Copolymer Vesicles Research Articles

Self-immolative polymersomes for high-efficiency triggered release and programmed enzymatic reactions.

Stimuli-triggered disassembly of block copolymer vesicles or polymersomes has been conventionally achieved via solubility switching of the bilayer-forming block, requiring cooperative changes of most of the repeating units. Herein we report an alternative approach by incorporating hydrophobic blocks exhibiting stimuli-triggered head-to-tail cascade depolymerization features. Amphiphilic block copolymers bearing this motif self-assemble into self-immolative polymersomes (SIPsomes). By modular design of terminal capping moieties, visible light, UV light, and reductive milieu can be utilized to actuate SIPsomes disintegration into water-soluble small molecules and hydrophilic blocks. The design of SIPsomes allows for triggered drug co-release and controllable access toward protons, oxygen, and enzymatic substrates. We also demonstrate programmed (OR-, AND-, and XOR-type logic) enzymatic reactions by integrating SIPsome encapsulation and trigger/capping moiety-selective cascade depolymerization events.

Spherical Blackberry-type Capsules Containing Block Copolymer Aggregates

The design, preparation, and properties of nanosized blackberry-like structures are described. These capsules are composed of two layers of individual block copolymer aggregates, relatively large core vesicles onto which is deposited a layer of smaller vesicles or micelles. The composition of the adjacent layers is such as to ensure strong electrostatic interactions between them. The core vesicles are typically composed of either PS-b-P4VP with a positively charged corona or of PS-b-PAA with a negatively charged corona, and are surrounded by a layer of smaller, oppositely charged block copolymer vesicles or micelles. These composite structures bear a strong resemblance to blackberries, hence the proposed name. The blackberry structures can be prepared in solution or on a flat surface, for example, a silicon wafer. Four compositional possibilities for the blackberries structures were studied, in which the positively or negatively charged core vesicles are covered either by a layer of oppositely charged micelles or by vesicles. These structures represent the earliest stage of a layer-by-layer approach of small spherical aggregates onto a larger spherical hollow core. The strong interaction between the contacting layers is achieved by electrostatic interactions or by complementary acid-base properties, for example, H-bonding. These multicompartmented capsules could be used potentially as delivery vehicles for multiple components; each layer of the capsules could be loaded with hydrophobic (in the core of the micelles or vesicle wall) or hydrophilic molecules (in the vesicle cavity). The overall size of such structures can vary, but in any case can be kept below 1 μm.

Concurrent Block Copolymer Polymersome Stabilization and Bilayer Permeabilization by Stimuli‐Regulated “Traceless” Crosslinking

AbstractThe fabrication of block copolymer (BCP) vesicles (polymersomes) exhibiting synchronized covalent crosslinking and bilayer permeabilization remains a considerable challenge as crosslinking typically leads to compromised membrane permeability. Herein it is demonstrated how to solve this dilemma by employing a stimuli‐triggered crosslinking strategy with amphiphilic BCPs containing photolabile carbamate‐caged primary amines. Upon self‐assembling into polymersomes, light‐triggered self‐immolative decaging reactions release primary amine moieties and extensive amidation reactions then occur due to suppressed amine pKa within hydrophobic milieu. This leads to serendipitous vesicle crosslinking and the process is associated with bilayer hydrophobicity‐to‐hydrophilicity transition and membrane permeabilization.

Concurrent Block Copolymer Polymersome Stabilization and Bilayer Permeabilization by Stimuli‐Regulated “Traceless” Crosslinking

The fabrication of block copolymer (BCP) vesicles (polymersomes) exhibiting synchronized covalent crosslinking and bilayer permeabilization remains a considerable challenge as crosslinking typically leads to compromised membrane permeability. Herein it is demonstrated how to solve this dilemma by employing a stimuli-triggered crosslinking strategy with amphiphilic BCPs containing photolabile carbamate-caged primary amines. Upon self-assembling into polymersomes, light-triggered self-immolative decaging reactions release primary amine moieties and extensive amidation reactions then occur due to suppressed amine pKa within hydrophobic milieu. This leads to serendipitous vesicle crosslinking and the process is associated with bilayer hydrophobicity-to-hydrophilicity transition and membrane permeabilization.

