Vesicles prepared in water from a series of diblock copolymers -"polymersomes" -are physically characterized and compared to lipid vesicles. With increasing molecular weightM n , the hydrophobic core thickness d for the self-assembled bilayers of poly(ethylene oxide)-polybutadiene (PEO-PBD) increases up to ≃20 nm -considerably greater than any previously studied lipid system. The mechanical responses of these membranes, specifically, the area elastic modulus K a and maximal areal strain α c are measured by micromanipulation. As expected for interface-dominated elasticity, K a (≃100 pN/nm) is found to be independent ofM n , but lower than the usual values for zwitterionic lipid membranes. Experiments on polymersomes show α c increases in a nearly linear fashion withM n , approaching a limiting value predicted by mean-field ideas which is universal and about 10-fold above that typical of lipids. Nonlinear responses and memory effects generally emerge with increasingM n , indicating the onset of chain entanglements at higherM n . The effects ofM n thus suggest a compromise between stability and fluidity for biomembranes. More generally, the results highlight the interfacial limits of self-assemblies at the nanoscale.

We investigated the micellar polymorphism of poly(ethylene oxide)(PEO)-based block copolymers to illustrate the possibility of a rational control of the aggregation structure through synthetic manipulation of the molecular characteristics. Boundaries for the micellar shape transitions from bilayers to cylinders to spheres with increasing PEO composition were determined with direct cryogenic transmission electron microscopic (cryo-TEM) imaging of the microstructures in the form of thin vitreous hydrated specimens. Analyses of cryo-TEM images lead to determination of the packing properties of the hydrophobic block in terms of the interfacial area per chain and the degree of chain stretching. Also, the micellar phases of the block copolymers are characterized by anomalous structural behaviors such as coexistence of different structures and formation of exotic compound structures, which are discussed in terms of metastability inherent in the system comprising polymeric materials.

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We demonstrate that synthetic soft materials can extend the utility of natural vesicles, from predominantly hydrophilic reservoirs to functional colloidal carriers that facilitate the biomedical application of large aqueous-insoluble compounds. Near-infrared (NIR)-emissive polymersomes (50-nm-to 50-m-diameter polymer vesicles) were generated through cooperative self assembly of amphiphilic diblock copolymers and conjugated multi(porphyrin)-based NIR fluorophores (NIRFs). When compared with natural vesicles comprised of phospholipids, polymersomes were uniquely capable of incorporating and uniformly distributing numerous large hydrophobic NIRFs exclusively in their lamellar membranes. Within these sequestered compartments, long polymer chains regulate the mean fluorophore-fluorophore interspatial separation as well as the fluorophore-localized electronic environment. Porphyrin-based NIRFs manifest photophysical properties within the polymersomal matrix akin to those established for these high-emission dipole strength fluorophores in organic solvents, thereby yielding uniquely emissive vesicles. Furthermore, the total fluorescence emanating from the assemblies gives rise to a localized optical signal of sufficient intensity to penetrate through the dense tumor tissue of a live animal. Robust NIR-emissive polymersomes thus define a soft matter platform with exceptional potential to facilitate deep-tissue fluorescence-based imaging for in vivo diagnostic and drug-delivery applications.porphyrin ͉ vesicles ͉ nanoscale ͉ diblock copolymer S upramolecular self assembly has revolutionized soft materials research by enabling the efficient and high-throughput fabrication of complex multicomponent nanostructures (1-3). For decades, self-assembled vesicles comprised of phospholipids (liposomes) or small-molecule surfactants (4) have been used for sequestering high concentrations of hydrophilic compounds (5) and controlling their temporal release and distribution for maximal therapeutic efficacy (6). More recently, amphiphilic peptides and polymers have been shown to form very elaborate architectures (7-9) and serve as useful nanocontainers in aqueous solution (10). In particular, self-assembled materials are ideal for carrying promising imaging and therapeutic agents whose biomedical utility has hitherto been hampered by inadequate aqueous solubility (11). Here, we demonstrate the unique ability of synthetic amphiphiles to assemble into functional vesicles that membrane-disperse numerous large hydrophobic fluorophores and enable their specialized application for deeptissue fluorescence-based in vivo imaging.Although visible probes enable exquisite imaging of live animals by intravital microscopy (12), their utility is significantly limited at greater than submillimeter tissue depths as a result of extensive light scattering and optical absorption. Because light scattering diminishes with increasing wavelength, and hemoglobin electronic and water vibrational overtone absorptions approach their nadir over the near-infrared (N...