Researchers from China and South Korea have developed a water-repellent smart sensor that can monitor underwater movement and provide an alert when a swimmer could be drowning. Such underwater movement sensors have a range of potential applications, including the remote monitoring of heart rate and tracking the activity and safety of swimmers and in water sports.


With the technology of wearable strain sensors for motion monitoring generating much interest in research, from small signals for joint movement and pulse in medical diagnostics up to whole body sensing, underwater motion sensing has remained somewhat of a challenge as electronic devices can lose effectiveness in a wet environment.


In addition, many of the flexible fitness trackers currently on the market are not able to be submerged in water because the electronics involved make the devices thick and impermeable to air, which can lead to skin irritation. Previous research has shown that thin and even layers of polydimethylsiloxane (PDMS) do offer sufficient water repulsion to protect flexible movement sensors underwater, but the coating can be uncomfortable. Here, the team instead looked to investigate PDMS as a water-repellent coating for a fabric-based sensor as part of a wireless underwater movement detection system.


The study, reported in the journal ACS Nano [Zhu et al. ACS Nano (2022) DOI: 10.1021/acsnano.2c0]was carried out by a team from Fuzhou University, Wenzhou University and Nantong University in China and Yonsei University in South Korea. It involved applying a thin and slippery coating to conductive fabric to produce a breathable underwater movement sensor. The sensor was integrated into a smart device that wirelessly alerts a smartphone app when a swimmer stops moving and could be drowning.


To achieve this, a piece of polyester knit fabric was dipped into a graphene oxide solution and then into hydroiodic acid, the latter being a solution containing PDMS microparticles and nanoparticles. Initial testing demonstrated that the coated fabric was both conductive and water-repellent but remained permeable to air, making it more comfortable. When a sample of the coated fabric was attached to a finger that was then bent while underwater, it produced a measurable electrical response.


The team also tested the smart device by attaching it to a motorized swimming doll, with the app tracking the doll’s kicking legs. When its kicking motion was tuned off to simulate a swimmer with difficulties, the fabric-based strain sensor transmitted a red warning message to a smartphone app.


They now hope to develop a flexible smart drowning-alarming system, the current wireless signal collector and transmitter system based on a hard substrate, which will improve comfortability. In addition, they will explore its potential for multifunctional applications, such as the monitoring of sweat composition to indicate the wearer’s health, and further electronic components that could be integrated into the sensor.

The fabric-based strain sensor was successfully tested by tracking a doll’s kicking legs
The fabric-based strain sensor was successfully tested by tracking a doll’s kicking legs