Photoinitiated destruction of composite porphyrin-protein polymersomes

Shape Transformation of Polymer Vesicles

Life activities, such as respiration, are accomplished through the continuous shape modulation of cells, tissues, and organs. Developing smart materials with shape-morphing capability is a pivotal step toward life-like systems and emerging technologies of wearable electronics, soft robotics, and biomimetic actuators. Drawing inspiration from cells, smart vesicular systems have been assembled to mimic the biological shape modulation. This would enable the understanding of cellular shape adaptation and guide the design of smart materials with shape-morphing capability. Polymer vesicles assembled by amphiphilic molecules are an example of remarkable vesicular systems. The chemical versatility, physical stability, and surface functionality promise their application in nanomedicine, nanoreactor, and biomimetic systems. However, it is difficult to drive polymer vesicles away from equilibrium to induce shape transformation due to the unfavorable energy landscapes caused by the low mobility of polymer chains and low permeability of the vesicular membrane. Extensive studies in the past decades have developed various methods including dialysis, chemical addition, temperature variation, polymerization, gas exchange, etc., to drive shape transformation. Polymer vesicles can now be engineered into a variety of nonspherical shapes. Despite the brilliant progress, most of the current studies regarding the shape transformation of polymer vesicles still lie in the trial-and-error stage. It is a grand challenge to predict and program the shape transformations of polymer vesicles. An in-depth understanding of the deformation pathway of polymer vesicles would facilitate the transition from the trial-and-error stage to the computing stage. In this Account, we introduce recent progress in the shape transformation of polymer vesicles. To provide an insightful analysis, the shape transformation of polymer vesicles is divided into basic and coupled deformation. First, we discuss the basic deformation of polymer vesicles with a focus on two deformation pathways: the oblate pathway and the prolate pathway. Strategies used to trigger different deformation pathways are introduced. Second, we discuss the origin of the selectivity of two deformation pathways and the strategies used to control the selectivity. Third, we discuss the coupled deformation of polymer vesicles with a focus on the switch and coupling of two basic deformation pathways. Last, we analyze the challenges and opportunities in the shape transformation of polymer vesicles. We envision that a systematic understanding of the deformation pathway would push the shape transformation of polymer vesicles from the trial-and-error stage to the computing stage. This would enable the prediction of deformation behaviors of nanoparticles in complex environments, like blood and interstitial tissue, and access to advanced architecture desirable for man-made applications.

Periodic Photobleaching with Structured Illumination for Diffusion Imaging

The use of periodically structured illumination coupled with spatial Fourier-transform fluorescence recovery after photobleaching (FT-FRAP) was shown to support diffusivity mapping within segmented domains of arbitrary shape. Periodic "comb-bleach" patterning of the excitation beam during photobleaching encoded spatial maps of diffusion onto harmonic peaks in the spatial Fourier transform. Diffusion manifests as a simple exponential decay of a given harmonic, improving the signal to noise ratio and simplifying mathematical analysis. Image segmentation prior to Fourier transformation was shown to support pooling for signal to noise enhancement for regions of arbitrary shape expected to exhibit similar diffusivity within a domain. Following proof-of-concept analyses based on simulations with known ground-truth maps, diffusion imaging by FT-FRAP was used to map spatially-resolved diffusion differences within phase-separated domains of model amorphous solid dispersion spin-cast thin films. Notably, multi-harmonic analysis by FT-FRAP was able to definitively discriminate and quantify the roles of internal diffusion and exchange to higher mobility interfacial layers in modeling the recovery kinetics within thin amorphous/amorphous phase-separated domains, with interfacial diffusion playing a critical role in recovery. These results have direct implications for the design of amorphous systems for stable storage and efficacious delivery of therapeutic molecules.

