Articles techniques et présentations

In a diagnostic system small magnetic beads of about 1µm diameter will be moved along a path in a viscose medium due to magnetic field actuators. Specific ligands on the beads will bound to specific samples. If bound, the diameter and mass of the beads rise and the bead movement is slower resulting in a shorter path in a fixed timeframe. The length of the path can be used to detect the samples or for sorting them in case of lateral deviations/curved movement. Goal of the simulation work is to understand the bead movement and derive first geometrical, magnetic, and electrical parameters of the diagnostic system. The inductors cause magnetic fields resulting in forces acting on the beads. Contrary to this, frictional forces on the interface between the viscose fluid and the beads act. Stokes’s law can be used to set the frictional forces in relation to the velocity of a bead. In a first model a sweep is done for a grid of radial and height positions of a bead to compute the magnetic forces acting on the bead in both directions and to export them to a file. In a second model the forces are used in interpolation functions to provide them to a global ODEs/DAEs section with the Stokes’s law stored in a global equations node. Utilizing this two-step approach it is possible to export the magnetic forces for different inductor currents and windings, bead radii and permeabilities as well as geometrical properties of the system. The movement of the beads in the viscous medium can be calculated for different bead and medium properties independently from the first step and a lot of simulation time will be saved in this way. The trace of a bead as well as the timing of the bead movement is very important for the application and can easily be visualized using COMSOL Multiphysics®. The feasibility of the concept could be shown. The bead movement was analyzed and depicted without building prototypes in advance and using microscopes to monitor the very small beads. Geometrical, magnetic, and electrical parameters where derived to build up a prototypical system.