Polymer Microfibers Research Articles

Efficient Spin Selectivity in Self-Assembled Superhelical Conducting Polymer Microfibers.

Chiral materials, natural or synthetic, have been widely studied since Pasteur's separation of enantiomers over a century ago. The connection between electron transmission and chirality was, however, established recently where one spin was preferably selected by the chiral molecules, displaying a typical chirality-induced spin selectivity (CISS) effect. Currently, this CISS effect was mainly demonstrated in the molecular-scale devices. Herein, we explored this effect in a microscale device where an efficient spin selectivity was found in the self-assembled superhelical conducting polyaniline (PANI) microfibers. A spin-selective efficiency up to 80% (not magnetoresistance) was achieved when spins traversed the ca. 2-6 μm-long helical channels at room temperature. Importantly, the long-range ordering of chiral PANI molecules is crucial to observe this efficient spin selectivity, whereas no selective transmission was found in the "amorphous" chiral PANIs. This efficient spin selectivity was subsequently rationalized by using an extended Su-Schrieffer-Heeger model where the Rashba spin-orbit coupling was considered. We expect these results could inspire the research of organic spintronics by using molecularly ordered, self-assembled, and chiral π-conjugated materials.

Mechanical evaluation of electrospun poly(ε-caprolactone) single fibers

In the scope of biomechanical applications of electrospun polymer microfibers, a new method for testing single electrospun microfibers was used to reliably establish mechanical properties of poly(ε-caprolactone) microfibers within a wide range of diameters from 0.4–6.0 μm. Cyclic mechanical tests were used to establish a bimodal linear relationship between fiber diameter and Young’s modulus. An inflection point was found at approximately 0.9 μm, where a sharp increase in modulus below this diameter was observed. The abrupt rise in modulus was not observed on the same scale in scaffolds produced with corresponding fibers and could not be explained only through a change in crystallinity of the polymer. Furthermore, the elastically recovered strain was assessed at maximum strain up to 20 %. It was found that polycaprolactone exhibits a viscoelastic range up to 13.17 ± 3.1 % after preconditioning, which is sufficient for most relevant biomechanical applications.

Energy losses and fluorescent efficiency of RhB-doped polymer microfibers via optical waveguiding excitation.

We demonstrate the dependencies of energy losses and fluorescent efficiencies on doping concentrations for rhodamine B (RhB)-doped polymer microfibers (PMFs) under optical waveguiding excitation. Compared with the four doping concentration groups (2.0 mg/g, 2.4 mg/g, 2.8 mg/g, and 3.2 mg/g), the 2.4 mg/g concentration group has the largest energy loss rates (∼0.102dB/µm and ∼0.036dB/µm for the excitation light and the fluorescence, respectively) and the highest fluorescence ratio at the coupling point. Further analysis demonstrates that the fluorescent emitting efficiency at the output end is approximately exponentially decaying with the propagation distance. The fluorescent emitting efficiency is also related to the doping concentration, which obtains the optimal fluorescent propagation effect at the doped PMF with a concentration of 2.8 mg/g. This work may provide a helpful reference for waveguiding circuit integration and active device design based on dye-doped micro/nanofibers.

Designing Intrinsically Microporous Polymer (PIM-1) Microfibers with Tunable Morphology and Porosity via Controlling Solvent/Nonsolvent/Polymer Interactions

In this study, we present the fabrication of fibers with tunable morphology and porosity from an intrinsically microporous polymer, PIM-1, using a simple one-step electrospinning technique. The met...

(Invited) Single-Strand Conducting Polymer Fiber-Based Wearable Sweat Sensors

Despite the great potential of polymer microfibers in human-friendly wearable electronics, most previous polymeric electronics have been limited to thin-film-based devices due to practical difficulties in fabricating microfibrillar devices, as well as defining the active channel dimensions in a reproducible manner. Herein, we report on conducting polymer microfiber-based organic electrochemical transistors (OECTs) and their application in single-strand fiber-type wearable ion concentration sensors. We developed a simple wet-spinning process to form very conductive poly(3,4- ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) microfibers using aqueous sulfuric acid solutions and carefully examined their electrical/electrochemical properties. In conjunction with fabricating substrate-free PEDOT:PSS microfiber-based OECT devices, the proposed novel characterization method demonstrated that the current variation ratio can be a reliable method for evaluating the device performance for sensing ion concentrations, regardless of the actual channel dimensions. Finally, we developed single-strand fiber-type skin-mountable OECTs by introducing a source-gate hybrid electrode and demonstrated that the resultant microfiber sensors can perform real-time repetitive measurements of the ion concentration in human sweat.

