How Cellets’ particle size influences the flow behavior in a Wurster fluid-bed coater

Cellets are inert starter cores made of microcrystalline cellulose (MCC). They play an important role in new formulations of solid dosage forms. As a carrier system for actives, the chemical inertness and surface smoothness are crucial parameters. Additionally, high level of robustness and sphericity simplify formulations and technical processes, such as fluidized bed technologies for coating and layering. In a joint study between the University of Hertfordshire and Freeman Technology (a Micromeritics company), the effect of pellets’ size on the behavior in a Wurster process is explained. Wurster fluid bed coating of Cellets with particle size larger than 400 µm is unproblematic. However, decreasing the particle size begins to complicate the coating process. So, powder rheology was used to compare Cellets with different particle sizes in terms of their effect on the powder flow in the Wurster fluid bed coater. For deeper knowledge, we strongly recommend reading investigations by V. Mohylyuk et al.

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Impact of the Cellets’ size
The impact of the Cellets’ size on bulk powder behavior can only be estimated by screening additional parameters. In addition to the mass flow rate, standard pharmacopoeia methods such as bulk/tapped density were initially employed for the characterization of the powder’s properties. This was extended to rotating drum measurements providing the dynamic angle of repose and dynamic cohesivity index. Via powder rheology the conditioned bulk density, basic flowability energy, specific energy, pressure drop, permeability and compressibility (Figure 4) were obtained [1].

By picking the compressibility of Cellets at an applied force of 10 kPa normal stress, two key points need to be mentioned: (a) smaller particle size induces a higher rate of compressibility; (b) Cellets are less compressible than the reference MCC powder.

These findings are part of the open question on powder flow in a Wurster process. It is expected, that Cellets with a lower compressibility will result in better flow behavior in the fluidized bed.

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References
[1] Mohylyuk V, Styliari ID, Novykov D, Pikett R, Dattani R. Assessment of the effect of Cellets’ particle size on the flow in a Wurster fluid-bed coater via powder rheology. J D Deliv Sci Tec. 2019; 54: 101320, doi: 10.1016/j.jddst.2019.101320.

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