The device, measuring just 400 nanometers thick, is capable of generating about 43 microwatts per square centimetre of electricity, which is the highest power density of any glucose fuel cell to date under ambient conditions.
The device is able to withstand temperatures up to 6000 Celsius and incorporated into a medical implant, the fuel cell is able to remain stable through the high-temperature sterilization process required for all implantable devices.
The device is made from ceramic, a material that retains its electrochemical properties even at high temperatures and miniature scales. The researchers said that the new design could be made into ultrathin films or coatings and wrapped around implants to passively power electronics, using the body’s abundant glucose supply.
“Glucose is everywhere in the body, and the idea is to harvest this readily available energy and use it to power implantable devices,” said Philipp Simons, who developed the design as part of his PhD thesis in MIT’s Department of Materials Science and Engineering (DMSE). “In our work we show a new glucose fuel cell electrochemistry.”
“Instead of using a battery, which can take up 90 percent of an implant’s volume, you could make a device with a thin film, and you’d have a power source with no volumetric footprint,” added Jennifer L.M. Rupp, a DMSE visiting professor, who is also an associate professor of solid-state electrolyte chemistry at Technical University Munich in Germany.
Glucose fuel cells were initially introduced in the 1960s and showed potential for converting glucose’s chemical energy into electrical energy. But glucose fuel cells at the 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 limit to how small they can be made, as their design requires the physical capacity to store energy.
“Fuel cells directly convert energy rather than storing it in a device, so you don’t need all that volume that’s required to store energy in a battery,” Rupp explained.
A glucose fuel cell’s basic design consists of three layers: a top anode, a middle electrolyte, and a bottom cathode. The anode reacts with glucose in bodily fluids, transforming the sugar into gluconic acid. This electrochemical conversion releases a pair of protons and a pair of electrons. The middle electrolyte acts to separate the protons from the electrons, conducting the protons through the fuel cell, where they combine with air to form molecules of water — a harmless by-product that flows away with the body’s fluid. Meanwhile, the isolated electrons flow to an external circuit, where they can be used to power an electronic device.
The team looked to improve on existing materials and designs by modifying the electrolyte layer, which is often made from polymers. Polymer properties, however, easily degrade at high temperatures, are difficult to retain when scaled down to the dimension of nanometers, and are hard to sterilize. Hence the decision to turn to ceramic as an electrolyte for glucose fuel cells.
The researchers have designed a glucose fuel cell with an electrolyte made from ceria, a ceramic material that possesses high ion 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.
“Ceria is actively studied in the cancer research community,” Simons noted. “It’s also similar to zirconia, which is used in tooth implants, and is biocompatible and safe.”
The team sandwiched the electrolyte with an anode and cathode made of platinum, a stable material that readily reacts with glucose. They fabricated 150 individual glucose fuel cells on a chip, each about 400 nanometers thin, and about 300 micrometers wide. They patterned the cells onto silicon wafers, showing that the devices can be paired with a common semiconductor material. They then measured the current produced by each cell as they flowed a solution of glucose over each wafer in a custom-fabricated test station.
The team found 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.
“Excitingly, we are able to draw power and current that’s sufficient to power implantable devices,” Simons said.
“It is the first time that proton conduction in electroceramic materials can be used for glucose-to-power conversion, defining a new type of electrochemstry,” Rupp explained. “It extends the material use-cases from hydrogen fuel cells to new, exciting glucose-conversion modes.”
