The Glatt Group has commercialized several improvements to the original Wurster bottom spray fluid bed technology invented by Dr. Dale Wurster in the 1950s. Early versions of the Wurster were used for tablet coating, particularly when coatings were applied using volatile organic solvents. However, the need for tablets to have very high hardness and low friability, and increased use of water-based coatings, led to greater interest in perforated pan coating equipment. Wurster was favored for producing or coating particles and pellets. Since Wurster offers a good distribution uniformity, interest in the technology for tablet coating re-emerged.
However, a Wurster designed for pellet coating could be problematic for controlled release formulations involving tablets, due to the potential for slow or no-flow zones, and tablet damage caused by impact with various insert components. These problems could be especially severe in cases where the coating determined the release of active ingredient and coating uniformity was critical, explains David Jones, vice president of Glatts Pharmaceutical Services Division, a division of Glatt Air Techniques, Inc. (Ramsey, N.J.).
To optimize the Wurster process for controlled release tablet coating, it was first necessary to study the deficiencies in radial and axial mixing, and to identify problem zones within the equipment. Jones used fiber optics and high speed photography to visualize process dynamics. One problem zone was traced to the base of the product container, at the perpendicular interface of the container wall and horizontal orifice plate (see Diagram above). A second involved the impact of horizontally flowing tablets with the vertically disposed spray nozzle. Based on findings, Jones and his colleagues designed an insert to improve coating uniformity, and minimize machine/tablet interactions.
Glatts tablet coating technologies include a patented Wurster HS (for high speed) system, which is said to provide for faster spray rates and reduced agglomeration. Glatts MacroWurster has been demonstrated to provide for a more uniform coating, and to reduce tablet attrition.
