Influence of temperature and electrolytes on the adsorption of poly(ethylene oxide)-poly(propylene oxide) block copolymer on polystyrene latex and on the stability of the polymer-coated particles

The effects of polymer concentration, shear rate and temperature on the gelation time of aqueous Silica-Poly(ethylene-oxide) "Shake-gels"

HYPOTHESIS

Aqueous mixtures of silica and Poly(ethylene-oxide) (PEO) are known as "Shake-gels" due to the formation of reversible gels when subject to an applied force, such as shaking. This shear-thickening effect can be observed using a rheometer, via distinct and abrupt increases in the viscosity of the material. Preliminary experiments qualitatively showed that the time elapsed before this occurs, termed the gelation time, varied depending on the conditions used. This paper reports on a systematic study into the effects of polymer concentration, shear rate and temperature on the gelation time, to quantify any relationships that exist between the variables and develop understanding of the gelation mechanism and kinetics.

EXPERIMENTS

Different constant shear rates were applied to samples at various polymer concentrations and temperatures using a rheometer with concentric cylinder geometry.

FINDINGS

The gelation time varied significantly from several seconds to an hour or more and was exponentially accelerated by shear rate. A peak in gelation time occurred at medium polymer concentrations of 0.35-0.40% (25% silica) and at a temperature about 20 °C. Higher temperatures also exponentially accelerated the gelation time as kinetic effects dominated the thermodynamic and structural resistances to gel formation.

Adsorption of poly(ethylene oxide)-containing amphiphilic polymers on solid-liquid interfaces: Fundamentals and applications

The adsorption of amphiphilic molecules of varying size on solid-liquid interfaces modulates the properties of colloidal systems. Nonionic, poly(ethylene oxide) (PEO)-based amphiphilic molecules are particularly useful because of their graded hydrophobic-hydrophilic nature, which allows for adsorption on a wide array of solid surfaces. Their adsorption also results in other useful properties, such as responsiveness to external stimuli and solubilization of hydrophobic compounds. This review focuses on the adsorption properties of PEO-based amphiphiles, beginning with a discussion of fundamental concepts pertaining to the adsorption of macromolecules on solid-liquid interfaces, and more specifically the adsorption of PEO homopolymers. The main portion of the review highlights studies on factors affecting the adsorption and surface self-assembly of PEO-PPO-PEO block copolymers, where PPO is poly(propylene oxide). Block copolymers of this type are commercially available and of interest in several fields, due to their low toxicity and compatibility in aqueous systems. Examples of applications relevant to the interfacial behavior of PEO-PPO-PEO block copolymers are paints and coatings, detergents, filtration, and drug delivery. The methods discussed herein for manipulating the adsorption properties of PEO-PPO-PEO are emphasized for their ability to shed light on molecular interactions at interfaces. Knowledge of these interactions guides the formulation of novel materials with useful mesoscale organization and micro- and macrophase properties.

A vaginal drug delivery model

Efficient drug delivery at vaginal cavity is often a challenge owing to its peculiar physiological variations including vast differences in pH. Keeping in view this attribute of the target site, the current work was aimed at developing formulation strategies which could overcome this and successfully deliver molecules like itraconazole through SLNs. Optimized SLNs with the given composition was selected for further development into mucoadhesive and thermosensitive gel. Stearic acid and Compritol 888 (1:1, w/w ratio) as lipid, a mixture of 3% Poloxomer 188 and 0.5% sodium taurocholate as surfactant and organic to aqueous ratio of 10:50 was taken. Carbopol 934 and Pluronic F 127 were taken for the development of gel. Optimized gel exhibited a desired gelling temperature (35 °C); viscosity (0.920 PaS) and appreciable in vitro drug release (62.2% in 20 h). MTT assay did not show any cytotoxic effect of the gel. When evaluated in vivo, it did not exhibit any irritation potential despite appreciable bioadhesion. A remarkable decrease in CFUs was also observed in comparison with control and marketed formulation when evaluated in rat infection model. Thus, the proposed study defines the challenges for developing a suitable formulation system overcoming the delivery barriers of the vaginal site.

