Liquid-Crystalline Side-Chain Silastomers, Largly Modifiable Diffusion Media for the Control of Drug Delivery

Abstract

Liquid-crystalline side-chain polysiloxane elastomers were synthesized in such a manner as to give thin foils that can be used as diffusion-controlling membranes or drug-loaded matrices. The structure and the properties of the silastomers can be modified by the type of polysiloxane main chain, the chain length of the dimethylpolysiloxane crosslinker and by the mesogenic group. The mesogens under investigation are 4-methoxyphenyl-4''-alkenyloxybenzoates with different lengths of the spacer. The type and the content of the mesogenic group strongly influence the permeability of the membrane. If polymers with the same type of mesogen are compared, the logarithms of the diffusion coefficients of the model drug, salicylic acid, in the LC-silastomers are linearly correlated to the mole fractions of mesogen, indicating low permeabilities at high mesogen contents. Further, the permeability depends on the state of the phases of the mesogenic domains: The diffusivity is much smaller and the activation energy much higher in the LC state at lower temperatures than in the isotropic state of the silastomer at higher temperatures; there is a distinct change in the region of phase transition. The saturation concentration of salicylic acid in the polymers increases with increasing mesogen content. Again, the type of the mesogenic group exerts an influence. Further, the solubility of the drug is higher in the LC state of the silastomer at lower temperatures than in the isotropic state. All these properties together offer multifarious possibilities to adjust the drug release to the pharmacodynamic and pharmacokinetic requirements. The importance of the state of order to the permeability of a diffusion medium and the comparability with other liquid-crystalline systems, such as biological membranes, are discussed.