How bacteria could one day replace batteries

Challenge: power mobile electric devices

Natural inspiration: bacteria

 

Girls on mobile phones © sanja gjenero, www.sxc.hu

Many of the mobile devices people use every day need electricity: watches, cameras, mobile phones, remote controls, pacemakers—the list goes on.  Most of these are powered by batteries.  Batteries generate electricity from the chemicals stored inside them. 1  The global battery industry was worth an estimated 71 billion US dollars in 2008, and is predicted to continue growing. However, the production and disposal of batteries is damaging to the environment:  manufacturing and recycling batteries uses significant amounts of energy (provided by fossil fuels), and batteries contain substances that are potentially harmful to biodiversity and to human health, such as cadmium and copper. 3Fuel cell used in Apollo spacecraft. © James Humphreys

 

 

 

Fuel cells are slightly different from batteries in that they do not store chemicals, but require a constant input of fuel from which they generate electricity. Fuel cells are highly efficient, but still have their problems: the chemicals used in them are expensive and toxic, and they need high operating temperatures. 4Common frog © Ellie Crane

 

 

 

 

 

 

 

Living creatures produce electricity as part of the process of converting food into energy.  The first recorded demonstration of this “bioelectricity” was in 1790, when Luigi Galvani noticed that a severed frog’s leg twitched when an electrical current was passed through it. 4  This discovery led to the idea that fuel cells could be powered by living organisms.  Unlike chemical fuel cells, biological fuel cells (BFCs) work at ambient temperature and pressure, and use cheap, non-toxic substances.  They convert the chemical energy of carbohydrates directly into electricity.4Fuel cell researchers © US Federal Government

 

 

 

BFCs first became popular in the 1960s, when NASA started exploring the possibilities of using algae or bacteria to turn organic waste into electricity on long-haul space flights.  The development of alternatives such as solar power meant that interest in BFCs waned, but it revived during the oil crisis of the 1970s and 80s. Research is now ongoing to improve the efficiency of BFCs to the point where they are becoming an increasingly attractive and commercially viable alternative for powering small, mobile devices. 4, 5

 

 

 

A yeast species © David O MorganElectricity is essentially the flow of electrons.  The effectiveness of a BFC depends on ‘harvesting’ the electrons generated as the organism digests the fuel, and directing them to the electrodes to drive a current.  Early BFC models were powered through fermentation by microscopic organisms such as yeast.  Yeast converts sugars into a mixture of waste products that contain some ‘spare’ electrons which can be used to create a current.  The efficiency at which fuel is converted into electricity is very low.  It can be improved by adding chemicals that ‘mop up’ the free electrons and carry them to the electrodes.  However, these chemicals are often toxic, meaning the BFC cannot be used in situations where its contents come into contact with the outside environment.  Another problem with this system is that waste products from the yeast’s digestive process build up inside the fuel cell, which limits the lifespan of the BFC.  In 2006, researchers investigating bacteria found certain strains with a different kind of digestive system. 6 Instead of partially breaking down their food and releasing a mix of waste products and electrons, these bacteria digest the fuel completely, releasing only carbon dioxide and electrons.  In nature, this strategy allows the bacteria to use more of the energy in their food for growth.  For scientists, it means a more efficient and long-lasting fuel cell with no need for toxic chemicals.  Fuel goes in and electrons come out, passing directly from the bacteria to the electrodes.  These types of bacteria are so effective at generating electrical current that scientists have named them ‘electricigens’. 7

 

E. coli © US Federal GovernmentScientists are experimenting with bacteria found in nature to improve the efficiency of BFCs. Geobacter species were first discovered in 1987.  These bacteria live in habitats including rivers and ditches and digest mineral substances such as iron oxides and petroleum.  This makes them interesting both from a BFC perspective, and because they could be used to clean up environmental pollution. 9  In 2009, Geobacter sulfurreducens was found to be an electricigen. 10  Thermincola ferriacetica, a bacterium found in hot springs on an island off the coast of Japan 11, is another. 12   Bacteria that can be used in BFCs have been found in temperate marine sediment, 13 hydrothermal vents 14 and deep ocean cold seeps. 15  Even Escherichia coli, better known for its ability to cause stomach upsets in humans, can generate electricity. 4 It seems that bacteria that generate electrical current may be common: one study showed that electrodes dipped into drinking water or compost developed a film of electricity-generating bacteria.16  Scientists have noticed that a mixed population of bacteria from different species generates more electricity than any one species could, and are investigating this phenomenon.17  At a larger scale, many studies of ecosystems have shown that landscapes with high biodiversity can be more productive:18 perhaps the same pattern applies at the microscopic scale.

 

exhaust fumes © Love Krittaya The possible applications of BFCs are numerous.  Inventors have already built various robots powered by BFCs. Although these are experimental designs built to test the theory, they demonstrate that a future where BFCs are put to practical use may not to be too far away. In the future, vehicles could be powered by BFCs.  Theoretically, a medium-sized car could travel more than 1000km on 50 litres of a strong sugar solution, with exhaust fumes consisting only of carbon dioxide and water. 4  A BFC with an area of just 0.07cm2 has been designed which can generate enough electricity to operate tiny devices such as microscopic drug-delivery systems.  These could be implanted into the bodies of people who need regular doses of drugs, for example AIDS patients.  The BFC runs on glucose, which means it can constantly be refuelled with sugar from the person’s bloodstream.  Compared to the batteries currently used for medical implants, the BFC is smaller, cheaper and has a longer shelf-life. 4 BFCs could one day be used in the treatment of waste water, digesting the pollutants and producing the water treatment plant’s electricity at the same time.  A pilot project is currently being run by the University of Queensland, Australia.   The fuel for this BFC is waste water from a brewery.  The process produces caustic soda, a widely-used and commercially valuable chemical. The electricity generated helps to power the system.  The project is being funded by the Queensland Sustainable Energy Innovation fund and by Fosters Brewery. 19

 

bacteria mat at Yellowstone © Gbaddorf Bacteria are found everywhere in the world: the tops of mountains, the bottom of ocean trenches, in ice, boiling hot springs and the bodies of other organisms.  They are absolutely vital to the functioning of life as we know it, playing a key role in the recycling of nutrients.  The oldest known fossils are of bacteria-like organisms from 3.5 billion years ago.20 Whether measured by total mass or number of individuals, the vast majority of life on earth is microscopic. 21 When biodiversity is discussed bacteria are often forgotten, but they are no less important than the most charismatic of animal or plant, and in many ways much more important.

 

 

 

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References

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