Fiber Lithography: A Facile Lithography Platform Based on Electromagnetic Phase Modulation Using a Highly Birefringent Electrospun Fiber.

Lithography plays a key role in advancing manufacturing as well as the semiconductor industry. However, the currently available state-of-the-art lithography methods still require access to expensive tools and facilities. Herein, we suggest a novel lithography method based on electromagnetic phase modulation of ultraviolet using a highly birefringent electrospun fiber to overcome such limitations. The optical birefringent effect, by which the phase of incident ultraviolet electromagnetic fields is retarded when passing through optically anisotropic media, is combined with semicrystalline poly(ethylene oxide) (PEO)-poly(ethylene glycol) (PEG) polymeric microfibers patterned in a programmable form using near-field electrospinning. By positioning the mask between two linear polarizers that are perpendicular to each other, only the UV waves that are passing through the fibers can be selectively utilized to exhibit lithographic property. Therefore, the UV intensity on the photoresist (PR) surface follows the shape of the fiber pattern, enabling precisely controlled patterning of the photoresist. Zero- to two-dimensional key features of lithography are achieved, including straight, curved, array, and isolated patterns. Facile optical alignments without using dedicated alignment marks are successfully demonstrated, as well as various applications including micro- to macroscale serpentine, tree, and antenna circuit patterns on a flexible substrate. The presented approach, packed in a table-top scale, is expected to provide a practical and affordable lithography solution by leveraging the direct-writing capability and tunable optical functionality of polymers, scalability, and the simple optical alignment method.

Multifunctional Scaffolds Based on Piezoelectric Electrospun Fibers modified with Biocompatible Drug Carriers for Regenerative Medicine

The incorporation of bioactive compounds onto polymer fibrous scaffolds is essential to improve the functionality of scaffolds for personalized drug therapy and regenerative medicine. In this study, SiO2 microcapsules were prepared and used as drug carriers, which are further deposited onto polymer microfiber scaffolds [polycaprolactone (PCL), poly(3-hydroxybutyrate) (PHB), and PHB doping with the conductive polyaniline (PANi) of 2 wt % (PHB-PANi)]. The number of immobilized microcapsules decreased with the increase of their ζ-potential due to the electrostatic repulsion forces between capsule wall and the negatively charged fiber surface, given the nature of the polymer used for the scaffold’s fabrication. Additionally, the immobilization of the capsules in dynamic mechanical conditions resulted in an increased number of the capsules attached on the fibers with the increasing of the scaffold piezoelectric response (PCL < PHB < PHB-PANi). Osteogenic factor dexamethasone (DEXA) is chosen for the microcapsules loading in order to demonstrate the osteogenesis of the developed scaffolds. The immobilization of microcapsules provides a simple and convenient way to incorporate bioactive compounds onto polymer scaffolds, which makes these multimodal materials suitable for the personalized drug therapy and bone tissue engineering.

Microfluidics-Based On-Demand Generation of Nonwoven and Single Polymer Microfibers.

In this work, we present a microfluidics-based microfiber fabrication method with the ability to control both the fiber size and the extent of coiling of the generated fiber. This latter feature allows on-demand generation of both nonwoven and single fiber within the same device, broadening the scope of application of the fabricated fibers. Using a hybrid poly(dimethylsiloxane) (PDMS)-glass microfluidic device, we implement a coflowing solvent removal technique to generate poly(ethylene oxide) (PEO) fibers. Characterization of fibers by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC) confirms the production of solvent-free, pure PEO fibers. Control over fiber size using both inner and outer liquid flow rates is demonstrated by scanning electron microscopy (SEM) imaging. More crucially, we employ a complementary flow toward the downstream end of the fiber solidification region to control the extent of coiling of the generated fiber. By simple variation of the complementary flow, we induce a transition from a nonwoven fiber to a single fiber. The presented technique is expected to broaden the scope of microfluidics as a tool for the continuous generation of microfibers with a wider range of applications than the existing microfluidics techniques.

A new generation of hollow polymeric microfibers produced by gas dissolution foaming.

