By Kyle Proffitt
February 14, 2022 | Imagine batteries made from completely safe, organic, non-combustible, and biodegradable materials. Now imagine that these batteries can be recharged not from an outlet, but from the excess energy of our bodies. Refueling by gathering kinetic energy from our movement, or thermal energy from body heat, or—in what might be the most sci-fi scenario—feeding on the biochemical surplus of sugars coursing through our veins, or even compounds present in our saliva, sweat, or other body fluids. This idea is the biobattery, and the potential uses and benefits are pretty remarkable.
Seokheun “Sean” Choi, professor of Electrical and Computer Engineering at Binghamton University in Binghamton, New York, is working at the forefront, driving research toward making biobattery commercialization and ubiquity feasible. Battery Power Online communicated with Choi via email to learn more about the latest research in this arena, particularly focusing on devices powered by biomolecules.
Choi defines the biobattery simply as an “energy-generating/harvesting device based on biological chemical reactions.” The concept is a relatively straightforward twist on the traditional battery. For the lithium-ion battery we’re more familiar with, lithium ions and electrons accumulate at the anode during charging, and the return movement of electrons through an external circuit, from anode to cathode, at the same time that the lithium ions return to the cathode through an ion-selective separator, powers your device.
For the typical biobattery or biofuel cell (when the device operates without replenishing the fuel, it is a biobattery; when fuel is replenished, it is a biofuel cell, Choi clarifies), you can think of it as existing only in a state analogous to a charged battery. The electrons are already at the anode, but they’re locked up in organic molecules such as glucose. Either isolated enzymes or enzymes present in whole microbes oxidize these molecules to release the electrons and protons. From here, the process is mostly equivalent to the lithium-ion battery. The electrons and protons ultimately recombine with an acceptor molecule at the cathode, although this cathode reduction process sometimes also includes enzymatic or microbial activity.
The scheme mimics and exploits the natural biochemical processes that allow organisms to harness energy from food—electrons are harvested from organic fuel and transferred to electron acceptors, such as the terminal acceptor oxygen we use in aerobic respiration. In organisms, the energy from this electron movement is ultimately captured in ATP, which provides the energy for most biochemical processes. However, just as electrons drive ATP synthesis in cells, they generate current in biobatteries by shuttling electrons through an external circuit.
Old Ideas, New Applications
The biobattery concept is not new. Sony made a headline-generating announcement in 2007 when it showed that a series of simple enzyme-based biobatteries using glucose as fuel could power small devices, including a flash-based mp3 player. The idea actually dates back much further, beginning with the appreciation by M.C. Potter in 1911 that electricity could be harnessed from Saccharomyces cerevisiae. However, despite the tenure of the concept, it hasn’t moved significantly beyond academic labs and prototypes. Sony never publicly followed up on their stated intentions to use their designs in small toys.
In one version of the biobattery, the primary motivations are moving away from fossil fuels and metals that are rare, toxic, or associated with geopolitical problems while reducing battery weight. For instance, CFD Research in Huntsville, Ala. is developing biobatteries with an eye toward military use. These cells run on glucose or alcohol and employ enzymes to pull electrons from the fuel source and create electricity. The key advantage of this concept compared with lithium-ion batteries is a “ten-fold or greater increased energy density”, they claim. Combined with solar cells, the net effect can be a lightened load for soldiers needing to power radios, GPS, night-vision goggles, vehicles, etc.
According to Choi, however, there are significant hurdles to this approach, and a major problem for the early biobattery hype was with the scope and anticipated use. “The pioneering work of biofuel cells did not have clear applications and was considered as a replacement of conventional high-power supplies (such as lithium-ion batteries). This was a wrong start and was not successful.”
Choi says the power available from devices on the scale developed by Sony and in his own lab is just too small to be thought of as replacing the battery in your smartphone. (Quick calculations suggest you might need 40,000 of some of the cells he’s developed, connected in series.) The key advantage exists, he says, in the “potentially infinite energy supplies”. It should be possible to design them so that they run forever.
To get closer to that target, instead of enzyme-based batteries, Choi specializes in and favors the use of microbial fuel cells (MFCs). The key to this approach is the identification and use of electrogenic microbes—most often bacteria that naturally transfer electrons across their cell membrane, just like Potter discovered yeast could do in 1911. The advantage of whole bacteria compared with a deconstructed metabolic machinery of core enzymes is that the bacteria are self-sustaining; they will grow and divide and perpetually rebuild the necessary machinery, whereas purified enzymes have a shelf life. “The enzymes suffer from degradation over time,” Choi says. So while a large and powerful enzyme-based biobattery can be made, it’s not likely to last very long, either prior to or during actual operation.
Paper and Spit
There is a tradeoff with MFCs, though. “Because the electrons must be transferred from the internal cells through the cell membrane, the electron transfer efficiency is much lower than that of the enzymatic fuel cells,” Choi says. Significant research efforts are being applied to improving this transfer efficiency, but the tradeoff is evidently one that Choi deems worthwhile in favor of battery longevity.
