[Image above] A silicon chip with 30 individual glucose micro fuel cells, seen as small silver squares inside each gray rectangle. Credits: Kent Dayton, Massachusetts Institute of Technology
By Laurel Sheppard
Since the first successful pacemaker was implanted in 1960, the implantable medical device market exploded thanks to numerous scientific and engineering innovations that expanded the types and sophistication of implantable devices.
Recently, the miniaturization of electronic circuits and mechanical structures instigated a shift in implantable medicine from passive to active implants, which will help devices self-manage incidents before they occur rather than remedying incidents afterward. However, to drive this revolution, new power sources are required that can deliver safe and stable energy to these miniaturized implants.
Traditionally, lithium-based battery systems are the primary power source for implantable medical devices, particularly the well-known lithium/iodine-polyvinylpyridine (Li–I2) battery for pacemakers. But batteries cannot be easily miniaturized without sacrificing substantial energy storage capacity. Also, the Li–I2 pacemaker battery is nonrechargeable, meaning replacement surgery is required once the battery’s energy is depleted, creating a risk for complications.
Because of these limitations, researchers have explored alternative energy systems, such as piezoelectric and triboelectric nanogenerators. Glucose fuel cells are another alternative. These fuel cells allow for significant volumetric scale-down because they do not physically store energy like batteries. Instead, they directly convert the sugar glucose, which is readily available inside the body, into electrical energy through oxidation.
Conventionally, glucose fuel cells use polymer proton-exchange membranes as the electrolyte. Despite good conductivity, these membranes present several drawbacks, including a limit on miniaturization, challenges integrating with silicon-based chip design, and inability for thermal sterilization.
Fortunately, these limitations may be overcome by moving away from polymer electrolytes. A new open-access study by researchers from the Massachusetts Institute of Technology and the Technical University of Munich demonstrates the potential of ceramic proton-conducting electrolytes to overcome these limitations.
Ceramic materials have a long history of use as electrolytes in solid oxide fuel cells and protonic ceramic fuel cells. However, they have not yet been considered for glucose fuel cells.
Makeup of a glucose fuel cell
A glucose fuel cell consists of three layers: a top anode, a middle electrolyte, and a bottom cathode. Glucose is converted into electrical energy through the following reactions.
1. At the anode, glucose is oxidized to form gluconic acid. This conversion releases pairs of protons and electrons.
2. The electrolyte acts to separate the protons from the electrons.
3. When the protons reach the cathode, they combine with air to form molecules of water.
4. The isolated electrons flow to an external circuit, where they can be used to power electronic devices.
For this study, the researchers chose to investigate the potential of ceria (CeO2), a widely used electrolyte in hydrogen fuel cells, because it is
- Nontoxic and biocompatible,
- Stable at temperatures exceeding 1,000°C,
- Displays proton conductivity that allows for operation at body temperature, and
- Has high mechanical stability.
Platinum was chosen for the anode and cathode because it is stable and readily reacts with glucose. The anode was fabricated as a layer of nanoporous platinum with a thickness of 100 nm using a reactive-sputtering method. Because no wet-etching step is required, the process is compatible with ceria. The ceria electrolyte was deposited as thin films of 250 ± 25 nm via pulsed laser deposition.
After optimizing all fabrication steps, the researchers successfully fabricated 150 visually intact glucose fuel cells. According to them, “Among both polymeric glucose fuel cells and hydrogen-based ceramic micro-fuel-cells, this number of tested prototypes is among the highest ever reported.”
Each cell had a thickness of 370 ± 40 nm and measured 300 µm × 300 µm in area. These dimensions made the cells two-orders-of-magnitude thinner than commercial polymer membranes and three times thinner than the next-thinnest device.
The fuel cells delivered a peak power density of up to 43 µW/cm2, which may be the highest power density of any glucose fuel cell to date under ambient conditions. In addition, each cell withstood temperatures up to 600°C. The cells were also exposed to glucose solution and characterized for up to 140 hours, indicating they have long-term stability.
On a structural level, the ceria films had a dense, columnar microstructure with a grain size diameter between 10 and 50 nm. This rough microstructure helped
- Enhance the overall proton conductivity of the electrolyte,
- Reduce mechanical stress in the fuel-cell membrane,
- Block chemical crosstalk between the electrodes,
- Maintain electrochemical potential, and
- Reduce the total device failure rate.
With the proof-of-concept a success, the researchers say there is more work to be done. First, the degradation behavior of the fuel cell system must be understood for future implantation. Then, in vivo studies will be required to verify the fuel cell can operate in a living organism.
Beyond implantable applications, the researchers state this study demonstrated ceria films can be used as the electrolyte of room-temperature energy-conversion devices, which could serve as a model for other small-scale low-temperature energy harvesters.
“Collectively, these ceramic glucose fuel cells constitute the smallest implantable power source to date and put new applications in highly miniaturized implantable devices into perspective,” they write.
The open-access paper, published in Advanced Materials, is “A ceramic-electrolyte glucose fuel cell for implantable electronics” (DOI: 10.1002/adma.202109075).