Defect-free Fibers Research Articles

Development of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Fibers for Skin Tissue Engineering: Effects of Topography, Mechanical, and Chemical Stimuli

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a biodegradable polyester, was electrospun to form defect-free fibers with high surface-area-to-volume ratio for skin regeneration. Several parameters such as solvent ratio, polymer concentration, applied voltage, flow rate, and tip-to-target distance were optimized to achieve defect-free morphology. The average diameter of the PHBV fibers was 724 ± 91 nm. PHBV was also solvent-cast to form 2-D films, and its mechanical properties, porosity, and degradation rates were compared with PHBV fibers. Our results demonstrate that PHBV fibers exhibited higher porosity, increased ductility, and faster degradation rate when compared with PHBV 2-D films (p < 0.05). In vitro studies with PHBV fibers and 2-D films were carried out to evaluate the adhesion, viability, proliferation, and gene expression of human skin fibroblasts. Cells adhered and proliferated on both PHBV fibers and 2-D films. However, the proliferation of cells on the surface of PHBV fibers was comparable to tissue culture polystyrene (TCPS, control) (p > 0.05). The gene expression of collagen I and elastin was significantly up-regulated when compared with TCPS control, whereas collagen III was down-regulated on PHBV fibers and 2-D film after 14 days in culture. The less ductile PHBV 2-D films showed higher levels of elastin expression. Furthermore, the PHBV fibers in the presence and absence of an angiogenesis factor (R-Spondin 1) were evaluated for their wound healing capacity in a rat model. The wound contracture in R-Spondin-1-loaded PHBV fibers was found to be significantly higher when compared with PHBV fibers alone after 7 days (p < 0.05). Furthermore, the presence of fibers promoted an increase in collagen and aided re-epithelialization. Thus our results demonstrate that the topography and mechanical and chemical stimuli have a pronounced influence on the cell proliferation, gene expression, and wound healing.

Toluene impurity effects on CO 2 separation using a hollow fiber membrane for natural gas

The performance of defect-free cross-linkable polyimide asymmetric hollow fiber membranes was characterized using an aggressive feed stream containing up to 1000 ppm toluene. The membrane was shown to be stable against toluene-induced plasticization compared with analogs made from Matrimid ®, a commercial polyimide. Permeation and sorption analysis suggest that the introduction of toluene vapors in the feed subjects the membrane to antiplasticization, as the permeance decreases significantly (to less than 30%) under the most aggressive conditions tested. Separation efficiencies reflected by permselectivities were less affected. The effect of the toluene on the membrane was shown to be reversible when the toluene was removed.

PZT Microfibre defect structure studied by Raman spectroscopy

Raman-scattering spectroscopy on the microscale is proposed as a method to study the distribution of defects and existing vacancies in lead zirconate titanate (PZT) fibres during industrial processes to monitor the volumetric structural order within the fibre and to estimate the spatial phase transition degree from rhombohedral to tetragonal phase.PZT fibres developed in two different sintering atmospheres, PbO and a mixture of PbZrO3 and ZrO2 (PZ+Z), were studied to determine optimal conditions for the production of defect-free PZT fibres. An estimation of defect distribution along the radius of the PZT fibres is presented using Raman spectroscopy and confirmed by x-ray-diffractometry measurements. The degradation of the spatial-mechanical properties is explained by structural changes produced by electrostatic interactions between Zr and Ti ions.

Electrospun Scaffolds of a Polyhydroxyalkanoate Consisting of ω-Hydroxylpentadecanoate Repeat Units: Fabrication and In Vitro Biocompatibility Studies

