A self-sustaining energy harvesting device that requires no physical exercise can accumulate 300 mJ of energy in a 10-hour sleep period.
Sensor collecting sweat. Image Credit: Lu Yin
Don’t sweat the small stuff – maximizing the productivity of energy harvesting devices
The human body produces consistent amounts, even at rest, the body can produce nearly 100 watts of power. Recent research efforts have attempted to collect some of that energy to use it, and the question of whether the human body could become a biological battery of sorts becomes increasingly possible.
Researchers have now achieved a key step towards harnessing some of the energy produced from the human body using a new device that harvests energy from the swat produced on fingertips. This device has shown to be the most efficient on-body energy harvester ever invented, producing 300 millijoules (mJ) of energy per square centimeter without any mechanical energy input during a 10-hour sleep and an additional 30 mJ of energy with a single press of a finger.
This amount of energy is sufficient to power some small electronic devices – but with more time and higher energy input, it can support more demanding electronic tools.
Previous devices have required intense activities to gather sufficient power generation but the large amount of energy required during running or biking easily negates the energy harvested, often resulting in an energy return on investment of less than 1%. However, this new device does not require such high levels of exercise, which directly improves the efficiency and productivity of energy gathering.
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. 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.'"
Joseph Wang, Professor of Nanoengineering, University of California San Diego
Developing the ‘holy grail’ of energy harvesting devices
This device can be categorized, as authors describe, into the holy grail" category of energy harvesters as it relies solely on the contact to fingertips and not on external, irregular sources such as sunlight or movement.
Although the choice of fingertips may seem odd, fingertips actually 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," says first co-author Lu Yin, a nanoengineering PhD student working in Wang's lab. "Sweat rates on the finger can reach as high as a few microliters per square centimeter per minute. This is significant compared to other locations on the body, where sweat rates are maybe two or three orders of magnitude smaller."
Researchers designed this energy harvester using a biofuel cell (BFC), which looks like a piece of foam connected to electrodes attached to the fingers. The BFC is powered directly by lactate, a compound derived from sweat, which is absorbed using a carbon nanotube material and a hydrogel that maximizes sweat absorption.
Upon absorbing sweat, each BFC undergoes a series of electrochemical reactions. The cells contain a bioenzyme that oxidizes the lactate, and a cathode that is deposited with a small amount of platinum to catalyze a reduction reaction that takes the electron to turn oxygen into water. The electrons then move from the lactate through the BFC circuit to create an electrical current.
Lu Yin describes; "The size of the device is about 1 centimeter 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,"
Not only does the entire process only require lactate and no additional energy to begin energy harvesting, but it is also equipped with piezoelectric generators, which convert mechanical energy into electricity to harvest up to 20% additional energy.
The entire circuit process allows for a single press of a finger once per hour that required only 0.5 mJ of energy, in turn, produces over 30 mJ of energy, a total of 6,000% return in investment from a very simple movement.
Researchers are optimistic to continue improving the device to develop even greater abilities in the future to maximize energy conversion. Developing such technology will directly help support health-related devices including glucose meters used for people with diabetes.
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. There's a lot of exciting potential. We have ten fingers to play with."
Improving the device in terms of surface area coverage, energy conversion, and adding other locations for sensors on the body, could in turn drastically increase energy harvesting. Such developments may prove particularly valuable as researchers are hoping to collect more of the energy produced by the human body, potentially turning biological processes into self-sustainable batteries of the future.