A new and straightforward route to produce polymeric hollow microfibers has been proposed. Polycaprolactone (PCL) hollow fibers are obtained for the first time using an environmentally friendly gas dissolution foaming approach, overcoming its limitations to induce porosity on samples in the micrometric range. Different porous morphologies are achieved from solid PCL microfibers with a well-controlled diameter obtained by conventional electrospinning. The optimization of the foaming parameters provides two sets of well-defined hollow fibers, one showing smooth surfaces and the other presenting an enhanced surface porosity. Accordingly, gas dissolution foaming proves to be not only suitable for the production of hollow polymeric microfibers, but is also capable of providing diverse porous morphologies from the same precursor, solid fibers. Moreover, a preliminary study about the suitability of this new generation of foamed hollow polymeric fibers for drug delivery is carried out, aiming to take advantage of the enhanced surface area and tunable morphology obtained by using the proposed new production method. It is found that the foamed microfibers can be loaded with up to 15 wt% of ibuprofen while preserving the morphology of each kind of fiber. Then, foamed PCL fibers presenting a hollow structure and surface porosity show a remarkable constant release of ibuprofen for almost one and a half days. In contrast, the original solid fibers do not present such behavior, releasing all the ibuprofen in about seven hours.

Whispering gallery mode emission from dye-doped polymer fiber cross-sections fabricated by near-field electrospinning.

Whispering gallery mode (WGM) resonators demonstrate great potential for photonic and sensing applications. Yet, these devices are often disadvantaged by costly materials or complex fabrication approaches, in addition to lack of manufacturing scalability. Near-field electrospinning (NFES), a recently emerged facile fiber fabrication method, offers a solution. Here, WGM resonances are reported in Rhodamine 6G-doped poly(vinyl) alcohol (PVA) microfibers via NFES. Diameters are tuned over a range of more than 10 μm by varying substrate stage speed. Fibers display uniform distribution of dye, smooth surfaces, and circular cross-sections, all critical for supporting WGMs. High quality (Q) resonances are confirmed within fiber cross-sections through polarization experiments, free-spectral range analysis, and Mie-theory-derived mode assignment. In addition to WGMs, groups of associated spiral or conical modes are observed due to taper-induced weak optical confinement along the fiber axis. Crosslinked, dye-doped PVA fibers are utilized to sense the ethanol concentration in ethanol-water mixtures and actuation mechanisms are evaluated by comparison to theoretical spectra. The demonstration of high-Q resonances within NFES polymer microfibers is a critical step toward simple, cost effective, high-volume fabrication of WGM resonators for optoelectronics and biomedical devices.

Controlling Polymer Microfiber Structure by Micro Solution Blow Spinning

AbstractRecent progress in microfluidic technology allows fabricating microfluidic devices to produce liquid microjets with unprecedented control of the jet diameter and velocity. Here it is demonstrated that microfluidic devices based on the gas dynamic virtual nozzle principle can be excellently used for micro solution blow spinning to continuously fabricate microfibers with excellent control of the fiber diameter and the internal crystalline alignment that determines the mechanical properties. Fiber spinning experiments with small‐ and wide‐angle X‐ray scattering are combined to directly relate the macroscopic spinning conditions to the bulk and molecular structure of the resulting fibers. The elongational rate is shown as the relevant parameter that transduces the nozzle flow conditions to the local macromolecular structure and orientation, and thus the mechanical properties of the resulting fiber. It is observed that the spinning process results in very uniform microfibers with a well‐defined shish–kebab crystal structure, which evolves into an extended chain crystal structure upon plastic deformation. Thus, the presented microfluidic spinning methodology has great implications for a precisely controlled production of microfibers using miniaturized spinning devices.

Properties of Mortar with Recycled Aggregates, and Polyacrylonitrile Microfibers Synthesized by Electrospinning.

