By Wei, Qufu Huang, Fenglin; Cai, Yibing; Gao, Weidong.
Polymer nanofibres have great potential for applications ranging from filtration, medical, tissue engineering, environmental protection and electronic textiles. Nanofibres can be constructed into various forms and structures to meet the needs for a variety of applications, say Qufu Wei, Fenglin Huang,Yibing Cai and Weidong Gao. Nanotechnology is the creation of functional materials, devices and systems through the control of matter on the nanometre (1 to 100+ nm) length scale, as well as the exploitation of novel properties and phenomena developed at that size. And there are a number of reasons why this length scale is so important:
* nanoscale components have large surface areas, making them ideal for use in filtration, separation, composites, catalyst systems, medicine and energy storage;
* by manipulating matter on the nanometre length scale, it is possible to alter the physical and chemical properties a material possesses. Nanostructured materials can be made stronger, more flexible, more porous, more reactive or to have different optical properties-all features that can render them unique and useful;
* the use of nanotechnology for the preparation and treatment of fibres, polymers and fabrics can open new and attractive perspectives for the whole textile world. Fabrics with new or better features such as hydrophobic, antistatic, flame retardant antibacterial, stain resistance, and specific optical or photo- thermo chromatic behaviours, are only a few examples of the possibilities.
Nanotechnology offers the industry the potential to enter a new phase of the development of added-value textiles.
NANOFIBRES AND THEIR FORMATION
There are several different types of textile fibres formed by nanotechnology, subject to their nanostructures. Nanostructured fibres may be broadly grouped as:
* nanocomposite fibres; or
* nanocoated fibres.
Among various processing techniques, electrospinning is widely used to produce polymer nanofibres. Electrospinning is a process by which polymer nanofibres are produced from a solution or melt and involves the application of a strong electric field. If electrostatic forces applied overcome the surface tension of the polymer (solution or melt) a charged jet is ejected that moves towards a grounded electrode. The electrospun nanofibres can be collected on a substrate located on the grounded electrode.
An electrospinning device developed by the Key Laboratory of Eco- textiles, Jiangnan University.
For a variety of applications it is desirable to produce such nanofibres with well-defined structures. Such fibres are also of interest in many technical applications as their structures affect properties such as water adsorption, filtration, electrical conductivity and biocompatibility.
Nanofibres can be manipulated to form unique features-including gradient pores within the material and interconnected pores leading to structures with very large surface:volume ratios. Electrospinning offers a great flexibility in controlling a fibre's diameter, porosity and orientation to meet different needs. In addition to the control of nanofibre construction, the chemical composition of electrospun nanofibres can also be adjusted through the use of various polymers, blends or nanocomposites made of organic or inorganic materials.
Nanofibres with gradient structures.
Gradient structures exist in various materials found in nature; for instance, silkworm cocoons exhibit functionally gradient fibrous structures which help to provide an optimal balance of stiffness, protection, permeability and durability. Other animals and plants also demonstrate gradient structures.
Nanofibres with gradient structures can be produced by adjusting spinning settings, such as voltages, injection volumes and rotation speeds of the collector. The nanofibres deposit onto a collector to form a web having a pore gradient. And a nanofibre web containing a pore size gradient has great potential for a wide range of applications.
In filtration applications, nanofibre materials with gradient pores offer better capture properties than the conventional filter media since natural particles also have their own gradient sizes in most cases. The size of a pore in sorbent materials defines the capillary rise of the water column. Smaller pores will enhance wicking properties and liquid retention.
Tissue engineering relies extensively on the use of porous scaffolds to provide the appropriate environment for the regeneration of tissues and organs. Nanofibre scaffolds have been increasingly used for the regeneration of tissues or organs. The traditional scaffolds with a uniform composition and pore structure are not suitable for tissues and organs with gradient structures, therefore developing anisotropic pore architectures for instructing complicated cell and tissue growth is of importance in tissue- engineering.
Poly(l-lactide) - coarse fibres - and polyvinyl alcohol nanofibres - fine fibres - are prepared by dual electrospinning and leave a gradient pore structure that is useful in filtration and tissue engineering uses.
Fibrous materials are typically porous materials used as sorbents, filters, coalescing media and in medical uses. Porous fibres have attracted a great deal of attention from scientists and engineers for a wide range of applications. Properties of porous materials are intimately related to their framework topological features and chemical compositions. Therefore, the development of porous materials with new compositions and topologies can lead to new applications or much improved properties for current applications.
Nanometre-scale fibres containing small pores can be synthesized from both natural and synthetic polymers. Porous nanofibres can be produced by using different solvents, and controlling the electrospinning conditions and environmental. These nanofibres have surface areas that are three to four orders of magnitude higher than conventional high specific surface materials.
