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Polymer webs on plants that surpass spider-webs in fineness, filters covered by a "whiff of nothing" (as once said by a renowned scientist) that increases their effectiveness immensely, wound dressings made of gossamer fibers carrying a medical agent which enable faster healing without formation of scale tissues, devices consisting of nanofibers that convert solar energy into electricity more efficiently and/or possess the capability of detecting trace amounts of hazard chemicals, extremely sensitive Raman directors made of nanofibers for the space-detection of minerals, microbes, and biomarkers, nanoscale continuous carbon fibers with superior mechanical strength that has yet been achieved by mankind: these are not fairy tales like "The Emperor’s New Clothes", but are examples from the rapidly growing field of "Electrospinning and Nanofibers".

The Materials-processing Technique of Electrospinning

The materials-processing technique of electrospinning provides a versatile approach for convenient preparation of polymer, ceramic, carbon/graphite, metallic, composite, and hierarchically-structured nanofibers with diameters in the range from nanometers to microns (commonly known as “Electrospun Nanofibers”). Electrospun nanofibers possess many extraordinary properties including small diameters and the related large specific surface areas, high degree of structural perfection and the resulting superior mechanical properties. Unlike nanowires, nanorods, and nanotubes, which are prepared by bottom-up synthetic methods, electrospun nanofibers are produced through a top-down nano-manufacturing process. Therefore, electrospun nanofibers are cost-effective; and they are also easy to align, assemble, and process into applications.

 

Electrospun Polymer Nanofibers and Their Applications

Many synthetic and natural polymers have been successfully electrospun into nanofibers. Polymer nanofibers can be prepared with various morphologies (e.g., cylinder-shaped, beaded, wrinkled, foamed, and ribbon-shaped). Different nano-fillers (e.g., layered silicates and carbon nanotubes) can be readily incorporated into polymer nanofibers with the fillers’ alignment along fiber axes. The non-woven mats/fabrics made of electrospun nanofibers offer unique capabilities to control the pore sizes among nanofibers. Additionally, polymer nanofibers can also serve as the templates for the preparation of various nanofibers and/or nanotubes. Consequently, electrospun polymer nanofibers have been of scientific, military, and commercial interests including, but not limited to, composites, filtration/separation, protective clothing, catalyses, agricultures, biomedical applications (e.g., tissue engineering and drug delivery), electronic applications (e.g., capacitors and transistors), and space applications.

 

Electrospun Ceramic Nanofibers and Their Applications

Ceramic nanofibers (e.g., SiO2, Al2O3, ZnO, and TiO2 nanofibers) can be prepared via electrospinning spin dopes containing their precursors into nanofibers followed by pyrolysis. For example, the precursor of TiO2, titanium tetraisopropoxide (Ti(OC3H7)4), can be co-electrospun with a carrying polymer of polyvinyl pyrrolidone using an ethanol/acetic acid mixture as the solvent.  Since titanium tetraisopropoxide can be rapidly hydrolyzed by the moisture in air, the networks (gels) of TiO2 can be formed in nanofibers during or shortly after electrospinning.  The organic components in the as-electrospun composite nanofibers can then be selectively removed via pyrolysis the samples in air at an elevated temperature, resulting in the formation of TiO2 nanofibers.  Additionally, by carefully controlling the gelation of Ti(OC3H7)4 aqueous solutions (e.g. through adjusting the pH value), TiO2 nanofibers can also be directly electrospun without the assistance of carrying polymer.

 