Electrospinning was used to fabricate fibrous scaffolds of lipase-catalyzed poly(ω-pentadecalactone) (PPDL). The slow resorbability of this biomaterial is expected to be valuable for tissue-engineering applications requiring long healing times. The effect of solvent systems and instrumental parameters on fiber morphology was investigated. PPDL electrospinning was optimized and defect-free fibers (diameter 410 ± 150 nm) were obtained by using a mixed three-solvent system. Scaffolds were characterized by scanning electron microscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXS). TGA showed no residual solvent in the scaffolds. DSC and WAXS results indicated that electrospun PPDL is semicrystalline. Biocompatibility of PPDL scaffolds was evaluated through indirect cytotoxicity tests using embryonic rat cardiac H9c2 cells. The ability of PPDL electrospun mats to support cell growth was verified by culturing H9c2 cells onto the scaffold. Cell adhesion, proliferation and morphology were evaluated. The results indicated that PPDL mats are not cytotoxic and they support proliferation of H9c2 cells. The cumulative results of this study suggest further exploration of PPDL fibrous mats as scaffolds for tissue-engineered constructs.

Optical waveguiding in bent-core liquid-crystal filaments

We demonstrate optical waveguiding in recently discovered free-standing bent-core liquid-crystal filaments. The bent-core liquid-crystal molecules in air self-assemble into a cylindrical geometry that is "solidlike" along the radial direction of the filament and liquid in the axial direction of the filament. These filaments are unique not only because they are fluids, but also because they are anisotropic. For this reason, their waveguiding properties not predictable need to be characterized. The light power transmitted through the filament was found to be independent of temperature from 180 degrees C to near room temperature. Initial defects of newly pulled filaments were found to self-anneal, thus leaving defect-free fibers, where light scattering was found to be insignificant. The absorbance was found to be strongly wavelength dependent in the visible regime and relatively small in the infrared range. A self-assembled optical waveguide with self-annealing fluid properties may have promising applications in optical communications or in optical microchips.

Polymerized Ionic Liquids: Solution Properties and Electrospinning

In this study, the solution properties and electrospinning of a polymerized ionic liquid was explored. Polymerized ionic liquids are synthesized from polymerizing ionic liquid monomers, where ionic liquids are of great interest due to their unique physiochemical properties. Compared to other polyelectrolyte solutions, this polymerized ionic liquid solution exhibits similar viscosity scaling relationships in the semidilute unentangled and semidilute entangled regimes. However, the electrospraying-electrospinning transition occurs at similar polymer solution concentrations compared to neutral polymers, where electrospinning produced beaded fibers and defect-free fibers at ∼1.25 and ∼2 times the entanglement concentration, respectively. Due to high solution conductivities, electrospinning produces fibers approximately an order of magnitude smaller than neutral polymers at equivalent normalized solution concentrations. In addition, a high ionic conductivity of the solid-state fiber mat was observed under dry conditions and even higher conductivities were observed for polyelectrolyte fiber mats produced from electrospinning polyelectrolyte−ionic liquid solutions, where both anion and cation are mobile species.

Mechanical examination of PZT microfibre defects structure

The major problem in the production process of efficient ultrasonic transducer is the preparation of defect-free PZT fibres. A considerable amount of empirical work is presently in progress to achieve this goal of special importance for high-sensitive transducers. However, there is a lack of basic research on the detection of residual stress and defects areas in these fibres due to difficulties in mechanical examination of such flexible elements. This work presents use of the nanoindenter for material characterisation of PZT fibres of 140 µm radius obtained by extrusion method. The sudden depth-excursions during indentation on the edge of fibres have been clarified using Piezoresponse Mode Atomic Force Microscopy method and XRD measurements. The nanoindentation method proves to be the efficient tool capable to detect contribution of defects along the radius, properly estimate hardness as well as corresponding Young's modulus and concluding on structural properties of the micrometre-range ceramics fibres.

Electrospinning of polyvinylidene difluoride-based nanocomposite fibers

Polyvinylidene difluoride fibers and composite fibers with Ni–Zn ferrite nanoparticles and rutile nanoparticles were prepared by electrospinning dimethyl formamide (DMF) solutions. To prevent agglomeration, the ferrite nanoparticles were coated with silica, allowing the formation of a stable ferrofluid in DMF as well as the formation of homogeneous fibers. The rutile nanoparticles could be spun with a uniform distribution within the fiber without silica coating. The effects of various solution properties (viscosity and solids loading for composite fibers) and processing parameters (flow rate and voltage) on fiber morphology and diameter were studied to identify a processing window that resulted in the formation of smooth, defect-free fibers. Of the variables examined, fiber diameter was found to be the most strongly dependent on the viscosity of the electrospinning solution. Infrared spectroscopy revealed that the inclusion of well-dispersed nanoparticles in the electrospun fibers enhanced the presence of the ferroelectric phase in the composite fibers.

