Researchers have developed a bio-fuel cell in the form of a film that can extract energy from the sweat of the fingertips. The tiny device is also equipped with a piezoelectric generator that produces additional electricity when it is pressed. The user does not have to do anything more than wear it on their fingers for everyday activities or when they are resting. The electricity is enough to run an electric wristwatch, for example. Portable sensors or mini displays can also be supplied with energy in this way, as the scientists explain.
Whether fitness bracelets, medical sensors or smartwatches: Small electronic devices that are worn on the body are becoming increasingly important. Most of these so-called wearables, however, have so far been dependent on an external power supply, usually in the form of batteries. Some devices rely on integrated solar collectors – with the disadvantage that they are dependent on the external factor light and cannot charge in the dark. Others are already taking advantage of the wearer’s movements. In order to generate enough electricity, however, the user often has to make a significant effort.
Energy without exertion
A team led by Lu Yin from the University of California San Diego has now developed a small device that can be worn on the finger that uses a bio-fuel cell to generate energy from sweat. An integrated piezoelectric generator also uses mechanical energy to generate additional electricity. “We wanted to create a device that is adapted to daily activity and requires almost no energy investment – you can completely forget about the device and go to sleep or do desk work such as typing and still generate energy. You can call it ‘energy from doing nothing’, ”says Yin’s colleague Joseph Wang. Unlike previous devices that rely on energy from sweat, the user does not have to perform any sweaty activities for the new wearable.
“By harnessing the sweat on the fingertip – which occurs naturally no matter where you are or what you are doing – this technology offers a net energy gain with no effort on the part of the user. That’s what we call a maximum energy return, ”says Wang. The researchers opted for the fingertips because the density of sweat glands is particularly high here. “The amount of sweat on the finger can be up to a few microliters per square centimeter per minute,” explains Yin. “That is significantly more than in other parts of the body where sweat rates are two or three orders of magnitude lower.”
Sweat from your fingertips
One challenge for the researchers, however, was to collect the sweat from such a small area and make it usable without the wearable becoming too big, too thick or too firm. This required innovative materials technology. In order to maximize the sweat yield, the researchers combined a special hydrogel with a foam made of carbon nanotubes. These absorb the sweat and pass it on to special electrodes to generate electricity. The anode is equipped with a bioenzyme that removes electrons from the lactate in sweat. At the cathode, these electrons convert oxygen into water. A small amount of platinum on the cathode serves as a catalyst. As long as there is lactate, the process runs automatically – and the flow of electrons creates an electrical current that is stored in a small capacitor and passed on to other devices when required.
“The device is about a square centimeter and the material is so flexible that you don’t have to worry about it being too rigid or feeling weird. You can wear it comfortably for a long period of time, ”says Yin. To do this, the energy collector can be wrapped around your finger like a plaster. In addition to the bio-fuel cell, piezoelectric generators are integrated that convert mechanical energy into electricity. If the generators are pressed – for example because the user is clicking a mouse or playing the piano – additional electricity is generated.
Energy source for clock, mini display or sensors
“The device collected 28.4 millijoules during one hour of desk work,” the researchers report. During this time, the test person typed on a keyboard and occasionally clicked the mouse. In another experiment, the test person wore the device while sleeping. The result: Without any mechanical input, the wearable collected 389 millijoules of energy within ten hours from sweat alone – enough to operate an electric wristwatch for 24 hours. The researchers demonstrated other possible applications by connecting small sensors to their energy collector. Using the energy from sweat and pressure alone, these successfully measured the test subject’s vitamin C level and the sodium content of a saline solution.
“Our goal is to establish the device for everyday use,” says Yin. “We want to show that it’s not just another cool gadget that can generate a small amount of energy and that’s it. We can actually use the energy to operate helpful electronics such as sensors and displays. ”One possible application would be glucose sensors for diabetics, for example. To make it easier to use, the researchers are planning to integrate the energy collector into gloves, for example. They also want to make it more efficient and durable, and pair it with other devices to create a new generation of self-sufficient portable systems.
Source: Lu Yin (University of California San Diego, USA) et al., Joule, doi: 10.1016 / j.joule.2021.06.004