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R.I. Bergman, S. Mannberg, M. Lundqvist, R.W. Shimizu; Critical Physico–Chemical Parameters to Define Cohesive Properties of an Ophthalmic Viscosurgical Device (OVD) . Invest. Ophthalmol. Vis. Sci. 2006;47(13):648.
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© ARVO (1962-2015); The Authors (2016-present)
The aim of this study is to show a correlation between cohesive and rheologically measured properties for OVDs and also to show the existence of a transition interval in molecular masses where there is a gradual change from cohesive to dispersive behavior.
A number of formulations of sodium hyaluronate (HA) in phosphate buffer differing in concentration and molecular mass were made and characterized rheologically. All solutions were formulated to belong to the "concentrated regime, i.e. above the "critical" concentration c* (Simha et al) for each molecular mass. Experimental data for sample formulations were compared to published data for some commercial formulations (Poyer et al). The oscillatory tests were performed in a Physica MCR 300 rheometer with a cone–plate arrangement (CP 25–1). The measurements were performed in the LVE (linear viscoelastic region) under controlled shear deformation (CSD). The elastic, G’, and the viscous moduli, G’’, were analyzed as a function of oscillation frequency. The mass average relative molecular mass (Mrm) of the HA formulations was measured with LALLS (low angular laser light scattering, Chromatix, KMX–6) for all but the above mentioned commercial products.
In the concentrated regime, the zero shear viscosity has a scaling exponent of 3.6, (Bothner–Wik et al) which is close to the theoretical value of 3.4 (Doi et al). All formulations in the present study are concentrated and follow the same cooperative scaling law for the zero shear viscosity but not all solutions can be classified as being "cohesive." The cross over frequency (CO), where G’ = G’’ meaning that the elastic and viscous properties are equal, is plotted versus the Mrm of the (HA) for each formulation. CO–frequencies close to and above 1 Hz are correlated to dispersive solutions (Vitrax, Viscoat). The corresponding molecular masses associated with dispersive properties are those having Mrm smaller than about 1.3 million.
Solutions with CO frequencies close to and below 0.1 Hz behave cohesively (Mrm > 1.9 million) commercial examples are Healon products and Provisc. OVD formulations between these limits are found to have a more dispersive than cohesive character when used in test surgery in cadaver eyes.
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