Deformation and height anomaly of soft surfaces studied with an AFM

Abstract

The authors have measured the force-versus-indentation curves of different elastomers (polyurethane), rubber, cartilage, and living cells and deduced a parabolic tip shape from these curves. They have also calculated the radius of curvature of the AFM tip to be 50-100 nm. The calculated ranges of the local Young's moduli E are 0.6-2.4 MPa for rubber, 0.16-0.6 MPa for cartilage, and 0.013-0.15 MPa for a living cell. This means that an applied force of 1-10 pN is required to obtain high-resolution images of a living cell with about 1 nm vertical deformation and only a few nm2 area of contact. Therefore, understanding the deformation mechanism is not only important for determining local elasticities, but also to understand the 'height' information and the resolution limits of AFM images. Deformation without strong adhesive forces causes a soft sample on a hard substrate to appear thinner, a fact which has already been observed in many AFM images.