Electrospinning Conditions Research Articles

Tailoring Pore Structure of Ultralight Electrospun Sponges by Solid Templating

AbstractFreeze‐casted nanofiber based sponges or aerogels exhibit a hierarchical porous structure. Pore formation is only partially understood. Therefore, we studied the underlying solid templating mechanism. We were able to tailor the secondary pore size between 9.5 and 123 μm while retaining the smaller primary pores known from electrospun nanofiber membranes. To understand the effect of microstructure on the sponges’ bulk properties, mass flow through the pores and interaction with the sponges’ internal surface were investigated. By solely altering the sponges’ microstructure we indeed found tunability in permeability by a factor 7 and in filtration efficiency by a factor of 220. Hence, pore architecture of nanofiber based sponges is a key element for their performance. The selected pullulan/PVA polymer blends and aqueous electrospinning conditions are benign and allow the facile adaptation of these ultralight highly porous sponges for a large number of applications.

Electrospun fibrous silk fibroin/poly(L-lactic acid) scaffold for cartilage tissue engineering.

For successful tissue engineering of articular cartilage, a scaffold with mechanical properties that match those of natural cartilage as closely as possible is needed. In the present study, we prepared a fibrous silk fibroin (SF)/poly(L-lactic acid) (PLLA) scaffold via electrospinning and investigated the morphological, mechanical, and degradation properties of the scaffolds fabricated using different electrospinning conditions, including collection distance, working voltage, and the SF:PLLA mass ratio. In addition, in vitro cell-scaffold interactions were evaluated in terms of chondrocyte adhesion to the scaffolds as well as the cytotoxicity and cytocompatibility of the scaffolds. The optimum electrospinning conditions for generating a fibrous SF/PLLA scaffold with the best surface morphology (ordered alignment and suitable diameter) and tensile strength (~1.5 MPa) were a collection distance of 20 cm, a working voltage of 15 kV, and a SF:PLLA mass ratio of S50P50. The degradation rate of the SF/PLLA scaffolds was found to be determined by the SF:PLLA mass ratio, and it could be increased by reducing the PLLA proportion. Furthermore, chondrocytes spread well on the fibrous SF/PLLA scaffolds and secreted extracellular matrix, indicating good adhesion to the scaffold. The cytotoxicity of SF/PLLA scaffold extract to chondrocytes over 24 and 48 h in culture was low, indicating that the SF/PLLA scaffolds are biocompatible. Chondrocytes grew well on the SF/PLLA scaffold after 1, 3, 5, and 7 days of direct contact, indicating the good cytocompatibility of the scaffold. These results demonstrate that the fibrous SF/PLLA scaffold represents a promising composite material for use in cartilage tissue engineering.

Stiffness, Strength, and Toughness of Electrospun Nanofibers: Effect of Flow-Induced Molecular Orientation

The simultaneous sharp rise in stiffness, strength, and toughness of electrospun nanofibers at small diameters is explained here as the result of the molecular orientation induced by the strong stretching of the electrospinning extensional flow. Differing from the common view that this phenomenon is related to the nanofibers size scale, we show by theoretical analysis that it is likely the result of an abrupt transition in polymer chain extension that occurs at high flow strain rates. Consequently, the molecular orientation and mechanical properties experience a matching transition, followed by a linear rise with the strain rate. The model compares well with published experimental data, supporting the assertion that the observed phenomena can be explained as the consequence of electrospinning conditions instead of size dependence. We show how the mechanical properties can be tuned by controlling the process as well as set the goal for future improvement in these properties.

Fabrication, Polarization of Electrospun Polyvinylidene Fluoride Electret Fibers and Effect on Capturing Nanoscale Solid Aerosols.

