Photoresponsive behavior of amphiphilic copolymers of azobenzene and N,N-dimethylacrylamide in aqueous solutions

Copolymers of 4-methacryloyloxyazobenzene and N,N-dimethylacrylamide (MOAB-DMA) can aggregate strongly in aqueous solution (they are soluble in water up to a MOAB molar fraction of 0.2) to give concentration-dependent aggregate size distributions and well-defined boundaries between the dilute and semidilute regimes, as determined by dynamic light scattering, surface tension, and probe solubilization experiments. The copolymers are strongly surface active, an uncommon observation for random copolymers, and exhibit pronounced photoviscosity effects at higher concentrations. The concentration dependence of the kinetic parameters for the reversible polymer rearrangement upon photoisomerization, as determined by electronic absorption spectroscopy, is attributed to steric hindrances. Trans-to-cis isomerization under UV light leads to partial dissociation of the azobenzene aggregates that cross-link the polymers, thereby significantly affecting the polymer solution rheology, with a consequent loss of viscoelasticity upon UV irradiation, especially in concentrated polymer solutions.

Novel one-pot synthesis of magnetite latex nanoparticles by ultrasound irradiation

A simple and efficacious procedure for the synthesis of magnetite nanoparticles has been achieved via a sonochemical miniemulsion polymerization process. The sonochemically synthesized magnetite encapsulated polymer latex particles exhibit excellent colloidal stability and strong magnetic properties, and are of a size that makes them technologically relevant. This novel method may be readily extended to the preparation of multiple combinations of different polymers and encapsulated materials.

Template-assisted synthesis of nanogels from Pluronic-modified poly(acrylic acid)

A series of novel polymeric nanogels with core-shell morphology was developed. Block ionomer complexes of comb-graft poly(ethylene oxide)-b-poly(polypropylene oxide)-b-poly(ethylene oxide)-g-poly(acrylic acid) copolymers (Pluronic-PAA) and divalent metal cations were utilized as micellar templates for the synthesis of nanogels with sizes ranging from 100 to 200 nm in diameter. The Pluronic-PAA nanogels were confirmed to possess ionic cross-linked PAA cores and flexible hydrophilic shells from the Pluronic copolymer chains. The ionic character of the core provided for pH-dependent swelling/collapse behavior of the nanogels. These prepared nanogels are expected to be of utility as carriers for charged therapeutic or diagnostic agents.

Controlled Release Camptothecin Tablets based on Pluronic and Poly(acrylic acid) Copolymer. Effect of Fabrication Technique on Drug Stability, Tablet Structure, and Release Mode

Poly(ethylene oxide)-b-poly(propylene oxide)-b-(polyethylene oxide)-g-poly(acrylic acid), a graft-comb copolymer of Pluronic 127 and poly(acrylic acid) (Pluronic-PAA), was explored as an excipient for tablet dosage form of camptothecin (CPT). The tablets were prepared by either direct compression of the drug-polymer physical blend, suspension in ethanol followed by evaporation, or compression after kneading and characterized with respect to their physical structures, drug stability, and release behavior. Porosity and water uptake rate were strongly dependent on the fabrication procedure, ranking in the order: direct compression of physical blend > compression after suspension/evaporation in ethanol > compression after kneading. Tablets prepared by compression of physical blends swelled in water with a rapid surface gel layer formation that impeded swelling and disintegration of the tablets core. These tablets were able to sustain the CPT release for a period of time longer than those observed with the tablets made by either suspension/evaporation or kneading, which disintegrated within a few minutes. Despite the tablet disintegration, the CPT release was impeded for at least 6 hr, which was attributed to the ability of the Pluronic-PAA copolymers to form micellar aggregates at the hydrated surface of the particles. Physical mixing did not alter the fraction of CPT being in the pharmaceutically active lactone form, whilst the preparation of the tablets by the other two methods caused a significant reduction in the lactone form content. Tablets prepared from the physical blends demonstrated CPT release rates increasing with the pH due to the PAA ionization leading to the increase in the rate and extent of the tablet swelling. The results obtained demonstrate the potential of the Pluronic-PAA copolymers for the oral administration of chemotherapeutic agents.

Reversible clustering of pH- and temperature-responsive Janus magnetic nanoparticles

Janus nanoparticles have been synthesized consisting of approximately 5 nm magnetite nanoparticles coated on one side with a pH-dependent and temperature-independent polymer (poly(acrylic acid), PAA), and functionalized on the other side by a second (tail) polymer that is either a pH-independent polymer (polystyrene sodium sulfonate, PSSNa) or a temperature-dependent polymer (poly(N-isopropyl acrylamide), PNIPAM). These Janus nanoparticles are dispersed stably as individual particles at high pH values and low temperatures, but can self-assemble at low pH values (PSSNa) or at high temperatures (>31 degrees C) (PNIPAM) to form stable dispersions of clusters of approximately 80-100 nm in hydrodynamic diameter. The Janus nanoparticle compositions were verified using FTIR and XPS, and their structures observed directly by TEM. Their clustering behavior is analyzed by dynamic light scattering and zeta potential measurements.