The effects of spinneret dimension and hollow fiber dimension on gas separation performance of ultra-thin defect-free Torlon ® hollow fiber membranes

A defect-free as-spun hollow fiber membrane with an ultra-thin dense-selective layer is the most desirable configuration in gas separation because it may potentially eliminate post-treatments such as silicone rubber costing, simplify membrane manufacture, and reduce production costs. However, the formation of defect-free as-spun hollow fiber membranes with an ultra-thin dense-selective layer is an extremely challenging task because of the complexity of phase inversion process during the hollow fiber fabrication and the trade-off between the formation of an ultra-thin dense-selective layer and the generation of defects. We have for the first time successfully produced defect-free as-spun Torlon ® hollow fiber membranes with an ultra-thin dense layer of around 540 Å from only a one polymer/one solvent binary system at reasonable take-up speeds of 10–50 m/min. The best O 2/N 2 permselectivity achieved is much higher than the intrinsic value of Torlon ® dense films. This is also a pioneering work systematically studying the effects of spinneret dimension and hollow fiber dimension on gas separation performance. Several interesting and important phenomena have been discovered and never been reported: (1) as the spinneret dimension increases, a higher elongation draw ratio is required to produce defect-free hollow fiber membranes; (2) the bigger the spinneret dimension, the higher the selectivity; (3) the bigger the spinneret dimension, the thinner the dense-selective layer. Mechanisms to explain the above observation have been elaborated. The keys to produce hollow fiber with enhanced permselectivity are to (1) remove die swell effects, (2) achieve finer monodisperse interstitial chain space at the dense-selective layer by an optimal draw ratio, and (3) control membrane formation by varying spinneret dimension.

Novel antibacterial fibers of quaternized chitosan and poly(vinyl pyrrolidone) prepared by electrospinning

The preparation of continuous defect-free fibers from quaternized chitosan derivative (QCh) has been achieved by electrospinning of mixed aqueous solutions of QCh with poly(vinyl pyrrolidone) (PVP). The average fiber diameter significantly decreases from 2800 to 1500 nm on increasing the polyelectrolyte content. In order to impart to QCh/PVP electrospun fibers stability to water and water vapor, the fibers have been crosslinked by incorporation of photo-crosslinking additives into QCh/PVP spinning solutions and subsequent UV irradiation of the electrospun fibers. Photo-crosslinked QCh-containing electrospun mats show high antibacterial activity against the Gram-positive bacteria Staphylococcus aureus and Gram-negative bacteria Escherichia coli.

Pervaporation Using Hollow-Fiber Membranes for Dehydrating Acetic Acid and Water Mixtures

Matrimid hollow fibers, originally developed for standard gas separation, were adapted to dehydrate acetic acid (HAc) and water mixtures in this work. These fibers, with an integrally defect-free skin layer supported on an open porous substrate, exhibit intrinsic selectivity for gases such as N2, O2, and He. When applied to separate HAc and water mixtures, the separation factor and water flux of ∼15 and 2.5 kg/m2h, respectively, were obtained for a regular 250 μm outer diameter and 20 cm long defect-free hollow fiber in a pervaporation process at 101.5 °C. A large bore pressure change along the axial direction of hollow fibers was shown to be negatively impacting separator performance. The separation factor and water flux were, therefore, increased by ∼150% using a similar fiber with a diameter twice as large as that of the regular gas separation fiber.

