What is the Wurster fluid-bed process?

The Wurster fluid-bed process is recognized by the pharmaceutical industry as the technology for precision application of a film coating onto particulate materials such as powders, crystals, or granules. The technology can be used to encapsulate solid materials with diameters ranging from near 20µm to several centimeters.

Film coating processes require evaporative removal of an organic solvent or aqueous vehicle as the film coat is deposited. The speed of a film coat application is related to the drying capacity of the process. Fluid-bed film coating processes have a greater drying capacity than other coating systems due to a relatively high fluidizing air volume that is used to both circulate particles and evaporate the coating vehicle. This increased drying capacity translates to more efficient film coat application.

Coating possibilities are almost unlimited including the ability to place a hydrophilic coating on a hydrophobic core, or a water-based coating on a water-soluble core. Film coat properties or product performance can be optimized with changes made to the coating formulation, processing conditions, or the use of multiple coating layers.

How does the Wurster fluid-bed process work?

The Wurster process is well suited to uniformly coat or microencapsulate individual particles. The technology is characterized by the location of a spray nozzle at the bottom of a fluidized bed of solid particles. The particles are moved with a fluidizing air stream that is designed to induce a cyclic particle flow upward past the spray nozzle. The nozzle sprays atomized droplets of coating solution or suspension concurrently with particle flow. Passing particles move upward into an expansion chamber as droplets deposit on their surfaces. The expansion chamber reduces air velocity to allow particles to circulate back to the coating chamber. It also allows particles to further separate from one another temporarily and minimize the potential for particle agglomeration and accretion. The organic solvent or aqueous coating vehicle is evaporated as the particles move into and through the expansion chamber to leave non-volatile coat formulation ingredients on the particle surface as part of the developing film coat. Process parameters are set for optimal vehicle removal and film coat characteristics. This batch process is continued until each particle is coated uniformly to the desired coat percentage or film thickness.