Figure 4. Stiffness tomography of a HeLa cell.

(a) Cartoon showing three indentation ranges: 0–80 nm, 80–160 nm, and beyond 160 nm, probed by the high–bandwidth cantilever. Deep indentation allows for probing stiffness of intracellular structures and extends nanomechanical imaging into the third dimension. (b) Normalized histograms of the loading stiffness (E*_load) images for the three indentation ranges. E*_load values for each pixel represented the statistically robustified mean of 25 consecutive taps estimated using a goodness–of –fit–weighted bootstrapping method. The E*_load value was corrected for close–to–substrate effects.²⁵ (c–e), Stiffness images for the three indentation ranges showing filamentous structures with shallow indentation and clusters of high stiffness structure with deep indentation. Scale bar is 2 µm and the stiffness image size is 665 × 200 pixels. (f) Height map calculated from the point of zero indentation. Three different lines in (f) are shown as depth tomograms in (g). The tomograms show features at different depths.