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Complex shaped and critical-sized bone defects have been a clinical challenge for many years. Scaffold-based strategies such as hydrogels provide localized drug release while filling complex defect shapes, but ultimately possess weaknesses in low mechanical strength alongside a lack of macroporous and collagen-mimicking nanofibrous structures. Thus, there is a demand for mechanically strong, extracellular matrix (ECM) mimicking scaffolds that can robustly fit complex shaped critical sized defects and simultaneously provide localized, sustained, multiple growth factor release. We therefore developed a composite, bi-phasic PCL/hydroxyapatite (HA) 3D nanofibrous (NF) scaffold for bone tissue regeneration by using our innovative electrospun-based thermally induced self-agglomeration (TISA) technique. One intriguing feature of our ECM-mimicking TISA scaffolds is that they are highly elastic and porous even after evenly coated with minerals and can easily be pressed to fit different defect shapes. Furthermore, the bio-mimetic mineral deposition technique allowed us to simultaneously encapsulate different type of drugs, e.g., proteins and small molecules, on TISA scaffolds under physiologically mild conditions. Compared to scaffolds with physically surface-adsorbed phenamil, a BMP2 signaling agonist, incorporated phenamil composite scaffolds indicated less burst release and longer lasting sustained release of phenamil with subsequently improved osteogenic differentiation of cells in vitro. Overall, our study indicated that the innovative press-fit 3D NF composite scaffold may be a robust tool for multiple-drug delivery and bone tissue engineering.

Three-dimensional (3D) scaffolds as artificial ECMs have been extensively studied to mimic the critical features of natural ECMs. To develop more clinically relevant 3D scaffolds, electrospun nanofibrous scaffolds with different weight ratios of PCL/PLA (i.e., 100/0, 60/40, and 20/80) were fabricated via the thermally induced (nanofiber) self-agglomeration (TISA) method. The hypothesis was that, with the weight ratio increase of stiffer and more bioactive PLA in the 3D PCL/PLA blend scaffolds, the osteogenic differentiation of human mesenchymal stem cells (hMSCs) would be enhanced. The results indicated that, all of the 3D scaffolds were elastic/resilient and possessed interconnected and hierarchical pores with sizes from sub-microns to ∼300 μm; therefore, the morphological structures of these scaffolds were similar to those of natural ECMs. The PLA80 scaffolds exhibited the best overall properties in terms of density, porosity, water absorption capacity, mechanical properties, bioactivity, and cell viability. Furthermore, with increasing the PLA weight ratio, the alkaline phosphatase (ALP) activity, calcium content, and gene expression level were also increased, probably due to the improved stiffness/bioactivity of scaffold. Hence, the novel 3D electrospun PLA80 nanofibrous scaffold might be desired/favorable for the osteogenic differentiation of hMSCs., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Nanofibrous scaffolds that are morphologically/structurally similar to natural ECM are highly interested for tissue engineering; however, the electrospinning technique has the difficulty in directly producing clinically relevant 3D nanofibrous scaffolds with desired structural properties. To address this challenge, we have developed an innovative technique of thermally induced nanofiber self-agglomeration (TISA) recently. The aim of this work was to prepare (via the TISA technique) and evaluate 3D electrospun PCL/PLA blend (mass ratio: 4/1) nanofibrous scaffolds having high porosity of ∼95.8% as well as interconnected and hierarchically structured pores with sizes from sub-micrometers to ∼300 μm for bone tissue engineering. The hypothesis was that the incorporation of PLA (with higher mechanical stiffness/modulus and bioactivity) into PCL nanofibers would significantly improve human mesenchymal stem cells (hMSCs) osteogenic differentiation in vitro and bone formation in vivo. Compared to neat PCL-3D scaffolds, PCL/PLA-3D blend scaffolds had higher mechanical properties and in vitro bioactivity; as a result, they not only enhanced the cell viability of hMSCs but also promoted the osteogenic differentiation. Furthermore, our in vivo studies revealed that PCL/PLA-3D scaffolds considerably facilitated new bone formation in a critical-sized cranial bone defect mouse model. In summary, both in vitro and in vivo results indicated that novel 3D electrospun PCL/PLA blend nanofibrous scaffolds would be strongly favorable/desired for hMSCs osteogenic differentiation and cranial bone formation., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

For the first time, electrospun polycaprolactone (PCL) 3D nanofibrous scaffold has been developed by an innovative and convenient approach (i.e., thermally induced nanofiber self-agglomeration followed by freeze drying), and the scaffold possesses interconnected and hierarchically structured pores including macropores with sizes up to ≈300 μm. The novel PCL 3D scaffold is soft and elastic with very high porosity of ≈96.4%, thus it is morphologically/structurally similar to natural extracellular matrix and well suited for cell functions and tissue formation. The in vitro studies reveal that the scaffold can lead to high cell viability; more importantly, it is able to promote more potent BMP2-induced chondrogenic than osteogenic differentiation of mouse bone marrow mesenchymal stem cells. Consistent to the in vitro findings, the in vivo results indicate that the electrospun PCL 3D scaffold acts as a favorable synthetic extracellular matrix for functional bone regeneration through the physiological endochondral ossification process., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In this study, a mat/membrane consisting of overlaid PVA/PA6-Cu(II) composite nanofibers was prepared via the electrospinning technique followed by coordination/chelation with Cu(II) ions; an enzyme of catalase (CAT) was then immobilized onto the PVA/PA6-Cu(II) nanofibrous membrane. The amount of immobilized catalase reached a high value of 64 ± 4.6 mg/g, while the kinetic parameters (Vmax and Km) of enzyme were 3774 μmol/mg·min and 41.13 mM, respectively. Furthermore, the thermal stability and storage stability of immobilized catalase were improved significantly. Thereafter, a plug-flow type of immobilized enzyme membrane reactor (IEMR) was assembled from the PVA/PA6-Cu(II)-CAT membrane. With the increase of operational pressure from 0.02 to 0.2 MPa, the flux value of IEMR increased from 0.20 ± 0.02 to 0.76 ± 0.04 L/m(2)·min, whereas the conversion ratio of H2O2 decreased slightly from 92 ± 2.5% to 87 ± 2.1%. After 5 repeating cycles, the production capacity of IEMR was merely decreased from 0.144 ± 0.006 to 0.102 ± 0.004 mol/m(2)·min. These results indicated that the assembled IEMR possessed high productivity and excellent reusability, suggesting that the IEMR based on electrospun PVA/PA6-Cu(II) nanofibrous membrane might have great potential for various applications, particularly those related to environmental protection.

