The Wurster process has been used for years to coat particles, spheres, granules, and tablets. Systems have been developed for use with a variety of coating formulations, including aqueous/organic solvents, hot saturated solutions and hot melts. The basic concept in Wurster coating is to separate the particles in the fluid bed from one another in an air (gas) stream. While the particles are suspended, a coating formulation is sprayed from the bottom of the bed up onto the particles (bottom-up spray).
The top spray coating process is less precise but an affordable alternative to Wurster Coating. Top Spray Coating is used when particles need a general protective layer but the exact application thickness is not critical. It works best when the particle size distribution of the product is narrow. Like top spray granulation, nozzles are located above the product bed pointed downward toward the bed. The particles are suspended in an airstream as they are slowly and gently spray coated before they dry and drop back down into the product bed. The particles are sprayed repeatedly until the desired coating or weight gain is achieved. Note that products with a wide particle size distribution may be top spray coated by pre-agglomerating the material using the same machine set-up at the beginning of the top coating process. Proper top spray agglomeration will narrow the particle size distribution and once this is achieved, the process parameters can be adjusted so that Top Spray Coating can occur.
Ultrasonic coating is a unique, relatively new approach to coating fine, particulate substances in a fluid bed using ultrasonic spray technology. Ultrasonic spray devices use very high frequency vibration instead of high pressure or compressed air to produce extremely small and uniform droplets which are ideal for high-precision coatings (such as Wurster coating) as well as vaporization, spray drying and humidification.
When a liquid binder, coating material or other liquid inside an Ultrasonic spray device vibrates at a specific frequency, capillary waves are produced on the surface of the liquid. During atomization, the capillary waves are transformed into droplets by increasing the amplitude until the peak of the wave forms droplets. The wavelength and subsequently, the droplet size, are determined by vibration frequency. High vibration frequency generates fine droplets and low vibration frequency generates coarse droplets.
The distribution graph below illustrates the average droplet size, in microns, at Dv 0.1, Dv 0.5, Dv 0.9, Dv 0.99 and the Dmax value at various pressures.
Tablet coating is a process that can be summarized as the application of an “edible paint” to the surface of a pharmaceutical dosage form. In the past, sugar coating was a common solution for coating tablets, but this process has many disadvantages. Modern tablet coatings are typically film coatings, flexible formulations that are applicable to a wide range of dosage forms (such as tablets, capsules, pellets, granules and drug crystals). The tablet coating process is complex, and involves parameters such as the spray pattern, drop size, and nozzle spacing (in addition to multiple other non-spray related parameters) which must all be precisely controlled in order to ensure uniform distribution of the coating material.