An investigation to revitalize the separation performance of hollow fibers with a thin mixed matrix composite skin for gas separation

Hollow fibers with a thin zeolite beta-polysulfone mixed matrix selective layer and enhanced selectivity for He/N 2 and O 2/N 2 separation have been developed in this study. It is believed that the heat treatment and the coating processes combine to bring out the separation properties of zeolite beta imbedded in the polymer matrix. The fibers were initially formed by using the dual-layer co-extrusion and dry-jet wet/wet spinning technology. Owing to the loose nature of the mixed matrix skin, the as-spun fibers show Knudsen diffusion even after silicone rubber coating. The effectiveness of heat treatment and surface coating in healing the hollow fiber has been investigated. SEM pictures show that heat treatment at above T g can significantly densify the loose mixed matrix layer and produce hollow fibers with a thin mixed matrix selective layer of around 1.5–12 μm. However, heat treatment at above T g does not necessarily produce defect-free hollow fiber membranes. Three types of silicone rubber coatings were applied and compared. It is found that, after a two-step coating, the ideal selectivity of mixed matrix fibers increases to surpass that of hollow fiber with a neat polysulfone outer layer. The sealing of the mixed matrix outer layer instead of the caulking of the inner layer is confirmed to be the cause for the selectivity increment. The reaction between the two chemicals in the two-step coating is contributive to the efficiency of this coating method. In addition, it is also observed that the thickness of the mixed matrix composite outer layer influences permeance and selectivity. The activation energies ( E p) for O 2 and N 2 calculated from the permeance or permeability at different temperatures indicate that the addition of zeolite beta in the polysulfone has reduced the energy barrier for O 2 permeation.

Effect of temperature on intrinsic permeation properties of 6FDA-Durene/1,3-phenylenediamine (mPDA) copolyimide and fabrication of its hollow fiber membranes for CO 2/CH 4 separation

We have determined the effect of temperature on intrinsic permeation properties of 6FDA-Durene/1,3-phenylenediamine (mPDA) 50/50 copolyimide dense film and fabricated high performance hollow fiber membranes of the copolyimide for CO 2/CH 4 separation. The hollow fiber membranes were wet-spun from a tertiary solution containing 6FDA-Durene/mPDA (PI), N-methyl-pyrrolidone (NMP) and tetrahydrofuran (THF) with a weight ratio of 20:50:30 at different shear rates within the spinneret. We observed the following facts: (1) the CO 2/CH 4 selectivity of the copolyimide dense film decreased significantly with an increase in temperature; (2) the performance of as-spun fibers was obviously influenced by the shear rate during spinning. For uncoated fibers, permeances of CH 4 and CO 2 decreased with increasing shear rate, while selectivity of CO 2/CH 4 sharply increased with shear rate until the shear rate reached 2169 s −1 and then the selectivity leveled off; (3) After silicone rubber coating, permeances of CH 4 and CO 2 decreased, the selectivity of CO 2/CH 4 was recovered to the inherent selectivity of its dense film. Both the permeances and selectivity with increasing shear rate followed their same trends as that before the coating; (4) there was an optimal shear rate at which a defect-free fiber with a selectivity of CO 2/CH 4 at 42.9 and permeance of CO 2 at 53.3 GPU could be obtained after the coating; and (5) the pressure durability of the resultant hollow fiber membranes could reach 1000 psia at room temperature.

A variation in fiber properties affects the performance of defect-free hollow fiber membrane modules for air separation

A flexible simulation method capable of modeling the performance of a gas separation module with variations in fiber properties is presented. Variations in fiber skin thickness, diameter and selectivity are considered. For separation of a high-pressure side product such as nitrogen from air, these variations can result in a considerable deviation from the ideal separation characteristic especially at high purities. It is shown that equal fiber quality is essential for the production of high purity nitrogen. The results presented in this publication are a part of a detailed and comprehensive sensitivity analysis on hollow fiber gas permeation modules.