Self-assembly of poly(ethylene oxide)-block-poly(acrylic acid) induced by CaCl2: mechanistic study

The interaction between CaCl 2 and double hydrophilic block copolymer, poly(ethylene oxide) 45- block-poly(acrylic acid) 70, PEO 45- b-PAA 70, was investigated. At a stoichiometric ratio of Ca2+:COO (-) = 0.5, Ca2+ ions were bound to COO (-) groups on PAA segments via electrostatic interaction. Small particles of 4-8 nm in diameter were observed, suggesting the formation of coil-like polymeric globule induced by charge neutralization. At Ca2+:COO (-) >or= 2.5, monodispersed aggregates of average hydrodynamic diameter of 52.0 +/- 7.4 nm were produced. The ISE, ITC, surface tension and fluorescence spectroscopic data confirmed that the formation of these aggregates is not the result of interaction between excess Ca2+ ions and the polymer, but rather it is due to changes in the water activity that triggers the structural rearrangement of Ca2+/PEO 45- b-PAA 70 complex.

Stop-flow lithography to generate cell-laden microgel particles

Encapsulating cells within hydrogels is important for generating three-dimensional (3D) tissue constructs for drug delivery and tissue engineering. This paper describes, for the first time, the fabrication of large numbers of cell-laden microgel particles using a continuous microfluidic process called stop-flow lithography (SFL). Prepolymer solution containing cells was flowed through a microfluidic device and arrays of individual particles were repeatedly defined using pulses of UV light through a transparency mask. Unlike photolithography, SFL can be used to synthesize microgel particles continuously while maintaining control over particle size, shape and anisotropy. Therefore, SFL may become a useful tool for generating cell-laden microgels for various biomedical applications.

Photochromic soft materials: flavylium compounds incorporated into pluronic F-127 hydrogel matrixes

The performance of flavylium-based photochromic systems is increased by their incorporation into Pluronic F-127 matrixes, which switch from polymeric solutions to micelles to gels with changes in temperature depending on copolymer concentration. Two flavylium compounds, 7,4'-dihydroxyflavylium and 7-(N,N-diethylamino)-4-hydroxyflavylium, both exhibiting a small thermal cis-trans isomerization barrier in water were investigated. In the first system the flavylium in the gel photoswitches from the colorless trans-chalcone (Ct) species to the yellow flavylium cation (AH+) with quantum yield Phi=0.04 (25 degrees C) at pH 2.2 or to the orange quinoidal base (A) with quantum yield Phi=0.015 (25 degrees C) at pH 5.2. The photoproducts revert back to their initial form by a thermal process characterized by first-order kinetics; the rate constants exhibit a bell shape variation with pH, with a maximum at pH 4.3 (lifetime 4.2 min). The second system, 7-(N,N-diethylamino)-4-hydroxyflavylium, does not exhibit photochemistry in water but, when incorporated into the Pluronic F-127 gel, switches from yellow to red with a quantum yield of Phi=0.01 at pH 4.9. The respective thermal back reaction takes place with a lifetime of 66.7 min1. The flavylium network of chemical reactions is a good sensor for the detection of not only the critical micelle temperature but also the gelation temperature of Pluronic and like solutions and, in some instances, the exposure to UV and visible radiation.

Temperature- and light-responsive blends of pluronic F127 and poly(N,N-dimethylacrylamide-co-methacryloyloxyazobenzene)

Photoresponsive poly(N,N-dimethylacrylamide-co-methacryloyloxyazobenzene) (DMA-MOAB) and temperature-responsive Pluronic F127 (F127) copolymers were blended to obtain systems responsive to both stimuli that are potentially useful for pharmaceutical formulations. The random DMA-MOAB copolymer undergoes a trans to cis isomerization when irradiated by 366 nm light, which modifies both the air-water interfacial behavior and the self-associative properties of the copolymer. Under dark conditions the azobenzene groups of DMA-MOAB in the trans conformation self-associate and the interactions with F127 are minimal. The cis conformation of the azobenzene groups of the DMA-MOAB copolymer is relatively more hydrophilic than the trans conformation, which causes the copolymer micelles to dissociate upon irradiation, allowing the unimers to form mixed micelles with the F127. This causes the sol-gel transition temperature of the DMA-MOAB:F127 blend to be 10 degrees C lower upon irradiation at 366 nm compared to that for the dark conditions. It has been found that F127 (10-12 wt %):DMA-MOAB (5-6 wt %) aqueous solutions have at body temperature a low viscosity when equilibrated in the dark and undergo a sol-gel transition when irradiated. Such a transition strongly alters the diffusion of solutes such as methylene blue within the solutions. This light-induced interaction between the azobenzene moieties of DMA-MOAB and F127 micelles disappears when hydroxypropyl-beta-cyclodextrin (HPbetaCD) is added to the medium. In the presence of HPbetaCD, the cis-azobenzene groups are hosted in the cyclodextrin cavities and the mixed micelles are not formed. Therefore, changes in HPbetaCD concentration could be used to modulate the response of the copolymer blends to light.

Preparation and Controlled Self-Assembly of Janus Magnetic Nanoparticles

Janus magnetic nanoparticles (~20 nm) were prepared by grafting either polystyrene sodium sulfonate (PSSNa) or polydimethylamino ethylmethacrylate (PDMAEMA) to the exposed surfaces of negatively charged poly(acrylic acid) (PAA)-coated magnetite nanoparticles adsorbed onto positively charged silica beads. Individually dispersed Janus nanoparticles were obtained by repulsion from the beads on reversal of the silica surface charge when the solution pH was increased. Controlled aggregation of the Janus nanoparticles was observed at low pH values, with the formation of stable clusters of approximately 2-4 times the initial size of the particles. Cluster formation was reversed, and individually dispersed nanoparticles recovered, by restoring the pH to high values. At intermediate pH values, PSSNa Janus nanoparticles showed moderate clustering, while PDMAEMA Janus nanoparticles aggregated uncontrollably due to dipolar interactions. The size of the stable clusters could be controlled by increasing the molecular weight of the grafted polymer, or by decreasing the magnetic nanoparticle surface availability for grafting, both of which yielded larger cluster sizes. The addition of small amounts of PAA-coated magnetic nanoparticles to the Janus nanoparticle suspension resulted in a further increase in the final cluster size. Monte Carlo simulation results compared favorably with experimental observations and showed the formation of small, elongated clusters similar in structure to those observed in cryo-TEM images.

