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The induction of dipolar coupling has proven to allow for the initiation of self-assembled, reconfigurable particle clusters of superparamagnetic microbeads suspended in a carrier liquid. The adjustment of the interplay between magnetic and hydrodynamic forces opens various possibilities for guiding strategies of these superstructures within microfluidic devices. In this work, the formation dynamics of such particle clusters under the influence of a rotating magnetic field are studied. Different agglomeration regimes are characterized by the dimensionality of the confined objects. The growth dynamics of the obtained agglomerates are analyzed quantitatively in order to deduce the microscopic growth mechanisms. The growth of two-dimensional clusters is governed by the addition of bead chains to previously formed agglomerates. Time scales for the cluster growth are characterized by the chain dissociation rate. Based on the experimental findings, we may conclude to a linear dependence of the chain dissociation rate on the rotation frequency of the applied magnetic field. [ABSTRACT FROM AUTHOR]

A microfluidic continuous flow lab-on-a-chip structure is presented, for combined reaction and separation implementation. Thereby, the geometry is optimized with respect to a long duration time in the reaction area and a narrow preconditioning along the separation site to enhance the separation yield. We theoretically predict high stability of separation and transport properties of the device against particle diffusion whereas the particle dynamics in the reaction site maintain a diffusive character. We derive a formula setting the limitations of the structure, relating the desired duration of the particles in the reaction site to the particle size under the condition of a narrow spatial particle distribution along the separation site. We experimentally realize the low diffusion limit to prove the theoretically predicted properties of the device. [ABSTRACT FROM AUTHOR]

Lab-on-a-chip immuno assays utilizing superparamagnetic beads as labels suffer from the fact that the majority of beads pass the sensing area without contacting the sensor surface. Different solutions, employing magnetic forces, ultrasonic standing waves, or hydrodynamic effects have been found over the past decades. The first category uses magnetic forces, created by on-chip conducting lines to attract beads towards the sensor surface. Modifications of the magnetic landscape allow for additional transport and separation of different bead species. The hydrodynamic approach uses changes in the channel geometry to enhance the capture volume. In acoustofluidics, ultrasonic standing waves force µm-sized particles onto a surface through radiation forces. As these approaches have their disadvantages, a new sensor concept that circumvents these problems is suggested. This concept is based on the granular giant magnetoresistance (GMR) effect that can be found in gels containing magnetic nanoparticles. The proposed design could be realized in the shape of paper-based test strips printed with gel-based GMR sensors.

Under the influence of homogeneous, rotating magnetic fields, superparamagnetic beads can be assembled into one- and two-dimensional superstructures on demand and used as dynamic components in microfluidic systems for colloidal separation. In this paper, the influence of the magnetic field strength and the rotation frequency on the device efficiency is studied. The optimum region is found to be between 100 and 200 rpm for a magnetic field strength of 330 Oe, while the highest value for separated mass per time (28 pg s(-1)) is achieved for a flow velocity of 370 μm s(-1) at a magnetic field strength of 690 Oe. Furthermore, the employment of superparamagnetic beads as a continuous-flow bioseparation device is shown in a proof-of-principle study.

Suspended magnetic beads are exposed to an external homogeneous magnetic field which rotates around the axis perpendicular to the field direction. Because of dipolar interactions, magnetic beads assemble in highly ordered two-dimensional hexagonal arrays perpendicular to the rotation axis. By continuous provision of the particle concentration, the growth modes of two-dimensional particle clusters and monolayers are observed. The structure of the resulting assembled objects is analyzed for different field frequencies and particle concentrations. We identify dynamic processes which enhance stability and reduce lattice distortions and, thus, allow for the application of these particle agglomerations as dynamic components in lab-on-a-chip technologies.