Mechanical tomography of human corneocytes with a nanoneedle.

Atomic force microscopy (AFM) can image biological samples and characterize their mechanical properties. However, the low aspect ratio of standard AFM probes typically limits these measurements to surface properties. Here, the intracellular mechanical behavior of human corneocytes is determined using "nanoneedle" AFM probes. The method evaluates the forces experienced by a nanoneedle as it is pushed into and then retracted from the cell. Indentation loops yield the stiffness profile and information on the elastic and nonelastic mechanical properties at a specific depth below the surface of the corneocytes. A clear difference between the softer ∼50-nm-thick external layer and the more rigid internal structure of corneocytes is apparent, which is consistent with the current understanding of the structure of these cells. There are also significant variations in the mechanical properties of corneocytes from different volunteers. The small diameter of the nanoneedle allows this "mechanical tomography" to be performed with high spatial resolution, potentially offering an opportunity to detect biomechanical changes in corneocytes because of, e.g., environmental factors, aging, or dermatological pathologies.