Thermoregulated Formation and Disintegration of Cationic Block Copolymer Vesicles: Fluorescence Resonance Energy Transfer Study

Formation and disintegration of self-assembled nanostructures in response to external stimuli are important phenomena that have been widely explored for a variety of biomedical applications. In this contribution, we report the thermally triggered assembly of block copolymer molecules in aqueous solution to form vesicles (polymersomes) and their disassembly on reduction of temperature. A new thermoresponsive diblock copolymer of poly(N-isopropylacrylamide) poly((3-methacrylamidopropyl)trimethylammonium chloride) (PNIPA-b-PMAPTAC) was synthesized by reversible addition-fragmentation chain transfer technique. The solution properties and self-assembling behavior of the block copolymer molecules were studied by turbidimetry, temperature-dependent proton nuclear magnetic resonance, fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. Fluorescence resonance energy transfer studies between coumarin-153 (C-153, donor) and rhodamine 6G (R6G, acceptor) have been performed by steady-state and picosecond-resolved fluorescence spectroscopy to probe the structural and dynamic heterogeneity of the vesicles. The occurrence of efficient energy transfer was evident from the shortening of donor lifetime in the presence of the acceptor. The capability of the vesicles to encapsulate both hydrophobic and hydrophilic molecules and release them in response to decrease in temperature makes them potentially useful as drug delivery vehicles.

Temperature-Sensitive Nanoparticle-to-Vesicle Transition of ABC Triblock Copolymer Corona–Shell–Core Nanoparticles Synthesized by Seeded Dispersion RAFT Polymerization

A new kind of heterogeneous RAFT polymerization called seeded dispersion RAFT polymerization to prepare triblock copolymer corona–shell–core nanoparticles of the temperature-sensitive poly(N,N-dimethylacrylamide)-block-polystyrene-block-poly[N-(4-vinylbenzyl)-N,N-diethylamine] (PDMA-b-PS-b-PVEA) is proposed, and the reversible nanoparticle-to-vesicle transition of the triblock copolymer is investigated. Well-controlled seeded dispersion RAFT polymerization as indicated by the pseudo-first-order kinetics, the linear increase in the triblock copolymer molecular weight with the monomer conversion and the narrow molecular weight distribution of the synthesized triblock copolymer was performed, and triblock copolymer corona–shell–core nanoparticles, in which the temperature-sensitive poly[N-(4-vinylbenzyl)-N,N-diethylamine] (PVEA) block forms the core, the solvophobic polystyrene block forms the shell, and the solvophilic poly(N,N-dimethylacrylamide) block forms the corona, were prepared. When the corona–shell–core nanoparticles were dispersed in alcoholic solvent at temperature below the lower critical solution temperature (LCST) of the core-forming PVEA block, the nanoparticle-to-vesicle transition occurred, and the unsymmetrical ABC triblock copolymer vesicles were formed. The nanoparticle-to-vesicle transition is ascribed to the initial nanoparticle swelling in the alcoholic solvent and the subsequent out-migration of the core-forming PVEA block from the core layer to corona, which was confirmed by the real-time 1H NMR analysis and XPS analysis. The parameters affecting the nanoparticle-to-vesicle transition were investigated, and it was found that the nanoparticle-to-vesicle transition took place in the alcoholic solvent at temperature below LCST when the triblock copolymer containing a long core-forming PVEA block.

Formation and Structural Characteristics of Thermosensitive Multiblock Copolymer Vesicles

The spontaneous vesicle formation of ABABA-type amphiphilic multiblock copolymers bearing thermosensitive hydrophilic A-block in a selective solvent is studied using dissipative particle dynamics (DPD) approach. The formation process of vesicle through nucleation and growth pathway is observed by varying the temperature. The simulation results show that spherical micelle takes shape at high temperature. As temperature decreases, vesicles with small aqueous cavity appear and the cavity expands as well as the membrane thickness decreases with the temperature further decreasing. This finding is in agreement with the experimental observation. Furthermore, by continuously varying the temperature and the length of the hydrophobic block, a phase diagram is constructed, which can indicate the thermodynamically stable region for vesicles. The morphological phase diagram shows that vesicles can form in a larger parameter scope. The relationship between the hydrophilic and hydrophobic block length versus the aqueous cavity size and vesicle size are revealed. Simulation results demonstrate that the copolymers with shorter hydrophobic blocks length or the higher hydrophilicity are more likely to form vesicles with larger aqueous cavity size and vesicle size as well as thinner wall thickness. However, the increase in A-block length results to form vesicles with smaller aqueous cavity size and larger vesicle size.

