Thin film tribology of pharmaceutical elastomeric seals

The primary purpose of valve seals in inhalation and other drug dispensing devices is to inhibit leakage of highly volatile formulation from pressurised canisters. This requirement often conflicts with smooth operation of valves because of poor lubrication of seals. The repercussions of this can be variability in dispensed dose as well as loss of prime and gradual wear of seals. Although a good volume of literature is available for general purpose o-ring seals, the characteristic behaviour of those used in pharmaceutical devices deviate from this significantly. The paper studies tribology of such seals, subjected to global fitment and canister pressure deformation and localised conjunctional elastohydrodynamic pressures. It is shown that ideally smooth seals would operate under iso-viscous elastic (soft EHL) regime of lubrication. However, the predicted ultra-thin films are insufficient to ensure fluid film lubrication because of rough micro-scale nature of elastomeric seal surface and poor lubricity of the usual bio-compatible formulations. The paper also shows that siliconisation of elastomeric contacting surface only marginally improves its tribological performance.