Purpose: : To investigate the viscoelastic properties of the human corneal tissue using Atomic Force Microscopy (AFM). Methods: : Analysis of the viscoelastic properties of four fresh human corneal tissues was performed in liquid environment using the NanoScope III (Veeco, Sunnyvale, CA) in the Force spectroscopy mode. Measurements were performed using rectangular silicon cantilevers with tip radius of 10 nm and nominal elastic constants between 20 and 80 Newton/meter (N/m). The measurements were done with a scanning speed between 4 and 80 ?μm/s. The force measurements were performed on the anterior stromal surface of the cornea after gently removing the epithelium. For each cornea, both the tissue deformation (??P) after load removal and the hysteresis (I) during the cycle were measured. Results: : By performing a large number of force measurements, at different scan rates and applied forces, on the central region of the anterior stroma, we derived the systematic dependence of ??P and I on either the maximum load or the load/unload rate. High-resolution imaging of the samples, performed immediately after the force measurements, confirmed how stromal surface morphology was not damaged during the course of the mechanical analysis. Conclusions: : All the viscoelasticity properties of the human cornea were observed at nanoscale level: asymmetry in the ramp loading-unloading response; corresponding hysteresis in the stress-strain response; time-dependent behaviour. The non-linear stress/strain response of the tissue was characterized by increasing stiffness with either increasing stress or pressure application rate. Nano-metric scale investigations of the human cornea may enhance our understanding on the biomechanical properties of the tissue and may ultimately be valuable in the design and development of improved bio-engineered corneas. Efforts in modeling the viscoelastic response of the human corneal tissue at nanoscale level are warranted.
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