Overview

= **ENZYMATIC FUEL CELLS** =

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**Why Enzymatic Fuel Cells?**
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In addition to standard fuel cells, enzymatic fuel cells (EFCs) are well suited to power low powered implantable devices such as muscle stimulators, drug delivery devices, biosensors, insulin pumps and pacemakers [1][2]. These fuel cells can potentially be operated using readily available reactants present in the body such as glucose and oxygen. Through the continuous supply of blood gluocse, EFCs will not need to be surgically removed and replaced periodically.

Compared to traditional metal catalysts, the positives attributes of powering fuel cells through enzymes include [2]:
 * extended long-term stability
 * bio-compatibility
 * integrable into medical devices
 * can be grown in quantity
 * abundant
 * inexpensive

Utilizing fuels and oxidants present in physiological fluids, Glucose Fuel Cells (GFCs) offer the promise of self-sustainable electrical power generation. Drawing from a continuous supply of blood glucose, GFCs and the implantable devices they power will not need periodic surgical removal.



A common enzyme that is used to catalyze the oxidation of glucose is glucose oxidase (GOx). The enzyme will catalyze glucose into gluconolactone, which hydrolyzes into gluconic acid. Oxygen accepts GOx's electrons and is reduced into hydrogen peroxide. Glucose oxidation can also be catalyzed with glutamate dehydrogenase (GDH) and cellobiose dehydrogenase (CDH) [2].



The benefits of using glucose include [2] :
 * highest energy density after complete oxidation as compared to ethanol or methanol (See Chart)
 * a continuous source of energy
 * high concentrations found in the bloodstream
 * less toxic compared to methanol or ethanol

**How do EFCs work??**
The diagram below shows how an enzymatic fuel cell works in the bloodstream. The picture [3] demonstrates the use of glucose as fuel and dissolved oxygen as an oxidant, along with mediators assisting the process. For more information on mediators, please visit our electron transport page.



The basic enzymatic fuel cell, pictured below, consists of an anode, cathode and a separator [7]. Instead of the traditional metallic electrocatalysts at the anode and cathode, oxidoreductase enzymes are employed at one or both terminals.



The simple glucose fuel cell reaction is as follows:

**Problems with EFC’s and the current applications**
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Glucose EFCs have the potential to be self contained power generators however current generation utilizing enzymes is complicated. Enzymes do not give up electrons as easily as metal catalysts. Not being able to generate sufficient current can cause unwanted problems in the fuel cells. In order to give up these electrons, the assistance of mediators (defined here) are required. Also, enzymes are specific, but are not use to staying put. In order for the enzymes to work properly, they must remain relatively stationary and in the proper orientation on the anode. Immobilizing the enzymes so that they can be held in a specific place for the reaction to occur, is therefore important.

Another problem in enzymatic fuel cells that needs to be addressed is the incomplete oxidation of glucose at the anode of the fuel cell. This produces low efficiency since only a single enzyme is incorporated. Sugars such as glucose have complex metabolic pathways and contain both oxidoreductase enzymes as well as other non-oxidoreductase enzymes. In addition, EFCs have failed previously as enzymes harvesting glucose rely on low pH, where as the body is too acidic to accommodate the enzymes. These enzymes are also inhibited by charged particles such as chloride or urate anions in the fluid surrounding the cells.

**//Recent Proof of Principle Experiments//**

 * A team of researchers from Tohoku University in Japan used blood sugar from a rabbit's ear as a power generator. A needle was inserted into the rabbit's blood vessel which acted as the anode required for glucose oxidation. [5]


 * In Grenoble, at Joseph Fourier University, a team successfully implanted a glucose EFC in the abdomen of two rats. The EFC used the glucose and oxygen present in the abdomen of the rats. Unfortunately these EFCs generated a very low voltage output of 6.5mV which is under the 10mV required for today`s pacemakers. [6]




 * Akermin, Inc developed an ethanol air fuel cell which powered an Ipod nano. This innovation goes to show the potential of enzymatic fuel cells as power generators. Similar devices may easily be developed that incorporate glucose air fuel cells into power small electronics. [7]



Glucose powered fuel cells have tubular consequences, which include the fact that they are able to operate optimally in room and body temperature. Although there are issues to consider including; short lifetimes, low power densities and low efficiency, many improvements have been made in recent years. Research in the area is primarily focused on immobilizing and stabilizing the enzymes so that they can function optimally in the fuel cells.

= = =References=

1.Shleev et al. 2008. A membrane-, mediator-, cofactor-less glucose/oxygen biofuel cell. Physical Chemistry Chemical Physics. 10(40): 6069-6200. 2. Oncescu V, Erickson D. 2011. A microfabricated low cost enzyme-free glucose fuel cell for powering low-power implantable devices. Journal of Power Sources. 196:9169-9175. 3. Sokic-Lazic D et al. 2008. Oxidation of Biofuels: Fuel Diversity and Effectiveness of Fuel Oxidation through Multiple Enzyme Cascades. Electroanalysis. 22: 757-764. 4. Yu E.H, Scott K. 2010. Enzymatic Biofuel Cells - Fabrication of Enzyme Electrodes. Energies. 3: 23-42. 5. Miyake et al. 2011. Enzymatic biofuel cells designed for direct power generation from bioﬂuids in living organisms. nergy and Environmental Science. 4:5008. 6. Cinquin et al. 2010. A glucose biofuel cell implanted in rats. PLOS ONE 5(5): e10476. doi:10.1371/journal.pone.0010476. 7. Atanassov et al. 2007. Enzymativ BioFuel Cells. The Electrochemical Society Interface. 1:28-31.