July 13 (UPI) -- Even the best air conditioning units are struggling to keep up with the heat waves that are melting much of the Western United States.
For those waking up to sweaty sheets, there's some good news.
While a new energy-harvesting device developed by scientists at the University of California, San Diego, won't make wearers any cooler, it could help them lower their energy bills and shrink their carbon footprint.
The device, described Tuesday in the journal Joule, harvests energy from the sweaty fingertips of wearers as they sleep.
Previous technologies designed to harvest energy from sweat were rather inefficient.
They required wearers to exercise vigorously, running or biking for long periods of time -- meaning wearers had to perform a lot of work to get just a little energy.
"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," senior author Joseph Wang said in a press release.
"But here, 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,'" said Wang, a professor of nanoengineering at the UCSD.
Engineers targeted the fingertips because of their high concentration sweat glands.
Scientists estimate humans evolved sweat glands in their hands and fingers to help them grip objects such as tools.
"Sweat rates on the finger can reach as high as a few microliters per square centimeter per minute," said co-author Lu Yin.
"This is significant compared to other locations on the body, where sweat rates are maybe two or three orders of magnitude smaller," said Yin, a nanoengineering doctoral student working in Wang's lab.
The new device relies on a biofuel cell, which is powered by lactate, a dissolved compound found in sweat.
The cell is surrounded by a carbon nanotube foam coated with hydrogel to maximize sweat absorption.
Inside the cell, an anode featuring bioenzyme oxidizes the lactate. As the electrons taken from the lactate flow toward the platinum cathode, they are used to trigger a reduction reaction that converts oxygen into water.
This reactions completes the circuit, returning the electrons to the lactate and generating an electric current.
Scientists attached piezoelectric generators, which convert mechanical energy to electricity, to the device to further boost its power-generating capabilities.
During sleep, the device can capture as much as 300 millijoules during a single night, enough to power a small electronic device.
During lab tests, researchers used the device to power a vitamin C- and sodium-sensing system.
Scientists hope to boost the technology's efficiency so that it can be used to power a variety of wearable electronics and biosensors.
"We want to make this device more tightly integrated in wearable forms, like gloves," Yin said. "We're also exploring the possibility of enabling wireless connection to mobile devices for extended continuous sensing."