When I think about the future I sometimes indulge in fantasies that include “bionic” type implants. Not so much artificial muscles that will enhance strength (check this out) but devices that will expand our mental capabilities. Implants that give us greater memory, faster thought processes, the ability to download skills and knowledge directly into our brains.
Perhaps I read too much science fiction.
The trouble with this utopian view of the future is a practical one, where will these devices receive their power from? If I start forgetting things because I haven’t plugged in a new 9volt, I won’t be happy. The ideal solution would be some sort of battery that can be inserted into the body and generate energy from the food I eat, just like the rest of my organs.
Enter: The BioFuel cell. Fuel cells have been around for a few decades and while most people have no dealings with them their essential mechanism is easily understood. Basically a chemical reaction is allowed to proceed under very controlled conditions to generate a flow of electrons (ie an electric current). The usual example given is reacting Hydrogen (the fuel) with Oxygen to generate water and electricity, but really almost any electron donor/acceptor pair will do.
Biofuel cells replace the electron donor (eg Hydrogen as above) with a biological molecule, glucose. These Glucose BioFuel Cells (GBFCs) could then in theory utilise glucose dissolved the blood as a fuel to generate electricity and power implanted devices. The fanciful science fiction devices I dream about above may not arrive on the scene any time soon but there are medical devices and synthetic organs that would benefit from such a power source now.
The device that immediately springs to mind is a pacemaker but the possibilities are much wider, ranging from the artificial urinary sphincter that recipients of Radical Prostatectomy surgery depend on to artificial kidneys which to be portable must currently be wearable because of (among many other reasons) the inability to effectively supply it with power inside the body.
Existing GBFCs have a draw back that the electrodes are inhibited (work less efficiently) by chloride or urate ions, both of which are present in your blood, or require low (acidic) pH to work whereas your body likes to be around neutral pH. This makes them ineffective in a real biological environment. Luckily a recent paper in Plos ONE, “A Glucose BioFuel Cell Implanted in Rats“, details an alternative type of fuel cell that overcomes these limitations and demonstrates it by implanting it inside a rat.
The GBFC produced a specific power of 24.4 µW mL−1 which, to put that in perspective, could power two pacemakers (just in case The Doctor gets into trouble). The mL−1 part refers to the volume of the fuel cell, this really means that the power output is related to the size of the fuel cell, just like regular batteries. The volume of this cell appears to be little more than a quarter of a millilitre (0.266mL, two electrodes of 0.133mL each), think about how much volume a normal 6 sided die takes up, imagine one quarter of that and you’ll be in the right ball park.
Inside the fuel cell electrodes are enzymes that react the glucose with dissolved oxygen also in the blood to produce an electric current. The glucose and oxygen required for the reaction diffuse through a membrane surrounding the electrodes while the waste products diffuse back out into the bloodstream to be taken care of by the body. In this way the fuel is constantly being replenished and so long as the enzymes retain their activity the fuel cell will continue to function and continuously produce energy. The time scales measured in this study were only a few months but experiments by others suggests that the enzymes will stay active in a device such as this for at least a year and possibly more.
The authors of the study consider that a scaled up version of the device would be able to power medical implants with much greater power requirements than a pacemaker, such as the artificial sphincter mentioned above that they calculate would need almost 10 times the amount of power of a pacemaker. Seems like the limit at the moment is how much room you have to spare inside your body to house the fuel cell. Early pacemaker batteries took up about 90mLs worth of volume, that’s roughly a quarter the size of a drink can. It is mentioned that an animal such as a pig could accommodate a fuel cell 133mL in size but it is not made clear if this is an experiment that will actually occur. RoboPig.
All this is pretty exciting and with that sort of potential in a first generation fuel cell, I’m betting I can get my memory expansion before I start going senile. Now I just have to figure out what to do in the mean time.
Cinquin P, Gondran C, Giroud F, Mazabrard S, Pellissier A, Boucher F, Alcaraz JP, Gorgy K, Lenouvel F, Mathé S, Porcu P, & Cosnier S (2010). A glucose biofuel cell implanted in rats. PloS one, 5 (5) PMID: 20454563
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