Two US engineers reported that they have found a simple way to use gravity or simple forces to sort microscopic particles and bits of biologic substances, including circulating tumor cells (CTCs).

In the May 25, 2012, online issue of the journal Physical Review Letters, Dr. German Drazer, an assistant professor of chemical and biomolecular engineering at Johns Hopkins University (Baltimore, MD, USA) and his doctoral student Jorge A. Bernate reported that they have developed a microfluidic device that can sort particles, cells or other tiny matter by physical means. By moving a liquid over a series of micrometer-scale high diagonal ramps the device causes microscopic material to separate into distinct categories, based on weight, size, or other factors, according to the scientists.

The process described in the journal article could be used to generate a medical diagnostic tool, according to the researchers. “The ultimate goal is to develop a simple device that can be used in routine checkups by healthcare providers,” said Mr. Bernate, who is lead author on the study. “It could be used to detect the handful of circulating tumor cells that have managed to survive among billions of normal blood cells. This could save millions of lives.”

Ideally, these cancer cells in the bloodstream could be detected and targeted for treatment before they have had a chance to metastasize, or spread cancer elsewhere. Detection at early stages of cancer is critical for successful treatment. Mr. Bernate explained that inside the microfluidic device, particles and cells that have been suspended in liquid flow along a “highway” that has speed-bump-like obstacles positioned diagonally, instead of perpendicular to, the path. The speed bumps differ in height, depending on the application.

“As different particles are driven over these diagonal speed bumps, heavier ones have a harder time getting over than the lighter ones,” Mr. Bernate stated. When the particles cannot get over the ramp, they begin to change course and travel diagonally along the length of the obstacle. As the process continues, particles end up fanning out in different directions. After the particles cross this section of the ‘highway,’ they end up in different ‘lanes’ and can take different ‘exits,’ which allows for their continuous separation.”
Gravity is not the only way to slow down and sort particles as they attempt to move the speed bumps. “Particles with an electrical charge or that are magnetic may also find it hard to go up over the obstacles in the presence of an electric or magnetic field,” Mr. Bernate said. For instance, cancer cells could be “weighted down” with magnetic beads and then sorted in a device with a magnetic field.

The ability to sort and separate things at the micro- and nano-scale is significant in many industries, ranging from solar power to bio-security. However, Mr. Bernate reported that a medical application is expected to have the most potential for immediate use. His doctoral adviser, Dr. Drazer, noted that, the research ultimately should lead to the development of a device that can easily sort whole blood into its components. A provisional patent has been filed for this device.

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