Film uses sweat to power wearables

  • January 5, 2021
  • Steve Rogerson
One application of the film is for shoe insoles where it changes colour from blue to pink after absorbing moisture, and can be reused more than 100 times.

Researchers from the National University of Singapore have created a film that helps evaporate sweat from skin and uses the moisture harvested to power wearable electronic devices such as watches and fitness trackers.

Sweating is a natural process for the body to reduce thermal stress.

“Sweat is mostly composed of water,” said research team leader Tan Swee Ching from the NUS materials science and engineering department. “When water is evaporated from the skin surface, it lowers the skin temperature and we feel cooler. In our new invention, we created a novel film that is extremely effective in evaporating sweat from our skin and then absorbing the moisture from sweat. We also take this one step further, by converting the moisture from sweat into energy that could be used to power small wearable devices.”

The main components of the thin film are two hygroscopic chemicals – cobalt chloride and ethanolamine. Besides being extremely moisture-absorbent, this film can rapidly release water when exposed to sunlight, and it can be regenerated and reused more than 100 times.

To make full use of the absorbed sweat, the NUS team has designed a wearable energy-harvesting device comprising eight electrochemical cells (ECs), using the film as the electrolyte. Each EC can generate about 0.57V of electricity upon absorbing moisture. The overall energy harvested by the device is sufficient to power an LED. This proof-of-concept illustrates the potential of battery-less wearables powered by human sweat.

Conventional hygroscopic materials such as zeolites and silica gels have low water uptake and bulk solid structures, making them unsuitable for absorbing moisture from sweat evaporation. In comparison, the new moisture-absorbing film takes in 15 times more moisture and does this six times faster than conventional materials.

In addition, the film changes colour upon absorbing moisture, from blue to purple, and finally pink. This feature can be used as an indicator of the degree of moisture absorption.

The NUS team packaged the film into breathable and waterproof polytetrafluoroethylene (PTFE) membranes, which are flexible and commonly used in clothing, and successfully demonstrated the application of the moisture-absorption film for underarm pads, shoe linings and shoe insoles.

“Underarm sweating is embarrassing and frustrating, and this condition contributes to the growth of bacteria and leads to unpleasant body odour,” said Tan Swee Ching. “Accumulation of perspiration in the shoes could give rise to health problems such as blisters, calluses and fungal infections. Using the underarm pad, shoe lining and shoe insole embedded with the moisture-absorbing film, the moisture from sweat evaporation is rapidly taken in, preventing an accumulation of sweat and provides a dry and cool microclimate for personal comfort.”

The NUS team now hopes to work with companies to incorporate the moisture-absorption film into consumer products.

“The prototype for the shoe insole was created using 3D printing,” said research team co-leader Ding Jun, also from the NUS materials science and engineering department. “The material used is a mixture of soft polymer and hard polymer, thus providing sufficient support and shock absorption.”

Tan Swee Ching (seated, left) and Ding Jun (seated, right) and their team from the NUS.