scholarly journals Effects of Age and Diabetes on Scleral Stiffness

2015 ◽  
Vol 137 (7) ◽  
Author(s):  
Baptiste Coudrillier ◽  
Jacek Pijanka ◽  
Joan Jefferys ◽  
Thomas Sorensen ◽  
Harry A. Quigley ◽  
...  
Keyword(s):  
Finite Element ◽  
Material Properties ◽  
Collagen Fiber ◽  
Older Age ◽  
Image Correlation ◽  
Mechanical Anisotropy ◽  
Matrix Stiffness ◽  
Fiber Alignment ◽  
Age Related ◽  
Related Increase

The effects of diabetes on the collagen structure and material properties of the sclera are unknown but may be important to elucidate whether diabetes is a risk factor for major ocular diseases such as glaucoma. This study provides a quantitative assessment of the changes in scleral stiffness and collagen fiber alignment associated with diabetes. Posterior scleral shells from five diabetic donors and seven non-diabetic donors were pressurized to 30 mm Hg. Three-dimensional surface displacements were calculated during inflation testing using digital image correlation (DIC). After testing, each specimen was subjected to wide-angle X-ray scattering (WAXS) measurements of its collagen organization. Specimen-specific finite element models of the posterior scleras were generated from the experimentally measured geometry. An inverse finite element analysis was developed to determine the material properties of the specimens, i.e., matrix and fiber stiffness, by matching DIC-measured and finite element predicted displacement fields. Effects of age and diabetes on the degree of fiber alignment, matrix and collagen fiber stiffness, and mechanical anisotropy were estimated using mixed effects models accounting for spatial autocorrelation. Older age was associated with a lower degree of fiber alignment and larger matrix stiffness for both diabetic and non-diabetic scleras. However, the age-related increase in matrix stiffness was 87% larger in diabetic specimens compared to non-diabetic controls and diabetic scleras had a significantly larger matrix stiffness (p = 0.01). Older age was associated with a nearly significant increase in collagen fiber stiffness for diabetic specimens only (p = 0.06), as well as a decrease in mechanical anisotropy for non-diabetic scleras only (p = 0.04). The interaction between age and diabetes was not significant for all outcomes. This study suggests that the age-related increase in scleral stiffness is accelerated in eyes with diabetes, which may have important implications in glaucoma.

scholarly journals Collagen Structure and Mechanical Properties of the Human Sclera: Analysis for the Effects of Age

2015 ◽  
Vol 137 (4) ◽  
Author(s):  
Baptiste Coudrillier ◽  
Jacek Pijanka ◽  
Joan Jefferys ◽  
Thomas Sorensen ◽  
Harry A. Quigley ◽  
...  
Keyword(s):  
Material Properties ◽  
Collagen Fiber ◽  
Image Correlation ◽  
Mechanical Anisotropy ◽  
Matrix Stiffness ◽  
Fiber Structure ◽  
Human Donor ◽  
Full Field ◽  
X Ray Scattering ◽  
The Difference

The objective of this study was to measure the collagen fiber structure and estimate the material properties of 7 human donor scleras, from age 53 to 91. The specimens were subjected to inflation testing, and the full-field displacement maps were measured by digital image correlation. After testing, the collagen fiber structure was mapped using wide-angle X-ray scattering. A specimen-specific inverse finite element method was applied to calculate the material properties of the collagen fibers and interfiber matrix by minimizing the difference between the experimental displacements and model predictions. Age effects on the fiber structure and material properties were estimated using multivariate models accounting for spatial autocorrelation. Older age was associated with a larger matrix stiffness (p = 0.001), a lower degree of fiber alignment in the peripapillary sclera (p = 0.01), and a lower mechanical anisotropy in the peripapillary sclera (p = 0.03).

scholarly journals Effects of Collagen Heterogeneity on Myocardial Infarct Mechanics in a Multiscale Fiber Network Model