Currently it is necessary to find alternatives towards a sustainable construction, in order to optimize the management of natural resources. Thus, using recycled fine aggregate (RFA) is a viable recycling option for the production of new cementitious materials. In addition, the use of polymeric microfibers would cause an increase in the properties of these materials. In this work, mortars were studied with 25% of RFA and an addition of polyacrylonitrile PAN microfibers of 0.05% in cement weight. The microfibers were obtained by the electrospinning method, which had an average diameter of 1.024 µm and were separated by means of a homogenizer to be added to the mortar. Cementing materials under study were evaluated for compressive strength, flexural strength, total porosity, effective porosity and capillary absorption, resistance to water penetration, sorptivity and carbonation. The results showed that using 25% of RFA causes decreases mechanical properties and durability, but adding PAN microfibers in 0.05% caused an increase of 2.9% and 30.8% of compressive strength and flexural strength respectively (with respect to the reference sample); a decrease in total porosity of 5.8% and effective porosity of 7.4%; and significant decreases in capillary absorption (approximately 23.3%), resistance to water penetration (25%) and carbonation (14.3% after 28 days of exposure). The results showed that the use of PAN microfibers in recycled mortars allowed it to increase the mechanical properties (because they increase the tensile strength), helped to fill pores or cavities and this causes them to be mortars with greater durability. Therefore, the use of PAN microfibers as a reinforcement in recycled cementitious materials would be a viable option to increase their applications.

Controllable Polarization of Lasing Emission From a Polymer Microfiber Laser

Microlasers with controllable polarization of output emission are vital for on-chip optical communications, optical sensors and optical switches. In this work, we report a high quality (Q) factor, low-threshold polymer microfiber laser and the possibility of achieving laser emission with a desired polarization. The microfiber is fabricated by direct drawing from a dye-doped polymer solution and it can generate whispering gallery mode (WGM) lasing under optical pulse excitation. When the microfiber is pumped from the side with pumping direction perpendicular to the microfiber’s axis, the polarization direction of the output laser is found to be the same as that of the pump laser. Lasing emission with either transverse electric (TE) or transverse magnetic (TM) modes can be obtained and these two polarization states can be switched over by tuning the pumping laser. Furthermore, emission with both TE and TM modes can also be observed by changing the orientation of the microfiber relatively to pumping direction. Our finding provides an effective approach for achieving microlasers that have high Q lasing modes with anticipated polarization.

Visually Adjusting Coupling Conditions in Light-Emitting Micro-Components

We demonstrate a directly visible approach to adjust the coupling conditions of photonic components based on fluorescent indication. Under an optical microscope, the fluorescence of the dye-doped polymer microfiber (PMF) is excited by the evanescent coupling method and is used to visualize the light path in the PMF. By simply moving the coupling position of input probe in a few microns according to the fluorescent indication, we adjust the coupling conditions of the components, such as $Y$ beam splitter and Mach–Zehnder interferometer, and the splitting ratio varies from 1.8 to 0.8. Besides, we directly visualize and adjust the energy coupling of two overlapping PMFs in the pig-tail microring via the fluorescent indication. The fluorescent indication in PMF demonstrated here simplifies the adjusting process of the component based on free standing PMFs and promises a potential for directly visible adjusting in microfibers-based photonic circuit integration for photonic neural networks.

Hydrogen Leakage Detectors Based on a Polymer Microfiber Decorated With Pd Nanoparticles

Taking advantage of hydrogen absorption properties of Pd, we developed herein a hydrogen sensor (0–1 vol%) based on a poly(methyl methacrylate) (PMMA) microfiber decorated with Pd nanoparticles. The sensitivity of the hydrogen sensor was determined by the diameter and sensing length of the Pd-decorated PMMA microfiber. Thus, the hydrogen sensor showed a sensitivity of $4.5 \times 10^{-4}$ nm/ppm for a microfiber having a diameter and a sensing length of 28 and $473~\mu \text{m}$ , respectively. The sensitivity was further increased up to $5.3\times 10^{-4}$ nm/ppm by extending the sensing length of the microfiber to $728~\mu \text{m}$ . When comparing the hydrogen sensing characteristics of pure- and decorated-PMMA microfibers, a significant improvement in sensitivity was obtained for the modified material which was attributed to the introduction of Pd nanoparticles.

Palladium nanoparticles doped polymer microfiber functioned as a hydrogen probe

Abstract Palladium (Pd) nanoparticles were elaborated into the amorphous poly(methyl methacrylate) (PMMA) microfiber to experimentally determine the hydrogen concentration during 0.2 vol%‒1 vol%. The respective high specific surface area and multi-void structure of the sensing probe effectively enhance its absorption and interaction effect for hydrogen molecules. The introduction of Pd nanoparticles has improved the sensitivity of the pure PMMA microfiber for more than 3 folds. The highest sensitivity of 5.2 × 10−4 nm/ppm has been demonstrated using a cut of Pd nanoparticles functioned PMMA microfiber with a diameter of 28 μm and a length of 473 μm. The optical fiber hydrogen sensor probe is expected to shorten the response time to several seconds and realize real-time monitoring of hydrogen leakage in trace concentration.