Formation of giant polymer vesicles by simple double emulsification using block copolymers as the sole surfactant

Polymer vesicles that mimic the function of cell membranes can be obtained through the self-assembly of amphiphilic block copolymers. The cell-like characteristics of polymer vesicles, such as the core-shell structure, semi-permeability and tunable surface chemistry make them excellent building blocks for artificial cells. However, the standard preparation methods for polymer vesicles can be time consuming, require special equipment, or have low encapsulation efficiency for large components, such as nanomaterials and proteins. Here, we introduce a new encapsulation strategy based on a simple double emulsification (SDE) approach which allows giant polymer vesicles to be formed in a short time and with basic laboratory equipment. The SDE method requires a single low molecular weight block copolymer that has the dual role of macromolecular surfactant and membrane building block. Giant polymer vesicles with diameters between 20-50 μm were produced, which allowed proteins and nanoparticles to be encapsulated. To demonstrate its practical application, we used the SDE method to assemble a simple artificial cell that mimics a two-step enzymatic cascade reaction. The SDE method described here introduces a new tool for simple and rapid fabrication of synthetic compartments.

Polymersome Poration and Rupture Mediated by Plasmonic Nanoparticles in Response to Single-Pulse Irradiation

The self-assembly of amphiphilic diblock copolymers into polymeric vesicles, commonly known as polymersomes, results in a versatile system for a variety of applications including drug delivery and microreactors. In this study, we show that the incorporation of hydrophobic plasmonic nanoparticles within the polymersome membrane facilitates light-stimulated release of vesicle encapsulants. This work seeks to achieve tunable, triggered release with non-invasive, spatiotemporal control using single-pulse irradiation. Gold nanoparticles (AuNPs) are incorporated as photosensitizers into the hydrophobic membrane of micron-scale polymersomes and the cargo release profile is controlled by varying the pulse energy and nanoparticle concentration. We have demonstrated the ability to achieve immediate vesicle rupture as well as vesicle poration resulting in temporal cargo diffusion. Additionally, changing the pulse duration, from femtosecond to nanosecond, provides mechanistic insight into the photothermal and photomechanical contributors that govern membrane disruption in this polymer-nanoparticle hybrid system.

Temperature-Dependent Dynamics of Giant Vesicles Composed of Hydrolysable Lipids Having an Amide Linkage

Various amphiphiles including surfactants and lipids have been designed and synthesized to improve and create new functionalities. In particular, the emergence of cell-like behaviors of giant vesicles (GVs) composed of synthetic lipids has drawn much attention in the development of chemical models for cells. The aim of this study was to measure temperature-dependent morphological changes of GVs induced by fragmentation and subsequent growth using hydrolysable cationic lipids having an amide linkage. Results from differential scanning calorimetry, fluorescence spectroscopy using an environment-responsive probe, and confocal Raman microscopy showed that the dynamics observed were due to changes in the vesicle membrane, including variation in the lipid composition, induced by thermal stimulation.

Photo-triggered polymer nanomedicines: From molecular mechanisms to therapeutic applications

The use of nanotechnology to improve treatment efficacy and reduce side effects is central to nanomedicine. In this context, stimuli-responsive drug delivery systems (DDS) such as chemical/physical gels or nanoparticles such as polymersomes, micelles or nanogels are particularly promising and are the focus of this review. Several stimuli have been considered but light as an exogenous trigger presents many advantages that are pertinent for clinical applications such as high spatial and temporal control and low cost. Underlying mechanisms required for the release of therapeutic agents in vitro and in vivo range from the molecular scale, namely photoisomerization, hydrophobicity photoswitching, photocleavage or heat generation via nanoheaters, through to the macromolecular scale. As well as these approaches, DDS destabilization, DDS permeation pore unblocking and formation are discussed.

Ruthenium-Crosslinked Hydrogels with Rapid, Visible-Light Degradation

Incorporation of photoresponsive molecules within soft materials can provide spatiotemporal control over bulk properties and address challenges in targeted delivery and mechanical variability. However, the kinetics of in situ photochemical reactions are often slow and typically employ ultraviolet wavelengths. Here, we present a novel photoactive crosslinker Ru(bipyridine)2 (3-pyridinaldehyde)2 (RuAldehyde), which was reacted with hydrazide-functionalized hyaluronic acid to form hydrogels capable of encapsulating protein cargo. Visible light irradiation (400-500 nm) initiated rapid ligand exchange on the ruthenium center, which degraded the hydrogel within seconds to minutes, depending on gel thickness. An exemplar enzyme cargo, TEM1 β-lactamase, was loaded into and photoreleased from the Ru-hydrogel. To expand their applications, Ru-hydrogels were also processed into microgels using a microfluidic platform.

