Figure 1: Applications of electrospun nanofibers .

Electrospinning is an advancement of electrospraying, where electric forces are used to disperse fine aerosols from a polymer solution, a technique invented in 1747 by Abbé Nollet . During electrospinning, fluid is drawn through an electrically charged spinneret with a conical shape at the tip from which a jet emerges . Based on Zeleny’s research, Geoffrey Taylor described in 1964 that the meniscus’ critical half-angle approaches 49.3° immediately before collapse. Thus, in a strong electric field, the tip of a polymer solution or melt extruded from a capillary changes from a spherical to a conical shape, the so-called Taylor cone .

The parameters involved in the electrospinning process can be divided into three categories, namely, electrospinning, solution, and environment. For electrospinning, factors such as applied electric field, distance between the needle and collector, flow rate, and needle diameter affect the fabrication of the nanofibrous sample. Solution parameters include the types of solvent, polymer concentration, viscosity, and solution conductivity. Finally, environmental parameters include relative humidity and temperature .

Chitosan, a widely utilized material in electrospun nanofiber membranes, is derived from the crystalline microfibrils of crustaceans, including crabs and prawns. It is biodegradable and exhibits a high capacity for adsorbing heavy metals and radionuclides . However, chitosan exhibits limited mechanical stability, is sensitive to pH variations, and tends to swell . To improve the spinnability of chitosan during the electrospinning process, it is commonly blended with other polymers, such as PVA . Additionally, PVA contributes to reducing the crystallinity of the chitosan structure .

Because of their unique and exceptional properties, nanofibrous membranes have become prominent materials for a wide range of applications . Throughout their time of use, electrospun fibers are exposed to environmental stresses in each of these applications. These forces could cause the nanofibers to fail or become permanently deformed, potentially rendering the entire device inoperative. Thus, before using these materials in particular fields, it is essential to take their mechanical properties into consideration .

Electrospun nanofibrous membranes typically have poor mechanical characteristics. The materials used, the non-Newtonian fluids in the electrospinning solutions, and the nanofibers’ random orientation are the three primary factors of the poor mechanical performance of electrospun nanofibers. For example, collagen is one substance used in electrospinning that has low strength . Its loose structure and weak composition are the reasons behind its inherent poor mechanical performance. Electrospinning solutions with non-Newtonian fluids come next. They exhibit unstable properties, including irregular fluid jet movement, variations in surface tension, and sensitivity to external parameters such as temperature, humidity, and airflow. These factors can disrupt the uniformity of the fiber formation. When the fiber jets are drawn in the direction of the electric field force, the polymer chains in the fibers are not properly aligned, resulting in non-uniform mechanical and structural properties . The incomplete crystallization of the fibers caused by the short and weak stretch process gives the fibers poor mechanical properties . Last, there is the fibers’ random orientation. When an external force is applied to electrospun nanofibers, the load is distributed across a network of thousands of fibers. However, because of their random orientation, the force is not evenly distributed. In some cases, the load is concentrated on a single fiber, which can result in significant stress on that fiber and weaken the overall structure .

With advancements in technology for producing nanofibers through electrospinning, various methods and techniques have been developed to enhance the mechanical properties of electrospun nanofibers. The objective of this review is to explore current techniques and methods for improving the mechanical properties of electrospun nanofibers, thereby enhancing their application in various fields.

Figure 2: Chemical structure of chitosan.

Table 1: Properties of chitosan studied in various applications.

Table 2: Applications of chitosan/PVA electrospun nanofibers.

Figure 3: Chemical structure of PVA.

The mechanical properties of PVA depend on several factors, such as molecular weight and retained moisture . Dry, fully hydrolyzed PVA exhibits ductile properties with an elongation at break of 25% . However, PVA has low mechanical strength under wet conditions. Because of its solubility in water, its applications are limited, and it can only be used as an internal layer in multilayer structures. A convenient strategy to overcome these limitations, without losing the advantages of PVA such as biodegradability, is to blend PVA with stiff and water-insoluble biodegradable polymers such as chitosan .

Figure 4: Schematic diagram of the electrospinning process for the fabrication of nanofibers.

