Self-powered heat sensor could help keep firefighters safe

Firefighters focused on putting out blazes and saving lives may not realize when their own protective clothing is getting damaged by the flames surrounding them. Now researchers have made a wearable, motion-powered sensor that can quickly warn the wearer of dangerously high heat (ACS Nano 2022, DOI: 10.1021/acsnano.1c10144). With a quick press, the fabric-based device could also be used to transmit the firefighter’s location in real-time to a fire station.

The textiles in firefighter suits—usually made of aromatic polyamide, or aramid, fibers—can withstand temperatures up to about 400 °C before they start breaking down. Researchers have made wearable electronic heat sensors with graphene oxide, but those sensors are rigid and difficult to integrate into protective textiles. Also, power sources for these sensors are easily damaged by high temperatures, says Jinfeng Wang of Wuhan Textile University.

To get around these issues, Wang and colleagues decided to make a flexible, self-powered fire alarm fabric based on calcium alginate–based aerogel fibers powered by a triboelectric nanogenerator (TENG). TENGs harvest mechanical energy from small movements and convert it into electrical energy via the triboelectric effect, the buildup of charge between two materials when they rub against each other and separate.

The researchers first incorporated iron oxide nanoparticles into the calcium alginate fibers and sprayed them with silver nanowires. The ultralight fibers become conductive at temperatures above 100 °C, so when a textile made from the fibers is exposed to a flame, it can complete an electrical circuit.

To make the TENG, the team put the alginate fabric next to a cotton fabric soaked in polytetrafluoroethylene. When the two materials rub together, opposite charges build up on the two surfaces, creating a small voltage of 3–10 V. The voltage that the device produced from harvesting the energy of wrist and elbow movements was enough to power a light that, if placed on a firefighter’s suit, could warn of elevated temperatures.

The researchers also demonstrated a system in which the TENG was connected to a small microchip that can transmit a location signal. In practice, a trapped firefighter could send a distress signal to a nearby fire station by pressing the TENG repeatedly, generating enough triboelectricity to power the chip.

“This fabrication strategy combines light weight and electrical properties, which are useful for sensing applications in wearable technology,” says Ren Cai, a chemist at Hunan University. However, the low mechanical strength of the alginate fibers might make it difficult to produce the textile on a large scale, he says.

Also, alginate fibers aren’t known to be as heat resistant as the aramid fibers used in firefighter suits, says Trisha Andrew, a chemist at University of Massachusetts Amherst.

Wang agrees and says that alginate, which is extracted from seaweed, is a natural, biodegradable, and inherently flame-retardant material, but it does not withstand temperatures past 500 °C well. The team is now working on making a self-powered fire alarm textile using aramid fibers, “which would do the same job but with much improved thermal stability for practical use.”