Coating Functionality

The polymer essentially determines the functionality of the coat. However, other ingredients can also impact release rate because of effects on properties such as hydrophilicity, porosity, and solubility. Such possibilities have to be considered during development and may be leveraged to achieve the desired release characteristics. Figure 13.7 illustrates how different antitack agents, present at functionality levels can influence release rates.

Reliable film functionality requires consistent coat thickness and integrity. Enteric coatings usually require thicknesses of 40-50 mm to confer gastroresistance.

Fig. 13.7 Influence of different antitacking agents on the release profile of phenylpropanolamine pellets coated with EUDRAGIT® RS 30 D

Fig. 13.8 SEM pictures: left: integer coating, right: cracks through insufficient plasticizer

Fig. 13.8 SEM pictures: left: integer coating, right: cracks through insufficient plasticizer

For controlled release or protective applications, coat thicknesses must be designed to deliver the required release profile or protection. Film stability is assured by its flexibility but few polymers are intrinsically flexible. The majority require the presence of a plasticizer to avoid cracks developing during coating, subsequent processing, or storage. If coated particles are to be compressed to multiparticulate tablets, even higher film flexibility is required to withstand mechanical stress during tablet-ing. Plasticizer needs to be matched with film forming polymer. Its impact can be checked by elongation-at-break measurements (ISO 527-3) or determination of Minimum Film forming Temperature (MFT) [27] (Fig. 13.8).

Pigments can affect coat porosity. Pigment binding capacity varies with polymer. Poly(meth)acrylates usually provide highest binding capacity, being capable of binding 300% of polymer weight. Cellulose-based polymers in contrast show only moderate binding capacities [27]. Loss or change of film functionality may be avoided by applying the pigments as a separate layer on top of the functional coat. Specific release profiles may require the sequential application of multiple coats.

Fig. 13.7 Influence of different antitacking agents on the release profile of phenylpropanolamine pellets coated with EUDRAGIT® RS 30 D

time [h]

An example is local treatment of inflammatory bowel diseases where a delayed release coat could ensure that drug release does not start before the formulation reaches the lower small intestine/colon. An inner insoluble coat can then prolong release during colonic transit [23].

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