Using this concept, Choi’s group reported in 2016 the development of a “papertronic” spit-powered biobattery. It consisted of freeze-dried electrogenic bacteria in a reservoir, a nickel and polypyrrole/carbon black anode, wax-based ion exchange membrane, and activated carbon based air cathode (oxygen as electron acceptor), all printed on paper that is easily folded to create the necessary interfaces. In addition to spit, practically any other body fluid, puddle water, or wastewater can be used to reanimate the bacteria while simultaneously providing the necessary fuel to enable electricity production—sufficient to power an LED for approximately 20 minutes, at least with a 16-battery stack. These paper batteries hold great promise for use in the internet of disposable things (IoDT)—single-use devices that could be used to track shipments, for point-of-care medical tests, for food and grocery monitoring, and in military surveillance operations. Choi is working on further variations of this design that can be powered with a single drop of dirty water, providing power for small devices in some of the most resource-limited places.
These paper batteries reported a maximum power density of 9.3 μW/m2. MFCs have been developed with densities as high as 561 μW/m2, and enzyme-based cells with densities of at least 1,300 µW/cm2 have been reported by other groups. This is still an order of magnitude less than a conventional lithium ion battery. There is also a biobattery design that uses neither enzymes nor microbes. Platinum can catalyze only a 2-electron oxidation of glucose (as opposed to the 24 electrons available from a full oxidation of glucose to CO2), and power density up to 180 µW/cm2 is attained. But again, Choi stresses that power density is not the major concern, “because the main applications are for the low-power one-time use or energy harvesting operation”.
The paper biobatteries demonstrate the single-use example, but Choi also envisions a future with perpetually self-powered biosensors, suggesting that “body fluids can work as an analyte for diagnostics and as a reagent for power generation.” In his view, it isn’t necessary to create super-efficient powerful cells as much as it is necessary that we deal with biocompatibility. “The biggest issue is how to generate a reliable and robust performance without toxicity problems.”
Especially when considering implantable, ingestible, or wearable devices, there are particular concerns surrounding the use of bacteria. The prototypical electrogenic bacteria are Shewanella oneidensis, which can be found in some pretty inhospitable environments, transferring their electrons to iron and manganese oxides, but a 2018 Nature study showed that diverse gram-positive bacteria also have this capacity, including the human gut-resident pathogen Listeria monocytogenes. You would not necessarily want to ingest or implant batteries powered by such pathogenic bacteria, but if we can identify naturally coexisting microorganisms with the capacity for extracellular electron transfer, then some of the concerns around biocompatibility and safety will diminish. Choi says he is “really interested in human bacteria for power generation.” He continues, saying that “our body is a home for billions of microorganisms, [and] we may use them for power generation.” Accordingly, his group has identified additional gut- and skin-dwelling bacteria with electrogenic capacity.
A second issue is the interface. For continuous use, the biofuel cells need steady replenishment of reducing agent—the fuel—at the anode, and of oxidant—the electron acceptor—such as oxygen, at the cathode. This requires some type of open system that allows material transfer in and out, without associated degradation or toxicity. However, Choi is optimistic. “There are many biodegradable/biocompatible metals for implantable/ingestible/transient devices like magnesium. Alternatively, many research groups are trying to develop conductive biocompatible polymers including bacteria. This will not be a significant problem.”
Essentially, Choi’s vision is that the scant energy available from individual biobatteries will find a nexus with miniaturized electronics, and the entirety of a process or device can be scattered into a mesh of nodes, each with its own biobattery power source. Choi says he believes that “with 6G technology in the near future, some electronics will have a significant size reduction,” even to the point of what is called “smart dust”—electronic devices so small they literally hang in the air. Choi says that even “a single bacterium may provide a power solution because of its comparable size and seamless integration with the device.” Choi elaborates this concept in a review article just published in Small. In this article, he talks about the idea of an entire body sensor network, a “technology that interconnects wearable and implantable sensor nodes, enabling continuous, autonomous, ubiquitous, and low-cost monitoring of human vital signs, radically improving conventional centralized and labor-intensive healthcare systems.”
And it gets better. With ongoing developments in bioengineering and synthetic biology, bacteria can even be programmed to self-assemble these tiny sensors, allowing further miniaturization and efficiency optimization.
The potential applications of this new era of biobatteries are staggering. They could be used for drug delivery, wound healing, and constant monitoring of glucose or any number of specific biomarkers and analytes. Bacteria can either be selected for or engineered to grow in the presence or absence of specific analytes, and the resulting electrical output will correlate with analyte concentration. Biobatteries could be incorporated into pacemakers, hearing aids, vision-enhancing devices, and other bionics—even brain implants. They could be used for animal tracking. In external devices, they can be used for environmental monitoring and surveillance, in wastewater treatment, and for any number of disposable devices in the IoDT.
From the Small article, Choi gives us a more complete glimpse of this future with biobatteries: “Stand-alone all-in-one electronics containing all functions but without distinct physical components will work similar to a living biological system that provides self-sustaining, self-assembling, self-repairing, and self-maintaining operational capabilities.” We live in exciting times.