Electrospun Nanofibers for Sensing and Biosensing Applications—A Review

Sensors and biosensors have found applications in many areas, e.g., in medicine and clinical diagnostics, or in environmental monitoring. To expand this field, nanotechnology has been employed in the construction of sensing platforms. Because of their properties, such as high surface area to volume ratio, nanofibers (NFs) have been studied and used to develop sensors with higher loading capacity, better sensitivity, and faster response time. They also allow to miniaturize designed platforms. One of the most commonly used techniques of the fabrication of NFs is electrospinning. Electrospun NFs can be used in different types of sensors and biosensors. This review presents recent studies concerning electrospun nanofiber-based electrochemical and optical sensing platforms for the detection of various medically and environmentally relevant compounds, including glucose, drugs, microorganisms, and toxic metal ions.     

Fabrication and DOE Optimization of Electrospun Chitosan/Gelatin/PVA Nanofibers for Skin Tissue Engineering

Chitosan/gelatin-based nanofibers display excellent biological performance in tissue engineering because of their biocompatible composition and nanofibrous structure with a high surface-to-volume ratio mimicking the native extracellular matrix. In this study, to save time and cost of experiments, a response surface methodology based on Box–Behnken design (BBD) is developed to predict the mean diameter of (chitosan:gelatin)/poly(vinyl alcohol) (PVA) nanofibers in three volume ratios of chitosan:gelatin by considering PVA percentage, applied voltage, and flow rate as input variables. The morphology and chemical composition of nanofibers are investigated through scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), respectively. The optimum conditions to yield the minimum diameter of nanofibers with chitosan:gelatin ratios of 25:75, 50:50, and 75:25 are found and result in 165, 121, and 92 nm, respectively, which show good accordance with BBD estimated results. The tensile testing indicates that nanofibers containing higher ratio of chitosan:gelatin result in higher tensile stress and lower toughness and tensile strain. The water contact angle analysis (WCA) shows the appropriate hydrophilicity of crosslinked nanofibers. The MTT assay shows excellent cell viability and cell attachment of nanofibers for mouse fibroblast (L929) cells. The results indicate that optimum nanofibers are potent candidates for wound healing applications.     

Optimization of Process-Control Parameters for the Diameter of Electrospun Hydrophilic Polymeric Composite Nanofibers

A composite nanofiber composed of three polymers, namely polyvinyl alcohol/polyvinyl pyrrolidone/polyethylene oxide, is produced. The experiments are constructed using three design of experiment techniques, Taguchi L₉, Taguchi L₂₇, and Screening method. The experiments are verified using the analysis of variance (ANOVA) method and later a mathematical model is developed using the regression method. The impact of electrospun processing parameters, namely applied voltage, flow rate, and working distance, on nanofibers' diameter is measured. The working distance is a significant factor in controlling the size of the fiber diameter, while the applied voltage has the lowest effect on it. As a result of the regression equation, a Genetic algorithm is used to find the optimum variables for the required fiber diameter, which is 156 nm for flow rate = 0.001 mL h⁻¹, voltage = 30 kV, and distance = 200 mm with a 3% difference from the experimental fiber diameter.     

Electrospun Poly(l‐Lactic Acid) Nanofibers for Nanogenerator and Diagnostic Sensor Applications

This paper presents for the first time that poly(l ‐lactic acid) (PLLA) nanofibers can show the piezoelectricity along the fiber direction (d₃₃) by using an electrospinning method. First, the electrospun fiber bundles are characterized by scanning electron microscope, X‐ray, and piezoelectric coefficient measurements. The data show that the supercritical CO₂ treatment can greatly enhance the piezoelectricity of electrospun PLLA fibers, which can be resulting from the increased crystallinity of the fibers. Later, it is found that the electrospun PLLA fiber can generate a current of 8 pA and a voltage of 20 mV by a simple push–release process. Further, a single PLLA fiber‐based blood pulse sensor is also fabricated and tested and shows around a 2 pA output for blood pulse. Due to easy fabrication and relatively simple structure, this device enables a broad range of promising future applications in the medical sensor area.

     

Highly‐Aligned Electrospun Luminescent Nanofibers Prepared from Polyfluorene/PMMA Blends: Fabrication,Morphology, Photophysical Properties and Sensory Applications

Highly‐aligned luminescent electrospun nanofibers were successfully prepared from two binary blends of PFO/PMMA and PF⁺/PMMA. The PFO/PMMA aligned electrospun fibers showed a core/shell structure but the PF⁺/PMMA fibers exhibited periodic aggregate domains in the fibers. The aligned fibers had polarized steady‐state luminescence with a polarized ratio as high as 4, much higher than the non‐woven electrospun fibers or spin‐coated film. Besides, the PF⁺/PMMA aligned electrospun fibers showed an enhanced sensitivity to plasmid DNA. Such aligned electrospun fibers could have potential applications in optoelectronic or sensory devices.

