Emerging paper-based electronics represent a simple, biodegradable and affordable alternative to mainstream electronics and could significantly reduce the amount of electronic waste produced each year. Scientists have particularly focused on the development of bio-batteries made of biodegradable paper.
In many parts of the world, simple electronic technologies like batteries are a luxury that few can afford. Such inaccessibility is particularly problematic for health professionals, whose diagnostic instruments cannot always be supplied in the most remote regions of the planet, whether for lack of availability or because of exorbitant prices.
New, inexpensive, portable and efficient power sources are therefore needed. It is for this purpose that scientists have turned to paper batteries. The idea is not new, however, but the batteries developed so far were not powerful enough and too complicated to produce for the concept to become truly viable. Today, new devices are emerging, combining power and ease of production.
At the University of Birmingham, Professors Seokheun “Sean” Choi and Omowunmi Sadik, respectively from the Department of Electrical Engineering and the Department of Chemistry, have developed a battery made of paper and biopolymers; such technology is therefore called "bio-battery". In addition, this battery has the particularity of being powered by bacteria.
“Paper has unique advantages as a material for biosensors explains Choi. “It’s affordable, readily available, flexible, and has a large surface area. However, sophisticated sensors require a power source. Commercial batteries are too expensive and produce too much waste, and cannot be incorporated into paper structures. The best solution therefore remains a paper bio-battery .
Scientists have already developed paper biosensors to easily diagnose certain diseases as well as environmental contaminants. Most of these devices rely on changing colors, but they are generally not very accurate. To improve their accuracy, biosensors require a power source.
The goal of Choi and his colleagues was to develop an affordable bacteria-powered paper battery that was easily integrated with these single-use biosensors.
The researchers therefore printed very thin layers of metals and other polymers on a paper surface. They then placed freeze-dried exoelectrogens on the paper.
Exoelectrogens are bacteria that can transfer electrons across their membrane. The electrons, generated when the bacterium produces energy for itself, are transferred outside the membrane and can thus come into contact with electrodes supplying the battery.
To activate the battery, the scientists added a small amount of water to the bacteria which revived and, within minutes, produced enough electrons to power a light diode and a calculator.
An equally important aspect of the device is its biodegradability. The researchers used two specific structural polymers, a PAA polymer (poly(amic acid)) and a PPDD polymer (poly(pyromellitic dianhydride-p-phenylenediamine)). Once immersed in water, the battery biodegraded within a few hours.
The tests also showed that oxygen was not a limiting obstacle. Indeed, by passing easily through the paper, the oxygen can capture the electrons leaving the bacterial membrane before they enter the electrodes.
However, the measurements revealed that although oxygen reduces the power generated somewhat, this reduction is negligible. This is due to the fact that, the bacteria being firmly bound to the fibers of the paper, the electrons produced manage to reach the anode before the oxygen intervenes.
This bacterial bio-battery can be used once and discarded in the environment. It has a lifespan of about 4 months. Choi is working on improving the survival and performance of freeze-dried bacteria to increase battery life.
“The generated power must further be increased by 1000 times before it can be put into common practical applications explains Choi. A threshold that could be reached by stacking several bio-batteries.