A simple method for the synthesis of porous polymeric vesicles and their application as MR contrast agents

Because of their low membrane permeability the use of polymeric vesicles in certain drug delivery and molecular imaging applications and as bioreactors is less than ideal. Here, we report a simple method to prepare porous polymeric vesicles that possess high membrane permeability. Specifically, porous vesicles were produced from the aqueous assembly of the diblock copolymer PEG-PBD, and the triblock copolymer PEG-PPO-PEG. It was found that PEG-PPO-PEG-doped polymersomes exhibited improved membrane permeability to molecules less than 5 kDa. Further, these porous vesicles retained molecules ≥10 kDa within their aqueous interiors with no significant leakage. To demonstrate its application, highly efficient magnetic resonance contrast agents were produced from porous polymersomes by encapsulating macromolecules labeled with gadolinium. Due to a fast water exchange rate with surrounding bulk water, these paramagnetic porous polymersomes exhibited higher r₁ relaxivity compared with Gd-encapsulated vesicles with no pores. Due to their simplicity, the porous polymersomes prepared with this method are expected to have additional useful applications.

Trigger chemistries for better industrial formulations

In recent years, innovations and consumer demands have led to increasingly complex liquid formulations. These growing complexities have provided industrial players and their customers access to new markets through product differentiation, improved performance, and compatibility/stability with other products. One strategy for enabling more complex formulations is the use of active encapsulation. When encapsulation is employed, strategies are required to effect the release of the active at the desired location and time of action. One particular route that has received significant academic research effort is the employment of triggers to induce active release upon a specific stimulus, though little has translated for industrial use to date. To address emerging industrial formulation needs, in this review, we discuss areas of trigger release chemistries and their applications specifically as relevant to industrial use. We focus the discussion on the use of heat, light, shear, and pH triggers as applied in several model polymeric systems for inducing active release. The goal is that through this review trends will emerge for how technologies can be better developed to maximize their value through industrial adaptation.

Caging metal ions with visible light-responsive nanopolymersomes

Polymersomes are bilayer vesicles that self-assemble from amphiphilic diblock copolymers, and provide an attractive system for the delivery of biological and nonbiological molecules due to their environmental compatibility, mechanical stability, synthetic tunability, large aqueous core, and hyperthick hydrophobic membrane. Herein, we report a nanoscale photoresponsive polymersome system featuring a meso-to-meso ethyne-bridged bis[(porphinato)zinc] (PZn2) fluorophore hydrophobic membrane solute and dextran in the aqueous core. Upon 488 nm irradiation in solution or in microinjected zebrafish embryos, the polymersomes underwent deformation, as monitored by a characteristic red-shifted PZn2 emission spectrum and confirmed by cryo-TEM. The versatility of this system was demonstrated through the encapsulation and photorelease of a fluorophore (FITC), as well as two different metal ions, Zn(2+) and Ca(2+).

Doubly responsive polymersomes towards monosaccharides and temperature under physiologically relevant conditions

A new class of doubly-responsive block copolymers could be utilized as new delivery vehicles for cargo molecules such as insulin.

Micrometer-size vesicle formation triggered by UV light

Vesicle formation is a fundamental kinetic process related to the vesicle budding and endocytosis in a cell. In the vesicle formation by artificial means, transformation of lamellar lipid aggregates into spherical architectures is a key process and known to be prompted by e.g. heat, infrared irradiation, and alternating electric field induction. Here we report UV-light-driven formation of vesicles from particles consisting of crumpled phospholipid multilayer membranes involving a photoactive amphiphilic compound composed of 1,4-bis(4-phenylethynyl)benzene (BPEB) units. The particles can readily be prepared from a mixture of these components, which is casted on the glass surface followed by addition of water under ultrasonic radiation. Interestingly, upon irradiation with UV light, micrometer-size vesicles were generated from the particles. Neither infrared light irradiation nor heating prompted the vesicle formation. Taking advantage of the benefits of light, we successfully demonstrated micrometer-scale spatiotemporal control of single vesicle formation. It is also revealed that the BPEB units in the amphiphile are essential for this phenomenon.

