A comparison of water binding and mobility in contact lens hydrogels from NMR measurements of the water self-diffusion coefficient P. McConville, J.M. Pope * Centre for Medical and Health Physics, School of Physical Sciences, Queensland University of Technology, Brisbane 4001, Australia Received 24 January 2000; received in revised form 10 March 2000; accepted 3 April 2000 Abstract Measurements of the water self-diffusion coefficient were made for a set of nine commercially available contact lens hydrogels, both at equilibrium water content (EWC) and as a function of reduced water content, using the pulsed field gradient NMR method. The data were shown to lie approximately on a universal curve, suggesting that water content (WC) itself was the predominant factor in determining the water diffusion coefficient. However, fitting of the data to a specific binding model suggested that subtle differences in the diffusional behaviour existed between the materials. These differences were measured in terms of the proportion of specifically bound water (p wb ). It was shown that the calculated proportions could be correlated with the EWC, and that they agreed reasonably well with the proportion of non- freezing water measured for similar materials using DSC.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Contact lens; Hydrogels; NMR 1. Introduction Hydrogels are most commonly used today for the fabri- cation of soft, or disposable contact lenses, but are also used in other biomedical applications [1] such as for artificial implants, and as matrix materials for controlled drug release. In recent times there has been much interest in the interaction between the polymer and the water it binds, as this interaction is believed to determine some of the most important properties of the hydrogel. A major disadvantage of hydrogel contact lenses is their tendency to dehydrate on the eye [2–6]. Although there have been many conflicting measurements of the extent of this dehydration, the majority of studies suggest that higher equilibrium water content (EWC) materials generally dehy- drate to a greater extent on the eye [4,7,8]. This dehydration is thought to be related to the state of binding and average mobility of the water molecules in the hydrogel polymer. There is therefore interest in obtaining a better understand- ing of the hydration properties of contact lens hydrogels [9,10]. Proton nuclear magnetic resonance (NMR) has the poten- tial to characterise this water binding and mobility in a directly quantifiable manner, and has been commonly used in studies of the water–polymer interaction [11–13]. In particular, the water self-diffusion coefficient (D), which can be measured using pulsed field gradient (PFG) NMR, is directly related to the potential for water to leave a hydrogel. Several NMR studies of diffusion in contact lens hydrogels have been performed in recent times [14,15]. The purpose of the current study is to use PFG NMR to measure the diffusional behaviour of water in a range of commercially available contact lens hydrogels of varying composition and EWC. It was anticipated that this would lead to a better understanding of those properties of the hydrogel that determine the rate of water diffusion, and how these properties might influence the potential of a parti- cular hydrogel for on-eye dehydration. 2. Experimental 2.1. Sample protocol A set of commercially available contact lens hydrogels was supplied by Benz Research and Development Corpora- tion, Sarasota, Fl, USA, Capricornia Contact Lens, Bris- bane, Australia, and the Cornea and Contact Lens Research Unit, Sydney, Australia. The materials used are shown in Table 1 with their EWCs and compositions. The Benz and IGEL materials were supplied in the form of contact lens blanks or ‘buttons’, whereas actual contact lenses were used for the remaining samples. Polymer 41 (2000) 9081–9088 0032-3861/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(00)00295-0 * Corresponding author. Fax: +61-7-3864-1521. E-mail address: j.pope@qut.edu.au (J.M. Pope). www.elsevier.nl/locate/polymer