Fig 1. Sketch of a typical force-displacement curve using nanometer-sized tip.

The black curve shows the force experienced by the nanometer-sized tip when approaching the sample, while the grey curve represents the force experienced on retracting from the sample. The main interaction force between the tip and the corneal specimen is the short-ranged van der Waals force. The black cross identifies the contact point; the absence of “jump-to-contact” indicates no adhesion between the tip and the stromal surface in 20% dextran solution. As the tip comes into contact with the stroma, there is a gradual increase in the deflection of the cantilever, as expected for soft biological specimens. In this study, the Young’s modulus was calculated by fitting the approach curve with the Hertz-Sneddon model for a conical indenter. Upon retraction, the approach and retraction curves do not overlap: this phenomenon is due to stromal hysteresis.