S22 Abstracts of the 2011 BCLA Annual Clinical Conference / Contact Lens & Anterior Eye 34, Supplement 1 (2011) S1–S43

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Investigating the mechanical properties of soft contact lenses usingnanoindentation

Alastair P Selby1,*, Carole Maldonado-Codina2, Brian Derby1

1School of Materials, University of Manchester, Manchester, UK; 2EurolensResearch, Faculty of Life Sciences, University of Manchester, Manchester, UK

*E-mail address: alastair.selby@postgrad.manchester.ac.uk

Purpose: Local mechanical measurements of hydrogel contact lenses can beachieved using nanoindentation.

Method: PolyHEMA/MAA hydrogel samples were characterised using aHysitron Triboscope nanoindenter using 50μm and 100μm conosphericaltips. Indents were performed on samples containing 0.2%, 0.4%, 1.0%, 2.0%and 5.0% ethylene glycol dimethacrylate (cross-linking agent). Further, sam-ples (from 50μm to 1000μm in thickness) 0.2% and 1.0% cross-linked werealso investigated. Values represent the mean ± SD.

Results: Hydrogel modulus increased with cross-link density. Modulusvalues for the 0.2% and 5.0% crosslink densities were 0.43MPa ± 0.01 and4.32MPa ± 0.05 respectively (50μm tip),and 0.44MPa ± 0.03 and 4.36MPa± 0.11, respectively (100μm tip). For the samples of varying thickness, the0.2% cross-linked gels were found to have a modulus of 0.42MPa ± 0.79 and0.43MPa ± 0.06 for the 50 μm and 100 μm tips. The 1.0% cross-linked gelswere found to have amodulus of 0.66MPa± 0.07 and 0.68MPa± 0.07 for the50μm and 100μm tips. Samples less than 400μm in thickness measured us-ing the 50μm tip and those of less than 500μm in thickness using the 100μmtip required a correction factor to account for the effect of the substrate onthe modulus value. ANOVA test confirms no significant differences with tipgeometry (p=0.37 and p=0.18 for the 0.2% and 1.0% cross-link densities).

Conclusions: Nanoindentation is a successful technique for measuring lo-cal mechanical properties of hydrogels. Tip geometry did not affect the mod-ulus values obtained; however, a correction factor is required in thinner sam-ples to account for substrate effects.

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Flexure characteristics of soft contact lenses

Marco A Miranda*, Hema Radhakrishnan, Clare O’Donnell

The University of Manchester, Manchester, UK

*E-mail address:mmir.99@gmail.com

Purpose: To assess the flexure characteristics of silicone and conventional hy-drogel lenses which are likely to differ in how they conform to the shape ofthe cornea.

Method: Measurements of lower-and higher-order aberrations (HOA)were performed on 60 lens-wearing subjects using an IRX-3 aberrometer(Imagine Eyes, France). Aberrations were then measured on lenses in vitrousing a ClearWave instrument (Wavefront Optics, USA). Subjects wore com-mercially available and custom-made lenses in vifilcon A (Focus® Montly),lotrafilcon A (Air Optix® Night & Day®), sifilcon A (Air Optix® Individual™)and balafilcon A (PureVision®) materials. Lotrafilcon A and sifilcon A lenseswere manufactured in two thicknesses. All lenses were plano (±0.25D) withthe exception of commercial Lotrafilcon A where ±3.00D lenses were alsoused.

Results: Small (≤0.30D) but significant differences in spherical equivalent(SE) were found after vifilcon A, sifilcon A, balafilcon A and custom-made lo-trafilcon A and sifilcon A lens wear (p<0.05). In general, differences wereattributed to the in vitro power although for −3.00D lotrafilcon A lenses, asignificant difference between the in vivo and in vitro power was found. Typ-ically, HOA did not change after lens application (p>0.05). However, a signif-icant effect of lens type on SE, RMS of HOA and secondary astigmatism wastypically observed (p<0.01).

Conclusions: Changes in SE and HOA were generally small, perhaps be-cause most lenses were plano (±0.25D) and aspheric. The observed changesare likely to be related to accommodation and/or lens decentration. The re-sults show that for silicone hydrogels, lens modulus and thickness may playa role in determining lens flexure effects.