Polymersomes as an effective drug delivery system for glioma--a review

Glioma is one of the most commonly occurring malignant brain tumours which need proper treatment strategy. The current therapies for treating glioma like surgical resection, radiotherapy, and chemotherapy have failed in achieving satisfactory results and this forms a rationale for the development of novel drug delivery systems. Among them, polymersomes are superior novel carriers with diverse functions like enhanced stability, low permeability, tunable membrane properties, surface functionality, and long blood circulation time which make them suitable for cancer therapy. These are bilayered vesicles capable of encapsulating both hydrophilic and hydrophobic drugs used to target glioma effectively. In this review, we have discussed on general preparation, characterization, and targeting aspects of surface modified polymersomes for effective delivery of therapeutic agents to glioma.

Progress toward synthetic cells

The complexity of even the simplest known life forms makes efforts to synthesize living cells from inanimate components seem like a daunting task. However, recent progress toward the creation of synthetic cells, ranging from simple protocells to artificial cells approaching the complexity of bacteria, suggests that the synthesis of life is now a realistic goal. Protocell research, fueled by advances in the biophysics of primitive membranes and the chemistry of nucleic acid replication, is providing new insights into the origin of cellular life. Parallel efforts to construct more complex artificial cells, incorporating translational machinery and protein enzymes, are providing information about the requirements for protein-based life. We discuss recent advances and remaining challenges in the synthesis of artificial cells, the possibility of creating new forms of life distinct from existing biology, and the promise of this research for gaining a deeper understanding of the nature of living systems.

Visible-light-induced disruption of diselenide-containing layer-by-layer films: toward combination of chemotherapy and photodynamic therapy

A photoresponsive polyelectrolyte multilayer film containing a diselenide functional group is fabricated using an unconventional layer-by-layer method. The polycation backbone is constructed through copolymerization of di-(1-hydroxylundecyl) diselenide and 1,4-bis(2-hydroxyethyl)piperazine with 2,4-diisocyanatotoluene. A common polyanion poly(styrene sulfonate) is selected as the polyanion. The obtained film can be gradually disrupted under the irradiation of mild visible light, and this process can be monitored with UV-vis spectroscopy. The residue of the film is estimated to be 17% after 5 h of irradiation. The intensity of the visible light can be as low as 50 mW cm⁻², which is even weaker than the sunlight. The cytotoxicity of the building blocks is evaluated in MTT assays using human hepatic cell line (L-02), and the results are satisfactory. Further tests show that cells can grow in a regular manner on this film, indicating good biocompatibility. In addition, the film can be used to achieve cargo loading and controlled release. Considering that light can not only trigger controlled release but also act as part of the therapy itself (photodynamic therapy), this system shows hope for further development into a platform for the combination of chemotherapy and photodynamic therapy, especially for applications concerning skin.

Exploiting endocytosis for nanomedicines

In this article, we briefly review the endocytic pathways used by cells, pointing out their defining characteristics and highlighting physical limitations that may direct the internalization of nanoparticles to a subset of these pathways. A more detailed description of these pathways is presented in the literature. We then focus on the endocytosis of nanomedicines and present how various nanomaterial parameters impact these endocytic processes. This topic is an area of active research, motivated by the recognition that an improved understanding of how nanomaterials interact at the molecular, cellular, and whole-organism level will lead to the design of better nanomedicines in the future. Next, we briefly review some of the important nanomedicines already on the market or in clinical development that serve to exemplify how endocytosis can be exploited for medical benefit. Finally, we present some key unanswered questions and remaining challenges to be addressed by the field.