Electrospun polyvinylidene fluoride (PVDF) fiber mats with average fiber diameters (≈200 nm, ≈2000 nm) were fabricated by controlled electrospinning conditions. These fiber mats were polarized using a custom-made device to enhance the formation of the electret β-phase ferroelectric property of the fibers by simultaneous uniaxial stretching of the fiber mat and heating the mat to the Curie temperature of the PVDF polymer in a strong electric field of 2.5 kV/cm. Scanning electron microscopy, Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry and Brunauer-Emmett-Teller (BET) surface area analyses were performed to characterize both the internal and external morphologies of the fiber mat samples to study polarization-associated changes. MATLAB simulations revealed the changes in the paths of the electric fields and the magnetic flux inside the polarization field with inclusion of the ferroelectric fiber mats. Both polarized and unpolarized fiber mats were challenged as filters against NaCl particles with average particle diameters of about 150 nm using a TSI 8130 to study capture efficiencies and relative pressure drops. Twelve filter experiments were conducted on each sample at one month time intervals between experiments to evaluate the reduction of the polarization enhancement over time. The results showed negligible polarization loss for the 200-nm fiber sample. The polarized mats had the highest filter efficiencies and lowest pressure drops.

Electrospinning of gelatin and SMPU with carbon nanotubes for tissue engineering scaffolds.

The nanofibres created by electrospinning technique are currently used for a variety of applications in tissue engineering; and Gelatin and Polyurethane Shape-Memory (SMPU) have important results in biomedicine. Similarly, carbon nanotubes combined with other biomaterials change important properties, opening new opportunities for biomedical applications. In this work, we constructed scaffold using electrospinning technique based in bovine-hide gelatin, SMPU and both materials hybrid with carbon nanotube. Morphology and cytotoxicity were evaluated and mechanical properties for two materials were obtained in scaffold building. Morphological, mechanical and citotoxic properties of the electrospun fibers were found to be dependent of alteration in materials concentration, electrospinning conditions and MWCNT concentration. According to morphological, cytotoxic and mechanical analysis, SMPU more MWCNT were the best material, with nanofibers of 451 nm, tensile strength of 1.912 MPa, and a high ratio surface volume.

Programmable Structure Control in Cigarlike TiO2 Nanofibers and UV-Light Photocatalysis Performance of Resultant Fabrics

Novel cigarlike nanofibers with an outer-shell and inner-continuous-pore structure and resultant fabrics have been fabricated by coupling the self-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) containing titanium precursors with the electrospinning technique in our previous work [You et al. ACS Appl. Mater. Interfaces 2013, 5, 2278]. In the current work, the structure control in these nanofibers has been investigated in detail using scanning electron microscopy, focused ion beam, and small angle X-ray scattering. Our results indicate that electrospinning conditions, the adopted solvent, the volume fraction of PS-b-PEO block copolymer, and the amount of titanium tetraisopropoxide in the mixture produce significant effects on both outer-shell and inner-continuous structures in the nanofibers. The parameters discussed above make it possible to achieve programmable structure control in the aspect of the diameter, thickness of the outer shell, and inner continuous pore. As a result, both microp...

Electrospinning of single and multilayered scaffolds for tissue engineering applications

Optimized electrospinning conditions were applied to produce single and multilayered (ML) scaffolds composed of polycaprolactone, collagen and elastin. The ML scaffold was cross-linked with glutaraldehyde to increase the stability. Morphological and structural characteristics of the scaffolds were measured by SEM and FTIR analyses. Results revealed that polymers combined to each other well and uniform fibers were obtained with the diameters ranging from 156 ± 53 to 1536 ± 293 nm. Contact angle measurements were performed to investigate the hydrophilic character of each structure. It was observed that incorporation of the natural polymers into the blends increased the hydrophilicity. Mechanical tests proved that collagen contributed to fabricate stiffer structures while elastin provided more elasticity. Biocompatibility of the scaffolds was examined by SEM analysis and WST-1 test with mouse fibroblast cells (L929) in vitro. Results exhibited that the addition of natural polymers increased the cell growth, and none of the single and ML scaffolds presented cytotoxic effect.

