an afm-based stiffness clamp for dynamic control of rigidity一个afm-based刚度夹动态刚度的控制.pdfVIP

An AFM-Based Stiffness Clamp for Dynamic Control of Rigidity 1 1 1,2,3 Kevin D. Webster , Ailey Crow , Daniel A. Fletcher * 1 Biophysics Graduate Group, University of California, Berkeley, California, United States of America, 2 Department of Bioengineering, University of California, Berkeley, California, United States of America, 3 Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States of America Abstract Atomic force microscopy (AFM) has become a powerful tool for measuring material properties in biology and imposing mechanical boundary conditions on samples from single molecules to cells and tissues. Constant force or constant height can be maintained in an AFM experiment through feedback control of cantilever deflection, known respectively as a ‘force clamp’ or ‘position clamp’. However, stiffness, the third variable in the Hookean relation F = kx that describes AFM cantilever deflection, has not been dynamically controllable in the same way. Here we present and demonstrate a ‘stiffness clamp’ that can vary the apparent stiffness of an AFM cantilever. This method, employable on any AFM system by modifying feedback control of the cantilever, allows rapid and reversible tuning of the stiffness exposed to the sample in a way that can decouple the role of stiffness from force and deformation. We demonstrated the AFM stiffness clamp on two different samples: a contracting fibroblast cell and an expanding polyacrylamide hydrogel. We found that the fibroblast, a cell type that secretes and organizes the extracellular matrix, exhibited a rapid, sub-second change in traction rate (dF/dt) and contraction velocity (dx/dt) in response to step changes in stiffness between 1–100 nN/mm.