Oscillatory shear experiments as criteria for potential vitreous substitutes

Polymers of N-vinyl-2-pyrrolidinone were synthesized as potential substitutes for the vitreous body of the eye. Certain polymers displayed typical gel behaviour and they were further characterized by dynamic shear analysis. The mechanical spectra of the fully hydrated samples indicated a covalently crosslinked network system, i.e. G' > G'' and slopes approaching zero. The gels were also subjected to preshearing by injection through a small needle, a procedure that is essential in the use of vitreous substitutes. The polymers crosslinked with ethylene dimethacrylate dramatically changed their characteristics following the injection process, as the values of shear moduli dropped substantially, and the gels behaved like free flowing fluids at higher frequencies (G'' > G'). The gels crosslinked with diallyl ether or divinyl glycol displayed only minor changes in their viscoelastic behaviour, but their moduli were very low before and after injection. The gels that were produced without crosslinking agents maintained their properties as virtually unchanged after injection, with no changes in the magnitude of G' and G'' and no rise In the loss tangent. These gels, which may be very lightly chemically crosslinked, due to divinyl contaminants, or physically cross[inked, displayed the most suitable mechanical characteristics as vitreous substitutes. Shear oscillatory tests appear as important criteria in the selection of such materials.