An investigation of the discrete and continuum models of water behavior in hydrogel contact lenses

Maldonado-Codina, Carole & Efron, Nathan (2005) An investigation of the discrete and continuum models of water behavior in hydrogel contact lenses. Eye and Contact Lens, 31(6), pp. 270-278.

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PURPOSE: To investigate the water behavior in hydrogels of differing equilibrium water content (EWC) and to use these results to investigate the opposing discrete model (thermodynamically different classes of water exist in hydrogels) and continuum model (water behaves as a consequence of nonequilibrium conditions) of nonfreezing water in swollen hydrogels. METHODS: Differential scanning calorimetry (DSC) was used to obtain melting thermograms for five lenses of each of 12 hydrogel lens types. The gravimetric content of the nonfreezing water was subsequently determined from an integrated endotherm for water. The effect of anneal time on the amount of nonfreezing water obtained in samples was investigated before undertaking these measurements. The glass transition temperature (Tg) of each of the lens types was obtained with DSC to investigate how the Tg was related to the amount of nonfreezing water found in the hydrogels. RESULTS: Melting enthalpy increased with increasing anneal time and leveled off at 6 hours. Low-EWC lenses showed endotherms with a single melting peak at 0 degrees C. Materials with a higher EWC showed more complicated melting endotherms, with a broad shoulder occurring at temperatures below 0 degrees C. There was a statistically significant positive correlation between EWC and freezing water (R=0.95, P<0.0001), but the amount of nonfreezing water was similar for all lens materials. There was a statistically significant positive correlation between the Tg of the hydrogels and the theoretically derived weight fraction of nonfreezing water in the hydrogels required to bring the Tg of the gels down to 0 degrees C (R=0.99, P<0.0001) but not with the experimentally derived values (R=0.29, P=0.07). CONCLUSIONS: Low-EWC hydrogels have the lowest free-to-bound water ratios. That the experimentally derived values of nonfreezing water are different from the theoretically derived values creates doubt with the continuum model theory. The best model probably employs a combination of the discrete and continuum theories.

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ID Code: 11067
Item Type: Journal Article
Refereed: Yes
Additional Information: For more information, please refer to the journal's website (see hypertext link) or contact the author.
Keywords: Bound water—Differential scanning calorimetry—, Glass transition—Hydrogel—Nonfreezing water—Soft contact lens—Water
DOI: 10.1097/01.ICL.0000163432.74952.1A
ISSN: 1542-2321
Subjects: Australian and New Zealand Standard Research Classification > MEDICAL AND HEALTH SCIENCES (110000) > OPTOMETRY AND OPHTHALMOLOGY (111300)
Divisions: Current > Research Centres > Centre for Health Research
Current > QUT Faculties and Divisions > Faculty of Health
Current > Institutes > Institute of Health and Biomedical Innovation
Copyright Owner: Copyright 2005 Lippincott Williams & Wilkins
Deposited On: 28 Nov 2007 00:00
Last Modified: 29 Feb 2012 13:29

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