Preparation of Core‐Sheath Composite Nanofibers by Emulsion Electrospinning

Summary: Uniform core‐sheath nanofibers are prepared by electrospinning a water‐in‐oil emulsion in which the aqueous phase consists of a poly(ethylene oxide) (PEO) solution in water and the oily phase is a chloroform solution of an amphiphilic poly(ethylene glycol)‐poly(L ‐lactic acid) (PEG‐PLA) diblock copolymer. The obtained fibers are composed of a PEO core and a PEG‐PLA sheath with a sharp boundary in between. By adjusting the emulsion composition and the emulsification parameters, the overall fiber size and the relative diameters of the core and the sheath can be changed. A mechanism is proposed to explain the process of transformation from the emulsion to the core‐sheath fibers, i.e., the stretching and evaporation induced de‐emulsification. In principle, this process can be applied to other systems to prepare core‐sheath fibers in place of concentric electrospinning and it is especially suitable for fabricating composite nanofibers that contain water‐soluble drugs.

Schematic mechanism for the formation of core‐sheath composite fibers during emulsion electrospinning.     

Fabrication of macroporous foam and microspheres of polystyrene by Pickering emulsion polymerization

Poly(styrene-co-methacrylic acid) (PS-co-MAA) particles were synthesized via surfactant-free emulsion polymerization and then used as particulate emulsifiers for preparation of Pickering emulsions. Our results showed that adjusting the solution pH can tune the wettability of PS-co-MAA particles to stabilize either water-in-oil (W/O) or oil-in-water (O/W) Pickering emulsions. Stable W/O emulsions were obtained with PS-co-MAA particles at low pH values due to their better affinity to the dispersed oil phase. In contrast, increasing the pH value significantly changed the stabilizing behavior of the PS-co-MAA particles, leading to the phase inversion and formation of stable O/W emulsions. We found that the oil/water ratio had a significant influence on pH value of the phase inversion. It decreased with decreasing the oil/water ratio, and no phase inversion occurred when the styrene volume fraction reduced to 10 %. Additionally, macroporous polystyrene (PS) foam and PS microspheres were obtained via polymerization of Pickering high internal phase emulsion (Pickering HIPE) and O/W Pickering emulsion, respectively.     

Free surface electrospinning of fibers containing microparticles

Many materials have been fabricated using electrospinning, including pharmaceutical formulations, superhydrophobic surfaces, catalysis supports, filters, and tissue engineering scaffolds. Often these materials can benefit from microparticles included within the electrospun fibers. In this work, we evaluate a high-throughput free surface electrospinning technique to prepare fibers containing microparticles. We investigate the spinnability of polyvinylpyrrolidone (PVP) solutions containing suspended polystyrene (PS) beads of 1, 3, 5, and 10 μm diameter in order to better understand free surface electrospinning of particle suspensions. PS bead suspensions with both 55 kDa PVP and 1.3 MDa PVP were spinnable at 1:10, 1:5, and 1:2 PS:PVP mass loadings for all particle sizes studied. The final average fiber diameters ranged from 0.47 to 1.2 μm and were independent of the particle size and particle loading, indicating that the fiber diameter can be smaller than the particles entrained and can furthermore be adjusted based on solution properties and electrospinning parameters, as is the case for electrospinning of solutions without particles.     

Microfluidic channels fabricated on mesoporous electrospun fiber mats: A facile route to microfluidic chips

We report a facile way of preparing microfluidic channels filled with electrospun functional fibers. Patterned elastic molds were in tight contact with electrospun fiber mats without any leak of the analyte solution. As an example of the simple devices, we demonstrated a microfluidic protein chip selectively purifying histidine‐tagged proteins. Highly mesoporous nitrilotriacetic acid‐functionalized polystyrene (PS‐NTA) fibers were produced by taking advantage of interpenetrating phase separation between PS and PEO during electrospinning. The specific interaction of Ni‐complexed PS‐NTA fibers with histidine enabled us to immobilize only target proteins from highly heterogeneous protein mixtures. The easy process to fabricate functionalized microchannels combined with the high production throughput from electrospinning may greatly contribute to chip‐based chromatographic and bioanalytical devices. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Emulsion electrospinning of a collagen-like protein/PLGA fibrous scaffold: empirical modeling and preliminary release assessment of encapsulated protein

