Bioreactors: Toward Miniaturation

How does a biofuel cell work?

Electrons generated at one electrode from metabolic energy are used to generate a current.

How does yours work?

We have not made a biofuel cell; rather, we have shown a proof-of-principle using a new class of metal reducing proteins that can convert metabolic energy directly into current at an electrode surface.

The major insight was the identification of outer membrane proteins in a class of bacteria that reduces metals, and the observation that once isolated that these proteins naturally bind to hematite (Fe2O3) and transfer charge (current) to the metal. This is a natural way for these bacteria to survive, since they can grow in the absence of oxygen and use extracellular metals to accept electron charge. From the standpoint of bioenergy, hematite is routinely used to coat electrodes, and these proteins could be used in a devise to couple metabolic energy into current. One might imagine that a small remote devise can couple the energy generated during normal cellular metabolism or on an industrial scale using in a bioreactor using immobilized enzymes to metabolize biomass. But the exciting thing in our mind is the discovery of this class of enzymes that directly interact with solid metals to transfer current.

This could make miniature bioreactor cells feasible. Can you elaborate?

The ability to immobilize the protein at the electrode surface makes it unnecessary to divide the two chambers of a fuel cell with a membrane, making miniaturation possible.

Figure: Artist's depiction of purified, electrified bacterial cell outer membrane protein binding with and passing electrons to the iron-rich mineral hematite. In this purified-protein fuel cell, the seal made by the protein coating on the electrode effectively acts in place of a membrane necessary in whole-organism biofuel cells. Eliminating the membrane could aid the design of bioreactors to power small electronic devices. Source: Pacific Northwest National Laboratory

[1] Yijia Xiong et al, Advance online edition of the Journal of the American Chemical Society (Web release date: October 11 2006). The abstract can be read here.

Thomas Squier works at the Pacific Nothwest National Laboratory

Interview by Thanh Tam Candice Vu and Gilles Prigent