Molecular Weight Dependence of Polymersome Membrane Structure, Elasticity, and Stability

Abstract

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.

Computer simulation of aqueous block copolymer assemblies: Length scales and methods

“…67 The model was used to study the effects of block length on different physical properties of the bilayer configuration, such as the area elastic modulus, the scaling of the hydrophobic core thickness, and the lateral chain mobility. 65 The scaling of the hydrophobic core thickness with hydrophobic block length in the bilayer configuration agreed with experimental results reported 18 on bilayers in the same block length regime as the simulations. The simulation data was also used to calculate the absolute value of the bilayer's area elastic modulus and it was found to be independent of the block length; both of these results were in good agreement with the experimental view.…”

“…Experimental characterization of block copolymer assemblies includes measurements on toughness, elasticity and bending rigidity, permeability and fluidity, interfacial viscosity, as well as their dependence on variables such as block length and crosslinking. Pipette aspiration results 18 showed that diblock copolymer membranes have similar area expansion moduli as lipid membranes, but their toughness (defined as the stored energy at which membrane rupture occurs) extends to $20% area strain, while lipid membranes can support strains of only $5%. A study 19 on the bending rigidity of diblock copolymer membranes reported a quadratic scaling with increasing hydrophobic thickness, with values similar to that of lipid membranes for comparable molecular weights.…”

“…10 Electroporation and other rupture techniques have allowed for studies of the dynamics of pore healing and more generally the deformation and hydrodynamic behavior of diblock copolymer vesicles. 21 FRAP studies 18 of probe molecules in diblock copolymer membranes have shown a decrease in the mobility parallel to the membrane plane with increasing molecular weight. At high molecular weights, chain entanglement leads to a crossover from Rouse to reptation dynamics, strongly influencing diffusivity.…”

Polymersomes: A new multi-functional tool for cancer diagnosis and therapy

“…Additional work in the field has led to the synthesis of a number of biocompatible PEO-based amphiphilic block copolymers that form aqueous vesicles dispersions, including poly(ethylene oxide)-block-poly(butadiene) (PEO-b-PBD) [7].…”

“…Polymersomes, polymer vesicles self-assembled from a diverse array of synthetic amphiphilic block copolymers containing hydrophilic and hydrophobic blocks [2][3][4], have been shown to possess superior biomaterial properties, including greater stability and storage capabilities [5][6][7], as well as prolonged circulation time, as compared to liposomes (vesicles derived from phospholipids) [8]. A particularly attractive storage feature, highlighted in Fig.…”