Electrospinning polyelectrolyte complexes: pH-responsive fibers

Charge transport in electrospinning of polyelectrolyte solutions

This study elucidates the electrical charge transport during electrospinning of weak polyelectrolyte (poly(acrylic acid) (PAA)) solutions by employing current emission measurements. With pH variation, the PAA ionization degree could be controlled from uncharged at low pH to weakly charged at intermediate solution pH. Electrospinning neutral poly(vinylpyrrolidone) (PVP) as a reference polymer solution confirmed established current-flow rate scaling relationships as shown by De La Mora and Loscertales (1994), I ∼ (γKQ)ν, independent of the applied electric field polarity, where ν = 0.5, K is the conductivity, γ is the surface tension, Q is the flow rate, and I is the current. Similarly, the uncharged PAA did not display any polarity dependence, yet ν ≈ 0.8. Negatively charged PAA, however, showed a marked deviation in the current-flow rate behavior, which was affected by the applied electric field polarity. In the case of negative polarity, ν = 0.99, whereas for a positive polarity ν = 0.68. Similarly, the voltage required for stable cone-jet electrospinning of charged PAA was significantly higher in the negative polarity configuration for all tested flow rates (300-1600 μL h-1). As opposed to merely surface charges typically considered when electrospinning leaky dielectric fluids, as suggested by Melcher and Taylor (1969), our results suggest that the measured current is also affected by volumetric charges from charged PAA in the bulk of the jet. The proposed additional charge transport might affect the orientational order within PE-based nanofibers and their diameter.

Scaling Relationship of Complex Coacervate Core Micelles: Role of Core Block Stretching

The scaling relationship of complex coacervate core micelles (C3Ms) has been investigated experimentally and theoretically. The C3Ms are formed by mixing two oppositely charged block copolyelectrolyte solutions (i.e., AB + AC system) and are characterized by small-angle neutron (SANS) and X-ray scattering (SAXS). Scaling relationships for micellar structure parameters, including core radius, total radius, corona thickness, and aggregation number, all with respect to the core block length, are determined. A scaling theory is also proposed by minimizing the free energy per chain, leading to four regimes depending on the core and corona chain conformations. Although the corona block is significantly longer than the core block, the structure of our C3Ms is consistent with that of the crew-cut I regime. A highly swollen core by water enables the core blocks to be stretched significantly and corona chains to be minimally overlapped.

Optical fiber pH sensor based on a multimode interference device with polymer overlay

An optical fiber pH sensor based on a multimode interference structure is presented. The sensitive element is a piece of no-core fiber (NCF) coated with a mixture of polyallylamine hydrochloride and polyacrylic acid by a modified layer-by-layer (LbL) self-assembly method. It is experimentally shown that by reducing the diameter of the NCF by chemical etching, the sensitivity is increased from -0.31n m/p H to -2n m/p H. The sensor exhibits a high linear response of 0.997 over a pH range from 5 to 11.3 with a rapid response time lower than 1 s.

Polymer Complex Fiber: Property, Functionality, and Applications

Polymer complex fibers (PCFs) are a novel kind of fiber material processed from polymer complexes that are assembled through noncovalent interactions. These can realize the synergy of functional components and miscibility on the molecular level. The dynamic character of noncovalent interactions endows PCFs with remarkable properties, such as reversibility, stimuli responsiveness, self-healing, and recyclability, enabling them to be applied in multidisciplinary fields. The objective of this article is to provide a review of recent progress in the field of PCFs. The classification based on chain interactions will be first introduced followed by highlights of the fabrication technologies and properties of PCFs. The effects of composition and preparation method on fiber properties are also discussed, with some emphasis on utilizing these for rational design. Finally, we carefully summarize recent advanced applications of PCFs in the fields of energy storage and sensors, water treatment, biomedical materials, artificial actuators, and biomimetic platforms. This review is expected to deepen the comprehension of PCF materials and open new avenues for developing PCFs with tailor-made properties for advanced application.