Yttrium Aluminum Garnet Fibers from Metalloorganic Precursors

Mixtures of yttrium acetate hydrate (Y(O2CCH3)34H2O) and aluminum formate hydrate (Al(O2CH)33H2O in H2O) or yttrium isobutyrate (Y[O2CCH(CH3)2]3) and aluminum isobutyrate (Al[O2CCH(CH3)2]3) in tetrahydrofuran were used as precursors to process yttrium aluminum garnet (YAG, Al5Y3O12) fibers. The pyrolytic decomposition patterns of Al(O2CH)33H2O, Y(O2CCH3)34H2O, and a [3Y(O2CCH3)34H2O/5Al(O2CH)33H2O] YAG stoichiometry mixture were assessed by heating samples to selected temperatures and characterizing the products by thermogravimetric analysis, differential thermal analysis, X‐ray diffractometry, and Fourier transform infrared spectroscopy. The YAG acetate/formate precursor decomposes to an amorphous intermediate at temperatures &gt;400°C and crystallizes (at ∼800°C) to phase‐pure YAG with a ceramic yield of 40% at 1000°C. YAG isobutyrate precursor fibers were extruded or hand drawn. YAG acetate/formate precursor fibers were formed using a commercial extruder. The pyrolysis behavior of both precursor fibers was studied to identify the best pyrolysis conditions for producing dense, defect‐free ceramic fibers. Only thin (diameter of &lt;30 μm) precursor fibers could be processed to dense, defect‐free, thin YAG fibers (diameter of &lt;20 μm). For the YAG isobutyrate precursor, crack‐free crystalline YAG fibers (diameter of ∼7 μm) were obtained at 1000°C. For YAG acetate/formate precursor fibers, dwell times of 2 h at temperatures of 400° and 900°C were necessary to process fully dense, defect‐free ceramic fibers. Heating the resulting 900°C fibers (at a rate of 30°C/min) to 1570°‐1650°C gave dense fibers with grain sizes of 0.7‐3.2 μm and bend strengths of up to 1.7 ± 0.2 GPa (for fibers that had a diameter of ∼10 μm and had undergone sintering at 1600°C).

Development of a defect-free 6FDA-durene asymmetric hollow fiber and its composite hollow fibers

We have developed 6FDA-durene polyimide and poly(4-vinylpyridine) (PVP)/6FDA-durene composite hollow fibers. The 6FDA-durene fiber was defect-free and had a selectivity of 4.3–5.1 for O 2/N 2. It had an average permeance for O 2 of ≈ 240×10 −6 cm 3 (STP)/cm 2 cmHg and its dense layer thickness was ≈ 2000–3000 μm. The dense layer location was at the outer surface. This fiber was wet-spun directly from an in-situ imidization dope consisting of 14.94 wt% 6FDA-durene in a (70.24/21.5/7.51/0.75) N-methyl-2-pyrrolidone (NMP)/propionic acid (PA)/β-picoline/propionic anhydride solvent mixture. The fiber diameter was ≈500 μm with a wall thickness ≈50–70 μm. The PVP/6FDA-durene composite fiber had a selectivity of 6.9 for O 2/N 2 and a permeance for O 2 of 14.5×10 ∂ cm 3 (STP)/cm 2 s cmHg. The newly developed technology suggests, for the first time, that a defect-free hollow fiber can be produced from a modified Lewis acid:base complex dope. Compared to most other conventional hollow fiber fabrication processes, our approach has many advantages because that (1) it is simpler and more economical, (2) spins fibers at low temperatures, (3) employs a dope directly from polymerization, and (4) requires no secondary operations, such as silicone coating and curing.

Statistical Theory of the Strength of Fiber Bundles

Following a review of statistical models of the failure of single fibers and bundles of these fibers, algebraic recurrence formulas are derived that generate expressions for the failure probabilities of bundles of classical fibers. One of these recurrence relations is suitable for the accurate numerical calculation of failure probabilities of bundles consisting of up to 500 single fibers. It is shown how account can be taken of the effect of defect-free fibers having finite strength. Numerical results are compared to three asymptotic analytic approximations, two of which have been proposed in the statistical literature and are now applied to fiber problems for the first time.