Progress in Research on Bacteria-Powered Motors

August 1, 2007

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

Progress has been made on several fronts in research on biomotors and especially on microscopic motors powered by bacteria. The progress consists mostly of advances in the art of attaching motile bacterial cells to surfaces in specific, pre-designed microarrays.

The accomplishments during the past four years are the following:

• The first advance was development of a method to fabricate microarrays capable of binding motile Escherichia coli cells. The microarrays were patterned with 16 mercaptohexadecanoic acid (MHA), then covalently functionalized with, variously, E. coli antibodies, lipopolysaccharide, or poly-Llysine (PLL). It was also found that the use of 11-mercaptoundecyl- penta(ethylene glycol) [PEG SH] or 11-mercapto- l-undecanol [MOU] as passivating molecules nearly completely inhibited non-specific binding of E. coli. Microcontact printing was used to prepare microarrays for adhesion of bacterial cells, and attachment of bacteria was examined by optical/fluorescence and atomic-force microscopy.
• It was shown that cells of the K-12 strain of E. Coli remain alive for more than four hours after initial adhesion to prefabricated surface structures.
• Attempts to bind E. Coli cells to PLL-MHA dots of various sizes revealed that the minimum surface feature size for binding of E. Coli is 1.3 μm. (Bacterial cells of species other than E. Coli may prefer different surface-feature sizes; this is the subject of a continuing investigation.)
• A substrate that contains a series of holes made by use of electron-beam lithography was prepared in order to use the holes as devices to hold bacterial cells in a "nose-on" orientation at the specific hole positions. The bottom surfaces of the holes were coated with gold and the gold was coated with MHA followed by PLL. The regions between the holes were passivated with penta(ethylene glycol). It was found to be possible to bind a single motile bacterial cell in a hole at a loose approximation of the "nose-on" orientation.
• The degree of adhesion of bacterial cells to surface features was found to depend on acidity or alkalinity: At a pH of 9, cells adhered to nearly all surface features, while at a pH of 4, nearly all the features were devoid of cells.
• CheY-deficient Pseudomonas aeruginosa cells, which have been characterized as "smooth swimming," have been attached to microarray surfaces. [CheY is an excitatory response regulator of chemotaxis; hence, CheY-deficient P. aeruginosa does not undergo chemotaxis.] P. aeruginosa are the bacteria of choice for micron-scale biomotors because, among other things, they swim about 40 times faster than do E. coli.
• Dip-pen nanolithography (DPN) was used to prepare microarrays for adhesion of bacterial cells. DPN is a particularly important soft lithographic method inasmuch as it enables placement of whole bacterial cells at designated sites on surface microarrays. It was found to be possible to attach a single motile P. aeruginosa cell to designated line and/or dot features. The cell can be bound via either its body or its flagellum. Cells bound in this way were found to remain alive for more than four hours.
• Biomotor devices consisting essentially of glass microscopeslide covers asymmetrically coated with Pseudomonas aeruginosa cells suspended in deionized water on gold threads were constructed. These devices were observed to spin in the water when the bacteria were alive, and not to spin when the bacteria were dead.

This work was done by Richard C. Holz of Utah State University for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp  under the Physical Sciences category. AFRL-0027

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Progress in Research on Bacteria-Powered Motors

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