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.     

"On the fly" continuous generation of alginate fibers using a microfluidic device

In this paper, we introduce a new continuous production technique of calcium alginate fibers with a microfluidic platform similar to a spider in nature. We have used a poly(dimethylsiloxane) (PDMS) microfluidic device embedded capillary glass pipet as the apparatus for fiber generation. As a sample flow, we introduced a sodium alginate solution, and, as a sheath flow, a CaCl2 solution was introduced. The coaxial flows were generated at the intersection of both flows, and the sodium alginate was solidified to calcium alginate by diffusion of the Ca2+ ions during traveling through the outlet pipet. The diameter changes in the sample and sheath flow variations were examined, and the size of alginate fibers was well regulated by changing both flow rates. In addition, we have measured the elasticity of dried fibers. We evaluated the potential use of alginate fibers as a cell carrier by loading human fibroblasts during the "on the fly" fabrication process. From the LIVE/DEAD assay, cells survived well during the fiber fabrication process. In addition, we evaluate the capability of loading the therapeutic materials onto the alginate fibers by immobilized bovine serum albumin-fluorescein isothiocyanate in the fibers.     

Self‐Assembling Supramolecular Hybrid Hydrogel Beads

With the goal of imposing shape and structure on supramolecular gels, we combine a low‐molecular‐weight gelator (LMWG) with the polymer gelator (PG) calcium alginate in a hybrid hydrogel. By imposing thermal and temporal control of the orthogonal gelation methods, the system either forms an extended interpenetrating network or core–shell‐structured gel beads—a rare example of a supramolecular gel formulated inside discrete gel spheres. The self‐assembled LMWG retains its unique properties within the beads, such as remediating Pd^II and reducing it in situ to yield catalytically active Pd⁰ nanoparticles. A single PdNP‐loaded gel bead can catalyse the Suzuki–Miyaura reaction, constituting a simple and easy‐to‐use reaction‐dosing form. These uniquely shaped and structured LMWG‐filled gel beads are a versatile platform technology with great potential in a range of applications.     

Preparation and properties of thermoresponsive and conductive composite fibers with core‐sheath structure

In this research, thermoresponsive and conductive fibers with core‐sheath structure were fabricated by coaxial electrospinning. For preparing the spinning sheath solution, poly‐(N‐isopropylacrylamide‐co‐N‐methylolacrylamide) (PNN) copolymer having thermoresponsive and cross‐linkable properties was synthesized by free‐radical polymerization using redox initiators; it was then mixed with the conductive poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) at different weight ratios in water. On the other hand, poly(butyl acrylate‐co‐styrene) (PBS) copolymer synthesized by emulsion polymerization was dissolved in chloroform and used as the spinning core solution. After electrospinning, the fibers were treated at 110 °C for 1 h to cross‐link the PNN portion in the sheath for strengthening the fibers. Well‐defined core‐sheath fibers were observed from SEM pictures; the outside and inside (core) diameters were 568 ± 24 and 290 ± 40 nm, respectively, as determined from TEM pictures. The fiber mats were further doped by DMSO to enhance their conductivity. For the fiber mat with the weight ratio of PEDOT:PSS/PNN at 0.20 in the sheath, its surface conductivity could reach 29.4 S/cm. In addition, the fiber mats exhibited thermoresponsive properties that both swelling ratio and electric resistance decreased with temperature. Furthermore, the fiber mats exhibited improved flexibility as evaluated via bending test. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1299–1307     

Electrospun core–sheath fibers for integrating the biocompatibility of silk fibroin and the mechanical properties of PLCL

