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Liquid crystal polymer vesicles and their bursting induced by light.

Min-Hui Li Institut Curie, CNRS UMR168

Polymer vesicles are stable and robust vesicles made of block copolymer amphiphiles. Recent progress in the chemical design of block copolymers opens up the exciting possibility of creating a wide variety of polymer vesicles with different fine structures, functionalities and geometries. Polymer vesicles not only constitute useful systems for drug delivery or micro/nano-reactors but also provide valuable models for exploring the physics of two-dimensional order in curved spaces. The objective of this talk is to discuss these two aspects with liquid crystal (LC) polymer vesicles.
By choosing suitable LC polymers for one of the copolymer components one can create vesicles with additional order in the two-dimensional membrane itself. We report ellipsoidal smectic polymer vesicles1 and faceted smectic polymer vesicles,2 formed from amphiphilic block copolymers in which the hydrophobic block is a smectic LC polymer. Smectic order on shapes of spherical topology inevitably possesses topological defects (disclinations). The competition between liquid crystal frank energy and membrane bending energy associated with the topological defects are responsible for the ellipsoidal shape and faceted shape observed. These smectic polymer vesicles offer novel examples of the interplay between orientational/positional order and the curved geometry of a two-dimensional membrane.
Polymer vesicles are excellent candidates as drug carriers and micro/nano-reactors if the controlled release of active substances is integrated. Common strategies have been to use amphiphilic copolymers that, under the effect of chemical stimuli (hydrolysis, oxidation or reduction reaction, and pH changes), can degrade or convert into hydrophilic moieties.3 and references therein The chemical stimuli require that the chemical environment be modified by additional reagents. These environment changes may not always be compatible with applications such as cancer treatment or synthesis of high value chemicals. In these applications, a fast and programmed release of entrapped species (drugs, catalysts or reactants) at a precise site induced by a remote stimulus is desirable to minimize the damage caused by therapeutic agents on the surrounding healthy tissue or to ensure the proper course of the chemical reaction. Therefore, we propose a new strategy of using copolymers with blocks responsive to physical stimuli, such as light. Novel block copolymers with nematic LC hydrophobic blocks, which are intrinsically sensitive to physical stimuli, were used as building blocks of polymer vesicles.4 This new strategy is highlighted by a new bursting mechanism we invented.5 The bursting results from a spontaneous curvature of the membrane induced by light, a remote stimulus, in an asymmetrical polymer vesicles. The versatility of this mechanism should broaden the range of applications of polymer vesicles in fields such as drug delivery, cosmetics and material chemistry.

References :

[1] Jia L., Cao A., Lévy D., Xu B., Albouy P.-A., Xing X., M. Bowick, Li M.-H, Soft Matter, 2009, 5, 3446 .

[2] Xu B., Piñol R., Nono-Djamen M., Pensec S., Keller P., Albouy P.-A., Lévy D., Li M.-H., Faraday Discussion, 2009, 143, in press . DOI : 10.1039/b902003a.

[3] Li M.-H. and Keller P., Soft Matter, 5 (2009), 927-937.

[4] Yang J., Lévy D., Deng W., Keller P., Li M.-H., Chem. Commun. 2005, 4345 – 4347.

[5] Mabrouk E., Cuvelier D., Brochard-Wyart F., Nassoy P. and Li M.-H., Proc. Natl. Acad. Sci. USA, 2009, 106, 7294-7298.