25-08-2022 | By Liam Critchey
New renewable strategies are constantly being devised to combat the need for fossil fuels and non-renewable energy sources in general. While renewable energy devices take many forms, they are typically focused around either large-scale energy production (solar farms, wind turbines etc.) that provide a lot of power to society, or they are smaller-scale devices that are suitable for charging single devices, devices that are located remotely, and wearable devices that can be attached to humans.
In recent years, wearable technologies have gathered a lot of interest in medical diagnostic and health monitoring applications because they offer a way of monitoring a patient remotely—and are suitable for aftercare protocols when the patient needn’t be in the hospital unnecessarily. A lot of effort has gone into creating energy harvesting systems that can power wearable devices, and much of the focus has been centred around using nanomaterials (especially 2D nanomaterials).
Many 2D materials have already been employed in small-scale energy harvesting devices, such as piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs), but a team of researchers have decided to take a different approach from the trending nanogenerators and have used microbial biofilms to generate electricity from evaporating water–including the water and moisture that is released by the skin of a human.
Water evaporation is a common process on Earth, and around 50% of the solar energy that gets absorbed on Earth results in some form of water evaporation. Harnessing this naturally occurring process using engineered materials has become a promising approach for generating electricity in a sustainable and renewable manner.
While it’s an interesting approach to generating electricity, a lot of the efforts so far have yielded devices with low power outputs (which is common for any new and developing technology). Many of the materials that are used in the creation of these energy harvesting devices are not sustainable. With the drive for a higher degree of sustainability in the way society harnesses energy, there’s also an interest in sustainably building the devices.
For these devices, the primary way of generating electricity is through evaporation-driven water flow at water-solid interfaces, as this process drives charge transport that ends up as an electrical current. For this process to be effective, a material with a large surface area is needed with mobile surface charges. For commercial purposes, materials that are low-cost with as few as possible processing steps for device integration are also favoured.
Materials with a large enough surface area, while small enough for small-scale devices, tend to be some form of thin films Many of the thin films to date have been created using functionalised thin film nanomaterials, much like the TENGs and PENGs that are being developed for energy harvesting technologies, but with hygroscopic properties induced through the functionalisation.
While the production of various nanomaterials is getting greener (greener synthetic processes and sustainable precursor materials), there is an interest in using more naturally occurring biomaterials. Wood has previously been trialled, but it was limited in energy density and scalability, and now researchers are looking at the potential of using microbial biofilms to generate electricity from evaporating water.
Microorganisms are found everywhere in the natural world, and there are certain electroactive microorganisms out there which can generate electricity—typically from the oxidation of organic matter. These types of microbes have been trialled for microbial fuel cells for powering microelectronic systems, but scientists have found it challenging to maintain the power of these fuel cells over time as a continuous feedstock of microbes is needed, and the bio-electrochemical reactions can only take place under certain conditions.
However, this is not the only way microbes can be used as a power source and a means of generating electricity. Researchers have now shown that microbial biofilms created from a sustainable feedstock of non-living G. sulfurreducens strains CL-1 can be used to generate electricity from the evaporation of water.
The biofilms were engineered to be flexible, around 40 microns thick, and could continuously produce electricity in the presence of evaporating water. A single biofilm sheet was used as the functional component in an energy harvesting skin-patch device and was sandwiched between a pair of mesh electrodes. The device maintained its energy production throughout its use and produced a power density of around 1 μW cm-2.
The device was able to sufficiently harvest moisture and sweat from the patient’s skin, and the energy output of the device was comparable to the values achieved with microbial fuel cells. The main difference between the two technologies is that there is no need for the skin patch to have a continuous organic feedstock to maintain cell viability as with microbial fuel cells. Therefore, the skin patch can harvest moisture and continually power the wearable device, much like PENGs and TENGs can in the presence of mechanical motion/mechanical forces.
It’s thought the biofilms could produce a greater amount of electricity if the microstructure of the biofilms were improved using a scaffold. The researchers developed a proof-of-concept device looking at this where planktonic cells were infiltrated within a tissue paper with a microporous structure. The newer scaffold-based devices produced a voltage and current that was 113% and 129% better, respectively, than when the biofilms were composed purely of G. sulfurreducens. Hybrid scaffold biofilms were also trialled using E. coli, and these improved the device capabilities further with 165% and 227% increases in voltage and current, respectively, compared to the original G. sulfurreducens device.
The results of the various devices show that engineering the biofilm can have differing effects and opens the door for new microbial films to be trialled—with or without a scaffold. The number of microbial agents and biofilms present in nature offers a wide range of potential biomaterials to create even better energy harvesting devices and could open up the possibility of using such devices for harvesting energy in a range of aqueous environments—not just on human skin.
Lovely D. R. et al., Microbial biofilms for electricity generation from water evaporation and power to wearables, Nature Communications, 13, (2022), 4369