Fabrication of trans-1,4-polyisoprene nanofibers by electrospinning and its crystallization behavior and mechanism

Trans-1,4-polyisoprene (TPI) nanofibers have been fabricated successfully through electrospinning technology. Through the control of electrospinning parameters, highly crystallized TPI fresh fibers composed mainly by β phase were produced. Morphology and diameter of TPI nanofibers can be controlled by adjusting the electrospinning conditions. The in situ observations of FTIR spectra revealed that the crystallinity of the TPI fibers decreased with aging. While for TPI nanofibers aging at 45 °C for 24 h, a decrease in crystallinity as well as β to α transformation was observed with aging and these changings happened in the first 50 h during aging. The mechanism for β-TPI formation during electrospinning process and the reduced crystallinity with aging were proposed.

Optimization of electrospinning parameters for polyacrylonitrile-MgO nanofibers applied in air filtration

ABSTRACTThe present study aimed to optimize the electrospinning parameters for polyacrylonitrile (PAN) nanofibers containing MgO nanoparticle to obtain the appropriate fiber diameter and mat porosity to be applied in air filtration. Optimization of applied voltage, solution concentration, and spinning distance was performed using response surface methodology. In total, 15 trials were done according to the prepared study design. Fiber diameter and porosity were measured using scanning electron microscopic (SEM) image analysis. For air filtration testing, the nanofiber mat was produced based on the suggested optimum conditions for electrospinning. According to the results, the lower solution concentration favored the thinner fiber. The larger diameter gave a higher porosity. At a given spinning distance, there was a negative correlation between fiber diameter and applied voltage. Moreover, there were curvilinear relationships between porosity and both spinning distance and applied voltage at any concentration. It was also concluded that the developed filter medium could be comparable to the high-efficiency particulate air (HEPA) filter in terms of collection efficiency and pressure drop. The empirical models presented in this study can provide an orientation to the subsequent experiments to form uniform and continuous nanofibers for future application in air purification.Implications: High-efficiency filtration is becoming more important, due to decreasing trends air quality. Effective filter media are increasingly needed in industries applying clean-air technologies, and the necessity for developing the high-performance air filters has been more and more felt. Nanofibrous filter media that are mostly fabricated via electrospinning technique have attracted considerable attention in the last decade. The present study aimed to develop the electrospun PAN-containing MgO nanoparticle (using the special functionalities such as absorption and adsorption characteristics, antibacterial functionality, and as a pore-forming agent) filter medium through experimental investigations for application in high-performance air filters.

Fabrication, Morphology Control, and Electroless Metal Deposition of Electrospun ABS Fibers

Acrylonitrile–butadiene–styrene (ABS) is a polymer composing of acrylonitrile, butadiene, and styrene. It has been widely used in industry because of its good mechanical and physical properties. The fabrication of ABS fibers, however, has been rarely studied. Here the fabrication of ABS fibers has been reported by an electrospinning technique, in which the sizes and morphologies of the fibers can be controlled by adjusting the electrospinning conditions. The morphologies of the ABS fibers can also be transformed by annealing the fibers on poly (methyl methacrylate) (PMMA) films. After annealing, the ABS fibers gradually transform to ABS particles embedded in the PMMA films by a mechanism similar to the Rayleigh‐instability‐type transformation. To extend the applications of the electrospun ABS fibers, electroless deposition of copper is also conducted, resulting in copper‐coated ABS fibers. image

Fabrication and chemical crosslinking of electrospun trans-polyisoprene nanofiber nonwoven

In this work, the optimal electrospinning conditions of trans-polyisoprene (TPI) solutions were evaluated nevertheless its lower glass transition temperature than the room temperature. Subsequently, chemical crosslinking of TPI nonwovens was firstly investigated by vulcanizing at high temperatures in the case of the persistence of nanofiber structure. For this purpose, curing agents of TPI were embedded in TPI nanofibers by co-electrospinning, and then a protect layer was coated on TPI nanofibers by filtering gelatin solution going through TPI nonwoven before the vulcanization at 140−160 °C. The results showed that the vulcanization of TPI fibrous nonwoven at high temperatures did not destroy the fiber morphology. Interestingly, TPI fibrous nonwovens after vulcanization showed excellent mechanical properties (~17 MPa of tensile strength) that could be comparable to or even higher than that of some bulk rubber materials.

