Microdevice with Half-Ring Shaped GMR Sensors for Magnetic Bead Manipulation and Detection

Abstract

Micro and nanosized superparamagnetic beads (MBs) have been used in several biomedical applications due to their comparable size to biomolecules and their ability to respond to external magnetic fields. The stray fields of magnetized MBs can be detected by a magnetic sensor, which is utilized for quantification of target biomolecules present in immunoassays when MBs are used as biomolecular labels.

In this chapter, we describe the design, fabrication and testing of a microdevice for manipulating, trapping and detecting MBs. Manipulation and trapping is accomplished with a microstructure comprising conducting rings to produce magnetic field gradients, which exert a force on MBs. Controlling the movement of MBs paves the way for their rapid detection, since the beads can be attracted and transported towards a sensing site. In order to ensure that the majority of the MBs trapped at the sensing site are detected, we designed a spin valve type giant magnetoresistance (GMR) sensor with half-ring geometry. Analytical and numerical analysis leading towards the fabrication of the microstructure and the half-ring GMR sensor are presented. Full characterization of a single half-ring sensing element showed a DC magnetoresistance of 5.9 %, a small signal AC sensitivity of 0.53 mV/mT and a noise level of 6 $nV/\surd$Hz. An analytical model backed up by experimental results is presented to characterize the behavior of MBs in solution. Experimental results showed that the half-ring GMR sensor detected the presence of 2 μm MBs, thus indicating the feasibility of integrating an MB manipulation microstructure with half-ring GMR sensors to optimize the active sensing site.