Planar biaxial testing of CXL strengthening effects.

The stiffening effect of corneal crosslinking (CXL) treatment, a therapeutic approach for managing the progression of keratoconus, has been primarily investigated using uniaxial tensile experiments. However, this testing technique has several drawbacks and is unable to measure the mechanical response of cornea under a multiaxial loading state. In this work, we used biaxial mechanical testing method to characterize biomechanical properties of porcine cornea before and after CXL treatment.

Regional differences in electroactive response of the sclera.

The sclera exhibits mechanical response when subjected to an external electric stimulation. The scleral electroactive response is a function of its charge density, mechanical properties, thickness, and strength of the applied electric voltage. The primary objective of the present work was to investigate the regional differences in the electroactive response of porcine sclera.

Finite Deformation of Scleral Tissue under Electrical Stimulation: An Arbitrary Lagrangian-Eulerian Finite Element Method.

The sclera is considered as the principal load-bearing tissue within the eye. The sclera is negatively charged; thus, it exhibits mechanical response to electrical stimulation. We recently demonstrated the electroactive behavior of sclera by performing experimental measurements that captured the deformation of the tip of scleral strips subjected to electric voltage.

On the Mechanical Roles of Glycosaminoglycans in the Tensile Properties of Porcine Corneal Stroma.

Purpose: The cornea is primarily composed of collagen fibrils that are embedded in a ground substance rich in proteoglycans and other glycoproteins. It is known that glycosaminoglycan (GAG) side chains of proteoglycans form anti-parallel duplexes between collagen fibrils. The present work was done in order to investigate the mechanical function of GAGs in defining the tensile properties of porcine corneal stroma.

Modeling and experimental investigation of electromechanical properties of scleral tissue; a CEM model using an anisotropic hyperelastic constitutive relation.

The sclera is a soft tissue primarily consisting of collagen fibers, elastin, and proteoglycans. The proteoglycans are composed of a core protein and negatively charged glycosaminoglycan side chains. The fixed electric charges inside the scleral extracellular matrix play a key role in its swelling and are expected to cause the tissue to deform in response to an electric field.

Effect of corneal collagen crosslinking on viscoelastic shear properties of the cornea.

The cornea is responsible for most of the refractive power in the eye and acts as a protective layer for internal contents of the eye. The cornea requires mechanical strength for maintaining its precise shape and for withstanding external and internal forces. Corneal collagen crosslinking (CXL) is a treatment option to improve corneal mechanical properties.

Experimental and numerical analysis of electroactive characteristics of scleral tissue.

The sclera provides mechanical support to retina and protects internal contents of the eye against external injuries. The scleral extracellular matrix is mainly composed of collagen fibers and proteoglycans (PGs). At physiological pH, collagen molecules are neutral but PGs contain negatively charged glycosaminoglycan chains.

Tensile Viscoelastic Properties of the Sclera after Glycosaminoglycan Depletion.

Purpose: Fibrillar collagen network and glycosaminoglycans (GAGs) are the primary components of extracellular matrix (ECM) of the sclera. The main goal of this study was to investigate the possible structural roles of GAGs in the scleral tensile properties as a function of preconditioning and displacement rate.

Methods: Four-step uniaxial stress-relaxation tests were used for characterizing the viscoelastic tensile response of the posterior porcine sclera with and without enzymatic GAG removal.

Regional Differences in the Glycosaminoglycan Role in Porcine Scleral Hydration and Mechanical Behavior.

Purpose: This study characterized the role of glycosaminoglycans (GAGs) in the hydration, thickness, and biomechanical properties of posterior and anterior porcine sclera.

Methods: The scleral discs and strips were obtained from the anterior and posterior parts of porcine eyes, and their initial hydration and thickness were measured. The anterior and posterior scleral discs were used to show the efficacy of the GAG removal protocol by quantifying their GAG content.

Nonlinear Mechanical Properties of Prestressed Branched Fibrous Networks.

Random fiber networks constitute the solid skeleton of many biological materials such as the cytoskeleton of cells and extracellular matrix of soft tissues. These random networks show unique mechanical properties such as nonlinear shear strain-stiffening and strain softening when subjected to preextension and precompression, respectively. In this study, we perform numerical simulations to characterize the influence of axial prestress on the nonlinear mechanical response of random network structures as a function of their micromechanical and geometrical properties.

Numerical Investigation on the Role of Mechanical Factors Contributing to Globe Flattening in States of Elevated Intracranial Pressure.

Flattening of the posterior eye globe in the magnetic resonance (MR) images is a sign associated with elevated intracranial pressure (ICP), often seen in people with idiopathic intracranial hypertension (IIH). The exact underlying mechanisms of globe flattening (GF) are not fully known but mechanical factors are believed to play a role. In the present study, we investigated the effects of material properties and pressure loads on GF.

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

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.

Mechanical properties of subisostatic random networks composed of nonlinear fibers.