2019 ◽  
Vol 141 (9) ◽  
Author(s):  
Christopher E. Korenczuk ◽  
Victor H. Barocas ◽  
William J. Richardson
Keyword(s):  
Myocardial Infarction ◽  
Finite Element ◽  
Homogeneous Model ◽  
Mechanical Anisotropy ◽  
Local Alignment ◽  
Fiber Density ◽  
Heterogeneous Model ◽  
Fiber Alignment ◽  
Sample Shape ◽  
Homogeneous Models

The scar that forms after a myocardial infarction is often characterized by a highly disordered architecture but generally exhibits some degree of collagen fiber orientation, with a resulting mechanical anisotropy. When viewed in finer detail, however, the heterogeneity of the sample is clear, with different subregions exhibiting different fiber orientations. In this work, we used a multiscale finite element model to explore the consequences of the heterogeneity in terms of mechanical behavior. To do so, we used previously obtained fiber alignment maps of rat myocardial scar slices (n = 15) to generate scar-specific finite element meshes that were populated with fiber models based on the local alignment state. These models were then compared to isotropic models with the same sample shape and fiber density, and to homogeneous models with the same sample shape, fiber density, and average fiber alignment as the scar-specific models. All simulations involved equibiaxial extension of the sample with free motion in the third dimension. We found that heterogeneity led to a lower degree of mechanical anisotropy and a higher level of local stress concentration than the corresponding homogeneous model, and also that fibers failed in the heterogeneous model at much lower macroscopic strains than in the isotropic and homogeneous models. Taken together, these results suggest that scar heterogeneity may impair myocardial mechanical function both in terms of anisotropy and strength, and that individual variations in scar heterogeneity could be an important consideration for understanding scar remodeling and designing therapeutic interventions for patients after myocardial infarction.

scholarly journals Extracting elastic constitutive parameters using the VFM within peridynamics

2019 ◽  
Author(s):  
Rolland Delorme ◽  
Patrick Diehl ◽  
Ilyass Tabiai ◽  
Louis Laberge Lebel ◽  
Martin Levesque
Keyword(s):  
Finite Element ◽  
Displacement Field ◽  
Material Properties ◽  
Noise Analysis ◽  
Image Correlation ◽  
Bending Test ◽  
Virtual Fields Method ◽  
Element Analysis ◽  
Constitutive Parameters

This paper implements the Virtual Fields Method within the ordinary state based peridynamic framework to identify material properties. The key equations derived in this approach are based on the principle of virtual works written under the ordinary state based peridynamic formalism for two-dimensional isotropic linear elasticity. In-house codes including a minimization process have also been developed to implement the method. A three-point bending test and two independent virtual fields arbitrarily chosen are used as a case study throughout the paper. The numerical validation of the virtual fields method has been performed on the case study by simulating the displacement field by finite element analysis. This field has been used to extract the elastic material properties and compared them to those used as input in the finite element model, showing the robustness of the approach. Noise analysis and the effect of the missing displacement fields on the specimen’s edges to simulate digital image correlation limitations have also been studied in the numerical part. This work focuses on pre-damage properties to demonstrate the feasibility of the method and provides a new tool for using full-field measurements within peridynamics with a reduced calculation time as there is no need to compute the displacement field. Future works will deal with damage properties which is the main strength of peridynamics.

Collagen Fiber Alignment Does Not Explain Mechanical Anisotropy in Fibroblast Populated Collagen Gels

2007 ◽  
Vol 129 (5) ◽  
pp. 642-650 ◽  
Author(s):  
Stavros Thomopoulos ◽  
Gregory M. Fomovsky ◽  
Preethi L. Chandran ◽  
Jeffrey W. Holmes
Keyword(s):  
Mechanical Behavior ◽  
Network Model ◽  
Continuum Model ◽  
Collagen Fiber ◽  
Collagen Gel ◽  
Mechanical Anisotropy ◽  
Collagen Gels ◽  
Fiber Alignment ◽  
High Degree ◽  
Collagen Fiber Alignment

