Polymersomes: Tough Vesicles Made from Diblock Copolymers

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Reviewed in R. Lipowsky and E. Sackmann Eds. Structure and Dynamics of Membranes—From Cells to Vesicles vol. 1 of the Handbook of Biological Physics (Elsevier Science Amsterdam 1995). Note (as tabulated on page 19 of this reference) that for a generic plasma membrane such as that of the red blood cell about three-fourths of the phospholipids have acyl chains of lengths 16 18 or 20 carbon atoms. Note also (as indicated on pages 229 or 658 of the same reference) that a bilayer of lipids with chains of 16 or 18 carbon atoms in the biologically relevant disordered liquid phase (L α ) may be estimated to have a thickness d ≈ 3 nm.

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. For EO 40 -EE 37 a polydispersity measure is given by M w / M n ∼ 1.10 where M w and M n are the weight-average and number-average molecular weights respectively. The PEO volume fraction is f EO = 0.39.

J. M. Harris and S. Zalipsky Eds. Poly(ethylene glycol): Chemistry and Biological Applications (American Chemical Society Washington DC 1997). Also as reviewed in D. D. Lasic in (3) chap. 10 PEG chain lengths that optimize the stealth of liposomes that is lipid-conjugated PEG leading to the longest blood circulation time of liposomes are in the approximate range of EO 34 to EO 114 .

Thin (about 10- to 300-nm) films of aqueous solution suspended in a microperforated grid were prepared in an isolated chamber with temperature and humidity control. The sample assembly was rapidly vitrified with liquid ethane at its melting temperature (∼90 K) and this was kept under liquid nitrogen until it was loaded onto a cryogenic sample holder (Gatan 626). We obtained images with a JEOL 1210 at 120 kV using a nominal underfocus of 6 μm and digital recording. For a more detailed description or related examples see

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EO 40 -EE 37 synthesized by a combination of anionic polymerization and catalytic hydrogenation was subsequently lyophilized into a solid and solubilized when needed in chloroform at 4 mg/ml. Evaporation of the solvent under nitrogen followed by vacuum drying for 3 to 48 hours was used to deposit a film on 1-mm-diameter platinum wire electrodes held in a Teflon frame (5 mm separation). The Teflon frame and electrodes were assembled into a chamber by sealing with coverslips and this was subsequently filled with 100 mM sucrose solution. To begin generating vesicles from the film we applied an alternating electric field to the electrodes (10 Hz 10 V) while the chamber was mounted and viewed on the stage of an inverted microscope. Giant vesicles attached to the film-coated electrode were visible after 15 to 60 min. These were dissociated from the electrodes by lowering the frequency to 3 to 5 Hz for at least 15 min. The electroformation method is adapted from that of M. I. Angelova S. Soleau P. Meleard J. F. Faucon and P. Bothorel [ Prog. Coll. Polm. Sci. 89 127 (1992)] as previously used by M. L. Longo A. J. Waring and D. A. Hammer [ Biophys. J. 73 1430 (1997)]. The vesicles were stable for at least several days when kept sealed from air in a gas-tight plastic syringe. Of additional note vesicles were stable when resuspended in physiological saline at temperatures ranging from 10° to 50°C. Micromanipulation was done with micropipette systems analogous to those described by M. L. Longo et al. and D. E. Discher N. Mohandas and E. A. Evans [ Science 266 1032 (1994)].

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. In this reference the self-consistent calculation models lipids as nearly symmetric diblock copolymers which are clearly closer in form to the molecules of this study.

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Because the average interfacial area per chain < A c > in the lamellar state has been estimated to be < A c > ≳ 2.5 nm 2 per molecule one estimates that E c ≳ 1.3 k B T.

Vesicles were prepared in 100 mOsm sucrose solution which established an initial internal osmolarity. They were then suspended in an open-edge chamber formed between cover slips and containing 100 mOsm glucose. A single vesicle was aspirated with a suction pressure sufficient to smooth membrane fluctuations; the pressure was then lowered to a small holding pressure. With a second transfer pipette the vesicle was moved to a second chamber with 120 mOsm glucose. Water flows out of the vesicle due to the osmotic gradient between inside and outside which leads to an increased projection length that is monitored over time. The exponential decrease in vesicle volume was calculated from video images and then fit to determine the permeability coefficient ( P f ) (4). For reference P f = 23.5 ± 1.7 μm/s for SOPC from this method.

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We thank E. A. Evans D. Needham P. Nelson and T. Lubensky for discussions. Support was primarily provided by the NSF-supported Materials Research Science and Engineering Center (MRSEC) at the University of Pennsylvania (DMR96-32598) and both the Center for Interfacial Engineering (CIE) and the MRSEC (DMR 98-09364) at the University of Minnesota. The work was also supported in part by grants from the Whitaker Foundation (D.D.) and National Institutes of Health [R01-HL62352-01(D.D.) and P01-HLI8208 (D.H.)].