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