Lithium-iodide batteries have 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. In recent years, scientists have been investigating glucose fuel cells as potentially smaller power sources, fueled directly by the body’s abundant glucose which is the sugar we absorb from the foods we eat. Glucose powers every cell in our bodies and could now power tomorrow’s medical implants.

Engineers at Massachusetts Institute of Technology (MIT, Cambridge, MA, USA) and the Technical University of Munich (Munich, Germany) have designed a new kind of glucose fuel cell that converts glucose directly into electricity. The device is smaller than other proposed glucose fuel cells, measuring just 400 nanometers thick, or about 1/100 the diameter of a human hair. The sugary power 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 resilient, able to withstand 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 researchers 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. 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 (about the width of 30 human hairs). 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. They found 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. The researchers envision 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% 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, Simons’ thesis supervisor and a DMSE visiting professor, who is also an associate professor of solid-state electrolyte chemistry at Technical University Munich in Germany.

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