Mechanically resilient mats consisting of overlaid electrospun nanofibers with self-generated TiC crystallites embedded in a carbon matrix are surface-decorated with Pt nanoparticles as a novel electrocatalytic system for oxygen reduction reaction. Electrocatalytic activities (e.g., on-set potential and current density) of Pt are substantially enhanced due to high specific surface area of the support and the synergetic effect of TiC and Pt on electrocatalysis.

The development of novel nanomaterials with well-controlled morphologies/structures to achieve excellent activities/sensitivities in surface-enhanced Raman scattering (SERS) is crucial in advancing the high-performance SERS detections of chemical and biological species. In this study, amidoxime surface-functionalized polyacrylonitrile (ASFPAN) nanofibrous membranes surface-decorated with silver nanoparticles (Ag NPs) were prepared via the technique of electrospinning followed by the method of seed-mediated electroless plating. High SERS activities/sensitivities were observed from the ASFPAN-Ag NPs nanofibrous membranes, while the density and size of Ag NPs had an important impact on the SERS activity/sensitivity. The results confirmed that the enhancement of Raman signals is due to the presence of hot spots between/among Ag NPs on the nanofiber surfaces. Electrospun nanofibrous membranes surface-decorated with Ag NPs were mechanical flexible/resilient and could be used as highly active/sensitive SERS substrates for a broad range of applications.

A composite counter electrode (CE) made of electrospun carbon nanofibers (ECNs) and platinum (Pt) nanoparticles has been demonstrated for the first time to improve the performance of dye-sensitized solar cells (DSCs). The new ECN-Pt composite CE exhibited a more efficient electro-catalytic performance with lower charge transfer resistance (R(ct)), larger surface area, and faster reaction rate than those of conventional Pt. It reduced the overall series resistance (R(se)), decreased dark saturation current density (J(0)) and increased shunt resistance (R(sh)) of the DSCs, thereby leading to a higher fill factor (FF) and larger open circuit voltage (V(oc)). The reduced electron transport resistance (R(s)) and faster charge transfer rate in the CE led to a smaller overall cell series resistance (R(se)) in the ECN-Pt composite based DSCs. The DSCs based on an ECN-Pt CE achieved a η of ∼8%, which was improved over those of pure Pt or ECN based cells.

Due to recent advances in the production of biotherapeutics, high capacity, high throughput adsorption media for efficient and economic separation of these medically important products are in great demand. One option that has been evaluated extensively is membrane/mat adsorption. While these media allow for rapid adsorption (due to the decreased internal diffusion) and high throughput processing (due to the open porous structure), they often suffer from low capacity and poor enrichment factors. Herein, we report the fabrication, characterization, and protein adsorption evaluation of an innovative type of membrane/mat adsorption media based on electrospun carbon nanofibers. By surface-functionalization of these nanofibers with a weak acid cation-exchange ligand, the capacity was doubled for binding a model protein (i.e., lysozyme) compared to commercial products; and the capacity value was over 200 mg lysozyme per gram of adsorption media. Meanwhile, the thin nanofibers (having diameters of ~300 nm) along with open pores among nanofibers in the mats (having sizes of ~10-15 μm) allowed for higher operating flow rates and lower pressure drops. Furthermore, the incorporation of higher ligand density and the addition of a non-ionic surfactant (i.e., Triton X-305) into the adsorption buffer eliminated the non-specific binding of a competing protein (bovine serum albumin). In combination, this study suggested that electrospun carbon nanofiber adsorption media would provide a promising alternative to packed resin beds for bioseparations., (Copyright © 2011 Elsevier B.V. All rights reserved.)

The annulus fibrosus comprises concentric lamellae that can be damaged due to intervertebral disc degeneration; to provide permanent repair of these acquired structural defects, one solution is to fabricate scaffolds that are designed to support the growth of annulus fibrosus cells. In this study, electrospun nanofibrous scaffolds of polycaprolactone are fabricated in random, aligned, and round-end configurations. Primary porcine annulus fibrosus cells are grown on the scaffolds and evaluated for attachment, proliferation, and production of extracellular matrix. The scaffold consisting of round-end nanofibers substantially outperforms the random and aligned scaffolds on cell adhesion; additionally, the scaffold with aligned nanofibers strongly affects the orientation of cells., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Continuous titanium carbide (TiC) nanofibers that possess an intriguing nanoribbon morphology with a width and thickness of approximately 300 nm and approximately 40 nm, respectively, and containing TiC crystallites with sizes ranging from 5 nm to 30 nm were synthesized through electrospinning followed by carbothermal reduction.

Electrospun nanofiber membranes surface functionalized with 3D nanolayers through ATRP provide adsorption capacities over 50-times higher than current commercial membrane adsorption systems and over 12-times higher than packed bed resins; additionally, the adsorption kinetics remain 10-times faster than packed bed resins and have over 15-times higher permeance.

