Polymer-Lipid Hybrid Materials

Abstract

Hierarchic self-assembly underpins much of the form and function seen in synthetic or biological soft materials. Lipids are paramount examples, building themselves in nature or synthetically in a variety of meso/nanostructures. Synthetic block copolymers capture many of lipid's structural and functional properties. Lipids are typically biocompatible and high molecular weight polymers are mechanically robust and chemically versatile. The development of new materials for applications like controlled drug/gene/protein delivery, biosensors, and artificial cells often requires the combination of lipids and polymers. The emergent composite material, a "polymer-lipid hybrid membrane", displays synergistic properties not seen in pure components. Specific examples include the observation that hybrid membranes undergo lateral phase separation that can correlate in registry across multiple layers into a three-dimensional phase-separated system with enhanced permeability of encapsulated drugs. It is timely to underpin these emergent properties in several categories of hybrid systems ranging from colloidal suspensions to supported hybrid films. In this review, we discuss the form and function of a vast number of polymer-lipid hybrid systems published to date. We rationalize the results to raise new fundamental understanding of hybrid self-assembling soft materials as well as to enable the design of new supramolecular systems and applications.

Figure 1.

Schematic illustration of polymer–lipid hybrid membranes: (i) homogeneous membrane and (ii) phase separated membrane (left) in bulk solution—vesicles, at liquid–air interfaces—monolayer films in suspension, and in air—solid supported films (right, from top to bottom).

Figure 2.

Schematic illustration of polymer–lipid hybrid membranes in solution; vesicles with phase separated polymer-rich/lipid-rich domains (left) and homogeneous membranes (right) depending on innate, extrinsic environmental and extrinsic molecular parameters.

Figure 3.

Representative membrane forms of GHUVs obtained by CLSM. The GHUVs are composed of PDMS-b-PEO and POPC. 2D cross-sectional image taken through the equator of the GHUV showing (a) a homogeneous membrane, (b) domains in a spherical vesicle, (c) domains during budding, and (d) and fission of a GHUV demonstrating a neat lipid vesicle and a neat BCP vesicle separately. (A–C) 3D reconstructed images along the z-axis, illustrating the different structures. Polymer-rich domains labeled with PDMS-NBD and lipid-rich domains dyed with rhodamine-PE. Note that the vesicle budding phenomenon can stop and results in a stable vesicle. Reproduced with permission from ref . Copyright 2020 The Royal Society of Chemistry.

Figure 4.

Schematic illustration of the membrane morphologies of copolymer–lipid GHUVs (A) in a homogeneously mixed state with nanodomains and (B) in microscale phase separation due to the hydrophobic mismatch between the hydrophobic segments of the polymer and lipid tails. The black line on the GHUVs represents a cross-sectional cut.

Figure 5.

Apparent phase diagrams of the GHUVs formed from POPC and three different PDMS-b-PEO diblock copolymers (left to right: PDMS₂₃-b-PEO_13,, PDMS₂₇-b-PEO₁₇, and PDMS₃₆-b-PEO₂₃). Reproduced with permission from ref . Copyright 2020 The Royal Society of Chemistry.

Figure 6.

A summary of hybrid vesicle morphologies as a function of composition lipid phase: (1–3) using DPPC in the gel state (4–5) using POPC in the fluid state. (1) Homogeneous membrane at 4 wt % DPPC/96 wt % PDMS-g-PEO. (2) Heterogeneous membrane at 7 wt % DPPC/93 wt % PDMS-g-PEO. (3) Heterogeneous membrane at 41 wt % DPPC/59 wt % PDMS-g-PEO. (4) Homogeneous membrane at 7 wt % POPC/93 wt % PDMS-g-PEO. (5) Large-scale phase-separated vesicle at 42 wt % POPC/58 wt % PDMS-g-PEO (75% POPC/25% PDMS-g-PEO) budding over several hours: (a) spherical biphasic vesicle, (b,c) two domains with different curvatures within the same vesicle, (d) flat contact area between two separated vesicles, and (e) emergence of a polymersome (green) and a liposome (red). (A) Fluorescein filter set, (B) triple band filter set, and (C) rhodamine filter set. Reproduced with permission from ref 47. Copyright 2012 The Royal Society of Chemistry.

Figure 7.

Organization of the lamellar phases of bilayered phospholipids (DPPC) in the fluid (L_α), ripple (P_β), gel (${\text{L}}_{{\beta }^{\prime }}$), and pseudocrystalline (L_c) states. The last column illustrates a top view of the packing of the hydrocarbon chains.

