Direct Observation of a Defect-Mediated Viscoelastic Transition in a Hydrogel of Lipid Membranes and Polymer Lipids
Physical Review Letters
We present the first direct imaging of a new hydrogel of lipid membranes containing polymer lipids. Freeze-fracture electron microscopy shows unambiguously that the hydrogel's surprisingly large viscoelasticity is explained by a novel defect topology of interconnections between defects. The defects are spherulites with high membrane curvatures which are either isotropic or cylinderlike. A lower concentration of dislocation-type defects was also observed. The interconnections between the defects
... between the defects distinguish the hydrogel from simple "onion" phases of multilamellar vesicles with a smaller viscoelasticity. [S0031-9007 (97) 03388-7] PACS numbers: 61.72.Ff, 61.16.Bg, 68.10.Et Polymer hydrogels constitute an important class of "soft" materials with uses as implants and tissue replacements, drug delivery systems, and bioseparation media [1, 2] . In particular, materials based upon lipids and polyethylene glycol (PEG) have biomedical applications due to low immunogenicity (e.g., "stealth liposomes") [1, 3, 4] . When PEG polymers are covalently attached to lipids, PEG-lipids are created. A new system with potential applications (tissue healing, drug delivery, or cosmetics applications) is the PEGlipid "lamellar hydrogel L a,g ." Warriner et al. recently used x-ray diffraction, polarized light microscopy, and rheometry to study this new hydrogel composed of multiple layers of surfactant membranes and small amounts of a polymer lipid (Fig. 1) . The hydrogel possesses a surprisingly high viscoelasticity, higher than that of multilamellar vesicle phases (MLVs or "onions," concentric shells of lipid membranes). The hydrogel also differs from "onion" phases in that it remains a single-phase gel at high water dilution (90%) and becomes more viscoelastic upon dilution. Using freeze-fracture electron microscopy and polarized optical microscopy, we show that the hydrogel's high viscosity is linked to a proliferation of membrane defects interconnected by bilayer sheets. The liquid-to-gel phase transition is driven by mesoscopic topological defects rather than by changing molecular properties. X-ray diffraction of the hydrogel found that the lipids' hydrocarbon chains remain liquidlike in both the liquid and gel phases (Fig. 1) . Thus, the addition of a PEGlipid does not affect bilayer fluidity, and, unlike L b 0 gels, gelation is not due to in-plane ordering of the lipids . PEG-lipid molecules diffuse freely in the plane of the membrane. Bilayers in this study are constructed of 4:1 pentanol:lipid, where the lipid is dimyristoylphosphatidylcholine (DMPC) and PEG-lipid. Pentanol reduces the bending rigidity of the lipid membranes ͑k ഠ k B T ͒ and increases thermal membrane undulations  . The PEG-lipid's PEG head has a molecular weight of 2053 g͞mol and its lipid base is dimyristoylphosphatidylethanolamine (DMPE). The addition of a PEG-lipid to flexible lipid bilayers creates a hydrogel. An unusual aspect of the hydrogel is an increase in viscoelasticity upon dilution with water, opposite the behavior of gels of isotropically dispersed polymer FIG. 1. Left: schematic of fluid membranes with polymer lipids. Right: phase diagram redrawn from Warriner et al.  . The weight fraction of water, F water , is plotted vs conc PEG-lipid c PEG 100 3 (number PEG-lipid molecules)͞(total number lipid molecules). The sample is biphasic (lamellae and excess water) only at very high water concentrations. In contrast, gels of frozen-chain L b 0 phase bilayers have an excess water phase at .40% water . At low water fractions ͑F water # 45%͒, membranes are separated by d water ഠ 25 Å which is too small to incorporate a swollen PEG polymer, and the system separates into two phases, lamellar and isotropic  . The hydrogel is reached from the liquid phase by dilution or by addition of PEG-lipid. Samples A, B, C, D, E, and F were investigated by freezefracture electron microscopy and polarized optical microscopy. Sample A contained F water 80% and c PEG 0.3%; sample B: F water 77% and c PEG 4.19%; sample C: F water 81% and c PEG 7.7%; sample D: F water 64% and c PEG 1.15%; sample E: F water 64% and c PEG 3.05%; and sample F: F water 60% and c PEG 8.1%.