Electrospun Metallic Nanofibers and Their Applications

In general, electrospun metallic nanofibers can be prepared using polymer and/or ceramic nanofibers as the templates.  For example, the porous silver nanofibers can be prepared using the following two approaches: (1) It is known that polymers containing amidoxime functional groups (-C(NH2)=NOH) on the surface possess high metal-adsorption capacity.  Amidoxime groups can be introduced onto the surface of electrospun polyacrylonitrile (PAN) nanofibers through the treatment with aqueous hydroxylamine (NH2OH) solution.  The nitrile (-C≡N) groups on the surface of PAN nanofibers react with NH2OH and lead to the formation of amidoxime groups, which can be used for chelating silver ions upon immersion in AgNO3 aqueous solution.  The chelated silver ions can then be reduced by hydrazine (NH2-NH2) into elemental silver nanoparticles.  Removal of PAN can be conducted via careful sintering of silver nanoparticles at lower temperature than the silver’s melting point, which is possible because silver nanoparticles exhibit much lower melting points than bulk silver.  (2) Thiol groups can be introduced onto the surface of electrospun SiO2 nanofibers through the treatment with 3-mercaptopropyltrimethoxysilane (MPTMS).  It is known that thiol groups (R-SH) bind strongly with silver ions.  The thiol-functionalized nanofibers can be immersed in AgNO3 aqueous solutions to adsorb silver ions.  Reduction of silver ions to metallic silver nanoparticles can be achieved using hydrazine.  The sintering of silver nanoparticles to nanotubes can be carried out similar to that in Approach I.  Electrospun SiO2 nanofibers will retain the fiber morphology during the sintering of silver nanoparticles.  The SiO2 templates can then be removed using hydrofluoric acid (HF) solution to obtain porous silver nanofibers.  Porous silver nanotube networks (PSNNs) with controlled nanostructures and superior surface-enhanced Raman scattering (SERS) activity can be utilized for in situ Raman detection of minerals, microbes, and biomarkers.

 

Electrospun Carbon/Graphite Nanofibers and Their Applications

Carbon/graphite nanofibers are made by carbonization/graphitization of their precursors of electrospun polymer nanofibers (e.g., polyacrylonitrile nanofiber). Two types of carbon/graphite nanofibers can be developed including (Type 1) continuous, nano-scaled carbon fibers with superior mechanical strength, and (Type 2) highly graphitic, extremely porous graphite nanofibers with specific surface areas of up to 2500 m2/g. For example, continuous nano-scaled carbon fibers can be developed by stabilization and carbonization of highly aligned and extensively stretched electrospun polyacrylonitrile copolymer nanofiber precursor under optimal tension. These carbon fibers with diameters being tens of nanometers possess a superior mechanical strength which is unlikely to be achieved through conventional approaches. This is because (1) the innovative precursor, with fiber diameter approximately 100 times smaller than that of conventional counterparts, possesses an extremely high degree of macromolecular orientation and a significantly reduced amount of structural imperfections; and (2) the ultra-small fiber diameter also effectively prevents the formation of structural inhomogeneity particularly sheath-core structures during stabilization and carbonization.

Nanofiber Separations, LLC.

With encouragement from South Dakota School of Mines and Technology (SDSM&T), Drs. Hao Fong and Todd J. Menkhaus (who is a faculty member in the Department of Chemical and Biological Engineering at SDSM&T) co-founded a company of Nanofiber Separations, LLC. in 2011. The company has been awarded the NSF SBIR grant entitled “Efficient and Scalable Production of Functionalized Electrospun Nanofiber Felts of Regenerated Cellulose with Superior Capacity and Elevated Throughput for Bioseparations.”

Nanofiber Separations, LLC produces cutting-edge separation media composed of functionalized nanofibers. The product is manufactured from a randomly overlaid mat of electrospun nanofibers, which provides a unique separation medium capable of both size-based as well as adsorptive separation mechanisms. The highly advanced and proprietary formulation provides the opportunity to greatly enhance process efficiencies and economics in the biopharmaceutical, water treatment, desalination, blood products, and air purification industries. Superior adsorptive binding capacity (allowing for a much lower amount of separation medium), elevated permeability (allowing for much faster throughput), improved selectivity (allowing for more streamlined and fewer operations), reduced waste generation, and simplification of processing operations (reducing the risks associated with operator error and improving product consistency) can all be achieved simultaneously with the company’s products. The lead application for the advanced nanofiber separation media/felts is the biopharmaceutical industry, where a large unmet market awaits advanced separation media needed to purify biologically-produced compounds such as proteins and nucleic acids as human or animal therapeutics. Based on process and economic modeling, it is estimated that the competitive advantages provided by the Nanofiber Separations product will reduce overall production costs within a standard biotherapeutics process by over 30% (accounting for over $110 million annual savings for a single product).


 
 
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