Many load-bearing soft tissues exhibit mechanical anisotropy. In order to understand the behavior of natural tissues and to create tissue engineered replacements, quantitative relationships must be developed between the tissue structures and their mechanical behavior. We used a novel collagen gel system to test the hypothesis that collagen fiber alignment is the primary mechanism for the mechanical anisotropy we have reported in structurally anisotropic gels. Loading constraints applied during culture were used to control the structural organization of the collagen fibers of fibroblast populated collagen gels. Gels constrained uniaxially during culture developed fiber alignment and a high degree of mechanical anisotropy, while gels constrained biaxially remained isotropic with randomly distributed collagen fibers. We hypothesized that the mechanical anisotropy that developed in these gels was due primarily to collagen fiber orientation. We tested this hypothesis using two mathematical models that incorporated measured collagen fiber orientations: a structural continuum model that assumes affine fiber kinematics and a network model that allows for nonaffine fiber kinematics. Collagen fiber mechanical properties were determined by fitting biaxial mechanical test data from isotropic collagen gels. The fiber properties of each isotropic gel were then used to predict the biaxial mechanical behavior of paired anisotropic gels. Both models accurately described the isotropic collagen gel behavior. However, the structural continuum model dramatically underestimated the level of mechanical anisotropy in aligned collagen gels despite incorporation of measured fiber orientations; when estimated remodeling-induced changes in collagen fiber length were included, the continuum model slightly overestimated mechanical anisotropy. The network model provided the closest match to experimental data from aligned collagen gels, but still did not fully explain the observed mechanics. Two different modeling approaches showed that the level of collagen fiber alignment in our uniaxially constrained gels cannot explain the high degree of mechanical anisotropy observed in these gels. Our modeling results suggest that remodeling-induced redistribution of collagen fiber lengths, nonaffine fiber kinematics, or some combination of these effects must also be considered in order to explain the dramatic mechanical anisotropy observed in this collagen gel model system.

Experimental Study on Displacement and Strain Distributions of Bone Model with Dental Implant

2011 ◽  
Vol 83 ◽  
pp. 73-77 ◽  
Author(s):  
Yasuyuki Morita ◽  
Yasuyuki Matsushita ◽  
Mitsugu Todo ◽  
Kiyoshi Koyano
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Dental Implants ◽  
Cancellous Bone ◽  
Material Properties ◽  
Alveolar Bone ◽  
Image Correlation ◽  
Element Analysis ◽  
Deformation Distribution ◽  
Natural Tooth

For normal healthy teeth, the percussive energy generated by mastication is attenuated by the periodontal ligament at the healthy bone/natural tooth interface. However, when a natural tooth must be replaced by an implant because of damage or disease, the ligament is lost and the implant will transmit the percussive forces to the bone directly. Studies have evaluated the deformation distribution of the alveolar bone in the vicinity of implants using finite element analysis and photoelasticity. However, finite element analysis requires clinical verification or a determination of material properties, and photoelastic materials generally have material properties and structure quite different from those of actual bone. Therefore, this study examined the deformation distribution around dental implants in cortical/cancellous bone experimentally using sawbone cortical/cancellous bone models. Dental implants were placed in the bone models and the displacement distribution was measured using the digital image correlation method, and the strain distribution was visualized under a compressive load that simulated the occlusion force.

scholarly journals Peripapillary and Posterior Scleral Mechanics—Part I: Development of an Anisotropic Hyperelastic Constitutive Model

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Michaël J. A. Girard ◽  
J. Crawford Downs ◽  
Claude F. Burgoyne ◽  
J.-K. Francis Suh
Keyword(s):  
Mechanical Properties ◽  
Finite Element ◽  
Optic Nerve ◽  
Constitutive Model ◽  
Optic Nerve Head ◽  
Collagen Fiber ◽  
Fiber Concentration ◽  
Fiber Alignment ◽  
Fiber Organization ◽  
Collagen Fiber Alignment