The effectiveness of a multifunctional scaffold produced by the electrospinning of emulsions composed of organic PLGA and aqueous collagen-like protein (denoted as Fol-8Col) solutions is demonstrated. The resultant Fol-8Col/PLGA fibrous scaffolds with homogeneous morphology have mean fiber diameters from 600 to 2,000 nm. A uniform distribution of encapsulated Fol-8Col in the fibers is observed by fluorescence microscopy. TEM is used to clarify the representative core/sheath structure of emulsion electrospun Fol-8Col/PLGA fibers. Preliminary release assessment of encapsulated Fol-8Col shows results of sustained release for more than one month from the Fol-8Col/PLGA fibrous mats. The cytocompatibility of fibroblast cell line L929 with the fibrous composite seems promosing.     

Photocurable pickering emulsion for colloidal particles with structural complexity

We prepared polymeric microparticles with coordinated patches using oil-in-water emulsion droplets which were stabilized by adsorbed colloidal polystyrene (PS) latex particles. The oil phase was photocurable ethoxylated trimethylolpropane triacrylate (ETPTA), and the particle-armored oil droplets were solidified by UV irradiation within a few seconds to produce ETPTA-PS composite microparticles without disturbing the structures. Large armored emulsion drops became raspberry-like particles, while small emulsion drops with a few anchored particles were transformed into colloidal clusters with well-coordinated patches. For high-molecular-weight PS particles with low chemical affinity to the ETPTA monomer, the morphology of the patchy particle was determined by the volume of the emulsion drop and the contact angle of the emulsion interface on the PS particle surface. Meanwhile, for low-molecular-weight PS particles with high affinity, the ETPTA monomers were likely to swell the adsorbed PS particles, and distinctive morphologies were induced during the shrinkage of emulsion drops and the phase separation of ETPTA from the swollen PS particles. In addition, colloidal particles with large open windows were produced by dissolving the PS particles from the patchy particles. We observed photoluminescent emission from the patchy particles in which dye molecules were dispersed in the ETPTA phase. Finally, we used Surface Evolver simulation to predict equilibrium structures of patchy particles and estimate surface energies which are essential to understand the underlying physics.     

Block Copolymers in Emulsion and Dispersion Polymerization

Summary: This article deals with recent progress including the authors' work concerning the application of block copolymers as polymeric surfactants in heterophase polymerizations. The synthesis methods for preparing block copolymers by emulsion and dispersion techniques are outlined, with emphasis on recently developed controlled free radical polymerizations in aqueous media. Specific characteristics of amphiphilic block copolymers are described, for example, micellization and emulsifying effects. A general overview of emulsion and dispersion polymerization in an aqueous and organic medium with ionic and nonionic block copolymers is presented for the preparation of electrosteric and sterically stabilized latex particles. Typical examples of microemulsion, miniemulsion, oil‐in‐oil emulsion, and micellar polymerizations are provided. Current and potential developments of so‐called “hairy latexes”, inverse‐, multiple‐, and solid emulsions, as well as of nonaqueous polymeric dispersions are also discussed.

PS foam obtained by free radical polymerization of water‐in‐styrene, stabilized with a PS–PEO diblock copolymer.     

Effects of poly(ethylene oxide) and pH on the electrospinning of whey protein isolate

Poly(ethylene oxide) (PEO) is known for facilitating the electrospinning of biopolymer solutions, which are otherwise not electrospinnable. The objective of this study was to improve the understanding of the positive effects of PEO on the electrospinning of whey protein isolate (WPI) solutions under different pH conditions. Alterations in protein secondary structure and polymer solution properties (viscosity, conductivity, and dynamic surface tension), as induced by pH changes, significantly affected the electrospinning behavior of WPI/PEO (10% w/w: 0.4% w/w PEO) solutions. Acidic solutions resulted in smooth fibers (707 ± 105 nm) while neutral solutions produced spheres (2.0 ± 1.0 μm) linked with ultrafine fibers (138 ± 32 nm). In comparison, alkaline solutions produced fibers (191 ± 36 nm) that were embedded with spindle‐like beads (1.0 ± 0.5 μm). ¹³C NMR and FTIR spectroscopies showed that the increase in random coil and α‐helix secondary structures in WPI were the main contributors to the formation of bead‐less electrospun fibers. The electrospinning‐enabling properties of PEO on aqueous WPI solutions were attributed to physical chain entanglement between the two polymers, rather than specific polymer–polymer interactions. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Encapsulation of proteins in poly(l-lactide-co-caprolactone) fibers by emulsion electrospinning