Axisymmetric instability in a thinning electrified jet.

The axisymmetric stability of an electrified jet is analyzed under electrospinning conditions using the linear stability theory. The fluid is considered Newtonian with a finite electrical conductivity, modeled as a leaky dielectric medium. While the previous studies impose axisymmetric disturbances on a cylindrical jet of uniform radius, referred to as the base state, in the present study the actual thinning jet profile, obtained as the steady-state solution of the one-dimensional slender filament model, is treated as the base state. The analysis takes into account the role of variation in the jet variables like radius, velocity, electric field, and surface charge density along the thinning jet in the stability behavior. The eigenspectrum of the axisymmetric disturbance growth rate is constructed from the linearized disturbance equations discretized using the Chebyshev collocation method. The most unstable growth rate for the thinning jet is significantly different from that for the uniform radius jet. For the same electrospinning conditions, while the uniform radius jet is predicted to be highly unstable, the thinning jet profile is found to be unstable but with a relatively very low growth rate. The stabilizing role of the thinning jet is attributed to the variation in the surface charge density as well as the extensional deformation rate in the fluid ignored in the uniform radius jet analysis. The dominant mode for the thinning jet is an oscillatory conducting mode driven by the field-charge coupling. The disturbance energy balance finds the electric force to be the dominant force responsible for the disturbance growth, potentially leading to bead formation along the fiber. The role of various material and process parameters in the stability behavior is also investigated.

Porous Carbon Interdigitated Electrode Arrays for Electrochemical Biosensors

In this paper, three-dimensional (3D) porous carbon interdigitated electrode arrays (IDEAs) are developed utilizing standard photolithography of electrospun mats of SU-8 photoresist nanofibers. Porous IDEAs have the potential to produce higher aspect ratio than non-porous electrode due to the reduced stress at the interface between the substrate and the carbon structure [1]. In a first step, a layer of polymeric nanofibers is deposited on a silicon substrate by optimizing the conditions for far-field electrospinning of SU-8 photoresist. Subsequently, the SU-8 mat is patterned using conventional photolithography to generate the polymer precursor for the carbon electrode structure. Finally, the polymer precursor is pyrolysed at 900°C in an inert atmosphere to produce a porous carbon IDEA [2]. The porous carbon IDEAs were optimized as a function of the nanofibers diameter, thickness of the electrospun SU-8 mat and width/gap ratio of the IDEAs. To characterize the porous carbon IDEAs, electrochemistry studies (cyclic voltammetry and electrical impedance spectroscopy) were conducted and the results were compared with traditional flat carbon IDEAs (the latter was fabricated by spin coating a non-porous SU-8 film on a silicon substrate). The redox amplification factor of both porous and flat carbon IDEAs was studied to gain an understanding of the effect of electrode porosity on the redox amplification. This factor was optimized for width and gap of the IDEAs and the nanofibers diameter. Acknowledgments This research is supported by Flagship grant project number FL001A-14AET, Transdisiplinary Research Grant Scheme (TRGS TR002A-2014B) from University of Malaya and Ministry of Science Technology and Innovation (MOSTI) Science Fund (SF-020-2014).

Catalytic Improvement on Counter Electrode of Dye-Sensitized Solar Cells Using Electrospun Pt Nano-Fibers.

