I present the design and characterization of a high force magnetic tweezers device that can apply controlled forces to magnetic beads embedded into soft materials or biological systems, while visualizing the resultant material deformation with microscopy. Using finite element analysis (FEA), I determined the effect of the geometry of the NdFeB magnet array, as well as the geometry of iron yokes designed to focus and shape the magnetic fields. Sixteen shape parameters including the magnet size, positioning and yoke curvature were defined and modeled using open-source magnetic FEA software. Parameter sweeps were performed using custom-written Matlab code. Geometries were optimized for the magnitude of the magnetic field gradient and the length scale over which the magnetic force operated. Once an optimal design was identified, the yoke was fabricated in-house and the FEA validated by mapping the device's magnetic field using a Hall probe.

To demonstrate the usefulness of this approach, I produced a magnetic tweezers device designed for use with optical microscopes available in a core imaging facility. The application demanded device portability and the ability to interface with a number of microscopes, thus imposing significant size restrictions on the magnets used. Iterative FEA delivered an optimal magnet-yoke geometry, which could be mounted to a carriage that advances or retracts on command, giving the operator fine control over the applied force. Such automation allows for rapid force switching, and also allows the effects of long periods of cyclical loading to be determined. The carriage design, automation and implementation were produced in collaboration with a summer intern, Timothy Thomas from the INSET program at UCSB.

In future work, such an FEA approach could easily be adapted to a range of design goals/restrictions to create an efficient means of testing possible magnet configurations, while streamlining the design and construction of specialized instrumentation for force-sensitive microscopy.