Microengines Forever Blowing Bubbles

  • ChemPubSoc Europe Logo
  • DOI: 10.1002/chemv.201000024
  • Author: David Bradley
  • Published Date: 11 August 2010
  • Source / Publisher: J. Am. Chem. Soc./ACS Publications
  • Copyright: Wiley-VCH Verlag GmbH & Co. KGaA
thumbnail image: Microengines Forever Blowing Bubbles
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A bubble-powered microengine, just 25 ┬Ám long, could drive technology towards controllable devices for environmental clean-up, medical diagnostics and chemical analysis. An international team based in China, Germany, and Japan, led by Samuel Sanchez of the Leibniz Institute for Integrative Nanosciences in Dresden, have demonstrated how the enzyme catalase covalently bound to the interior of a microtube can release oxygen bubbles from hydrogen peroxide and cause the microengine to move. The team suggests that further modifications could be carried out to exploit fuels other than peroxide depending on specific applications.


There are numerous examples of microengines in nature, cilia, microflagellae, and proteins that transport materials within cells, for instance. Motor proteins, powered by the chemical fuel provided by a cell, have inspired researchers to try and mimic their behavior in the laboratory. The first generation of such nanomotors required a platinum metal catalyst, but Sanchez and colleagues hoped to circumvent that expense and the inconvenience of metal ions that might interfere with other processes within a device.


"Hybrid machines that couple catalytic biomolecules such as enzymes and artificial nano/microdevices are a promising alternative to the first generation devices," the team explains, "and could give us higher efficiency conversion, versatile configurations, more physiological conditions, and biocompatible fuels."


Diffusion Problems


One of the problems faced by researchers working on machinery at such a small scale are diffusion effects, such as Brownian motion, which often outweigh any attempt to control the motion of a microengine. Other researchers have exploited enzymes for locomotion to impel carbon fibers and bubbles have been used as a propulsion mechanism for multiwalled carbon nanotubes. The trajectories of these objects were strongly affected by Brownian motion and the location of the enzymes.


Sanchez and colleagues previously demonstrated propulsion in a microengine based on a platinum catalyst in which they could externally control the motion using a magnetic field, and therefore direct the microengines to specific targets. "The platinum decomposes hydrogen peroxide generating bubbles which propels the microengine fast enough to overcome Brownian motion. We can also load, transport and delivery these objects to defined sites by turning the magnetic field quickly," Sanchez explains.


Enzyme Motor


The researchers have now addressed the second issue, the use of a precious metal, by turning to a biological system for energy production based on the enzyme catalase. Their new device, formed by rolling up a thin titanium/gold films into a microtube, also demonstrated its locomotive prowess in moving 10 times as quickly as the platinum-based devices across the surface of a body of water. They have also demonstrated how the tiny device can nudge microscopic polystyrene beads across a water surface.


"This is the first report on the effective use of enzymes as catalysts in self-propelled microengines," the team says. "Catalase is one of the most efficient enzymes found in cells since each catalase molecule can decompose millions of hydrogen peroxide molecules every second to water and oxygen," the researchers add. This is an important point for devices that might be used in the body, as it allows the amount of on-board fuel, the toxic hydrogen peroxide, to be kept to a minimum, although the team is also investigating other potential fuels for their microengines.


MEMS pioneer Ayusman Sen of Pennsylvania State University told ChemViews that while enzyme-powered motors have been reported by others the level of control that Sanchez and his team have achieved at the micrometer scale for such motors is entirely new. "This is an important advance in the field," Sen told us. "Because of their high turnover rates, enzymes are almost the ideal candidates to power nano and micro-sized objects," he adds.


Sen points out that among the many important anticipated applications for such motors, medical applications, including sensing and drug delivery are high on the list. "These applications require that the motors be biocompatible," he points out. Unfortunately, hydrogen peroxide does not fit this criterion. "Thus, the search goes on for other classes of enzyme-powered motors," he concludes.


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