Carbon nanofibers with diameters that fall into submicron and nanometer range have attracted growing attention in recent years due to their superior chemical, electrical, and mechanical properties in combination with their unique 1D nanostructures. Unlike catalytic synthesis, electrospinning polyacrylonitrile (PAN) followed by stabilization and carbonization has become a straightforward and convenient route to make continuous carbon nanofibers. This paper is a comprehensive and state-of-the-art review of the latest advances made in development and application of electrospun PAN-based carbon nanofibers. Our goal is to demonstrate an objective and overall picture of current research work on both functional carbon nanofibers and high-strength carbon nanofibers from the viewpoint of a materials scientist. Strategies to make a variety of carbon nanofibrous materials for energy conversion and storage, catalysis, sensor, adsorption/separation, and biomedical applications as well as attempts to achieve high-strength carbon nanofibers are addressed. [ABSTRACT FROM AUTHOR]

In this study, electrospun glass (structurally amorphous SiO2) nanofibers (EGNFs) with diameters of ∼ 400 nm were incorporated into epoxy resin for reinforcement and/or toughening purposes; the effects of silanization treatment (including different functional groups in silane molecules) and mass fraction of EGNFs on strength, stiffness, and toughness of the resulting nano-epoxy composite resins were investigated. The experimental results revealed that EGNFs substantially outperformed conventional glass fibers (CGFs, with diameters of ∼ 10 μm) in both tension and impact tests, and led to the same trend of improvements in strength, stiffness, and toughness at small mass fractions of 0.5 and 1%. The tensile strength, Young's modulus, work of fracture, and impact strength of the nano-epoxy composite resins with EGNFs were improved by up to 40, 201, 67, and 363%, respectively. In general, the silanized EGNFs with epoxy end groups (G-EGNFs) showed a higher degree of toughening effect, while the silanized EGNFs with amine end groups (A-EGNFs) showed a higher degree of reinforcement effect. The study suggested that electrospun glass nanofibers could be used as reinforcement and/or toughening agent for making innovative nano-epoxy composite resins, which would be further used for the development of high-performance polymer composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012 [ABSTRACT FROM AUTHOR]

Titania (TiO2) nanofibers were fabricated by electrospinning three representative spin dopes made of titanium (IV) n-butoxide (TNBT) and polyvinylpyrrolidone (PVP) with the TNBT/PVP mass ratio being 1/2 in three solvent systems including N,N-dimethylformamide (DMF), isopropanol, and DMF/isopropanol (1/1 mass ratio) mixture, followed by pyrolysis at 500 °C. The detailed morphological and structural properties of both the as-electrospun precursor nanofibers and the resulting final TiO2 nanofibers were characterized by SEM, TEM, and XRD. The results indicated that the precursor nanofibers and the final TiO2 nanofibers made from the spin dopes containing DMF alone or DMF/isopropanol mixture as the solvent had the common cylindrical morphology with diameters ranging from tens to hundreds of nanometers, while those made from the spin dope containing isopropanol alone as the solvent had an abnormal concave morphology with sizes/widths ranging from sub-microns to microns. Despite the morphological discrepancies, all precursor nanofibers were structurally amorphous without distinguishable phase separation, while all final TiO2 nanofibers consisted of anatase-phased TiO2 single-crystalline grains with sizes of approximately 10 nm. The electrospun TiO2 nanofiber mat is expected to significantly outperform other forms (such as powder and film) of TiO2 for the solar cell (particularly dye-sensitized solar cell) and photo-catalysis applications. [ABSTRACT FROM AUTHOR]

Herein, we report the transient photocurrent and photovoltage studies on the charge transport in dye sensitized solar cells (DSSCs) made from the composites of electrospun titanium oxide (TiO2) nanofibers, and conventional TiO2 nanoparticles. The results on charge transport parameters (including the charge recombination lifetime, the electron transport lifetime, and the diffusion length) indicated that the addition of TiO2 nanofibers into composite photoanodes led to substantial improvement on the efficiency of charge collection, suggesting that the composites of nanoparticles and one-dimensional nanostructures are promising materials for the development of DSSCs with high efficiency. [ABSTRACT FROM AUTHOR]

Abstract: A newly developed electrospun polyimide nanofiber mat/membrane along with three commercial 0.22µm microfiltration membranes have been investigated for applications within a renewable biorefinery to complete solid–liquid clarification at two locations: (1) following enzymatic hydrolysis (pre-fermentation) and (2) post-ethanol fermentation. Performance was assessed by evaluating process flux, reversible and irreversible fouling, and the ability to completely remove insoluble solids. Lignin (the primary insoluble solid following enzymatic hydrolysis of a biomass feedstock), yeast cells, and cell debris (following fermentation to ethanol), along with soluble phenolic compounds, all being very hydrophobic in nature, caused extensive fouling, which led to permanent irreversible flux decline for the three commercial membranes. In addition, all three commercial membranes showed a small amount of insoluble solids in the permeate, indicating incomplete removal of the insoluble solids. In contrast, the polyimide nanofiber membrane showed virtually no irreversible fouling and complete retention of insoluble solids, suggesting the tremendous potential for biorefinery applications. [Copyright &y& Elsevier]

Abstract: Adsorptive membranes have shown great promise for bioseparations as an alternative to packed bed chromatography. Here we evaluate an adsorptive membrane (felt) made from electrospun cellulose acetate nanofibers as an ion-exchange medium for protein separations. The as-electrospun cellulose acetate nanofibers had diameters ranging from tens of nanometers to microns, and the pore sizes within the nanofiber felts were in the range from sub-microns to microns. The cellulose acetate nanofibers were hydrolyzed/deacetylated to yield regenerated cellulose nanofibers, which were then surface functionalized with diethylaminoethyl (DEAE) anion-exchange ligand. SEM imaging, along with FT-IR spectroscopy, and nitrogen content analysis were used to follow each stage of the process. For comparison, a regenerated cellulose microfiber adsorption medium, a commercially available regenerated cellulose adsorptive membrane, and bleached absorbent cotton balls, were all similarly treated and evaluated. The results indicated that the functionalized nanofiber felt had the highest static binding capacity of 40.0mg/g for bovine serum albumin (BSA), compared to 33.5mg/g, 14.5mg/g, and 15.5mg/g for the functionalized commercial membrane, cellulose microfiber medium, and cotton balls, respectively; and the DEAE nanofiber felt had over five times higher permeability of 10mM Tris buffer, pH 8.0 than the DEAE commercial membrane. It was also found that detrimental system dispersion, as determined by calculation of the Peclet number, could be limited by increasing the number of felt layers used for the adsorption bed, without compromising pressure limitations. Dynamic adsorption of BSA, at 10% breakthrough, was higher for the DEAE nanofiber felt (26.9mg/g) compared to the DEAE commercial membrane (20.9mg/g). [Copyright &y& Elsevier]