Poly(acrylic acid)-block-poly(l-valine): Evaluation of β-Sheet Formation and Its Stability Using Circular Dichroism Technique

Secondary structure formation in four novel hybrid poly(acrylic acid)-b-poly(L-valine) (PAA-b-PLVAL) block copolymers, that is, PAA(40)-PLVAL(100), PAA(80)-PLVAL(100), PAA(80)-PLVAL(80), and PAA(80)-PLVAL(60), was investigated by circular dichroism. The formation of stable and well-defined beta-sheet structure in the PLVAL hydrophobic domains was observed for all the copolymers. At pH 5, PAA(80)-PLVAL(60) with the lowest PLVAL/PAA molar ratio possessed the lowest beta-sheet content of 12%, and it increased to 62% for PAA(40)-PLVAL(100) system. The beta-sheet formation in the block copolymers was controlled by both random PAA-PLVAL hydrogen bonds at low pH and electrostatic repulsive forces on the PAA segment at high pH; hence, the beta-sheet structure was most stable at intermediate pH. The length of PAA segments was critical in the beta-sheet solubilization and in providing sufficient shielding of the hydrophobic core from denaturing agents such as urea.

Temperature dependence of aggregation and dynamic surface tension in a photoresponsive surfactant system

The response of a nonionic photoresponsive surfactant system to changes in temperature is reported. This surfactant contains the light-sensitive azobenzene group, and when exposed to light, a solution of this surfactant contains a mixture of the cis and trans photoisomers of this group. The temperature of the surfactant solution has a strong impact on the time needed for the surfactant to diffuse and adsorb to a freshly formed interface. At surfactant concentrations that give rise to trans aggregates but not to cis aggregates, the transport of cis and of trans isomers to the surface of a pendant bubble have quite different temperature dependencies, owing largely to the difference in their aggregation states in bulk solution. Diffusion and adsorption of the cis isomer are described reasonably well by a simple diffusion model that accounts for the effect of temperature on the diffusion coefficient. The trans isomer, which was primarily bound in aggregates during these measurements, exhibits a stronger dependence of this adsorption time scale on the temperature of the solution. This temperature dependence of trans diffusion and adsorption is quantitatively consistent between samples containing only the trans isomer and samples containing a mixture of isomers. Fluorescence studies were done to determine the effect of temperature on the cmc of the surfactant. The critical concentration associated with the formation of cis-dominant aggregates increases modestly with increasing temperature. The cmc of the trans isomer also increases with increasing temperature, most significantly when the temperature exceeds about 35 degrees C. These trans cmc temperature-dependence data were incorporated into diffusion models that account for the potential roles of aggregates in the adsorption process. The observed temperature dependency of the trans adsorption time scale is consistent with a model that includes the effect of temperature on both the diffusivity and the supply of monomer via its effect on the cmc. Specifically, the results suggest that the dissolution of trans-dominant aggregates is important to the trans adsorption process. Further fluorescence studies were performed in which surfactant solutions containing aggregates were diluted rapidly, and the rate of dissolution of these aggregates was inferred from fluorescence decay. Aggregate breakup in colder trans samples is slower than in warmer samples, but these dissolution time scales are significantly shorter than those associated with the adsorption process. This is consistent with the assumption that aggregation kinetics do not contribute to the observed adsorption kinetics.

Stop-flow lithography in a microfluidic device

Polymeric particles in custom designed geometries and with tunable chemical anisotropy are expected to enable a variety of new technologies in diverse areas such as photonics, diagnostics and functional materials. We present a simple, high throughput and high resolution microfluidic method to synthesize such polymeric particles. Building off earlier work that we have done on continuous flow lithography (CFL) (D. Dendukuri, D. C. Pregibon, J. Collins, T. A. Hatton, P. S. Doyle, Nat. Mater., 2006, 5, 365-369; ref. 1), we have devised and implemented a new setup that uses compressed air driven flows in preference to syringe pumps to synthesize particles using a technique that we call stop-flow lithography (SFL). A flowing stream of oligomer is stopped before polymerizing an array of particles into it, providing for much improved resolution over particles synthesized in flow. The formed particles are then flushed out at high flow rates before the cycle of stop-polymerize-flow is repeated. The high flow rates enable orders-of-magnitude improvements in particle throughput over CFL. However, the deformation of the PDMS elastomer due to the imposed pressure restricts how quickly the flow can be stopped before each polymerization event. We have developed a simple model that captures the dependence of the time required to stop the flow on geometric parameters such as the height, length and width of the microchannel, as well as on the externally imposed pressure. Further, we show that SFL proves to be superior to CFL even for the synthesis of chemically anisotropic particles with sharp interfaces between distinct sections.