A dye-sensitized solar cell is one of cost-competitive photovoltaic devices. For higher performance, all components have been actively studied and improved. However, Pt is still a dominant catalyst since first development although some catalytic materials were studied so far. Catalytic materials of counter electrode play an important role in the performance because it supplies electrons from counter electrode to electrolyte. Therefore, the catalytic activation of counter electrode is closely connected with the performance enhancement. In this work, Pt nano-fiber was fabricated by electrospinning and applied for the counter electrode. Its wide surface area is advantageous for good conductivity and catalytic activation. Morphological characteristics of nano-fibers were analyzed according to electrospinning conditions. Photovoltaic properties, cyclic voltammetry, impedance analysis verified the catalytic activation. Consequently, dye-sensitized solar cell with Pt nano-fiber electrospun at 5.0 kV of applied voltage had higher performance than conventional dye-sensitized solar cell with Pt thin film. This work is significant for related researches because all nano-fibers counter electrode material proposed so far never exceeded the performance of conventional Pt counter electrode.

Nitrogen- and Oxygen-Containing Porous Ultrafine Carbon Nanofiber: A Highly Flexible Electrode Material for Supercapacitor

Herein, we report a simple and effective preparation of ultrafine CNFs (u-CNFs) with high surface area via electrospinning of two immiscible polymers [polyacrylonitrile (PAN) and poly(methyl methacrylate) (PMMA)] followed by calcination at high temperature in an inert atmosphere. Various electrospinning conditions were optimized in detail. Four different kinds of PAN/PMMA ratios (10/0, 7:3, 5:5 and 3:7) were chosen and found that the PAN/PMMA ratio of 3:7 (PAN/PMMA-3:7) is the optimum one. BET analysis showed the specific surface area of the u-CNFs-3:7 was 467.57 m2/g with an excellent pore volume (1.15 cm3 g−1) and an average pore size (9.48 nm): it is about 25 times higher than the conventional CNFs (c-CNFs). TEM and FE-SEM images confirmed the ultrafine structure of the CNFs with a thinner fiber diameter of ~50 nm. The graphitic nature and atomic arrangement of the u-CNFs were investigated by Raman and XPS analyses. For the supercapacitor application, unlike the common electrode preparation methods, the u-CNFs-3:7 was used without any activation, chemical or mechanical modifications. The u-CNFs-3:7 showed a better specific capacitance of 86 F/g in 1 mol/L H2SO4 when compared to pure CNFs. The excellent physicochemical properties make the u-CNFs-3:7 an alternative choice to the existing CNFs for the supercapacitors.

Controllable growth of highly organized ZnO nanowires using templates of electrospun nanofibers

We present a straightforward and accessible method to fabricate high density ZnO nanowires (NWs) with tunable morphology. Our approach includes (1) electrospinning of zinc salts embedded polymer nanofibers, (2) high temperature calcination for the polymer removal and the formation of ZnO seeds, and (3) hydrothermal growth of ZnO NWs. The high resolution transmission electron emission and selected-area electron diffraction characterization results indicate that the obtained ZnO NWs have a single crystalline structure. The method could be applied to produce highly dense and organized ZnO NWs with different pattern morphologies, including aligned, uniform film and hierarchical structured ZnO NWs mostly depending on the initial electrospinning conditions. The length of the ZnO NWs could be controlled in the range of 1–8 μm by different growth time, which are able to generate the hydrophobic surface with different wetting properties. Due to the convenient preparation and large surface areas, the ZnO NWs fabricated by this method are suitable for a range of energy applications.

Polyvinylbutyral assisted synthesis and characterization of kesterite quaternary semiconductor Cu2ZnSnSe4 nanofibers by electrospinning route

Kesterite quaternary semiconductor Cu2ZnSnSe4 nanocrystals were successfully prepared via a relatively simple electrospinning route and characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). Semiconductor Cu2ZnSnSe4 nanofibers were obtained for PVB/ Cu2ZnSnSe4 precursor at an applied voltage of 20kV after annealing treatment at 500°C for 2h. The optical property of Cu2ZnSnSe4 nanocrystals was also recorded by means of UV–vis absorption spectroscopy. It was shown that the dimension effect of composites could be enhanced by increasing the electrospinning conditions (voltage, distances, concentrations, viscosity) and the CZTSe nanofibers prepared at annealing process exhibited more intense optical properties. Temperature dependent studies were conducted and the annealing of composites was found to be the key factor in determining the phase formation.

