Wearable Sensors Are Making a Big Splash

Wearable sensors are often tasked with monitoring peoples’ actions or behaviors for a variety of applications. These applications include health monitoring, human-computer interaction, and gesture recognition, to name a few. While the raw data needed to fuel these use cases can be obtained with cameras or other advanced sensors, the large amounts of data that they produce require a lot of processing power to make sense of. That is far from ideal when working with a heavily resource-constrained wearable device with a tight power budget.

For this reason, simpler instruments, like strain sensors, are getting a lot of attention from developers. A high-precision strain sensor can even monitor the flow through tiny blood vessels, so it will have no problem handling nearly any application that involves measuring how something stretches, bends or moves. However, most existing strain sensors do not deal well with getting wet. This can cause a problem for any wearable if the user gets caught in a rainstorm. But beyond that, it also keeps these sensors from being used in underwater settings, or in implantable medical devices.

A novel amphibious strain sensor developed by a team at North Carolina State University is not limited in this way, however. Their flexible sensing system can move between the air and aqueous environments without causing any damage or impacting its performance. These qualities could make the sensors ideal for implantable and underwater applications, in particular, where few good options presently exist.

Existing solutions frequently use gel-based materials to waterproof the sensors. But these gels can swell in the water and dry out in the air, so they have limited useful lifespans. Some other devices are encapsulated as a means of waterproofing them. While this works, it also limits the flexibility of the sensor and severely impacts its sensitivity.

A key part of the researchers’ sensor is a network of silver nanowires embedded in a flexible material called polydimethylsiloxane. To make the sensor even more effective, small, sharp cuts are made in this material. These cuts guide the flow of electricity in a specific way that allows the sensor to detect when it is being stretched. When the sensor is stretched, the cracks in the material get bigger, which increases the resistance. This change in resistance is what allows the sensor to measure strain.

To protect the sensitive parts of the sensor and make it work well in wet environments, the entire structure is encapsulated in a material called thermoplastic polyurethane (TPU). TPU is very stretchy, easy to work with, and resistant to water, making it ideal for this kind of sensor. The encapsulation also helps the sensor maintain its sensitivity and durability, even after being stretched many times or exposed to water.

To showcase the capabilities of their sensing platform, the team built it into a number of devices. In one, a strain sensor was utilized to measure the blood pressure in a pig’s aorta. Another demonstration showed how the sensors can be built into gloves that allow scuba divers to communicate with each other via hand gestures.

The unique design of these sensors could enable sensing in many applications where it was previously impractical. Toward that goal, the researchers are presently working with partners in industry to incorporate their system into a variety of commercial applications.