Complexation and release of doxorubicin from its complexes with pluronic P85-b-poly(acrylic acid) block copolymers

Poly(acrylic acid) (PAA) was attached on both termini of Pluronic P85 copolymer (EO27PO39EO27) via atom transfer radical polymerization (ATRP) to produce a novel block copolymer, PAA-b-P85-b-PAA (P85PAA). The P85PAA-DOX complex formation and drug loading were strongly dependent on the PAA segment length and pH, where the protonation of carboxyl groups in the PAA segment at pH < 7.2 reduced the binding sites of DOX onto P85PAA chains, resulting in a diminished DOX uptake at low pH. The composition of copolymer-DOX complexes at pH 7.2 was close to the stoichiometric 1:1 DOX:carboxyl molar ratio, confirming the dominance of electrostatic interactions between cationic DOX molecules and carboxyl groups. The stability study of the copolymer-DOX complex suggested that non-polyelectrolyte interactions may also participate in the complexation of drug and P85PAA block copolymer. DOX loading at pH 5.0 decreased to 60% of the total binding capacity, indicating that protonation of carboxyl groups reduced the DOX binding to P85PAA block copolymer. DOX release from the complex is a pH-responsive process, where the protonation of carboxyl groups at mildly acidic condition resulted in a faster dissociation of copolymer-DOX complex, leading to an accelerated release of DOX at pH 5.0. Thus, complexation of DOX with P85PAA yielded a drug delivery system affording a pH-triggered release of DOX in an acidic environment of pH 5.0.

Dynamic surface tension behavior in a photoresponsive surfactant system

The surface properties of a nonionic photoresponsive surfactant that incorporates the light-sensitive azobenzene group into its tail have been investigated. Cis-trans photoisomerization of this azobenzene group alters the ability of the surfactant to pack into adsorbed monolayers at an air/water interface or into aggregates in solution, thereby causing a significant variation in surface and bulk properties following a change in the illumination conditions. NMR studies indicate that a solution left in the dark for an extended period of time contains the trans isomer almost exclusively, whereas samples exposed to light of fixed wavelength eventually reach a photostationary equilibrium in which significant amounts of both isomers are present. At concentrations well above the cmc but under different illumination conditions (dark, UV light, visible light), freshly formed surfaces exhibit profoundly different surface tension trajectories as they approach essentially identical equilibrium states. This common equilibrium state corresponds to a surface saturated with the trans (more surface active) isomer. The dark sample shows a simple, single-step relaxation in surface tension after the creation of a fresh interface, whereas the UV and visible samples exhibit a more rapid initial decrease in tension, followed by a plateau of nearly constant tension, and finally end with a second relaxation to equilibrium. It is hypothesized that this behavior of the UV and visible samples is caused by competitive adsorption between the cis and trans isomers present in these mixtures. The cis surfactant reaches the interface more quickly, leading to an initially cis-dominated interface having a tension value corresponding to the intermediate plateau, but is ultimately displaced by the trans isomer. Fluorescence studies are used for cmc determination in the samples, and the results suggest that the two isomers segregate into distinct aggregate phases. The critical concentration associated with the formation of cis-rich aggregates is much larger than that of the trans-rich aggregates, which accounts for the faster diffusion of the cis isomer to a fresh interface. Models of the diffusion and adsorption of surfactant are developed. These consider the role of aggregates in the adsorption process by examining the limiting behavior of three aggregate properties: dissolution rate, mobility, and ability to incorporate into the interface. These models are used to analyze the surface tension relaxation of dark and UV samples, and the predictions are found to be in agreement with the observed characteristic relaxation time scales for these samples, though the results are inconclusive regarding the specific role of aggregates. High-intensity illumination focused on a surface saturated with surfactant is used to drive photoisomerization of the adsorbed surfactant, and rapid, substantial changes in surface tension result. These changes are consistent with proposed conformations of the adsorbed surfactant and with monolayer studies performed with a Langmuir film balance.

Synthesis and self-assembly of amphiphilic polymeric microparticles

We report the synthesis and self-assembly of amphiphilic, nonspherical, polymeric microparticles. Wedge-shaped particles bearing segregated hydrophilic and hydrophobic sections were synthesized in a microfludic channel by polymerizing across laminar coflowing streams of hydrophilic and hydrophobic polymers using continuous flow lithography (CFL). Particle monodispersity was characterized by measuring both the size of the particles formed and the extent of amphiphilicity. The coefficient of variation (COV) was found to be less than 2.5% in all measured dimensions. Particle structure was further characterized by measuring the curvature of the interface between the sections and the extent of cross-linking using FTIR spectroscopy. The amphiphilic particles were allowed to self-assemble in water or at water-oil interfaces. In water, the geometry of the particles enabled the formation of micelle-like structures, while in emulsions, the particles migrated to the oil-water interface and oriented themselves to minimize their surface energy.

Synthesis and aggregation behavior of pluronic F87/poly(acrylic acid) block copolymer in the presence of doxorubicin

Poly(acrylic acid) (PAA) was polymerized on both termini of Pluronic F87 copolymer using the atom transfer radical polymerization technique to produce a novel block copolymer, PAA-b-F87-b-PAA (F87PAA). The loading of a cationic anticancer drug, doxorubicin (DOX), to F87PAA at different pH values was investigated using isothermal titration calorimetry (ITC), laser light scattering techniques, and UV-vis spectroscopy. At pH of 4.3-7.1, the ITC profile exhibited a significant exothermic peak, which indicated that the drug loading is an enthalpically driven process. At a pH of 4.3, the enthalpy maximum was significantly reduced in the presence of 2 M urea, indicating the existence of hydrogen bonds between the DOX and F87PAA copolymer. At a pH of 7.1, the fraction of bound DOX was close to the stoichiometric proportion of 1:1 to the molar concentration of carboxyl groups in the copolymer, where the drug loading is governed by electrostatic and stacking interactions. The TEM image of the complex indicated the formation of large compound micelles induced by the binding of DOX to the PAA segments.