Shear Flow Controlled Morphological Polydispersity of Amphiphilic ABA Triblock Copolymer Vesicles

Self-assembled polymeric aggregates are generally polydisperse in morphology due to the existence of many metastable states in the system. This shortcoming becomes a bottleneck for preparing high quality self-assembled polymeric materials. An important concern is the possibility of controlling morphological polydispersity through the modulation of the metastable states. In this study, both simulative and experimental results show that the metastable states can be modulated. As a typical example, the morphological polydispersity of amphiphilic ABA triblock copolymer vesicles have been successfully controlled by shear flow. A higher shear rate results in more uniform and smaller vesicles. However, if the shear rate is extremely high, small spheres and short rods can be observed. These findings not only give a deeper insight into the metastable behavior of self-assembled polymeric aggregates but also provide a new strategy for improving the uniformity of vesicles.

Protein–Polymer Supramolecular Assemblies: A Key Combination for Multifunctionality

2D/3D structures resulting from self-assembly of amphiphilic block copolymers can be combined with bioactive compounds, such as proteins and enzymes, to create supramolecular assemblies with specific desired properties and functionality. Chemical tuning of the architecture and properties of supramolecular assemblies to accommodate sensitive biomolecules allows the development of new soft hybrid materials that benefit from the robustness of polymers and from the functionality of biomolecules. The encapsulation/insertion of biomolecules (enzymes, mimics, proteins) in self-assembling block copolymer vesicles enables design of 'nanoreactors' both in solutions and at surfaces for highly diverse applications, ranging from production of antibiotics to creation of artificial organelles. When membrane proteins are inserted into polymer membranes, it is possible to generate functional membranes or active surfaces with a rapid and specific response. In addition, the selective binding of ligand-terminated polymers holds potential for targeted delivery of drugs, or for immobilization on solid support, to provide functional 3D assemblies on an extended surface.

Stimuli-Responsive Poly(caprolactone) Vesicles for Dual Drug Delivery under the Gastrointestinal Tract

We report the first example of carboxylic functionalized poly(caprolactone) (PCL) block copolymer vesicles as a novel dual drug delivery pH responsive vehicle for oral administration under the gastrointestinal (GI) tract. A new carboxylic functionalized caprolactone monomer was custom designed through multistep organic reactions and polymerized under controlled ROP using polyethylene glycol (PEG-2000) to produce amphiphilic diblocks, PEG-b-CPCLx, with x = 25, 50, 75, and 100. These carboxylic PCL block copolymers were self-organized into 100-250 nm vesicular assemblies in water. The size and shape of the vesicular assemblies were confirmed by light scattering, zeta potential, and electron microscopes. These vesicles were capable of loading both hydrophilic molecules (Rhodamine B, Rh-B) and hydrophobic drugs such as ibuprofen (IBU) and camptothecin (CPT) in the core and layer, respectively. These pH-responsive PCL vesicles were stable in strong acidic conditions (pH < 2.0, stomach) and ruptured to release the loaded cargoes under neutral or basic pH (7.0 ≤ pH, similar to that of small intestine). The drug release kinetics under simulated GI tract revealed that the individual drug loaded vesicles followed the combination of diffusion and erosion pathway, whereas the dual drug loaded vesicles predominantly followed the diffusion controlled process. Thus, the custom designed PCL vesicles open up new area of pH stimuli responsive polymer vehicles for delivering multiple drugs in oral drug delivery which are yet to be explored for biomedical applications.

