Every five to eight years, pacemaker users must undergo minor surgery to replace the battery on their lifesaving implant. To create a pacemaker that can stay in place without ever needing a new power source, a number of research groups worldwide are working to develop tiny biobatteries that can convert chemical energy into electricity inside the blood vessels, using biocatalysts (enzymes or microorganisms) to speed up the chemical reactions and generate an electrical current. One of the most promising projects is being developed by the team headed by Professor Frank Crespilho, coordinator of the Bioelectrochemistry and Interfaces Group at the São Carlos Institute of Chemistry (IQ-SC) of the University of São Paulo (USP). The group also includes researchers from the Federal University of the ABC (UFABC) in the city of Santo André, São Paulo State. They are researching a biofuel cell (BFC) that produces energy from blood glucose in rats. To test the device, the researchers implanted it in the jugular vein of a rodent.
Crespilho first started his work on these cells in 2008 and began developing implantable microcells in late 2010. “The main objective was to develop a biofuel cell and use it as an alternative power source for pacemakers, insulin pumps, neural implants, electric biostimulators, and devices for controlled release of medications,” he explains. “Implantable BFCs powered by glucose and oxygen, like the one we are developing, are desirable because they can generate a potential difference of more than 1.0 volt [an AA battery generates 1.5 volts, for example]. Also, both glucose and molecular oxygen are readily available in many regions of the human body.”
The innovations in the BFC developed by Crespilho’s group include the scale and size of its components. “Our biofuel cell is called a ‘microcell’ because it operates with microvolumes. And the size of the electrodes makes it possible to implant the device in the vein of a rat,” he explains. The electrodes are 20 micrometers in diameter (six times thinner than a strand of hair), inserted in a catheter 0.5 millimeter in thickness by 0.6 millimeter in length. Like common batteries, the BFC created in São Carlos has two electrodes – one positive (cathode) and one negative (anode). The cathode is made of platinum nanoparticles and the anode is based on the enzyme glucose oxidase. Both are coated in a polymer and affixed to the electrode’s flexible carbon fiber frame. “Blood cells, such as the red or white cells, may adhere to the electrodes’ surface and block the diffusion of glucose,” Crespilho explains. “So our strategy was to use a special polymer called a dendrimer, which prevents adhesion and blockage of the electrodes.”
The microcell’s flexible carbon fibers are an additional innovation created by the group. According to Crespilho, when the team decided to develop biofuel cells for medical applications, their first insight was that they needed to create flexible electrodes that were compatible with biological systems. “From there, we began using flexible carbon fibers,” he says. The researchers were already well acquainted with carbon fibers and electrodes. But in the scientific literature, there was no reported use of these flexible fibers in biological implants. Using new techniques in micromanipulation, they extracted different types of fibers from commercially available carbon fabrics used for manufacturing highly resistant, lightweight materials including F1 race cars, surfboards, and bicycle frames.
Devising a flexible carbon fiber with the right properties for use in BFCs was one of the most complicated parts of the project. They could not simply use any commercially available material. “It took us at least two years to find the ideal fabric, as the electrodes strongly depend on how the carbon atoms are aligned, as well as the quality of the materials used to produce the fibers,” Crespilho clarifies. “We had to develop a technique to obtain these fibers. Once selected, they undergo a chemical treatment that makes them suitable for use in a biofuel cell.” When the BFC is ready, it is implanted in a rat’s jugular vein. “The blood goes through it and brings glucose, which is the fuel for the anode, and the oxygen in the blood reacts with the cathode,” the professor explains. “The glucose ‘donates’ electrons to the BFC by reacting with the anode’s surface which contains the glucose oxidase enzyme, in a process known as oxidation. The cathode reduces an oxidizing agent, which in this case is the oxygen dissolved in the animal’s blood. In this reaction, the oxygen receives electrons.”
The two electrodes enable electrons to pass from one extremity of the BFC to the other. The combined reactions of oxidation and reduction (or simply “redox”) create an electric current that can be used to power an external circuit — such as a pacemaker. For this to happen, the generated electricity is transported from the BFC to the device through wires that cross the vein’s walls. This means that the biofuel cell is constantly “fed” by the blood’s oxygen and glucose, which are continuously replenished as an animal breathes and eats.
In their paper published in the journal Lab on a Chip, the researchers also mention that further studies are still needed in order to discover alternatives for preventing inflammation and the formation of fibrous tissue around the electrodes implanted in blood vessels, which would reduce the devices’ useful life. In addition to the group from USP São Carlos and UFABC, biofuel cells are also being developed by two research teams in the United States and one in France. The pioneer of this field was Professor Serge Cosnier of Joseph Fourier University in France, who implanted a BFC in a rat’s abdomen in 2010. In 2012, Daniel Scherson from Case Western Reserve University in the U.S. did the same to a cockroach. That same year, the group led by Evgeny Katz from Clarkson University, also in the U.S., implanted a BFC in a snail. “In all this work, our group developed the implantable biobatteries with the highest power density reported until now, roughly 100 microwatts per square centimeter,” Crespilho assures. To implement their project, the researchers in São Carlos and Santo André received funding from FAPESP, the National Institute for Organic Electronics (Ineo), the Nanobiomedicine Network (Nanobiomed), and the Coordinating Agency for the Improvement of Higher Education Personnel (Capes).
Biomolecules and cellular system interaction with nanostructures 0D, 1D and 2D by using electrochemical methods (nº 2009/15558-1); Grant mechanism Regular Line of Research Project Award; Coordinator Frank Crespilho/USP; Investment R$ 92,262.80 and R$ 50,821.57 (FAPESP).
SALES, F. C. et al. An intravenous implantable glucose/dioxygen biofuel cell with modified flexible carbon fiber electrodes. Lab on a Chip. v. 13, p. 468-74, 2013.