Micromotor Uses Biocompatible Fuel

Miniature rockets zooming through a person’s body are still the stuff of science fiction. With the goal of using the devices to deliver drugs, researchers have built some versions in the lab, but the tiny motors typically propel themselves with toxic fuels such as hydrogen peroxide, or they emit harmful metals such as gallium in their exhaust. A team led by Ayusman Sen of Pennsylvania State University recently developed a micromotor that carries its own fuel: a polymer approved by the Food & Drug Administration for biomedical applications. And the tiny racer expels low levels of ethanol and formaldehyde, which are naturally occurring compounds found in food and drink (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja4089549). To make the motor, the researchers soaked a 300-µm-diameter anion-exchange bead in the fuel, poly(2-ethyl cyano­acrylate) (PECA). Then, they coated half of the bead with an inert compound, poly(methyl methacrylate), to give the motor directionality. When floated atop a high-concentration salt solution, the bead releases hydroxide ions from its uncoated half. The ions break down PECA, releasing ethanol, formaldehyde, and cyanoacrylate monomer. These compounds, the researchers believe, create a surface tension gradient around the motor that propels it forward at some 500 body lengths per second.