Dynamic Actuation of Magnetic Beads for Immunoassays on-chip

This thesis was carried out in the frame of the European project DetectHIV aiming the development of a new biosensor platform for the highly sensitive detection of the HIV capsid protein p24. We explore the implementation of a magnetic bead-based lab-on-a-chip system, offering significant advantages compared to conventional systems, mainly through the possibility of controlled manipulation of the magnetic carriers on-chip. In particular, microfluidic immunoassays using functionalized magnetic beads raise increasing interest. In this thesis, we present a microsystem for the magnetic manipulation of superparamagnetic beads on-chip. A highly confined and dynamically actuated plug of biochemically functionalized beads is formed in a microchannel. This plug extends over the channel cross-section, thus allowing efficient analyte capture from the flow. Subsequent immobilization of the plug for incubation modifies the colloidal state (agglutination test). Dynamic actuation of beads is enabled by superposing a static magnetic field and a time-varying magnetic field. The latter field is highly focused and concentrated across the microchannel by means of soft magnetic microtips. A new method for the fabrication of rigid monolithic SU-8 microchannels allows control and ready mechanical integration of the microtips with the microfluidic structure. A protocol for performing magnetic bead-based immuno-agglutination assays on-chip using our system was developed. A simple detection method based on the swelling of the released plug after agglutination is presented. We demonstrate the feasibility of on-chip agglutination tests by means of a streptavidin/biotinylated-bovine serum albumin (bBSA) model assay. A detection limit of about 200 pg/mL (≈3 pM) was obtained. Furthermore, the potential of the magnetic actuation method was emphasized by implementing a heterogeneous immunoassay with a dendritic amplification mechanism. Dendritic amplification aims to increase the detection sensitivity. Amplification of the detection signal is achieved by alternating exposure of the beads to a flow of fluorescently labeled streptavidin molecules and biotin conjugated anti-streptavidin, respectively. The magnetic system developed in the frame of this thesis was integrated in the final biosensor platform of the DetectHIV project. This platform comprises an integrated chip cartridge with an optical detection module. This design is outlined in the last part of the thesis.