Morphology of core-shell-corona aqueous micelles: II. Addition of core-forming homopolymer

Amphiphilic Block Copolymers: Their Structures, and Self-Assembly to Polymeric Micelles and Polymersomes as Drug Delivery Vehicles

Self-assembly of amphiphilic block copolymers display a multiplicity of nanoscale periodic patterns proposed as a dominant tool for the 'bottom-up' fabrication of nanomaterials with different levels of ordering. The present review article focuses on the recent updates to the self-association of amphiphilic block copolymers in aqueous media into varied core-shell morphologies. We briefly describe the block copolymers, their types, microdomain formation in bulk and micellization in selective solvents. We also discuss the characteristic features of block copolymers nanoaggregates viz., polymer micelles (PMs) and polymersomes. Amphiphilic block copolymers (with a variety of hydrophobic blocks and hydrophilic blocks; often polyethylene oxide) self-assemble in water to micelles/niosomes similar to conventional nonionic surfactants with high drug loading capacity. Double hydrophilic block copolymers (DHBCs) made of neutral block-neutral block or neutral block-charged block can transform one block to become hydrophobic under the influence of a stimulus (physical/chemical/biological), and thus induced amphiphilicity and display self-assembly are discussed. Different kinds of polymer micelles (viz. shell and core-cross-linked, core-shell-corona, schizophrenic, crew cut, Janus) are presented in detail. Updates on polymerization-induced self-assembly (PISA) and crystallization-driven self-assembly (CDSA) are also provided. Polyion complexes (PICs) and polyion complex micelles (PICMs) are discussed. Applications of these block copolymeric micelles and polymersomes as nanocarriers in drug delivery systems are described.

Synthesis and Self-Assembly of Amphiphilic Triblock Terpolymers with Complex Macromolecular Architecture

Two star triblock terpolymers (PS-b-P2VP-b-PEO)₃ and one dendritic-like terpolymer [PS-b-P2VP-b-(PEO)₂]₃ of PS (polystyrene), P2VP (poly(2-vinylpyridine)), and PEO (poly(ethylene oxide)), never reported before, were synthesized by combining atom transfer radical and anionic polymerizations. The synthesis involves the transformation of the -Br groups of the previously reported Br-terminated 3-arm star diblock copolymers to one or two -OH groups, followed by anionic polymerization of ethylene oxide to afford the star or dendritic structure, respectively. The well-defined structure of the terpolymers was confirmed by static light scattering, size exclusion chromatography, and NMR spectroscopy. The self-assembly in solution and the morphology in bulk of the terpolymers, studied by dynamic light scattering and transmission electron microscopy, respectively, reveal new insights in the phase separation of these materials with complex macromolecular architecture.

Online rheological investigation on ion-induced micelle transition for amphiphilic polystyrene-block-poly(acrylic acid) diblock copolymer in dilute solution

The ion-induced micellar transition is online-investigated by the time dependence of the viscosity of the solution under shear flow for the first time. During the morphological transition, the change in the micellar structure can be tracked by the change in viscosity. Adding HCl or CaCl2 into pre-prepared spherical micelle solution from the self-assembly of polystyrene-block-poly(acrylic acid) (PS144-b-PAA22) in the N,N-dimethylformamide (DMF)/water mixture, the micellar structures change into short cylinders, long, entangled cylinders, and then lamellae or vesicles, corresponding to the viscosity increasing first and then declining. When HCl or CaCl2 is added to the pre-prepared spherical micelle solution formed by PS144-b-PAA50 in the dioxane/water mixture, the micellar structures are quickly transformed into cylinders or lamellae before carrying out the rheological measurement and then are turned to vesicles or spheres under the shearing, corresponding to a gradual decline in viscosity. This study shows that the rheology can be a very simple and effective online method on the investigation of the micellization, which plays an important role in understanding the micellization mechanism and micellar transition pathway of block copolymers in dilute solution.

ABC triblock terpolymer self-assembled core-shell-corona nanotubes with high aspect ratios

Nanotubes have attracted considerable attention due to their unique 1D hollow structure; however, the fabrication of pure nanotubes via block copolymer self-assembly remains a challenge. In this work, the successful preparation of core-shell-corona (CSC) nanotubular micelles with uniform diameter and high aspect ratio is reported, which is achieved via self-assembly of a poly (styrene-b-4-vinyl pyridine-b-ethylene oxide) triblock terpolymer in binary organic solvents with assistance of solution thermal annealing. Via direct visualization of trapped intermediates, the nanotube is believed to be formed via large sphere-large solid cylinderical aggregates-nanotube transformations, wherein the unique solid to hollow transition accompanied with the unidirectional growth is distinct from conventional pathway. In addition, by virtue of the CSC structure, gold nanoparticles are able to be selectively incorporated into different micellar domains of the nanotubes, which may have potential applications in nanoscience and nanotechnology.