Figure 8.

(A) 3D reconstructed CLSM images of the GHUV including mPEG-b-PCL and DPPC demonstrating micrometer as well as nanoscale phase separation. The leftmost inset is the graphical illustration of the merged image. RL-FITC and RL-Rhod B channels are separately shown (upper). (mPEG-b-PCL)-FITC and 16:0 Liss Rhod PE preferentially partitioned polymer domains and lipid domains, respectively. The scale bar is 10 μm. (B) Schematic illustration of the hybrid nanostructures composed of mPEG-b-PCL and DPPC (left) and the crystallographic information of each component (right). Crystalline PCL adopting an orthorhombic unit cell of lattice parameters: a = 7.5 Å, b = 5 Å, and c = 17.3 Å; gel phase DPPC packed in a pseudohexagonal unit cell with a = 8.46 and b = 4.71 Å. Reproduced with permission from ref . Copyright 2020 MDPI.

Figure 9.

Schematic representation of the structure of lipid nanodomains formed by (A) glycerophospholipids, sphingomyelins, and cholesterol, (B) ceramides, (C) glycosphingolipids, and (D) phosphoinositides. Reproduced with permission from ref . Copyright 2018 American Chemical Society.

Figure 10.

CLSM images of the GHUVs composed of 5 mol % DSPE-PEG200-biotin, 25 mol % POPC, and 70 mol % PBD-b-PEO at different neutravidin incubation times tagged with Oregon Green DHPE (green) and Rh-NA (red). Lipid domains are formed during incubation induced by neutravidin binding. (a) 2D cross sectional and (b) 3D reconstructed images of the vesicle before incubation. 2D cross sectional image of (c) green channel after 1 h incubation, (d) red channel after 1 h incubation, (e) green channel after 12 h incubation, and (f) red channel after 12 h incubation. 3D reconstruction after 12 h incubation of (g) green channel and (h) red channel. The scale bar is 5 μm. Reproduced with permission from ref . Copyright 2010 American Chemical Society.

Figure 11.

CLSM images of the GHUVs. (a) Homogeneous vesicle composed of PBD-b-PEO:DPPC (6:4) at ~50 °C. (b) Irregular, star-shaped DPPC-rich domains in PBD-b-PEO:DPPC (6:4) vesicles at room temperature. (c) Spherical DPPC-rich domains in a PBD-b-PEO:DPPC:Chol (5:3:2) vesicle at room temperature. (d) Homogeneous PBD-b-PEO:POPC (5:3) vesicle at room temperature. (e,f) PBD-b-PEO:POPC:Chol (1:1:1) GHUVs at room temperature. (a–c) show the red channel (Rh-DOPE, which probes the copolymer domain). (d–f) Overlay the red channel (Rh-DOPE) and the green channel (OG-DHPE, which partitions into POPC-rich domains). (a–c,f) 3D reconstructed images. (d,e) 2D cross-sectional images through the equator of the GHUVs. The scale bar is 10 μm. Reproduced with permission from ref . Copyright 2012 The Royal Society of Chemistry.

Figure 12.

β-CD triggered morphology change in the GHUVs including PBD-b-PEO/POPC/Chol (4:3:3) at room temperature. CLSM image of (a) a heterogeneous phase-separated hybrid membrane before the reaction and (b) homogeneously mixed outermost membrane after the reaction. The β-CDs cannot penetrate into the inner leaflet of the vesicle membrane. The scale bars represent 10 μm. Reproduced with permission from ref . Copyright 2012 The Royal Society of Chemistry.

Figure 13.

(A) Fluorescence microscopic images of GHUVs composed of PDMS-g-PEO and DPPC compared with DOPC/DPPC vesicles for various compositions at cooling rates of 5 °C/min (upper) and 1 °C/min (lower). (B) Membrane tension GHUVs (70 mol % DPPC) after cooling-down to 35 °C near the initial phase separation temperature; in osmotically conditioned sucrose buffer (left) and DI water (right) with different cooling rates. (C) Images of the GHUVs (70 mol % DPPC) in different cooling and osmotic control. The scale bar is 10 μm. Reproduced with permission from ref. Copyright 2015 The Royal Society of Chemistry.

Figure 14.

Impact of cooling rates on the domain size and morphology on the GHUVs composed of PBD-b-PEO and DPPC (copolymer to lipid ratio as 6:4). (a–c) Epifluorescence microscopy images labeled with Rh-DOPE prepared at different cooling rates. The scale bars are 10 μm. (d) Number and apparent area fraction of individual domains (A/A_o) against cooling rate, where A is the apparent area of individual domains and A_o is the total area of observable membrane. Reproduced with permission from ref . Copyright 2012 The Royal Society of Chemistry.

