Electrospun Nanofibers for Integrated Sensing, Storage, and Computing Applications

Electrospun nanofibers have become the most promising building blocks for future high-performance electronic devices because of the advantages of larger specific surface area, higher porosity, more flexibility, and stronger mechanical strength over conventional film-based materials. Moreover, along with the properties of ease of fabrication and cost-effectiveness, a broad range of applications based on nanomaterials by electrospinning have sprung up. In this review, we aim to summarize basic principles, influence factors, and advanced methods of electrospinning to produce hundreds of nanofibers with different structures and arrangements. In addition, electrospun nanofiber based electronics composed of both two-terminal and three-terminal devices and their practical applications are discussed in the fields of sensing, storage, and computing, which give rise to the further integration to realize a comprehensive and brain-like system. Last but not least, the emulation of biological synapses through artificial synaptic transistors and additionally optoelectronics in recent years are included as an important step toward the construction of large-scale, multifunctional systems.

Review on Electrospun Nanofiber-Applied Products

Electrospinning technology, which was previously known as a scientific interdisciplinary research approach, is now ready to move towards a practice-based interdisciplinary approach in a variety of fields, progressively. Electrospun nanofiber-applied products are made directly from a nonwoven fabric-based membranes prepared from polymeric liquids involving the application of sufficiently high voltages during electrospinning. Today, electrospun nanofiber-based materials are of remarkable interest across multiple fields of applications, such as in electronics, sensors, functional garments, sound proofing, filters, wound dressing and scaffolds. This article presents such a review for summarizing the current progress on the manufacturing scalability of electrospun nanofibers and the commercialization of electrospun nanofiber products by dedicated companies globally. Despite the clear potential and limitless possibilities for electrospun nanofiber applications, the uptake of electrospinning by the industry is still limited due to the challenges in the manufacturing and turning of electrospun nanofibers into physical products. The recent developments in the field of electrospinning, such as the prominent nonwoven technology, personal views and the potential path forward for the growth of commercially applied products based on electrospun nanofibers, are also highlighted.

Optimization of electrospinning process & parameters for producing defect-free chitosan/polyethylene oxide nanofibers for bone tissue engineering

Chitosan (CS) nanofibers were electrospun from aqueous chitosan solution using concentrated acetic acid solution as a solvent. Polyethylene oxide (PEO) with varying weight content from 10- 60 wt% was mixed with chitosan solution that acted as a plasticizer to improve spinability of the prepared chitosan solution. With the increase in PEO content from 10-50 wt% the viscosity of the resultant CS/PEO solution was decreased from 0.938 Pa-s to 0.272 Pa-s, whereas higher the concentration of acetic acid lower was the surface tension of resultant chitosan solution. It was found beadless nanofibrous chitosan mat was obtained not less than 85% acetic acid concentration, 50 wt% PEO and at 0.2 wt% NaCl and 5 wt% total polymer concentration. From field emission scanning electron microscopy (FESEM) investigation, it was observed that chitosan fibers with an average diameter of 149 nm were produced at an applied voltage of 22.5 KV, while that varied between 17.5- 25 KV. On the other hand, a minimum of 110 nm of average diameter chitosan nanofiber was obtained at a needle tip to rotor collector distance of 15 cm by the method of electrospining. In terms of solution flow rate, 0.4 mL/h was found to be optimum in obtaining defect-free electrospun fiber with lower average diameter. As a whole, smooth and uniform chitosan nanofibers were obtained from 50/50 CS/PEO solution prepared by using 90% acetic acid and electrospun at 20 kV applied voltage, 15 cm needle tip-to- rotor collector distance with 0.2 mm inner diameter needle and 0.4 mL/h feeding rate. After crosslinking with 1 wt% glutaraldehyde (GTA), the ultimate tensile strength and Young's modulus of chitosan scaffold increased upto 9.47 MPa and 147.75 MPa respectively. From MTT assay and alkaline phosphatase expression analysis upto 11 days of cell culture period it was evident that thus prepared electrospun CS scaffolds supported MG 63 cell proliferation and its differentiation into mature osteoblast.

A review of evolution of electrospun tissue engineering scaffold: From two dimensions to three dimensions

The potential of electrospinning process to fabricate ultrafine fibers as building blocks for tissue engineering scaffolds is well recognized. The scaffold construct produced by electrospinning process depends on the quality of the fibers. In electrospinning, material selection and parameter setting are among many factors that contribute to the quality of the ultrafine fibers, which eventually determine the performance of the tissue engineering scaffolds. The major challenge of conventional electrospun scaffolds is the nature of electrospinning process which can only produce two-dimensional electrospun mats, hence limiting their applications. Researchers have started to focus on overcoming this limitation by combining electrospinning with other techniques to fabricate three-dimensional scaffold constructs. This article reviews various polymeric materials and their composites/blends that have been successfully electrospun for tissue engineering scaffolds, their mechanical properties, and the various parameters settings that influence the fiber morphology. This review also highlights the secondary processes to electrospinning that have been used to develop three-dimensional tissue engineering scaffolds as well as the steps undertaken to overcome electrospinning limitations.

