**2.1. Electrospinning**

Electrospinning is an electrostatically driven method of fabricating polymer nanobers. Nanofibers are formed from a liquid polymer solution or melt that is feed through a capillary tube into a region of high electric field [39]. The electric field is most commonly generated by connecting a high voltage power source in the kilovolt range to the capillary tip (Fig. 1). As electrostatic forces overcome the surface tension of the liquid, a Taylor cone is formed and a thin jet is rapidly accelerated to a grounded or oppositely charged collecting target. Instabilities in this jet cause violent whipping motions that elongate and thin the jet allowing the evaporation of some of the solvent or cooling of melts to form solid nanobers on the target site. Nanober size and microstructure can be controlled by several processing parameters including: solution viscosity, voltage, feed rate, solution conductivity, capillary to collector distance, and orice size [40]. The electrospinning technique is very versatile and a wide range of polymer and copolymermaterials with a wide range of fiber diameters (several nanometers to several microns) can be fabricated using this technique. Many different types of molecules can be easily incorporated during the electrospinning fabrication process to produce functionalized nanofibers. Electrospun nanobers are usually collected from an electrospinning jet as non-woven randomly or uniaxially aligned sheets or arrays.

### **2.2. Phase separation**

Nanofibrous foam materials have been fabricated by a technique called thermally induced liquid-liquid phase separation [41]. This fabrication procedure involves (a) the dissolution of polymer in solvent (b) phase separation and polymer gelatination in low temperature (c) solvent exchange by immersion in water and (d) freezing and freeze-drying (Fig. 2). The morphology of these structures can be controlled by fabrication parameters such as gelatination temperature and polymer concentration. Interconnected porous nanofiber networks have been formed from polymers such as, poly-L-lactide acid (PLLA), poly-lacticco-glycolic acid (PLGA), and poly-DL-lactic acid (PDLLA) with fiber diameters from 50–500 nm, and porosities up to 98.5%.

**Figure 1.** (a) schematic of a standard electrospinning setup [39] and a scanning electron microscope (SEM) image (b) of electrospun polyurethane nanofibers.

**Figure 2.** A schematic (a) of nanofiber formation by phase separation [42], and an SEM image (b) of nanofibrous structure fabricated by this technique [41].

## **2.3. Self assembly**

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nanofiber architecture [38].

**2.1. Electrospinning** 

arrays.

**2.2. Phase separation** 

Polymer MNFs with diameter down to several nanometers and length up to hundreds of millimeters have been reported. Besides its low cost and tiny size, polymer MNFs are easy to couple with other general optical devices, enabling them to fulfill their potentials in optical functions. Due to their good flexibility and large tunability, many shapes and structures can be achieved from polymer MNFs, including optical splitters [18,30], resonator [31], couplers [32], Mach–Zehnder Interferometers (MZI) [33], light-emitting polymer nanofibers [34], photodetector [35], organic nanofiber laser [36], sensors [37,38], polymeric

In this chapter, we will introduce the fabrication techniques of polymer MNFs. Then we will focus on polymer MNF-based elements and assess their potential used as passive and active

A number of different techniques including physical, chemical, thermal, and electrostatic method have been used to fabricate polymer MNFs. Ten of these fabrication methods are

Electrospinning is an electrostatically driven method of fabricating polymer nanobers. Nanofibers are formed from a liquid polymer solution or melt that is feed through a capillary tube into a region of high electric field [39]. The electric field is most commonly generated by connecting a high voltage power source in the kilovolt range to the capillary tip (Fig. 1). As electrostatic forces overcome the surface tension of the liquid, a Taylor cone is formed and a thin jet is rapidly accelerated to a grounded or oppositely charged collecting target. Instabilities in this jet cause violent whipping motions that elongate and thin the jet allowing the evaporation of some of the solvent or cooling of melts to form solid nanobers on the target site. Nanober size and microstructure can be controlled by several processing parameters including: solution viscosity, voltage, feed rate, solution conductivity, capillary to collector distance, and orice size [40]. The electrospinning technique is very versatile and a wide range of polymer and copolymermaterials with a wide range of fiber diameters (several nanometers to several microns) can be fabricated using this technique. Many different types of molecules can be easily incorporated during the electrospinning fabrication process to produce functionalized nanofibers. Electrospun nanobers are usually collected from an electrospinning jet as non-woven randomly or uniaxially aligned sheets or