This study was aimed at investigating emulsion electrospinning to prepare biodegradable fibrous mats with encapsulation of human-nerve growth factor (NGF). One of the best methods for fabricating a bio-functional tissue engineering scaffold is to load bioactive agent into the scaffold. In this work, the feasibility of incorporating NGF into poly(l-lactide-co-caprolactone) fibers by emulsion electrospinning has been studied. The release behavior of encapsulated bovine serum albumin (BSA) was investigated. The bioactivity of NGF released from fibrous mats was verified by testing the neurite outgrowth of rat pheochromocytoma cells (PC12). Furthermore, the process of fiber forming during emulsion electrospinning was discussed. The results demonstrate that emulsion electrospun fibers can successfully encapsulate proteins and release them in a sustained manner. The bioactivity of NGF released from emulsion electrospun fibers was confirmed by PC12 bioassays.     

Liquid crystal fibers produced by using electrospinning technique

This paper investigates the electrospinning process of liquid crystalline polysiloxane with cholesterol as side chain (LCPC) and the influence on the morphology of the formed fibers by mixing LCPC solution with small-molecule liquid crystal, triethylamine, and poly(ethylene oxide)(PEO). The mechanical properties of single fibers were characterized by a novel approach. The results indicate that, under appropriate conditions, fine liquid crystal fibers can be obtained and the preferable mechanical properties can be achieved, especially after annealing. WXRD was used to investigate the orientation of polymer molecules in the formed fibers, suggesting that strong interaction exists between LCPC and PEO molecule in the resulting composite fibers, and polymer molecular tends to arrange regularly during electrospinning processing. This research work provides a new and facile method of using electrospinning to prepare liquid crystal fibers, which would be useful for designing the related high-performance materials.     

High internal phase emulsions: catastrophic phase inversion, stability, and triggered destabilization

We have investigated the formation, drop sizes, and stability of emulsions prepared by hand shaking in a closed vessel in which the emulsion is in contact with a single type of surface during its formation. The emulsions undergo catastrophic phase inversion from oil-in-water (o/w) to water-in-oil (w/o) as the oil volume fraction is increased. We find that the oil volume fraction required for catastrophic inversion exhibits a linear correlation with the oil-water-solid surface contact angle. W/o high internal phase emulsions (HIPEs) prepared in this way contain water drops of diameters in the range 10-100 μm; emulsion drop size depends on the surfactant concentration and method of preparation. W/o HIPEs with large water drops show water separation but w/o HIPEs with small water drops are stable with respect to water separation for more than 100 days. The destabilization of the w/o HIPEs can be triggered by either evaporation of the oil continuous phase or by contact the emulsion with a solid surface of the "wrong" wettability.     

Pickering emulsion polymerization of graphene oxide-stabilized styrene

Polystyrene (PS) particles were prepared via Pickering emulsion polymerization using graphene oxide (GO) as the stabilizer. The results show that pH is an important factor in the stability of Pickering emulsions. The effects of two different phase initiators, the water phase initiator potassium persulfate and the oil phase initiator azobisisobutyronitrile, on the morphology of PS particles in Pickering emulsion polymerization had been investigated in detail. Wrinkled particles were prepared using the water phase initiator, and spherical particles were prepared using the oil phase initiator. In addition, hexadecane was used as the auxiliary stabilizer in the polymerization, which narrowed the diameter distribution of the PS spheres, and the hollow PS spheres were fabricated. The size of the GO particles also influenced the final morphology of the particles. Nano-sized polymer particles were grafted onto the surface of micro-sized GO. Small GO particles were suitable for Pickering emulsion polymerization to prepare the composite particles. The thermogravimetric analysis of the prepared particles confirmed that they were PS/GO composite particles, which could have a wide range of potential applications, such as in catalysts, sensors, environmental remediation, and energy storage.     

