Wurster process coating

Air-Suspension Coa.ting. The Wurster process utilizes a cylindrical chamber in which the cores are suspended in a controlled stream of air. Film coatings are appHed by introducing the coating solution into the airstream, where the solvent evaporates quickly. The process is much quicker than film coating however, care must be taken to avoid destmction of the cores by attrition in the air stream. 

That said, the task should not be overcomplicated, and many good instances exist to illustrate successful conclusions to such efforts. For example, the data shown in Table 12 illustrate the scaling-up of the Wurster process in which an aqueous latex coating has been applied to drug-loaded pellets in order to prepare a modified-release product. It is appropriate to point out that since the 32" Wurster essentially comprises three 18" Wurster units, the airflow used in the former represents approximately a threefold increase over that of the latter, with the result that the spray rate is scaled... 

Table 12 Coating Process Conditions Used in the Scaling-Up of the Wurster Process for Application of an Aqueous Latex Coating to Drug Loaded Pellets...

Figure 16 Comparison of drug release characteristics of pellets coated with an aqueous ethylcellulose dispersion using a laboratory-, pilot-, and production-scale Wurster process.

Effective product and process optimization play a prominent role in any successful scale-up study. As an illustration, this case study summarizes the initial development, and subsequent scale-up, of a Wurster process designed to facilitate the application of an aqueous ethylcellulose dispersion to drug-loaded pellets. At the same time, it was intended to deal, up front, with some of the idiosyncrasies of such a coating system that often influence the functionality of the final dosage form. 

Table 16 Details of Coating Procedures Used in Scaling-Up the Wurster Process...

Figure 17 Drug release characteristics of pellets coated on various scales of the Wurster process when the laboratory-scale process, used as the basis for scale-up, has been fully optimized.

The bottom-spray process, commonly called the Wurster process and the only one specifically designed as a coating process... 

FIGURE 5.87 Schematic of fluidized bed coating chamber (Wurster process). (After Thies, C. 1989. Biomaterials and Medical Applications, Encyclopedia Reprint Series, J. I. Kroschwitz, ed., pp. 346-367. John Wiley, New York.)... 

A spouted bed process is similar to the Wurster process. The principal difference is that an air velocity differential needed to properly circulate particles through the nozzle spray zone is created by a tapered coat chamber design rather than an air distribution pattern created by the fluidization plate. An illustration of a spouted bed is provided in Figure 7.5. Process ciir entering through the fluidization plate passes upward past the nozzle, Ccurying particles with it as they feed in from the periphery of the plate and lower chamber. Spray is directed upward with the direction of pmticle flow. Vertical increase in cross-sectional area reduces the air and particle velocity to allow pmticles... 

Bottom spray This process is known as the Wurster process/bottom spray when the spray nozzle is located at the bottom of the fluidized bed of particles. As the particles move upward through the perforated bottom plate and pass the nozzle area, they are encapsulated by the coating material. Although it is a time-consuming process, the multilayer coating procedure helps in reducing particle defects. 

---