Fingertip sweat used to power wearable devices

The team claims their device is “the most efficient on-body energy harvester ever invented” as it has been shown to produce 300 millijoules (mJ) of energy per square centimetre without any mechanical energy input during a 10-hour sleep, as well as an additional 30mJ of energy with a single press of a finger. They believe it could represent “a significant step forward” for self-sustainable wearable electronics.

“Normally, you want maximum return on investment in energy. You don’t want to expend a lot of energy through exercise to get only a little energy back,” said Joseph Wang, the paper’s senior author.

“We wanted to create a device adapted to daily activity that requires almost no energy investment: you can completely forget about the device and go to sleep or do desk work like typing, yet still continue to generate energy. You can call it ‘power from doing nothing.’”

Previous sweat-based energy devices required intense exercise, such as a great deal of running or biking, before the user sweated enough to activate power generation. However, the large amount of energy consumed during exercise can easily cancel out the energy produced, often resulting in energy return on investment of less than 1 per cent.

Instead of relying on external, irregular sources like sunlight or movement, all the device needs is finger contact to collect more than 300mJ of energy during sleep which should be enough to power some small wearable electronics.

The fingertips were chosen as the source of this sweat because they have the highest concentration of sweat glands compared to anywhere else on the body.

“Generating more sweat at the fingers probably evolved to help us better grip things,” said first co-author Lu Yin. “Sweat rates on the finger can reach as high as a few microliters per square centimetre per minute. This is significant compared to other locations on the body, where sweat rates are maybe two or three orders of magnitude smaller.”

The device the researchers developed in this study is a type of energy harvester called a biofuel cell (BFC) and is powered by lactate, a dissolved compound in sweat.

“The size of the device is about 1cm squared. Its material is flexible as well, so you don’t need to worry about it being too rigid or feeling weird. You can comfortably wear it for an extended period of time,” Yin said.

Within the device, a series of electrochemical reactions occur. The cells are equipped with a bioenzyme on the anode that oxidises, or removes electrons from, the lactate; the cathode is deposited with a small amount of platinum to catalyse a reduction reaction that takes the electron to turn oxygen into water.

Once this happens, electrons flow from the lactate through the circuit, creating a current of electricity. This process occurs spontaneously: as long as there is lactate, no additional energy is needed to kickstart the process.

Piezoelectric generators, which convert mechanical energy into electricity, are also attached to the device to harvest up to 20 per cent additional energy.

The researchers were able to use the device to power effective vitamin C and sodium-sensing systems. They are optimistic about improving the device to have even greater abilities in the future, which might make it suitable for health and wellness applications such as glucose meters for people with diabetes.