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can be directly produced through nanofiber spinning process (e.g. electrospinning, centrifugal spinning), whereafter randomly stacked nanofiber fabrics can be collected. With the scale‐up of spinning capability, the commercialized production of nanofiber nonwoven fabrics is no longer a fantasy (Zhou and Gong 2008). Also, nanofiber nonwoven fabrics have been studied for many applications, such as filtrations, tissue engineering scaffolds, release control, catalyst, and enzyme carriers, sensors, and energy storage (Feng 2017; Zhu et al. 2016; Yanilmaz et al. 2016).

Nonwoven fabrics can be categorized into single layer and multilayers. Yanilmaz et al. prepared a thin silica/PAN nanofiber fabric as a separator for secondary batteries by electrospinning (Yanilmaz et al. 2016). Briefly, PAN was first dissolved in dimethylformamide (DMF) to form 10 wt% PAN solution. Meanwhile, tetraethyl orthosilicate (TEOS) was dissolved in DMF and HCl solution (37 wt% in water) for generating SiO₂. Then TEOS solution was added to the PAN solution to obtain a homogeneous precursor followed by electrospinning into nanofibers with a flow rate of 0.75 ml/h, a voltage of 16 kV, and a tip‐to‐collector distance of 25 cm. PAN nanofiber nonwoven separator possessed higher porosity and better wettability compared with traditional microporous polyolefin membranes. The addition of SiO₂ into the nanofiber nonwoven was for giving separators higher electrochemical oxidation limit, larger liquid electrolyte uptake, and lower interfacial resistance. The thickness of electrospun nanofiber fabrics was about 65 μm. With the increment of SiO₂, the diameter of electrospun nanofibers was decreased from 324 to 308, 246, and 187 nm when the proportion of SiO₂ was adding to 16, 19, and 27 wt%, while the tensile strength decreased from 4.5 to 3.5 MPa. Hassan et al. fabricated self‐bonded nonwoven fabrics directly from polymer resins by melt blowing process, shown in Figure 2.12 (Hassan et al. 2013). The resin was first poured into the extruder and melt with the increment of heating temperature. When the molten was squeezed from the spinneret, the high‐velocity air was conducted and the drag force stretched the fiber rapidly, which reduced the fiber diameter to 0.5–10 μm. Fibers were then gathered on the drum collector to form nonwoven fabrics.

There are also multilayer fabrics, which attach nanofiber nonwoven onto standard fabrics to achieve higher strength or for specialized applications. Lee et al. incorporated an electrospun PU fiber layer onto a PP nonwoven for use in protective clothing systems for agricultural workers (Lee and Obendorf 2007a). In this work, a nonwoven PP layer was used as a substrate and electrospun PU fibers were made from PE‐based thermoplastic PU dissolved in DMF. As shown in Figure 2.13, nanofibers with the average diameter of around 300 nm were covered onto the substrate. Nanofiber nonwoven fabrics were often used as protective textile barriers for preventing liquid penetration (Lee and Obendorf 2007b). Bagherzadeh et al. combined electrospun nanofibers with woven sheets to develop a water repellent breathable fabric. PAN was dissolved in DMF and electrospun to form a nanofiber layer, which was placed between two woven sheets to form a sandwich structure (Bagherzadeh et al. 2011).

Figure 2.12 Melt blowing process for producing nanofiber nonwovens.

      Source: Hassan et al. (2013).

Figure 2.13 Layered fabric structure containing electrospun PU nanofibers.

      Source: Reproduced with permission from Lee and Obendorf (2007b). Copyright 2007, Springer.

      2.4.2 Nanofibrous Woven Fabrics

      Based on the nanofiber yarn formation, nanofiber woven fabrics are also a promising trend, especially for the future textile industry. They have numerous potential applications, including but not limited to artificial leather, filters, wiping cloths, bone tissue engineering, etc. (Zhou and Gong 2008). Nanofiber fabrics with plain‐woven structures have been achieved in recent years.

Shao et al. wove a scaffold from nanofiber yarns of blended PLA and tussah silk fibroin (TSF) to achieve good biocompatibility (Figure 2.14) (Shao et al. 2016). First, nanofiber yarns were obtained by electrospinning and winding processes. Then, the warp and weft nanofiber yarns were interwoven vertically into a single‐layer woven fabric, three layers of which were then jointed into a three‐dimensional (3D) woven structure. This 3D woven structure demonstrated excellent mechanical properties (Young's modulus of 417.65 MPa and tensile strength of 180.36 MPa). In the structure, warp, and weft densities were 300 root/10 cm and 500 root/10 cm, respectively, the entire thickness was 2.0 ± 0.1 mm. Because of the use of PLA and TSF, this structure also exhibited biomimetic architecture and good biocompatibility.

Zhao et al. applied AgNPs nanofiber yarns into a 3D woven structure by the process shown in Figure 2.15 (Zhao et al. 2017). Nylon‐6,6 yarns were placed on the collecting drum. The precursor containing AgNPs and 8 wt% of PAN was then electrospun onto nylon‐6,6 yarns. Subsequently, nylon‐6,6 monofilament was used as warp yarn and Z yarn, and nylon‐6,6 draw textured yarn with silver ion as the weft yarn to make 3D woven filters. Antibacterial testing was also carried out by incubating the obtained 3D woven structure in the Staphylococcus aureus cultural gelation for 24 hours. It was observed that there was blank space around the AgNPs nanofiber‐contained 3D woven structure, indicating the antimicrobial property of the product was ideal for the application of wastewater filtration.

Figure 2.14 (a) Photograph of PLA/TSF nanofiber woven fabric; SEM images of (b) the surface of the fabric, (c) cross section of the fabric, and (d) cross section of the yarns in the fabric.

      Source: Reproduced with permission from Shao et al. (2016). Copyright 2016, Elsevier.

Figure 2.15 Illustration of the fabrication process for the 3D woven fabric filter with AgNPs/PAN nanofiber wrapped yarns.

      Source: Reproduced with permission from Zhao et al. (2017). Copyright 2017, Elsevier.

Figure 2.16 (a) The weaving of the electrospun nanofibers in succession (from 1 to 4), (b) Schematic image of the overall deposition pattern.

      Source: An et al. (2016).

      (c, d) SEM images of the deposited PAN fibers.

      Source: Reproduced with permission from An et al. (2016). Copyright 2016, Elsevier.

Interestingly, a single nanofiber can also be fabricated

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