Biobatteries: Harnessing the Power of Nature

Kerry Hebden speaks to researchers who are developing novel batteries for military and environmental applications

High in the Chilean mountains, in an imposing landscape dotted with volcanoes, is the Salar de Atacama – the largest salt flat in Chile. It is an impressive sight, yet aerial pictures showing the multi-hued pools of salty groundwater used to extract lithium, offer a stark reminder of the transformation this key ingredient for batteries is having on its surroundings. Along with concerns that as a highly mobile element, lithium will be released into the environment when extracted, there is also the huge volumes of groundwater that the extraction requires [DOI 10.1088/1748-9326/aae9b1]. As the flats are one of the driest desert regions in the world, the increasingly erratic supply of water caused by lithium mining is having devastating effects on wildlife and local communities. It’s a situation that is likely to be compounded further as Chile is one of the top producers of lithium and demand is expected to rise from 540,000 t of lithium carbonate equivalent (LCE) in 2021, to 1.5m t by 2025, and over 3m t by 2030 – the majority of which is earmarked for batteries.

Right now, lithium battery use is so ingrained, it could not simply be replaced with a suitable alternative. But with a clear need to develop alternatives to lithium batteries for reliable and renewable portable power sources, scientists are not only finding ways to improve current battery technologies but they are also re-envisaging them completely. One of these potential solutions is biobatteries.

The power of nature

Biobatteries fall into two main groups – those that use bacteria as a fuel source and those that use enzymes. Regardless of the method used, biobatteries work in generally the same way by generating electricity from the breakdown of complex fuels, such as carbohydrates, fatty acids and alcohols.

Lactic acid, or lactate, which is present in most biological fluids, such as blood, tears, and sweat, has been investigated as a promising fuel to power health devices, including self-powered lactate biosensors, fitness monitors, and drug release systems because it is abundant and easy to obtain. However, although lactate has high fuel energy density, by itself it doesn’t release many electrons as it oxidises on precious metal catalysts, and many are left in the remaining waste product.

One team researching ways to make the lactate oxidation process more efficient is the Minteer research group at the University of Utah, US. Led by Shelley Minteer, the group is working on finding bio-inspired alternatives to traditional chemical batteries, which in the US are thrown away in the billions each year. “Traditional batteries contain metals and those metals need to be recycled. But in the United States we’re not particularly good at recycling batteries which means they are ending up in our landfills and they will actually break down and cause environmental hazards long-term,” Minteer said.

Past studies into using organic catalysts such as 4-amino-TEMPO (Amino-TEMPO) to oxidise fuels has shown that while they have the ability to oxidise a wide range of oxygen, nitrogen, and sulfur-containing functional groups, they can’t cleave carbon bonds.

Enzyme cascades on the other hand were found to be a good way to release more electrons by obtaining a higher degree of oxidation, but this approach too had its drawbacks. Cascade reactions are a series of chemical reactions in which the products of one reaction are consumed in the next reaction, and for biobatteries to work well, this process must be efficient. However, a single enzyme can’t completely oxidise fuels in biological fluids – it takes a few different ones. You also need the right enzymes as many of the ones tested by the team in the cascade reactions, were found to be limited by their stability, high substrate specificity, pH, and/or temperature sensitivity.

But the team did find that a hybrid approach using oxalate decarboxylase (OxDc), a metal binding protein, combined with an organic catalyst – 4-amino-TEMPO (Amino-TEMPO) – showed the complete oxidation of lactate to CO2. One of the reasons OxDc is used is that it is a promiscuous recombinant enzyme meaning that it can catalyse other reactions in addition to its main reaction. The process also benefits from collecting 12 electrons from the fuel molecule instead of the two released when only lactate oxidase in the lactate oxidation process is used.