To develop tissue engineering scaffolds that possess similar morphological structures to natural extracellular matrices (ECMs) is a major technological challenge. Herein, the feasibility of utilizing polycaprolactone (PCL) as biocompatible and biodegradable binding agent to fabricate electrospun three-dimensional (3D) scaffolds has been demonstrated. The obtained 3D scaffolds are soft while elastic, and they possess interconnected and hierarchically structured pores with sizes in the range from sub-microns to hundreds of microns; hence, they are morphologically similar to natural ECMs thus well suited for cell functions and tissue formation. It is envisioned that various thermoplastic polymers could be fabricated into 3D nanofibrous scaffolds/structures by first making blend nanofibers with PCL followed by processing via the thermally induced (nanofiber) self-agglomeration (TISA) method and finally being thermally stabilized, and the resulting electrospun 3D nanofibrous scaffolds/structures might to be useful for a variety of applications (particularly those related to tissue engineering). [ABSTRACT FROM AUTHOR]

In this study, an innovative nano-material was prepared, which was ultilized to removal of heavy metal ions from wastewater. Polyacrylonitrile/cellulose acetate (PAN/CA) composite nanofibrous membranes were generated by the electronspinning technique first, and then amidoxime ployarcylonitrile/regenerate cellulose (AOPAN/RC) composite nanofibrous membranes were prepared by combining hydrolysis and amidoximation modification. The modification of composite nanofibers (AOPAN/RC) were consequently used in heavy metal ions adsorption. The characterizations of various different nanofibers were analyzed using scanning electron microscopy, Fourier transform infrared spectroscopy, surface area and pore size distribution analyzer and energy dispersive X-ray spectroscopy. Meantime, the adsorption equilibrium studies were studied. In addition, the saturation adsorption amount of nanofibrous membranes (at 25 °C) for Fe(III), Cu(II) and Cd(II) of 7.47, 4.26 and 1.13 mmolg −1 , respectively. The effects of pH value of solution, adsorption time and ions concentration on adsorption capacity were also investigated. Furthermore, the composite nanofibrous membranes after five times consecutive adsorption and desorption tests, the desorption rate of the Fe(III), Cu(II) and Cd(II) mental ions maintained more than 80% of their first desorption rate, AOPAN/RC composite nanofibrous reflected excellent resuability. [ABSTRACT FROM AUTHOR]

The aim of this study is to develop high-performance carrier/support for immobilization of enzyme ( e.g. , laccase). The hypothesis is that electrospun blend nanofiber membrane consisting of polyurethane (PU), amidoxime polyarcylonitrile (AOPAN), and β-cyclodextrin (β-CD) would possess the desired morphological, mechanical, and chemical properties for the coordination/chelation of Fe(III) ions and the subsequent immobilization of laccase molecules. To test this hypothesis, the Fe(III)-PU/AOPAN/β-CD nanofiber membrane has been prepared and characterized first; thereafter, the membrane’s performance as laccase immobilization carrier/support has been investigated. The results indicate that the prepared nanofiber membrane can efficiently immobilize laccase, and it also possesses excellent morphological/mechanical properties and regeneration capability. In specific, the membrane’s immobilization amount of laccase reaches a high value of 186.34 mg/g; moreover, the immobilized laccase exhibits reasonably high catalytic activity and significantly improved resistance against the variations of temperature and p H value than the free laccase. Additionally, thermal stability, storage stability, and reusability of the immobilized laccase are substantially improved; and the membrane carrier/support can also be regenerated/reused for several times. Therefore, the developed electrospun PU/AOPAN/β-CD blend nanofiber membrane is a high-performance carrier/support for efficient and reusable immobilization of laccase. [ABSTRACT FROM AUTHOR]

A technique of electro-spinning/netting (ESN) has been investigated to prepare poly(vinyl alcohol) (PVA) nanofibrous membrane with unique morphological structure of nano-fiber/net (NFN). The prepared NFN membrane consists of both common electrospun nanofibers (with diameters in hundreds of nanometers) and two-dimensional nano-nets (with nanofibril diameters smaller than 50 nm). It is revealed that formic acid has dual effects ( i.e. , as solvent and crosslinking agent) on solution properties and membrane morphological structures, and systematic analyses on rheological properties of three different 10 wt% PVA solutions with solvents being water, formic acid, and their 1/1 (wt./wt.) mixture have been conducted. Results indicate that the molecular interactions (particularly hydrogen bonding) among water, formic acid, and PVA macromolecular chains, as well as the acetal formation ( i.e. , chemical crosslinking), have significant influences on macromolecular conformation, solution rheological properties, and morphological structure of membranes. The acetal formation, which leads to improved tensile strength of PVA NFN membrane, has been verified by FT-IR and DSC. Additionally, the PVA NFN membrane has been assembled into a gravity-driven filtration device; and the membrane exhibits high rejection ratio (> 99%) for polystyrene microspheres (with diameters of ~ 0.2 μm) and high water flux (3773 L/m 2 ∙ h) under the pressure of 40 kPa. [ABSTRACT FROM AUTHOR]

Electrospun nanofiber membranes (with varied fiber diameters and different membrane porosities and thicknesses) have been explored as innovative microfiltration media for the highly efficient and cost-effective removal of contaminations with particle sizes of 0.2 μm (even 0.1 μm) from water. For the first time, the membrane porosity has been judiciously adjusted/controlled by a facile hot-pressing method; and the relationships between membrane properties and microfiltration performances have been systematically investigated. Upon the hot-pressing process, the membrane porosity could be significantly decreased from ∼86% to as low as ∼34%, further leading to the substantial increase of rejection fraction from 0% to ∼100% for 0.2 μm particles; moreover, some of these membranes (consisting of small diameter nanofibers) that were hot-pressed with relatively high pressure could even reject all of 0.1 μm particles. As compared to conventional microfiltration media, these membranes exhibited significantly higher flux values and substantially lower degrees of fouling without distinguishably sacrificing particle rejection fractions. For example, the rejection fraction and permeate flux value of one membrane (fiber diameters: ∼150–250 nm, membrane porosity: ∼40%) were ∼99.3% and ∼71.2·L·h −1 ·m −2 ·psi −1 at 13 mL volume fed, respectively, indicating that electrospun nanofiber membranes might be highly promising for microfiltration applications (particularly water purification). [ABSTRACT FROM AUTHOR]