Effect of silica particles on stability of highly concentrated water-in-oil emulsions with non-ionic surfactant

Water-in-oil, high internal phase emulsion made of super-cooled aqueous solution containing a mixture of inorganic salts and stabilized with non-ionic surfactant (sorbitan monooleate) alone was investigated. It was not possible to produce a highly concentrated emulsion (with aqueous phase fraction = 94 wt %), stabilized with surface-treated silica, solely: we were able to form an emulsion with a maximal aqueous phase mass fraction of 85 wt % (emulsion inverts/breaks above this concentration). The inversion point is dependent on the silica particle concentration, presence of salt in the aqueous phase, and does not depend on the pH of the dispersed phase. All emulsions stabilized by the nanoparticles solely were unstable to shear. So, the rheological properties and stability of the emulsions containing super-cooled dispersed phase, with regards to crystallization, were determined for an emulsion stabilized by non-ionic surfactant only. The results were compared to the properties obtained for emulsions stabilized by surface treated (relatively hydrophobic) silica nanoparticles as a co-surfactant to sorbitan monooleate. The influence of the particle concentration, type of silica surface treatment, particle/surfactant ratio on emulsification and emulsion rheological properties was studied. The presence of the particles as a co-stabilizer increases the stability of all emulsions. Also, it was found that the particle/surfactant ratio is important since the most stable emulsions are those where particles dominate over the surfactant, when the surfactant’s role is to create bridging flocculation of the particles. The combination of the two types of hydrophobic silica particles as co-surfactants is: one that resides at the water/oil interface and provides a steric boundary and another that remains in the oil phase creating a 3D-network throughout the oil phase, which is even more beneficiary in terms of the emulsion stability.     

Effect of dispersion pH on the formation and stability of Pickering emulsions stabilized by layered double hydroxides particles

Using positively charged plate-like layered double hydroxides (LDHs) particles as emulsifier, liquid paraffin-in-water emulsions stabilized solely by such particles are successfully prepared. The effects of the pH of LDHs aqueous dispersions on the formation and stability of the emulsions are investigated here. The properties of the LDHs dispersions at different pHs are described, including particle zeta potential, particle aggregation, particle contact angle, flow behavior of the dispersions and particle adsorption at a planar oil/water interface. The zeta potential decreases with increasing pH, leading to the aggregation of LDHs particles into large flocs. The structural strength of LDHs dispersions is enhanced by increasing pH and particle concentration. The three-phase contact angle of LDHs also increases with increasing pH, but the variation is very small. Visual observation and SEM images of the interfacial particle layers show that the adsorption behavior of LDHs particles at the planar oil/water interface is controlled by dispersion pH. We consider that the particle-particle (at the interface) and particle-interface electrostatic interactions are well controlled by adjusting the dispersion pH, leading to pH-tailored colloid adsorption. The formation of an adsorbed particle layer around the oil drops is crucial for the formation and stability of the emulsions. Emulsion stability improves with increasing pH and particle concentration because more particles are available to be adsorbed at the oil/water interface. The structural strength of LDHs dispersions and the gel-like structure of emulsions also influence the stability of the emulsions, but they are not necessary for the formation of emulsions. The emulsions cannot be demulsified by adjusting emulsion pH due to the irreversible adsorption of LDHs particles at the oil/water interface. TEM images of the emulsion drops show that a thick particle layer forms around the oil drops, confirming that Pickering emulsions are stabilized by the adsorbed particle layers. The thick adsorbed particle layer may be composed of a stable inner particle layer which is in direct contact with the oil phase and a relatively unstable outer particle layer surrounding the inner layer.     

Naturally occurring spore particles at planar fluid interfaces and in emulsions

We have investigated the potential of utilizing naturally occurring spore particles of Lycopodium clavatum as sole emulsifiers of oil and water mixtures. The preferred emulsions, prepared from either oil-borne or aqueous-borne dispersions of the monodispersed particles of diameter 30 microm, are oil-in-water. The particles act as efficient stabilizers for oils of different polarity. Droplets as large as several millimeters are stable to coalescence indefinitely, despite the low coverage of interfaces by particles observed microscopically. Consistent with the emulsion findings, we discover that particles spontaneously adsorb to bare oil-water interfaces of single drops from oil dispersions, whereas adsorption is less spontaneous and extensive from aqueous dispersions. Monolayers of the spore particles at both air-water and oil-water planar interfaces contain particles in an aggregated state forming clusters and chains. The influence of particle concentration, oil/water ratio, and additives in the aqueous phase is studied.     

