Wearable electronics powered by miniaturized electromechanical energy storage devices have their limitations. The source of power, whether batteries or supercapacitors, inevitably run out of power.
That is why scientists are working on ways to enable self-sustained systems, including ways to harvest energy from sunlight, movement, temperature gradients or biofuels to power sensors and electronics. But these harvesters, they have found, are problematic in that their external environments are inconsistent and they cannot rely on the energy supply to be available on demand. By the same token, mechanical and biological energy harvesters can be impractical options because they require vigorous movement and high mechanical energy input.
Instead, nanoengineering researchers at the University of California San Diego have devised an alternative energy harvester that relies on passive constant input from the human body. Their work, published in Joule, explains how they were able to reap high levels of energy using a natural, passive harvesting device. The device is powered by fingertip sweat and offers a high energy return on investment (ROI).
Fabrication
The device is powered by lactate, a dissolved compound in sweat, and is constructed with a flexible, porous, water-wicking 3D carbon nanotube foam, which collects perspiration from a finger. Lactate-based biofuels (BFCs) have shown promise both as self-powered sensors and bioenergy harvesters for powering electronics, noted the researchers. They also use a porous polyvinyl alcohol (PVA) hydrogel to help maximize sweat absorption, their paper reported.
In contrast, the natural perspiration from fingertips, noted the authors, have been demonstrated to have advantages over other sweat simulation methods, such as exercise and iontophoresis (a method of using electricity to push medication through your skin), or heat. For example, the energy consumed during running cancels out energy produced; the energy ROI is less than 1%. The sweat rate on the fingertip is considerably high (80–160 g h−1), so the fingertip energy harvester could collect more than 300 mJ of energy during sleep, noted the researchers. That’s enough to power some small wearables.
Harvesting Energy
A series of electrochemical reactions are set off within the device when a bioenzyme on the anode oxidizes, or removes electrons from the lactate. The researchers explain further how the cathode is deposited with a small amount of platinum to catalyze a reduction reaction that takes the electron to turn oxygen into water. Then, electrons flow from the lactate through the circuit and create an electrical current. The process is spontaneous since lactate replaces the need for additional energy to activate the process.
In addition, the researchers were able to boost their energy yield by attaching piezoelectric generators (which convert mechanical energy into electricity) to the device. In this case, intuitive finger pinching motions and typing can be used to generate up to 20% additional energy. According to the paper, a single press of a finger once per hour required only 0.5 mJ of energy.
The device is the most efficient on-body energy harvester ever invented, said the researchers. It can produce 300 millijoules (mJ) of energy per square centimeter without any mechanical energy input during a 10-hour sleep. An additional 30 mJ of energy was produced with a single press of a finger—a 6,000% ROI.
“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,” commented senior author Joseph Wang, a nanoengineering professor at the University of California San Diego. “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.’”
Tracking Healthcare and Wellness
Further experimentation showed the device could power effective vitamin C- and sodium-sensing systems. The researchers are optimistic about improving the device so it can be suitable for health and wellness applications such as glucose meters for people with diabetes.
The device represents a significant step forward for self-sustainable wearable electronics, noted the authors.
“We want to make this device more tightly integrated in wearable forms, like gloves. We’re also exploring the possibility of enabling wireless connection to mobile devices for extended continuous sensing,” said first co-author Lu Yin, a nanoengineering PhD student working in Wang’s lab.
Editor's Note: Click here to access the paper, and be sure to check out this video, which shows the process of wrapping the BFC onto the fingertip using a stretchable, waterproof film.
