Issue Date: November 10, 2008
Computer Sensor Goes Biological
A NEW DEVICE could be the first step toward medical diagnostic tests that use readers similar to magnetic card-swipe machines, according to researchers at the University of Utah. Such tests might include immunoassays, DNA hybridization, and other biological assays.
Chemistry and chemical engineering professor Marc D. Porter, staff scientist Michael C. Granger, and coworkers base their device on the giant magnetoresistance (GMR) effect, by which the resistivity of a material undergoes large changes in response to an applied magnetic field. Sensors based on the GMR effect contain alternating thin layers of magnetic and conducting materials and are widely used as high-speed read heads in computer hard disk drives.
When such GMR sensors read data from the billions of nanometer-sized magnetic bits that make up a computer's hard drive, "it's incredibly fast," Porter says. "Can we take that technology, which is well developed, mature, rugged, small, and uses little power, and turn it into a diagnostic device? The advantages in the long run would be size, portability, and reading speed."
The device consists of four GMR sensors deposited in a serpentine pattern and wired together as a Wheatstone bridge, which is an electrical circuit that measures changes in resistance. Two interdigitated GMR sensors serve as sense resistors, which detect the sample, and the other two GMR sensors serve as reference resistors. GMR sensors are susceptible to changes in temperature, and the two reference resistors help compensate for these changes. The researchers apply an external magnetic field that is oriented along the long axis of the device. Then they move a sample-carrying Pyrex stick across the sense resistors, much like swiping a credit card through a card reader.
The researchers use the device to detect biological samples, which must be magnetically labeled (Anal. Chem. 2008, 80, 7930 and 7940). In one example, they attach biotin capture probes to gold squares, which have no GMR signature of their own, on the sample stick. When streptavidin-coated magnetic particles bind to the biotin, the magnetic particles screen the external magnetic field and cause a resistivity change as the sample moves past the sensor. Such streptavidin-coated magnetic particles could be used as a universal label for many types of biological assays.
"This technique shows significant promise for medical point-of-care diagnostics," says Mark T. McDermott, a chemistry professor at the University of Alberta. "The internal calibration, easy-to-handle sample stick, and fast analysis times demonstrated in this work are some of the characteristics that will be incorporated in the next generation of diagnostic detection platforms."
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