In order to use this hybrid architecture as a battery though, you need a lot of enzymes, and enzymes are fragile. If enzymes are exposed to extremes of pH or high temperatures, the shape of their active site may irrevocably change. This is called denaturing, and if it happens, the catalytic process slows down or stops. So enzymes need to be immobilised in some sort of substance like a polymer to stabilise and protect them. Although a wide variety of polymers were considered by the team, none were the right fit with the large quantities of enzymes needed. And then a solution was found – DNA hydrogels.

Locked in

DNA molecules are polymers, that in turn are made up of monomers called nucleotides, joined one after another into a very long chain. DNA hydrogels are formed by the self-assembly of DNA under physiological conditions. This makes DNA hydrogels an ideal material for locking in cells or biomolecules such as DNA or proteins in situ. Although the hydrogel is solid enough to act as a scaffold and immobilise enzymes, it is also liquid enough to allow smaller molecules and electrons to move through it, making it an ideal material for a biobattery. “Our strategy was to use the DNA to accommodate whole cascades, to achieve a deeper complete oxidation of complex fuels,” Minteer said. “This was really important to the technology, but at the time, it was prohibitively expensive to get the DNA to be able to produce the hydrogel.” It wasn’t long after publishing a paper on the subject however, that this problem too was solved as Minteer was contacted by UK biotechnology firm Touchlight about a potential partnership.

Touchlight specialise in the enzymatic manufacture of DNA, and they can do so at scale and speed, explains Tom Adie, head of platform discovery. The idea for the DNA biobattery was originally conceived as a joint project between Touchlight, the Minteer group, the UK Defence Science and Technology Laboratory (Dstl) and the Office of Naval Research (ONR) from the US Department of Defense as a potential replacement for lithium batteries carried by soldiers in war or disaster situations. More and more technical equipment is being carried by soldiers and normal batteries can explode if shot, said the Dstl and the ONR, who are funding the project. A biobattery on the other hand would simply go inert – or may even still be able to function.

“The core idea behind the structure is that the DNA encodes little motifs which are recognised by zinc finger bind domains attached to the enzymes,” Adie said. “We then attach the enzymes to DNA to co-localise them.”

Called fingers due to their finger-like protrusions, zinc finger domains are relatively small protein motifs that bind selectively to certain sequences of DNA. Touchlight encode recognition sites for multiple zinc finger binding motifs in their DNA scaffold constructs, and except for these sites the DNA sequence is broadly irrelevant, said Adie.

Once the electrons have been produced in the cascade reactions, they are then passed to the electrode via carbon nanotubes which are mixed into the hydrogel phase to make a composite on the electrode surface. Put simply, the DNA scaffold holds the enzymes onto the electrode by wrapping round it.

It’s not all been smooth sailing, for example the team encountered problems discharging the battery effectively, as the electrons flowing out of the battery drove the DNA off the carbon nanotube. This was overcome by anchoring the DNA to the electrode using pyrene, which was attached to a high affinity peptide nucleic acid primer and hybridised to the DNA.

The next steps are for both partners to work on their respective areas with the goal of producing a working demo in a couple of years. For Minteer, that means working on the cathode in the battery. “Our work to date has really focused on the anode,” Minteer says. “Any electrochemical cell has to have an anode where oxidation occurs, and a cathode where reduction occurs. In these biobatteries the cathode is actually what we call an air breathing cathode, so it breathes the air and uses the oxygen in the air to do the reduction reaction. Our job now is to work on the cathode and marry everything together.”

Meanwhile for Touchlight, they are scaling up their side of things and are scoping process improvements to enable the potential requirement for kilogram-scale production of pure DNA at “nanomaterial grade”, for future implementation of the programme.

Further down the line, Touchlight has plans to investigate a freeze-dried version of the biobattery which could be dehydrated, making it very light and portable, and then hydrated again with seawater or even dirty water, wherever in the world it was required.

“It could provide a charge for anything needed and then be discarded without making an environmental impact,” Adie said. “It’s something which has clear applications, and it could even find niches in civilian life eventually.”