Nile Blue-Based Nanosized pH Sensors for Simultaneous Far-Red and Near-Infrared Live Bioimaging

Diblock copolymer vesicles are tagged with pH-responsive Nile Blue-based labels and used as a new type of pH-responsive colorimetric/fluorescent biosensor for far-red and near-infrared imaging of live cells. The diblock copolymer vesicles described herein are based on poly(2-(methacryloyloxy)ethyl phosphorylcholine-block-2-(diisopropylamino)ethyl methacrylate) [PMPC-PDPA]: the biomimetic PMPC block is known to facilitate rapid cell uptake for a wide range of cell lines, while the PDPA block constitutes the pH-responsive component that enables facile vesicle self-assembly in aqueous solution. These biocompatible vesicles can be utilized to detect interstitial hypoxic/acidic regions in a tumor model via a pH-dependent colorimetric shift. In addition, they are also useful for selective intracellular staining of lysosomes and early endosomes via subtle changes in fluorescence emission. Such nanoparticles combine efficient cellular uptake with a pH-responsive Nile Blue dye label to produce a highly versatile dual capability probe. This is in marked contrast to small molecule dyes, which are usually poorly uptaken by cells, frequently exhibit cytotoxicity, and are characterized by intracellular distributions invariably dictated by their hydrophilic/hydrophobic balance.

Hybrid, Nanoscale Phospholipid/Block Copolymer Vesicles

Hybrid phospholipid/block copolymer vesicles, in which the polymeric membrane is blended with phospholipids, display interesting self-assembly behavior, incorporating the robustness and chemical versatility of polymersomes with the softness and biocompatibility of liposomes. Such structures can be conveniently characterized by preparing giant unilamellar vesicles (GUVs) via electroformation. Here, we are interested in exploring the self-assembly and properties of the analogous nanoscale hybrid vesicles (ca. 100 nm in diameter) of the same composition prepared by film-hydration and extrusion. We show that the self-assembly and content-release behavior of nanoscale polybutadiene-b-poly(ethylene oxide) (PB-PEO)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) hybrid phospholipid/block copolymer vesicles can be tuned by the mixing ratio of the amphiphiles. In brief, these hybrids may provide alternative tools for drug delivery purposes and molecular imaging/sensing applications and clearly open up new avenues for further investigation.

Controlled Release Characteristics of PLA-Pluronic-PLA Nano-Sized Vesicles&lt;i&gt; In Vitro&lt;/i&gt;

In the present study, controlled release characteristics of new nanosized PLA-Pluronic-PLA block copolymer vesicles comprising of amphiphilic poly (lactic acid) (PLA) and Pluronic block copolymers (PEO-PPO-PEO) have been evaluated as an oral insulin carrier. The mean size of vesicles was 78 nm for PLA-F127-PLA and 165 nm for PLA-P85-PLA copolymer. The mean insulin entrapment efficiency was 59.6% for PLA-P85-PLA and 26.4% for PLA-F127-PLA. The in vitro release characteristics of insulin from vesicles exhibited an initial burst in the range of pH 1.2-7.4 dissolution mediums. The presence of PLA-Pluronic-PLA vesicles improved the stability of insulin in the gastrointestinal fluids than that of the phosphate buffer solution (PBS) of insulin. More importantly, the released insulin from the vesicles maintained their biological activity. The results from this studies demonstrated that biodegradable PLA-Pluronic-PLA can self-assemble with insulin, form insulin-encapsulated vesicles, and is good carrier materials for oral insulin/protein delivery.

Polymersome‐mediated intracellular delivery of antibiotics to treat Porphyromonas gingivalis ‐infected oral epithelial cells

The gram-negative anaerobe Porphyromonas gingivalis colonizes the gingival crevice and is etiologically associated with periodontal disease that can lead to alveolar bone damage and resorption, promoting tooth loss. Although susceptible to antibiotics, P. gingivalis can evade antibiotic killing by residing within gingival keratinocytes. This provides a reservoir of organisms that may recolonize the gingival crevice once antibiotic therapy is complete. Polymersomes are nanosized amphiphilic block copolymer vesicles that can encapsulate drugs. Cells internalize polymersomes by endocytosis into early endosomes, where they are disassembled by the low pH, causing intracellular release of their drug load. In this study, polymersomes were used as vehicles to deliver antibiotics in an attempt to kill intracellular P. gingivalis within monolayers of keratinocytes and organotypic oral mucosal models. Polymersome-encapsulated metronidazole or doxycycline, free metronidazole, or doxycycline, or polymersomes alone as controls, were used, and the number of surviving intracellular P. gingivalis was quantified after host cell lysis. Polymersome-encapsulated metronidazole or doxycycline significantly (P<0.05) reduced the number of intracellular P. gingivalis in both monolayer and organotypic cultures compared to free antibiotic or polymersome alone controls. Polymersomes are effective delivery vehicles for antibiotics that do not normally gain entry to host cells. This approach could be used to treat recurrent periodontitis or other diseases caused by intracellular-dwelling organisms.

