Power type strain energy function model and prediction of the anisotropic mechanical properties of skin using uniaxial extension data

STUDY OF THE TRANSVERSAL DEFORMATION OF CORNEAL STRIP UNDER UNIAXIAL LOADING

Uniaxial test is easy to access and to obtain accuracy data, but it is difficult to acquire two-dimensional deformation information. We investigated the relationship between the two strain components of corneal strip in uniaxial tests, which is the basis for determining of anisotropic strain energy function of cornea via uniaxial tests. Nine rabbits were taken. The left and right corneas were cut along superior-inferior (SI) and nasal-temporal (NT) direction, respectively. For each strip the uniaxial test was carried out, and the tensile displacements, strip images and loads were recorded. Then the stretching strain, the transversal strains and stress were obtained. Optimization based inverse analysis was utilized to find the best among six fitting models that characterizes the relationship between two strain components in uniaxial tests. All models fitted well the experimental data gathered for corneal strips ([Formula: see text]). According to the model selection index, the power model achieved the best performance index: 0.1268 for SI strips and 0.1063 for NT strips versus 0.151 (SI strips) and 0.107 (NT strips) found at most by other models. Thus, it is the most suitable one for describing the relationship between the two strain components of corneal strip during uniaxial stretching.

UNDERSTANDING THE VISCOELASTIC PROPERTIES OF RABBIT CORNEA BASED ON STRESS RELAXATION TESTS AND CYCLIC UNIAXIAL TESTS

Background: Determining the viscoelastic properties of cornea is important in the fields of understanding of the tissue’s response to mechanical actions and the accurate numerical simulation of corneal biomechanical behavior under the effects of keratoconus and refractive surgery. To address this need, we present an approach to model the viscoelastic response of rabbit cornea from uniaxial test data. Methods: The corneal strip samples from six rabbits were obtained to perform cyclic uniaxial tension tests and stress relaxation tests. We investigated the suitability of six constitutive models, including empirical models and hyperelastic models, by a quasi-linear viscoelastic law. Applying non-linear optimization techniques, we found material parameters for each different strip sample. Results and conclusions: The model gave a better fit to loading data with [Formula: see text], and predicted the unloading data in the cyclic uniaxial tests with errors-of-fit ranging from 0.03 to 0.06. The results indicate that the best model is the power of the first invariant of strain with Prony form relaxation model, and that the method to identify the material parameters are valid for modeling the visoelastic response of cornea from uniaxial test data.

DIFFERENCES BETWEEN PULMONARY ARTERIAL AND AORTIC MATERIAL PROPERTIES

To investigate the differences of mechanical responses between pulmonary artery and aorta to different biaxial loading conditions, we simulated the process of human pulmonary artery and aorta subjected to biaxial loading based on four-family fiber strain density function model determined from uniaxial extension data of arterial walls. It was shown that different stress–strain curves of pulmonary artery and aorta under biaxial loading conditions: different loading ratios between the loads in two perpendicular directions and displacement-controlled equibiaxial stretch. Tissue stiffness, defined as the first derivative of the stress–strain response at a strain point, of human pulmonary artery and aorta were obtained when they were subjected to biaxial loads (systemic pressure). The two-dimensional mechanical response of artery can be acquired by determination of the four-family fiber strain density function model of the tissue based on uniaxial extensile data. There are differences between material properties of pulmonary artery and aorta: aorta is stiffer circumferentially than longitudinally, and pulmonary artery is more compliant circumferentially than longitudinally.