Quenched Assembly of NIR-Active Gold Nanoclusters Capped with Strongly Bound Ligands by Tuning Particle Charge via pH and Salinity

Gold nanospheres coated with a binary monolayer of bound citrate and cysteine ligands were assembled into nanoclusters, in which the size and near-infrared (NIR) extinction were tuned by varying the pH and concentration of added NaCl. During full evaporation of an aqueous dispersion of 4.5 ± 1.8 nm Au primary particles, the nanoclusters were formed and quenched by the triblock copolymer polylactic acid (PLA)(1K)-b-poly(ethylene glycol) (PEG)(10K)-b-PLA(1K), which also provided steric stabilization. The short-ranged depletion and van der Waals attractive forces were balanced against longer ranged electrostatic repulsion to tune the nanocluster diameter and NIR extinction. Upon lowering the pH from 7 to 5 at a given salinity, the magnitude of the charge on the primary particles decreased, such that the weaker electrostatic repulsion increased the hydrodynamic diameter and, consequently, NIR extinction of the clusters. At a given pH, as the concentration of NaCl was increased, the NIR extinction decreased monotonically. Furthermore, the greater screening of the charges on the nanoclusters weakened the interactions with PLA(1K)-b-PEG(10K)-b-PLA(1K) and thus lowered the amount of adsorbed polymer on the nanocluster surface. The generalization of the concept of self-assembly of small NIR-active nanoclusters to include a strongly bound thiol and the manipulation of the morphologies and NIR extinction by variation of pH and salinity not only is of fundamental interest but also is important for optical biomedical imaging and therapy.

Docetaxel-nicotinamide complex-loaded nanostructured lipid carriers for transdermal delivery

Docetaxel (DTX) proved one of the most effective active pharmaceutical ingredients (APIs) for the treatment of cancers. However, in respect of its low solubility and high lipophilic property, nicotinamide (NCT) was chosen as the co-former to form the docetaxel-nicotinamide complex to handle the drawbacks. As was analyzed by Fourier Transform Infrared spectrometer, thermal analysis and saturated solubility, the complex proved stable. Then, docetaxel-nicotinamide complex nanostructured lipid carriers (DN-NLCs) were prepared by emulsion-evaporation at low temperature method. The average drug entrapment efficiency, particle size and drug loading of docetaxel-NLCs (D-NLCs) and DN-NLCs were 81.41-79.48%, 61.45-59.48nm and 1.60-1.63%, respectively. The physicochemical characteristics of nanoparticles were valued by transmission electron microscope and Powder X Ray Diffraction. The in vitro drug-release profile of nanoparticle formulations fitted the Weibull dynamic equation. The skin permeability test was performed by Vertical Franz-type diffusion cells. It demonstrated that DN-NLCs transported drugs more easily than D-NLCs. Confocal Laser Scanning Microscopy observation showed DN-NLCs permeated more effectively than D-NLCs. In vivo study demonstrated that DN-NLCs maintained most in the skin. These results suggest that the DN-NLCs can be a useful method to increase skin permeation of docetaxel.

Nanostructured lipid carriers (NLC) based topical gel of flurbiprofen: design, characterization and in vivo evaluation

Nanostructured lipid carriers (NLC)-based gel was developed as potential topical system for flurbiprofen (FP) topical delivery. The characterizations of the prepared semisolid formulation for topical application on skin were assessed by means of particle size distribution, zeta potential analysis, X-ray analysis, in vitro percutaneous penetration, rheological study, skin irritation test, in vivo pharmacodynamic evaluation and in vivo pharmacokinetic study. The NLC remained within the colloidal range and it was uniformly dispersed after suitably gelled by carbopol preparation. It was indicated in vitro permeation studies through rat skin that FP-NLC-gel had a more pronounced permeation profile compared with that of FP-loaded common gel. Pseudoplastic flows with thixotropy were obtained for all NLC-gels after storage at three different temperatures. No oedema and erythema were observed after administration of FP-NLC-gel on the rabbit skin, and the ovalbumin induced rat paw edema could be inhibited by the gel. The maximum concentration in plasma was 29.44 μg/ml and 2.49 μg/ml after oral and topical administration, respectively. While the amount of drug accumulated in skin after topical application was much higher than oral application. In conclusion, NLC-based gel could be a promising vehicle for topical delivery of FP.

Dispersions, novel nanomaterial sensors and nanoconjugates based on carbon nanotubes