The combination of porosity with flexibility in the porous nanofibres opens up more applications in many industries. Porous nanofibres be used as separators or solid electrolytes in batteries due to their open channels for ion movement. The new porous materials possess some structural features for filtration and separation. The unique structure of porous nanofibres will also expand technical applications in areas such as selective chemical conversion, solid support catalysts, membrane-supported smart materials, scaffolds for tissue/cell growth, and membranes for immobilizing biological and pharmacological active agents and molecules.
Aligned and oriented nanofibres
As described above, electrospinning uses a static electric field to draw a polymer solution or melt jet from an injection tip to a fibre collector. The driving force of the jet is provided by a high voltage source. Nanofibres are formed by this elongation action, which are typically collected in a random fashion on the grounded screen to obtain nonwoven structured materials. In some applications it would be advantageous to orient nanofibres in a certain direction to form aligned or oriented nanofibres.
Poly(l-lactide) porous nanofibres as viewed (left) under a scanning electron microscope (SEM) and (right) by an atomic force microscope (AFM)i.
Randomly oriented polyamide 6/montmorillonite nanofibres (left) and aligned polyamide 6/montmorillonite nanofibres (below).
Adjusting the flow rate of the polymer solution, the magnitude of the electric field and the rotation speed of the collector drum can help to obtain aligned or oriented nanofibres deposited on the collector. Aligned or oriented nanofibres have great potential in such applications as tissue engineering, high strength nanocomposites, electronic and sensing applications.
In tissue engineering,for instance, it has been found that cells grow preferentially in the direction of the fibre alignment on a fibrous scaffold. This is very useful in the construction of scaffolds in tissue engineering as some cells found in the body are aligned in certain tissues.
The alignment of nanofibres has also found a useful application in electronic textiles. Well-aligned nanofibre arrays can be constructed in the same direction or in several different directions. The aligned or oriented nanofibres have many prospective applications in electronic devices, such as sensors and circuit elements.
For a variety of applications it is also desirable to produce nanofibres with well-defined chemical structures and various techniques can be employed to achieve this. Using various polymers, polymer blends, or nanocomposites made of organic or inorganic materials are the most common approaches. Nanocomposites based on the combination of inorganic nanoparticles and organic polymers have attracted more attention since significant reinforcements (mechanical and thermal properties) and even novel properties can be achieved in this way. Nanoparticles can serve as matrix reinforcements, as well as to change the electrical, optical or biomedical behaviour of the base materials.
There is a great technical challenge in obtaining intimate blending of inorganic particles and polymer matrices. Various techniques have been developed to make composite nanofibres, such as melt-compounding, solution mixing, solgel, layer-by-layer deposition and physical and chemical vapour deposition. Workers in the Key Laboratory of Science & Technology at Southern Yangtze University have successfully applied sputter coatings of metals, metal oxides and polymers to make functional nanofibres. Conductive metals, photo- catalytic metallic oxides and bioactive materials have been used to make composite nanofibres. The resulting improvement in surface conductivity, optical properties and bioactivity of nanofibres opens up new possibilities for expanded applications in many industries.
Polyamide 6 nanofibres and polyamide 6/silver composite nanofibres observed with an atomic force microscope (AFM)i.
Electrospinning combined with other techniques provide new approaches to make nanofibre materials with well-defined structures. These specially designed nanofibre materials have great potential for a variety of applications.
J. Doshi and D.H. Reneker, Eiectrospinning process and applications of electrospun fibres. Journal of Electrostatics, 1995; Volume 35: pages 151-160.
W.J. Li, C.T. Laurencin, E.J. Caterson, R.S. Tuan and F.K. Ko, Electrospun nanofbrous structure: novel scaffold for tissue engineering, Journal of Biomedical Materials Research, 2002; Volume 60: pages 613-621.
N. Bunyan, I. Chen, J. Chen, S. Farboodmanesh and K. White, Control of deposition & orientation of electrospun nano-fbres, Fibre Society Symposium-(Abstracts), 2002; pages 36-37.
J.F. Hagewood, Polymeric nanofibres: fantasy or future, International Fibres journal, 2002; Volume 12: pages 62-63.
1 See also, Technical Textiles International, September 2006, Dynamic characterization of fibres using electron microscopy, pages 15-18;
Technical Textiles International, December 2005, The functionalization of the surfaces of polymer nanofibres, pages 21- 23;
Technical Textiles International, May 2005, Functionalization of textile fibres using plasma-based technology, pages 27-29;
Technical Textiles International, May 2004, Surface nanostructures seen by atomic force microscopy, pages 33-35;
The authors wish to thank the specialized Research Fund for the Doctoral Program of higher education (No. 20060295005) and the Program for New Century Excellent Talents in University (NCET-06- 0485) for their financial support.
Qufu Wei, Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi 214122, PR China. Tel: +86-510-8591-2007. Fax: +86-510-8591 -3200.
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