In the process of preparing core–sheath fibers via coaxial electrospinning, the relative evaporation rates of core and sheath solvents play a key role in the formation of the core–sheath structure of the fiber. Both silk fibroin (SF) and poly(lactide‐co‐ε‐caprolactone) (PLCL) have good biocompatibility and biodegradability. SF has better cell affinity than PLCL, whereas PLCL has higher breaking strength and elongation than SF. In this work, hexafluoroisopropanol (HFIP)‐formic acid (volume ratio 8:2), HFIP and HFIP–dichloromethane (volume ratio 8:2) were used to dissolve PLCL as the core solutions, and HFIP was used to dissolve SF as the sheath solution. Then, core–sheath structured SF/PLCL (C‐SF/PLCL) fibers were prepared by coaxial electrospinning with the core and sheath solutions. Transmission electron microscopy images indicated the existence of the core–shell structure of the fibers, and energy dispersive X‐ray analysis results revealed that the fiber mat with the greatest content of core–shell structure fibers was obtained when the core solvent was HFIP–dichloromethane (volume ratio 8:2). Tensile tests showed that the C‐SF/PLCL fiber mat displayed improved tensile properties, with strength and elongation that were significantly higher than those of the pure SF mat. The C‐SF/PLCL fiber mat was further investigated as a scaffold for culturing EA.hy926 cells, and the results showed that the fiber mat permitted cellular adhesion, proliferation and spreading in a manner similar to that of the pure SF fiber mat. These results indicated that the coaxial electrospun SF/PLCL fiber mat could be considered a promising candidate for tissue engineering scaffolds for blood vessels. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis and rheological investigation of self‐healable deferoxamine grafted alginate hydrogel

Deferoxamine grafted alginate (SA‐DFA) was successfully synthesized via amidation of sodium alginate with deferoxamine mesylate as determined by H‐NMR and elemental analysis. SA‐DFA with different graft yield was obtained by adjusting the ratio of sodium alginate and deferoxamine mesylate. It was found that aqueous solution of SA‐DFA could form hydrogel spontaneously due to hydrogen bonding interactions, which also endowed the SA‐DFA hydrogel with self‐healing capability. The healing efficiency of SA‐DFA hydrogels ranged from 53.64 to 90.16%. In addition, surface morphologies of SA‐DFA hydrogels before/after self‐healing process were demonstrated by SEM images. We anticipated that such self‐healable alginate hydrogel would be applied in the field of wound healing. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 856–865     

Mesomorphism,polymerization, and chirality induction in α‐cyanostilbene‐functionalized diacetylene‐assembled films: Photo‐triggered Z/E isomerization

Engineering of molecular stacking arrangement via environmental stimuli is of particular interest in stimuli‐responsive self‐assembling architectures. A novel dual photo‐functionalized diacetylene ((Z)‐CNBE‐DA) molecule was synthesized, in which photo‐responsive cyanostilbene moieties exhibited interesting Z‐E isomerization upon UV light irradiation and could be utilized to modulate mesomorphism, molecular stacking arrangement and resulting polymerization behavior. Rod‐like (Z)‐CNBE‐DA could self‐assemble into well‐defined lamellar structures and the helical polydiacetylene (PDA) chains could be formed upon irradiation with circularly polarized ultraviolet light (CPUL). However, the bent‐shaped (E)‐CNBE‐DA molecules only self‐assembled into irregular loose packing, inhibiting the formation of ordered helical PDA chains upon CPUL irradiation. In this work, we established the links between chemical structures, molecular packing engineering and photophysical properties, which would be of great fundamental value for the rational design of smart soft materials. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2458–2466     

Surface amine‐functionalization of UHMWPE fiber by bio‐inspired polydopamine and grafted hexamethylene diamine

Ultrahigh molecular weight polyethylene (UHMWPE) fibers exhibit excellent mechanical property, but their low surface activity limits the application in many fields. In this work, an efficient method was used to improve the surface activity and adhesion property of UHMWPE fibers. The amine functionalized UHMWPE fibers were prepared by the combination of bio‐inspired polydopamine (PDA) and grafted hexamethylene diamine (HMDA). The chemical structure of UHMWPE fibers was characterized by X‐ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy. The surface morphologies and mechanical property of the fibers were investigated by scanning electron microscopy and tensile testing respectively. In addition, a single‐fiber pull‐out test was carried out to investigate the adhesion property of the fibers with epoxy resin matrix. The results showed that PDA was coated on the surface of UHMWPE fibers and then HMDA was successfully grafted on the PDA layers. The excellent mechanical property of UHMWPE fibers had no obvious change. Compared with the pristine UHMWPE fibers, the interfacial shear strength of the PDA coated UHMWPE fibers with the epoxy resin matrix improved by 28.3%, while the IFSS of the HMDA grafted UHMWPE fibers had an increase of 82.7%. Copyright © 2017 John Wiley & Sons, Ltd.     

