Steering reactions with light

Waves of electricity or chemicals ripple through living tissues in many processes, including heart-muscle contractions, nerve signaling, and cell metabolism. To learn how those waves form and propagate, researchers examine analogous wave motions in the lab (SN: 2/11/95, p. 84).

WAVE WAYS. This combined sequence of colorized snapshots, each separated by 40 seconds, depicts the moving front of a chemical reaction. T. Sakurai et al./Science

In new experiments, Kenneth Showalter of West Virginia University in Morgantown and his colleagues have used light to program the motion of such waves. By varying the pattern and intensity of light projected onto a fingernail-size patch of chemical gel, the team both initiated and steered tiny, bright arcs of reacting chemicals.

Aspects of the new control techniques may ultimately find a place in medical therapies, such as preventing epileptic seizures, Showalter says.

For their studies, Showalter and his colleagues used the most well-studied wave-forming chemical reaction, known as the Belousov-Zhabotinsky (BZ) reaction (SN: 2/21/98, p. 116:

It unfolds as a complex interplay among bromate ions, malonic acid, and a catalyst.

To orchestrate the reaction, the researchers direct a flow of the reactants continuously over a silica gel impregnated with a light-sensitive catalyst.

Meanwhile, they illuminate the setup, which suppresses the catalyst’s action. To initiate the BZ reaction in any specific spot in the gel, the researchers dim the illumination at that spot, spurring the catalyst to act.

In the reaction, the catalyst temporarily oxidizes and turns green against the otherwise orange gel. A green arc of catalyst a millimeter or two wide shows the researchers, at any given moment, where the leading edge of the chemical wave front is.

Such wave fronts spontaneously move in a straight line. However, the researchers report in a forthcoming Science, that the waves turn toward the dark side of a light gradation. So, by varying the illumination, “we can make [the wave fronts] go anywhere we want,” Showalter says.

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