Three-Dimensional Magnetic Focusing of Superparamagnetic Beads for On-Chip Agglutination Assays

We present a new method for three-dimensional (3D) magnetic focusing of magnetic microparticles in a microfluidic system. On-chip magnetic particle manipulation in themicrochannel is achieved by a high-gradientmagnetic field generated by means of a micromachined field concentrator. The system allows retention of functionalized beads in a dense plug while flowing through buffer or analyte. Slowly reducing the magnetic retention force in the presence of a flow results in controlled release of the particles into a fine streamline with regular longitudinal interparticle spacing. Alignment at half-height of the channel is readily obtained through the symmetry of the magnetic field. A single lateral sheath flow is required to provide full 3D focusing of the microparticles in the middle of the microchannel with a maximum deviation of ±5 μm from the center position. With the use of this system, a new approach for performing an immunoagglutination assay on-chip has been implemented. Three-dimensional focusing allowed reliable counting of singlets and agglutinated doublets. We demonstrate the potential of the agglutination assay in a microfluidic format using a streptavidin/biotinylated bovine serum albumin (bBSA) model system. A detection limit of about 400 pg/mL (6 pM) is achieved.

Separation and Focusing of Magnetic Beads for Agglutination Tests

Rana Afshar Ghasemlouy

Functional magnetic micro- and nanoparticles are used in bioanalytical applications as solid carriers for capture, transport and detection of biomolecules or magnetically labeled cells. Colloidal suspensions of such particles provide a large specific surface for chemical binding and therefore allow highly efficient interactions with target molecules in a sample solution. Controlled actuation and manipulation of these mobile substrates in the microfluidic format offers interesting new opportunities for on-chip bioassays with previously unmatched properties. Separation of functional magnetic particles or magnetically labeled entities is therefore a key feature for bioanalytical or biomedical applications and also an important component of lab-on-a-chip devices for biological applications. In this thesis we present two novel integrated microfluidic magnetic bead manipulation devices. The first system consists of dosing of magnetic particles, controlled release and subsequent magnetophoretic size separation with high resolution. On-chip integrated soft-magnetic microtips with different shapes provide the magnetic driving force for the bead manipulation. The system is designed to meet the requirements of specific bioassays, in particular of on-chip agglutination assays for the detection of rare analytes, in which the latter can be quantified via the counting of the particle doublets. In a second approach, magneto-microfluidic three-dimensional (3D) focusing of microparticles has been developed. In this system, magnetic microparticles from a dense plug are released into a single streamline with longitudinal inter-particle spacing. Plug formation is induced by a high-gradient magnetic field generated at the sidewall of a microchannel by a micromachined magnetic tip that is connected to an electromagnet. Controlled release of the microparticles is achieved using an exponential damping protocol of the magnetic retention force in the presence of an applied flow. Carefully balancing the relative strengths of the drag force imposed by the flow and the magnetic retention force moreover allows in-flow size separation of the microparticles. Adding subsequently a lateral sheath flow microchannel focuses the microparticles into a single stream situated within 0plusmn; 5 µm from the channel center axis. Our system for 3D focusing and in-flow separation of magnetic microparticles has been used for performing an immuno-agglutination assay on-chip. 3D focusing was of the basis of reliable in-flow counting of singlets and agglutinated doublets. We demonstrated the potential of the agglutination assay in a microfluidic format using a streptavidin/biotinylated-bovine serum albumin (bBSA) model system. A bBSA detection limit of about 400 pg/mL (6 pM) is achieved.

EPFL2011

Separation of magnetic microparticles in segmented flow using asymmetric splitting regimes

Matteo Cornaglia, Martinus Gijs

A magnetic microparticle-based bioassay requires the separation of the microparticles from the sample matrix, after the microparticles have specifically captured the target of interest. For the implementation of such an assay in water-in-oil droplet segmented flow microfluidics, the particles must be separated from the aqueous sample droplets during a purification step. Current magnetic separation methods pose limits to purification, as only a limited part of the sample volume is removed in the purification step. Combining asymmetric droplet splitting in a T-junction-shaped microfluidic channel with magnetic separation, as induced by a permanent magnet positioned close to the microfluidic channel, is a promising and elegant solution for extracting the magnetic microparticles. However, retaining a high separation efficiency is a challenge and yet untried. In this paper, we describe a microfluidic and magnetic setup to separate superparamagnetic microparticles from the sample droplets, removing up to 90 % of the original sample volume in a single purification step, while keeping the separation efficiency constant. First, the conditions for particle aggregation, attraction and immobilization are determined and used to predict good separation conditions. Second, the magnetic forces at the splitting zone on the microfluidic chip are simulated for different permanent magnet positions and orientations; hereafter, the most promising setups are experimentally realized in polydimethylsiloxane microchannels, tested and the results considering different splitting regimes compared.

Springer Heidelberg2015

Separation and Focusing of Magnetic Beads for Agglutination Tests

Rana Afshar Ghasemlouy

EPFL2011