Polymeric micelle assembly for the direct synthesis of platinum-decorated mesoporous TiO(2) toward highly selective sensing of acetaldehyde

Platinum-decorated mesoporous TiO2 is synthesized by the self-assembly of polymeric micelles of an asymmetric triblock copolymer with three chemically distinct units in an acidic tetrahydrofuran solution. The strong hydrophobic interaction of platinum(II) 2,4-pentanedionate with a polystyrene core and electrostatic interaction of titanium tetraisopropoxide with a poly(vinylpyridine) shell enable us to directly synthesize crystalline mesoporous TiO2 with platinum nanoparticles. A thermally stable block copolymer prevents collapse of the ordered mesostructure during the calcination process. The platinum source is in situ reduced to form the platinum nanoparticles on the TiO2 walls. The sensing performance of platinum-decorated mesoporous TiO2 is studied in detail using a quartz crystal microbalance technique, and it is found that it shows excellent sensitivity for acetaldehyde.

Aqueous nanomicellar formulation for topical delivery of biotinylated lipid prodrug of acyclovir: formulation development and ocular biocompatibility

PURPOSE

The objective of this study was to develop a clear, aqueous nanomicellar formulation and evaluate its in vitro ocular biocompatibility as a novel carrier for topical ocular delivery of biotinylated lipid prodrug for the treatment of herpetic keratitis.

METHODS

Micellar formulation of Biotin-12Hydroxystearic acid-acyclovir (B-12HS-ACV) was prepared by solvent evaporation/film hydration method with two nonionic surfactants, vitamin E TPGS and octoxynol-40. The optimized formulation was characterized for various parameters including micelle size, polydispersity index (PDI), and zeta-potential and in vitro prodrug release. Human corneal epithelial cells (HCECs) were employed for studying the cytotoxicity of the formulation. Further, mRNA expression levels of various cytokines were also studied with quantitative real-time PCR (qPCR).

RESULTS

Average size was 10.46±0.05 nm with a PDI of 0.086 for blank nanomicelles, and 10.78±0.09 nm with a PDI of 0.075 for prodrug-loaded nanomicelles. Both unloaded and prodrug-loaded nanomicelles had low negative zeta potential. Prodrug encapsulation efficiency of mixed nanomicelles was calculated to be ∼90%. Transmission electron microscopy analysis revealed that nanomicelles were spherical, homogenous, and devoid of aggregates. B-12HS-ACV release from nanomicelles was slow with no significant burst effect. Results show a sustained release of the prodrug from nanomicelles over a period of 4 days. Neither the blank formulation nor the prodrug-loaded micellar formulation demonstrated any cytotoxic effects. Further, incubation of HCECs with blank and prodrug-loaded nanomicellar groups did not significantly alter the expression levels of IL-1β, IL-6, IL-8, IL-17, TNF-α, and IFN-γ.

CONCLUSIONS

In summary, a topical clear, aqueous nanomicellar formulation comprised of vitamin E TPGS and octoxynol-40 loaded with 0.1% B-12HS-ACV was successfully developed. B-12HS-ACV-loaded nanomicelles are small in size, spherical, and homogenous, without any aggregates. The micellar formulations were perfectly transparent similar to pure water. Ocular biocompatibility studies indicated that mixed nanomicelles were nontoxic and noninflammatory to corneal epithelial cells. Therefore, nanomicellar technology represents a promising strategy for the delivery of biotinylated lipid prodrugs of ACV.