Electrospun Carbon Nanotube-Reinforced Nanofiber

We fabricated multi-walled carbon nanotube (MWNT) reinforced polyurethane (PU) nanofiber (MWNT-PU) web via electrospinning. In order to optimize the electrospinning conditions, we investigated the effects of various parameters including kind of solvent, viscosity of the spinning solution, and flow rate on the spinnability and properties of nanofiber. N,N-dimethylformamide (DMF), tetrahydrofuran (THF) and their mixture with various volume ratio were used as the spinning solvent. Morphology of the nanofiber was studied using scanning electron microscope (SEM) and transmission electron microscope (TEM), confirming successful fabrication of MWNT-PU nanofiber web with uniform dispersion of MWNT in longitudinal direction of the fiber. The MWNT-PU nanofiber web exhibited two times higher tensile strength than PU nanofiber web. We also fabricated electrically conducting MWNT-PU nanofiber web by coating poly(3,4-ehtylenedioxythiophene) (PEDOT) on the surface of MWNT-PU nanofiber web for electromagnetic interference (EMI) shielding application. The electromagnetic interference shielding effectiveness (EMI SE) was quite high as 25 dB in the frequency range from 50 MHz to 10 GHz.

Methyl cellulose nanofibrous mat for lipase immobilization via cross-linked enzyme aggregates

Nanofibers prepared from cellulose derivatives are a suitable support for enzyme immobilization. We selected a methyl cellulose (MC) nanofibrous mat for enzyme immobilization. First, we established the electrospinning conditions for the preparation of the MC nanofibrous mat. A mixture of ethanol and water at a volume ratio of 1:1 was selected as the solvent of MC for electrospinning. An MC concentration of 5 wt% and a flow rate of 0.5 mL/h were employed to ensure stable electrospinning without bead formation. By varying the applied voltage, the sizes of the MC nanofibers could be controlled in the range of 50 to 80 nm, and an applied voltage of 18 kV was selected for further experimentation. The MC nanofibrous mats were cross-linked with glutaraldehyde under acidic conditions for 12 h in order to increase the stability of the mats in water. Lipase was directly immobilized onto cross-linked MC nanofibrous mats without a further activation step, and 34.82 µg of lipase was immobilized per mg of support. The reusability test showed an unexpected loss of activity after second reuse due to a loss of lipase during the washing procedure promoted by the surface erosion of the MC nanofiber. This problem was solved by introducing crosslinked enzyme aggregates (CLEA) to the surfaces of MC nanofibers. CLEA formation enhanced the activity of lipase per mg of support by almost 5-fold compared with the original lipase-immobilized MC nanofibrous mat. The reusability was also enhanced: more than 90% of the initial activity was retained after seven reuses.

エレクトロスピニング法による導電性ナノファイバーの開発

Nanofibers have exceptional properties due to their diameter and large surface area to mass ratio. Thus, nanofibers can potentially be used as electrodes in highly sensitive biosensing systems, bio-fuel cells, and high-power devices. In this study, conductive polyurethane-FeCl3 composite nanofibers were fabricated by electrospinning. The optimal electrospinning conditions were investigated, and the physical and chemical properties of the composite were evaluated. Conductive polyurethane/FeCl3 fibers of 1-5 µm in diameter were obtained, and their conductivity was 13.7±1.47×10-3 S/m. The effects of the morphology of the material were also monitored, and it was found that a fiber mat was 10 times more conductive than a film, because the crystallinity of the polyurethane and the Fe3+ distribution affect the electrical properties. The conductive nanofibers are flexible because the fiber substrate is made of polyurethane. Therefore, the fibers demonstrated in this study could make it possible to control the electric properties by modifying the nanofiber morphology.