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 partial derivative B-x/partial derivative 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 X 4 mm(2) in the focal plane of the microscope and 0.05 mm along the microscope optic axis. Within this space the maximum force on a 2.8 mu m 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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3469792]