue it can monitor. Image used courtesy of Nature Biomedical Engineering

The study found that the phased-array technique offered better results than single-element transducers. By delivering a safe 0.47W cm-1 of ultrasonic energy to deep tissues (according to FDA’s standards), the device penetrates down to 14cm in human tissue. The researchers note that this finding might be used to monitor cardiac activity and central blood flow.


Apple Tracks Hydration Levels

Wearable sensor innovations aren't just happening in the research realm, however. Apple was recently granted a patent that tracks a user’s hydration level with the Apple Watch


Hydration measurement with Apple Watch

Hydration measurement with Apple Watch. Image used courtesy of Patently Apple


Apple’s new technology works by sampling the user’s perspiration via electrodes. High conductance in the perspiration indicates a high concentration of electrolytes, implying lower hydration levels. Conversely, low perspiration conductance indicates low electrolyte concentration and high hydration levels. Using this principle, the wearable sensor can efficiently measure and track a user's hydration throughout the day—and especially during physical exertion.



Size and user safety are often a top concern for wearable developers. If you've worked with wearable sensors in the past, what constraints did you have to work around? Share your experience in the comments below.