Functionalization of monodisperse magnetic nanoparticles

We report a new strategy for the preparation of monodisperse, water-soluble magnetic nanoparticles. Oleic acid-stabilized magnetic nanocrystals were prepared by the organic synthesis route proposed by Sun et al. (J. Am. Chem. Soc. 2004, 126, 273.), with size control obtained via seeded-mediated growth. The oleic groups initially present on the nanoparticle surfaces were replaced via ligand exchange reactions with various capping agents bearing reactive hydroxyl moieties. These hydroxyl groups were (i) exploited to initiate ring opening polymerization (ROP) of polylactic acid from the nanoparticle surfaces and (ii) esterified by acylation to permit the addition of alkyl halide moieties to transform the nanoparticle surfaces into macroinitiators for atom transfer radical polymerization (ATRP). By appropriate selection of the ligand properties, the nanoparticle surfaces can be polymerized in various solvents, providing an opportunity for the growth of a wide variety of water-soluble polymers and polylectrolyte brushes (both cationic and anionic) from the nanoparticle surfaces. The nanoparticles were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), electron microscopy, and light scattering. Light scattering measurements indicate that the nanoparticles are mostly present as individual nonclustered units in water. With pH-responsive polymers grown on the nanoparticle surfaces, reversible aggregation of nanoparticles could be induced by suitable swings in the pH between the stable and unstable regions.

Correlating Transfection Barriers and Biophysical Properties of Cationic Polymethacrylates

Transfection efficiencies of several polymeric gene carriers were compared and correlated quantitatively to the amounts of cellular accumulation of plasmid DNA and to the expression of mRNA by quantitative real-time polymerase chain reaction (real-time PCR). Three polycations polymers with similar chemical structure were used in this study: poly(dimethylamino)ethyl methacrylate (PDMA) homopolymer, PEO-b-PDMA copolymer, and PEO-b-poly(diethylamino)ethyl methacrylate (PEO-b-PDEA) copolymer. Despite their similar chemical structures, the transfection efficiencies were significantly different. PEO-b-PDEA copolymer was significantly less efficient as gene carrier as compared to both PDMA and PEO-b-PDMA. Correlations between cytotoxicity, cellular uptake of plasmid DNA, expression levels of transgene and protein, and the physical properties of the polymers were observed. With the PEO-b-PDEA studies, cytotoxicity was due primarily to the excess of polymers that did not participate in the DNA binding. In addition, the inability of the polymer/DNA polyplexes to interact with cell effectively was identified as a critical barrier for high efficiency of transfection. This study demonstrated that the use of quantitative real-time PCR in combination with physical characterization techniques could provide useful insights into the transfection barrier at different cellular levels.

Daniel I.C. Wang: a tribute to an inspirational leader and colleague

Daniel I.C. Wang has been an influential leader of the biotechnology industry over the past four decades through his inspirational research activities, the legions of students and other researchers that have studied under him, his development of many research and educational initiatives, both nationally and internationally, and the advice he has given worldwide to companies, research institutions, universities, and governments. He has played an important role in the mentoring and nurturing of junior faculty members, and has been a supportive collaborator in research and teaching. This two-part article provides a brief overview of Danny Wang's many contributions to furthering the global development of biotechnology, with particular emphasis on his recent activities in Asia, and concludes with an account of his research collaborations with the author over the past two decades.

Solution behavior of linear-dendritic rod diblock copolymers in methanol

The solution behavior of spherical dendrimers as well as hybrid-linear dendritic diblock copolymers has been extensively studied, and the size, shape, and ability of these polymers to encapsulate small molecules have led to their comparison with traditional micelles. We have recently reported the synthesis of a new dendritic copolymer architecture, the linear-dendritic rod diblock copolymer, and in this work, we examine the solution behavior of these unique polymers in methanol at 25 degrees C, using dynamic light scattering and intrinsic viscosity measurements. The diblock copolymers consist of a linear poly(ethylene oxide)-poly(ethylene imine) diblock copolymer backbone around which poly(amido amine) branches have been divergently synthesized from the poly(ethylene imine) block. The hydrodynamic radii and the viscometric radii of the polymers were found to increase slowly with increasing generation up to generation 3.5; however, after generation 3.5, the radii were found to increase very rapidly. This increase can be explained by an elongation of the dendritic block into a more rodlike configuration and a corresponding breakdown of the spherical approximation used to calculate the radii. The intrinsic viscosity of the amine and ester terminated polymers was found to follow two very different trends at low generation; however, at higher generations, they followed similar, yet slightly different, curves with the values for the amine terminated polymers only a little larger than those of the ester terminated polymers. At low generations, the chemistry of the end groups and its interaction with the solvent were found to be more important, whereas at higher generations, the highly branched nature of the dendritic block was the more important factor. For the ester terminated polymers, a maximum in the intrinsic viscosity occurred at generation 1.5. Since this maximum occurred at a much lower generation number than is traditionally seen for spherical dendrimers, new scaling relations for the intrinsic viscosity of dendritic rod polymers were developed and were found to support this observation. A minimum in the intrinsic viscosity was also observed at generation 3.5 for the ester terminated polymers and a minimum or leveling off in the intrinsic viscosity at generation 4.0 was found for the amine terminated polymers, which can be attributed to the transitioning of the polymers to a more elongated, rodlike shape and the increased influence of the shape factor on the intrinsic viscosity.

