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Pooria Sharif-Kashani, Jean-Pierre Hubschman, H. Pirouz Kavehpour; Viscoelasticity and Macromolecule Organization of the Vitreous Gel. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5344.
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© ARVO (1962-2015); The Authors (2016-present)
To study the viscoelasticity and macromolecular organization of the vitreous gel using shear rheometery and to understand its relation to age-related vitreous degeneration and the process of liquefaction.
30 freshly harvested porcine eyes were dissected and the vitreous was cut directly onto the rheometer plate. A stressed-controlled shear rheometer with 20 mm parallel disc geometry was used to obtain the rheological properties. The parallel discs were covered with 600-grit silicon carbide sandpaper to minimize the slippage of the sample. In order to minimize the effect of liquid evaporation and loss, a solvent trap sealed with vacuum oil was used to enclose the sample. All the experiments were done at the elevated temperature of 37 oC to mimic in-vivo conditions. Failure analyses were performed to determine the minimum shear stress and strain required to destroy the tissue. Additionally, frequency tests were conducted to obtain storage and loss moduli as a function of frequency. Creep compliance experiments were performed for constant shear stresses of 0.5, 1, and 2 Pa.
We modeled the creep deformation of the vitreous gel using the two-element retardation spectrum model. By associating each element of the model to an individual biopolymeric system in the vitreous gel, a distinct response to the applied stress was observed from each component. We hypothesized that the first viscoelastic response with the short time scale (~1 second) is associated with the collagen structure, while the second viscoelastic response with longer time scale (~100 seconds) is related to the microfibrils and hyaluronan network. We also observed the same time scales in the failure analyses and oscillation experiments.
Our results show that both collagen structure and hyaluronan network contribute to the overall viscoelasticity of vitreous gel and therefore, both biopolymer systems must be accounted for in the process of liquefaction. The time scales obtained from the creep deformation are both quantitative and reproducible. In addition to resolving the effects of each vitreous component from the overall properties, these time scales are useful in quantifying the effects of vitreous related pathology, as well as the short-term and long-term effects of pharmacological agents on the properties of the vitreous humor.
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