Shear and extensional rheometry of PDMS tamponade agents used in vitroretinal surgery

The emulsification of low molar mass silicone oil (PDMS)-based tamponade agents used in the treatment of complex retinal detachments is a significant clinical problem leading to the patient suffering impairment of vision whilst the tamponade is in place. This is particularly the case in temporal postoperative applications where the tamponade can remain in the ocular cavity for several months. The majority of clinicians prefer to use a PDMS fluid of kinematic viscosity 1000cS, which offers ease of manual injection. Work is progressing towards the development of tamponades with a reduced tendency to emulsify, through specific tailoring of rheology and interfacial properties. Greater knowledge of the mechanism of intraocular emulsification is being elucidated and has led to the development of 'polymer modified' tamponades with enhanced performance. Such materials are formulated by the addition of a high molecular weight PDMS to the base PDMS fluid. Measurement of the shear viscosity at moderate shear rates is useful in predicting performance during manual injection into the eye. However, the determination of the behaviour in both shear and extension at high strain rates is useful in predicting resistance to intraocular emulsification and in informing future modeling studies. Initial experiments focused on rotational rheometry (shear) and capillary breakup rheometry (CaBER - extension). A range of polymer modified tamponades were characterised - based on PDMS base oils from 100-5000 cS and containing up to 30% by weight PDMS of molecular weight varying in the range 100-800 kDa. Even though the volume occupancy of the polymeric additive ranged from dilute to significantly interpenetrated, a linear response was observed in each case. However, subsequent experiments using capillary rhoometry lead to the observation of non-linear behaviour and significant differences in the type of response. Systems with enhanced high strain rate tensile viscosities gave improved performance in in-vitro emulsification tests and are now the subject of clinical trials. The thermodynamic interaction between the polymeric 'solute' and the PDMS fluid 'solvent' was also considered. Interestingly, the PDMS fluids appeared to behave as relatively 'good' solvents, the chains of the polymeric additives being considerably expanded from their unperturbed dimensions.