Nanomaterials are structures with dimensions characteristically much below 100 nm. The unique physical properties (e.g., conductivity, reactivity) have placed these nanomaterials in the forefront of emerging technologies. Significant enhancement of optical, mechanical, electrical, structural, and magnetic properties are commonly found through the use of novel nanomaterials. One of the most exciting classes of nanomaterials is represented by the carbon nanotubes. Carbon nanotubes, including single-wall carbon nanotubes, multi-wall carbon nanotubes, and concentric tubes have been shown to possess superior electronic, thermal, and mechanical properties to be attractive for a wide range of potential applications They sometimes bunch to form "ropes" and show great potential for use as highly sensitive electronic (bio)sensors due to the very small diameter, directly comparable to the size of single analyte molecules and that every single carbon atom is in direct contact with the environment, allowing optimal interaction with nearby molecules. Composite materials based on integration of carbon nanotubes and some other materials to possess properties of the individual components with a synergistic effect have gained growing interest. Materials for such purposes include conducting polymers, redox mediators and metal nanoparticles. These tubes provide the necessary building blocks for electronic circuits and afford new opportunities for chip miniaturization, which can dramatically improve the scaling prospects for the semiconductor technologies and the fabrication of devices, including field-effect transistors and sensors. Carbon nanotubes are one of the ideal materials for the preparation of nanoelectronic devices and nanosensors due to the unique electrical properties, outstanding electrocatalytic properties, high chemical stability and larger specific surface area of nanotubes. Carbon nanotubes are attractive material for supercapacitors due to their unique one-dimensional mesoporous structure, high specific surface area, low resistivity and good chemical stability. Nanoscaled composite materials based on carbon nanotubes have been broadly used due to their high chemical inertness, non-swelling effect, high purity and rigidity. The integration of carbon nanotubes with organics, biomaterials and metal nanoparticles has led to the development of new hybrid materials and sensors. Hybrid nanoscale materials are well established in various processes such as organic and inorganic compounds, nucleic acid detachment, protein separation, and immobilization of enzymes. Those nanostructures can be used as the building blocks for electronics and nanodevices because uniform organic and metal coatings with the small and monodisperse domain sizes are crucial to optimize nanoparticle conductivity and to detect changes in conductivity and absorption induced by analyte adsorption on these surfaces. The highly ordered assembly of zero-dimensional and one-dimensional nanoparticles is not only necessary for making functional devices, but also presents an opportunity to develop novel collective properties.

Colloidal stability of pluronic F68-coated PLGA nanoparticles: a variety of stabilisation mechanisms

Poloxamers are a family of polypropylene oxide (PPO) and polyethylene oxide (PEO) tri-block copolymers that are usually employed in the micro- and nanoparticulate engineering for drug delivery systems. The aim of this work is to study the electrophoretic mobility (mu(e)) and colloidal stability of complexes formed by adsorbing a poloxamer (Pluronic F68) onto poly(d,l-lactic-co-glycolic acid) (PLGA) nanoparticles. A variety of stabilisation mechanisms have been observed for the Pluronic-coated PLGA nanoparticles, where DLVO interactions, solvent-polymer segment interactions and hydration forces play different roles as a function of the adsorbed amount of Pluronic. In addition, the mu(e) and stability data of these complexes have been compared to those obtained previously using a PLGA-Pluronic F68 blend formulation. As both the mu(e) and the stability data are identical between the two systems, a phase separation of both components in the PLGA-Pluronic blend formulation is suggested, being the PLGA located in the core of the particles and the Pluronic in an adsorbed shell.

Effect of steric, double-layer, and depletion interactions on the stability of colloids in systems containing a polymer and an electrolyte

Experiments carried out by Stenkamp et al. [Stenkamp, V. S.; McGuiggan, P.; Berg, J. C. Langmuir 2001, 17, 637.] have shown that polystyrene latexes can be restabilized at sufficiently high electrolyte concentrations in the presence of an amphiphilic block copolymer [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO)] At even higher electrolyte concentrations, the systems can again be destabilized. The present paper attempts to explain the restabilization through the dominance of steric interactions and the destabilization through the dominance of depletion interactions. Because of salting out, as the concentration of electrolyte increases, the polymer molecules are increasingly precipitated onto the surface of the latex particles and, at sufficiently high electrolyte concentrations, form, in addition, aggregates. The precipitation onto the latex particles generates steric repulsion, which is responsible for the restabilization, whereas the formation of aggregates generates depletion interactions, which are responsible for destabilization.

Surfactant effects on carbon nanotube interactions with human keratinocytes

Interactions of multiwalled carbon nanotubes (MWCNTs) with human epidermal keratinocytes (HEKs) were studied with respect to the effect of surfactant on dispersion of MWCNT aggregates and cytotoxicity. Our earlier studies had shown that the unmodified MWCNTs were localized within the cytoplasmic vacuoles of HEKs and elicited an inflammatory response. However, MWCNTs in solution tend to aggregate and, therefore, cells are exposed to large MWCNT aggregates. The purpose of this study was to find a surfactant that prevents the formation of large aggregates of MWCNTs without being toxic to the HEKs. HEKs were exposed to serial dilutions (10% to 0.1%) of L61, L92, and F127 Pluronic and 20 or 60 Tween for 24 hours. HEK viability, proportional to surfactant concentration, ranged from 27.1% to 98.5% with Pluronic F127; viability with the other surfactants was less than 10%. Surfactants dispersed and reduced MWCNT aggregation in medium. MWCNTs at 0.4 mg/mL in 5% or 1% Pluronic F127 were incubated with HEKs and assayed for interleukin 8 (IL-8). MWCNTs were cytotoxic to HEKs independent of surfactant exposure. In contrast, MWCNT-induced IL-8 release was reduced when exposed to 1% or 5% Pluronic F127 (P < .05). However, both MWCNTs and surfactant, alone or in combination, increased IL-8 release compared with control exposures at 12 and 24 hours. These results suggest that the surfactant-MWCNT interaction is more complex than simple dispersion alone and should be investigated to determine the mode of interaction.