Aromatic polyimides (PIs) are high-performance polymers with rigid heterocyclic imide rings and aromatic benzene rings in their macromolecular backbones. Owing to excellent mechanical properties and thermal stability, as well as readily adjustable molecular structures, PIs have been widely adopted for many applications related to electronics, aerospace, automobile, and other industries. In recent years, PI fibers prepared by electrospinning of polyamic acid (PAA) precursor nanofibers followed by imidization (commonly known as electrospun PI nanofibers) have attracted growing interests. Herein, the preparation, evaluation, and application of electrospun PI nanofibers are reviewed. PI polymers and the electrospinning technique are introduced first followed by the preparation of electrospun nanofibers of homo-PI, co-PI, blend-PI, and PI composite. Subsequently, the mechanical and thermal properties of electrospun PI nanofibers are presented; in particular, the mechanical properties of individual electrospun PI nanofibers are highlighted. Thereafter, various applications of electrospun PI nanofibers are outlined, including reinforcement of composites, Li-ion battery separators, fuel cell proton exchange membranes, sensors, microelectronics, high-temperature filtration media, super-hydrophobic PI nanofibers, and PI-based carbon nanofibers. In the final section of conclusions and perspectives, future research endeavors and high-value applications of electrospun PI nanofibers are discussed. [ABSTRACT FROM AUTHOR]

The ultimate goal of this research is to develop continuous carbon nanofibers with superior mechanical strength. The hypothesis is that aligned and stretched polyacrylonitrile (PAN) copolymer nanofibers may be an innovative type of precursor, because the nanofibers possess high degrees of morphological and structural perfections; and due to size effect ( i.e. , the nanofibers are ~ 20 times thinner than the conventional microfibers), the formation of structural heterogeneities ( e.g. , sheath/core structure) can be effectively prevented/mitigated. In this study, a bundle consisting of aligned PAN copolymer nanofibers was first prepared by electrospinning (with nanofibers collected by a flowing water system) followed by 3 times stretching; the oxidative stabilization of precursor nanofiber bundle was then investigated. Compared to conventional microfibers, electrospun nanofibers had smoother surface, higher density, and higher degrees of crystallinity and macromolecular orientation. As a result, the activation energy for cyclization of nitrile groups was higher; nevertheless, the diffusion of oxygen molecules was easier and the distribution of oxygen was more uniform particularly when the stabilization temperature was 240 °C or higher. Such a situation led to the reduction of activation energy for oxidative dehydrogenation, facilitated the structural conversion into aromatic and supra-molecular structures, and also prevented the formation of sheath/core structure. [ABSTRACT FROM AUTHOR]

Electrospun polyacrylonitrile (PAN) copolymer nanofibers (particularly those after the post-spinning stretching process) are expected to be an innovative type of precursors for making continuous carbon nanofibers with superior mechanical properties. In this study, three types of unidirectionally aligned PAN-based nanofibers including homo-PAN, binary-PAN (a PAN copolymer with itaconic acid units), and ternary-PAN (a PAN copolymer with itaconic acid and methyl acrylate units) were prepared by the electrospinning technique; thereafter, the nanofibers (in the form of bundles) were stretched into 3 times of their original lengths in hot water at 97 °C. The effects of chemical composition and post-spinning stretching process on morphological, structural, and thermo-chemical properties of these precursor nanofibers were investigated. The results indicated that, the stretched electrospun binary-PAN nanofibers possessed the most desired thermo-chemical properties, macromolecular regularity, crystal and macromolecule orientations, and dimensional stability; and thus they are suitable for the development of high-performance carbon nanofibers. [ABSTRACT FROM AUTHOR]

Electrospun SiO 2 nanofibers (in the form of overlaid mat) absorbed with fatty acid eutectics were studied to fabricate form-stable phase change materials (PCMs) for storage and retrieval of thermal energy. The SiO 2 nanofibers were made via electrospinning of precursor nanofibers followed by pyrolysis, while four fatty acid eutectics of capric acid–lauric acid (CA–LA), capric acid–myristic acid (CA–MA), capric acid–palmitic acid (CA–PA), and capric acid–stearic acid (CA–SA) were studied as the model PCMs. The morphological structure, absorption capacity, thermal energy storage property, and thermal energy storage/retrieval rates of the developed form-stable composite PCMs were characterized by scanning electron microscopy, differential scanning calorimetry, and measurement of melting/freezing time. The results indicated that the electrospun SiO 2 nanofibrous mats were highly porous and capable of absorbing a large amount of PCMs, and the absorption capacities of SiO 2 nanofibers for the four fatty acid eutectics ( i.e. , CA–LA, CA–MA, CA–PA and CA–SA) were 82.0, 83.7, 84.2, and 83.1%, respectively. The experimental values of heat enthalpy for the fabricated composite PCMs were slightly lower than the corresponding theoretical values, whereas there were no appreciable changes on phase transition temperatures. In comparison with fatty acid eutectics (without supporting materials), the melting and freezing time periods of the corresponding CA–LA/SiO 2 , CA–MA/SiO 2 , CA–PA/SiO 2 , CA–SA/SiO 2 were shortened by 28.6% and 30.8%, 20.0% and 44.4%, 62.5% and 50.0%, and 50.0% and 33.3%, respectively. This study suggested that the fabricated form-stable composite PCMs possessed appropriate phase transition temperatures and high heat enthalpy values; hence, they would have potential applications for low-temperature thermal energy storage/retrieval. [ABSTRACT FROM AUTHOR]

Abstract: A hybrid mat consisting of anatase-phase TiO2 nanofibers (~75wt.%) and structurally amorphous SiO2 nanofibers (~25wt.%) was prepared for the first time by electrospinning with dual spinnerets followed by pyrolysis in air at 500°C. Unlike many other TiO2 nanostructures, the TiO2/SiO2 hybrid mat was mechanically flexible; after being sensitized with N719 dye, the dye-sensitized mat was then assembled as a photo-detector for visible light. The fabricated photo-detector was also flexible; it exhibited excellent sensitivity and reproducibility/reversibility under varied light intensities, and it also showed high performance under the bending condition. Additionally, it is envisioned that the reported method could be a general approach for the preparation of flexible hybrid mats with different electrospun nanofibers of semiconducting metal oxides (e.g., TiO2 and ZnO) for various applications. [Copyright &y& Elsevier]

