Inner diameter of hypodermic needles: A critical factor in prefilled syringe design

In the frame of pharmaceutical device development, the evaluation of component properties and their effects on overall device performance characteristics poses a substantial challenge. The hydrodynamic inner diameter (IDh) of a staked-in needle prefilled syringe (PFS) represents such a parameter affecting device performance, especially in the case of autoinjectors, where injection times can vary by up to 30% depending on the IDh. Herein, measurement systems to detect variation of IDh in samling based, AQL-driven incoming goods control were evaluated. To ensure the capability of such fluid-based IDh measurement systems, measurement system analysis studies combined with µ-computer tomography of reference samples was applied. A round robin study assessing different IDh measurement methods was performed, identifying the semi-automated measurement system as suited for reliable and accurate measurement. A second study examined possible variations between PFS batches and different manufacturers regarding the IDh. The study revealed statistically significant variation of IDh ranging from 259 to 300 µm for 27 G special thin wall needles. For auto injectors, lack of proper control of IDh might lead to more variable injection time and even device stalling. This may ultimately result in an unsatisfactory patient experience and incomplete dosing. Hence, control over needle parameters can have a beneficial impact on patient experience and ultimately on treatment success.