Polymer-based stimuli-responsive nanosystems for biomedical applications

The application of organic polymers and inorganic/organic hybrid systems in numerous fields of biotechnology has seen a considerable growth in recent years. Typically, organic polymers with diverse structures, compositional variations and differing molecular weights have been utilized to assemble polymeric nanosystems such as polymeric micelles, polymersomes, and nanohydrogels with unique features and structural properties. The architecture of these polymeric nanosystems involves the use of both hydrophobic and hydrophilic polymeric blocks, making them suitable as vehicles for diagnostic and therapeutic applications. Recently, "smart" or "intelligent" polymers have attracted significant attention in the biomedical field wherein careful introduction of specific polymeric modalities changes a banal polymeric nanosystem to an advanced stimuli-responsive nanosystem capable of performing extraordinary functions in response to an internal or external trigger such as pH, temperature, redox, enzymes, light, magnetic, or ultrasound. Further, incorporation of inorganic nanoparticles such as gold, silica, or iron oxide with surface-bound stimuli-responsive polymers offers additional advantages and multifunctionality in the field of nanomedicine. This review covers the physical properties and applications of both organic and organic/inorganic hybrid nanosystems with specific recent breakthroughs in drug delivery, imaging, tissue engineering, and separations and provides a brief discussion on the future direction.

Biodegradable Polymersomes for the Delivery of Gemcitabine to Panc-1 Cells

Traditional anticancer chemotherapy often displays toxic side effects, poor bioavailability, and a low therapeutic index. Targeting and controlled release of a chemotherapeutic agent can increase drug bioavailability, mitigate undesirable side effects, and increase the therapeutic index. Here we report a polymersome-based system to deliver gemcitabine to Panc-1 cells in vitro. The polymersomes were self-assembled from a biocompatible and completely biodegradable polymer, poly(ethylene oxide)-poly(caprolactone), PEO-PCL. We showed that we can encapsulate gemcitabine within stable 200 nm vesicles with a 10% loading efficiency. These vesicles displayed a controlled release of gemcitabine with 60% release after 2 days at physiological pH. Upon treatment of Panc-1 cells in vitro, vesicles were internalized as verified with fluorescently labeled polymersomes. Clonogenic assays to determine cell survival were performed by treating Panc-1 cells with varying concentrations of unencapsulated gemcitabine (FreeGem) and polymersome-encapsulated gemcitabine (PolyGem) for 48 hours. 1 μM PolyGem was equivalent in tumor cell toxicity to 1 μM FreeGem, with a one log cell kill observed. These studies suggest that further investigation on polymersome-based drug formulations is warranted for chemotherapy of pancreatic cancer.

Precision intracellular delivery based on optofluidic polymersome rupture

We present an optical approach for intracellular delivery of molecules contained within oxidation-sensitive polymersomes. The photosensitizer ethyl eosin is associated with the polymersome membrane to oxidatively increase the hydrophilicity of the hydrophobic block under optical excitation. This optofluidic interaction induces rapid polymersome rupture and payload release via the reorganization of the aggregate structure into smaller diameter vesicles and micelles. When the particles are endocytosed by phagocytes, such as RAW macrophages and dendritic cells, the polymersomes' payload escapes the endosome and is released in the cell cytosol within a few seconds of illumination. The released payload is rapidly distributed throughout the cytosol within milliseconds. The presented optofluidic method enables fast delivery and distribution throughout the cytosol of individual cells, comparable to photochemical internalization, but a factor of 100 faster than similar carrier mediated delivery methods (e.g., liposomes, polymersomes, or nanoparticles). Due to the ability to simultaneously induce payload delivery and endosomal escape, this approach can find applications in detailed characterizations of intra- and intercellular processes. As an example in quantitative cell biology, a peptide antigen was delivered in dendritic cells and MHC I presentation kinetics were measured at the single cell and single complex level.

Towards an artificial cell

We are on the verge of producing "synthetic cells," or protocells, in which some, many or all of the tasks of a real biological cell are harnessed into a synthetic platform. Such advances are made possible through genetic engineering, microfabrication technologies, and the development of cellular membranes from new surfactants that extend beyond phospholipids in stability and chemical control, and can be used to introduce designer functionality into membranes and cells. We review some of the recent advances in the development of synthetic cells and suggest future exciting directions.

