Magnet polepiece design for uniform magnetic force on superparamagnetic beads

Here we report construction of a simple electromagnet with novel polepieces which apply a spatially uniform force to superparamagnetic beads in an optical microscope. The wedge-shaped gap was designed to keep $∂Bx/∂y$ constant and $B$ large enough to saturate the bead. We achieved fields of 300–600 mT and constant gradients of 67 T/m over a sample space of $0.5×4mm2$ in the focal plane of the microscope and 0.05 mm along the microscope optic axis. Within this space the maximum force on a $2.8μm$ diameter Dynabead was 12 pN with a spatial variation of approximately 10%. Use of the magnet in a biophysical experiment is illustrated by showing that gliding microtubules propelled by the molecular motor kinesin can be stopped by the force of an attached magnetic bead.