Probing the dynamics of cellular traction forces with magnetic and non-magnetic micropost arrays

The process of force transduction by living cells is linked to changes in cellular function. To study the cellular response to applied forces we utilized substrates which consisted of arrays of flexible cantilevers (microposts), in conjunction with ferromagnetic nanowires, to simultaneously apply forces to living cells and to measure the cellular response. Cells seeded on chemically functionalized micropost arrays spread on the tops of the posts, and exerted cellular traction forces, displacing the posts. Knowledge of the mechanical properties of the posts enabled measurement of the spatially-resolved forces over time. The arrays were fabricated by a process of replica molding. Magnetic nanowires, cylinders of magnetic material made by electrodeposition through a porous template, experience a torque in a uniform magnetic field due to their shape anisotropy. Nanowires were embedded lengthwise in a fraction of the posts, and under a uniform magnetic field, applied a force to an adherent cell.;Fluorescent time-lapse imaging was performed to establish baseline activity of the cell and to monitor cellular reaction after force application. Permanent magnets applied a single step force to adherent 3T3 mouse fibroblast cells and a relaxation response (decrease in contractile energy of the cell) was measured in 54% of cells. The reaction was observed to take place over a short (<15 seconds) or long (∼10 minutes) time scale, and was neither local to the site of force stimulation nor homogeneously distributed throughout the cell.;Smooth muscle cells (SMCs) lining arteries in the human body experience periodic forces and can undergo a mechanical feedback response to increased forces which can result in harmful conditions such as atherosclerosis. A dual magnetic tweezers system was designed to apply localized, time-varying forces to SMCs cultured on micropost arrays. The cells were stimulated by internalized 5mum or 20 mum long magnetic nanowires, which experienced rotation within the cell under the applied 0.5 Hz oscillating field. A global reinforcement response was observed for SMCs under these conditions. The response was not local to the site of force application, but was concentrated in regions of high cellular contractility (predominately on the periphery of the cell), indicating that the structure of the cytoskeleton was not dramatically altered due to the applied forces. However, the reaction was also not a linear increase in forces, indicating an adaptive cytoskeletal response, perhaps due to a diffusive signaling mechanism.