The sugary energy source generates about 43 microwatts per square centimeter of electricity, achieving the highest power density of any glucose fuel cell to date under ambient conditions.
The new device is also durable, capable of withstanding temperatures up to 600 degrees Celsius. If incorporated into a medical implant, the fuel cell could remain stable through the high-temperature sterilization process required for all implantable devices.
The core of the new device is made of ceramic, a material that retains its electrochemical properties even at high temperatures and on a miniature scale. Researchers envision that the new design could be made into ultrathin films or coatings and wrapped around implants to passively power the electronics, utilizing the body's abundant glucose supply.
"Glucose is present throughout the body, and the idea is to collect this readily available energy and use it to power implantable devices," explains Philipp Simons, who developed the design as part of his doctoral thesis in MIT's Department of Materials Science and Engineering (DMSE). "In our work, we demonstrate a novel electrochemistry of glucose fuel cells.".
"Instead of using a battery, which can take up 90 percent of the volume of an implant, you could manufacture a device with a thin film, and you would have a power source with no volumetric footprint," says Jennifer LM Rupp, Simons's thesis supervisor and a visiting professor at DMSE, who is also an associate professor of solid-state electrolyte chemistry at the Technical University of Munich (Germany).
Simons and his colleagues detail their design today in the journal Advanced Materials. Co-authors of the study include Rupp, Steven Schenk, Marco Gysel, and Lorenz Olbrich.
A "tough" separation
The inspiration for the new fuel cell came in 2016, when Rupp, who specializes in ceramics and electrochemical devices, went to have a routine glucose test toward the end of her pregnancy.
"At the doctor's office, I got really bored with electrochemistry, wondering what could be done with sugar and electrochemistry," Rupp recalls. "Then I realized it would be great to have a solid-state device powered by glucose. So Philipp and I got together for coffee and sketched the first drawings on a napkin.".
The team is not the first to conceive of a glucose fuel cell, which was initially introduced in the 1960s and showed potential for converting the chemical energy of glucose into electrical energy. But glucose fuel cells at that time were based on soft polymers and were quickly eclipsed by lithium-iodide batteries, which would become the standard power source for medical implants, most notably the cardiac pacemaker.
However, batteries have a size limit, as their design requires the physical capacity to store energy.
"Fuel cells directly convert energy instead of storing it in a device, so you don't need all that volume that's required to store energy in a battery," says Rupp.
In recent years, scientists have returned to studying glucose fuel cells as potentially smaller energy sources, powered directly by the body's abundant glucose.
The basic design of a glucose fuel cell consists of three layers: an upper anode, an intermediate electrolyte, and a lower cathode. The anode reacts with glucose from body fluids, transforming the sugar into gluconic acid. This electrochemical conversion releases a pair of protons and a pair of electrons. The intermediate electrolyte separates the protons from the electrons, conducting the protons through the fuel cell, where they combine with air to form water molecules, a harmless byproduct that flows with the body fluid. Meanwhile, the isolated electrons flow to an external circuit, where they can be used to power an electronic device.
The team sought to improve existing materials and designs by modifying the electrolytic layer, which is typically made of polymers. However, the properties of polymers, along with their ability to conduct protons, degrade easily at high temperatures, are difficult to preserve when reduced to nanometer dimensions, and are challenging to sterilize. The researchers wondered if a ceramic—a heat-resistant material that can naturally conduct protons—could be used as an electrolyte for glucose fuel cells.
"When you think about ceramics for a fuel cell of this type, they have the advantage of long-term stability, small scalability, and silicon chip integration," Rupp notes. "They are tough and robust.".
Maximum power
The researchers designed a glucose fuel cell with an electrolyte made of ceria, a ceramic material that possesses high ionic conductivity, is mechanically robust, and, as such, is widely used as an electrolyte in hydrogen fuel cells. It has also been shown to be biocompatible.
"Cerium is actively being studied in the cancer research community," Simons notes. "It's also similar to zirconia, which is used in dental implants, and is biocompatible and safe.".
The team placed the electrolyte with a platinum anode and cathode, a stable material that reacts readily with glucose. They fabricated 150 individual glucose fuel cells on a chip, each about 400 nanometers thick and about 300 micrometers wide (roughly the width of 30 human hairs). The cells were placed on silicon wafers, demonstrating that the devices can be combined with a common semiconductor material. They then measured the current produced by each cell by flowing a glucose solution over each wafer in a custom-built test station.
They discovered that many cells produced a peak voltage of about 80 millivolts. Given the tiny size of each cell, this output is the highest power density of any existing glucose fuel cell design.
"It's exciting that we're able to generate enough power and current to power implantable devices," says Simons.
"This is the first time that proton conduction in electroceramic materials can be used to convert glucose into energy, defining a new type of electrochemistry," says Rupp. "It expands the use cases for materials from hydrogen fuel cells to exciting new modes of glucose conversion.".
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Written by Jennifer Chu, MIT News Office
