The lyophilised microsphere samples were analysed as powders using a helium pycnometer (Micromeritics Accupyc II 1340, Norcross, GA, US) to determine the density and specific volume of the microparticles. Approximately 20 mg of each sample were placed in the instrument compartment and measurements were performed with helium gas at 298 K at a pressure of 10 mbar. The measurements were repeated for each sample five times. The average volume of the five repeat measurements was used to determine the density of each material. For comparison, polymer films of PLGA 50:50 and PLGA 75:25 were prepared using the vacuum compression moulding (VCM) tool (MeltPrep, Graz, Austria), and these were also analysed using helium pycnometry.