(586ak) An Alternate Coating Method for Tablets: Wurster Technology

An Alternate Coating Method for Tablets: Wurster Technology

Tablets are routinely coated for appearance, controlled release and in some cases with active ingredients.  While pan coating is an ancient process which has been updated through the use of perforated pan technology to meet modern pharmaceutical processing; air suspension coating, such as that accomplished in a Wurster device has rarely been applied.  While the mechanical forces which a tablet could experience are of concern and are most often cited as the cause for discounting the Wurster process for tablets, the proper selection and setup of equipment and design of tablet properties can assure a successful coating process.

Although the basic coating technology - applying an atomized liquid to moving tablets - is the same for each of the techniques, the airflow, tablet flow, and contact with the atomized liquid is different for the Wurster process and can actually impart some advantages.  For example the droplet flight to the tablet is typically shorter for a Wurster compared to a coating pan, which can reduce the amount of spray dried coating generated during the process.  The contact between liquid and tablet in a Wurster occurs when the tablets are spatially separated in lean phase pneumatic transport upwards in the central column, rather than the coating pan flow of tumbling against each other in a packed bed, which widens the range of acceptable coating parameters.  Unlike tablets tumbling in a pan the material flow in the Wurster device has a definite pattern which means it can approximate an idealized plug flow reactor rather than a continuous stirred tank.

Experiments have shown that small scale Wurster devices can be used to prepare small samples of product quickly allowing the screening of prototypes in early development while consuming a minimal amount of material.  The selection of equipment and conditions for coating tablets in a Wurster and the required tablet properties to obtain successful coating results will be discussed.  Tablets of up to 1200 mg weight were coated without compromising the quality of the product.  A comparison of tablets coated using pan and Wurster technology at similar scales will also be discussed.