Coating functionality
Optimum functionality is a primary goal for many types of excipients. Take into account, for example, materials used in coating
systems. The industry's basic needs in terms of functional coating are generally the same as they have always been. There
is still demand for coating systems to aid in achieving various drug-release profiles while providing good flexibility, taste
masking, adhesion, mechanical strength, and ability to withstand small variations in processing. Industry does not always
agree, however, that these needs are actually being met. "Although the fundamental needs have not changed, needs are not currently
being met satisfactory to the industry overall," says John Brown, marketing director at International Specialty Products (Wayne,
NJ). "Those manufacturers have been able to work with what is available, but if you ask them they would tell you that it is
not delivering 100% of what they are looking for. In some cases, it has been a product-driven market, so there is still a
void. As the industry comes under a considerable amount of pressure because of cost savings, brand extension, and patent expiration,
companies are looking to gain functionality."
There are several ways to increase the functionality of coating systems and other excipients, both for immediate-release and
for modified-release purposes. The following describes a chemical and a statistical technique.
A polymer approach.
Coatings are continuously improving in terms of the types of polymer systems, and excipient users and makers seek materials
that will provide better performance at faster coating times. Hydroxypropyl methylcellulose (HPMC), for example, is one of
the most commonly used polymers for tablet coating because of it forms films easily. However, the disadvantage with HPMC is
that it has low flexibility, and brittle tablets with HPMC coatings may swell under the high humidity storage conditions present
in some areas of the world. Swollen tablets then crack the coating. To avoid the effects of humidity, tablets must either
be packaged in 100% sealed blisters, which require an expensive polymer or plastic foil as opposed to the typical PVC blisters
that don't protect against humidity, or plasticizers must be added to the coating formula. However, as Folttmann observes,
the disadvantage of plasticizers is that they may either migrate into the tablet core and interact with the active ingredient
or they may migrate out of the film, outside of the tablet, making the coating brittle again.
In addition, aqueous coating polymers are spray dried, and the process time, energy, and amount of material required varies
greatly, depending on the particular polymer used. For example, HPMC is diluted in water and then a plasticizer, color, and
other materials are added before being sprayed onto tablets. However, only very low concentrations (~12%) of HPMC solution
can be made, because when diluted in water, HPMC greatly increases the viscosity of the solution. Moreover, in a spray-drying
coating process there is a lot of heat involved with the warm inlet air and the colder exhaust air and the energy that is
used to remove the water. "When you work with a low concentration of a polymer such as HPMC, the spray process is rather long
to get enough polymer onto the tablets, and you have to remove quite a lot of water to end up with a solid coating," explains
Folttmann.
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