**2. Pharmaceutically used plasticizers**

The primary role of all plasticizers as low molecular weight non-volatile additives is to improve the flexibility and processability of polymers by lowering the second order transition temperature (glass transition temperature, Tg) (Rosen, 1993). The extent of Tg reduction in the presence of a plasticizer can be used as a parameter to assess the plasticization efficiency (Senichev Tereshatov, 2004).

When incorporated into a polymeric material, a plasticizer improves the workability and flexibility of the polymer by increasing the intermolecular separation of the polymer molecules. This results in a reduction in elastic modulus, tensile strength, polymer melt viscosity and Tg. The polymer toughness and flexibility is improved and lower thermal processing temperatures can be employed. (Zhu et al., 2002).

The attributes of an ideal plasticizer are changed with each application. When selecting an appropriate plasticizer, the compatibility with the polymer and plasticization efficiency are the pivotal criteria. Incompatibility is commonly evidenced by phase separation between the biopolymer and plasticizer, presented in the form of exudated drops on the surface of the product immediately after its blending or during final application (Wilson, 1995).

Historically the first plasticized polymer used as a medicinal preparation since the 19th century has been cellulose nitrate. Its solution in a mixture of ethanol and diethyl ether is called collodion and used to cover wounds. After administration, the solvents quickly evaporate and cause unpleasant tension. That is why its plasticizing with 5% castor oil was introduced (Murray, 1867). The composition after incorporation of an antimicrobial substance functions as a protective barrier.
