Film forming emulsions consist of an O/W emulsion with the api dissolved in the dispersed oil phase. The emulsions are stabilized by a water soluble polymer (polyvinyl alcohol) which also acts as a polymeric emulsifier. To allow the formation of a water insoluble film on skin the dispersions of sustained release polymers, namely, Ethyl Acrylate and Methyl Methacrylate Copolymer Dispersion (Eudragit^® NE 30D (NE), Evonik Röhm GmbH, Germany) and Ammonio Methacrylate Copolymer Dispersion, Type B (Eudragit^® RS 30D (RS), Evonik Röhm GmbH, Germany) are added to the aqueous phase. Therefore, the preparation of film forming emulsions poses a number of challenges to the producer. First, the drug needs to be incorporated into the inner oil-phase. Second, polymer dispersions are sensible to shear stress, the dispersions may only be added to the emulsion after homogenization. Third, the dispersions (NE and RS) are not compatible with each other unless pH is adjusted to pH 5–6 and polysorbate 80 is added to both dispersions prior to mixing (Lehmann and Dreher 1986). Fourth, the glass transition temperature of the polymers needs to be lowered to <20 °C by the addition of a plasticizer (Triethyl citrate, Sigma-Aldrich Chemie GmbH, Germany) to ensure complete film formation on the skin. Therefore, a three-step manufacturing process needed to be applied. First, the emulsion itself was made by homogenizing the drug solution in oil and the aqueous polymer solution. Second, the pH of the dispersions (NE and RS) was adjusted to pH 5–6 by addition of hydrochloric acid and the dispersions were plasticized. For that purpose, triethyl citrate was added to RS and the dispersion was stirred. Subsequently, polysorbate 80 was added to the plasticized RS and the dispersion was again stirred. Similarly, polysorbate 80 was added to NE and the dispersion was stirred. No triethyl citrate was added to NE. Third, the dispersions were added to the emulsion in order to obtain the final film forming emulsion. Figure 19.2 gives a scheme of the preparation of film forming emulsions. Compositions of film forming emulsions are given in Table 19.2.

Film forming emulsions were characterized by laser diffraction to verify absence of flocculation. The data show that droplet sizes of emulsions before and after addition of Eudragit^® were similar (Fig. 19.3). The addition of the dispersion of Eudragit^® resulted in a supplementary peak at approx. 120 nm which corresponds to polymer particles of the Eudragit^® dispersions. No additional peaks were detected. This indicates that no flocculation occurred and stable emulsions containing a combination of NE and RS were formed.

Film forming emulsions form a film on skin. In this film, the oil droplets are encapsulated in a dry polymeric matrix. The api which is dissolved in the oil phase needs to diffuse through this matrix in order to reach the skin. As this is a slow process, drug release is sustained. Even more important than the sustained release characteristics is an adequate substantivity which the emulsion films need to ensure in order to allow prolonged contact of the api to the skin. To achieve the required substantivity, complete film formation needs to occur at skin surface temperature (30–32 °C), the film needs to adhere strongly to the skin and needs to show sufficient elasticity to be able to follow the movements of the skin. The emulsion films were therefore examined concerning glass transition temperature (T _g), built-up, adherence and elongation to identify promising candidates for in vitro release testing. Compositions of the tested emulsions are given in Table 19.1.

The T _g of pure RS is well above 20 °C and triethyl citrate was used to lower T _g to a sufficient extent. Polysorbate 80, which had to be added to the Eudragit^® dispersions to prevent flocculation exhibits, a plasticizing effect similar to that of triethyl citrate (Grützmann and Wagner 2005) and polyvinyl alcohol may also contribute to changes in T _g. Therefore, measurements of T _g were taken out on films obtained from mixtures containing all excipients of the aqueous phase in the composition used in the preparation of film forming emulsions. Results are shown in Fig. 19.4.