**2.7. Extraction**

Nanofibers can be extracted from natural materials using chemical and mechanical treatments. Cellulose fibrils can be disintegrated from plant cell walls. In one example, cellulose nanofibers were extracted from wheat straw and soy hull with diameters ranging from 10 to 120 nm and lengths up to a few thousand nanometers (Fig. 7) [54]. Invertebrates have also been used as a source for the extraction of nanofibers. Chitin nanofibers 3−4 nm in diameter and a few micrometers in length were extracted from squid pen and Poly-N-acetyl glucosamine nanofibers isolated from a marine diatom demonstrated prothrombotic interactions with red blood cells [55,56].

**Figure 7.** Images of natural wheat straw [57], wheat straw microfibers [54] after chemical treatment and wheat straw nanofibers [54] after chemical treatment.

### **2.8. Conventional chemical oxidative polymerization of aniline**

Chemical oxidative polymerization of aniline is a traditional method for synthesizing polyaniline and during the early stages of this synthesis process polyaniline nanofibers are formed (Fig. 8). Optimization of polymerization conditions such as temperature, mixing speed, and mechanical agitation allows the end stage formation of polyaniline nanofibers with diameters in the range of 30–120 nm [58,59].

**Figure 8.** Schematic showing the nucleation of polyaniline nanofibers [59]. (I) Under non-ideal nucleation conditions aggregate formation is present. (II) When ideal nucleation conditions are predominant, well-dispersed polyaniline nanofibers are formed. Typical images of the reaction vials and microstructure are displayed next to the schematic.

#### **2.9. Bacterial cellulose**

294 Optical Communication

**2.7. Extraction** 

interactions with red blood cells [55,56].

wheat straw nanofibers [54] after chemical treatment.

**2.6. Vapor-phase polymerization** 

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

Polymer MNFs have also been fabricated from vapor-phase polymerization. Plasmainduced polymerization of vapor phase vinyltrichlorosilane produced organosiloxane fibers with diameters around 25 nm and typical lengths of 400–600 nm and cyanoacrylate fibers with diameters from 100 to 400 nm and lengths of hundreds of microns (Fig. 6) [52,53].

**Figure 6.** (a) Schematic describing a proposed mechanism for nanofiber formation by vapor-phase polymerization (b) Arial (1) and side views (2) of polymer nanofibers fabricated from vapor-phase

Nanofibers can be extracted from natural materials using chemical and mechanical treatments. Cellulose fibrils can be disintegrated from plant cell walls. In one example, cellulose nanofibers were extracted from wheat straw and soy hull with diameters ranging from 10 to 120 nm and lengths up to a few thousand nanometers (Fig. 7) [54]. Invertebrates have also been used as a source for the extraction of nanofibers. Chitin nanofibers 3−4 nm in diameter and a few micrometers in length were extracted from squid pen and Poly-N-acetyl glucosamine nanofibers isolated from a marine diatom demonstrated prothrombotic

**Figure 7.** Images of natural wheat straw [57], wheat straw microfibers [54] after chemical treatment and

polymerization at high (b), intermediate (c) and low (d) packing densities [52].

can be controlled as a function of parameters such as melt time and temperature [51].

Cellulose nanofibers produced by bacteria have been long used in a variety of applications, including biomedical applications [60]. Cellulose synthesis by Acetobacter involves the polymerization of glucose residues into chains, followed by the extracellullar secretion, assembly and crystallization of the chains into hierarchically composed ribbons (Fig. 9). Networks of cellulose nanofibers with diameters less than 100nmare readily produced, and fibers with different characteristics may be produced by different strains of bacteria [49]. Copolymers have been produced by adding polymers to the growth media of the cellulose producing bacteria [50,51].

#### **2.10. Kinetically controlled solution synthesis**

Nanofibers and nanowires have been fabricated in solution using linear aligned substrates as templating agents such as iron-cation absorbed reverse cylindrical micelles and silver nanoparticles [61]. Poly(vinyl alchohol)-poly(methyl methacrylate) nanofibers were fabricated using silver nanoparticle that were linearly aligned in solution by vigorous magnetic stirring (Fig. 10) [62]. These nanoparticle chain assemblies acted as a template for further polymerization of nanofibers with diameters from 10 to 30 nm and lengths up to 60 μm.

**Figure 9.** (a) Schematic of Acetobacter cells depositing cellulose nanofibers, (b) an SEM image of a cellulose nanofiber mesh produced by bacteria [60].

**Figure 10.** (a) Schematic of silver nanoparticle embedded polymer nanofibers fabrication (b) SEM and TEM images of silver nanoparticle embedded Polymer nanofibers [62].

### **3. Polymer MNF based photonic components and devices**

Optical fiber based components and devices have been very successful in the past 30 years and will surely continue to thrive in a variety of applications including optical communications, optical sensing, power delivery and nonlinear optics [63-65]. With increasing requirements for higher performance, wider applicability and lower energy consumption, there is a strong demand for the miniaturization of fiber-optic components or devices. When operated on a smaller spatial scale, a photonic circuit can circulate, process and respond to optical signals on a smaller time scale. Only at wavelength or subwavelength size does the photonic structure manifest evident near-field features that can be utilized for interlinking and processing optical signals highly efficiently. For example, it was estimated that to reach an optical data transmission rate as high as 10 Tb/s, the size of photonic matrix switching devices should be reduced to 100-nm scale [66]. At the same time, to perform a given function that relies on a certain kind of light-matter interaction, usually less energy is required when smaller quantities of matter are involved. MNFs featured at subwavelength scale, provide a number of interesting properties such as mechanical flexibility, high optical cross-sections, large and ultrafast nonlinear responses, and broad spectral tunability that are highly desirable for functionalizing high density and miniaturized PICs [67]. Besides its low cost and tiny size, MNFs are also easy to couple with other general optical devices, providing excellent compatibility with standard optical fiber systems.This section gives an up-to-date review of polymer MNF based photonic components/devices that have been investigated very recently.