Micelle-like nanoparticles of star-branched PEO-PLA copolymers as chemotherapeutic carrier

Four-armed (star-branched) block copolymers of l-PLA and PEO were synthesized using ring opening polymerization with different LA/EO ratio. Micellar aggregates were prepared from these block copolymers and characterized. Some surface segregation of PEG was found : the extent depends on the state of the material (whether it is in film or particle form), as well as on molecular geometry. The degradation behavior of star-shaped copolymer was studied over a three week period and compared to its linear counterpart. Anti-cancer drugs 5-FU and paclitaxel were loaded into the micellar nanoparticles. The drug release profile showed that the release of paclitaxel from these polymers could be controlled over 2 weeks. The kinetics of drug release for star-branched, tri- and di-block copolymers were compared. The micelles from star-shaped branch showed more complete release of drug than the diblock copolymers; also, the lower hydrodynamic radius of star-shaped polymers may result in better clearance of the carrier polymer from the body.

Micelle-like nanoparticles of PLA–PEG–PLA triblock copolymer as chemotherapeutic carrier

Triblock copolymer PLA-PEG-PLA were synthesized using ring opening polymerization with different LA/EG ratio. Micellar aggregates were prepared from these block copolymers and characterized. The degradation characteristics of selected copolymers were assessed in both micellar and film forms. Surface segregation of PEG was also quantified as a function of copolymer composition. Anti-cancer drugs 5-FU and paclitaxel were loaded into the micellar nanospheres with good efficiency. The drug release profile showed good control over the release of paclitaxel from these polymers.

Thin wetting films from aqueous solutions of a polyoxyethylene-polyoxypropylene block copolymer on silicon carbide surface

Thin wetting films from aqueous solutions of polyoxyethylene-polyoxypropylene ABA-block copolymer+NaCl in different concentrations on silicon carbide substrate have been studied. The polymer adsorption at the silicon carbide/aqueous solution interface has been studied using the spectrophotometric method, based on the colored polymer-iodine complex. The area per molecule, the conformation of the adsorbed molecules and the thickness of the adsorption layer at the solid/liquid interface were determined. The three-layer model of the structure of thin liquid films on the flat SiC surface has been used for calculation of the film thickness from the data obtained by the microinterferometric method. The dependence of the film thickness on time has been determined. The influence of the electrolyte concentration on the equilibrium film thickness has been studied. It was found that for NaCl concentrations higher than 3 x 10(-2) M the film thickness remained constant, i.e. the electrostatic disjoining pressure was fully suppressed. The disjoining pressure/thickness-isotherm has been obtained experimentally using the dynamic method. It has been interpreted by the superposition of a negative van der Waals component and a positive steric component of the disjoining pressure.

Adsorbed pluronics on the skin of human volunteers: effects on bacterial adhesion

An amphiphilic copolymer, Pluronic F127, has been deposited, by adsorption, to the skin of human volunteers and the ability of the coated skin to resist bacterial colonisation has been evaluated. In parallel, the ability of the same copolymer to act as a bacterial release agent has been evaluated. In both cases, F127 proved to be of little added value in formulations designed to suppress the bacterial colonisation of human skin.

The effect of surface coverage and conformation of poly(ethylene oxide) (PEO) chains of poloxamer 407 on the biological fate of model colloidal drug carriers