Fabricating high mechanical strength γ-Fe2O3 nanoparticles filled poly(vinyl alcohol) nanofiber using electrospinning process potentially for tissue engineering scaffold

The use of electrospinning has gained substantial interest in the development of tissue engineering scaffolds due to its ability to produce nanoscale fibers which can mimic the geometry of extracellular tissues. Besides geometry, mechanical property is one of the main elements to be considered when developing tissue engineering scaffolds. In this study, the electrospinning process parameter settings were varied in order to find the optimum setting which can produce electrospun nanofibrous mats with good mechanical properties. Maghemite (γ-Fe2O3) was mixed with poly(vinyl alcohol) and then electrospun to form nanofibers. The five input variable factors involved were nanoparticles content, voltage, flow rate, spinning distance, and rotating speed, while the response variable considered was Young’s modulus. The performance of electrospinning process was systematically screened and optimized using response surface methodology. This work truly demonstrated the sequential nature of designed experimentation. Additionally, the application of various designs of experiment techniques and concepts was also demonstrated. Results revealed that electrospun nanofibrous mats with maximum Young’s modulus (273.51 MPa) was obtained at optimum input settings: 9 v/v% nanoparticle content, 35 kV voltage, 2 mL/h volume flow rate, 8 cm spinning distance, and 3539 r/min of rotating speed. The model was verified successfully by performing confirmation experiments. The nanofibers characterization demonstrated that the nanoparticles were well dispersed inside the nanofibers, and it also showed that the presence of defects on the nanofibers can decrease their mechanical strength. The biocompatibility performance was also evaluated and it was proven that the presence of γ-Fe2O3 enhanced the cell viability and cell growth rate. The developed poly(vinyl alcohol)/γ-Fe2O3 electrospun nanofiber mat has a good potential for tissue engineering scaffolds.

Mechanical properties and biocompatibility of co-axially electrospun polyvinyl alcohol/maghemite

Electrospinning is a simple and efficient process in producing nanofibers. To fabricate nanofibers made of a blend of two constituent materials, co-axial electrospinning method is an option. In this method, the constituent materials contained in separate barrels are simultaneously injected using two syringe nozzles arranged co-axially and the materials mix during the spraying process forming core and shell of the nanofibers. In this study, co-axial electrospinning method is used to fabricate nanofibers made of polyvinyl alcohol and maghemite (γ-Fe2O3). The concentration of polyvinyl alcohol and amount of maghemite nanoparticle loading were varied, at 5 and 10 w/v% and at 1–10 v/v%, respectively. The mechanical properties (strength and Young’s modulus), porosity, and biocompatibility properties (contact angle and cell viability) of the electrospun mats were evaluated, with the same mats fabricated by regular single-nozzle electrospinning method as the control. The co-axial electrospinning method is able to fabricate the expected polyvinyl alcohol/maghemite nanofiber mats. It was noticed that the polyvinyl alcohol/maghemite electrospun mats have lower mechanical properties (i.e. strength and stiffness) and porosity, more hydrophilicity (i.e. lower contact angle), and similar cell viability compared to the mats fabricated by single-nozzle electrospinning method.

Fabrication of biocompatible nanohybrid shish-kebab-structured carbon nanotubes with a mussel-inspired layer

The first report investigating the biocompatibility of the (polydopamine coated) carbon nanotubes/polymer nanohybrid shish-kebab structure for tissue engineering.

Fabrication of conductive polymer nanofibers through SWNT supramolecular functionalization and aqueous solution processing