Figure 15.

Molecular structures of (a) POPC and (b) PBD₄₆-b-PEO₃₀. CLSM images of (c) a 2D cross-sectional image at the vesicle equator and (d) a 3D reconstructed image of a vesicle hemisphere using 0.5 mol % Oregon Green DHPE in hybrid PBD₄₆-b-PEO₃₀/POPC vesicles (70 mol % PBD₄₆-b-PEO₃₀). Scale bars are 10 μm. (e) Stretching elasticity modulus (K_A, ■) and lipid lateral diffusion coefficient (D_l, ○) with different amounts of polymer. (f) Critical lysis tension (τ_c, ■) and lysis strain (α_c, □) of GHUVs comprising PBD₄₆-b-PEO₃₀ and POPC. Reproduced with permission from ref . Copyright 2010 American Chemical Society.

Figure 16.

Mechanical properties of the GHUVs in various combinations of diblock or triblock copolymers (PDMS-b-PEO or PEO-b-PDMS-b-PEO) with POPC. (A) Stretching modulus, (B) lysis strain, (C) lysis stress, and (D) Toughness at various lipid fractions. The dashed red line shows the typical toughness of a liposome. Reproduced with permission from ref . Copyright 2020 The Royal Society of Chemistry.

Figure 17.

(A) Bending rigidity (κ_B) of PDMS₂₆-g-(PEO₁₂)₂/soy PC GHUVs without protein (w/o bo₃; white area) and protein-functionalized (w/bo₃; gray area) compared to lipid/polymer-only systems. Each square displays a measurement on a single GUV. Half-filled squares refer to the average of all evaluated GUVs. (B) Cryo-TEM images of soy PC, PDMS₂₆-g-(PEO₁₂)₂/soy PC, and PDMS₂₆-g-(PEO₁₂)₂ LUVs (from top to bottom). Scale bar is 100 nm with a −2 μm defocus. Reproduced with permission from ref . Copyright 2020 National Academy of Sciences.

Figure 18.

FRAP/FCS analyses of Rh-DHPE-labeled DPPC and PIB₈₇-b-PEO₁₇ hybrid membranes at 0–40 mol % BCP. (A) Normalized fluorescence vs time plots during FRAP experiments. Near zero BCP content (black curve), low fluorescence recovery of the bleached area appears after 1 min. At 18 mol % (green curve) and 40 mol % of BCP (blue curve), the recovery has a half-time of around 4 s and is almost complete after 1 min. (B) Fluorescence intensity obtained for neat DPPC and hybrid vesicle membranes with 18 and 40 mol % BCP. (C) Fluorescence autocorrelation functions for selected compositions of 18 mol % (black) and 40 mol % BCP (red). The lateral mobility increases with increasing amount of the polymer due to the hybridized BCP molecules loosening the rigid lipid packing in the DPPC membranes, as shown in (D). Reproduced with permission from ref . Copyright 2013 Wiley-VCH.

Figure 19.

(a) CLSM images of the GHUVs composed of oligo(Asp)-b-PPO and DOPC with NBD-DOPE (green channel) and rhodamine-polymer (red channel) labeling. Scale bar is 10 μm. (b) FRET analysis indicating the presence of polymer-rich heterogeneous domains. The fluorescence spectra of the GHUVs is labeled with NBD and rhodamine (red line), the polymer vesicles are labeled with NBD and rhodamine (green line), and the hybrid vesicles are labeled with NBD (black line). (c) Absorbance of 2-nitro-5-thiobenzoate in PBS buffer at 412 nm for AchE in the presence of ATC (open circles) and AchE-loaded GHUVs in the presence of ATC (red circles). Reproduced with permission from ref . Copyright 2019 American Chemical Society.

Figure 20.

Phase diagram (surface pressure vs mixture composition) of the PMOXA₆₅-b-PDMS₁₆₅-b-PMOXA₆₅/DPPC binary monolayer system. The inset shows Brewster angle microscopy images of mixed monolayers at 25 mN/m: (A) 10 mol % polymer, (B) 20 mol %, (C) 30 mol %, (D) 40 mol %, (E) 50 mol % and 10 mN/m, (F) 10 mol % polymer, (G) 20 mol %, and (H) 30 mol %. Reproduced with permission from ref . Copyright 2009 American Chemical Society.

Figure 21.