Biodegradable polymersomes with an ionizable membrane: facile preparation, superior protein loading, and endosomal pH-responsive protein release

Novel biodegradable polymersomes containing an ionizable membrane were developed for efficient loading and rapid intracellular release of proteins. The polymersomes were prepared from poly(ethylene glycol)-b-poly(trimethylene carbonate) (PEG-PTMC) block copolymer derivatives containing acrylate, carboxylic acid, and amine groups along PTMC block, which are denoted as PEG-PTMC(AC), PEG-PTMC(COOH), and PEG-PTMC(NH(2)), respectively. Notably, nano-sized polymersomes (95.1-111.6nm) were formed by directly dispersing these copolymers in phosphate buffer at room temperature. Both FITC-labeled bovine serum albumin (FITC-BSA) and cytochrome C (FITC-CC) were readily loaded into PEG-PTMC(COOH) and PEG-PTMC(NH(2)) polymersomes with remarkably high loading levels. Interestingly, in vitro release studies showed that PEG-PTMC(COOH) and PEG-PTMC(NH(2)) polymersomes had pH-responsive protein release behaviors in which significantly faster protein release was observed at endosomal pH than at physiological pH. MTT assays indicated that these polymersomes had low cytotoxicity. Furthermore, confocal laser scanning microscope (CLSM) observations revealed that FITC-CC loaded polymersomes efficiently delivered proteins into MCF-7 cells following 24h incubation. Importantly, flow cytometry showed that CC-loaded polymersomes induced markedly enhanced apoptosis in MCF-7 cells as compared to free CC. These novel membrane ionizable biodegradable polymersomes have appeared as highly promising nanocarriers for efficient intracellular protein delivery.

Endocytosis at the nanoscale

Endocytosis is a fundamental process in which eukaryotic cells internalise molecules and macromolecules via deformation of the membrane and generation of membrane-bound carriers. Functional aspects are not only limited to uptake of nutrients, but also play a primary role in evolutionary conserved processes such as the regulation of plasma membrane protein activity (i.e. signal-transducing receptors, small-molecule transporters and ion channels), cell motility and mitosis. The macromolecular nature of the material transported by endocytosis makes this route one of the most important targets for nanomedicine. Indeed, many nanoparticle formulations have been customised to enter cells through endocytosis and deliver the cargo within the cell. In this critical review, we present an overview of the biology of endocytosis and discuss its implications in cell internalisation of nanoparticles. We discuss how nanoparticle size, shape and surface chemistry can control this process effectively. Finally, we discuss different drug delivery strategies on how to evade lysosomal degradation to promote effective release of the cargo (376 references).

Improved tumor targeting of polymer-based nanovesicles using polymer-lipid blends

Block copolymer-based vesicles have recently garnered a great deal of interest as nanoplatforms for drug delivery and molecular imaging applications due to their unique structural properties. These nanovesicles have been shown to direct their cargo to disease sites either through enhanced permeability and retention or even more efficiently via active targeting. Here, we show that the efficacy of nanovesicle targeting can be significantly improved when prepared from polymer-lipid blends compared with block copolymer alone. Polymer-lipid hybrid nanovesicles were produced from the aqueous coassembly of the diblock copolymer, poly(ethylene oxide)-block-polybutadiene (PEO-PBD), and the phospholipid, hydrogenated soy phosphatidylcholine (HSPC). The PEG-based vesicles, 117 nm in diameter, were functionalized with either folic acid or anti-HER2/neu affibodies as targeting ligands to confer specificity for cancer cells. Our results revealed that nanovesicles prepared from polymer-lipid blends led to significant improvement in cell binding compared to nanovesicles prepared from block copolymer alone in both in vitro cell studies and murine tumor models. Therefore, it is envisioned that nanovesicles composed of polymer-lipid blends may constitute a preferred embodiment for targeted drug delivery and molecular imaging applications.