Continuous bundles of aligned electrospun polyacrylonitrile (PAN) copolymer nanofibers were prepared via collection with flowing water at varied temperatures. The morphological and structural properties, as well as the density and residual solvent amount, of electrospun nanofibers in different bundles were investigated. The results indicated that with the increase of water temperature, both diffusion coefficients of solvent (i.e., N,N-dimethylformamide) and non-solvent (i.e., H2O) increased; the surface roughness of nanofibers also increased, and the cross-section of electrospun nanofibers changed in shape from circular to irregular. With the decrease of water temperature from 25 to 2 °C, the average radius of gyration value (related to the average size of micro-voids in the nanofibers) decreased from 10.9 to 6.8 nm, the crystallinity value increased from 54.6% to 57.9%, and the nanofiber density increased from 1.191 to 1.202 g/cm3. It is envisioned that the continuous bundles of aligned electrospun PAN copolymer nanofibers collected with the flowing water bath at a low temperature (e.g., 2 °C) might be an innovative type of precursor for making the carbon nanofibers with superior mechanical strength. [ABSTRACT FROM AUTHOR]

Abstract: Herein we report an effective method for the preparation of composite nanofibers of polybutadiene rubber (BR) containing uniformly distributed Ag nanoparticles (Ag NPs). These nanofibers were morphologically stable and were made from electrospun core–sheath nanofibers with the sheath component being polyvinyl pyrrolidone (PVP) and the core component being BR, in situ synthesized Ag NPs, and a photo-curing agent of camphorquinone. The method included (1) co-axial electrospinning of core–sheath nanofibers, (2) in situ chemical crosslinking of BR in the core component of nanofibers, and (3) removal of the sheath component of PVP from nanofibers. The experimental observation indicated that, when the flow rate of inner spin dope was slower than that of the outer spin dope, the co-axial electrospinning process would be stable, the resulting nanofibers would not be curled; and there were few conglutination locations among the core–sheath nanofibers. Both electrospun core–sheath nanofibers and the resulting composite nanofibers possessed the cylindrical morphology with smooth surface. The study also revealed that the Ag in the composite nanofibers primarily existed in the elemental form. [Copyright &y& Elsevier]

Abstract: In this study, two types of polyimide (PI) nanofiber mats, including (1) the mats consisting of (almost) randomly overlaid PI nanofibers and (2) the mats consisting of highly aligned PI nanofibers, were prepared by the materials-processing technique of electrospinning. The nanofiber mats were subsequently used to develop composites with polyamide 6 (PA6) via the composites – fabrication method of polymer melt infiltration lamination (PMIL). Owing to superior mechanical properties (i.e., the tensile strength and modulus were 1.7GPa and 37.0GPa, respectively) and large specific surface area of electrospun PI nanofibers, the PI/PA6 composites with PI nanofiber mats as skeletal framework demonstrated excellent mechanical properties. In particular, the PI/PA6 composite containing 50wt.% of aligned PI nanofibers had the tensile strength and modulus of 447MPa and 3.0GPa along the longitudinal direction, representing ∼700% and ∼500% improvements as compared to neat PA6. [Copyright &y& Elsevier]

Abstract: Continuous bundles of aligned and stretched electrospun polyacrylonitrile (PAN) precursor nanofibers were prepared in an attempt to develop carbon nanofibers with superior strength. The bundles were prepared through collection of electrospun nanofibers with a flowing water bath followed by stretching in water at 97°C. Their morphologies, structures, and thermo-chemical properties were characterized by SEM, XRD, and DSC. The shrinkages in boiling water and the amounts of residual solvent were also measured. The results indicated that, the nanofibers in the bundles were uniform with smooth surfaces and small variations in diameters; after stretching the bundles by 4 times, the average fiber diameter was reduced to 56%, while the crystallinity of PAN was improved by 72%. The post-spinning stretching process facilitated the stabilization of PAN, as evidenced by the shift of the cyclization reaction to a lower temperature with smaller activation energy and larger enthalpy change. In comparison with the commonly adopted nanofiber collection method of a rotating drum, the flowing water bath method results in higher degree of uni-axial alignment and more desired structures of nanofibers. [Copyright &y& Elsevier]

Abstract: The ultrafine composite fibers consisting of lauric acid (LA), polyethylene terephthalate (PET), and silica nanoparticles (nano-SiO2) were prepared through the materials processing technique of electrospinning as an innovative type of form-stable phase change materials (PCMs). The effects of nano-SiO2 on morphology, thermal energy storage, thermal stability, and combustion properties of electrospun LA/PET/SiO2 composite fibers were studied. SEM images revealed that the LA/PET/SiO2 composite fibers with nano-SiO2 possessed desired morphologies with reduced average fiber diameters as compared to the LA/PET fibers without nano-SiO2. DSC measurements indicated that the amount of nano-SiO2 in the fibers had an influence on the crystallization of LA, and played an important role on the heat enthalpies of the composite fibers; while it had no appreciable effect on the phase change temperatures. TGA results suggested that the incorporation of nano-SiO2 increased the onset thermal degradation temperature, maximum weight loss temperature, and charred residue at 700°C of the composite fibers, indicating the improved thermal stability of the fibers. MCC tests showed that the heat resistance effect and/or barrier property generated by nano-SiO2 resulted in an increase of initial combustion temperature and a decrease of the heat release rate for the electrospun ultrafine composite fibers. [Copyright &y& Elsevier]

