**4. Polymeric membranes for transdermal system**

Transdermal systems are based on the adhesion of the system containing the active ingredient onto the intact skin and the transfer of the dissolved active ingredient through the skin. The limiting factor for the velocity of transdermal absorption is most frequently the thin membrane or a polymeric matrix structure. To fulfil the necessary precondition of the barrier principle as the controlling system of transdermal absorption, the velocity of permeation through the membrane or matrix must be lower than the velocity of penetration into the skin structures. Plasticizers are of extraordinary importance as they ensure not only suitable mechanical properties, in particular flexibility of the system, but also in connection with a decreased value of Tg increase the diffusivity of polymers.

In the development of a suitable membrane for transdermal use, cellulose acetate casted from a chloroform solution was used. The polymer was plasticized with dibutyl phthalate, polyethylene glycol 600 and propylene glycol in a concentration of 40 %. The prepared membranes were characterized from the standpoint of mechanical properties, transfer of water vapours and permeability of diltiazem hydrochloride and indomethacin. Permeability of the active ingredient was the highest with the use of polyethylene glycol as the plasticizer, the lowest in the case of dibutyl phthalate (Rao & Diwan, 1997). The plasticizer of membranes from cellulose acetate was poly(caprolactone triol), and the pore forming agent was water (Meier et al., 2004). On the basis of testing the permeability of paracetamol, water was demonstrated to be a suitable porosigen with a possible regulation of porosity by its concentration, the plasticizer suitably acting as the modulator of permeation of the active ingredient. A combination of these two excipients yielded the membranes whose values of the coefficients of permeation of paracetamol ranged from 10-7 to 10-5 cm s-1.

The matrix system from polyvinyl acetate containing polyethylene-2 oleyl ether as the enhancer of permeation and triethyl citrate as the plasticizer resulted in a significant increase in the bioavailability of the antihistamine drug triprolidine. The effect was demonstrated after the administration of the system to the abdominal rabbit skin (Shin & Choi, 2005). Roughness, mechanical and adhesive properties of skin patch were studied. An adhesive plaster was prepared from a blend of polyvinyl alcohol and polyvinylpyrrolidon with glycerol or polyethylene glycols 200 or 400 as the plasticizers. The adhesivity of the system was ascertained only in contact with wet surfaces. Glycerol in a concentration of 10 % did not influence the very smooth texture of the surface of the plaster. The effect of the plasticizer was increased with decreasing molecular mass (Gal & Nussinovitch, 2009).