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Mechanical properties of contact lenses: what do we need to measure?

Tarnveer S Bhamra*, Brian J Tighe

Biomaterials Research Unit CEAC Aston University, Birmingham, UK

*E-mail address: bhamrats@aston.ac.uk

Purpose:Mechanical properties of hydrogel lenses are customarily measuredby simple tensile testing; the resultant modulus providing a measure of stiff-ness. Silicone hydrogels show more complicated behaviour because inclu-sion of silicone rubber “fragments” enhances the elastic (related to springi-ness) contribution to viscoelastic behaviour, in contrast to the viscous (re-lated to conformability) contribution. In consequence behaviour under shear(typified by the eyelid moving across the lens surface) becomes an importantaspect of silicone hydrogel characterisation.

Method: Conventional hydrogels and current silicone hydrogels havebeen characterised under tension, compression (eyelid load) and shear.Shear-dependent behaviour was measured using the Bohlin CVO rheometerat 34°C and a range of shear rates (0.5–20Hz) reflecting deformation duringhandling and wear.

Results: Conventional hydrogel behaviour is typified by polymacon (poly-HEMA) which has a modulus of 0.5MPa under tension and compression. Thecomplex modulus under shear shows little change as deformation rate is in-creased. In particular the elastic component is characteristically stable risingonly from 20kPa at 0.5Hz to 23kPa at 10 Hz. In contrast all silicone hydrogelsshow a typical “fingerprint” marked by a rise in elastic modulus with shearrate. Thus balafilcon A which has a similar water content to polymacon hasan elastic component that rises from 30 kPa at 0.5 Hz to 50 kPa at 10 Hz. Thiselastomeric behaviour is characteristic of the constituent siloxy componentsand reduced somewhat at higher EWCs.

Conclusions: Elastomeric behaviour of silicone hydrogels is important tocharacterise and understand. It contributes to SiHy-related phenomena suchas SEALS and mucin balls.

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Does soft contact lens daily wear impact corneal curvature?

Fabian Conrad1,*, Percy Lazon de la Jara1,2, Padmaja Sankaridurg1,2,3, BrienHolden1,2,3

1Brien Holden Vision Institute, Sydney, Australia; 2School of Optometry andVision Science, University of New South Wales, Sydney, Australia; 3VisionCo-operative Research Centre, Sydney, Australia

*E-mail address: f.conrad@brienholdenvision.org

Purpose: Corneal curvature changes following extended wear of both conven-tional hydrogels and silicone hydrogels contact lenses have been attributedto hypoxia (hydrogel) or mechanical pressure (silicone hydrogel). This studyaimed to investigate the effect of daily wear of both high and low oxygentransmissible contact lenses (lotrafilcon B (AIR OPTIX™ Aqua (AO), AO Mul-tifocal), lotrafilcon A (Night & Day™), enfilcon A (Avaira™) and etafilcon A(Acuvue 2®) contact lenses on corneal curvature.

Method: Twenty-eight subjectswere enrolled in a prospective, open label,contralateral, cross-over, randomised study. Corneal topography was cap-tured using a Medmont E300 topographer. Subjects had to wear the lensesfor at least six hours on each day and return to the clinic after approximatelyone week. Simulated keratometry readings (K) were extracted from the to-pographical data. Custom developed software was used to extract the hori-zontal and vertical meridian of each map.

Results: Statistically significant flattening in K (mean ±SD) was producedby Acuvue 2 (flat K −0.09±0.14D p=0.004, steep K −0.17±0.19D p<0.001), AOMultifocal (steep K −0.10±0.12D p<0.001) and Avaira (steep K −0.07±0.15Dp=0.025). Analysis of the horizontal and vertical meridian over the central 5mm of the cornea revealed localised flattening across all lenses.

Conclusions: After one week of daily wear both conventional hydrogelsand silicone hydrogels were seen to flatten the cornea in some areas withinthe central 5 mm. Comparison of corneal K readings alone is insufficient todescribe localised changes in corneal shape.