Aggregation of water-soluble block copolymers in aqueous solutions: recent trends

This review summarizes recent literature and some of our own results on aggregation behavior on water-soluble block copolymers belonging to three different classes viz. hydrophilic-hydrophobic (AB, ABA and BAB) block copolymers, double hydrophilic block copolymers (DHBCs) and ABC triblock copolymers. In the case of amphiphilic copolymers, special attention has been focussed on aggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronics) and their aggregation in aqueous solutions at different temperatures as well as in the presence of various additives. Recent studies based on modern techniques viz. scattering (static and dynamic light scattering and small angle neutron scattering), spectral methods, e.g., fluorescence (static and time resolved), nuclear magnetic resonance and Fourier transform infrared spectroscopies, thermal methods e.g., differential scanning calorimetry and isothermal titration calorimetry, cryotransmission electron microscopy, ultrasonic absorption along with general physical properties like surface tension, viscosity and dye solubilization are summarized. For the DHBCs where one of the blocks is usually a polyion, complex formation by adding oppositely charged ions induces the formation of nanoaggregates. Characterization of such nanoaggregates of polyion complexes of DHBCs and their potential use for incorporation of ionic solutes in the micellar core are reviewed. The formation and characteristics of core-shell-corona micelles of ABC triblock copolymers and their applications as vehicles for controlled drug release are also discussed.

Nanoscale Patterning of Two Metals on Silicon Surfaces Using an ABC Triblock Copolymer Template

Patterning technologically important semiconductor interfaces with nanoscale metal films is important for applications such as metallic interconnects and sensing applications. Self-assembling block copolymer templates are utilized to pattern an aqueous metal reduction reaction, galvanic displacement, on silicon surfaces. Utilization of a triblock copolymer monolayer film, polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP-b-PEO), with two blocks capable of selective transport of different metal complexes to the surface (PEO and P2VP), allows for chemical discrimination and nanoscale patterning. Different regions of the self-assembled structure discriminate between metal complexes at the silicon surface, at which time they undergo the spontaneous reaction at the interface. Gold deposition from gold(III) compounds such as HAuCl4(aq) in the presence of hydrofluoric acid mirrors the parent block copolymer core structure, whereas silver deposition from Ag(I) salts such as AgNO3(aq) does the opposite, localizing exclusively under the corona. By carrying out gold deposition first and silver second, sub-100-nm gold features surrounded by silver films can be produced. The chemical selectivity was extended to other metals, including copper, palladium, and platinum. The interfaces were characterized by a variety of methods, including scanning electron microscopy, scanning Auger microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.

Amphiphilic Block Copolymers that Form Micelles but Are not Surface Active and Bind Normal Surfactants

The phase diagram and the properties of the aqueous phases of the AB block copolymer poly-n-butylacrylate-block-polyacrylic acid (nBuA100-b-AA150) has been studied as a function of the neutralisation degree α with NaOH. Aqueous solutions of the block copolymer with α < 0.1 are turbid and separate into two phases while solutions with α > 0.1 are transparent and single phase. The viscosities of such solutions increase with α and pass over a maximum at α = 0.5. Solutions with a concentration of Cp ≥ 1% and at α = 0.5 are highly viscoelastic and have a yield stress value. Cryo-TEM micrographs show that globular micelles are present in the viscoelastic solutions. The micelles have a polydisperse size distribution. The radius of the hydrophobic core varies from about 100 Å to 200 Å what is the length of the fully stretched hydrophobic block.

The rheological properties of the solutions are explained on the basis of the globular core–shell micelles where the polyelectrolyte chains form a corona around the hydrophobic block. Most of the counter ions are in the condensed and free state inside the corona. Some of the counter ions are outside of the corona and form an electrical double layer whose thickness depends on α. The viscoelastic properties of the solutions are thus given by the dense packing of the charged globular micelles. The decrease of the viscosity for α > 0.5 comes about by the shrinkage of the thickness of the electrical double layer. With increase of excess salt the double layer and the corona also shrink and the viscoelastic properties break down. With adsorption of zwitterionic surfactants the viscoelastic properties also disappear. Cryo-TEM micrographs of the block copolymers at α = 0.5 with adsorbed zwitterionic surfactants show that the polyelectrolyte chains of the block copolymers are wrapped around small globular micelles of the zwitterionic surfactants. The supra-molecular structures look like raspberry-like micelles.

The block copolymers show a very unusual amphiphilic behavior. For a neutralisation degree of α > 0.1 the compounds do not seem to adsorb at the water surface even though the compounds form micelles in the bulk solution. The micellar solutions have the same surface tension of water. The acidic form of the compound however is surface active.

Triblock terpolymer micelles: a personal outlook

This outlook paper focuses on micelles formed by ABC triblock copolymers (triblock terpolymers) and related systems resulting from mixtures of diblock copolymers. Micelles with different internal structure such as micelles with a heterogeneous core and a homogeneous corona or micelles with a homogeneous core and a mixed corona are presented. More complex nanoobjects such as vesicles and Janus particles are also reviewed. Finally, potential applications of these objects are discussed.