Investigating polymer optical fibre Bragg grating technology for freeze drying applications

Abstract

Pharmaceuticals and biologics that are unstable in aqueous form but liable to thermal degradation are generally freeze dried or lyophilised to extend their shelf life. Primary drying is the most expensive and longest stage of the freeze drying process, prompting extensive research to reduce production costs and increase productivity. This work investigated current commercially available technologies used to detect sublimation of water vapour and determine the end of primary drying time. The investigation identified the potential cost benefits of using polymer optical fibre Bragg gratings (POFBGs), as non-invasive sensors that monitor batch sublimation of water vapour during primary drying. The thesis initially discusses the fabrication of POFBG sensors, the protocols and housing structure developed to protect the sensors in this extreme environment. This was achieved by studying the behaviour of custom-built anemometers, temperature data loggers and sensors that detect changes in gas composition. The next stage involved the characterisation of PMMA and TOPAS POFBGs to changes in temperature and pressure within the chamber. This provided information on the sensitivity of the sensors to these parameters at dry sub-atmospheric pressures. The studies revealed red shifts in the Bragg wavelengths as temperature decreased from an inlet shelf temperature of 20oC to -20oC and temperature drifts from radiative heat effects. The pressure studies revealed non-linear Bragg wavelength sensitivity within the region of 200μbar to 2500μbar, several orders of magnitude greater than reported within literature in high pressure chambers. The POFBGs were exposed to water vapour subliming from samples of water, mannitol and native collagen, which were frozen and attached to the access port or placed onto the shelf and freeze dried. The studies reveal the potential for POFBG technology to be used to detect sublimation and end of primary drying, with future work focusing on modifications to the design and temperature compensation techniques