Electrospinning is a versatile technique for producing microfibers and nanofibers, but the process, especially with the conventional single-nozzle electrospinning, has several limitations. Some of the challenges include low throughput, which hinders industrial-scale production, poor fiber uniformity, and limited control over fiber alignment and orientation . Random fiber alignment and orientation, in addition to low crystallinity of electrospun structures, tend to result in poor mechanical strength . To address the challenges of conventional electrospinning, researchers have explored alternative approaches with distinct fiber-producing capabilities such as centrifugal electrospinning, alternating current electrospinning, near-field electrospinning, multinozzle electrospinning, and needleless electrospinning. Other improvement efforts include modifications on the solution or melts used for electrospinning, innovative designs of the electrospinning setup and components, and various post-treatment methods. These advancements aim to improve productivity, safety, and control over fiber properties, potentially enabling broader industrial applications of nanofibers .

The use of a single polymer material as the electrospinning solution produces fibers with limited functional properties . This limits the practical use of the fibers, especially to solve advanced issues that arise in line with the rapid growth in research and advanced technologies. Multicomponent fibers, composed of two or more materials, are designed to enhance the functionality and properties of the fibers, thereby broadening their applications in areas such as antibacterial treatments, water purification, and biomedical engineering . Fabrication of multicomponent polymer nanofibers like chitosan/PVA can be carried out through methods such as blend electrospinning, sequential electrospinning, co-electrospinning, and emulsion electrospinning.

Figure 5: Electrospinning polymer blend solutions by (a) mixing two polymer materials in a single solvent system and (b) mixing two solutions prepared from separate solvent systems.

Figure 6: Two-step sequential electrospinning for the fabrication of multilayered nanofibers.

Figure 7: Co-electrospinning with two nozzles (a) facing each other on the opposite sides and (b) next to each other on the same side of the collector.

Multichamber electrospinning incorporates special nozzle designs to fabricate multicomponent fibers, where a single nozzle is partitioned into separate chambers, with the most popular technique being the coaxial electrospinning. Coaxial electrospinning involves the use of a compound coannular nozzle to fabricate core–shell fibers . For a two-solution system, the solution from the inner tube forms the core and the annular solution from the outer tube forms the shell of the resultant fibers. Although it appears as if only a single nozzle is used, the method essentially involves simultaneous electrospinning from multiple coaxial nozzles of different diameters; hence, it is sometimes considered a subcategory of co-electrospinning. The special design of the coaxial electrospinning spinneret enables fiber formation from polymer solutions that are considered almost impossible to electrospin on their own . In this case, the shell fluid, which is usually the solution with better electrospinning capability, will assist in the entrainment of the core fluid to form core–shell fiber jets. Three-material core–sheath fibers are reported to have been successfully electrospun using triaxial nozzles . Other design variants include having multiple nozzles enclosed within one outer nozzle, producing fibers with multiple cores encapsulated by a sheath material .

Figure 8: Multichamber nozzles used to prepare multicomponent electrospun fibers.

Emulsion electrospinning is similar to blend electrospinning in principle, but instead of a single homogenous mixture, the emulsion is based on multiple phases that are not mixed during the process . To prepare an emulsion, two immiscible fluids, for example, oil and water are mixed with the use of stabilizers . Emulsion electrospinning is a potentially sustainable method of electrospinning as it limits the use of organic solvents. This method also allows for the fabrication of core–shell fibers from a standard nozzle, without the need of coaxial nozzles. Emulsion electrospinning is a promising method for fabricating nanofibers with advantages such as protection, controlled release, and high loading efficiency for food, pharmaceutical, and biomedical applications .

Table 3: Advantages and disadvantages of different electrospinning methods for multicomponent fiber fabrication.

Chitosan nanofibers offer numerous advantages, including biocompatibility, biodegradability, and similarity to the extracellular matrix (ECM) . Electrospinning pure chitosan without any other precursor material, however, presents significant challenges due to its inherent properties. Chitosan has high viscosity and low solubility in most solvents, and it tends to form gels, which complicates the electrospinning process. Repulsive forces between ionic groups due to high electric fields during electrospinning also tend to form discontinuous chitosan fibers . Solvents used in the electrospinning of pure chitosan include hydrochloric acid, acetic acid, formic acid, hexafluoro isopropanol (HFIP), trifluoroacetic acid (TFA), and dichloromethane (DCM). TFA is a good solvent to produce homogenous fibers, but it is a highly toxic organic substance . Because of the difficulties regarding the processing of chitosan, it is almost always mixed with another material prior to electrospinning, with polyethylene oxide (PEO) and PVA as popular precursor material candidates. Other disadvantages of chitosan nanofibers include low mechanical strength and poor solubility in water .

