Design and performance of GMR sensors for the detection of magnetic microbeads in biosensors

We are developing a biosensor system, the Bead ARray Counter (BARC), based on the capture and detection of micron-sized, paramagnetic beads on a chip containing an array of giant magnetoresistive (GMR) sensors. Here we describe the design and performance of our current chip with 64 sensor zones, compare its performance with the previous chip design, and discuss a simple analytical model of the sensor micromagnetics. With assay-ready Dynal M-280 microbeads (2.8 μm diameter), our threshold for detection is approximately 10 beads per 200 μm-diameter sensor. Single beads made of solid Ni 30Fe 70 can easily be detected, but they must be made biocompatible. The relatively large size of our sensors helps to improve their practical sensitivity compared with other microsensor-based magnetic particle detectors.