A biobattery generates electricity using biological/organic compound materials and processes. Biofuel cells (BFCs) produce electrical energy through oxidation and reduction reactions at two electrodes using substances like glucose, bacteria, and enzymes. These enzymes present like they do in the human body and which break down glucose, then break down into electrons and protons which are released. Although the batteries have never been commercially sold, they are still being tested, and several research teams and engineers are working to further advance the development of these batteries. Related energy-harvesting devices that can aid in the creation of bio batteries development. For example, thermoelectric generators are not traditional chemical batteries but devices that convert fluctuations in temperature directly into electricity. Triboelectric nanogenerators and piezoelectric generators are devices that convert mechanical motion, pressure, or friction into electrical energy. These devices are starting to be researched by researchers for wearable devices and implanted sensors.
History
Biobatteries origins stem from the earliest experiments with bioelectricity, when scientists first began to wonder whether living organisms could produce electricity. In the late 1700s, Italian scientist Luigi Galvani observed that frog legs would twitch when they came into contact with different metals, which led him to believe in the idea of animal electricity. This curiously eventually inspired the development of the frog battery, where multiple frog muscles were connected in series to create a small but measurable electric current. In 1837, Carlo Matteucci improved this setup into a more reliable device by arranging frog tissues in a way that acted like a chain of electrochemical cells. Researchers also experimented with other animal tissues, including the ox-head battery, where freshly severed ox heads were used to demonstrate that biological tissue could carry and produce electrical signals. Although these experiments were later understood to rely on chemical reactions rather than a life supplying the energy, it allowed the scientists to gain a perspective on how electricity can come from living systems that follow the same principles of electrochemical that's found in commercial batteries. As the understanding of these systems improved, the experiments developed into simplest and more ethical demonstrations like the lemon battery. It is set up with two different metal electrodes, usually copper inserted into a lemon, and the fruit's acidic base acts as an electrolyte that allows electrons to flow through a circuit. The energy actually comes from the oxidation of zinc and reduction for hydrogen ions, not from the lemon itself which the fruit provides the medium as the place the reaction occurs. This does not only work for lemons, but a wide variety of different acidic fruits like oranges, grapefruits that have the same properties that can function as electrolytes in simple electrochemical cells.
Workings Like any battery, bio-batteries consist of an anode, cathode, separator, and electrolyte with each component layered on top of another. Anodes and cathodes are the positive and negative areas on a battery that allow electrons to flow in and out. The anode is located at the top of the battery and the cathode is located at the bottom of the battery. Anodes allow conventional current to flow in from outside the battery, whereas cathodes allow conventional current to flow out from the battery. As conventional current is opposite to electron flow, this means that cathodes allow electrons to flow into the battery and anodes allow electrons to flow out of the battery.
Between the anode and the cathode lies the electrolyte which contains a separator. The main function of the separator is to keep the cathode and anode separated, to avoid electrical short circuits. This system as a whole, allows for a flow of protons (H+) and electrons (e−) which ultimately generates electricity.
Types of Biobatteries Biobatteries utilize a wide range of chemistries, and use different methods to produce current. The efficiencies of these batteries are commonly measured as the voltage and current output, performance stability, and sustainability. Biobatteries serve many functions which will be discussed more in detail. Some biobatteries under development are intended to replace lithium ion batteries and therefore needing a high voltage, while low voltage designs are intended to identify gastrointestinal diseases. Depending on the application for its intended use, the performance and chemistry of biobatteries are specific. While there have been studies done on the efficiency of biobatteries, because of the many different types of biobatteries, there are only a few studies covering any one chemistry.
In general biobatteries tend to record a much lower voltage than lithium ion batteries. Some batteries cannot operate for longer than a few hours, but other batteries are capable of running off of sweat and mechanical energy allowing them to run for years, and potentially decades. Because biobatteries are made from biological agents and/or harvest their energy from biological agents, across the board, they tend to be more sustainable than their metal counterparts. They are currently used in low power technologies like pacemakers and insulin pumps, but are generally insufficient for technologies with significant power consumption like smartphones. While biobatteries have been studied for over a decade—and will likely take a significant amount of time to scale to the efficiency needed for applications like smartphones—biobatteries are proficient at operating in room temperature, which makes them useful tools for communities that need to power small medical devices but don't have access to constant refrigeration like developing countries.
