scholarly journals A microstructural model of cross-link interaction between collagen fibrils in the human cornea

2019 ◽  
Vol 377 (2144) ◽  
pp. 20180079 ◽  
Author(s):  
A. Pandolfi ◽  
A. Gizzi ◽  
M. Vasta
Keyword(s):  
Mechanical Response ◽  
Applied Mathematics ◽  
Micromechanical Model ◽  
Collagen Fibrils ◽  
Structural Function ◽  
Human Cornea ◽  
Corneal Tissue ◽  
Cross Link ◽  
Stromal Tissue ◽  
Microstructural Model

We propose a simplified micromechanical model of the fibrous reinforcement of the corneal tissue. We restrict our consideration to the structural function of the collagen fibrils located in the stroma and disregard the other all-important components of the cornea. The reinforcing structure is modelled with two sets of parallel fibrils, connected by transversal bonds within the single fibril family (inter-cross-link) and across the two families (intra-cross-link). The particular design chosen for this ideal structure relies on the fact that its ability to sustain loads is dependent on the degree of the cross-link and, therefore, on the density and stiffness of the bonds. We analyse the mechanical response of the system according to the type of interlacing and on the stiffness of fibres and bonds. Results show that the weakening of transversal bonds is associated with a marked increase of the deformability of the system. In particular, the deterioration of transversal bonds due to mechanical, chemical or enzymatic reasons can justify the loss of stiffness of the stromal tissue resulting in localized thinning and bulging typically observed in keratoconus corneas. This article is part of the theme issue ‘Rivlin's legacy in continuum mechanics and applied mathematics’.

scholarly journals Diffusion-based degeneration of the collagen reinforcement in the pathologic human cornea

2021 ◽  
Vol 127 (1) ◽  
Author(s):  
Alessio Gizzi ◽  
Maria Laura De Bellis ◽  
Marcello Vasta ◽  
Anna Pandolfi
Keyword(s):  
Scalar Field ◽  
Mechanical Response ◽  
Mechanical Stability ◽  
Shape Change ◽  
Collagen Fibrils ◽  
Human Cornea ◽  
Collagen Structure ◽  
Cross Link ◽  
Mechanical Stiffness ◽  
Microstructural Model

AbstractWe describe a multiphysics model of the collagen structure of the cornea undergoing a progressive localized reduction of the stiffness, preluding to the development of ectasia and keratoconus. The architecture of the stromal collagen is assumed to follow the simplified two-family model proposed in Pandolfi et al. (A microstructural model of cross-link interaction between collagen fibrils in the human cornea. Philos Trans R Soc A 377:20180079, 2019), where the mechanical stiffness of the structure is supplied by transversal bonds within the fibrils of the same family (inter-crosslink bonds) and across the fibrils of the two families (intra-crosslink bonds). In Pandolfi et al. (A microstructural model of cross-link interaction between collagen fibrils in the human cornea. Philos Trans R Soc A 377:20180079, 2019), it was shown that the loss of the spherical shape due to the protrusion of a cone can be ascribed to the mechanical weakening of the intra-crosslink bonds in the central region of the collagen structure. In the present study, the reduction of bond stiffness is coupled to an evolutive pathologic phenomenon, modeled as a reaction–diffusion process of a normalized scalar field. We assume that the scalar field is a concentration-like measure of the degeneration of the chemical bonds stabilizing the structural collagen. We follow the evolution of the mechanical response of the system in terms of shape change, according to the propagation of the degeneration field, and identify the critical loss of mechanical stability resulting in the typical bulging of keratoconus corneas.

Constitutive Modeling of Corneal Tissue: Influence of Three-Dimensional Collagen Fiber Microstructure

2020 ◽  
Vol 143 (3) ◽  
Author(s):  
Shuolun Wang ◽  
Hamed Hatami-Marbini
Keyword(s):  
Extracellular Matrix ◽  
Constitutive Modeling ◽  
Mechanical Response ◽  
Thickness Distribution ◽  
Three Dimensional ◽  
Vital Role ◽  
Collagen Fibrils ◽  
Model Parameters ◽  
Corneal Tissue ◽  
Numerical Predictions

Abstract The cornea, the transparent tissue in the front of the eye, along with the sclera, plays a vital role in protecting the inner structures of the eyeball. The precise shape and mechanical strength of this tissue are mostly determined by the unique microstructure of its extracellular matrix. A clear picture of the 3D arrangement of collagen fibrils within the corneal extracellular matrix has recently been obtained from the secondary harmonic generation images. However, this important information about the through-thickness distribution of collagen fibrils was seldom taken into account in the constitutive modeling of the corneal behavior. This work creates a generalized structure tensor (GST) model to investigate the mechanical influence of collagen fibril through-thickness distribution. It then uses numerical simulations of the corneal mechanical response in inflation experiments to assess the efficacy of the proposed model. A parametric study is also done to investigate the influence of model parameters on numerical predictions. Finally, a brief comparison between the performance of this new constitutive model and a recent angular integration (AI) model from the literature is given.

