Water properties of soft contact lens materials

Ioannis Tranoudis, Nathan Efron

    Research output: Contribution to journalArticlepeer-review


    The properties of water in soft contact lenses such as the water content, free-to-bound water ratio, and the extent to which soft lenses dehydrate during wear, are key determinants of their in eye performance and oxygen transmissibility characteristics. This study describes clinical and laboratory experiments that were conducted in order to examine the state of water in eight soft contact lenses manufactured from different materials. Specifically, lenses made from the following eight materials (and nominal water contents) were used: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% [HEMA = 2-hydroxy-ethyl methacrylate, VP = vinyl pyrrolidone, MMA = methyl methacrylate, MAA = methacrylic acid]. Differential scanning calorimetry (DSC) was used for measuring the free water content in the materials listed above. Some noticeable differences in water properties among soft contact lens materials having approximately the same water contents were revealed. Low water content materials exhibited a simple endotherm and all water melted around 0 °C. On the other hand, medium and high water content materials exhibited multiple melting endotherms, representing a broad range of interactions between water and the polymer. Low water content soft contact lenses have approximately the same amount of bound water as those with much higher water contents. Six subjects were then fitted with the same lenses for one day. In vitro measurements of water content and oxygen transmissibility were taken at 35 °C, both before lens fitting and after 6 h of lens wear. Water content and oxygen transmissibility were correlated with the water properties of the soft contact lens materials. The relative change in lens water content (%ΔWC) and relative change in lens oxygen transmissibility (%ΔDk/t) were calculated and correlated with the water properties of the eight soft contact lens materials. It was concluded that (a) oxygen transmissibility, free water content and free-to-bound water ratio are increased when the water content of a contact lens is increased and (b) water content, free water content and free-to-bound water ratio cannot be used for the prediction of soft contact lens dehydration in vivo. © 2004 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
    Original languageEnglish
    Pages (from-to)193-208
    Number of pages15
    JournalContact Lens and Anterior Eye
    Issue number4
    Publication statusPublished - Dec 2004


    • Bound water content
    • Differential scanning calorimetry
    • Free water content
    • Free-to-bound water ratio
    • Hydrogel contact lens materials
    • Lens dehydration
    • Water properties


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