magnet polepiece design for uniform magnetic force on superparamagnetic beads

• DOI: 10.17615/0nz5-zp64
• Publication Date: 2010-07-01
• Authors: David B. Hill; Jed C. Macosko; Matthew R. Steen; Todd Fallesen; Keith Bonin; George Holzwarth
• Keywords: Magnetics; Microscopy; Motion; 0303 health sciences; 03 medical and health sciences; Imaging, Three-Dimensional; Microspheres

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 0.5×4 mm2 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 μm 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.