A magnetic separation technique developed by researchers at Duke University's Pratt School of Engineering and Purdue University makes it relatively simple to sort through beads hundreds of times smaller than the period at the end of this sentence.
The method could lead to new technologies for medical or environmental testing, according to the researchers. For example, specially coated magnetic particles designed to attract particular viruses or bacteria might be used in tailored combinations to simultaneously test for multiple infectious pathogens in a blood or water sample.
Benjamin Yellen, assistant professor of mechanical engineering and materials science at Duke, and Gil Lee, associate professor of chemical engineering at Purdue, report their findings in the December issue of the journal Lab on a Chip (see http://www.rsc.org/Publishing/Journals/LC/article.asp?doi=b713547e).
"If there were five viruses that a patient might have been exposed to, you could potentially develop a technique to look for those five viruses all at the same time," Yellen said. In principle, such a test could be done with just a single drop of blood, as long as there was virus in the sample.
As an initial demonstration of the concept, the researchers attached two "model pathogens," a baker's yeast and a soil bacterium, to magnetic beads, and used their technique to selectively isolate them.
The magnetic separation method, which the researchers call magnetophoresis, uses a rotating magnetic field and a microchip containing an array of miniature magnets to separate tiny magnetic beads based on their size within a matter of minutes.
The physics behind the technique is as interesting as its potential applications, Yellen added. "The method causes certain particles to become essentially immobile -- just jittering back and forth -- while others move off the chip where they can be isolated. It implies that we could achieve effectively infinite separation between particle types. We thought our technique would work well for bioseparation, but we hadn't predicted it would be this good."
While the researchers know how to precisely control which particles move and which stay put, by varying the frequency of the magnetic field they apply, the underlying physics responsible for the behavior remains partly unexplained and will be the subject of future investigation, Yellen said.
Micrometer and nanometer sized "superparamagnetic" beads already are used widely to magnetically separate biological molecules and cells from complex fluid mixtures, Yellen said. Superparamagnetism is a form of magnetic behavior which occurs primarily in materials composed of very small magnetic grains. Such materials are commonly used for drug delivery and imaging applications and in biomedical devices because they become magnetized only in the presence of an externally applied magnetic field, which helps prevent clumping.
Over the past few decades, however, there have been few new developments in the field of magnetic separation, according to the researchers, with most of the efforts focused on using stronger and stronger magnetic fields and field gradients.
"Now, we've demonstrated a fundamentally new and different approach to magnetic separation, which can dramatically increase the separation efficiency, not by exploiting stronger fields and field gradients, but rather by precisely tuning the mobility of beads and exploiting the non-linear dynamics of particles moving in a traveling wave," Yellen said.