     

Gellan-Xanthan Hydrogel Conduits with Intraluminal Electrospun Nanofibers as Physical,Chemical and Therapeutic Cues for Peripheral Nerve Repair

Optimal levels of functional recovery in peripheral nerve injuries remain elusive due to the architectural complexity of the neuronal environment. Commercial nerve repair conduits lack essential guidance cues for the regenerating axons. In this study, the regenerative potential of a biosimulated nerve repair system providing three types of regenerative cues was evaluated in a 10 mm sciatic nerve-gap model over 4 weeks. A thermo-ionically crosslinked gellan-xanthan hydrogel conduit loaded with electrospun PHBV-magnesium oleate-N-acetyl-cysteine (PHBV-MgOl-NAC) nanofibers was assessed for mechanical properties, nerve growth factor (NGF) release kinetics and PC12 viability. In vivo functional recovery was based on walking track analysis, gastrocnemius muscle mass and histological analysis. As an intraluminal filler, PHBV-MgOl-NAC nanofibers improved matrix resilience, deformation and fracture of the hydrogel conduit. NGF release was sustained over 4 weeks, governed by Fickian diffusion and Case-II relaxational release for the hollow conduit and the nanofiber-loaded conduit, respectively. The intraluminal fibers supported PC12 proliferation by 49% compared to the control, preserved up to 43% muscle mass and gradually improved functional recovery. The combined elements of physical guidance (nanofibrous scaffolding), chemical cues (N-acetyl-cysteine and magnesium oleate) and therapeutic cues (NGF and diclofenac sodium) offers a promising strategy for the regeneration of severed peripheral nerves.     

Comparison of the Effects of an Ionic Liquid and Other Salts on the Properties of Electrospun Fibers, 2 – Poly(vinyl alcohol)

Understanding the effect of conductivity in electrospinning solutions is crucial in order to improve or control the electrospinning process. In this paper the effect of adding small amounts (0.039–0.259 mol · kg^?1) of three different conductive additives to aqueous solutions of polyvinyl alcohol has been investigated. The salts were HMICl (a room temperature ionic liquid), TEBAC (a quaternary ammonium salt) and KCl. Addition of these salts caused a steady increase in the solution conductivity but the fiber diameter was typically greater than that of PVA alone, and exhibited an oscillatory trend. The oscillatory trend on the fiber diameter is attributed to fiber backbuilding and fusion that occurs prior to deposition on the collector.

     

Comparison of the Effects of an Ionic Liquid and Triethylbenzylammonium Chloride on the Properties of Electrospun Fibers, 1 – Poly(lactic acid)

Improving the conductivity of electrospinning solutions is often achieved by adding small amounts of conductive additives. HMIMCl, a room temperature ionic liquid, and TEBAC, a quaternary ammonium salt, were added to polylactic acid in chloroform and their effects on solution properties, electrospinning, and fiber properties were investigated. Both additives increased the conductivity which decreased the fiber diameter, but differences were observed on the fiber dispersity and fiber morphology. The conductive solutions caused fiber backbuilding with aggregation and fiber fusion. Reasons for the differences in fiber diameter and fiber morphology are discussed.

     

An Inexpensive,Portable Device for Point‐of‐Need Generation of Silver‐Nanoparticle Doped Cellulose Acetate Nanofibers for Advanced Wound Dressing

This short communication describes the design and assembly of a new, miniaturized electrospinner to produce nanofibers at the site of need for drug delivery and wound dressing applications. The portable apparatus would eliminate the storage and transportation concerns with regards to the delicate nature of drug‐loaded nanofibers, thereby preserving product integrity at the site of use. Furthermore, the setup features a smaller size, a cheaper price, and components that are readily obtainable off‐the‐shelf, compared to those of available devices that are custom‐built and more expensive, making it desirable and accessible for other users in the field. As a proof‐of‐concept for wound care, the device is successfully used to electrospin three types of nanofibers comprised of pure cellulose acetate (CA), and CA respectively doped with 0.75 and 1.5 wt% silver nanoparticles. The miniaturized device is useful on account of the popularity of electrospinning as well as the potential to minimize wound infection due to the reduced manipulation of both the dressing and the wound from product generation to the point of need. Work is in progress to further develop the portable device and compare its product performance with traditional wound dressing materials for clinical translation.     

