Characterization Of Visco-Hyperelastic Biomechanical Properties For Human Corneal Stroma Before And After Corneal Cross-Linking Treatment

Human cornea,controlling the 70 % refractive power of visual system,exhibits transparence,continuous non-uniformity,anisotropy and soft shell with multi-scale composite structure.The anatomical microstructure and biomechanical behavior of the human cornea directly affect visual quality.With the increasing popularity of corneal refractive surgery,corneal collagen cross-linking surgery,corneal injury suture,keratoplasty,etc.,require the precise characterization of the corneal biomechanical properties.In this paper,the lenticule,extracted from corneal stroma of young myopic patients after small incision lenticule extraction(SMILE)corneal refractive surgery,were tested as samples to explore the hyperelastic mechanical model of the corneal anterior central stroma and its performance regurity.Considering the patients’ individualized corneal geometry,the experimental stress–stretch curves of the corneal stroma in the fibril direction(NT and SI)was obtained by uniaxial tension.The morphology of the collagen fibirils inside the stromal lamellas was observed by transmission electron microscope,and Gaussian distribution characteristic of crimping degree was evaluated.Combining the crimping morphology of collagen fibirils with macroscopic hyperelastic behavior,the collagen fibril crimping constitutive model was developed,and the constitutive parameters of each tensile sample were fitted.Compared with the other constitutive models,this model can more accurately characterize the hyperelastic behavior of the corneal physiological deformation range,and each constitutive parameter owns clear physical meaning.The constitutive model,programmed as a user material subroutine,realizes the finite element numerical simulation of the tensile behavior of the corneal tissue.The results show that the hyperelastic mechanical properties in two fibril main direction of the corneal stroma were the same,and the hyperelastic mechanical properties of the matrix and collagen fibrils show a downward trend in the depth direction of the corneal stroma.In this paper,standard corneal collagen cross-linking with four different irradiance energies were adopt and "treat" the corneal stromal lenticules,to explore the changes of the visco-hyperelastic mechanical behavior of the central stroma.Along the main fibril direction(SI),double strip samples with symmetric geometry and microstructure were cut out simultaneously,and enrolled for cross-linking treatment and control respectively.First,some strips were loading to failure by tensile test,and the mechanical properties of the strips were obtained.Next,the special uniaxial tensile test with sequential quasi-static loading–unloading,instantaneous loading and stress relaxation was followed to obtain the visco-hyperelastic constitutive parameters of subsequent strips.Comparisons between the experiment and the control show that the corneal strength and hyperelastic stiffness were significantly enhanced with the increase of cross-linking energies,but the toughness and viscosity were significantly reduced.The results provide the surgical design of SMILE refractive surgery,postoperative visual quality assessment,quantitative analysis of corneal cross-linking,and rehabilitation prediction of keratoconus treatment,etc.,with basic biomechanical constitutive models,quantitative analysis methods and experimental results.The results of this paper deepen the understanding of the biomechanical behavior of human cornea.