Edible Batteries In traditional batteries, energy is stored in the redox state. However, in edible batteries, researchers are looking to develop batteries that store energy in chemical bonds. At the Italian Institute of Technology, researchers are looking into ingestible electronics, a small pill that is swallowed to gather information on your digestive tract. By placing a microprocessor or LED, you can put a mini camera inside. They currently have very limited potential as they have very high costs, and may be environmentally costly as well. Caironi's team decided to create something without metal electrodes that people already have at home and has the possibility of recharging through energy harvesting. Researchers discovered that riboflavin (Vitamin B2) could serve as the battery anode and quercetin as the cathode. The electrodes are encapsulated in beeswax and the separator is made from nori seaweed used in sushi. The cell operates 0.65 Volts which is proven to not create problems in the body. It provides a current of 48 microAmps for 12 minutes or a lower level for more than an hour, enough to power low-power LEDs.
These batteries are being studied for use in biomedical applications, specifically for testing the pH of the stomach, and after further development, for diagnosing and treating gastrointestinal tract diseases. The advantage of these batteries is that they run on low voltages. Inside the body there is a risk of the battery breaking down and releasing its toxic materials. This is particularly risky above around 1.2 V. The safety of edible batteries, compared to other commercial batteries, also opens up possible applications in monitoring food quality and in edible soft robotics.
The efficiency of edible batteries should not be measured by the maximum voltage they can generate as they are made to generate voltages under around 1.2 V. While edible batteries might be capable of producing a higher voltage, a low voltage is necessary to prevent tissue damage, and worse side effects such as death, post-ingestion. The capacity for edible batteries ranges from 10 micro Ah to over 20 micro Ah. Some edible batteries have also been made to be rechargeable, indicating that they can be used multiple times.
Electric Eel Researchers at Penn State use a fabrication method to layer multiple types of hydrogels in a pattern that replicates the ionic process we see in nature from electric eels when generating electrical bursts. Their work has been published in Advanced Science and their approach creates a power source that is flexible and environmentally stable.
To research this, Penn State looked into the biology of electric fish however the challenge is that the eel inspired devices produce limited power that would functionally support human design. By creating ultra thin hydrogel sheets, they were able to produce more power without the need of mechanical supports. With this, they were able to generate over 600 volts of electricity from small volumes. To ensure the batteries remained non-toxic, they were made only from hydrogel, allowing them to be more flexible despite being powerful.
The process is done via spin coating, layering material on a spinning surface of 20 mm thick. This process ensures high power while preserving mechanical strength and flexibility. Due to the gel like nature of hydrogels, they typically need external support structures to actually output high outputs. Viscosity of the gel plays the role in getting the layers just right because their first iterations would spin right off the table, so they had to increase the viscosity and mechanical strength to ensure the layer could be created.
By using equipment in the Materials Research Institute at Penn State, they collected electrochemical measurements from the power sources such as discharge rate, power density and conductive potential. With this improvement in the viscosity, the power densities were measure higher than before at 44 kW/m3. This power density allowed for implanted medical sensors and soft robotics controllers and wearable electronics to be powered.
To address concerns about hydrogels expected drying when exposed to air, the team incorporated the chemical glycerol, which allowed the hydrogel to remain functional from 80 C without freezing and is able to retain water longer than most hydrogels. As of today, this is the only hydrogel that acts as a power source entirely contained in a hydrogel solution that requires no external support.
Electric eel batteries are flexible and biocompatible, making them potentially usable for biomedical applications. These batteries are also environmentally stable, and with more development, would also ideally be able to recharge in their environments. This makes this potentially suitable for biological applications.
Electric eels are capable of generating over 600 V of electricity in one burst, and are capable of very high power densities. Multiple studies have found that battery cells made by electric eel electrolytes and hydrogels can produce over 100 mV of potential and maximum power densities ranging from 0.001 kW/ m^3 to 1500 kW/ m^3. These cells can be linked together to make cells with large voltage outputs, with some studies reporting voltages of over 100 V.
Bacteria Batteries Microbial fuel cells (MFCs) function as rechargeable biobatteries powered by renewable resources. Electrogenic microbial biofilms oxidize organic matter and transfer the generated electrons to external circuits. The biocatalysts naturally self assemble, self repair and adapt to fluctuating environments. There are troubles with the anode interface due to challenges ensuring adequate nutrient diffusion, efficient waste removal, mechanical robustness, high electrical conductivity, compatibility with standard microfabrication processes and sustained microbial viability during harsh fabrication and storage conditions.