Fibrous protein networks provide structural integrity to different biological materials such as soft tissues. These networks display an unusual exponential strain-stiffening behavior when subjected to mechanical loads. This nonlinear strain-stiffening behavior has so far been explained in terms of the network microstructure and the flexibility of constituting fibers.

A computational study of the behavior of colloidal gel networks at low volume fraction.

Colloidal gel networks appear in different scientific and industrial applications because of their unique properties. Molecular dynamics simulations could reveal the relation between molecular level and macroscopic properties of these systems. Nevertheless, the predictions of numerical simulations might depend on the specific form and parameters of interaction potentials.

Hydration related changes in tensile response of posterior porcine sclera.

It has been shown that there exists significance dependence between hydration and biomechanical properties of hydrated tissues such as cornea. The primary purpose of this study was to determine hydration effects on mechanical properties of sclera. Scleral strips, dissected from the posterior part of pig eyes along the superior-inferior direction, were divided into four hydration groups by first drying them and then soaking them in PBS until their hydration reached to 75%, 100%, 150%, and 200%.

Colloidal particle gel models using many-body potential interactions.

Many-body effective interactions are commonly used in a molecular dynamics simulation study of gel networks formed by colloidal particles. Here we report an interaction potential that can be used to investigate the mechanical response of colloidal gel networks under shear deformation. We then investigate the dependence of the numerical simulation results on the form of mathematical expression used to define the interparticle interactions.

The contribution of sGAGs to stress-controlled tensile response of posterior porcine sclera.

Despite the significant progress in characterizing mechanical functions of individual scleral extracellular matrix (ECM) components, the biomechanical contribution of sulfated glycosaminoglycans (sGAGs) is still poorly understood. The primary purpose of this study was to determine the possible function of sGAGs in scleral mechanical response by characterizing the tensile behavior of normal and sGAG-depleted samples. We used chondroitinase ABC solution to remove sGAGs from scleral samples that were dissected from posterior porcine eyes.

Influence of Microstructure on Stiffening Effects of Corneal Cross-linking Treatment.

Purpose: To investigate the stiffening effects of corneal cross-linking (CXL) on tensile properties of anterior and posterior corneal flaps.

Methods: A Descemet stripping automated endothelial keratoplasty system was used to prepare anterior and posterior flaps from porcine corneas. The flaps were subjected to CXL and their mechanical behavior was assessed by conducting uniaxial tensile experiments.

Relationship between initial corneal hydration and stiffening effects of corneal crosslinking treatment.

Purpose: To characterize the mechanical property improvement of riboflavin and ultraviolet light corneal crosslinking (CXL) procedure in artificially swollen human and porcine corneas.

Setting: Computational Biomechanics Research Laboratory, Mechanical and Industrial Engineering Department, University of Illinois at Chicago, Illinois, USA.

Design: Experimental study.

UVA/riboflavin collagen crosslinking stiffening effects on anterior and posterior corneal flaps.

The UVA/riboflavin collagen crosslinking (CXL) is one of the treatment procedure for stopping the progression of keratoconus. The inclusion criterion for this procedure is a minimum corneal thickness of 400 μm, which is not often met in patients with advanced keratoconus. Preoperatively swelling thin corneas was shown to stabilize the keratectasia without any postoperative endothelial damage.

Effect of UVA/Riboflavin Collagen Crosslinking on Biomechanics of Artificially Swollen Corneas.

Purpose: The purpose of this study was to characterize the relation between corneal hydration and stiffening effects of the UVA/riboflavin collagen crosslinking treatment and to investigate how artificially swelling the cornea prior to this treatment procedure affects tensile property improvement.

Methods: Porcine corneas were collagen crosslinked in vitro at different hydration levels using a number of hypoosmolar and isoosmolar riboflavin solutions. Thickness of the specimens prior to crosslinking was taken as a proxy for their hydration and was used to divide them into different thickness groups.

Nonaffine behavior of three-dimensional semiflexible polymer networks.

Three-dimensional semiflexible polymer networks are the structural building blocks of various biological and structural materials. Previous studies have primarily used two-dimensional models for understanding the behavior of these networks. In this paper, we develop a three-dimensional nonaffinity measure capable of providing direct comparison with continuum level homogenized quantities, i.

Interrelation of Hydration, Collagen Cross-Linking Treatment, and Biomechanical Properties of the Cornea.

Purpose: The present study was designed to investigate the effects of hydration and collagen cross-linking treatment on biomechanical properties of the cornea.

Methods: The original corneal collagen cross-linking protocol was used to induce cross-links in bovine corneas. The thickness of samples was used as a measure of their hydration and five different thickness groups (n = 5 each) were considered.

Collagen cross-linking treatment effects on corneal dynamic biomechanical properties.

Cornea is a soft tissue with the principal function of transmitting and refracting light rays. The objective of the current study was to characterize possible effects of the riboflavin/UVA collagen cross-linking on corneal dynamic properties. The original corneal cross-linking protocol was used to induce cross-links in the anterior portion of the bovine cornea.