Size Selective Incorporation of Gold Nanoparticles in Diblock Copolymer Vesicle Wall

A systematic study is conducted to reveal how far the polymeric vesicle wall can embed gold nanoparticles (AuNPs) with different sizes by combining experiments and self-consistent field simulations. Both the experimental and simulative results indicate that the location of AuNPs in vesicle wall or in spherical micelle is heavily size dependent. Whether the AuNPs enter the vesicle wall or not is determined by a ratio of the diameter of AuNPs (D0) to the thickness of the vesicle wall (d(w0)). The 1-dodecanethiol-coated AuNPs (Au(x)R) with D0/d(w0) < 0.3 will stably disperse in the vesicle walls. For polystyrene-coated AuNPs (Au(x)S), a criterion of D0/d(w0) is proposed based on the phase diagram; i.e., the Au(x)S with D0/d(w0) < 0.5 can be located in the vesicle wall. Otherwise, the Au(x)R and the Au(x)S prefer to locate in spherical micelles. Moreover, the contributions of enthalpy and entropy to the total free energy of the system are respectively calculated to reveal the mechanism of the size selective distribution of AuNPs. The results demonstrate that the escape of AuNPs from vesicle walls and their selective distribution in spherical micelles is an entropy-driven process. Our study provides an important guideline for fabricating nanoparticle/block copolymer hybrid vesicles in dilute solution.

Photoreaction of a Hydroxyalkyphenone with the Membrane of Polymersomes: A Versatile Method To Generate Semipermeable Nanoreactors

Block copolymer vesicles can be turned into nanoreactors when a catalyst is encapsulated in these hollow nanostructures. However the membranes of these polymersomes are most often impermeable to small organic molecules, while applications as nanoreactor, as artificial organelles, or as drug-delivery devices require an exchange of substances between the outside and the inside of polymersomes. Here, a simple and versatile method is presented to render polymersomes semipermeable. It does not require complex membrane proteins or pose requirements on the chemical nature of the polymers. Vesicles made from three different amphiphilic block copolymers (α,ω-hydroxy-end-capped poly(2-methyl-2-oxazoline)-block-poly(dimethylsiloxane)-block-poly(2-methyl-2-oxazoline) (PMOXA-b-PDMS-b-PMOXA), α,ω-acrylate-end-capped PMOXA-b-PDMS-b-PMOXA, and poly(ethylene oxide)-block-poly(butadiene) (PEO-b-PB)) were reacted with externally added 2-hydroxy-4'-2-(hydroxyethoxy)-2-methylpropiophenone under UV-irradiation. The photoreactive compound incorporated into the block copolymer membranes independently of their chemical nature or the presence of double bonds. This treatment of polymersomes resulted in substantial increase in permeability for organic compounds while not disturbing the size and the shape of the vesicles. Permeability was assessed by encapsulating horseradish peroxidase into vesicles and measuring the accessibility of substrates to the enzyme. The permeability of photoreacted polymersomes for ABTS, AEC, pyrogallol, and TMB was determined to be between 1.9 and 38.2 nm s(-1). It correlated with the hydrophobicity of the compounds. Moreover, fluorescent dyes were released at higher rates from permeabilized polymersomes compared to unmodified ones. The permeabilized nanoreactors retained their ability to protect encapsulated biocatalysts from degradation by proteases.