Polydiacetylene‐Based Electrospun Fibers for Detection of HCl Gas

Polydiacetylenes (PDAs), a family of conjugated polymers, are very intriguing materials in several aspects. Especially, the stimulus‐induced apparent blue‐to‐red transition of the PDAs has led to the development of a variety of PDA‐based chemosensors. In the current work, we synthesized PDA monomers bearing trimethyl amine (PCDA‐DMEDA) and incorporated them with Poly(ethylene oxide) (PEO) into electrospun fibers. For the first time, we successfully demonstrated that PDA‐based electrospun fibers can be used for the naked‐eye detection of HCl gas by simple color change (blue to red).     

Study on structure and performance of surface‐metallized carbon fibers reinforced rigid polyurethane composites

The simultaneous promotion in mechanical and electrical properties of rigid polyurethane (RPU) is an important task for expanding potential application. In this work, carbon fibers (CFs) reinforced RPU composites were prepared with the goal of improving mechanical and electrical properties. Metallized CFs meet our performance requirements and can be easily achieved via electrodeposition. However, the weak bonding strength in fiber‐metal‐RPU interface restricts their application. Inspired by the reducibility and wonderful adhesion of dopamine (DA), we proposed a new and efficient electrochemical method to fabricate metallized CFs, where DA polymerization was simultaneously integrated coupled with the reduction of metal ions (Ni²⁺). The characterization results helped us to gain insight about the reaction mechanism, which was never reported as far as we know. Compared with pure RPU, the tensile, interlaminar shear and impact strength of polydopamine (PDA)‐nickel (Ni) modified CFs/RPU composites were improved by 11.2%, 21.0%, and 78.0%, respectively, which attributed to the strong interfacial adhesion, including mechanical interlocking and chemical crosslinking between treated CFs and RPU. In addition, the PDA‐Ni surface treatment method also affected the dispersion of short CFs in the RPU, which increased the possibility of conductor contact and reduced insulator between fibers networks, resulting in higher electrical conductivity.     

Ultrafine hydrogel fibers with dual temperature‐ and pH‐responsive swelling behaviors

Ultrafine hydrogel fibers that were responsive to both temperature and pH signals were prepared through the electrospinning of poly(N‐isopropylacrylamide) (PNIPAAm) and poly(acrylic acid) mixtures in dimethylformamide. Both the diameters (700 nm to 1.2 μm) and packing of the fibers could be controlled through changes in the polymer compositions and PNIPAAm molecular weights. These fibers were rendered water‐insoluble by the addition of either Na₂HPO₄ or poly(vinyl alcohol) (PVA) to the solution, followed by the heat curing of the fibers. The fibers crosslinked with Na₂HPO₄ swelled to 30–120 times in water; this was significantly higher than the swelling of those crosslinked with PVA. The PVA‐crosslinked hydrogel fibers, however, exhibited faster swelling kinetics; that is, they reached equilibrium swelling in less than 5 min at 25 °C. They were also more stable after 1 week of water exposure; that is, they lost less mass and retained their fibrous form better. All the hydrogel fibers showed a drastic increase in the swelling between pH 4 and 5. The PVA‐crosslinked hydrogel fibers exhibited distinct temperature‐responsive phase‐transition behavior of PNIPAAm, whereas the Na₂HPO₄‐crosslinked hydrogel fibers showed altered two‐stage phase transitions that reflected side‐chain modification of PNIPAAm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6331–6339, 2004     

Thermally induced core-shell type hydrogel beads having interpenetrating polymer network (IPN) structure

Monodisperse hydrogel beads composed of calcium alginate and crosslinked polyNIPAAm (N-isopropylacrylamide) were synthesized based on a simultaneous interpenetrating network process. With increasing the temperature above the phase transition temperature of polyNIPAAm, a core-shell type of hydrogel beads was developed; polyNIPAAm-enriched core region and Ca-alginate-enriched outer shell layer were observed. The thermally reversible formation of the core-shell double structure in the IPN hydrogel beads was applied for the temperature modulated drug release using indomethacin as a model drug.     

