A NEW METHOD FOR APPLICATION OF FORCE TO CELLS VIA FERRIC-OXIDE BEADS

We describe a new method that uses straight-forward physics to apply f orce to substrate-attached cells. In this method, collagen-coated magn etic ferric oxide beads attach to the dorsal surface of cells via rece ptors of the integrin family, and a magnetic field gradient is applied to produce a force. In this paper we present a complete characterizat ion of the method in a configuration that is easy to use, in which a p ermanent magnet provides a fairly uniform gradient over a relatively l arge area. This allows a fairly uniform average force that can be cont rolled in magnitude, direction, and duration to be applied to a large number of cells. We show how to determine the applied force per cell b y measuring the force per unit volume of magnetic bead, the distributi on of bead diameters, and the distribution of beads per cell. We also show how to calculate the force per unit volume of bead in a three-dim ensional region near the permanent magnet on the basis of field measur ements, and present results for three of the magnets. An upward force applied to fibroblasts by this method produces a measurable time-depen dent increase in attachment of cytoskeletal actin filaments to the for ce application points, and an increase in actin cross-linking. This is accompanied by an actin-dependent retraction of the force-induced upw ard movement of the dorsal surface of the cells.