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.

Vesicles made completely from diblock copolymers-polymersomes-can be stably prepared by a wide range of techniques common to liposomes. Processes such as film rehydration, sonication, and extrusion can generate many-micron giants as well as monodisperse, approximately 100 nm vesicles of PEO-PEE (polyethyleneoxide-polyethylethylene) or PEO-PBD (polyethyleneoxide-polybutadiene). These thick-walled vesicles of polymer can encapsulate macromolecules just as liposomes can but, unlike many pure liposome systems, these polymersomes exhibit no in-surface thermal transitions and a subpopulation even survive autoclaving. Suspension in blood plasma has no immediate ill-effect on vesicle stability, and neither adhesion nor stimulation of phagocytes are apparent when giant polymersomes are held in direct, protracted contact. Proliferating cells, in addition, are unaffected when cultured for an extended time with an excess of polymersomes. The effects are consistent with the steric stabilization that PEG-lipid can impart to liposomes, but the present single-component polymersomes are far more stable mechanically and are not limited by PEG-driven micellization. The results potentiate a broad new class of technologically useful, polymer-based vesicles.

Massively cross-linked and property-tunable membranes have been fabricated by free radical polymerization of self-assembled, block copolymer vesiclespolymersomes. Similar efforts with cross-linkable lipids would appear frustrated in the past due to at least two factors: limited reactivity and membrane fragility under local stresses of nano-confined cross-linking. We describe here a diblock copolymer of poly(ethylene oxide)polybutadiene that has a hydrophilic weight fraction like that of lipids and forms robust fluid phase membranes in water. The polymersomes sustain free radical polymerization of the hydrophobic butadiene, thereby generating a semipermeable nano-shell. Cross-linked giant vesicles prove stable in chloroform and can also be dehydrated and re-hydrated without rendering the ∼9 nm thick membrane core; the results imply defectfree membranes many microns-squared in area. Surface elastic moduli as well as sustainable wall stresses up to 10 3 Atm, orders of magnitude greater than any natural lipid membrane, appear consistent with strong tethering between close-packed neighbors. The enormous stability of the giant vesicles can be tuned down for application: blending in the hydrogenated analogue poly(ethylene oxide)-polyethylethylene modulates the effective elastic constants as well as the rupture strength by orders of magnitude. The results appear consistent with rigidity percolation through a finite-layer stack of two-dimensional lattices. Moreover, below the percolation limit, a regime of hyper-instability emerges, reflecting perhaps nanoscale demixing and suggestive of the limitations encountered with low reactivity lipids. The results provide general insights into covalent cross-linking within self-assembled nanostructures.

The extracellular matrix of dense, avascular tissues presents a barrier to entry for polymer-based therapeutics, such as drugs encapsulated within polymeric particles. Here, we present an approach by which polymer nanoparticles, sufficiently small to enter the matrix of the targeted tissue, here articular cartilage, are further modified with a biomolecular ligand for matrix binding. This combination of ultrasmall size and biomolecular binding converts the matrix from a barrier into a reservoir, resisting rapid release of the nanoparticles and clearance from the tissue site. Phage display of a peptide library was used to discover appropriate targeting ligands by biopanning on denuded cartilage. The ligand WYRGRL was selected in 94 of 96 clones sequenced after five rounds of biopanning and was demonstrated to bind to collagen II alpha1. Peptide-functionalized nanoparticles targeted articular cartilage up to 72-fold more than nanoparticles displaying a scrambled peptide sequence following intra-articular injection in the mouse.

The ability of nucleases to perform their catalytic functions depends on the sequence and structural features of target DNA substrates. Due to their size and shape, several DNA tetrahedra are resistant to the action of specific and non-specific nucleases. Such enhanced stability is a key requirement for DNA nanostructures to be useful as delivery vehicles.

The bending rigidity k(c) of bilayer vesicles self-assembled from amphiphilic diblock copolymers has been measured using single- and dual-micropipet techniques. These copolymers are nearly a factor of 5 greater in hydrophobic membrane thickness d than their lipid counterparts and an order of magnitude larger in molecular weight M(n). The macromolecular structure of these amphiphiles lends insight into and extends relationships for traditional surfactant behavior. We find the scaling of k(c) with thickness to be nearly quadratic, in good agreement with existing theories for bilayer membranes. The results here are key to understanding and designing soft interfaces such as biomembrane mimetics.

Giant cell-like polymer vesicles, polymersomes, made from the diblock copolymer poly(ethylene oxide)-polybutadiene (PEO-PBD), have bilayer structures similar to the cell membrane but have superior and tunable properties for storage and stability. We have modified the terminal hydroxyl of the hydrophilic block with biotin-lysine (biocytin), a biologically derived group that imparts specific adhesiveness to a polymer colloid coated with avidin. The functionalized polymer will form vesicles, either on its own or when mixed with unmodified block copolymers that also form vesicles. The incorporation and mixing of the functionalized polymer into vesicle bilayers is measured using a fluorescent version ofbiocytin with confocal microscopy. The fluorescence signal associated with the vesicle is in proportion with the concentration of functional polymer added during vesicle construction. The adhesiveness of polymer vesicles containing functionalized biotinylated polymer to avidin coated microspheres is measured with micropipet aspiration. Two types of polymer vesicles were constructed: one where the functionalized polymer (molecular weight (MW), 10400 Da) was longer than the surrounding unfunctionalized polymer (MW, 3600 Da) and one where the functionalized polymer (MW, 10400 Da) was the same length as the unfunctionalized polymer. In all cases, the avidin-biotin bonds form kinetically trapped crossbridges that impart little tension as they form but require significantly more tension to break. The relative length of the functionalized polymer on the surface of the vesicle is an important determinant for the adhesion of a polymer vesicle but not for the adsorption of soluble avidin. Greater adhesion strengths are seen where the functionalized polymer is longer than the surrounding polymer. The concentration of functionalized polymer at which adhesion is maximal depends on the relative lengths of the polymers. When the functionalized polymer is the same length as the surface brush of the polymersome membrane, the critical tension is maximal at 10 mol % functionalized polymer concentration. However, when the biocytin groups are attached to a polymer which is larger than the surface brush, the critical tension is maximal at 55 mol % functionalized polymer. These results indicate that polymer mixing and length can control the interfacial adhesion of polymer brushes and must be understood to tune polymersome adhesiveness.

Self-assembled membranes of amphiphilic diblock copolymers enable comparisons of cohesiveness with lipid membranes over the range of hydrophobic thicknesses d=3-15 nm. At zero mechanical tension the breakdown potential V_c for polymersomes with d=15 nm is 9 V compared to 1 V for liposomes with d=3 nm. Nonetheless, electromechanical stresses at breakdown universally exhibit a square dependence on V_c, and membrane capacitance shows the expected strong d-dependence, conforming to simple thermodynamic models. The viscous nature of the diblock membranes is apparent in the protracted post-poration dynamics.Comment: 13 pages, 4 figures, 2 tables and 1 chair. Submitted to PRL (May 24, 2001