title: Magnet polepiece design for uniform magnetic force on superparamagnetic beads.
abstract: 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 differential B(x)/ partial differential 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 0.5x4 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 microm 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.
authors.affiliation: Department of Physics, Wake Forest University, Winston-Salem, North Carolina 27109, USA.
authors.forename: Todd
authors.initials: T
authors.lastname: Fallesen
authors.forename: David B
authors.initials: DB
authors.lastname: Hill
authors.forename: Matthew
authors.initials: M
authors.lastname: Steen
authors.forename: Jed C
authors.initials: JC
authors.lastname: Macosko
authors.forename: Keith
authors.initials: K
authors.lastname: Bonin
authors.forename: George
authors.initials: G
authors.lastname: Holzwarth
mesh_terms: D021621:Imaging, Three-Dimensional; D008280:Magnetics; D008853:Microscopy; D008863:Microspheres; D009038:Motion