Microfluidic synthesis of tail‐shaped alginate microparticles using slow sedimentation

This study reports the synthesis of tail‐shaped alginate particles using a microfluidic platform combined with a sedimentation strategy. By utilizing microfluidic emulsification in the cross‐junction channel, the formation of regular droplets was achieved. Following a facile and convenient sedimentation process and an ionic crosslinking process, sodium‐alginate droplets became tail‐shaped and then gradually developed into calcium‐alginate microparticles. The effects of the concentration of the CaCl₂ crosslinker and the viscosity of the alginate solution on the shape and/or size of the particles were further investigated. The proposed synthesis methodology has the advantages of actively controlling the tail‐shape formation, having a narrow size distribution, as well as being a facile and convenient process with a high throughput. This approach can be applied to many applications in the pharmaceutical and biomedical arena.     

Mussel‐Inspired One‐Pot Synthesis of a Fluorescent and Water‐Soluble Polydopamine–Polyethyleneimine Copolymer

Inspired by the molecular mechanics of mussel adhesive formation, a novel water‐soluble fluorescent macromolecule (polydopamine–polyethyleneimine (PDA–PEI)) is prepared by one‐pot copolymerization of dopamine (DA) and PEI. In this method, DA is polymerized to form PDA, which is then coupled with PEI mainly through Michael addition. The fluorescence property of PDA–PEI is mainly attributed to the Michael addition of PEI on the 5,6‐dihydroxyindole (DHI) units of PDA, where PEI can form hydrogen bonds with oxidative products such as DHI and force the DHI units to twist out of plane, resulting in a decrease in the intra‐ and intermolecular coupling of PDA. In addition, the influence of various metal cations on the fluorescence of the PDA–PEI copolymer is investigated. This work may facilitate the development of new strategies for controlling the emission characteristics of PDA.

     

Processing of a Strong Biodegradable Poly[(R)‐3‐hydroxybutyrate] Fiber and a New Fiber Structure Revealed by Micro‐Beam X‐Ray Diffraction with Synchrotron Radiation

Summary: Biodegradable poly[(R)‐3‐hydroxybutyrate] (P(3HB)) fibers with high tensile strength of 1.32 GPa were processed from ultra‐high‐molecular‐weight P(3HB) by a method combining cold‐drawing and two‐step‐drawing procedures at room temperature. The distribution of molecular structures in a mono‐filament was analyzed by micro‐beam X‐ray diffraction with synchrotron radiation. It was revealed that the P(3HB) fiber has a new core‐sheath structure consistent with two types of molecular conformations: a 2₁ helix conformation in the sheath region and a planar zigzag conformation in the core region.

P(3HB) fiber processed by cold‐drawing in ice water and two‐step drawing at room temperature, and subsequently annealing at 50 °C.     

Fabrication of novel core–shell PLGA and alginate fiber for dual‐drug delivery system

Biodegradable fibers for the controlled delivery of anti‐inflammatory agent dexamethasone were developed and studied. Mono and core–shell structure fiber are prepared by wet‐spinning solutions of hydrophobic poly (lactide‐co‐glycolide) and hydrophilic alginic acid shell. The two model drugs, dexamethasone and dexamethasone‐21‐phosphate, were entrapped in core and shell, respectively. These fibers were characterized in terms of morphology, diameters, mechanical properties, in vitro degradation, and drug release. The optical microscopy and scanning electron microscopy photos revealed directly that fibers possessed core–shell structure. The release of dexamethasone and dexamethasone‐21‐phosphate was investigated, and the results showed that alginate shell retarded dexamethasone release significantly in both early and late stages. The core–shell structure fiber release shows a two stage release of dexamethasone and dexamethasone‐21‐phosphate with distinctly different release rates, and minimal initial burst release is observed. The results indicated that the prepared fibers are efficient carrier for both types of dexamethasone. Copyright © 2016 John Wiley & Sons, Ltd.     

