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Abstract. "Polymersomes" are vesicular structures made from the self-assembly of block copolymers. Such structures present outstanding interest for different applications such as micro-or nano-reactor, drug release or can simply be used as tool for understanding basic biological mechanisms. The use of polymersomes in such applications is strongly related to the way their membrane properties are controlled and tuned either by a precise molecular design of the constituting block or by addition of specific components inside the membrane (formulation approaches). Typical membrane properties of polymersomes obtained from the self-assembly of "coil coil" block copolymer since the end of the nineties will be first briefly reviewed and compared to those of their lipidic analogues, named liposomes. Therefore the different approaches able to modulate their permeability, mechanical properties or ability to release loaded drugs, using macromolecular engineering or formulations, are detailed. To conclude, the most recent advances to modulate the polymersomes' properties and systems that appear very promising especially for biomedical application or for the development of complex and bio-mimetic structures are presented.

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Natural inspiration: Amphiphilic polysaccharide-block-polypeptide copolymers were synthesized by click chemistry from dextran end-functionalized with an alkyne group and poly(gamma-benzyl L-glutamate) end-functionalized with an azide group. The ability of these copolymers to self-assemble into small vesicles (see picture) suggests the possibility of a new generation of drug- and gene-delivery systems whose structure mimics that of viruses.

Using "click chemistry" as an easy and versatile synthetic strategy to combine hyaluronan and polyglutamate blocks, we have prepared nanovesicles (polymersomes) that present a controlled size, excellent colloidal stability, and a high loading capacity for hydrophilic and hydrophobic drugs. The unique feature of our concept is the use of hyaluronan, a polysaccharide with known capacity for targeting cancer-related protein receptors, as the hydrophilic portion of a block copolymer system. The cytotoxicity and internalization mechanism of doxorubicin-loaded polymersomes have been evaluated in C6 glioma tumor cell lines. The dual purpose served by hyaluronan, as both a hydrophilic block critical to vesicle formation and a binding agent for biological targets, breaks new ground in terms of multifunctional nanomaterial design for drug delivery.

Hybrid polymer/lipid giant unilamellar vesicles (GUV) were developed using lipids of respectively low and high melting transition temperature (DPPC:1,2-dipalmitoyl-sn-glycero-3 phosphocholine, T m =41°C, and POPC : palmitoyl-2-oleoyl-sn-glycero-3phosphocholine; T m =-2°C) and a copolymer poly(dimethylsiloxane)-graft-poly(ethylene oxide) (PDMS-g-PEO) well known to selfassemble into vesicular structures. Using epifluorescence microscopy as well as differential scanning calorimetry (DSC), different structures have been identified depending on the molar composition and on the fluid or gel state of the lipid used. The most promising objects are hybrid vesicles with copolymer as major component, in which lipids are either randomly distributed or present "raft-like" domains in the polymer-rich membrane. The results are discussed on the basis of the fluidity of the different components and of their respective membrane thickness. .

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Poly(trimethylene carbonate)-b-poly(L-glutamic acid) (PTMC-b-PGA) diblock copolymers have been synthesized by ring-opening polymerization (ROP) of gamma-benzyl-L-glutamate N-carboxyanhydride (BLG) initiated by amino functionalized PTMC and subsequent hydrogenation. Self-assembly in water gave well-defined vesicles which have been studied combining light and neutron scattering techniques with electron microscopy imaging. The size and dispersity of vesicles have been tuned by varying preparation conditions, direct dissolution, or nanoprecipitation. In addition, PGA conformation could be reversibly manipulated as a function of environmental changes such as pH and ionic strength. Vesicles showed high tolerance and stability toward nonionic surfactant and pH due to a thick membrane and were revealed to be nonpermeable to water. Nevertheless, they can be rapidly degraded by enzymatic hydrolysis of the polycarbonate block. The ability to tune their size through the formation process, their stimuli responsiveness, their high stability, and their biodegradability make them suitable for biomedical applications.