Amphiphilic Block Copolymer Vesicles for Active Wound Dressings: Synthesis of Model Systems and Studies of Encapsulation and Release

SummaryPolystyrene‐block‐poly(acrylic acid) (PS‐b‐PAA) diblock copolymers were synthesized via atom transfer radical polymerization (ATRP). Self‐assembly of these block copolymers into polymeric vesicles, so‐called polymersomes, was performed by addition of water to a solution of the block copolymers in tetrahydrofuran (THF). Various antimicrobial compounds and dyes were incorporated in the polymersomes and in addition to the corresponding size distributions, as determined by dynamic light scattering (DLS), the maximum attainable loading concentrations were determined. Using the release of incorporated calcein, which is a pH responsive dye, the principle of a light up probe approach for advanced in situ diagnostics was demonstrated. The release from PS‐b‐PAA vesicles via a vesicle‐to‐micelle transition was triggered by heating a vesicle suspension after addition of the plasticizer THF to 50 °C. Furthermore, a biocompatible and degradable block copolymer system, which contains degradable ester bonds in the main chain, was synthesized and investigated. In particular, poly(ethylene glycol)‐block‐polycaprolactone (PEG‐b‐PCL) copolymers were synthesized via ring‐opening polymerization (ROP). These copolymers were shown to form polymersomes as well and represent a promising family of degradable amphiphilic block copolymers for active wound dressings and related applications.

Photo/pH dual-responsive behavior of azopyridine-containing copolymer vesicles

The amphiphilic diblock copolymer composed of poly(ethylene oxide) as the hydrophilic chain and polymethacrylate with azopyridine moieties in the side groups as the hydrophobic chain self-assembled to vesicles in the mixture of THF/water. The dual photo- and pH-responsive behavior of the copolymeric vesicles was investigated by UV–Vis absorption spectra and optical microscope. It was found that the trans–cis photoisomerization of azopyridine in the vesicles was influenced by the water content in the mixture and the intensity of light irradiation. Regard to the pH-responsive behavior, the copolymer vesicles dissociated into solution gradually triggered by acid, while they reformed in a basic environment. Furthermore, the UV light irradiation accelerated the process of pH-induced dissociation of the vesicles.

Morphology transformation of polystyrene-block-poly(ethylene oxide) vesicle on surface

Morphology transformation of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) vesicle on solid surface induced by vapor annealing was investigated. The PS-b-PEO diblock copolymer vesicles, prepared by dissolving in a co-solvent and adding a selective solvent, were transferred to silicon wafer surface by spin cast and then incubated in three different vapors, tetrahydrofuran (THF), dioxane (DIOX), and N, N-dimethylformamide (DMF) separately. In THF or DIOX, the vesicular structures quickly fuse together into a droplet which then spreads on the surface. While in DMF vapor, vesicle-cylinder-sphere transformation is present. The vesicle morphology transformation in DMF vapor is very sensitive to water content. A small amount of water in DMF would slow down the transformation rate, and water content would affect the cylinder packing.

Superhydrophobic Hierarchically Assembled Films of Diblock Copolymer Hollow Nanospheres and Nanotubes

Reported are the formation of rough particulate films from cross-linked diblock copolymer vesicles and nanotubes and the wetting properties of the resultant films. The diblock copolymers used were F66M200 and F95A135, where the subscripts denote the repeat unit numbers, whereas M, A, and F denote poly(2-cinnamoyloxyethyl methacrylate), poly(2-cinnamoyloxyethyl acrylate), and poly(2,2,2-trifluoroethyl methacrylate), respectively. The precursory polymers to F66M200 and F95A135 were prepared by atom transfer radical polymerization. In 2,2,2-trifluoroethyl methacrylate (FEMA), a selective solvent for F, vesicles and tubular micelles were prepared from F66M200 and F95A135, respectively. Photo-cross-linking the M and A blocks of these aggregates yielded hollow nanospheres and nanotubes bearing F coronal chains. These particles were dispersed into CH2Cl2/methanol, where CH2Cl2 was a good solvent for both blocks and methanol was a poor solvent for F. Casting CH2Cl2/methanol dispersions of these particles yielded films consisting of hierarchically assembled diblock copolymer nanoparticles. For example, the hollow nanospheres fused into microspheres bearing nanobumps after being cast from CH2Cl2/methanol at methanol volume fractions of 30 and 50%. The roughness of these films increased as the methanol volume fraction increased. The films that were cast at high methanol contents were superhydrophobic, possessing water contact angles of ∼160° and water sliding angles of ∼3°.