Electrospun Polymer Nanofibers: Processing, Properties, and Applications

Electrospun polymer nanofibers (EPNF) constitute one of the most important nanomaterials with diverse applications. An overall review of EPNF is presented here, starting with an introduction to the most attractive features of these materials, which include the high aspect ratio and area to volume ratio as well as excellent processability through various production techniques. A review of these techniques is featured with a focus on electrospinning, which is the most widely used, with a detailed description and different types of the process. Polymers used in electrospinning are also reviewed with the solvent effect highlighted, followed by a discussion of the parameters of the electrospinning process. The mechanical properties of EPNF are discussed in detail with a focus on tests and techniques used for determining them, followed by a section for other properties including electrical, chemical, and optical properties. The final section is dedicated to the most important applications for EPNF, which constitute the driver for the relentless pursuit of their continuous development and improvement. These applications include biomedical application such as tissue engineering, wound healing and dressing, and drug delivery systems. In addition, sensors and biosensors applications, air filtration, defense applications, and energy devices are reviewed. A brief conclusion is presented at the end with the most important findings and directions for future research.

Injectable Hydrogels Based on Inter-Polyelectrolyte Interactions between Hyaluronic Acid, Gelatin, and Cationic Cellulose Nanocrystals

The present work dealt with the development of physically cross-linked injectable hydrogels with potential applications in tissue engineering. The hydrogels were composed of a ternary mixture of a polyanion and a polyampholyte, hyaluronic acid (HA) and gelatin, respectively, bridged by cationic cellulose nanocrystals (cCNCs). A 3D network is formed by employing attractive electrostatic interactions and hydrogen bonding between these components under physiological conditions. The hydrogels demonstrated low viscosity at high stresses, enabling easy injection, structural stability at low stresses (<15 Pa), and nearly complete structure recovery within several minutes. Increasing the cCNC content (>3%) reduced hydrogel swelling and decelerated the degradation in phosphate-buffered saline as compared to that in pure HA and HA-gelatin samples. Biological evaluation of the hydrogel elutions showed excellent cell viability. The proliferation of fibroblasts exposed to elutions of hydrogels with 5% cCNCs reached ∼200% compared to that in the positive control after 11 days. Considering these results, the prepared hydrogels hold great potential in biomedical applications, such as injectable dermal fillers, 3D bioprintable inks, or 3D scaffolds to support and promote soft tissue regeneration.

Electrostatically crosslinked cellulose nanocrystal and polyelectrolyte complex sponges with pH responsiveness

This work focuses on the development of a responsive sponge made of an anionic cellulose nanocrystal (CNC) skeleton that is electrostatically crosslinked by a pH-responsive poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) polyelectrolyte complex (PEC). The results prove the formation of a global percolated network comprised of disordered CNC rods crosslinked by PEC clusters. The bulk density of the freeze-dried CNC-PEC sponges increases from 35 to 93 mg/cm³ with PEC concentration, while the compression modulus of dry specimens increases from 7 up to 62 kPa. At the lowest PEC concentration of 1 wt%, at pH 2.0, the compression modulus decreases to 0.9 kPa, whereas at pH 5.5, it increases to 42 kPa. The intensive complexation between sponge constituents is also reflected in a reduced ability to bind charged dyes at neutral pH values. Decreasing the pH results in an increased adsorption efficiency for anionic dyes, while raising the pH improves the cationic dye adsorption.

Antimicrobial PAA/PAH Electrospun Fiber Containing Green Synthesized Zinc Oxide Nanoparticles for Wound Healing

Wound infections are the main complication when treating skin wounds. This work reports a novel antimicrobial material using green synthesized zinc oxide nanoparticles (ZnONPs) incorporated in polymeric fibers for wound healing purposes. ZnONPs are a promising antimicrobial nanomaterial with high activity against a range of microorganisms, including drug-resistant bacteria. The electrospun fibers were obtained using polyacrylic acid (PAA) and polyallylamine hydrochloride (PAH) and were loaded with ZnONPs green synthesized from Ilex paraguariensis leaves with a spherical shape and ~18 nm diameter size. The fibers were produced using the electrospinning technique and SEM images showed a uniform morphology with a diameter of ~230 nm. EDS analysis proved a consistent dispersion of Zn in the fiber mat, however, particle agglomerates with varying sizes were observed. FTIR spectra confirmed the interaction of PAA carboxylic groups with the amine of PAH molecules. Although ZnONPs presented higher antimicrobial activity against S. aureus than E. coli, resazurin viability assay revealed that the PAA/PAH/ZnONPs composite successfully inhibited both bacteria strains growth. Photomicrographs support these results where bacteria clusters were observed only in the control samples. The PAA/PAH/ZnONPs composite developed presents antimicrobial activity and mimics the extracellular matrix morphology of skin tissue, showing potential for wound healing treatments.