Ion-Exchange Purification of Proteins Using Magnetic Nanoclusters

Polymer-coated magnetic nanoclusters were used for recovery and purification of proteins from both model systems and cell-free Pichia pastoris fermentation broth. The nanoclusters exhibited extremely high capacity for proteins, up to 900 mg/mL adsorbent, and were recovered by high gradient magnetic separation (HGMS) at flow rates of up to 3,600 cm(3)/cm(2) h (flow rates up to 15,000 cm(3)/cm(2) h are possible). The nanoclusters were coated with a primary coating of poly(acrylic acid-co-styrenesulfonic acid-co-vinylsulfonic acid), which allowed both electrostatic and hydrophobic interactions with the protein to be used to enhance specificity for targeted products. With this dual mode separation, nearly pure protein could be recovered from complex mixtures, such as fermentation broth, in a few quick steps.

Block ionomer complexes as prospective nanocontainers for drug delivery

Nanosized environmentally responsive materials are of special interest for various applications, including drug delivery. Block ionomer complexes (BIC) composed of graft-comb copolymers of Pluronic and poly(acrylic acid) (Pluronic-PAA) and a model cationic surfactant, hexadecyltrimethylammonium bromide (HTAB), were synthesized by mixing the polymer and surfactant in aqueous media. According to TEM, the resulting BIC represented spherical particles of nanoscale size (50 to 100 nm). The stability of the BIC in the aqueous dispersion depended on the lengths of the hydrophilic poly(ethylene oxide) and hydrophobic poly(propylene oxide) chains in Pluronic molecules as well as on the surface charge of the resulting complexes. The latter was controlled by changing the ratio of the Pluronic-PAA and HTAB in the BIC and by changing the pH due to reversible ionization of the PAA chains. The acidification of the media below pH 6.0 resulted in the appearance of a strong positive charge on the BIC, which in the intracellular environment can trigger interaction of such BIC with the cell membranes. An efficient solubilization of a model hydrophobic molecule, Sudan III, and a drug, Etoposide, in such BIC was demonstrated with the loading capacities of about 6 to 15% by weight of the dispersed complex. Overall, these BIC wield a promise as environmentally responsive nanocarriers for pharmaceuticals.

Continuous-flow lithography for high-throughput microparticle synthesis

Precisely shaped polymeric particles and structures are widely used for applications in photonic materials, MEMS, biomaterials and self-assembly. Current approaches for particle synthesis are either batch processes or flow-through microfluidic schemes that are based on two-phase systems, limiting the throughput, shape and functionality of the particles. We report a one-phase method that combines the advantages of microscope projection photolithography and microfluidics to continuously form morphologically complex or multifunctional particles down to the colloidal length scale. Exploiting the inhibition of free-radical polymerization near PDMS surfaces, we are able to repeatedly pattern and flow rows of particles in less than 0.1 s, affording a throughput of near 100 particles per second using the simplest of device designs. Polymerization was also carried out across laminar, co-flowing streams to generate Janus particles containing different chemistries, whose relative proportions could be easily tuned. This new high-throughput technique offers unprecedented control over particle size, shape and anisotropy.

Aggregation behavior and thermodynamics of binding between poly(ethylene oxide)-block-poly(2-(diethylamino)ethyl methacrylate) and plasmid DNA

The aggregation behavior and the thermodynamics of binding between poly(ethylene oxide)-block-poly(2-(diethylamino)ethyl methacrylate) (PEO-b-PDEAEMA) block copolymers and plasmid DNA were examined. Binding between the polymer and DNA were confirmed by gel electrophoresis. The high affinity between the polymer and DNA was demonstrated through the ethidium bromide (EtBr) displacement assay, and the binding was found to be related to the stoichiometric balance between the amine group of the polymer and the DNA nucleotide molar ratio (N/P molar ratio). The light scattering and TEM results showed that, at low polymer concentration, the hydrodynamic radii (R(h)) of the polymer/DNA complexes was around 90 nm; however, at sufficiently high polymer concentration, the complexes condensed to around 35 nm induced by a structural rearrangement of the amphiphilic nature of the block copolymer. The isothermal titration calorimetric results showed that the binding between the polymer and DNA is driven by a large favorable enthalpy.

Self-assembly of a nonionic photoresponsive surfactant under varying irradiation conditions: a small-angle neutron scattering and cryo-TEM study

We have used small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) to determine the structure of aggregates formed by the photoresponsive surfactants diethylene glycol mono(4',4-butyloxy, butyl-azobenzene) (C4AzoOC4E2) and diethylene glycol mono(4',4-hexyloxy, butyl-azobenzene) (C4AzoOC6E2) under different illumination conditions. At high concentrations, the self-assembly behavior of these surfactants changes remarkably in response to different radiation conditions. The trans isomers assemble into bilamellar (C4AzoOC4E2) and unilamellar (C4AzoOC6E2) vesicles, while the cis isomers (under UV light) form bicontinuous phases. These light-induced structural changes are attributed to a change in the sign of the Gaussian rigidity, which is the direct result of azobenzene photoisomerization.