PVA is a good precursor material in electrospinning as it helps in improving the spinnability of fibers by reducing repulsive forces within charged polymer solutions . While PVA nanofibers offer advantages such as ease of fabrication and biocompatibility, applications of single-material PVA nanofibers are usually limited by their poor mechanical properties, high water solubility and hydrophilicity, and low thermal stability . The disadvantages often necessitate the combination of PVA with other materials or the application of post-processing techniques such as cross-linking or coating for performance improvement . As a countermeasure, Rafieian et al. proposed electrospinning only PVA, and preparing a separate film containing aloe vera and chitosan using a film applicator. The electrospun PVA was then layered on top of the aloe vera/chitosan film to improve mechanical properties. The chitosan/aloe vera film fabricated using this method, however, is in the form of a non-fibrous thin film; hence. it may not suit applications that highly depend on the properties of a nanofibrous structure, such as large surface area-to-volume ratio or high permeability of the material. Similarly, Kang et al. fabricated chitosan-coated PVA nanofibers by coating the heat-treated PVA nanofibrous matrix with a chitosan solution to create a biomimetic nanofibrous wound dressing.

Electrospinning of chitosan/PVA blend nanofibers has been successfully reported by many researchers. In these studies, two of the most common solvents used to dissolve chitosan and PVA are water and acetic acid . Thien et al. prepared chitosan and PVA solutions separately, dissolving chitosan in acetic acid and PVA in water. The two solutions were then subsequently mixed to form a homogenous solution for electrospinning . The method of mixing the two solutions can also be adopted if the same solvent is used for the two materials, as reported by and with acetic acid or water as the only solvent, respectively. Vu et al. prepared a solution by directly mixing chitosan and different concentrations of PVA in a single solvent system of acetic acid/water.

The use of multiple nozzles to fabricate chitosan/PVA nanofibers are also reported, but these studies involved electrospinning of chitosan and PVA together as a single blend from one nozzle, while the other nozzle contained different materials such as polycaprolactone (PCL) . There are currently no documented studies reporting the simultaneous electrospinning of chitosan and PVA from separate nozzles, likely because of the good compatibility of these two materials in forming blends. Co-electrospinning of chitosan and PVA may complicate the process, since it is difficult to electrospin chitosan on its own. There have been reports of co-axial electrospinning of chitosan/PVA, although additional materials are usually added to aid the electrospinning of chitosan. Zhu et al. used a chitosan/PCL blend as the sheath material and PVA as the core material for the production of guided bone generation membranes. Kuo et al. added small amounts of gum arabic to significantly reduce the viscosity of the chitosan shell solution and to ease the electrospinning process while maintaining high chitosan contents. Interestingly, Chen et al. fabricated trilayered fiber membranes via the combination of sequential, multinozzle, and blend electrospinning. The resultant composite was made up of chitosan fibers as the bottom layer, co-electropun chitosan and PVA fibers as the middle layer, and blended PVA/nanobioglass fibers as the top layer.

Table 4: Methods and solvents for electrospinning of chitosan/PVA nanofibers from the literature.

Electrospun nanofibers possess unique mechanical properties, which depend on their material composition, fiber diameter, and structural arrangement. The mechanical properties of PVA/chitosan-based nanofibers, such as tensile strength, elasticity, and elongation at break, are crucial for determining their suitability for various applications.

Figure 9: (a) Paper frame with test specimen, tesile test (b) before start and (c) during test. Reprinted from Composites Part B: Engineering, vol. 166, by E. Maccaferri; L. Mazzocchetti; T. Benelli; A. Zucchelli; L. Giorgini, “Morphology, thermal, mechanical properties and ageing of Nylon 6,6/graphene nanofibers as Nano2 materials”, p. 120-129, Copyright (2019), with permission from Elsevier. This content is not subject to CC BY 4.0.

Tensile properties measured by UTM include ultimate tensile strength, Young’s modulus, and elongation at break . Ultimate tensile strength is the maximum stress the specimen can withstand before rupturing. Young's modulus (or elastic modulus) is a measurement of elasticity, which describes the stiffness of nanofibers and is calculated as the ratio of stress to strain in the elastic deformation region, which is the slope of the initial li