Abstract: In this study, polyacrylonitrile (PAN) nano-fibrous membranes with fiber diameters of ∼450nm were prepared by the technique of electrospinning; amidoxime nano-fibrous membranes were then prepared through treatment of PAN nano-fibrous membranes in hydroxylamine (NH2OH) aqueous solution. The –Con the surface of PAN nanofibers reacted with NH2OH molecules and led to the formation of –C(NH2)OH groups, which were used for coordination of Ag+ ions. Subsequently, the coordinated Ag+ ions were converted into silver nanoparticles (AgNP) with sizes being tens of nanometers. Morphologies, structures, and antimicrobial efficacies (against Staphylococcus aureus and Escherichia coli) of the membranes of electrospun PAN (ESPAN) nanofibers, ESPAN surface functionalized with amidoxime groups (ASFPAN), ASFPAN coordinated with silver ions (ASFPAN–Ag+), and ASFPAN attached with silver nanoparticles (ASFPAN–AgNP) were investigated. The study revealed that, with treatment of ESPAN membranes in 1M NH2OH aqueous solution for 5min, the resulting ASFPAN membranes became antimicrobial without distinguishable morphological variations; further treatment of ASFPAN membranes in 0.1M AgNO3 aqueous solution for 1h and the subsequent treatment in 0.01M KBr aqueous solution for 2h followed by photo-decomposition made the respective membranes of ASFPAN–Ag+ and ASFPAN–AgNP highly antimicrobial, which were capable of killing the tested microorganisms in 30min. The water permeability test indicated that these membranes possessed adequate transport properties for filtration applications. This study demonstrated a convenient and cost-effective approach to develop antimicrobial nano-fibrous membranes that are particularly useful for the filtration of water and/or air. [Copyright &y& Elsevier]

Abstract: Herein we report that hierarchical electrospun SiO2 nanofibers incorporated with SiO2 nanoparticles with fiber diameters being ∼500nm and particle sizes being tens of nanometers were developed through the combination of sol–gel process and electrospinning technique followed by high-temperature pyrolysis; and their morphologies and BET surface areas were examined. The study revealed that the pre-gelation of tetraethyl orthosilicate (TEOS) in spin dopes was important to achieve the morphological consistence of the electrospun precursor nanofibers and the resulting final SiO2 nanofibers; additionally, SiO2 nanoparticles appeared to be enriched on the fiber surface, while the surface-roughness and/or porosity of the nanofibers could be controlled through adjusting the incorporation amount of SiO2 nanoparticles. The developed hierarchical electrospun SiO2 nanofibers are expected to have important applications in composites (particularly dental composites) as well as catalyst support and adsorption. [Copyright &y& Elsevier]

Abstract: Graphitic carbon nanofibers (GCNFs) with diameters of approximately 300 nm were developed using bundles of aligned electrospun polyacrylonitrile (PAN) nanofibers containing phosphoric acid (PA) as the innovative precursors through thermal treatments of stabilization, carbonization, and graphitization. The morphological, structural, and mechanical properties of GCNFs were systematically characterized and/or evaluated. The GCNFs made from the electrospun PAN precursor nanofibers containing 1.5 wt.% of PA exhibited mechanical strength that was 62.3% higher than that of the GCNFs made from the precursor nanofibers without PA. The molecules of PA in the electrospun PAN precursor nanofibers initiated the cyclization and induced the aromatization during stabilization, as indicated by the FT-IR and TGA results. The stabilized PAN nanofibers possessed regularly oriented ladder structures, which facilitated the further formation of ordered graphitic structures in GCNFs during carbonization and graphitization, as indicated by the TEM, XRD, and Raman results. [Copyright &y& Elsevier]

Abstract: We report a study of the fluorescence properties of the conjugated polymer poly [2-methoxy-5-(2′-ethyl hexyloxy)-p-phenylene vinylene] (MEH-PPV) and polyethylene oxide (PEO) nanofibers. MEH-PPV/PEO nanofibers with different compositions have been fabricated by the electrospinning technique. The fluorescence spectra of the nanofibers show that the emission shoulder at ∼630nm blue-shifts ∼45nm, whereas the main emission peak around 590nm blue-shifts ∼15nm with decreasing concentration of MEH-PPV in the nanofiber. In addition, confocal microscopic studies of a single MEH-PPV/PEO electrospun nanofiber indicate that the fluorescence spectra of the nanofiber do not show any polarization dependence. The results are discussed in terms of the aggregation of MEH-PPV in an inert matrix. [Copyright &y& Elsevier]

Abstract: Carbon nanofibers with diameters of 200–300nm were developed through stabilization and carbonization of aligned electrospun polyacrylonitrile (PAN) nanofiber bundles. Prior to the oxidative stabilization in air, the electrospun PAN nanofiber bundle was tightly wrapped onto a glass rod, so that tension existed during the stabilization. We also investigated several carbonization procedures by varying final carbonization temperatures in the range from 1000 to 2200°C. The study revealed that: (1) with increase of the final carbonization temperature, the carbon nanofibers became more graphitic and structurally ordered; (2) the carbon nanofiber bundles possessed anisotropic electrical conductivities, and the differences between the parallel and perpendicular directions to the bundle axes were over 20 times; and (3) the tensile strengths and Young''s moduli of the prepared carbon nanofiber bundles were in the ranges of 300–600MPa and 40–60GPa, respectively. [Copyright &y& Elsevier]

Abstract: Thermo-chemical reactions occurring during the oxidative stabilization of electrospun polyacrylonitrile (PAN) precursor nanofibers with diameters of approximately 300nm were investigated as well as the resulting structural conversions, and the results were compared to those of conventional SAF 3K (Courtaulds) precursor fibers. The study revealed that: (1) the nitrile groups in the electrospun nanofibers possessed a higher reactivity than those in the SAF 3K fibers; (2) the macromolecules in the electrospun nanofibers predominantly underwent inter-molecular cyclization/crosslinking while those in the SAF 3K fibers underwent intra-molecular cyclization during the early stages of stabilization; and (3) under the same stabilization conditions, the structural conversion from linear macromolecules to aromatic ring/ladder structures in the electrospun nanofibers occurred faster and more thoroughly than in the SAF 3K fibers. These characteristics combined with other properties, including small diameter and high degree of structural perfection, suggest that electrospun PAN precursor nanofibers may be used to develop continuous nano-scale carbon fibers with superior mechanical strength, especially if the electrospun nanofibers could be further aligned and stretched. [Copyright &y& Elsevier]