Polymeric thin films and nanostructured composites with excellent electrical properties are required for the development of advanced optoelectronic devices, flexible electronics, wearable sensors, and tissue engineering scaffolds. Because most polymers available for fabrication are insulating, one of the biggest challenges remains the preparation of inexpensive polymer composites with good electrical conductivity. Among the nanomaterials used to enhance composite performance, single walled carbon nanotubes (SWNTs) are ideal due to their unique physical and electrical properties. Yet, a barrier to their widespread application is that they do not readily disperse in solvents traditionally used for polymer processing. In this study, we employed supramolecular functionalization of SWNTs with a conjugated polyelectrolyte as a simple approach to produce stable aqueous nanotube suspensions, that could be effortlessly blended with the polymer poly(ethyleneoxide) (PEO). The homogeneous SWNT:PEO mixtures were used to fabricate conductive thin films and nanofibers with improved conductivities through drop casting and electrospinning. The physical characterization of electrospun nanofibers through Raman spectroscopy and SEM revealed that the SWNTs were uniformly incorporated throughout the composites. The electrical characterization of SWNT:PEO thin films allowed us to assess their conductivity and establish a percolation threshold of 0.1 wt% SWNT. Similarly, measurement of the nanofiber conductivity showed that the electrospinning process improved the contact between nanotube complexes, resulting in conductivities in the S m(-1) range with much lower weight loading of SWNTs than their thin film counterparts. The methods reported for the fabrication of conductive nanofibers are simple, inexpensive, and enable SWNT processing in aqueous solutions, and offer great potential for nanofiber use in applications involving flexible electronics, sensing devices, and tissue engineering scaffolds.

The effect of stereocomplex polylactide particles on the mechanical properties of poly(lactide-co-glycolide) copolymer

The effect of the addition of stereocomplex polylactide with different contents and particle sizes to a poly(lactide-co-glycolide) matrix was investigated. The organic filler stereocomplex polylactide affects the mechanical properties of poly(lactide-co-glycolide) as an organic nucleating agent without any surface modification. A low content of organic stereocomplex polylactide particles in the poly(lactide-co-glycolide) blends enhanced their mechanical properties. A smaller stereocomplex polylactide particle size improves the mechanical properties of poly(lactide-co-glycolide) blends more effectively. The improvement in the mechanical properties of the poly(lactide-co-glycolide) blends stemmed from the nucleating effects of the stereocomplex polylactide particles in the poly(lactide-co-glycolide) matrix. The optimum content and homogeneous distribution of stereocomplex polylactide particles increased Young’s modulus and tensile strength by approximately 15% and 30%, respectively. The presence of stereocomplex polylactide particles also accelerated the hydrolytic degradation, as represented by a decrease in the molecular weight. The improvements in the mechanical properties and hydrolytic degradation acceleration match the requirements of poly(lactide-co-glycolide) applications as bone fracture fixation materials.

Electrospun polymer mat as a SERS platform for the immobilization and detection of bacteria from fluids

A new class of SERS substrates is presented that allows for the simultaneous filtration of bacteria from any solution (blood, urine, water, or milk), immobilization of bacteria on the SERS platform, and enhancing the Raman signal of bacteria.

Antibiofilm activity of quercetin-encapsulated cytocompatible nanofibers against Candida albicans

In this study, nanofibers against pro dimorphic fungal sessile growth were developed. Quercetin was successfully encapsulated within poly(d,l-lactide- co-glycolide)–poly(ε-caprolactone) nanofibers using an electrospinning technique. Field emission scanning electron microscopy, fluorescent microscopy, and Fourier-transformed infrared spectrometer were used to confirm the formation as well as encapsulation of quercetin within the nanofibers. These fabricated nanofibers were further evaluated to determine the effectiveness of the antibiofilm activity against Candida albicans. The cytocompatibility of quercetin-encapsulated nanofibers was found to be similar to control and pure polymeric nanofibers based on 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay against human embryonic kidney (HEK-293) cell lines. These fabricated nanofibers potentially could be used as coatings on biomedical devices to inhibit microbial contaminations.

Trapping tetracycline-loaded nanoparticles into polycaprolactone fiber networks for periodontal regeneration therapy

The controlled delivery of antibiotics, anti-inflammatory agents, or chemotherapeutic agents to the periodontal site is a recognized strategy to improve the efficiency of regenerative processes of hard tissues. A novel approach based on the trapping of tetracycline hydrochloride–loaded particles in polycaprolactone nanofibers was used to guide the regeneration processes of periodontal tissue at the gum interface. Chitosan nanoparticles loaded with different levels of tetracycline hydrochloride (up to 5% wt) were prepared by solution nebulization induced by electrical forces (i.e. electrospraying). The fine tuning of process parameters allows to obtain nanoparticles with tailored sizes ranging from 0.485 ± 0.147 µm to 0.639 ± 0.154 µm. The tetracycline hydrochloride release profile had a predominant burst effect for the first 70% of release followed by a relatively slow release over 24 h, which is promising for oral drug delivery. We also demonstrated that trapping tetracycline hydrochloride–loaded particles with submicrometer diameters into a polycaprolactone fiber network contributed to slowing the release of tetracycline hydrochloride from the nanoparticles, thus providing a more prolonged release in the periodontal pocket during clinical therapy. Preliminary studies on human mesenchymal stem cells confirm the viability of cells up to 5 days after culture, and thereby, validate the use of nanoparticle-/nanofiber-integrated systems in periodontal therapies.

