Fitness trackers powered by sweat

by | Dec 14, 2023

The generator harnesses energy from water and is built with a fiberform material derived from the straps of disposable medical masks.
A man lifting weights.

Fitness trackers monitor your activity, sleep, and can even alert you to possible health problems. But one thing they can’t do is track your activities 24 hours a day, 365 days a year.

All fitness trackers have to be periodically taken off and charged, and they’re not always practical for activities involving long stretches without access to electricity, such as camping or backpacking trips.

A team of researchers at Xi’an Jiaotong University in Xi’an, China have developed a device that could provide a solution to this problem: A fiber-based, knittable, stretchable generator powered by human sweat. Their research was recently published in Small.

“Most […]devices [powered by moving water] are made of rigid or non-stretchable substrates and functional materials,” said Guoxi Luo, who led the study. This makes them impractical for use in wearable sensors, which need to be able to flex with the person wearing them.

Powered by perspiration

The device, known as a hydrovoltaic generator, was developed by Luo and his colleagues and is made from a fiberform material that contains a capillary network of tiny water diffusion channels. “We can integrate the prepared prototype with clothing to generate electricity using sweat produced during exercise,” said Luo.

When the material gets wet, liquid flows through the capillary network. Carbon-based coatings selectively attract the positive ions present in the wearer’s sweat, producing an electrical charge. “Meanwhile, the evaporation of sweat from the wet-dry interface maintains the capillary pressure difference required for liquid diffusion, realizing the continuation of power production,” said Luo.

The material can also take a beating. “[It] breaks the barrier of most current hydrovoltaic nanogenerators to stably work under large mechanical deformations, such as bending, twisting, and stretching,” the researchers wrote in their paper.

Stretching and releasing the material reverses the electrical output, which converts motion into quantifiable signals. This means the device is not only self-powered, it can also function as an activity tracker.

“Stretchable items are good for wearable sensors, especially those that sense deformation,” said Cindy Harnett, professor at the University of Louisville J.B. Speed School of Engineering, who was not involved in the study. “These are responsive materials that generate electrical energy when deformed, so they need to be able to move like clothing does.”

Sustainable materials

The generator is sustainable too; the fiber is made from the straps of disposable medical masks, like those that saw widespread use during the COVID-19 pandemic.

“According to WHO estimates, the number of masks used and discarded globally each month amounts to about 129 billion, most of which are disposable,” said Luo. “If they are buried in landfills, the mask straps prepared from synthetic fibers need decades or centuries to be naturally degraded, which will pollute the soil, air, and water, and seriously damage the ecological environment, so the disposal of discarded masks inevitably becomes a global issue.”

Luo said recycled straps are an ideal material for the technology. “The mask strap is a typically 1D fiber fabric made of spandex and polyester yarns combined into bundles on the micron to millimeter scale and woven into ropes by warp/weft knitting.”

Spandex and polyester composites, he said, have high water retention and can be easily flexed and stretched. The recycled straps, he added, can be cost-effectively sterilized with alcohol before being repurposed.

Harnett, however, said other materials would also work. “The use of mask strap material is interesting from a recycling perspective but not necessary,” she said.

Potential obstacles

The technology may also need to overcome commercial and marketing hurdles. Jason Heikenfeld, professor of Engineering and Applied Science at the University of Cincinnati, who was also not involved in the study, said similar technologies have been historically slow to take hold.

He compared the technology to the development of flexible electronic displays in the early 2000s, which have only recently been adopted by manufacturers.

“While research […] continues to advance what is possible for stretchable and self-powered wearables, commercially there has been almost no adoption,” he said. “Rigid watches, rigid patches such as the leading glucose monitors, all rely on batteries and do not require stretchable form factors, nor are companies or consumers clamoring for those attributes.”

Creating consumer demand for self-powered wearables is a significant challenge, but if the technology catches on, the researchers said it could represent a “sustainable, renewable, and clean energy” way to reduce reliance on environmentally unfriendly batteries as the sole power source for small, portable devices like fitness trackers. As these devices become more popular, this remains an important consideration.

Reference: Min Li, Libo Zhao, Kaifei Wang, et al., Highly Stretchable, Knittable, Wearable Fiberform Hydrovoltaic Generators Driven by Water Transpiration for Portable Self-Power Supply and Self-Powered Strain Sensor, Small (2023). DOI: 10.1002/smll.202306318

Feature image credit: Victor Freitas on Unsplash