Enhancement of β‐Phase Crystalline Structure and Piezoelectric Properties of Flexible PVDF/Ionic Liquid Surfactant Composite Nanofibers for Potential Application in Sensing and Self‐Powering

Polyvinylidene fluoride (PVDF) is a piezo‐polymer which among its crystalline phases, the β‐phase has been researched for the improvement of piezoelectric properties. In this study, to improve the β‐phase contents and thereby the piezoelectric response of the polymer, the effect of adding self‐synthesized ionic liquid surfactant (ILS) in PVDF nanofibers is studied. This material is added in different weight percentages into the PVDF solution and the nanofibers are produced by electrospinning to prepare active piezoelectric thin layers. SEM, XRD, FTIR, and piezo‐tests are employed for assessing the effect of the ILS on the enhancement of β‐phase in electrospun nanofibers and their piezoelectric performance. The results indicate ≈98.6% β‐phase formation in the sample containing 4 wt% ILS and in comparison with the pure nanofibers, the output voltage and its power density are improved 186.9% and 275%, respectively. Considering the results, it is suggested that the ILS can improve the piezoelectric response of the polymer in the fabricated structure by simple mixing in solution compared to other additives.     

Synthesis and Optimization of 2-ethylhexyl Ester as Base Oil for Drilling Fluid Formulation

A stable ester was synthesized to overcome the ester hydrolysis problem during the drilling of oil or gas wells using a conventional ester-based drilling fluid. The thermal and hydrolytic stability of the produced ester was high owing to the transesterification method employed in this study. The reaction was performed using 2-ethylhexanol and methyl laureate esters in the presence of sodium methoxide as a catalyst. In order to obtain the optimum synthesis conditions, a response surface methodology (RSM) was appraised based on the central composite design (CCD). The optimum conditions were determined as follows: 0.6 wt.% catalyst, 70°C reaction temperature, 1:1.5 molar ratio, and 11.5 min of reaction time. The results of 77 wt.% 2-ethylhexyl ester (2-EH) illustrated a high agreement between the experimental and RSM models. The reaction product contained 77 wt.% 2-EH and 23% 2-ethylhexanol. The kinematic viscosity was 5 mm²/s at 40°C and 1.5 mm²/sec at 100°C; the specific gravity was 0.854, flash point was 170°C, and pour point was ?7°C. The produced product showed similar properties to the available commercial product. However, it was observed that the mud formulation using the synthesized base oil had superior rheological properties at 121°C.     

Silver Loaded Nanofibrous Curdlan Mat for Diabetic Wound Healing: An In Vitro and In Vivo Study

A fibrous scaffold of curdlan/poly(vinyl alcohol) (PVA) blend is prepared by electrospinning technique and antimicrobial property is imparted to it by the addition of silver nitrate (1, 3, and 5 wt%). All the scaffolds except the PVA/curdlan with 5 wt% AgNO₃ show good viability of Swiss 3T3 fibroblast cells. Significant reductions in the growth of Staphylococcus aureus and Escherichia coli are also observed in all the scaffolds. In vitro scratch assay and cell adhesion studies indicate that the scaffold containing 1% AgNO₃ shows significant wound healing and better cell spreading. The in vivo results also show faster healing of excision wounds in diabetic rats treated with the same material when compared to the control and the commercial sample. Furthermore, downregulation of proinflammatory cytokines and upregulation of anti‐inflammatory cytokines on the skin of the treated animals confirm that PVA/curdlan/1% AgNO₃ electrospun mat could be a promising material for diabetic wound healing.     

How Nanofibers Carry the Load: Toward a Universal and Reliable Approach for Tensile Testing of Polymeric Nanofibrous Membranes

Nanofibrous nonwovens show high versatility and outstanding properties, with reduced weight. Porous morphology, high material flexibility and deformability challenge their mechanical testing, severely affecting results reliability. Still today, a specific technical standard method to carry out tensile testing of nonwoven nanofibrous mats is lacking, as well as studies concerning tensile test data reliability. In this work, an accurate, systematic, and critical study is presented concerning tensile testing of nonwovens, using electrospun Nylon 66 random nanofibrous mats as a case study. Nanofibers diameter and specimen geometry are investigated to thoroughly describe the nanomat tensile behavior, also considering the polymer thermal properties, and the nanofibers crossings number as a function of the nanofibers diameter. Below a threshold value, which lies between 150 and 250 nm, the overall mat mechanical behavior changes from ductile to brittle, showing enhanced elastic modulus for a high number of nanofibers crossings. While specimen geometry does not affect tensile results. Stress–strain data are analyzed using a phenomenological data fitting model to better interpret the tensile behavior. The experimental results demonstrate the high reliability of the proposed mass-based load normalization, providing a simple, effective, and universally suitable method for obtaining high reproducible tensile stress–strain curves.     

