par Lynn Shapiro
, Writer | November 03, 2008
The prototype does not yet look like a credit card reader or card-swipe device. Instead, it is used to "read" a Pyrex glass sample stick about three-quarters-inch long and one-eighth-inch wide. Biological samples can be placed on the sample stick, which then is "scanned much like a credit card reader," Porter says.
In the first study, instead of holding blood or other medical samples, the sample stick had 15 raised spots of iron-nickel "permalloy," a magnetic material that produces a magnetic signature read by GMR sensors. "We are simulating a signal to test the card-swipe device," Granger noting, "it's not a real sample."
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The study determined how measurements by the GMR sensors -- the heart of a future card-swipe device -- can be calibrated to account for variations in the size of the permalloy spots, the amount of separation between the sensors and the sample stick, and on the angle of the sample stick as it is scanned by the sensors. Those factors determine how consistently and accurately a card-swipe device would detect minute amounts of substances associated with diseases.
Simulating a Disease Test
In the second study, the sample stick's alloy spots were replaced by the material that would be used on real medical test cards: microscopic spots or "addresses" of gold that were no longer than the smallest known bacterium. The widths were varied to test which size of addresses could be "read" most accurately.
A substance named biotin or vitamin B-7 was bound chemically to the gold spots on the sample stick. Tiny drops of magnetic particles coated with streptavidin - a protein found in eggs - were placed on the gold spots.
"The gold address has no magnetic signature," Granger says. "Once the particles are bound to it, GMR picks up that magnetic signature. The experiment was repeated hundreds of times with different concentrations of magnetic particles bound to the biological substance.
"We could detect as few as 800 magnetic particles on an address," Porter says. "We believe that with further development, we can get down to single-particle detection."
The Utah scientists cite examples of how a GMR card reader might be used medically for humans and animals:
* Current tests for prostate specific antigen (PSA) - an indicator for possible prostate cancer - only look for "free" PSA but not for other forms. A card-like sample stick with multiple "addresses," each with an antibody that binds to a different kind of PSA, would be able to test a blood sample for multiple forms of PSA and for their relative abundances, and thus be more reliable in predicting prostate cancer, Porter says.