The film coating, which has just been awarded a US patent (No 6,884,288)) is made by adding a cellulose polymer such as hydroxypropyl methylcellulose (HPMC) to the gum acacia, and incorporating a plasticizer such as propylene glycol.
"The resulting formulation is a dry, free flowing powder that can be put into solution and applied to a tablet or other substrate without an extended waiting period. The resulting film coating is clear, shiny, durable and extremely economical," according to the patent.
Cellulose polymers such as HPMC have been used for many years as aqueous film coatings for pharmaceutical tablets, but have the drawback of being rather expensive. The available alternative for a non-enteric coating is a sugar coating.
However, although a sugar coating can be less expensive and has additional beneficial properties such as preventing moisture migration into the tablet and masking bad flavors, it is not as desirable as a film coating for all uses, according to the patent. For example, sugar coatings cannot be applied as thinly as a film coating and are not clear. Furthermore, sugar coatings have a higher risk of cracking than do film coatings.
"Manufacturers faced with the need to provide a durable, virtually non-caloric, thin or clear coat on tablets or confectionaries must pay a relatively high amount per pound for HPMC, or live with the drawbacks of sugar coatings," it continues.
Gum acacia is approved as a pharmaceutical ingredient by the US Food and Drug Administration (FDA),but to date has largely been used as a wetting or thickening agent, as an emulsifier, or as a binder.