One-Dimensional Programmable Polymeric Microfiber Waveguide with Optically Reconfigurable Photonic Functions.

Programmable materials and reconfigurable photonic components, which can change their physicochemical properties and functionalities upon external stimuli, are a major topic of interest in modern science. However, most conventional reconfigurable photonic components rely heavily on mechanical deformation, restricting their application. Herein, a novel strategy based on a dynamically tunable fluorescence resonance energy transfer process to design and fabricate programmable fluorescent micropatterns within single polymer microfiber is proposed. A set of reconfigurable photonic components (including optical switchable waveguide systems, photonic analogies of diodes and transistors, as well as one-dimensional (1D) optical encoding) can be achieved within a single polymeric waveguide microfiber straightforwardly, in which such photonic components can be written, erased, and rewritten as 1D binary patterns with tailored external optical stimuli. These results might be of great fundamental value for the rational design of novel reconfigurable photonic devices with numerous potential applications in highly integrated optical communication components and optical computing devices.

Preparation and Characterization of Polymeric Microfibers of PLGA and PLGA/PPy Composite Fabricated by Solution Blow Spinning

In this study, microfibers of poly(L‐lactic‐co‐glycolic acid) (PLGA) and PLGA/polypyrrole (PPy) composite (90/10 wt%) are produced by using the solution blow spinning (SBS) technique. PPy is synthesized by the oxidative polymerization method using p‐toluenesulfonic acid (p‐TSA) as the dopant and FeCl3 as the oxidant. The prepared PPy showed microfibers with globular particles morphology. Mixtures of porous and nonporous microfibers and microfibers incorporated with PPy are obtained. A wettability test shows that the PLGA and PLGA/PPy fibrous mats are hydrophobic. The electrical conductivity of the PLGA/PPy composite is of the same order as that of pure PLGA (≈10−10 S cm−1), indicating that the electrical percolation threshold is not reached for PPy loading of 10 wt%. The incorporation of PPy into PLGA microfibers improved the thermal stability of the composite and also increases the PLGA crystalline phase.

Polymer microfiber coated with ZnO for humidity sensing

A PMMA microfiber that is coated with ZnO is employed as a relative humidity sensor. The PMMA microfiber with a diameter of 60 μm is fabricated by using a direct drawing method from molten PMMA. Then, the fiber is coated with ZnO that is prepared from sol - gel immersion method. There are two types of coating done for this work, namely ZnO nanostructures and nanorods respectively. The experimental setup for the proposed sensor comprises of an amplified spontaneous emission (ASE) light from Erbium-doped filber amplifier (EDFA), sealed chamber and an optical spectrum analyser (OSA). The variation of the output power of the ASE with relative humidity is recorded. It is observed that the output power for PMMA microfiber coated with ZnO nanostructures decreases from -22.43 to -27.87 dBm as the humidity is increased from 50% and 80%. The performance of the proposed sensor is enhanced after being coated with ZnO, for both nanostructures and nanorods. The results show that the sensitivity of the sensor when it is coated with ZnO nanostructures has improved from 0.1191 dBm/% to 0.1791 dBm/%. When ZnO nanorods are coated onto the PMMA fiber, the performances of the proposed fiber further improved with the sensitivity increases to 0.2159 dBm/%. This proposed sensor provides numerous advantages such as higher mechanical strength and low cost of production. In addition, it is highly suitable for continuous monitoring of relative humidity in compact areas.

Active Polymer Microfiber with Controlled Polarization Sensitivity

Adv. Optical Mater. 2016, 4(3), 371–377 Recently the authors found that Figures 3 and 4 are the same in the published version of their article, while these obviously should be different figures: one is for polarized fluorescence and the other is for polarized Raman spectra. Furthermore, Figure 5 is wrong: in the published paper, Figure 5 is the corrected version of Figure 4, and the original Figure 5 has gone lost. These unfortunate mistakes occurred during proof-correction processing, much to the regret of the authors and editors. The correct Figures 3-5 are shown below.