The kinetics and release behaviour of curcumin loaded pH-responsive PLGA/chitosan fibers with antitumor activity against HT-29 cells

The bioavailability and clinical effect of curcumin (Cur) are greatly restricted due to its physicochemical instability and high hydrophobicity. To overcome the disadvantages, the nanofibers of poly(lactide-glycolide)/chitosan loaded with Cur (PLGA/CS/Cur) was developed here by electrospinning technique for controlled Cur delivery. The incorporated Cur was well-dispersed and maintained crystalline form in PLGA/CS fiber matrix by hydrogen bonding. The incorporation of Cur had no obvious influence on the fiber size and morphology but exerted impacts on thermal stability. At pH 7.4, the release followed Fickian diffusion mechanism; while at pH 2.0, the release followed the coexistence of diffusion and erosion mechanisms. In addition, the amount of Cur released at pH 2.0 was much higher than that at pH 7.4. As a result, the nanofibers demonstrated higher anticancer activity at acidic environment. Therefore, the PLGA/CS/Cur nanofibers may be served as a potential pH responsive vehicle for the controlled drug delivery.

Polymer-free cyclodextrin and natural polymer-cyclodextrin electrospun nanofibers: A comprehensive review on current applications and future perspectives

The present review discusses the use of cyclodextrins and their derivatives to prepare electrospun nanofibers with specific features. Cyclodextrins, owing to their unique capability to form inclusion complexes with hydrophobic and volatile molecules, can indeed facilitate the encapsulation of bioactive compounds in electrospun nanofibers allowing fast-dissolving products for food, biomedical, and pharmaceutical purposes, filtering materials for wastewater and air purification, as well as a variety of other technological applications. Additionally, cyclodextrins can improve the processability of naturally occurring biopolymers helping the fabrication of "green" materials with a strong industrial relevance. Hence, this review provides a comprehensive state-of-the-art of different cyclodextrins-based nanofibers including those made of pure cyclodextrins, of polycyclodextrins, and those made of natural biopolymer functionalized with cyclodextrins. To this end, the advantages and disadvantages of such approaches and their possible applications are investigated along with the current limitations in the exploitation of electrospinning at the industrial level.

Calcium alginate nanoparticle crosslinked phosphorylated polyallylamine to the controlled release of clindamycin for osteomyelitis treatment

Osteomyelitis is one of the infections of the bone, and the treatment needs to the infection problems. Here, a local therapeutic approach for efficient drug delivery systems was designed to enhance the antibiotic drug's therapeutic activity. Calcium-Alginate nanoparticle (Ca-Alg) crosslinked phosphorylated polyallylamine (PPAA) was prepared through the salting-out technique, and it achieved 82.55% encapsulation of Clindamycin drug. The physicochemical characterizations of FTIR, SEM/EDX, TEM, and XRD were investigated to confirm the materials nature and formation. Clindamycin loaded Ca-Alg/PPAA system showed sustained Clindamycin release from the carrier. Cell viability was assessed in bone-related cells by Trypan blue assay and MTT assay analysis method. Both assay results exhibited better cell viability of synthesized materials against MG63 cells. MIC value of Ca-Alg/PPAA/Clindamycin in the Methicillin-resistant Staphylococcus aureus (MRSA) pathogen was 275 µg/mL, and it was 120 µg/mL for Enterobacter cloacae pathogen. The materials promising material for Osteomyelitis affected bone regeneration without any destructive effect and speedy recovery of infected parts from these investigations.