Sensing membrane stress with near IR-emissive porphyrins

Probes embedded within a structure can enable prediction of material behavior or failure. Carefully assembled composites that respond intelligently to physical changes within a material could be useful as intrinsic sensors. Molecular rotors are one such tool that can respond optically to physical environmental changes. Here, we propose to use molecular rotors within a polymersome membrane to report membrane stress. Using supermolecular porphyrin-based fluorophores as rotors, we characterize changes in the optical emission of these near-infrared (NIR) emissive probes embedded within the hydrophobic core of the polymersome membrane. The configuration of entrapped fluorophore depends on the available space within the membrane; in response to increased volume, emission is blue shifted. We used this feature to study how shifts in fluorescence correlate to membrane integrity, imparted by membrane stress. We monitored changes in emission of these porphyrin-based fluorophores resulting from membrane stress produced through a range of physical and chemical perturbations, including surfactant-induced lysis, hydrolytic lysis, thermal degradation, and applied stress by micropipette aspiration. This paper comprehensively illustrates the potential for supermolecular porphyrin-based fluorophores to detect intrinsic physical changes in a wide variety of environments, and suggests how molecular rotors may be used in soft materials science and biology as sensors.

Recent trends in the tuning of polymersomes' membrane properties

"Polymersomes" are vesicular structures made from the self-assembly of block copolymers. Such structures present outstanding interest for different applications such as micro- or nano-reactor, drug release or can simply be used as tool for understanding basic biological mechanisms. The use of polymersomes in such applications is strongly related to the way their membrane properties are controlled and tuned either by a precise molecular design of the constituting block or by addition of specific components inside the membrane (formulation approaches). Typical membrane properties of polymersomes obtained from the self-assembly of "coil coil" block copolymer since the end of the nineties will be first briefly reviewed and compared to those of their lipidic analogues, named liposomes. Therefore the different approaches able to modulate their permeability, mechanical properties or ability to release loaded drugs, using macromolecular engineering or formulations, are detailed. To conclude, the most recent advances to modulate the polymersomes' properties and systems that appear very promising especially for biomedical application or for the development of complex and bio-mimetic structures are presented.

NANOSCALE SELF-ASSEMBLY FOR DELIVERY OF THERAPEUTICS AND IMAGING AGENTS

Self-assemblies are complex structures spontaneously organized from simpler subcomponents, primarily through noncovalent interactions. These structures are being exploited as delivery vehicles of therapeutic and imaging agents. They have two unique advantages in comparison to other vehicles: 1) they are able to assume complex structures that are difficult to attain by chemical synthesis, and 2) the dissociation of self-assembled structures can be triggered by external stimuli, which can serve as a mechanism of payload release. In this review, we discuss two naturally occurring (proteins and viral capsids) and five engineered self-assemblies (vesicles, micelles, proteins, hydrogels, and inclusion complexes) that are under development for delivery of drugs and imaging agents. For each class of self-assembled supramolecular structures, we examine its structural and physicochemical properties, and potential applications within the context of assembly, loading, and payload release.

Two-photon microscopy of oxygen: polymersomes as probe carrier vehicles

Oxygen concentration distributions in biological systems can be imaged by the phosphorescence quenching method in combination with two-photon laser scanning microscopy. In this paper, we identified the excitation regime in which the signal of a two-photon-enhanced phosphorescent probe (Finikova, O. S.; Lebedev, A. Y.; Aprelev, A.; Troxler, T.; Gao, F.; Garnacho, C.; Muro, S.; Hochstrasser, R. M.; Vinogradov, S. A. ChemPhysChem 2008, 9, 1673-1679) is dependent quadratically on the excitation power (quadratic regime), and performed simulations that relate the photophysical properties of the probe to the imaging resolution. Further, we characterized polymersomes as a method of probe encapsulation and delivery. Photophysical and oxygen sensing properties of the probe were found unchanged when the probe is encapsulated in polymersomes. Polymersomes were found capable of sustaining high probe concentrations, thereby serving to improve the signal-to-noise ratios for oxygen detection compared to the previously employed probe delivery methods. Imaging of polymersomes loaded with the probe was used as a test-bed for a new method.

