Wurster process

Wurster process Wurster salts Wurtz-Fittigcoupling Wurtz-Fittig reaction Wurtzite... 

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. 

Rekhi, G. S., Mendes, R. W., Porter, S. C., and Jambhekar, S. S., Aqueous Polymeric Dispersions for Controlled Drug Delivery - Wurster Process, Pharm. Technol., 13(3) 112-125 (1989)... 

Figure 13 Photographs illustrating equipment used during the scale-up of the Wurster process.

Since batch size constraints may be more significant in the Wurster process, this processing concept will be used primarily as the basis for further discussion on the subject of defining appropriate batch sizes. 

For the Wurster process, calculating batch volume on scale-up can be calculated using Equation (2)... 

When scaling-up the fluid-bed process, a major requirement is to produce fluidization behavior on the larger machines equivalent to that used on the scale that provided the basis for process development. To achieve this goal, and minimize attritional effects, the same air velocities for each scale of equipment are required. Thus, the overall increase in air volume required during scale-up will be related to the increase in area of the perforated base plate, and, in the case of the Wurster process, the open area of the partition plate immediately beneath each of the inner partitions. Such calculations are simplified when scaling-up from an 18" pilot scale machine to, say, a 32" machine, since the latter represents a three-multiple of the former, and thus would require a threefold increase in airflow. 

Figure 14 Influence of spray nozzle type and atomizing air pressure on the mean droplet size of water sprayed from guns used in a Wurster process.

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... 

Although these examples are more specific to the Wurster process, similar guidelines can be applied to the top-spray process, in which case the definition for working capacity will be different. When dealing with the tangential-spray process, the quantity of product that is sufficient to ensure that the spray nozzles are completely immersed when the product is in motion will define the minimum starting batch weight. 

Ichikawa, H. Fukumori, Y. Microagglomeration of pulverized pharmaceutical powders using the Wurster process. I. Preparation of highly drug-incorporated, subsieve-sized core particles for subsequent microencapsulation by filmcoating. Int. J. Pharm. 1999,180, 195-210. 

Fukumori, Y. Ichikawa, H. Jono, K. Takeuchi, Y. Fukuda, T. Computer simulation of agglomeration in the Wurster process. Chem. Pharm. Bull. 1992, 40 (8), 2159-2163. 

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