Generation of Three-Dimensional Microstructure Model for Discontinuously Reinforced Composite by Modified Random Sequential Absorption Method

2016 ◽  
Vol 138 (2) ◽  
Author(s):  
Jiming Zhou ◽  
Lehua Qi ◽  
Arun M. Gokhale
Keyword(s):  
Carbon Fibers ◽  
Matrix Composite ◽  
Mechanical Response ◽  
Short Fibers ◽  
Alloy Matrix ◽  
Rsa Algorithm ◽  
Reinforced Composite ◽  
Microstructural Model ◽  
Microstructure Model ◽  
Short Carbon

Computer simulation of mechanical behavior of discontinuously reinforced composites containing randomly oriented short-fibers/whiskers presents an attractive opportunity for reduction of the number of experiments and resources required for microstructure design of such advanced materials. It is desirable to perform such simulations using microstructure model that accounts for randomness in angular orientations and locations of the short fibers/whiskers. In this contribution, a methodology is presented for efficient simulation of the required microstructural model through modification of well-known random sequential adsorption (RSA) algorithm for microstructure simulation through its application to the microstructure of Mg–alloy matrix composite containing randomly oriented short carbon fibers. The modified RSA algorithm enhances accuracy and efficiency of the complex geometric details of the randomly oriented short-fiber reinforced composite microstructure. Simulated microstructural model of composite is implemented in abaqus to simulate the mechanical response of the Mg–matrix composite containing randomly oriented short carbon fibers. The generated complex microstructure model in abaqus code is sliced into thin slices for reducing computing resources. The simulated results from multiple sliced models were averaged to approximate the result for the full volume element. The simulated mechanical response by use of multiple sliced models is validated via comparison with the experimental data.

scholarly journals The effect of semicarbazide on the nature and stability of collagen fibrils

1970 ◽  
Vol 118 (1) ◽  
pp. 61-65 ◽  
Author(s):  
Shirley Ayad ◽  
C. H. Wynn
Keyword(s):  
Collagen Fibril ◽  
Collagen Fibrils ◽  
Cross Linking ◽  
Unsaturated Aldehyde ◽  
Cross Link ◽  
Soluble Collagen ◽  
Acid Soluble Collagen ◽  
Direct Binding ◽  
Aldehyde Groups

Semicarbazide affected both the final width and stability of fibrils reconstituted from solutions of acid-soluble collagen. Fibril width was increased after semicarbazide treatment at pH2.6 and 4.3, whereas after treatment at pH8.9 it decreased. Fibril stability was decreased after semicarbazide treatment at all values of pH and temperature, indicating the inhibition of intermolecular cross-linking. A direct binding of semicarbazide to the αβ-unsaturated aldehyde groups in the intramolecular cross-link was demonstrated.

EMBEDDED COLLAGEN DEFORMATION MODELS FOR COMPUTATIONAL MODELING OF HEALTHY, KERATOCONIC, AND CROSSLINKED CORNEAS

2013 ◽  
Vol 13 (03) ◽  
pp. 1350024 ◽  
Author(s):  
CRAIG D. FOSTER ◽  
DIPKA GONGAL ◽  
TEDI BEGAJ ◽  
MICHELLE LUO
Keyword(s):  
Crosslink Density ◽  
Tensile Tests ◽  
Collagen Fibrils ◽  
Corneal Tissue ◽  
Normal Vision ◽  
Finite Element Program ◽  
Strain Behavior ◽  
Element Program ◽  
Deformation Models ◽  
Healthy Eyes

Collagen plays an extremely important role in carrying forces and maintaining the shape of the cornea. In keratoconus, the cornea shape can become distorted to the extent that normal vision is impossible, and the amount crosslinking between collagen fibrils are generally lower than in healthy eyes. In contrast, riboflavin-induced crosslinks can strengthen and stiffen the cornea. This article examined quantitatively how the extent of crosslinking in collagen fibrils influences the overall mechanical behavior of corneal tissue. Three models for the stress–strain behavior of the fibrils were examined, which is a function of the crosslink density within the fibrils. These models were then embedded in a matrix model, and tensile tests of cornea strips were examined using a finite element program. Results were compared with experiments from the literature for both normal and crosslinked corneas.

scholarly journals Examining the Transmission of Visible Light through Electrospun Nanofibrous PCL Scaffolds for Corneal Tissue Engineering

