Rheological Behavior of a Novel Viscoelastic Formulation for Ophthalmic Surgery

Purpose: To evaluate the rheological behavior of a novel viscoelastic formulation (VISC26), based on sodium hyaluronate (SH) and hydroxypropylmethylcellulose (HPMC), in comparison to Viscoat®, based on sodium hyaluronate (SH) and chondroitin sulfate (CDS). Methods: Rheological properties were evaluated on a strain–controlled rheometer (Bohlin VOR) at a temperature of 34 °C. The measuring systems were cone and plate (CP 5/30 cell). Small amplitude oscillatory shear experiments allowed the measurement of the dynamic response of the samples and hence the determination of linear viscoelastic properties. The non–linear flow properties of the investigated formulations were evaluated through steady shear measurements to determine the viscosity as function of shear rate. Results: Pseudoplastic behavior (shear thinning) was observed for both the novel formulation and Viscoat®. VISC26 exhibits a more marked viscoelastic behavior than Viscoat®; indeed its pseudoplasticity index is higher than Viscoat® (3551 for VISC26 and 77 for Viscoat). At 0.009 s–1 shear rate the viscosity of VISC26 is higher than Viscoat® (3310 Pa·s for VISC26 and 64 Pa·s for Viscoat®) and drops more rapidly as shear rate increases. The mechanical spectra show that the solutions behave as transient–network polymer solution or entangled solution having a predominantly viscous character (G’’>G’) at low frequencies and an essentially elastic behavior (G’>G’’) at higher frequencies. This transition occurred at a given value of frequency (cross–over frequency). The novel formulation (VISC26) has both moduli higher than that of Viscoat® and the crossing of the moduli curve occurring at lower frequency than that of Viscoat® (0.04 Hz for VISC26 and 3 Hz for Viscoat®). Conclusions: The new formulation VISC26 (SH–HPMC) shows improved viscoelastic properties when compared to Viscoat® (SH–CDS), allowing it to better fulfill the requirement for ophthalmic surgery