The sclera is the white outer shell and principal load-bearing tissue of the eye as it sustains the intraocular pressure. We have hypothesized that the mechanical properties of the posterior sclera play a significant role in and are altered by the development of glaucoma—an ocular disease manifested by structural damage to the optic nerve head. An anisotropic hyperelastic constitutive model is presented to simulate the mechanical behavior of the posterior sclera under acute elevations of intraocular pressure. The constitutive model is derived from fiber-reinforced composite theory, and incorporates stretch-induced stiffening of the reinforcing collagen fibers. Collagen fiber alignment was assumed to be multidirectional at local material points, confined within the plane tangent to the scleral surface, and described by the semicircular von Mises distribution. The introduction of a model parameter, namely, the fiber concentration factor, was used to control collagen fiber alignment along a preferred fiber orientation. To investigate the effects of scleral collagen fiber alignment on the overall behaviors of the posterior sclera and optic nerve head, finite element simulations of an idealized eye were performed. The four output quantities analyzed were the scleral canal expansion, the scleral canal twist, the posterior scleral canal deformation, and the posterior laminar deformation. A circumferential fiber organization in the sclera restrained scleral canal expansion but created posterior laminar deformation, whereas the opposite was observed with a meridional fiber organization. Additionally, the fiber concentration factor acted as an amplifying parameter on the considered outputs. The present model simulation suggests that the posterior sclera has a large impact on the overall behavior of the optic nerve head. It is therefore primordial to provide accurate mechanical properties for this tissue. In a companion paper (Girard, Downs, Bottlang, Burgoyne, and Suh, 2009, “Peripapillary and Posterior Scleral Mechanics—Part II: Experimental and Inverse Finite Element Characterization,” ASME J. Biomech. Eng., 131, p. 051012), we present a method to measure the 3D deformations of monkey posterior sclera and extract mechanical properties based on the proposed constitutive model with an inverse finite element method.

Reliability Evaluation of a 3D SIC Package by the Combination of the SEM-DIC and the FEM

2013 ◽  
Author(s):  
Toru Ikeda ◽  
Masatoshi Oka ◽  
Noriyuki Miyazaki
Keyword(s):  
Finite Element ◽  
Cross Section ◽  
Material Properties ◽  
Test Specimen ◽  
Thermal Strain ◽  
Curing Temperature ◽  
Image Correlation ◽  
Test Chip ◽  
Uf Resin ◽  
The Cross

Numerical methods such as the finite element method (FEM) have been used to evaluate the reliability of electronic packages. The accuracy of the analyses should be verified by some experimental measurements. In this study, we evaluated the thermal strain of a test chip for three-dimensional stacked integrated circuits (3D SIC) with both measurement and an analysis. First, the distribution of thermal strain on the cross-section of a test chip was measured using scanning electron microscope (SEM) and the digital image correlation. Then, the distribution of strain of the test chip was also analyzed by the FEM considering the viscoelastic material properties of underfill (UF) resin measured with the stress relaxation test and the elastic-plastic material properties of components measured with the nano-indentation tests. The accuracy of the nonlinear finite element analysis was verified using the strain measurements with the SEM-DICM. A test specimen for the 3D SIC packages was built and cut out a part of the test specimen and polished its cross-section. We took digital images using a SEM (FEI Quanta 200) to measure the strain distributions on the cross-section of a specimen by the DICM. The specimen was subjected to thermal loading in a heat chamber. The temperature in the chamber was raised from 30° C to 130°C. The FE analyses were carried out using MSC.Marc™. We assumed the initial temperature of the analysis to be 150°C, which was the curing temperature of the UF resin, and decreased the temperature to 30°C during 100 seconds. Then, the temperature was raised up to 130°C, which is the same with the experiment. We compared the numerical result with the measurement and modified the model of the FE analyses.

Sign in / Sign up

Export Citation Format

Share Document