Poloxamer 407 was adsorbed onto the surface of model colloidal drug carriers, polystyrene nanoparticles of 40, 70 and 137 nm in diameter, and the effect of the degree of surface coverage and the conformation of the poly(ethylene oxide) (PEO) chains on biological fate was studied. The relationship between the physicochemical and the biological properties of the nanoparticle systems was also investigated. The adsorbed layer of poloxamer 407 was characterised in terms of percentage surface coverage, thickness of the adsorbed layer and average surface area per PEO chain. Computer modelling of the adsorbed layer was performed (applying the self-consistent field technique), to obtain the structural information of the PEO chains in the layer. The in vitro interaction of the nanoparticles with different degrees of poloxamer 407 surface coverage with serum components and the in vivo biodistribution in the rat model were assessed. The results demonstrated that an increase in the surface coverage with poloxamer 407 resulted in an increased volume fraction of the PEO in the adsorbed layer, further extension of the PEO chains from the surface and closer packing of the chains at the surface. With regard to the interaction with the serum components, an increased surface coverage resulted in a reduction of the amount of serum proteins adsorbed, and, importantly, affected the type of proteins adsorbed. High molecular weight proteins were not adsorbed onto the nanoparticles with a surface coverage above approx. 25%. Following the intravenous administration to rats, even the nanoparticles with the lowest degree of surface coverage (approx. 5%) showed improved circulation profiles relative to the uncoated nanoparticles. The effect was more pronounced for the 40 nm nanoparticles. A further increase in the surface coverage to approx. 25% resulted in a significant increase in circulation time, as compared to uncoated and 5% coated systems, for all sizes of nanoparticles. Importantly, it was found that a long in vivo blood circulation time could be achieved for nanoparticles with a relatively low degree of surface coverage with PEO chains.

Synthesis of Porous Zirconia Spheres-Mechanism and Prospects for Multistep Processing

We have previously reported the polymerization-induced colloid aggregation process (1, 2) to synthesize monodisperse, porous zirconia particles. One drawback is that it is difficult to simultaneously achieve a larger particle size and a higher zirconia yield. In this paper, we develop strategies to achieve these dual goals. We elucidate the interplay between the polymerization reaction and the various polymer-colloid interactions which result in the formation of the monodisperse particles. We explain the role of the polymer and the colloid in the process and suggest reasons for the limitations observed with a single-step process in achieving the dual goals of larger particle size and increased zirconia yield. Finally, we demonstrate a multistep process to overcome these limitations and achieve these dual goals. Copyright 1999 Academic Press.

Stability determination of solid lipid nanoparticles (SLN) in aqueous dispersion after addition of electrolyte

The contribution of mono-, di- and trivalent ions to the destabilization of solid lipid nanoparticle (SLN) dispersions was investigated, i.e. particle growth and subsequent formation of semi-solid gels. Sodium, calcium and aluminium chloride were added in varying concentrations to a Compritol formulation which had proved to be highly sensitive towards destabilizing effects. Dispersions containing up to 10(-3) M sodium chloride remained stable for 14 days. The same concentrations of calcium or aluminium induced slight and rapid particle growth, respectively. Generally, a pronounced destabilizing effect was observed with increasing electrolyte concentration and increasing valence. Higher concentrations of electrolyte (10(-2), 10(-1) M) induced gelation of the systems. The extent of solidification was highly dependent on the crystallinity of the lipid phase. The recrystalization indices of the gels were distinctly higher compared to the liquid systems. Additionally, unstable modifications, being present in liquid dispersions, were transformed into stable ones with increasing solidification. The mechanisms of the destabilizing effect of the electrolytes are reduced electrostatic repulsion and transformation of the lipid Compritol to the beta' modification promoting gel formation.

Biodegradable polymeric microparticles for drug delivery and vaccine formulation: the surface attachment of hydrophilic species using the concept of poly(ethylene glycol) anchoring segments

Poly(ethylene glycol)-dextran (PEG-DEX) conjugates have been used as a combined stabilizer and surface modifier to produce resorbable poly(DL-lactide-co-glycolide) (PLG) microparticles by an emulsification/solvent evaporation technique. The use of PEG or dextran polymers alone was incapable of producing microparticles. Particle size measurements revealed smaller mean particle sizes (480 nm) and improved polydispersity when using a 1.2% PEG substituted conjugate relative to a 9% substituted material (680 nm). PLG microparticles modified by post-adsorbed PEG-DEX conjugates flocculated in 0.01 M salt solutions, whereas PLG microparticles prepared using PEG-DEX as a surfactant were stable in at least 0.5 M NaCl solutions. Surface modification of PLG microparticles was confirmed by zeta potential measurements and surface analysis using X-ray photoelectron spectroscopy. The presence of surface exposed dextran was confirmed by an immunological detection method using a dextran-specific antiserum in an enzyme-linked immunosorbent assay. The findings support a model in which the PEG component of the PEG-DEX conjugate provides an anchor to the microparticle surface while the dextran component extends from the particle surface to contribute a steric stabilization function. This approach offers opportunities for attaching hydrophilic species such as targeting moieties to biodegradable microparticles to improve the interaction of drug carriers and vaccines with specific tissue sites.