Nanofibrous foam materials have been fabricated by a technique called thermally induced liquid-liquid phase separation [41]. This fabrication procedure involves (a) the dissolution of polymer in solvent (b) phase separation and polymer gelatination in low temperature (c)

components in miniaturized photonic devices. Final is a perspective.

**2. Fabrication methods of polymer MNFs** 

briefly described in the following section [29].

Self assembly is a process by which molecules organize and arrange themselves into patterns or structures through non-covalent forces such as hydrogen bonding, hydrophobic forces, and electrostatic reactions. Dialkyl chain amphiphiles containing peptides were developed to mimic the ECM. These peptide amphiphiles (PA), derived from a collagen ligand, allow for a self assembling system that consists of a hydrophobic tail group and a hydrophilic head group [43]. Figure 3 shows the hierarchy of structure found in the selfassembled PA nanofiber networks [44]. The specific composition of amino acid chains in peptide amphiphile systems determines the assembly, chemical, and biological properties of the system, and therefore PA systems can be tailored to specific applications [45,46]. Nanofibers with diameters around 5–25 nm can be formed by the self assembly process. Cells can be encapsulated in a nanofibrous PA structureif they are added during the self assembly process and PA can also be injected in vivo where they subsequently self assemble into a nanofibrous network. It has been demonstrated that self assembled peptide nanofibers can spontaneously undergo reassembly back to a nanofibers scaffold after destruction by sonication, and after multiple cycles of destruction and reassembly, the peptide nanofibers scaffolds were still indistiquishable from their original structures [47].

**Figure 3.** Schematics of the (a) molecular structure and (b) nanostructure, and images of the (c) micro and macro structure of a self assembling peptide amphiphile nanofiber network [44].

## **2.4. One-step drawing technique**

Nanofibers can be mechanically drawn from viscous polymer liquids directly [48]. In one example, nanofibers were drawn directly when a rod was placed in a polymer melt and moved up forming a thin filament that cooled to form a nanofiber [17]. Figure 4 shows the schematic illustration of the drawing process. Fig. 4a shows a vertical direction tip-drawing process. Poly(trimethylene terephthalate) (PTT) pellets (melt temperature Tm = 225°C) was melt by a heating plate and thetemperature was kept at around 250°C during the wire drawing. First, an iron or silicarod/tip with radius of about 125 μm is being approached and its tip is immerged into the molten PTT. Then the rod tip is retracted from the molten PTT with a speed of 0.1−1 m/s, leaving a PTT wire extending between the molten PTT and the tip. The extended PTT wire is quickly quenched in air and finally, a naked amorphous PTT nanowire is formed. PTT nanofibers with diameters as low as 60 nm, and lengths up to 500 mm have been achieved. A SEM image (Fig. 4b) shows part of the coiled nanofiber with a length of about 200 mm and an average diameter of 280 nm. The diameter variation ratio is about 8.410-8. Figure 4c demonstrates flexible and elastic connection by pulling the polymer nanofibers with diameters of 140 and 170 nm. To examine surface roughness of the polymer nanofibers, high-magnification transmission electron microscope (TEM) was done. Figure 4d shows a TEM image of a 190-nm-diameter nanowire, indicating no visible defect and irregularity on the surface of the polymer nanofiber. Typical average sidewall root-meansquare roughness of the polymer nanofiber is 0.28 nm. The electron diffraction pattern (inset of Fig. 4d) demonstrates that the obtained PTT MNF is amorphous. The results demonstrate that the obtained polymer nanofibers exhibit high surface smoothness, length uniformity, high mechanical properties, and excellent flexibility.