Polycaprolactone/Chitosan Composite Nanofiber Membrane as a Preferred Scaffold for the Culture of Mesothelial Cells and the Repair of Damaged Mesothelium

Mesothelial cells are specific epithelial cells lining the serosal cavity and internal organs. Nonetheless, few studies have explored the possibility to culture mesothelial cells in a nanostructure scaffold for tissue engineering applications. Therefore, this study aims to fabricate nanofibers from a polycaprolactone (PCL) and PCL/chitosan (CS) blend by electrospinning, and to elucidate the effect of CS on the cellular response of mesothelial cells. The results demonstrate that a PCL and PCL/CS nanofiber membrane scaffold could be prepared with a comparable fiber diameter (~300 nm) and porosity for cell culture. Blending CS with PCL influenced the mechanical properties of the scaffold due to interference of PCL crystallinity in the nanofibers. However, CS substantially improves scaffold hydrophilicity and results in a ~6-times-higher cell attachment rate in PCL/CS. The mesothelial cells maintain high viability in both nanofiber membranes, but PCL/CS provides better maintenance of cobblestone-like mesothelial morphology. From gene expression analysis and immunofluorescence staining, the incorporation of CS also results in the upregulated expression of mesothelial marker genes and the enhanced production of key mesothelial maker proteins, endorsing PCL/CS to better maintain the mesothelial phenotype. The PCL/CS scaffold was therefore chosen for the in vivo studies, which involved transplanting a cell/scaffold construct containing allograft mesothelial cells for mesothelium reconstruction in rats. In the absence of mesothelial cells, the mesothelium wound covered with PCL/CS showed an inflammatory response. In contrast, a mesothelium layer similar to native mesothelium tissue could be obtained by implanting the cell/scaffold construct, based on hematoxylin and eosin (H&E) and immunohistochemical staining.     

Process Optimization and Empirical Modeling for Electrospun Poly(D,L‐lactide) Fibers using Response Surface Methodology

Summary: Ultrafine fibers were spun from poly(D ,L ‐lactide) (PDLA) solution using a homemade electrospinning set‐up. Fibers with diameter ranging from 350 to 1 900 nm were obtained. Morphologies of fibers and distribution of fiber diameters were investigated varying concentration and applied voltage by scanning electron microscopy (SEM). Average fiber diameter and distribution were determined from about 100 measurements of the random fibers with an image analyzer (SemAfore 5.0, JEOL). A more systematic understanding of process parameters of the electrospinning was obtained and a quantitative relationship between electrospinning parameters and average fiber diameter was established by response surface methodology (RSM). It was concluded that the concentration of polymer solution played an important role in the diameter of fibers and standard deviation of fiber diameter. Lower concentration tended to facilitate the formation of bead‐on‐string structures. Fiber diameter tended to increase with polymer concentration and decrease with applied voltage. Fibers with lower variation in diameter can be obtained at lower concentration regardless of applied voltage. Fibers with uniform diameter and lower variation in diameter can be obtained at higher concentration and higher applied voltage. Process conditions for electrospinning of PDLA could be chosen according to the model in this study.

Contour plots of average fiber diameter as a function of concentration and applied voltage.     

Assessing the Suitability of Electrospun Poly(Ethylene Terephthalate) and Polystyrene as Cell Carrier Substrates for Potential Subsequent Implantation as a Synthetic Bruch's Membrane

Electrospun poly(ethylene terephthalate) (PET) and polystyrene (PS) were tested for suitability as cell carrier substrates. Membranes were UV/ozone treated, which improved protein adsorption, with aminolysis observed on PET. PET demonstrated greater handling and durability compared to PS. Treated and untreated PET supported cell proliferation, with cells exhibiting the desired monolayer morphology. Untreated PS did not support cell proliferation and although treated PS did, the resultant RPE cell morphology was undesirable. Preliminary tests investigating thickness of mats were also undertaken, with PET exhibiting better results. Electrospun PET exhibited cytocompatibility, and could prove to be a suitable candidate for potential subsequent implantation.     

