A team of engineers from MIT and the Technical University of Munich have designed an ultra-thin glucose fuel cell that converts the sugar directly into electricity.
Just 400 nanometres thick, the device generates about 43 microwatts per square centimetre, 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°C.This is important for its potential use to power medical implants, as the fuel cell could remain stable through the high-temperature sterilisation process required for all implantable devices.
The researchers suggest that the new design could be made into ultra-thin 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 research.
‘Instead of using a battery, which can take up 90 per cent 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,’ said Jennifer LM Rupp, Simons’ thesis supervisor and an associate professor of solid-state electrolyte chemistry at Technical University Munich.
The inspiration for the new fuel cell came in 2016, when Rupp, who specialises in ceramics and electrochemical devices, went to take a routine glucose test toward the end of her pregnancy. ‘In the doctor’s office, I was a very bored electrochemist, thinking what you could do with sugar and electrochemistry,’ Rupp recalled. ‘Then I realised, it would be good to have a glucose-powered solid-state device. And Philipp and I met over coffee and wrote out on a napkin the first drawings.’
Glucose fuel cells, based on soft polymers, were first developed during the 1960s, but were quickly eclipsed by lithium-iodide batteries, which would become the standard power source for medical implants, most notably the cardiac pacemaker. However, batteries are size-limited 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,’ said Rupp.
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 water, which 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. Polymers quickly degrade at high temperatures, lose their useful properties when scaled down to nanometre dimensions and are difficult to sterilise, so theresearchers turned to ceramics, which are heat resistant and can naturally conduct protons.
‘When you think of ceramics for such a glucose fuel cell, they have the advantage of long-term stability, small scalability and silicon chip integration,’ Rupp said. ‘They’re hard and robust.’
The new fuel cell features an electrolyte made from ceria, a ceramic material widely used as an electrolyte in hydrogen fuel cells that possesses high ion conductivity and is mechanically robust. It has also been shown to be biocompatible. ‘Ceria is actively studied in the cancer research community,’ said Simons. ‘It’s also similar to zirconia, which is used in tooth implants and is biocompatible and safe.’
The team sandwiched the electrolyte between 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 nanometres thick and about 300 micrometres 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.
They 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 electro-ceramic materials can be used for glucose-to-power conversion, defining a new type of electrochemistry,’ Rupp said. ‘It extends the material use-cases from hydrogen fuel cells to new, exciting glucose-conversion modes.’
The research has been published in Advanced Materials.