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Diagnostic Chip Based on Vibrating Beads

Scientists have developed a lab-on-a-chip technology based on superparamagnetic microbeads that they suggest could form the basis of a rapid diagnostic platform. The technology hinges on the principle that if two balls of different sizes are submerged in water (or another fluid) and subjected to a kinetic force, the smaller ball will move further because it has a smaller cross section and thus isn’t affected as much by the fluid’s resistance.

Massachusetts Institute of Technology (MIT) researchers have exploited this phenomenon to develop a technology in which superparamagnetic beads (the balls) are coated with capture probes for the analytes being tested, and subjected to a magnetic field that sets them oscillating. The test fluid is then added, and if any target molecules are present they bind to the beads, increasing the bead size, which changes the resonant frequency by a detectable amount.

In order to make the platform workable for chip-based biomedical applications, Elizabeth Rapoport, Ph.D., Geoffrey Beach, Ph.D., and Daniel Montana combined the bead concept with a technology they developed earlier this year for creating magnetic tracks on a microchip surface, along which the beads can be transported rapidly and positioned. Controlling the magnetic field directions in closely spaced adjacent regions enables the creation of minute areas with very strong magnetic fields, known as magnetic domain walls that can be moved along the track, to transport and capture the beads. These domain walls also act as the sensors.

The final device would thus consist of the microchip itself, overlaid with reservoirs that contain the magnetic beads and biological samples. However, rather than pumping the fluid and particles through the channels, the beads in the MIT team’s device would be directed through the chip by changing the applied magnetic fields.

The team says their device could feasibly offer much faster assays than existing chip-based platforms, and require much smaller volumes of biological sample. And while most biochips are designed to detect just one analyte, the magnetic bead-based system could be a true multitasker. After completing one test, the chip could be flushed through and magnetic beads for a different analyte added. “You’d just use it, wash it off, and use it again,” Dr. Rapoport suggests.

The MIT investigators haven’t yet tested the system on biological samples, but have provided proof of principle that it will work, by using magnetic beads of differing sizes to represent the increases in bead size caused by bound biomolecules. “We now have all the elements required to make a sensing device,” professor Beach says. “The next step is to combine the pieces in an operational device and demonstrate its performance.”

The team describes its technology in Lab on a Chip, in a paper titled “Integrated capture, transport, and magneto-mechanical resonant sensing of superparamagnetic microbeads using magnetic domain walls.”

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