Electrospun Poly(styrene) Fibers as a Protection for the First- and the Second-Generation Grubbs’ Catalyst

The study presents a novel method for protection of the first- and the second-generation Grubbs’ catalyst, by incorporation in poly(styrene) fibers through electrospinning technique. Both catalysts are sensitive to the presence of the amine hardeners in the epoxy-based self-healing composites and require protection from deactivation to retain their ability to promote polymerization reaction of the healing agent. Comparison of healing efficiencies of both catalysts suggested that poly(styrene) fibers offer better protection and dispersion for the first-generation Grubbs’ catalyst, although all the samples exhibited high-healing efficiency. Difference in stereoselectivity between two catalysts was also indicated.     

Electrospun Composite Mats of Poly[(D,L‐lactide)‐co‐glycolide] and Collagen with High Porosity as Potential Scaffolds for Skin Tissue Engineering

Electrospun composite mats of poly[(D,L ‐lactide)‐co‐glycolide] and collagen with high porosities of 85–90% and extended pore sizes of 90–130 µm were prepared to mimic the ECM morphologically and chemically. The existence of collagen molecules on the fiber surface was confirmed, enabling the cells to find enhanced binding sites for their integrin receptors. The mechanical data for the blended fibrous mats indicated that they were sufficiently durable for dermal tissue engineering. Fibroblasts derived from GFP transgenic C57BL/6 mice were used to directly observe cell proliferation, and the inoculation of collagen enhanced cell attachment, proliferation and extracellular matrix secretion, which were found to be dependent on the amount of collagen in the composite scaffold.

     

Preparation of a CaO Nanocatalyst and Its Application for Biodiesel Production Using Butea monosperma Oil: An Optimization Study

CaO nanoparticles (NP) were synthesized through solution combustion using crude glycerin (biodiesel by‐product) as the combustion fuel. The synthesized CaO NP were characterized using Fourier transform infrared spectrometer (FTIR), X‐ray diffractometer (XRD), temperature programmed desorption of carbon dioxide (CO₂‐TPD), scanning electron microscope (SEM), and transmission electron microscope (TEM). The CaO NP were successfully used as a catalyst for biodiesel synthesis. Response surface methodology was used to determine the optimal conditions for biodiesel production from Butea monosperma oil (BMO) using central composite design. A total of 20 experiments were designed and conducted to study the effects of the methanol to BMO molar ratio, reaction time, and catalyst loading conditions on the biodiesel yield. A yield of 96.2% of Butea monosperma methyl ester (BMME or biodiesel) was obtained under optimum conditions, namely a molar ratio (methanol to BMO) of 9:1, a reaction time of 70 min, a catalyst loading of 1.60 wt%, a constant temperature of 65 °C, and an agitation speed of 600 rpm. The fatty‐acid composition of BMO was characterized through gas chromatography. Finally, BMME was characterized using FTIR, ¹H NMR, and ¹³C NMR, and the fuel properties of BMME were determined using the test methods of the American Society for Testing and Materials.     

PVDF and P(VDF-TrFE) Electrospun Scaffolds for Nerve Graft Engineering: A Comparative Study on Piezoelectric and Structural Properties,and In Vitro Biocompatibility

Polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene (P(VDF-TrFE)) are considered as promising biomaterials for supporting nerve regeneration because of their proven biocompatibility and piezoelectric properties that could stimulate cell ingrowth due to their electrical activity upon mechanical deformation. For the first time, this study reports on the comparative analysis of PVDF and P(VDF-TrFE) electrospun scaffolds in terms of structural and piezoelectric properties as well as their in vitro performance. A dynamic impact test machine was developed, validated, and utilised, to evaluate the generation of an electrical voltage upon the application of an impact load (varying load magnitude and frequency) onto the electrospun PVDF (15–20 wt%) and P(VDF-TrFE) (10–20 wt%) scaffolds. The cytotoxicity and in vitro performance of the scaffolds was evaluated with neonatal rat (nrSCs) and adult human Schwann cells (ahSCs). The neurite outgrowth behaviour from sensory rat dorsal root ganglion neurons cultured on the scaffolds was analysed qualitatively. The results showed (i) a significant increase of the β-phase content in the PVDF after electrospinning as well as a zeta potential similar to P(VDF-TrFE), (ii) a non-constant behaviour of the longitudinal piezoelectric strain constant d₃₃, depending on the load and the load frequency, and (iii) biocompatibility with cultured Schwann cells and guiding properties for sensory neurite outgrowth. In summary, the electrospun PVDF-based scaffolds, representing piezoelectric activity, can be considered as promising materials for the development of artificial nerve conduits for the peripheral nerve injury repair.