High‐Internal‐Phase Pickering Emulsions Stabilized Solely by Peanut‐Protein‐Isolate Microgel Particles with Multiple Potential Applications

High‐internal‐phase Pickering emulsions have various applications in materials science. However, the biocompatibility and biodegradability of inorganic or synthetic stabilizers limit their applications. Herein, we describe high‐internal‐phase Pickering emulsions with 87 % edible oil or 88 % n‐hexane in water stabilized by peanut‐protein‐isolate microgel particles. These dispersed phase fractions are the highest in all known food‐grade Pickering emulsions. The protein‐based microgel particles are in different aggregate states depending on the pH value. The emulsions can be utilized for multiple potential applications simply by changing the internal‐phase composition. A substitute for partially hydrogenated vegetable oils is obtained when the internal phase is an edible oil. If the internal phase is n‐hexane, the emulsion can be used as a template to produce porous materials, which are advantageous for tissue engineering.     

One-step inversion process to a Janus emulsion with two mutually insoluble oils

High internal phase ratio (HIPR) aqueous Janus emulsions of two immiscible oils, silicone oil (SO) and a vegetable oil (VO), were prepared using a vibration mixer. The simple HIPR Janus emulsions, (VO + SO)/W, were found at weight fractions of the aqueous phase in excess of 0.3, while at a corresponding fraction of 0.1, a triple emulsion was obtained with the Janus emulsion forming a drop inside the vegetable oil to give a double Janus emulsion, (VO + SO)/W/VO, which in turn formed drops in the silicone oil resulting in a triple Janus emulsion (VO + SO)/W/VO/SO. Increasing the aqueous-phase fraction from 0.1 to 0.3 consequently meant an inversion, of which one intermediate stage was observed: a more complex configuration, e.g., one in which large SO drops with highly distorted VO drops attached were dispersed in a regular aqueous emulsion with spherical Janus (VO + SO) drops. A preliminary investigation was made into the destabilization process of the triple emulsions.     

After‐Rinsing Hair Growth Promotions of Hinokitiol‐Containing Vesicles and Emulsions

Two kinds of topical dosage forms of hinokitiol (HKL), namely vesicles and oil‐in‐water (O/W) emulsions, were prepared. Behenyl trimethylammonium chloride (BTMAC) and fatty acids were used as bilayer‐forming materials of the vesicles, and oils were employed as oil phases of the emulsions. The substantivity of HKL in the preparations was evaluated in vitro using hairless mouse skins. After applying the preparations onto the skin and rinsing it, the amount of HKL left on the skin was determined using high performance liquid chromatography (HPLC). It was higher when HKL was encapsulated in cationic vesicles rather than in nonionic vehicles, emulsions. An ionic interaction between the cationic vehicle and negatively charged skin is likely to account for the high substantivity. Among the emulsion preparations, an emulsion having octyl salicylate as oil phase exhibited the highest substantivity of HKL. This is probably because that the oil is a good solvent for HKL and it is skin‐retentive. In vivo hair growth‐promotion effect of each dosage form was investigated, where the sample application onto the clipped backs of female mice (C57BL6) and the subsequent rinsing of the backs were done once a day for 30 days. Only HKL in the cationic vesicles had hair growth promotion effect, possibly due to the significant substantivity.     

Phase inversion of the Pickering emulsions stabilized by plate-shaped clay particles

We investigated the phase inversion of Pickering emulsions stabilized by plate-shaped clay particles. Addition of water induced a phase inversion from a water-in-oil (W/O) emulsion to an oil-in-water (O/W) emulsion when the amount of the oil phase exceeded a limiting amount of oil absorption to solid particles. On the other hand, a phase inversion from a powdery state to an O/W emulsion state through an oil-separated state is observed when the amount of an oil phase is less than the limiting amount of the oil absorption. Interestingly, the oil separated is re-dispersed as emulsion droplets into the O/W emulsion phase. This type of phase inversion, which is a feature of the Pickering emulsions stabilized by the clay particles, is caused by a change in the aggregate structures of particles.     

SiO2纳米颗粒与聚氧乙烯对Pickering乳液的协同稳定作用简

研究了聚氧乙烯(PEO)与SiO2纳米颗粒对水/二甲苯体系Pickering乳液的协同稳定作用. 实验发现,PEO的存在减小了乳液液滴的平均直径,抑制了乳液的相反转,有效阻止了乳液的熟化,使乳液具有更好的稳定性. 进一步对纳米颗粒膜的流变性质进行研究,结果表明,PEO高分子促进了纳米颗粒形成更大尺寸的聚集结构,提高了其在界面上的吸附性,增强了颗粒膜的力学性能,在较小颗粒用量条件下使得Gibbs稳定性判据得到满足.