Deformation and height anomaly of soft surfaces studied with an AFM

The atomic force microscope (AFM) can easily image 'hard' sample surfaces with atomic or molecular resolution. For ‘soft’ samples, such as polymers or biological objects, this resolution power is very difficult to reach, because the AFM tip can cause large deformation. This deformation is revealed in the touching regime of the force-versus-distance curve, or more directly in the force-versus-indentation curves. To obtain the force-versus-indentation curve of a soft sample, its force-versus-distance curve is corrected with the force-versus-distance curve measured with the same cantilever on a hard sample. We 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. We 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 0.62.4 MPa for rubber, 0.160.6 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. Furthermore, if deformation of the soft sample occurs the applied force is overestimated. Therefore, high-resolution imaging of soft samples remains a challenge since the applied force needs to be in the pN range.