Schematic illustration of the location of polymer-functionalized CdSe nanoparticles in mixed DPPC/PIB₈₇-b-PEO₁₇ BCP monolayers. (a) Specific location of hydrophobically modified NPs on top of the polymer domains, (b) interaction of water-soluble NPs with mixed polymer–lipid monolayers within the subphase, and (c) unspecific location of amphiphilic NPs in mixed monolayers at the air–water interface. Reproduced with permission from ref . Copyright 2012 American Chemical Society.

Figure 22.

(upper) Illustration of the formation of domains in a hybrid polymer–lipid monolayer, from low to high surface pressures (I to IV). Lipid molecules are marked in green, while amphiphilic block copolymers are in red–blue. (lower) CLSM images demonstrating protein distribution in films consisting of mixtures of PDMS₆₅-b-PMOXA₁₂ and (a) DPPC (x_DPPC = 0.75), (b) DPPC (x_DPPC = 0.5), (c) DPPE (x_DPPE = 0.25), (d) DOPC (x_DOPC = 0.25), and (e) POPE (x_POPE = 0.25). (f) PDMS₃₇-b-PMOXA₉ hybridized with DPPE (x_DPPE = 0.5). Films were transferred at a surface pressure of 35 mN/m. Scale bars are 50 μm. Reproduced with permission from ref . Copyright 2015 American Chemical Society.

Figure 23.

AFM topographical images of hybrid DPPC/PEO₄₀-b-PPO₂₇-b-PEO₄₀ monolayers supported onto a mica substrate. The film was transferred from the air–water interface at (A) π = 24 mN/m (A_DPPC = 53 Ų), (B) π = 22.1 mN/m (A_DPPC = 62 Ų), and (C,D) π = 21.8 mN/m (A_DPPC = 82 Ų). The data suggests that BCP is incorporated into the DPPC monolayer in (B–D) but not in (A). Images (C,D) were obtained from different spots in the same sample. Arrows in (C) point to three representative height levels. The hybrid film shows heterogeneity arising from the presence of lipid-rich domains and polymer-rich domains. Reproduced with permission from ref . Copyright 2009 American Chemical Society.

Figure 24.

Cryo-EM images of mesoporous silica nanoparticles (MSNP) with and without PBD₃₇-b-PEO₂₂/DOPC hybrid bilayers (10, 25, 50, 75, and 100 mol % polymer). The arrows show bilayer coatings on the MSNPs. The samples with higher polymer fraction (75 and 100 mol % polymer) display some vesicle adsorption. The scale bar is 100 nm. Reproduced with permission from ref . Copyright 2018 The Royal Society of Chemistry.

Figure 25.

(A) CLSM images acquired at different z-depths of PBD-b-PEO/DPPC hybrid membrane (1:1 molar ratio) films with NBD-DPPE probes (0.1 mol %). Binary spatial patterns remain throughout the membrane normal, indicating domain alignment across multilamellar films. Scale bars are 50 μm. (B) 2D GISAXS data obtained for hybrid membranes (left) and 1D I(q) plots obtained for hybrid, lipid, and polymer films (right) (>95% relative humidity). (C) Phase contrast AFM images overlaid onto pseudo-3D topographical images of hybrid membranes (top). Cross-sectional profiles of phase and topography along the red arrow marker (bottom). Reproduced with permission from ref . Copyright 2017 American Chemical Society.

Figure 26.

CLSM images of carboxyfluorescein dye release from GHUVs comprising PBD-b-PEO/POPC/Chol (5:3:2) (a) before and (c) after lipid enzyme phospholipase A₂ (PLA₂) treatment (0.05 mg/mL). (b) and (d) demonstrate the fluorescence intensities of Rh-DOPE and carboxyfluorescein along the dashed lines shown in (a) and (c), respectively. Upon addition of PLA₂, carboxyfluorescein dyes are released from the GHUVs when the phase separated lipid domains are simultaneously dissolved. The scale bars are 10 μm. Reproduced with permission from ref . Copyright 2012 The Royal Society of Chemistry.

Figure 27.

Schematic representation of light-induced membrane permeabilization. The irradiation at 436 nm on the GHUVs results in a permeable membrane with releasing internal contents while the UV light maintained the membrane impermeable. Reproduced with permission from ref . Copyright 2010 American Chemical Society.

Figure 28.

Graphical illustration of solid-supported polymer–lipid bilayer films integrating Cytochrome c by (A) insertion to the lipid membrane and (B) covalent conjugation using EDC/NHS coupling. Reproduced with permission from ref . Copyright 2020 American Chemical Society.