Electrospun poly(lactic-co-glycolic acid)/wool keratin fibrous composite scaffolds potential for bone tissue engineering applications

Biocomposite scaffolds consist of poly(lactic- co-glycolic acid) and wool keratin were obtained by an electrospinning process. Scanning electron microscopy images showed that the poly(lactic- co-glycolic acid)/wool keratin fibers had relatively rougher surfaces and smaller diameters. Thermogravimetric analysis showed higher thermal stabilities of the developed biocomposites compared to neat poly(lactic- co-glycolic acid). Mechanical tests showed that when the wool keratin content increased from 0% to 0.5% w/v, the tensile strength and elongation at break of the poly(lactic- co-glycolic acid)/0.5% wool keratin scaffolds increased with maxima of 6.59 MPa and 104.44%, respectively, which was an increase of 8.2% and 570% over the poly(lactic- co-glycolic acid) scaffold. The biological response of bone mesenchymal stem cells to the poly(lactic- co-glycolic acid)/1.5% wool keratin biocomposites was superior when compared to pure poly(lactic- co-glycolic acid) scaffold in terms of improved cell attachment and higher proliferation. These observations suggest that the addition of wool keratin to a poly(lactic- co-glycolic acid) matrix can improve several properties of the electrospun poly(lactic- co-glycolic acid) fibers, and the poly(lactic- co-glycolic acid)/wool keratin biocomposites could make excellent materials for tissue engineering applications.

Functionalization of electrospun poly(ε-caprolactone) scaffold with heparin and vascular endothelial growth factors for potential application as vascular grafts

In this study, a heparin-conjugated poly(ε-caprolactone) electrospun fiber was constructed to develop a functional scaffold for controlled release of vascular endothelial growth factors. The immobilization of vascular endothelial growth factor was achieved through affinity binding between heparin and vascular endothelial growth factor molecules. The sustained release of vascular endothelial growth factor from the scaffold was followed for up to 15 days. The endothelial cell adhesion and proliferation assay demonstrated that immobilized vascular endothelial growth factor maintained its activity. The blood compatibility of the scaffold was evaluated by activated partial thromboplastin time, platelet adhesion test, and arteriovenous shunt, and the functionalized scaffold showed improved anticoagulation properties. The biocompatibility was evaluated by subcutaneous implantation. Results showed that this vascular endothelial growth factor–releasing scaffold stimulated neovascularization with minimum immunological rejection compared to the unmodified poly(ε-caprolactone) scaffold. The present study demonstrated a new strategy of building bioactive scaffolds for the development of small-diameter vascular graft.

Electrospun nanofibers in drug delivery: recent developments and perspectives

In this review article, some key challenges in drug delivery are first introduced and methods that have been applied in attempts to solve them enumerated. Particularly intractable problems are highlighted: these include issues of solubility, targeting and drug degradation. The technique of electrospinning is subsequently introduced, and the influence of processing parameters on the fibers produced discussed. The potential of electrospun nanofibers in drug delivery is then explored, with examples given from the recent literature to illustrate how fibers can be used to overcome hurdles in drug solubility, degradation and targeting. Future perspectives and challenges are also considered.

Evaluation of biological activity of bone morphogenetic proteins on exposure to commonly used electrospinning solvents

Bone tissue engineering is one of the emerging strategies for developing functionally viable bone substitutes. The recent trend in bone tissue engineering is to combine the benefits of a three-dimensional nanofibrous scaffold with biologically active molecules and responsive stem cells. Electrospinning is the most versatile of the scaffold fabrication strategies and may involve the use of an organic solvent at one stage or another. In spite of all distinct advantages of electrospinning, valid concerns about potentially denaturing interactions between the organic solvent and the biomolecules exist. Efforts are ongoing to incorporate osteoinductive molecules, such as bone morphogenetic proteins (BMPs), during the electrospinning process. The challenge lies in ensuring that the biological activity of these incorporated molecules survives the process. This study was specifically designed to investigate the effects of exposure to commonly used organic solvents on heterodimeric BMP-2/7 using slot-blot assay quantified by infrared imaging and on embryonic myoblasts stably transfected with BMP-specific response element linked to a luciferase reporter – C2C12BRA. Overall, the biological activity of these molecules significantly decreased when exposed to organic solvents but can be restored to their original values by increasing the polarity of the solvent. It was found that an aqueous buffer can effectively overcome the deleterious effects of organic solvents on BMPs, thus generating osteoinductive bone scaffolds.