A review of smart electrospun fibers toward textiles

Electrospinning as a versatile technology has attracted a large amount of attention in the past few decades due to the facile way to produce micro- and nano-scale fibers featuring flexibility, large specific surface area and high porosity. Stimuli-responsive polymers are a class of smart materials that are capable of sensing surround environment and interacting with them. Therefore, the combination of electrospinning and smart materials could have a great deal of benefits over the development of smart fibers. In this review, it offers a comprehensive understanding of smart electrospun fibers toward textile applications. Firstly, the definition of smart fibers and the differences between interactive fibers and passive interactive fibers are briefly introduced. Then some interactive fibers made from temperature-, pH-, light-, electric field/electricity-, magnetic field-, multi-responsive polymers, as well as some polymers featuring piezoelectric and triboelectric effect which are suitable flexible electrics, are emphasized with their applications in the form of electrospun fibers. Afterwards, some passive and hybrid smart electrospun fibers are introduced. Finally, associated challenges and perspectives are summarized and discussed.

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Understanding of passive smart electrospun fibers and interactive smart electrospun fibers.

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The recent progress in flexible electronics from electrospun fibers.

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The recent progress in stimuli-responsive polymers applied in interactive smart electrospun fibers.

Humidity-Triggered Relaxation of Polyelectrolyte Complexes as a Robust Approach to Generate Extracellular Matrix Biomimetic Films

Generating a biofunctional film that can mimic the extracellular matrix (ECM) in an efficient and robust technique that may have great potential for medical devices, tissue engineering, and regenerative medicines. Herein, a facile approach to generate ECM biomimetic films based on the humidity-triggered relaxation of polyelectrolyte complex (PEC) nanoparticles is reported. The poly(l-lysine) and hyaluronan are precomplexed and sprayed onto a substrate, which, via a trigger of vaporous water, can be transformed into an even and stable film. The spontaneous polymer chain interfusion (diffusion coefficient ≈1.01 × 10^-9  cm²  s^-1 ) under saturated humidity, allowing for the rapid reorganization (within 30 min) of film morphology and structure is demonstrated. A controllable and scalable way for the loading of diversified bioactive agents, as well as on-demand modulation of stiffness is further presented. Moreover, the high-throughput arrays and programmed patterns can be easily completed, suggesting huge potentials that surpass those of state-of-the-art methods. Combined with high efficiency and flexible functionalization, it is believed that this approach should be beneficial for extending the practical applications of PEC films, such as medical implants, chip detectors, and so on.

Hydrogen-Bonded, Mechanically Strong Nanofibers with Tunable Antioxidant Activity

We report on mechanically strong, water-insoluble hydrogen-bonded nanofiber mats composed of a hydrophilic polymer and a natural polyphenol that exhibit prolonged antioxidant activity. The high performance of fibrous mats resulted from the formation of a network of hydrogen bonds between a low-molecular-weight polyphenol (tannic acid, TA) and a water-soluble polymer (polyvinylpyrrolidone, PVP) and could be precisely controlled by the TA-to-PVP ratio. Dramatic enhancement (5- to 10-fold) in tensile strength, toughness, and Young's moduli of the PVP/TA fiber mats (as compared to those of pristine PVP fibers) was achieved at the maximum density of hydrogen bonds, which occurred at ∼0.2-0.4 molar fractions of TA. The formation of hydrogen bonds was confirmed by an increase in the glass-transition temperature of the polymer after binding with TA. When exposed to water, the fibers exhibited composition- and pH-dependent stabilities, with the TA-enriched fibers fully preserving their integrity in acidic and neutral media. Importantly, the fiber mats exhibited strong antioxidant activity with dual (burst and prolonged) activity profiles, which could be controlled via fiber composition, a feature useful for controlling radical-scavenging rates in environmental and biological applications.

Structure and Rheology of Polyelectrolyte Complexes in the Presence of a Hydrogen-Bonded Co-Solvent

Intermolecular interactions as well as macromolecular conformation affect the rheological and microstructural properties of polyelectrolyte complexes (PECs) solutions. The properties of semi-dilute solutions of weakly charged PECs can be controlled by the degree of ionization and solvent composition. In this work, we examined the effect of ethanol as a co-solvent on PECs composed of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) at low pH. The aqueous PECs solution was turbid, indicating formation of large aggregates, whereas PECs solution in water/ethanol (60:40 w/w) was transparent, implying no aggregation, and demonstrated higher relative viscosity than the aqueous solution, implying pronounced network formation. Imaging PECs solution by transmission electron microscopy (TEM) demonstrated aggregation, whereas the solution prepared with the mixed solvent revealed almost no phase contrast. Small-angle X-ray scattering (SAXS) of PECs in the aqueous solution indicated the presence of aggregates, while PECs in mixed solvent demonstrated a swelled macromolecular conformation with diminished aggregation. PECs with no ionic interactions in the mixed solvent assumes a homogenous network structure, which enables PECs solution processing by electrospinning.

