Hydration related changes in tensile response of posterior porcine sclera

Abstract

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%. The strips were subjected to ten consecutive cycles of loading and unloading up to 1 MPa. The response of samples at the tenth cycle was used to compute the tangent modulus, maximum strain, and hysteresis as a function of hydration. The experiments were done in oil in order to prevent hydration changes during the mechanical tests. The mechanical response of strips right after dissection, control group, was also measured. In general, significant softening of scleral strips was found with increasing hydration (p < 0.05). The stress-strain response of control group was between those of samples with hydration 150% and 200%. The experimental stress-strain data were successfully represented numerically with an exponential mathematical relation with R² > 0.99. The present study showed that hydration would significantly alter the tensile response of scleral tissue. Thus, the hydration of scleral specimens during mechanical experimental measurements should be carefully controlled.

Introduction

The sclera plays an important role in protecting internal components of the eyeball against external forces. It forms about 80% of the outer surface area of eye globe and is constantly subjected to intraocular pressure as well as forces from extraocular muscles. The mechanical properties of sclera are extremely important for the proper functioning of the eye. Ocular diseases such as myopia and glaucoma have been found to be initiated or advance because scleral mechanical properties were compromised (Avetisov et al., 1982; Campbell et al., 2014; Coudrillier et al., 2015; Downs et al., 2001; Girard et al., 2011; Phillips and McBrien, 1995; Rada et al., 2006). Myopia is a common eye problem in which patients see distant objects blurry. This refractive error occurs because the eyeball is too long and light rays do not focus correctly on the retina. Significant changes in the mechanical properties of sclera have been observed during myopia development. In addition to myopia, the amount of damage of retinal ganglion cells at the optic nerve head in glaucoma eyes seems to be correlated with significant changes in biomechanical response of sclera. Thus, it is of great interest to better understand the origins of the mechanical properties of scleral tissue.

Previous reports on biomechanical properties of sclera show large variation. In particular, the inflation, uniaxial testing, and unconfined compression gave Young's modulus varying from about 1 KPa to 50 MPa (Eilaghi et al., 2010; Friberg and Lace, 1988; Geraghty et al., 2012; Mortazavi et al., 2009; Schultz et al., 2008; Woo et al., 1972). This large variation could be because of differences in testing protocols as well as intrinsic differences in the mechanical response of scleral samples from different species. The sclera is also an anisotropic viscoelastic material whose biomechanical properties show regional variations that depend on the age of specimens (Curtin, 1969; Elsheikh et al., 2010; Geraghty et al., 2012; Girard et al., 2009; Grytz et al., 2014). Not controlling the hydration of samples during experimental studies might be another contributing factor to the existing variation in the mechanical repose of sclera.

The extracellular matrix of sclera is primarily composed of collagen fibrils and proteoglycans (Watson and Young, 2004). The proteoglycans consist of a core protein to which sulfated glycosaminoglycans (GAGs) are covalently attached. The primary proteoglycans in human sclera are decorin and biglycan, which have only few GAG side chains. GAGs play a crucial role in collagen fibrillogenesis and tissue hydration. The sulfated GAGs become negatively charged in the aqueous environment and attract cations, which subsequently attract water molecules inside the tissue. The negatively charged GAG chains also repel each other, which increases the volume of the solid skeleton, and attract water inside. It is found that hydration variation in sclera affects the solute diffusion and fluid movement (Boubriak et al., 2000). Specifically, increasing hydration increases diffusion coefficients. Nevertheless, possible effects of hydration on the biomechanical response of sclera have not yet been determined. Due to similarities between the extracellular matrices of the sclera and cornea and previous studies on the effect of hydration on mechanical response of the cornea (Hatami-Marbini, 2014; Hatami-Marbini and Etebu, 2013; Hatami-Marbini and Rahimi, 2014b), we assumed that hydration would significantly influence the scleral biomechanical properties and play a role in their accurate characterization. In order to investigate the above assumption, we conducted tensile experiments on porcine scleral samples at different hydration levels in the present study. The samples were prepared from the region near the posterior pole of similar age porcine eyes in order to prevent known regional variations in the mechanical properties.