Supramolecular assemblies of block copolymers in aqueous media as nanocontainers relevant to biological applications

Synthesis of Core - Shell Nanoparticles with Polystyrene Core and PEO Corona from Core-Crosslinked Micelles by the RAFT Process

Core–shell nanoparticles have been synthesized by core crosslinking of micelles. The underlying block copolymer, poly(oligo(ethylene glycol methyl ether methacrylate))-block-polystyrene (POEGMA-b-PS), was synthesized successfully by the reversible addition–fragmentation chain transfer (RAFT) process, using POEGMA as a macro-RAFT agent. The block copolymers were self-assembled into micelles in aqueous media and the resulting micelles and the RAFT endgroup, located in the core of the micelle, were used for the subsequent crosslinking step using a crosslinker, divinyl benzene (DVB). The rate of the crosslinking reaction was found to be slow with less than 20% conversion being achieved after 72 h. Nevertheless, crosslinked micelles were obtained and only a small fraction of free block copolymers remained. Cytotoxicity tests confirmed the biocompatibility of the prepared core-crosslinked micelles. In addition the crosslinked micelles were taken up by L929 cells without causing any signs of cell damage.

Relationship between wall thickness and size in block copolymer vesicles

In this study, we report a new phenomenon dealing with the size-dependent behavior of the wall thickness of block copolymer vesicles, especially the decrease in wall thickness with decreasing vesicle size. Four vesicle-forming copolymers from the polystyrene-b-poly-(acrylic acid) (PS-b-PAA) family (i.e., PS(500)-b-PAA(50), PS(310)-b-PAA(28), PS(240)-b-PAA(15), and PS(412)-b-PAA(46)) were chosen for study. The sizes and wall thicknesses of the vesicles after quenching were determined from the TEM micrographs, and plots were made of the wall thickness versus size for each family of vesicles made from each of the various blocks. First, the effect of the length of the PAA block on the relationship between the wall thickness and the size was examined. In the vesicles prepared from PS(500)-b-PAA(50), the copolymer with the longest PAA block that yields the smallest vesicles, the wall thickness decreases strongly with decreasing size. By contrast, in the case of vesicles made from PS(240)-b-PAA(15), for which a wide size distribution is obtained, only a weak size dependence of the wall thickness is seen. For vesicles made from the copolymer with intermediate PAA block length (i.e., PS(310)-b-PAA(28)), both strong and weak behavior regions are observed depending on the vesicle size range. We suggest that this new phenomenon of the size dependence of the wall thickness can be considered to be another stabilization mechanism for very small vesicles, under conditions where chain segregation is insufficient to stabilize the size. The vesicles can be stabilized by decreasing the wall thickness for very small vesicles, resulting in the increase in area per corona chain, thus decreasing the corona repulsion on the inside. The effects of additives such as NaCl, HCl, or NaOH on the relationship between the wall thickness and the size were also investigated. By shielding the electrostatic repulsion among corona chains in the presence of NaCl, the strong behavior of the vesicles prepared from PS(412)-b-PAA(46) changes to a weak one as the width of the vesicle size distribution increases. In a NaCl concentration region around 10 mM, an opposite effect is seen relative to that observed in small vesicles in that the wall thickness decreases with increasing vesicle size for vesicles larger than ca. 300 nm, an effect ascribed to corona repulsion among the external corona chains. The addition of HCl also drives the relationship to be weaker through the protonation of the carboxylate groups of PAA chains, in an effect similar to that of NaCl. The presence of NaOH is expected to strengthen the relationship via the deprotonation of PAA, which increases the corona repulsion. However, because of the very short length of PAA chains in the system where a weak effect is seen, no significant effect of NaOH addition was observed because the size distribution remained broad.

Noninvasive monitoring of the fate of 111In-labeled block copolymer micelles by high resolution and high sensitivity microSPECT/CT imaging

The validation of high sensitivity and high resolution microSPECT/CT imaging for tracking the in vivo pathway and fate of an 111Indium-labeled (111In) amphiphilic diblock copolymer micelle formulation is presented. Heterobifunctional poly(ethylene glycol) was used to initiate cationic ring opening polymerization of epsilon-caprolactone, which was then conjugated to 2-(4-isothiocyanatobenzyl)-diethylenetriaminepentaacetic acid (p-SCN-Bn-DTPA) for chelation with 111In. The micelles were characterized in terms of their physicochemical properties including size, size distribution, zeta-potential, and radiochemical purity. Elimination kinetics and tissue deposition were evaluated in healthy mice following administration of 111In-micelles, 111In-DTPA-b-PCL unimers (i.e., administered under the critical micelle concentration) or 111In-Bn-DTPA. Healthy and MDA-MB-231 tumor-bearing mice were imaged using microSPECT/CT following iv administration of 111In-micelles or 111In-Bn-DTPA. Overall, incorporation of 111In onto the surface of thermodynamically stable micelles results in long plasma residence times for the radionuclide and preferential localization within the spleen (22 +/- 5% i.d/g), liver (13 +/- 3% i.d./g), and tumor (9 +/- 2% i.d./g). MicroSPECT/CT imaging provided noninvasive longitudinal visualization of circulation dynamics and tissue deposition. A strong correlation between image-based region of interest (ROI) analysis and biodistribution data was found, implying that nuclear imaging can be used as a noninvasive tool to accurately quantify tissue distribution. As well, the image-based assessment provided unique insight into the intratumoral distribution of the micelles in vivo.

