Nanofibres fibres

Electrospinning (solvent) Long, continuous nanofibres Fibre diameters are quite uniform Wide range of fibre diameters possible Needleless systems have high throughput Small pore size Uniform fibres with diameters <50 nm difficult Needle systems, low productivity Jet instability Solvent recovery issues Low mechanical properties... 

Figure 7.5 shows, for the same period, the relative number of recent patents per fibre type. Nanotubes and nanocomposites, particularly carbon nanotubes, are generating intense research activity whereas research is definitely weaker for nanofibres. Figure 7.6 shows, for the same period, the recent patents for the different nano-reinforcements. 

Figure 37 Electron micrographs of self-assembled fibres, (c) Electron micrograph showing the dense network of long helical fibres in a steroid/cyclohexane organogel, (d) Preparation of single fibres, demonstrating their helical superstructure [425]. (e) Cyclic oligopeptides, selfassembling into nanofibres [435,436], Reproduced with permission of The Royal Society of Chemistry and The American Chemical Society. Parts (c) and (d) 1986 American Chemical Society...

Teo WE, Ramakrishna S (2005) Electrospun fibre bundle made of aligned nanofibres over two fixed points. Nanotechnology 16(9) 1878-1884... 

The formation of filamentous carbon deposits on transition metal catalysts (Fe, Co, Ni) and their alloys have been investigated in some detail over the past two decades.21,38-40 Among them, nickel is the most promising candidate since it forms carbon deposits at temperatures as low as 723-823 K using CH4, C2H6 or CO + H2 feeds. Carbon fibres are usually produced during these reactions. Typical forms of the carbon produced from CH4 decomposition on silica-supported Ni catalysts are shown in Fig. 7.1. The pyrolysis of methane at temperatures somewhat lower than 873 K produces fish-bone type nanofibres.41 The Ni metal particles are present at the tip of each carbon fibre, and catalyse methane decomposition as well as growth... 

The present work indicated that titania fibers were produced successfully by the titania suspension at 18KV at distance of 8cm of the electric field. Although majority of the fibres were in mirco, there are some nanofibre in the midst of the web. 

Pyrolysis of hydrocarbons (e.g. benzene, acetylene, naphthalene, ethylene, etc.) in the presence of catalysts (e.g. Co, Ni and Fe deposited on substrates such as silicon, graphite or silica) provides an additional route to fullerenes and carbon nanotubes. Prior to the discovery of fullerenes in 1985, pyrolytically grown nanofibres/nanotubes had actually been observed and structurally identified by several authors [74, 89-91]. Even at that early stage, a growth mechanism was postulated involving metal (catalyst) particles, which were held to be responsible for the agglomeration of carbon and subsequent axial growth of the fibre. 

Nanofibres can be produced by a number of methods. They can be extracted from natural materials (e.g., cellulose or protein fibres) via physical separation and/or chemical extraction. They can also be produced by means of drawing, template synthesis, phase separation, self-assembly and electrospinning. The details are briefly described below. 

Drawing is a process similar to dry-spinning, in which nanofibres are drawn slowly from the droplet of a polymer solution by a micropipetter. The polymer solution is made from a viscoelastic material (i.e., one exhibiting both viscous and elastic characteristics upon deformation) that can accommodate the extensive deformation caused by drawing and retain the integrated form of an ultrafine fibre (Ondarcuhu and Joachim, 1998). 

Self-assembly is an intricate technique to build nanofibres from small molecules or polymers into bricks. There are various patterns in which molecules are assembled into nanofibres. For example, a designed amphiphile (i.e., a molecule with both hydrophilic and hydrophobic blocks and properties) can be induced to self-assemble into a cylindrical nanosized fibre. It can be seen that, at any cross section, the amphiphiles have... 

In general, textiles used in wound-dressing products include fibres, nanofibres, filaments, yarns, and woven/knitted/non-woven and composite materials. 

Nanofibre mats are produced by the electrospinning process as a result of the potential gradient between needle tip and collector. Various polymers have been successfully electrospun into ultrafine fibres mostly from solvent solution and some in melt form... 

Textile materials can be used in moist wound management as fibres themselves (advanced fibres such as alginate and chitosan fibres), or conventional/advanced fibres can be modified or coated with various substances such as honey or hydrogels to obtain special properties such as ultra-absorbency, drag release, etc. In general, textiles used in wound-dressing products come in all possible forms, including fibres, nanofibres, filaments, yarns, and woven/knitted/non-woven and composite materials. 

The electrospinning process was established in 1934 when the first patent on electrospinning was filed (Formhals, 1934). Nanofibres based on polymeric materials can be produced featuring a wide range of dimensions. The diameter of the fibre can range from 10 up to several hundred nanometres. Electrospinning can be carried out... 

In a more recent work, MWNTs have been incorporated into surface-modified, reactive P(St-co-GMA) nanofibres by electrospinning. Then resulting nanofibres have been functionalised with epoxide groups and added to the epoxy matrix producing reinforced epoxy resins. The polymer composites have demonstrated over a 20% increase in flexural modulus, when compared with neat epoxy, despite a very low composite fibre weight fraction (at approximately 0.2% by a single-layer fibrous mat). The increase is attributed to the combined effect of the well-dispersed MWNTs and the surface chemistry of the electrospun fibres that enabled an effective cross-linking between the polymer matrix and the nanofibres. 

Electrospinning of PLA and copolymers [12] and the PLA stereocomplex [145,146] has been used to prepare nanofibres or nanomats. Similar to the effect of pores on the mechanical properties of PLA-based materials, the formation of nanomats effectively lowers the E and elevates the 8b value. The morphology of the fibres and nanomats and the mechanical properties are affected by the applied voltage, the effluent rate and concentration of the solution, the type of solvent and so on (Figure 8.12 [147]). 

Due to its ease of implementation, electrospinning has received a lot of attention as a technique to produce nanoflbres [83]. When the diameter of polymer fibre materials shrinks from the microscale to the submicro or nanoscale, several new characteristics appear, such as enhanced surface area-to-volume ratio and a superior mechanical performance [84]. Therefore, biopolymer nanofibrous mats show great potential to be used as particle filters, nanocomposite reinforcing fibres, protective clothing and in biomedical applications like wound dressings, sutures, tissue engineering scaffolds, implantable devices and drug delivery [83-85]. 

Figure 12.5 Scanning electron micrograph of as-spun SELP-47K fibres (a) and representative tensile stress-strain curves (b) of MeOH- (1), CPA- (2), and MeOH-GTA-treated (3) SELP-47K nanofibrous scaffold. Scale bar 5 xm. (Adapted with permission from 164], Copyright 2010, American Chemical Society and [56], Copyright 2011 American Institute of Physics.)...

Qin et have studied the filtration characteristics of nanofibre layers with different area densities that were electrospun onto spunlaid or meltblown substrates. The fibre diameter, pore diameter, filtration efficiency, and filtration resistance of nanofibre webs and sublayers were determined. It was found that the pore diameter of nanofibre web is much smaller than sublayers and the coefficient of variation of the pore diameter of nanofibre webs is also smaller than that of the sublayers. Consequently, the filtration efficiency and filtration resistance of sublayers are lower than the nanofibre webs. These researchers also found that there is an optimum region for the maximum filtration efficiency at minimum pressure drop and that this optimum is at a lower add-on weight for meltblown webs than for spunlaid webs. 

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