This thesis presents comprehensive investigations into the mechanical behaviour of freeze dried (FD) biopharmaceuticals with respect to processing conditions (freezing rate, mode of ice nucleation and primary drying temperature). The contribution of formulation composition, cake density and individual excipients on the mechanical properties were studied. The interactive effect of processing conditions and model formulation composition was also studied using statistically designed experiments.
A mechanical indentation test using a flat faced punch was developed and validated to test fragile freeze-dried cakes in-situ (within vial) without any sample preparation. FD cakes were made from sucrose, trehalose, mannitol and model formulations with lysozyme and bovine serum albumin (BSA) under typical industrial FD process conditions.
Freeze-dried cakes exhibited failure by a brittle crushing mechanism. An initial near linear rise in the stress-strain curve represented cake elastic behaviour and transitioned into a steady stress plateau as strain was increased, with intermittent cracking and crushing failures representative of cells failures in the z axial direction.
FD trehalose, sucrose and mannitol have distinctly different Young’s modulus and maximum stresses at failure. Higher primary drying temperature (-40°C) causes cell micro-collapse hence reduces the Young’s modulus and crushing stress compared to -50°C. Cakes manufactured at faster cooling rates (0.9°C/min) produce smaller pore sizes but more in number resulting in higher Young’s modulus and crushing stress compared to slower cooling rates of 0.09°C/min with less numerically but larger pore sizes. Controlled nucleation creates a narrow pore size distribution resulting in a uniform Young’s modulus and similar crushing stress compared to cakes with wider pore size distributions (spontaneous nucleation). FD lysozyme and BSA products cakes are strongest when mannitol and sucrose are used in combination at ~1:1 ratio compared to other formulation compositions.
In summary, the findings offer a systematic understanding of the relationship of mechanical behaviour of FD cakes, processing conditions and formulation composition. The mechanical test method developed can potentially provide a quantitative critical quality attribute related to internal FD cake structure hence its robustness for handling and transportation. It will provide information on suitability of biopharmaceutical formulations including excipients for manufacture of robust FD cakes.