Hydrophobically modified biodegradable poly(ethylene glycol) copolymers that form temperature-responsive Nanogels

Biodegradable copolymers consisting of a hydrophilic poly[l-aspartic acid-alt-poly(ethylene glycol)] (poly(l-Asp-alt-PEG)) backbone and hydrophobic capryl units as side chains were synthesized. The amphiphilic copolymer was found to form nanosized hydrogel particles (nanogels) of approximately 15 nm in size by self-assembly at 20 degrees C in aqueous media, and the nanogel solutions displayed phase-transition in response to temperature. The transition of the nanogel solution was reversible and tunable in the range from 19 to 55 degrees C by variation of the amounts of capryl units introduced and the solution concentration. The nanogels were gradually degraded within days in a phosphate buffer solution (PBS) at 37 degrees C. Temperature-responsive biodegradable nanogel systems consisting of biocompatible PEG may have potential utility for high biocompatibility temperature-responsive nanodevices such as microreactor systems, molecular-chaperones, and drug delivery vehicles.

Cellular uptake of functional nanogels prepared by inverse miniemulsion ATRP with encapsulated proteins, carbohydrates, and gold nanoparticles

Atom transfer radical polymerization (ATRP) was used to produce a versatile drug delivery system capable of encapsulating a range of molecules. Inverse miniemulsion ATRP permitted the synthesis of biocompatible and uniformly cross-linked poly(ethylene oxide)-based nanogels entrapping gold nanoparticles, bovine serum albumin, rhodamine B isothiocyanate-dextran, or fluoresceine isothiocyanate-dextran. These moieties were entrapped to validate several biological outcomes and to model delivery of range of molecules. Cellular uptake of nanogels was verified by transmission electron microscopy, gel electrophoresis, Western blotting, confocal microscopy, and flow cytometry. Fluorescent colocalization of nanogels with a fluorophore-conjugated antibody for clathrin indicated clathrin-mediated endocytosis. Furthermore, internalization of nanogels either with or without GRGDS cell attachment-mediating peptides was quantified using flow cytometry. After 45 min of incubation, the uptake of unmodified FITC-Dx-loaded nanogels was 62%, whereas cellular uptake increased to >95% with the same concentration of GRGDS-modified FITC-Dx nanogels. In addition, a spheroidal coculture of human umbilical vascular endothelial cells (HUVECs) and human mesenchymal stem cells (hMSCs) validated cell endocytosis. Application of ATRP enabled the synthesis of a functionalized drug delivery system with a uniform network that is capable of encapsulating and delivering inorganic, organic, and biological molecules.

Polyhydroxyethylaspartamide-based micelles for ocular drug delivery

In this paper three copolymers of polyhydroxyethylaspartamide (PHEA), bearing in the side chains polyethylene glycol (PEG) and/or hexadecylamine (C(16)) (PHEA-PEG, PHEA-PEG-C(16) and PHEA-C(16) respectively) have been studied as potential colloidal drug carriers for ocular drug delivery. The physical characterization of all three PHEA derivatives, using the Langmuir trough (LT) and micellar affinity capillary electrophoresis (MACE) techniques allowed to assume that whereas alone PHEA backbone is an inert polymer with respect to the interactions with lipid membranes and drug complexation, when PHEA chains are grafted with long alkyl chains like C(16) or in combination C(16) chains and hydrophilic chains like PEG, copolymers with lipid membrane interaction ability and drug complexation capability are obtained. In vitro permeability studies performed on primary cultured rabbit conjunctival and corneal epithelia cells, using PHEA-C(16) and PHEA-PEG-C(16) as micelle carriers for netilmicin sulphate, dexamethasone alcohol and dexamethasone phosphate, demonstrated that in all cases drug loaded PHEA-C(16) and PHEA-PEG-C(16) micelles provide a drug permeation across ocular epithelia greater than simple drug solutions or suspensions. In particular PHEA-PEG-C(16) acts as the best permeability enhancer in our experimental model. In vivo bioavailability studies conducted with PHEA-PEG-C(16) micelles loaded with dexamethasone alcohol, confirmed that this system also provides a drug bioavailability greater in comparison with that obtained with water suspension of the same drug after ocular administration to rabbits.