**Figure 4.** Schematic of nanofiber fabrication by the drawing technique. (b) SEM images of a nanofiber with average diameter of 280 nm coiled on a 12-μm-diameter PTT bending rod, the length of the nanofiber displayed is about 200 mm. (c) Flexible and elastic enough nanofiber connection with diameters of 140 and 170 nm. (d) Transmission electron microscope (TEM) image of a 190-nm-diameter fiber. The insetshows its electron diffraction pattern [17,18].

**Figure 5.** (A) Schematic of the fabrication of polymer nanofibers using a nondestructive templating technique (grey: alumina template, green: resin, blue: polymer nanofibers, pink: silica replica template. (B-E) SEM images of 120 nm (b&c) and 1μm (d&e) polymer fibers fabricated by the above technique [49].

### **2.5. Templating**

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the system, and therefore PA systems can be tailored to specific applications [45,46]. Nanofibers with diameters around 5–25 nm can be formed by the self assembly process. Cells can be encapsulated in a nanofibrous PA structureif they are added during the self assembly process and PA can also be injected in vivo where they subsequently self assemble into a nanofibrous network. It has been demonstrated that self assembled peptide nanofibers can spontaneously undergo reassembly back to a nanofibers scaffold after destruction by sonication, and after multiple cycles of destruction and reassembly, the peptide nanofibers scaffolds were still indistiquishable from their original structures [47].

**Figure 3.** Schematics of the (a) molecular structure and (b) nanostructure, and images of the (c) micro

Nanofibers can be mechanically drawn from viscous polymer liquids directly [48]. In one example, nanofibers were drawn directly when a rod was placed in a polymer melt and moved up forming a thin filament that cooled to form a nanofiber [17]. Figure 4 shows the schematic illustration of the drawing process. Fig. 4a shows a vertical direction tip-drawing process. Poly(trimethylene terephthalate) (PTT) pellets (melt temperature Tm = 225°C) was melt by a heating plate and thetemperature was kept at around 250°C during the wire drawing. First, an iron or silicarod/tip with radius of about 125 μm is being approached and its tip is immerged into the molten PTT. Then the rod tip is retracted from the molten PTT with a speed of 0.1−1 m/s, leaving a PTT wire extending between the molten PTT and the tip. The extended PTT wire is quickly quenched in air and finally, a naked amorphous PTT nanowire is formed. PTT nanofibers with diameters as low as 60 nm, and lengths up to 500 mm have been achieved. A SEM image (Fig. 4b) shows part of the coiled nanofiber with a length of about 200 mm and an average diameter of 280 nm. The diameter variation ratio is about 8.410-8. Figure 4c demonstrates flexible and elastic connection by pulling the polymer nanofibers with diameters of 140 and 170 nm. To examine surface roughness of the polymer nanofibers, high-magnification transmission electron microscope (TEM) was done. Figure 4d shows a TEM image of a 190-nm-diameter nanowire, indicating no visible defect and irregularity on the surface of the polymer nanofiber. Typical average sidewall root-meansquare roughness of the polymer nanofiber is 0.28 nm. The electron diffraction pattern (inset of Fig. 4d) demonstrates that the obtained PTT MNF is amorphous. The results demonstrate

and macro structure of a self assembling peptide amphiphile nanofiber network [44].

**2.4. One-step drawing technique** 

Polymer nanofibers can be fabricated using templates such as self-ordered porous alumina. Alumina network templates with pore diameters from 25 to 400 nm, and pore depths from around 100 nm to several 100 μm have been be fabricated. Polymer nanofiber arrays can be released from these molds by destruction of the molds or mechanical detachment (Fig. 5) [49,50]. The length of polycaprolactone (PCL) nanofibers fabricated from alumina templates can be controlled as a function of parameters such as melt time and temperature [51].