Nanomaterials ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 3191
Author(s):  
Marcus Himmler ◽  
Dirk W. Schubert ◽  
Thomas A. Fuchsluger
Keyword(s):  
Tissue Engineering ◽  
Visible Light ◽  
Light Transmission ◽  
Fiber Diameter ◽  
Surrounding Medium ◽  
Wound Dressings ◽  
Human Cornea ◽  
Corneal Tissue ◽  
Nanofibrous Scaffolds ◽  
Vis Spectroscopy

The transparency of nanofibrous scaffolds is of highest interest for potential applications like corneal wound dressings in corneal tissue engineering. In this study, we provide a detailed analysis of light transmission through electrospun polycaprolactone (PCL) scaffolds. PCL scaffolds were produced via electrospinning, with fiber diameters in the range from (35 ± 13) nm to (167 ± 35) nm. Light transmission measurements were conducted using UV–vis spectroscopy in the range of visible light and analyzed with respect to the influence of scaffold thickness, fiber diameter, and surrounding medium. Contour plots were compiled for a straightforward access to light transmission values for arbitrary scaffold thicknesses. Depending on the fiber diameter, transmission values between 15% and 75% were observed for scaffold thicknesses of 10 µm. With a decreasing fiber diameter, light transmission could be improved, as well as with matching refractive indices of fiber material and medium. For corneal tissue engineering, scaffolds should be designed as thin as possible and fabricated from polymers with a matching refractive index to that of the human cornea. Concerning fiber diameter, smaller fiber diameters should be favored for maximizing graft transparency. Finally, a novel, semi-empirical formulation of light transmission through nanofibrous scaffolds is presented.

On the Connection Between Geometry and Statically Determined Membrane Stresses in the Human Cornea

2020 ◽  
Vol 142 (5) ◽  
Author(s):  
M. Angelillo ◽  
A. Montanino ◽  
A. Pandolfi
Keyword(s):  
Intraocular Pressure ◽  
Stress State ◽  
Principal Stress ◽  
Collagen Fibrils ◽  
Equilibrium Analysis ◽  
Time Dependent ◽  
Patient Specific ◽  
Shear Stresses ◽  
Human Cornea ◽  
The Matrix

Abstract Under the action of the intraocular pressure (IOP), the human cornea is stressed and deforms acquiring a quasi-spherical configuration. If the stressed configuration is known, and the cornea is regarded as a membrane, disregarding flexural behaviors with an equilibrium analysis only is possible to estimate the distribution of the average stress across the thickness. In the cornea, the action of the intraocular pressure is supported by collagen fibrils, immersed into an elastin-proteoglycan matrix, and organized in a very precise architecture to provide the necessary confinement and transparency to the light. With the goal of understanding the static consequences of shape modifications due to pathological dilatation (ectasia), we present a simplified stress analysis of the human cornea modeled as a membrane. A numerical investigation over 40 patient-specific corneas (20 normal and 20 ectatic) is carried out to establish a relationship between the physiological geometry and the distribution of the membrane stresses, and to assess the possibility to obtain information on the stress state based on topographic images only. Comparative analyses reveal that, with respect to normal corneas, in ectatic corneas the pattern of the principal stress lines is modified markedly showing a deviation from the hypothetical dominant orientation of the collagen fibrils. The rotation of the principal stress with respect to the fibril orientation can be thought as responsible of the transmission of a large amount of shear stresses onto the elastin-proteoglycan matrix. The anomalous loading of the matrix could be correlated to the evolution of time-dependent shape modifications leading to ectasia.

scholarly journals Implementing a micromechanical model into a finite element code to simulate the mechanical and microstructural response of arteries

2020 ◽  
Vol 19 (6) ◽  
pp. 2553-2566
Author(s):  
Daniele Bianchi ◽  
Claire Morin ◽  
Pierre Badel
Keyword(s):  
Finite Element ◽  
Mechanical Response ◽  
Micromechanical Model ◽  
Mechanical Tests ◽  
Material Behavior ◽  
The Finite Element Method ◽  
Experimental Findings ◽  
Constitutive Formulation ◽  
Constitutive Parameters ◽  
Computational Strategy

Abstract A computational strategy based on the finite element method for simulating the mechanical response of arterial tissues is herein proposed. The adopted constitutive formulation accounts for rotations of the adventitial collagen fibers and introduces parameters which are directly measurable or well established. Moreover, the refined constitutive model is readily utilized in finite element analyses, enabling the simulation of mechanical tests to reveal the influence of microstructural and histological features on macroscopic material behavior. Employing constitutive parameters supported by histological examinations, the results herein validate the model’s ability to predict the micro- and macroscopic mechanical behavior, closely matching previously observed experimental findings. Finally, the capabilities of the adopted constitutive description are shown investigating the influence of some collagen disorders on the macroscopic mechanical response of the arterial tissues.

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