Chemistry moves micro machines

Taking a pill can be a relatively coarse-grained way to get a substance where it is needed in the body. Many teams of researchers are working on ways to more finely target the drug delivery process.

Researchers from the Isaac Newton Institute for Mathematical Sciences in England, The Institute for Advanced Studies in Basic Sciences in Iran, the University of Leeds in England, and the Laboratory for Theoretical Physical Chemistry in France have devised a way to propel a molecular machine that does not require an outside control mechanism.

The method could eventually be used for drug delivery in the human body, and for stirring small volumes of liquid on biochips.

The researchers showed that it is possible to design a spherical device that can propel itself by a chemical reaction that concentrates a substance on one side of the machine. As the products of the reaction diffuse in the solution, the sphere is pushed away from the reaction site. This simple design is akin to inflating a balloon, then letting it loose without tying it off, so that the release of pressurized air propels the balloon.

Key to the method is that it does not require a global control mechanism, and can operate by itself once it is made and is placed in the right solution. The amount of time the machine can be propelled this way in one direction is determined by the size of machine. The method could eventually be used to propel tiny machines in liquid for brief periods of time, and could also be used to propel machines on rails, according to the researchers.

The researchers calculated that a four-micron-diameter sphere would travel in one direction for about 50 seconds at a rate of about half a micron per second. A micron is one thousandth of a millimeter.

The method can be ready for practical application in one or two decades, according to the researchers. The work appeared in the June 10, 2005 issue of Physical Review Letters (Propulsion of a Molecular Machine by Asymmetric Distribution of Reaction Products).