A green approach to obtain stable and hydrophilic cellulose-based electrospun nanofibrous substrates for sustained release of therapeutic molecules

Stable and (bio)-compatible nanofibrous matrices showing effective incorporation and release of nonsteroidal anti-inflammatory drugs (NSAIDs) hold a huge potential in tissue regeneration and wound healing.

Electrospun Fibers of Cyclodextrins and Poly(cyclodextrins)

Cyclodextrins (CDs) can endow electrospun fibers with outstanding performance characteristics that rely on their ability to form inclusion complexes. The inclusion complexes can be blended with electrospinnable polymers or used themselves as main components of electrospun nanofibers. In general, the presence of CDs promotes drug release in aqueous media, but they may also play other roles such as protection of the drug against adverse agents during and after electrospinning, and retention of volatile fragrances or therapeutic agents to be slowly released to the environment. Moreover, fibers prepared with empty CDs appear particularly suitable for affinity separation. The interest for CD-containing nanofibers is exponentially increasing as the scope of applications is widening. The aim of this review is to provide an overview of the state-of-the-art on CD-containing electrospun mats. The information has been classified into three main sections: (i) fibers of mixtures of CDs and polymers, including polypseudorotaxanes and post-functionalization; (ii) fibers of polymer-free CDs; and (iii) fibers of CD-based polymers (namely, polycyclodextrins). Processing conditions and applications are analyzed, including possibilities of development of stimuli-responsive fibers.

Low-Voltage Continuous Electrospinning Patterning

Electrospinning is a versatile technique for the construction of microfibrous and nanofibrous structures with considerable potential in applications ranging from textile manufacturing to tissue engineering scaffolds. In the simplest form, electrospinning uses a high voltage of tens of thousands volts to draw out ultrafine polymer fibers over a large distance. However, the high voltage limits the flexible combination of material selection, deposition substrate, and control of patterns. Prior studies show that by performing electrospinning with a well-defined "near-field" condition, the operation voltage can be decreased to the kilovolt range, and further enable more precise patterning of fibril structures on a planar surface. In this work, by using solution dependent "initiators", we demonstrate a further lowering of voltage with an ultralow voltage continuous electrospinning patterning (LEP) technique, which reduces the applied voltage threshold to as low as 50 V, simultaneously permitting direct fiber patterning. The versatility of LEP is shown using a wide range of combination of polymer and solvent systems for thermoplastics and biopolymers. Novel functionalities are also incorporated when a low voltage mode is used in place of a high voltage mode, such as direct printing of living bacteria; the construction of suspended single fibers and membrane networks. The LEP technique reported here should open up new avenues in the patterning of bioelements and free-form nano- to microscale fibrous structures.

Ratiometric Organic Fibers for Localized and Reversible Ion Sensing with Micrometer-Scale Spatial Resolution

A fundamental issue in biomedical and environmental sciences is the development of sensitive and robust sensors able to probe the analyte of interest, under physiological and pathological conditions or in environmental samples, and with very high spatial resolution. In this work, novel hybrid organic fibers that can effectively report the analyte concentration within the local microenvironment are reported. The nanostructured and flexible wires are prepared by embedding fluorescent pH sensors based on seminaphtho-rhodafluor-1-dextran conjugate. By adjusting capsule/polymer ratio and spinning conditions, the diameter of the fibers and the alignment of the reporting capsules are both tuned. The hybrid wires display excellent stability, high sensitivity, as well as reversible response, and their operation relies on effective diffusional kinetic coupling of the sensing regions and the embedding polymer matrix. These devices are believed to be a powerful new sensing platform for clinical diagnostics, bioassays and environmental monitoring.