Rigid, superparamagnetic chains of permanently linked beads coated with magnetic nanoparticles. Synthesis and rotational dynamics under applied magnetic fields

An inexpensive and versatile approach is reported for the synthesis of monodisperse magnetoresponsive rods of desired diameter, length, and magnetic susceptibility based on the confined alignment of magnetic beads in microchannels of selected channel height, followed by localized hydrolysis of sol-gel precursors within polyelectrolyte shells adsorbed on the beads. The layer-by-layer technique was used to coat the polystyrene beads with polyelectrolytes of alternating charge and with charged magnetic nanoparticles, and the polystyrene cores could be removed either by solvent dissolution or by calcination to form hollow-shelled chains. The reorientation dynamics of single and clustered chains following the application of an external magnetic field was evaluated theoretically, with favorable comparisons with the experimental data.

Synthesis of flexible magnetic nanowires of permanently linked core-shell magnetic beads tethered to a glass surface patterned by microcontact printing

We have developed an efficient, one-step method to create magnetic nanowires consisting of permanently linked chains of magnetic beads of varying flexibility tethered to a patterned glass surface using simple amidation chemistry. The flexibility of the nanowire was governed by the molecular weight of the molecule used to covalently link the beads and its length by the height of the microchannel in which it was synthesized. The nanowire diameter was determined both by the bead size and by the number of beads adhering to each dot in the microstamped, patterned array. Longer nanowires can form loops attached at two points on the glass surface. Both single flexible chains and flexible loops can adopt different configurations (straight, hairpin, S-shaped, etc.) when subjected to magnetic fields, the configurations depending on the directions of these fields. Shorter, less flexible nanowires align with the field always and do not exhibit the more exotic configurations seen for long, flexible chains and loops. These magnetic nanowires can have potential use in microfluidic pumping and mixing processes and in microparticle manipulation.

Pluronic-g-poly(acrylic acid) copolymers as novel excipients for site specific, sustained release tablets

Potential utility of copolymers comprising Pluronic (PEO-PPO-PEO) surfactants covalently conjugated with poly(acrylic acid) (PAA) as excipients for sustained-release tablets was explored. Apparent particle density, particle size distribution, Carr index, thermal stability, and compression behavior of the Pluronic-PAA copolymers were characterized. Tablets prepared by direct compression of blends of Pluronic-PAA copolymers were evaluated on the basis of their thermomechanical profile, crushing strength, friability, and drug release properties. Small molecular weight drugs of aqueous solubility decreasing in the order theophylline>hydrochlorothiazide>nitrofurantoin were incorporated to the tablets. For comparison purposes, tablets were also prepared from PAA of Carbopol 71G (C71G), and mixtures of C71G and Pluronic F127, with each of the above three drugs. The Pluronic-PAA aggregates are stabilized by hydrophobic associations between poly(propylene oxide) (PPO) segments in aqueous solutions, and thus require higher ionization of the carboxylic groups to overcome the associations and swell. The swelling pattern of the Pluronic-PAA copolymers is more dramatically pH-dependent than that of Carbopol lacking any hydrophobic associations. The drug retention in and release from the Pluronic-PAA based tablets is profoundly pH-dependent and hence specific to the pH exceeding that of the pK(a)>5 of these copolymers. Theophylline- and hydrochlorotiazide-containing tablets made with Pluronic-PAA copolymers showed a reduced release rate under acidic conditions compared to the neutral or alkaline conditions, while the opposite pattern was observed with the Carbopol-based tablets due to the different pH-dependent swelling behavior of the polymers. Nitrofurantoin-containing tablets showed a remarkably low drug release rate owing to the strong hydrophobic character of nitrofurantoin and of its complexes with the copolymers. Integrity of the nitrofurantoin-containing tablets was maintained during the 24h release test. Zero-order kinetics of the cumulative release profile of all drugs under study was observed with the Pluronic-PAA as a tablet excipient. Adequate mechanical properties, the self-assembling behavior, and the pH-sensitiveness of the Pluronic-PAA copolymers make them promising excipients for tablets with preferential delivery into a neutral to alkaline pH environment.

Controlled clustering and enhanced stability of polymer-coated magnetic nanoparticles

The clustering and stability of magnetic nanoparticles coated with random copolymers of acrylic acid, styrenesulfonic acid, and vinylsulfonic acid has been studied. Clusters larger than 50 nm are formed when the coatings are made using too low or too high molecular weight polymers or using insufficient amounts of polymer. Low-molecular-weight polymers result in thin coatings that do not sufficiently screen van der Waals attractive forces, while high-molecular-weight polymers bridge between particles, and insufficient polymer results in bare patches on the magnetite surface. The stability of the resulting clusters is poor, but when an insufficient polymer is used as primary coating, and a secondary polymer is added to coat remaining bare magnetite, the clusters are stable in high salt concentrations (>5 M NaCl), while retaining the necessary cluster size for efficient magnetic recovery. The magnetite cores were characterized by TEM and vibrating sample magnetometry, while the clusters were characterized by dynamic light scattering. The clustering and stability are interpreted in terms of the particle-particle interaction forces, and the optimal polymer size can be predicted well on the basis of these forces and the solution structure and hydrophobicity of the polymer. The size of aggregates formed by limited polymer can be predicted with a diffusion-limited colloidal aggregation model modified with a sticking probability based on fractional coating of the magnetite cores.

