Coating technologies have many applications in the pharmaceutical industry, such as taste masking and drug release control. Fluid bed coating is particularly suitable for smaller particles and pellets. In this article, Dr Harald Stahl explains how the unique airflow within the GEA PRECISION-COATER™ can provide significant improvements in coating performance and overcome many of the coat quality and scale-up problems experienced with traditional coating systems.

Particle Coating Expertise

The pharmaceutical industry has had technology that can coat particles to improve appearance, mask unpleasant tastes or provide sustained release drugs for almost 50 years. However, with the dramatic rise in the number of drugs requiring this technology, and the increased cost of processing time and the compounds used, technologists are now under growing pressure to increase yields, improve coat quality and reduce processing times.

Quality Coating

Coating products can be expensive and the volume of product needed is inversely proportional to the particle size of the material to be coated. For example, the surface area of a 1 cm cube is 6 cm. If, however, that cube was made up of smaller 1 mm cubes, the total surface area would increase ten-fold. For particles smaller than 100 microns, the volume of the coating material can easily exceed the volume of the active ingredient. Coating quality is, therefore, of paramount importance.

The aim of the process is to coat the particle with just enough material to achieve the desired result, but no more. In the case of functional coatings, it must also be evenly distributed over the entire surface of the product. An uneven application can seriously affect the performance (release profile) of the drug. One cause of uneven coating can be the failure to generate sufficient rotation (spin) of the product during the coating process. A solution would be to increase the airflow; however, this will, in turn, increase the losses due to attrition through the generation of small, dust particles and, thus, reduce yield.

Product Yield

The product yield is simply the product, which meets the required specification, produced, expressed as a percentage of the total material mass introduced to the process. The difference represents the product losses that occur during processing. In a traditional fluid bed coater, product can be lost as dust (caused by attrition of the core material or the spray dried particles) or by the agglomeration of larger, damp particles as they impact upon themselves or the equipment.

Agglomeration is caused when the spray rate is too high and the product is still damp when it meets either another particle in the bed or the surface of the coater. In some systems, the spray nozzle itself can also cause using a ‘venturi’ effect, sucking particles into the zone of very high humidity around the nozzle where they collide and agglomerate.

Again, a solution would be to increase the airflow rate to keep the particles separate. However, this can also increase attrition-derived losses. The alternative is to reduce the humidity in the vicinity of the spray nozzle by reducing the spray rate; however, the downside is an increase in processing time.

Traditional Coating Technology

Traditional bottom spray coating systems, such as those that have been in use since the 1950s, suffer from all of these problems and more. In addition to the difficulties of agglomeration, attrition, uneven product quality and long processing times, they are also difficult to scale-up from pilot to full production, they cannot operate as multi-tube systems and it is impossible to inspect the nozzles for agglomeration during processing. All of these failings have been addressed in the new breed of coating technology from GEA: the PRECISION-COATER™.

How the PRECISION-COATER™ Works

Liquid Layering Web

The key element of the PRECISION-COATER™ is the Swirl Accelerator that accurately controls the airflow properties and, as such, the behaviour of the particles flowing into the coating column. In a traditional fluidized bed coater, a large proportion of the air flows into the bed around the column, which is then fully fluidized and allows the particles to flood into the area around the spray nozzle by virtue of the hydrostatic head of the bed.

With the PRECISION-COATER™, however, much more of the air is directed into the coating column via an insert plate and the swirl accelerator. This design creates a much higher process air speed adjacent to the nozzle and generates a low-pressure zone that sucks particles into the core of the coating tube. The high speed of the airflow causes the individual particles to spin, allowing a much more even coating. The swirl accelerator imparts a rotational component to the air stream that, when combined with the high speed of the air stream, ensures that the individual particles are kept separate so that it is possible to operate a much higher humidity local to the nozzle without the risk of agglomeration. Tests show that the high speed of the air stream also generates a much higher level of evaporation so that particles are dry and will not agglomerate long before they return to the bed around the coating tube.

The system has been proved to provide an even coating of a prescribed thickness in a significantly reduced process time compared with traditional coating systems. The use of a high speed / low pressure zone to suck the particles into the coating area, enables the multi-tube PRECISION-COATER™ system to avoid dead zones with low particle flow which can be found on traditional multi-tube systems. As a result the PRECISION-COATER™ system can be scaled up from test, through pilot and up to full production using similar sized coating tubes. Configuring multi-tube production units and single column pilot scale systems to use the same air flows and patterns in the coating tube can eliminate many scale up difficulties experienced with systems which have much larger coating tubes in production units.

Conclusion

The PRECISION-COATER™ provides higher yields through reduced agglomeration and attrition; it improves productivity by coating and drying product faster; and it significantly improves product quality by ensuring a coating of uniform thickness for optimum clinical efficacy. It is ideal for coating particles from 50 microns to 3 mm in diameter. GEA believes that no other product is better suited to the industry’s need for a highly effective coating system with excellent scale-up characteristics. 

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