A magnetic manipulator for studying local rheology and micromechanical properties of biological systems

A magnetic micromanipulator capable of generating two-dimensional translational and rotational motions on a microscope stage is described. With 3 mu m-diam paramagnetic beads, forces in the piconewton range and torques on the order of 10(-14) N m are obtained and can be modulated in time at moderate frequencies (<5 Hz). Typical magnetic fields between 0.1 and 0.2 T, and gradients between 10 and 20 T m(-1) are created by four independent feedback-controlled electromagnets. Video microscopy and computerized image analysis are used to locate the beads on each image with a resolution of 0.1 pixel (20 nm). The device is primarily designed to study, at a microscopic scale, the local mechanical properties of biological polymers such as actin in solution, and of cell cytoplasm. Possible applications include the in situ manipulation of intracellular organelles. (C) 1996 American Institute of Physics.