Fluid Bed Precision-Coating™ Technology for Improved Yield, Enhanced
Product Quality, Reduced Process Time and Easier Scale-up.
Coating technology has many applications in the pharmaceutical industry
for taste masking and drug release control. Fluid bed coating is particularly
suitable for smaller particles and pellets. In this article, Dr Harald Stahl
from GEA Pharma Systems, explains how the unique airflow within the
Aeromatic-Fielder Precision-Coater™ can provide significant improvements in
coating performance and overcome many of the coat quality and scale up problems
experienced with traditional coating tubes.
The pharmaceutical industry has had technology that can coat particles in
order to improve appearance, mask unpleasant tastes or to 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 yield, improve coat quality and reduce process time.
Coating Quality
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 1cm cube is
6cm. If, however, that cube were made up of smaller cubes each of 1mm, 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 problem is to coat the particle with just enough coating material to
achieve the desired result, but no more. In the case of functional coatings the
coating must, also, be evenly distributed over the entire surface of the
product. An uneven application can seriously affect the performance of the coat
e.g. - the 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. One solution to this can 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 can
collide and agglomerate.
Again one cure for this can be to increase the airflow rate in order to keep
the particles separate. However this can also increase the losses through
attrition the alternative is to reduce humidity in the vicinity of the spray
nozzle by reducing the spray rate, however the downside of this is an increase
in the process time.
Traditional coating technology
Traditional bottom spray
coating systems, such as those that have been in use since the 1950s, suffer
from all 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 can not
operate as a 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 Pharma Systems - the
Precision-Coater™.
How the Precision-Coater™ Works The key element
of the new Precision-Coater™ is the Swirl Accelerator that controls accurately
the airflow properties and so the behaviour of 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 area around the spray nozzle by virtue of
the hydrostatic head of the bed.
However in the Precision-Coater™ 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 which 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.
This very fast drying effect is the same process that causes clothes to dry
faster on a windy day.
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.
In the event of a spray nozzle problem during processing the unique design
of the Precision Coater™ allows the whole nozzle assembly to be removed,
cleaned and replaced without stopping the process and thereby allows minimum
production time and top quality production.
Conclusion
The Precision-Coater™ provides higher yield 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 effectiveness. It is ideal
for uses in coating particles from 50 microns to 3mm diameter. GEA Pharma
Systems Systems believes that no other product currently available is better
suited to the industry’s need for a highly effective coating system with
excellent scale up characteristics.
Article by Dr Harald Stahl Senior
Pharmaceutical Technologist GEA Pharma Systems