Complex coacervate core micelles

In this review we present an overview of the literature on the co-assembly of neutral-ionic block, graft, and random copolymers with oppositely charged species in aqueous solution. Oppositely charged species include synthetic (co)polymers of various architectures, biopolymers - such as proteins, enzymes and DNA - multivalent ions, metallic nanoparticles, low molecular weight surfactants, polyelectrolyte block copolymer micelles, metallo-supramolecular polymers, equilibrium polymers, etcetera. The resultant structures are termed complex coacervate core/polyion complex/block ionomer complex/interpolyelectrolyte complex micelles (or vesicles); i.e., in short C3Ms (or C3Vs) and PIC, BIC or IPEC micelles (and vesicles). Formation, structure, dynamics, properties, and function will be discussed. We focus on experimental work; theory and modelling will not be discussed. Recent developments in applications and micelles with heterogeneous coronas are emphasized.

Bioconjugation of biotin to the interfaces of polymeric micelles via in situ click chemistry

Azido-containing amphiphilic triblock copolymer poly(ethylene glycol)-b-poly(azidoethyl methacrylate)-b-poly(methyl methacrylate) (PEG-b-PAzEMA-b-PMMA) was prepared by postpolymerization functionalization of poly(ethylene glycol)-b-poly(hydroxyethyl methacrylate)-b-poly(methyl methacrylate) (PEG-b-PHEMA-b-PMMA). In aqueous media, PEG-b-PAzEMA-b-PMMA self-assembled into spherical micelles with the azide groups at the hydrophobic/hydrophilic interface due to the molecular architecture. Biotin was conjugated to the micelles by in situ click chemistry between azide groups and alkynated biotin, resulting in the formation of a functional interface between the hydrophilic shell and the hydrophobic core. The bioavailability of biotin to avidin was demonstrated by an avidin/4'-hydroxyazobenzene-2-carboxylic acid (avidin/HABA) assay, transmission electron microscopy, and dynamic light scattering investigations.

Polymeric core-shell assemblies mediated by host-guest interactions: versatile nanocarriers for drug delivery

Novel hydrophilic-hydrophilic block copolymer with one block containing β-cyclodextrin was synthesized. Core-shell structured nano-assemblies with chemical sensitivity could be constructed by this copolymer in the presence of hydrophobic compounds. By selecting appropriate guest components, polyion complex micelles could also be assembled. These results suggest the potential versatile applications of this type of copolymers in pharmaceutics, nanomedicine, and nano-biotechnology.

Galactosylated fluorescent labeled micelles as a liver targeting drug carrier

Galactosylated and fluorescein isothiocyanate (FITC) labeled polycaprolactone-g-dextran (Gal-PCL-g-Dex-FITC) polymers were synthesized. The grafted polymers can self-assemble into stable micelles in aqueous medium and in serum. Transmission electron microscopy (TEM) images showed that the self-assembled micelles were regularly spherical in shape. Micelle size determined by size analysis was around 120 nm. The anti-inflammation drug prednisone acetate as a model drug was loaded in the polymeric micelles, and the in vitro drug release was investigated. The galactosylated micelles could be selectively recognized by HepG2 cells and subsequently accumulate in HepG2 cells. The in vivo study demonstrated the relative uptake of the micelles by liver is much higher than the other tissues, indicating that the galactosylated micelles have great potential as a liver targeting drug carrier.

pH-dependent self-assembly of polystyrene-block-poly((sulfamate-carboxylate)isoprene) copolymer in aqueous media

The amphiphilic polystyrene- block-poly((sulfamate-carboxylate)isoprene) (PS-PISC) diblock copolymer was synthesized from the precursor diblock copolymer polystyrene- block-isoprene by reaction with chlorosulfonyl isocyanate. The structure and behavior of self-assembled PS-PISC nanoparticles was studied in alkaline and acidic aqueous solutions by a combination of static and dynamic light scattering, analytical ultracentrifugation, atomic force and cryogenic transmission electron microscopies, NMR spectroscopy, potentiometric titration, and fluorometry using pyrene as a polarity-sensitive fluorescent probe. It was found that PS-PISC exists in aqueous solutions in the form of micellar aggregates. The aggregation tendency increases with decreasing effective charge density in the shell, that is, with decreasing pH of the solution, and aggregates found in alkaline aqueous media have much smaller molar masses than those formed in acidic media. The latter are dense, collapsed structures with immobile PISC domains in which most of the COOH and NH 2 (+)SO 3 (-) groups are buried inside of the nanoparticles. The swelling of PISC domains and disentanglement of PISC chains after addition of a base are slow processes occurring on the time scale of days.

