Nanofibers

What is Nanotechnology ?The naive and direct answer to the frequently posed question what exactly is Nanotechnology is to say that it is a technology concerning processes which are relevant to physics, chemistry and biology taking place at a length scale of one divided by 1000 million (billion) of a meter. Thus, 1 nanometer = 10–9 meter

• An obvious phenomenon is the remarkably large surface-to-volume ratio of nanomaterials. Consider a fiber with radius of 1 mm and length of 10 mm. Its surface area is:

• S=2rL=2 *10-3*10-2=2 *10-5 m2 • Now we divide the fiber into nanofibers with radius of 10

nm. The number of nanofibers can be calculated as: • n= r2/rₒ2=10-6/10-16=1010 • So the total surface of the nanofibers is: • S= 2 nrₒL= 2 *1010*10-8*10-2= 2 m2

•The volume remains unchanged, while the surface area• increases remarkably by the factor:• Sₒ / S= 105

Nano fibersare defined as fibers with diameters on the order of 100 nanometers. See figure (1):

•Figure (1) Fiber diameter comparison

• The nano fibers can be produced by :• – Interfacial polymerization• – Drawing• – Phase Separation• – Self-Assembly• – Template synthesis• – Electro spinning

• 1) Interfacial Polymerization:- • In the interfacial polymerization technique the wall is formed

from monomers that are dissolved in the two separate phases (oil and water phase) and they polymerize at the interface of the emulsion droplets. For example, monomers such as diamine can be dissolved in the water and the aqueous phase is dispersed in the oil phase. The second monomer that is oil-soluble, for example diacid chloride, is then added at the interface forming the wall material. Different types of polymers may be produced by selecting different monomers but most publications refer to polyamide membrane and reacts with the first monomer

Figure 2. Polyaniline nanofiber synthesis. A. Interfacial polymerization occurs through an interfacial region (oil/water) yielding anisotropic polymer nanoparticles; this procedure is carried out at ambient conditions. The intrinsic morphology of the polymer is nanofibrillar due to homogeneously nucleated growth, the nanofibers form within seconds and can be purified by centrifugation. B. Transmission electron micrograph of polyaniline

nanofibers.

• 2) Drawing:- • In the drawing process, the fibers are fabricated by contacting a

previously deposited polymer solution droplet with a sharp tip and drawing it as a liquid fiber which is then solidified by rapid evaporation of the solvent due to the high surface area. The drawn fiber can be connected to another previously deposited polymer solution droplet thus forming a suspended fiber. Here, the predisposition of droplets significantly limits the ability to extend this technique, especially in free dimensional configurations and hard to access spatial geometries. Furthermore, there is a specific time in which the fibers can be pulled. Viscosity of the droplet continuously increases with time due to solvent evaporation from the deposited droplet. The continual shrinkage in the volume of the polymer solution droplet affects the diameter of the fiber drawn and limits the continuous drawing of fibers.

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• To overcome the above-mentioned limitation is appropriate to use hollow glass micropipettes with a continuous polymer dosage. It provides greater flexibility in drawing continuous fibers in any configuration. Moreover, this method offers increased flexibility in the control of key parameters of drawing such as waiting time before drawing (due to the required viscosity of the polymer edge drops), the drawing speed or viscosity, thus enabling repeatability and control on the dimensions of the fabricated fibers, figure (3).

• 3) Phase Separation:- • Nanofibrous foam materials have been fabricated by a

technique called thermally induced liquid-liquid phase separation. 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. 4). 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-lactic-co-glycolic acid (PLGA), and poly-DL-lactic acid (PDLLA) with fiber diameters from 50–500 nm, and porosities up to 98.5%.

Figure (4) a schematic (a) of nanofiber formation by phase separation, and an SEM image (b) of nanofibrous structure fabricated by this technique.

• 4) Self-Assembly:- • 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. Figure 5 shows the hierarchy of structure found in the self-assembled PA nanofiber networks. 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. Nanofibers with diameters around 5–25 nm can be formed by the self-assembly process.

• Cells can be encapsulated in a nanofibrous PA structure if 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.

• 5) Template Synthesis:- • 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 fabricated. Polymer nanofiber arrays can be released from these molds by destruction of the molds or mechanical detachment, Figure (6). The length of polycaprolactone (PCL) nanofibers fabricated from alumina templates can be controlled as a function of parameters such as melt time and temperature.

• 6) Electrospinning:- • Electrospinning is an electrostatically driven method of fabricating polymer

nanofibers. Nanofibers are formed from a liquid polymer solution or melt that is feed through a capillary tube into a region of high electric field. The electric field is most commonly generated by connecting a high voltage power source in the kilovolt range to the capillary tip (Fig. 7). 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 nanofibers on the target site. Nanofiber size and microstructure can be controlled by several processing parameters including: solution viscosity, voltage, feed rate, solution conductivity, capillary to collector distance, and orifice size. The electrospinning technique is very versatile and a wide range of polymer and copolymer materials 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 nanofibers are usually collected from an electrospinning jet as non-woven randomly or uniaxially aligned sheets or arrays.