Inorganic–organic hybrid alginate beads with LCST near human body temperature for sustained dual‐sensitive drug delivery

In order to obtain dual‐stimuli‐responsive (temperature/pH) alginate beads that exhibit LCST close to human body temperature for sustained drug release applications, poly (NIPAAm‐co‐AAm) hydrogel (with LCST 37.5°C) were selected and associated with calcium alginate to prepare inorganic–organic hybrid biomineralized polysaccharide alginate beads via a one‐step method in this paper. Scanning electron microscopy (SEM) and energy dispersive X‐ray spectrometer (EDS) results demonstrated that calcium phosphate could not only be found in the surface but also in the cross‐section of biomineralized polysaccharide beads. Both equilibrium swelling and indomethacin release behavior were found to be pH‐ and thermo‐responsive. In addition, indomethacin release profile could be sustained with a inorganic–organic hybrid membrane: the release amount reached 96% within 4 hr for the unmineralized beads, while a drug release of only 64% obtained after subjecting the biomineralized polysaccharide beads to the same treatment. These results indicate that the biomineralized polysaccharide membrane could prevent the permeability of the encapsulated drug and reduce the drug release rate effectively. The studied system has the potential to be used as an effective smart sustainable delivery system for biomedical applications. Copyright © 2008 John Wiley & Sons, Ltd.     

Shape-controlled production of biodegradable calcium alginate gel microparticles using a novel microfluidic device

In this paper we describe a novel method of manufacturing shape-controlled calcium alginate gel microparticles in a microfluidic device. Both manufacturing shape-controlled microparticles and synthesizing hydrogel microparticles could be performed simultaneously in the microfluidic device. The novel microfluidic device comprised of two individual flow-focusing channels and a synthesizing channel was successfully applied as a continuous microfluidic reactor to synthesize gel microparticles with size and shape control. By passive control based on the microchannel geometric confinement and liquid-phase flow rates, we succeeded in producing monodisperse sodium alginate microparticles with diverse shapes (such as plugs, disks, microspheres, rods, and threads) in the flow-focusing channels of the microfluidic device. The shape and size of the sodium alginate microparticles could be tuned by adjusting the flow rates of the various streams. Further stages of the chemical reaction could be initiated by mixing sodium alginate microparticles and calcium chloride (CaCl2) solution in the synthesizing channel. The shapes of the sodium alginate microparticles could be permanently preserved by the synthesis of calcium alginate gel microparticles. The preparation conditions of size- and shape-controlled calcium alginate microparticles and influence factors were studied.     

Simple Radical Polymerization of Poly(Alginate‐Graft‐N‐Isopropylacrylamide) Injectable Thermoresponsive Hydrogel with the Potential for Localized and Sustained Delivery of Stem Cells and Bioactive Molecules

In this study, thermoresponsive copolymers that are fully injectable, biocompatible, and biodegradable and are synthesized via graft copolymerization of poly(N‐isopropylacrylamide) onto alginate using a free‐radical reaction are presented. This new synthesis method does not involve multisteps or associated toxicity issues, and has the potential to reduce scale‐up difficulties. Chemical and physical analyses verify the resultant graft copolymer structure. The lower critical solution temperature, which is a characteristic of sol–gel transition, is observed at 32 °C. The degradation properties indicate suitable degradation kinetics for drug delivery and bone tissue engineering applications. The synthesized P(Alg‐g‐NIPAAm) hydrogel is noncytotoxic with both human osteosarcoma (MG63) cells and porcine bone marrow derived mesenchymal stem cells (pBMSCs). pBMSCs encapsulated in the P(Alg‐g‐NIPAAm) hydrogel remain viable, show uniform distribution within the injected hydrogel, and undergo osteogenic and chondrogenic differentiation under appropriate culture conditions. Furthermore, for the first time, this work will explore the influence of alginate viscosity on the viscoelastic properties of the resulting copolymer hydrogels, which influences the rate of medical device formation and subsequent drug release. Together the results of this study indicate that the newly synthesized P(Alg‐g‐NIPAAm) hydrogel has potential to serve as a versatile and improved injectable platform for drug delivery and bone tissue engineering applications.     

Spatial Structuring of a Supramolecular Hydrogel by using a Visible‐Light Triggered Catalyst

Spatial control over the self‐assembly of synthetic molecular fibers through the use of light‐switchable catalysts can lead to the controlled formation of micropatterns made up of hydrogel structures. A photochromic switch, capable of reversibly releasing a proton upon irradiation, can act as a catalyst for in situ chemical bond formation between otherwise soluble building blocks, thereby leading to fiber formation and gelation in water. The use of a photoswitchable catalyst allows control over the distribution as well as the mechanical properties of the hydrogel material. By using homemade photomasks, spatially structured hydrogels were formed starting from bulk solutions of small molecule gelator precursors through light‐triggered local catalyst activation.