Giant Phospholipid/Block Copolymer Hybrid Vesicles: Mixing Behavior and Domain Formation

Abstract

Lipids and block copolymers can be individually assembled into unsupported, spherical membranes (liposomes or polymersomes), each having their own particular benefits and limitations. Here we demonstrate the preparation of microscale, hybrid "lipopolymersomes" composed of the common lipid POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine) and the commercially available copolymer PBd-b-PEO (polybutadiene-b-poly(ethylene oxide)) with the goal of incorporating the advantageous qualities of the unitary systems into mixed-membrane capsules. We investigate the lipopolymersomes using confocal fluorescence microscopy and demonstrate that these hybrid membranes are well mixed on nanoscopic length scales within the permittable compositional windows for hybrid vesicle formation. We measure the intramembrane dynamics and mechanical properties of these hybrid membranes by fluorescence recovery after photobleaching (FRAP) and micropipet aspiration, respectively. For the first time, we demonstrate the demixing of lipid-rich and polymer-rich membrane domains within the same vesicle membrane. This is achieved by the biotinylation of one of the constituent species and cross linking with the protein NeutrAvidin. The resultant domain patterning is dependent upon which component carries the biotin functionality: cross linking of the copolymer species results in domains that ripen into a single, large, copolymer-rich island, and cross linking of the lipids yields many small, "spot-like", lipid-rich domains within a copolymer-rich matrix. We discuss these morphological differences in terms of the fluidity and mechanical properties of the membrane phases and the possible resultant interdomain interactions within the membrane. These heterogeneous hybrid lipopolymersomes could find applications in fields such as targeted delivery, controlled release, and environmental detection assays where these capsules possess the characteristics of biocompatible lipid membranes combined with enhanced mechanical strength and stability from the copolymer matrix.

Hybrid, Nanoscale Phospholipid/Block Copolymer Vesicles

“…Studies reported to date on hybrid vesicles have not yielded detailed information on the efficiency of hybrid vesicle formation in bulk solution [14][15][16][17][18][19][20][21]. Flow cytometry has been demonstrated as a useful tool to study vesicle populations [23].…”

“…It may be concluded that liposomes and polymersomes have their own particular benefits and limitations, which is why the preparation of hybrid membranes consisting of phospholipids and block copolymers is attracting increasing attention [14][15][16][17][18][19][20][21]. In this way, the robustness and chemical versatility of polymersomes can be combined with the softness and biocompatibility of liposomes.…”

“…Using fluorescence microscopy, several recent studies observed the formation of hybrid lipid/polymer giant unilamellar vesicles (GUVs) of 10-50 µm, prepared via electroformation, over a certain range of compositions [14][15][16][17][18]. Parameters controlling the self-assembly of lipids with block copolymers (forming hybrids GUVs), and the resulting phase behavior, are the mole fraction of the constituting amphiphiles, as well as the nature of the amphiphiles themselves [14][15][16][17].…”

“…Parameters controlling the self-assembly of lipids with block copolymers (forming hybrids GUVs), and the resulting phase behavior, are the mole fraction of the constituting amphiphiles, as well as the nature of the amphiphiles themselves [14][15][16][17]. Nam et al [15] observed efficient mixing of PB 46 -PEO 30 [polybutadiene-b-poly(ethylene oxide)] and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine) to occur only at polymer concentrations above 65%, while Schulz et al [17] found hybrid GUVs to form at all investigated molar ratios of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and polyisobutylene-b-PEO. The membrane thickness of the polymers and the melting transition temperature of the lipids have been demonstrated to significantly affect the formation, morphology and/or stability of hybrid GUVs, as well [14].…”

Asymmetric Hybrid Polymer–Lipid Giant Vesicles as Cell Membrane Mimics

“…For POPC/PBut 2.5 ‐ b ‐PEO 1.3 aGHUV we found D = 1.8 ± 0.50 µm 2 s −1 for DOPE‐rhod at 25 °C. Under the same experimental conditions, the diffusion coefficient for DOPE‐rhod in pure PBut 2.5 ‐ b ‐PEO 1.3 giant vesicles was found to be D = 0.46 ± 0.055 µm 2 s −1 and it has been shown that for pure POPC giant vesicles D = 9.8 ± 1.7 µm 2 s −1 61. Therefore, as for GHUV, an intermediate diffusion coefficient value is found, and shows that despite the asymmetric character of the membrane, the lipid lateral diffusion coefficient is lowered by the copolymer chains, suggesting some interdigitation between lipids and copolymer chains in the membrane.…”