Characterization of freeze-dried pharmaceutical product structures by an FFT-imaging approach

This paper describes an imaging-based approach for the characterization of the pharmaceutical product structures obtained by freeze-drying. The analysis of the dried cake structure and particularly the estimation of its porosity and permeability, is fundamental for the selection of the optimal process conditions and also for the product quality control. Furthermore, the product structure has a dramatic impact on the drying behavior as it determines the resistance offered by the dried product to the vapor flow generated by the ice sublimation. This parameter, that allows one to relate sublimation rate to partial pressure gradients, is gaining growing importance in recent years, as it is necessary for the application of the most modern model-based methods for the design of the freeze-drying cycle. This parameter is difficult to be measured so new measuring approaches are required to make its estimation easier. In this study a bi-dimensional Fourier analysis is used for image processing of the internal structure of lyophilised samples and, then, for the estimation of their mass transfer coefficient.