The new fuel cell, which provides twice as much power as conventional biofuel cells according to the researchers from the Georgia Institute of Technology and Korea University, could be paired with batteries or supercapacitors to provide a hybrid power source for the medical devices.
The team used gold nanoparticles assembled on the cotton to create high-conductivity electrodes that helped improve the fuel cell's efficiency. That allowed them to address one of the major challenges limiting the performance of biofuel cells - connecting the enzyme used to oxidise glucose with an electrode.
A layer-by-layer assembly technique used to fabricate the gold electrodes - which provide both the electrocatalytic cathode and the conductive substrate for the anode - helped boost the power capacity to as much as 3.7 milliwatts per square centimetre.
"We could use this device as a continuous power source for converting chemical energy from glucose in the body to electrical energy," said Assistant Professor Seung Woo Lee of Georgia Tech. "The layer-by-layer deposition technique precisely controls deposition of both the gold nanoparticle and enzyme, dramatically increasing the power density of this fuel cell."
Fabrication of the electrodes begins with porous cotton fibre composed of multiple hydrophilic microfibrils - cellulose fibres containing hydroxyl groups. Gold nanoparticles about eight nanometers in diameter are then assembled onto the fibres using organic linker materials.
To create the anode for oxidising the glucose, the researchers apply glucose oxidase enzyme in layers alternating with an amine-functionalised small molecule known as TREN. The cathode, where the oxygen reduction reaction takes place, used the gold-covered electrodes, which have electrocatalytic capabilities.
"We precisely control the loading of the enzyme," Assist Prof. Lee said. "We produce a very thin layer so that the charge transport between the conductive substrate and the enzyme is improved. We have made a very close connection between the materials so the transport of electrons is easier."
The porosity of the cotton allowed an increase in the number of gold layers compared to a nylon fibre. "Cotton has many pores that can support activity in electrochemical devices," explained Yongmin Ko, one of the paper's co-authors. "The cotton fibre is hydrophilic, meaning the electrolyte easily wets the surface."
Beyond improving the conductivity of the electrodes, the cotton fibre could improve the biocompatibility of the device, which is designed to operate at low temperature to allow use inside the body.
Implantable biofuel cells suffer from degradation over time, and the new cell developed by the US and Korean team offers improved long-term stability. "We have a record high power performance, and the lifetime should be improved for biomedical applications such as pacemakers," Assist Prof. Lee said.
Pacemakers and other implantable devices are now powered by batteries that last years, but may still require replacement in a procedure that requires surgery. The biofuel cell could provide a continuous charge for those batteries, potentially extending the time that devices may operate without battery replacement, Assist Prof. Lee added.
In addition, the biofuel cell could be used to power devices intended for temporary use. Such devices might be implanted to provide timed release of a drug, but would biodegrade over time without requiring surgical removal. For these applications, no battery would be included, and the limited power required could be provided by the biofuel cell.
Future goals of the research include demonstrating operation of the biofuel cell with an energy storage device, and development of a functional implantable power source. "We want to develop other biological applications for this," said Assist Prof Lee. "We'd like to go further with other applications including batteries and high-performance storage."