Surface modification of polymeric vesicles via host-guest inclusion complexation

We have prepared a novel kind of building block (CD-PI-CD) of polyether imide (PI) with beta-cyclodextrin (beta-CD) at the both ends that self-assembles into vesicles in water. The vesicles are "reactive" in supramolecular chemistry because the host groups of beta-CD equally distribute on the inside and outside surfaces of the vesicles. Isothermal titration calorimetry (ITC) and static light scattering (SLS) studies demonstrate that both the inner and outer surfaces of the vesicles can be further modified noncovalently with adamantane-ended polyethlene glycol (Ada-PEG) via a host-guest inclusion interaction between beta-CD and adamantane.

Self-assembly behavior of A-B diblock and C-D random copolymer mixtures in the solution state through mediated hydrogen bonding

We have synthesized poly(methyl methacrylate- b-4-vinylpyridine) (PMMA- b-P4VP) and poly(styrene- r-vinylphenol) (PS- r-PVPh) copolymers by using anionic and free radical polymerizations, respectively. Well-defined micelles through hydrogen bonding have been prepared by mixing PMMA- b-P4VP diblock copolymer and PS- r-PVPh random copolymer in a single solvent. Block copolymers were mixed with random copolymers, with various [N]/[OH] ratios (4/1, 2/1, 1/1, and 1/4) in which "[N]/[OH]" represents the molar ratio of pyridine groups on P4VP to hydroxyl groups on PVPh. The presence distribution of PVPh/P4VP and PVPh/PMMA hydrogen bonding depends on the feeding ratio of PVPh to P4VP. When the PVPh content is lower than that of P4VP, hydrogen bonding occurs only between PVPh and P4VP; with excess PVPh, additional hydrogen bonding between PVPh and PMMA would occur. Furthermore, the effect of the solvent quality on the self-assembly behavior of PMMA- b-P4VP/PS- r-PVPh blends is investigated by considering tetrahydrofuran (THF) and dimethylformamide (DMF) as common solvents. We can mediate the strength of hydrogen bonding in blend systems by adopting different solvents and inducing different morphology transitions.

Complex coacervate core micro-emulsions

Complex coacervate core micelles form in aqueous solutions from poly(acrylic acid)-block-poly(acrylamide) (PAAxPAAmy, x and y denote degree of polymerization) and poly(N,N-dimethyl aminoethyl methacrylate) (PDMAEMA150) around the stoichiometric charge ratio of the two components. The hydrodynamic radius, Rh, can be increased by adding oppositely charged homopolyelectrolytes, PAA140 and PDMAEMA150, at the stoichiometric charge ratio. Mixing the components in NaNO3 gives particles in highly aggregated metastable states, whose Rh remain unchanged (less than 5% deviation) for at least 1 month. The Rh increases more strongly with increasing addition of oppositely charged homopolyelectrolytes than is predicted by a geometrical packing model, which relates surface and volume of the particles. Preparation in a phosphate buffer - known to weaken the electrostatic interactions between PAA and PDMAEMA - yields swollen particles called complex coacervate core micro-emulsions (C3-μEs) whose Rh increase is close to that predicted by the model. These are believed to be in the stable state (lowest free energy). A two-regime increase in Rh is observed, which is attributed to a transition from more star-like to crew-cut-like, as shown by self-consistent field calculations. Varying the length of the neutral and polyelectrolyte block in electrophoretic mobility measurements shows that for long neutral blocks (PAA26PAAm405 and PAA39PAAm381) the ζ-potential is nearly zero. For shorter neutral blocks the ζ-potential is around -10 mV. This shows that the C3-μEs have excess charge, which can be almost completely screened by long enough neutral blocks.

Spontaneous self-assembly of water-soluble nucleotide-calixarene conjugates in small micelles coalescing to microspheres

Spontaneous self-assembly of calix[4]arenes bearing four 2'-deoxythymidine or 2'-deoxyadenosine nucleotide pendants is investigated using (1)H NMR, exchange NMR, and diffusion ordered NMR spectroscopies and dynamic light scattering. In aqueous medium, the nucleotide-calixarene conjugates, by noncovalent interactions involving both nucleobases and calixarene skeleton, form dimers which self-organize in micelles by increasing the concentration. Microscopic images (scanning electron microscopy and transmission electron microscopy) show that the nucleobase affects the aggregate morphology in the solid state.