Biophysical characterization of complexation of DNA with block copolymers of poly(2-dimethylaminoethyl) methacrylate, poly(ethylene oxide), and poly(propylene oxide)

The interactions of DNA (salmon testes) with two new cationic block copolymers made of poly(2-dimethylaminoethyl) methacrylate and poly(ethylene oxide), PEO-pDMAEMA, or poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), L92-pDMAEMA, were studied with the aim to understand their different in vitro transfection efficiencies when used as nonviral delivery vectors. PEO-pDMAEMA does not show surface activity while L92-pDMAEMA is as surface active as its parent Pluronic L92. Surface tension, titration microcalorimetry, ethidium bromide displacement, and zeta-potential measurements were carried out in phosphate buffers at pH 5 and 7. The association of L92-pDMAEMA with DNA was strongly exothermic at both pHs; the critical aggregation concentration (CAC) corresponded to a N/P ratio of 0.3, the maximum energy evolved was reached for N/P ratios of 0.82 and 1.27 at pH 5 and pH 7, respectively, and the saturation occurred for N/P ratios close to 2. The presence of L92 in the structure of this new block copolymer apparently did not modify the thermodynamic parameters of the interaction with DNA. In contrast, the interaction with PEO-pDMAEMA was significantly less exothermic, and CAC and saturation occurred for N/Ps equal to 0.43 and 1.37, respectively. The strong affinity of L92-pDMAEMA for DNA was reflected in its capacity to displace ethidium bromide and in the jump in the values of the zeta potential when N/P is near 1. Above the N/P ratio at which electroneutral polyplexes are formed, only at pH 5 an excess of L92-pDMAEMA is incorporated in the complexes, resulting in positively charged complexes. The profile of the zeta-potential values obtained for mixtures of L92-pDMAEMA with Pluronic P123 showed a shift to a lower N/P ratio, owing to an easier interaction of L92-pDMAEMA molecules with DNA in the presence of P123. Additionally, a visual inspection of the systems indicates that P123 contributes to stabilize/solubilize the DNA/cationic polymer aggregates, by avoiding the typical phase separation near the charge neutralization point. The information obtained can be particularly useful to optimize the conditions to form efficient polyplexes for gene delivery systems.

Polycationic Block Copolymers of Poly(ethylene oxide) and Poly(propylene oxide) for Cell Transfection

A facile, one-step synthesis of cationic block copolymers of poly(2-N-(dimethylaminoethyl) methacrylate) (pDMAEMA) and copolymers of poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO) has been developed. The PEO-PPO-PEO-pDMAEMA (L92-pDMAEMA) and PEO-pDMAEMA copolymers were obtained via free radical polymerization of DMAEMA initiated by polyether radicals generated by cerium(IV). Over 95% of the copolymer fraction was of molecular mass ranging from 6.9 to 7.1 kDa in size, indicating the prevalence of the polyether-monoradical initiation mechanism. The L92-pDMAEMA copolymers possess parent surfactant-like surface activity. In contrast, the PEO-pDMAEMA copolymers lack significant surface activity. Both copolymers can complex with DNA. Hydrodynamic radii of the complexes of the L92-pDMAEMA and PEO-pDMAEMA with plasmid DNA ranged in size from 60 to 400 nm, depending on the copolymer/DNA ratio. Addition of Pluronic P123 to the L92-pDMAEMA complexes with DNA masked charges and decreased the tendency of the complex to aggregate, even at stoichiometric polycation/DNA ratios. The transfection efficiency of the L92-pDMAEMA copolymer was by far greater than that of the PEO-pDMAEMA copolymer. An extra added Pluronic P123 further increased the transfecton efficacy of L92-pDMAEMA, but did not affect that of PEO-pDMAEMA.

Controlled synthesis of nonspherical microparticles using microfluidics

The controlled synthesis of nonspherical microparticles using microfluidics processing is described. Polymer droplets, formed by shearing a photopolymer using a continuous water phase at a T-junction, were constrained to adopt nonspherical shapes by confining them using appropriate microchannel geometries. Plugs were obtained by shearing the polymer phase at low shear rates, while disks were obtained by flattening droplets using a channel of low height. The nonspherical shapes formed were permanently preserved by photopolymerizing the constrained droplets in situ using ultraviolet light. Monodisperse plugs and disks of different lengths and diameters were obtained by varying the flow rates of the two phases.

Kinetics of swelling of polyether-modified poly(acrylic acid) microgels with permanent and degradable cross-links

Spherical particles of 50-100 mum size composed of poly(acrylic acid) networks covalently bonded to Pluronic polyether copolymers were tested for swelling in aqueous media. The microgels were cross-linked either by permanent ethylene glycol dimethacrylate (EGDMA) cross-links alone or by EDGMA together with reversible disulfide or biodegradable azoaromatic cross-links. Optimum conditions for a rapid, diffusion-limited swelling of the pH- and temperature-sensitive microgels with nondegradable cross-links were found. The microgels cross-linked by disulfide groups and equilibrium-swollen in the buffer solution exhibited degradation-limited kinetics of swelling under physiological conditions, with a first-order reaction constant, k(1), linearly proportional to the concentration of reducing agents such as dithiotreitol and tris(2-carboxyethyl)phosphine (TCEP). A severalfold faster swelling in the presence of more powerful reducing agent, TCEP, was observed, indicating the chemical specificity of the microgel swelling. The reoxidation of the thiol groups into disulfide cross-links by sodium hypochlorite led to the restoration of the microgels' diameter measured prior to the reduction-reoxidation cycle, which confirms the shape memory of the microgels. Enzymatically degradable azoaromatic cross-links enabled slow microgel swelling due to degradation of the cross-links by azoreductases from the rat intestinal cecum. The low rate of swelling of the Pluronic-containing microgels can enable sustained drug release in colon-specific drug delivery.