Abstract: The aim of this study was to investigate the preparation, characterization, and encapsulation/release performance of an electrospun composite nanofiber mat. The hypothesis was that the composite nanofiber mat with nano-scaled drug particles impregnated in biocompatible and biodegradable polymer nanofibers can serve as an innovative type of tissue engineering scaffold with desired and controllable drug encapsulation/release properties. To test the hypothesis, the composite nanofiber mat electrospun from an emulsion consisting of poly(lactic-co-glycolic acid) (PLGA) Rhodamine B (a model compound to simulate drugs), sorbitan monooleate (Span-80, a non-ionic emulsifier/surfactant that is presumably non-toxic/safe for cell-growth), chloroform, DMF, and distilled water was prepared and characterized; and the Rhodamine B encapsulation/release profile in phosphate buffered saline (pH=7.4) was recorded and analyzed. For comparison purposes, two additional nanofiber mats electrospun from (1) a solution containing PLGA and Rhodamine B, and (2) a solution containing PLGA, Rhodamine B, and Span-80 were also prepared and assessed as the control samples. The results indicated that the composite nanofiber mat electrospun from the emulsion had the most desired and controllable Rhodamine B encapsulation/release profile and the excellent morphological sustainability; thus, it could be utilized as both a drug encapsulation/release vehicle and a tissue engineering scaffold. [Copyright &y& Elsevier]

Herein, we report an innovative method to produce three-dimensional (3D), elastic, and conductive nanofibrous sponges with high sensitivity, great mechanical properties, and sweatproof feature for wearable tactile pressure sensor. The 3D sponges were prepared by assembling the shortened polyacrylonitrile/polyimide (PAN/PI) electrospun nanofibers and reduced graphene oxide (rGO) nanosheets. In specific, the PAN/PI nanofibers and graphene oxide (GO) nanosheets were mixed in aqueous circumstance first and then freeze dried to form a 3D porous sponge. Subsequently, the sponge was thermally treated at 100 °C and 210 °C successively to reduce the GO nanosheets to conductive rGO nanosheets. Thereafter, the polydimethylsiloxane (PDMS) treatment was carried out to provide the sponge with elasticity/robustness and hydrophobicity. Upon varying the amount of rGO, three types of sponges were fabricated to study their mechanical and electrical properties. The obtained 3D conductive sponge with the most rGO amount (3-rGO/NF) possessed light weight (11.50 mg/cm3) and high porosity (99.23%). The mechanical strength and current change ratio during the cyclic compression process were investigated, and the results were closely correlated with the amount of rGO existed in the sponge. The 3-rGO/NF had the largest current change ratio when compressed, leading to the highest sensitivity even at a relatively small compressive strain. It is important to note that the prepared 3D conductive sponges were sensitive, bendable, and sweatproof, which would be particularly suitable for making wearable tactile pressure sensors. [Display omitted] • Superhydrophobic and elastic 3D conductive nanofibrous sponges were fabricated. • The sponges were made from electrospun PAN/PI nanofibers and GO/rGO nanosheets. • Tunable conductivity and high sensitivity could be achieved for these sponges. • The PDMS treatment improved mechanical robustness/elasticity and hydrophobicity. • The 3D sponges exhibited excellent performance for making wearable pressure sensors. [ABSTRACT FROM AUTHOR]

The objective of this study was to develop highly efficient and cost-effective ultrafiltration (UF) media/membranes from electrospun nanofibers alone without using expensive/complicated chemical modification methods. The hypothesis was that, during hot-pressing hybrid electrospun nanofiber membranes (ENMs) consisting of a thermoplastic polymer (e.g. , PVDF) and a skeleton polymer (e.g. , PAN), the PVDF nanofibers could be partially melted/softened even broken, leading to the conglutination/fusion of nanofibers. This would further result in the reduction of membrane porosities and the variation of membrane morphologies/structures; consequently, the membrane equivalent/apparent pore sizes would also be decreased. To test the hypothesis, as-collected PAN/PVDF hybrid ENMs (PPHENMs) were hot-pressed at 180 °C (above the PVDF melting point of ~177 °C) by using different pressures of 2.5, 5, and 10 MPa; for comparison, PPHENMs were also prepared upon being hot-pressed at 60 °C. The PPHENMs180 hot-pressed under 10 MPa had the lowest porosity of ~16.7 vol%, indicating that nanofibers occupied ~5/6 vol in the membrane; and the membrane exhibited high pure water flux value of ~200 L m−2∙h−1 under the applied pressure of 4 bar. Moreover, this membrane could completely reject/remove polystyrene particles with sizes larger than 20 nm from water; and the rejection percentages for bovine serum albumin and bovine γ-globulin molecules reached ~60% and ~75%, respectively. Therefore, hot-pressed hybrid ENMs made of thermoplastic nanofibers and skeleton nanofibers could provide an innovative and facile approach for the development of high-performance UF media/membranes. Image 1 • Hot-pressed PAN/PVDF hybrid electrospun nanofiber membranes were prepared. • The membranes were prepared simply by electrospinning followed by hot-pressing. • The preparation process did not involve any chemical treatments. • The membranes exhibited high performance as filtration/ultrafiltration media. • They could remove particles/species (≥20 nm) from water with large flux values. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) monolithic structures (i.e. , aerogels/sponges/scaffolds) assembled from fragmented electrospun nanofiber mats/membranes represent an emerging research topic in the electrospinning field. Owing to extremely high porosity, as well as excellent structural flexibility and stability, these 3D nanofibrous structures have attracted significant interests for various applications. In this review, the preparation of 3D monolithic structures are thoroughly discussed; and the properties of 3D structures and their various applications in the fields of environment (e.g. , organic compound removal, dye adsorption, and filtration and separation), energy (e.g. , supercapacitor), electronics (e.g. , pressure sensor), chemical engineering (e.g. , catalyst support, thermal insulator, and Joule heater), and biomedical engineering (e.g. , tissue engineering, hydrogel, and drug delivery) are summarized. Additionally, the future perspectives and challenges are also presented. It is envisioned that, this review will provide important guidance in designing novel 3D electrospun nanofibrous structures and exploring their potential applications. [ABSTRACT FROM AUTHOR]