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When we think of the term fuel cell, the image that comes to our mind is that of a bulky car-battery-sized device which is normally not portable. But recently, researchers turned that visual into a thing of the past by coming up with a novel invention: flexible fuel cells that turn sweat into electricity, thus utilizing an often-unpleasant substance to harness power.
These epidermal bio-fuel cells generate ten times more power than any other available wearable bio-fuel cells. Therefore, this innovation can be safely termed as a major revolution in the flourishing field of bio-electrochemistry. So, how was this feat achieved?
Scientists from the University of California, San Diego invented a resilient electronic base by employing lithography and screen-printing to make 3D carbon nanotube-based cathode and anode arrays. To convert perspiration into power, bio-fuel cells are fabricated to chemically react with lactate, a component of sweat. The ultimate aim of generating current is achieved by removing electrons from the lactic acid present in sweat which is accomplished by an enzyme, lactate oxidase.
As the objective of the project was to develop a bio-fuel cell to run wearable devices, the technology was designed and fabricated in a soft, stretchable format — one that can easily blend with the soft, curvilinear nature of the human skin. For creating such a structure, the engineers used a ‘bridge and island’ design. Essentially, the cell consists of rows of dots that are each connected by spring-shaped structures. Half of the dots make up the cell’s anode and the other half, the cathode. So, it gives the impression of islands that are connected by spring-like structures (bridges).
When the island-bridge structure is stretched, most of the strain is accommodated by the serpentine interconnects, leaving the islands unharmed. The spring-like gold structures, manufactured via lithography, stretch and bend — making the cell flexible without deforming the anode and cathode. Since the above configuration permits negligible strain on the ‘islands’, the active anode and cathode materials can be densely packed without the fear of them experiencing mechanical stress and the subsequent degradation due to the stretching of the device during routine use.
Carbon nanotube-based cathode and anode arrays, developed through the above-mentioned lithography and screen-printing processes, form a vital portion of the technology. To increase power density, the researchers screen-printed a 3 D carbon nanotube structure over the anodes and cathodes. The three-dimensional nature of the carbon-nanotube pellet system permits higher loadings of the lactate oxidase enzyme and therefore, higher electron transfer. The arrangement subsequently leads to power density levels higher than previously reported devices: approximately 1 mW/cm² of power from human sweat.
To test the technology, the UCSD researchers equipped four subjects with a bio-fuel-cell-and-board combination. While exercising on a stationary bike, the participants were able to power a blue LED for about four minutes.
The team still has a long way to go, according to one of the engineers. One of the main hurdles they hope to cross is the stabilization of the lactate oxidase enzyme, which degrades over time. They also aspire to increase the system’s power density. Combining the cell with other forms of wearable energy systems, such as wearable solar cells and thermo-electrics is also a great idea, as an integrated energy harvesting system can harness energy from multiple sources.
These bio-fuel cells have been proven to be capable of powering a range of wearable devices such as LED’s and Bluetooth radios.
With a hopeful increase in the power density, this range can be broadened, and it sure seems that the developments in this field are going in the right direction. Such leaps in these upcoming fields are essential in the present, where the existing conventional sources of power are diminishing, and convince us that scientific advancements will never cease to amaze us in the near future!
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