Compressive behaviour of soft contact lenses and its effect on refractive power on the eye and handling off the eye.

Ahmad H Shihab,Ahmed Abass,Ashkan Eliasy,Steve Jones,Lynn White,Brendan Geraghty,Ahmed Elsheikh,Richard Wu,Bernardo T Lopes,Akram Joda,Krishna Garikipati

doi:10.1371/journal.pone.0247194

Ahmad H Shihab, Ahmed Abass + Show 9 more

Open Access

https://doi.org/10.1371/journal.pone.0247194

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Abstract

PurposeTo investigate the stress-strain behaviour of 9 soft contact lens materials, that are commonly used in the market, under uniaxial compression loading.MethodsSeven types of hydrogel and two types of silicone-hydrogel soft contact lens materials were hydrated in phosphate-buffered saline (PBS) solution then subjected to uniaxial compression loads. The load rate was set to 16.0 N/min starting with two consecutive initial 5.0 N loading cycles followed by three relaxation periods of 4.0 min within which there were two more 5.0 N loading cycles and eventually, a full loading cycle that stopped at a load of 49.0 N. The load and contraction data obtained experimentally were analysed to derive the stress-strain behaviour. Finite Element (FE) analysis was then utilised to evaluate the performance of soft contact lenses on the human eye and handling lenses off the eye.ResultsUnlike tensile tests, all tested materials showed nonlinear behaviour when tested under compression. When fitted to first-order Ogden hyperelastic model, parameter μ was found to be varying in the range 0.12 to 0.74 MPa and material parameter α was found to be varying in the range 8.2 to 20.326 among the nine tested materials. Compression modulus of elasticity was 2.2 times higher than the tensile modulus of elasticity on average. FE simulation with nonlinear Ogden constitutive model showed a limited change (8%~12%) in the optical performance when compared to other material models, however, it predicted higher stress when the lens was simulated under bending during off-eye handling.ConclusionsCompression tests revealed slightly nonlinear behaviour when materials were strained under compression stress down to 15% ~ 30% of their nominal heights. Considering the physiological compression loading range of 8 mmHg, secant moduli of elasticity were 1.5% to 6.9% higher than the tension moduli of elasticity depending on the material. Tensile-based moduli of elasticity could be used in FE analysis as a step towards simulating the optical performance of soft contact lenses on-eye. However, nonlinear compression-based material models are recommended for FE analysis of soft contact lenses when lens-handling is investigated off-eye.

Highlights

Material stiffness was not of particular interest when contact lenses first became widely commercially available [1, 2]
Tensilebased moduli of elasticity could be used in Finite Element (FE) analysis as a step towards simulating the optical performance of soft contact lenses on-eye
Nonlinear compression-based material models are recommended for FE analysis of soft contact lenses when lens-handling is investigated off-eye

Summary

Introduction

Material stiffness was not of particular interest when contact lenses first became widely commercially available [1, 2]. As soft contact lenses became more commercially successful, the main driver in terms of physical characteristics was oxygen permeability (DK) [3, 4] in a bid to maintain a healthy corneal metabolism. It was to this end that silicone hydrogel materials were developed, which provided higher DK values [5]. The downside of this development was that silicone hydrogel materials had much higher moduli of elasticity than hydrogels and this brought up issues of comfort and decreased wettability, as silicon is hydrophobic [6, 7]

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