Mini Oven Heats Fluids for LoC

By embedding a thin-film microwave transmission line between a glass substrate and a polymer block, scientists have created what could be the world's smallest microwave oven. The tiny mechanism can heat a pinhead-sized drop of liquid inside a container slightly shorter than an ant and half as wide as a single hair. The "micro microwave" is intended for lab-on-a-chip (LoC) devices that perform chemical analyses on tiny samples.

The NIST micro microwave oven. The gold traces on the glass circle are microwave transmission lines. The 1.25-cm-wide polymer block over the transmission line in the center houses a miniature chamber in which a pinhead-sized drop of fluid is heated. (Photo courtesy NIST)

In a paper in the November 2007 Journal of Micromechanics and Microengineering, the National Institute of Standards of Technology (NIST) and George Mason University research team led by NIST engineer Michael Gaitan describes for the first time how a tiny dielectric microwave heater can be successfully integrated with a microfluidic channel to control selectively and precisely the temperature of fluid volumes ranging from a few microliters (millionth of a liter) to sub-nanoliters (less than a billionth of a liter). Sample heating is an essential step in a wide range of analytic techniques that could be built into microfluidic devices, including the high-efficiency polymerase chain reaction (PCR) process that rapidly amplifies tiny samples of DNA for forensic work, and methods to break cells open to release their contents for study.

After they embedded the microwave line between the glass substrate and polymer block, the researchers made a trapezoidal-shaped cut in the polymer block only 7-µm across at its narrowest -- the diameter of a red blood cell -- and nearly 4-mm long (about the length of an ant) to serve as the chamber for the fluid to be heated.

Based on classical theory of how microwave energy is absorbed by fluids, they developed a model to explain how their miniature oven would work. They predicted that electromagnetic fields localized in the gap would directly heat the fluid in a selected portion of the microchannel while leaving the surrounding area unaffected. Measurements of the microwaves produced by the system and their effect on the fluid temperature in the microchannel validated the model by showing that the increase in temperature of the fluid was predominantly due to the absorbed microwave power.

Once the new technology is more refined, the researchers hope to use it to design a microfluidic microwave heater that can cycle temperatures rapidly and efficiently for a host of applications.

The work is supported by the Office of Science and Technology at the Department of Justice’s National Institute of Justice.

For more information, visit: www.nist.gov