Tunable leuko-polymersomes that adhere specifically to inflammatory markers

The polymersome, a fully synthetic cell mimetic, is a tunable platform for drug delivery vehicles to detect and treat disease (theranostics). Here, we design a leuko-polymersome, a polymersome with the adhesive properties of leukocytes, which can effectively bind to inflammatory sites under flow. We hypothesize that optimal leukocyte adhesion can be recreated with ligands that mimic receptors of the two major leukocyte molecular adhesion pathways, the selectins and the integrins. Polymersomes functionalized with sialyl Lewis X and an antibody against ICAM-1 adhere avidly and selectively to surfaces coated with inflammatory adhesion molecules P-selectin and ICAM-1 under flow. We find that maximal adhesion occurs at intermediate densities of both sialyl Lewis X and anti-ICAM-1, owing to synergistic binding effects between the two ligands. Leuko-polymersomes bearing these two receptor mimetics adhere under physiological shear rates to inflamed endothelium in an in vitro flow chamber at a rate 7.5 times higher than those to uninflamed endothelium. This work clearly demonstrates that polymersomes bearing only a single ligand bind less avidly and with lower selectivity, thus suggesting proper mimicry of leukocyte adhesion requires contributions from both pathways. This work establishes a basis for the design of polymersomes for targeted drug delivery in inflammation.

Efficient encapsulation of plasmid DNA in pH-sensitive PMPC-PDPA polymersomes: study of the effect of PDPA block length on copolymer-DNA binding affinity

We report the self-assembly of a series of amphiphilic diblock copolymers comprising a biocompatible, hydrophilic block, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and a pH-sensitive block, poly(2-(diisopropylamino)ethyl methacrylate) (PDPA), into a dispersion of colloidally stable, nanometer-sized polymersomes at physiological pH and salt concentration. The pH-sensitivity of the PDPA block affords the electrostatic interaction of these block copolymers with nucleic acids at endocytic pH, as a result of the protonation of its tertiary amine groups at pH values below its pK(a). Herein we investigate the effect of PDPA block length on the binding affinity of the block copolymer to plasmid DNA.

A Generalized System for Photo-Responsive Membrane Rupture in Polymersomes

Polymersomes are vesicles whose membranes are comprised of self-assembled block co-polymers. We recently showed that co-encapsulating conjugated multi-porphyrin dyes in a polymersome membrane with ferritin protein in the aqueous lumen confers photo-lability to the polymersome. In the present study, we illustrate that the photo-lability can be extended to vesicles containing dextran, an inert and inexpensive polysaccharide, as the luminal solute. Here we explore how structural features of the polymersome/porphyrin/dextran composite affect its photo-response. Increasing dextran molecular weight, decreasing block copolymer molecular weight, and altering fluorophore-membrane interactions results in increasing the photo-responsiveness of the polymersomes. Amphiphilic interactions of the luminal encapsulant with the membrane coupled with localized heat production in the hydrophobic bilayer likely cause differential thermal expansion in the membrane and the subsequent membrane rupture. This study suggests a general approach to impart photo-responsiveness to any biomimetic vesicle system without chemical modification, as well as a simple, bio-inert method for constructing photo-sensitive carriers for controlled release of encapsulants.

Modular synthesis of biodegradable diblock copolymers for designing functional polymersomes

Polymer vesicles, or polymersomes, are promising candidates for applications in drug delivery and tissue imaging. While a vast variety of polymers have been explored for their ability to assemble into polymersomes, relatively little research on the functionalization of these polymers has been reported. We present here a novel route for the synthesis of poly(caprolactone)-b-poly(ethylene glycol) (PCL-b-PEG) diblock copolymers that allows for the insertion of functional groups at the block junctions and the assembly of functional membranes. This modular synthesis has been developed on the basis of solid-phase peptide synthesis techniques and is accomplished through the formation of two peptide bonds, one between an amine-terminated PEG and the carboxyl moiety of the functional group and the other between the functional group amine and a carboxy-terminated PCL. As a demonstration of the potential utility of the resulting vesicles, we incorporated two different amino acid functional groups at the junction. 2-Nitrophenylalanine was utilized to create UV-responsive membranes in which the vesicles were destabilized and released encapsulated contents upon irradiation. A fluorescein-conjugated lysine was also utilized to create stable fluorescent membranes in which the fluorescence was built into the polymer. This method should contribute to our ability to further develop smart, functional membranes.