High Dimension

Ultra-thin fuel cell generates electricity from your body’s sugar

Silicon chip with 30 individual micro glucose fuel cells, seen as small silver squares inside each gray rectangle. Credit: Kent Dayton

Engineers have created a glucose power source that could power miniature implants and electronic sensors.

Glucose is a sugar that we absorb from the foods we eat. It is the fuel that powers every cell in our body. Could glucose also power the medical implants of the future?

Engineers at MIT and the Technical University of Munich think so. They have designed a new type of glucose fuel cell that converts glucose directly into electricity. The device is smaller than other glucose fuel cells on offer, measuring just 400 nanometers thick, or about 1/100 the width of a human hair. The sugary power source generates approximately 43 microwatts per square centimeter of electricity, achieving the highest power density of any glucose fuel cell under ambient conditions to date.

The new device is also tough, able to withstand temperatures of up to 600 degrees. Celsius (1,112 Fahrenheit). If incorporated into a medical implant, this high heat tolerance would allow the fuel cell to remain stable through the high temperature sterilization process required for all implantable devices.

The heart of the new device is ceramic, a material that retains its electrochemical properties even at high temperatures and at miniature scales. The researchers envision the new design could be made into ultra-thin films or coatings and wrapped around implants to passively power the electronics, using the body’s abundant supply of glucose.

Silicon-chip micro-glucose fuel cell experiment

Custom experimental setup used to characterize 30 rapid sequence glucose fuel cells. Credit: Kent Dayton

“Glucose is everywhere in the body, and the idea is to harvest this readily available energy and use it to power implantable devices,” says Philipp Simons, who developed the design as part of his doctoral thesis at MIT Department of Materials Science and Engineering (DMSE). “In our work, we show a new electrochemistry of glucose fuel cells.”

“Instead of using a battery, which can take up 90% of an implant’s volume, you could create a device with a thin film, and you would have a power source with no volumetric footprint,” says Jennifer LM Rupp, of Simons thesis. DMSE supervisor and visiting professor, who is also an associate professor of solid-state electrolyte chemistry at the Technical University of Munich in Germany.

Simons and his colleagues detailed their design recently in the newspaper Advanced materials. The co-authors of the study are Rupp, Steven Schenk, Marco Gysel and Lorenz Olbrich.

A “hard” separation

The inspiration for the new fuel cell came in 2016, when Rupp, which specializes in ceramics and electrochemical devices, went for a routine blood sugar test towards the end of her pregnancy.

“In the doctor’s office, I was a very bored electrochemist, thinking about what you could do with sugar and electrochemistry,” Rupp recalled. “Then I realized that it would be good to have a glucose-powered solid-state device. And Philipp and I met over coffee and wrote down the first drawings on a napkin.

The team is not the first to design a glucose fuel cell, which was originally introduced in the 1960s and showed potential for converting chemical energy from glucose 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, including pacemakers.

However, batteries have a limit to their size, as their design requires the physical ability to store energy.

“Fuel cells convert energy directly rather than storing it in a device, so you don’t need all that bulk to store energy in a battery,” Rupp says.

In recent years, scientists have re-examined glucose fuel cells as potentially smaller power sources, powered directly by the body’s abundant glucose.

The basic design of a glucose fuel cell consists of three layers: an upper anode, a central electrolyte, and a lower cathode. The anode reacts with glucose in body fluids, turning sugar into gluconic acid. This electrochemical conversion releases a pair of protons and a pair of electrons. The intermediate electrolyte acts to separate the protons from the electrons, driving the protons through the fuel cell, where they combine with air to form water molecules – a harmless byproduct that flows with fluids bodily. 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 electrolyte layer, which is often made of polymers. But the properties of polymers, as well as their ability to conduct protons, degrade easily at high temperatures, are difficult to maintain when reduced to nanometer size, and are difficult to sterilize. The researchers wondered if a ceramic – a heat-resistant material that can naturally conduct protons – could be made into an electrolyte for glucose fuel cells.

“When you think of ceramic for such a glucose fuel cell, it has the advantage of long-term stability, small scalability, and silicon chip integration,” Rupp notes. “They are tough and sturdy.”

peak power

Researchers designed a glucose fuel cell with an electrolyte based on cerium oxide, a ceramic material that has high ionic conductivity, is mechanically robust, and as such is widely used as an electrolyte in fuel cells to hydrogen. It has also been shown to be biocompatible.

“Ceria is being actively studied in the cancer research community,” notes Simons. “It is also similar to zirconia, which is used in dental implants, and is biocompatible and safe.”

The team sandwiched the electrolyte with an anode and a cathode made of platinum, a stable material that reacts easily with glucose. They fabricated 150 individual glucose fuel cells on a chip, each about 400 nanometers thick and about 300 micrometers wide (about the width of 30 human hairs). They patterned the cells on 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 ran glucose solution over each wafer in a custom-made test station.

They found that many cells produced a peak voltage of around 80 millivolts. Given the small size of each cell, this output is the highest power density of any existing glucose fuel cell design.

“Excitingly, we are able to draw sufficient power and current to drive implantable devices,” says Simons.

“This is the first time that proton conduction in electroceramic materials can be used for the conversion of glucose into power, defining a new kind of electrochemistry,” Rupp says. “It expands use cases for the materials, from hydrogen fuel cells to exciting new modes of glucose conversion.”

The researchers “have opened a new path to miniature power sources for implanted sensors and possibly other functions,” says Truls Norby, a professor of chemistry at University of Oslo in Norway, which did not contribute to the work. “The ceramics used are non-toxic, inexpensive and above all inert to body conditions and sterilization conditions prior to implantation. The concept and demonstration so far are indeed promising.

Reference: “A Ceramic-Electrolyte Glucose Fuel Cell for Implantable Electronics” by Philipp Simons, Steven A. Schenk, Marco A. Gysel, Lorenz F. Olbrich and Jennifer LM Rupp, April 5, 2022, Advanced materials.
DOI: 10.1002/adma.202109075