Table 1.
Characteristics of the eye bank donor corneal tissues.
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.
Fig 2.
Plots of the Young’s modulus before and after riboflavin/UV-A corneal cross-linking.
A) Average values of the Young’s modulus at different approach speeds in all five corneal specimens before riboflavin/UV-A corneal cross-linking. B) Average values of the Young’s modulus after treatment. Bars indicate standard deviation.
Fig 3.
Representative force-indentation curves obtained before and after riboflavin/UV-A corneal cross-linking.
In cornea 5, the force curves were acquired at approach speeds of 1.7- and 12.3-μm/s both before (left column) and after (right column) riboflavin/UV-A corneal cross-linking. After treatment, the approach curves became steeper, thus demonstrating an increased stiffness of the stroma; in addition, hysteresis decreased at the scale of stromal molecular interactions.
Table 2.
Young’s modulus (E; MPa; M±SD) obtained for each sample before and after cross-linking.
Fig 4.
Hysteresis plotted as a function of the approach speed before and after riboflavin/UV-A corneal cross-linking.
A) Hysteresis of cornea 1 at an applied load of 1.9 μN. B) Hysteresis of cornea 2 at an applied load of 3.6 μN. The changes of hysteresis of the anterior stroma were evaluated by comparing the measurements obtained at specific ranges of applied loads both before (squares) and after (triangles) treatment in each specimen. Bars indicate standard deviation.
Fig 5.
Representative force-displacement (F-D) curves before and after chemical corneal cross-linking using glutaraldehyde 2.5%.
In cornea 6, the F-D curves were acquired before (A) and after (B) chemical cross-linking at an approach speed of 3.5 μm/s and an applied load of 6.5 μN. After treatment, the slope of the approach curve becomes steeper and the area under the curves becomes smaller than baseline measurements. The tip-stromal interaction demonstrates both an increase of the elastic response and a decrease of the viscous response of the chemically cross-linked stroma.
Table 3.
Hysteresis and indentation depth values (M±SD) obtained at same ranges of applied loads before and after riboflavin/UV-A corneal cross-linking.