Devices that support various functions of our bodies are being used increasingly. Today, they include cardiac pacemakers or hearing aids. Tomorrow, they may be contact lenses with automatically changing focal length or computer-controlled displays generating images directly in the eye. But, none of these devices will work if not coupled to an efficient and long-lasting power supply source. Researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw say that the best solution seems to be miniaturized biofuel cells that consume substances naturally occurring in the human body or in its immediate surroundings.
They have developed an efficient electrode to be used in the construction of biofuel cells or zinc-oxygen biobatteries. After installation in a cell, the new biocathode generates a voltage, over many hours, that is higher than what can be obtained in existing power sources of similar design. The most interesting feature, they say, is that the device is air-breathing—it works at full efficiency when it can take in oxygen directly from the air.
Common batteries and rechargeable batteries are unsuitable, they say, to power implants inside the human body because they use strong bases or acids. The battery housing must, therefore, be tightly sealed.
Here is where biofuel cells offer an essential advantage: they do not require a housing. To get electricity, it is enough to insert the electrodes into the body, they say.
“One of the most popular experiments in electrochemistry is to make a battery by sticking appropriately selected electrodes into a potato. We are doing something similar. The difference is that we are focusing on biofuel cells and the improvement of the cathode. And, of course, to have the whole project working, we’d rather replace the potato with...a human being,” says Dr. Martin Jönsson-Niedziółka, a research associate in the Department of Eletrode Processes, IPC PAS.
In the experiments, his group uses zinc-oxygen batteries. The principle of their operation is not new. Batteries constructed in this way have been popular since before alkaline power sources came.
“At present, many laboratories work on glucose-oxygen biofuel cells. In the best case they generate a voltage of 0.6-0.7 V. A zinc-oxygen biobattery with our cathode is able to generate 1.75 V for many hours,” says Adrianna Złoczewska, a PhD student at IPC PAS, whose research has been supported under the International PhD Projects Programme of the Foundation for Polish Science. (See Figure 1)
The main component of the biocathode developed at the IPC PAS is an enzyme surrounded by carbon nanotubes and encapsulated in a porous structure—a silicate matrix deposited on an oxygen permeable membrane. Their group has been working for many years on the techniques needed to construct the cathode using enzymes, carbon nanotubes, and silicate matrices.
An electrode so constructed is installed in a wall of a small container. To have the biofuel cell working, it is enough to pour an electrolyte (here: a solution containing hydrogen ions) and insert the zinc electrode in the electrolyte. The pores in the silicate matrix enable oxygen supply from the air and H+ ions from the solution to active centers of the enzyme, where oxygen reduction takes place. Carbon nanotubes then facilitate the transport of electrons from the surface of the semipermeable membrane.
A cell with the new biocathode is able to supply power with a voltage of 1.6 V, for a minimum one and a half weeks. The cell efficiency decreases with time, likely because of gradual deactivation of the enzyme on the biocathode.
In the experiments carried out so far, a stack of four batteries connected in series successfully powered a lamp composed of two LEDs. However, before the biofuel cells based on the design developed at the IPC PAS get commercialized, the researchers must solve the problem of relatively low electric power that is common to all types of biofuel cells.
The research being conducted may someday create miniaturized power supply sources for medical implants, biosensors, or light-emitting tattoos.