Indian Journal of Fibre & Texti le Research Vol. 3 1 , March 2006, pp. 4 1 -5 1

Functionalized nanofibers for advanced applications Mohammad Munim Hussain & S S Ramkumar"

The Institute of Environmental and Human Health, Texas Tech University, Lubbock, TX 79409-1 1 63. USA

Electrospinning process is a fairly well established experimental method to produce submicron fibers. Fiber diameters are usually in the range of 1 00-500 nm that enable them to find out applications as filters, protective fabric l iners, face masks. etc. More recently, adding functional i ty to nanofibers has gained increased attention from the research community. This review paper reports the state-of-the-art research on functionalized nanofibers and their fabrication. characteristics and high-end applications in chemical process i ndustries. chemical protective clothing. biomaterials. drug delivery and tissue engineering.

Keywords: Drug delivery, Electrospinning. Nanofibers, Nanotubes. Scaffolds, Tissue engineering

fPC Code: Int. CI 8 8828. DO I D5/00

1 Introduction Nanotechnology has gained tremendous

momentum in the last decade due to the support from both public and private sector organizations. According to a statement made by the U S Government Advisory Panel, the nanocomposites, nanomembranes & fi lters, medical diagnostic devices, and chemical & biological sensors are the areas where the most progress in the next five years is expected. !

Electrospun nanofibers can be used in a wide range of applications, and making these nanofibers smarter with added functionalities is an emerging area in thi s field today. Nanofibers have created attraction due to their several unique properties, such as high surface area to volume ratio, fi lm thinness, fiber diameter at nano scale, porous structure, l ighter weight. etc. l

Although the electrospinning process was invented in 1 934, there has not been wide spread research interest in this field ti l l the mid 1 990s. A major upsurge in the research on the fundamental knowledge, mechanics of the jet motion and applications has taken place since the mid 1990s. A typical electrospinning set-up i s shown i n Fig. 1 . The basic theory of the nanofiber spinning process and the parameters affecting the process, properties of polymer solution, thermal and mechanical properties of electrospun materials and their different applications have recently been reviewed.3,4

As i s evident from the recent reviews, the subject of functionalizing nanofibers is sti l l at i ts infancy and

aTo whom all the correspondence should be addressed . E-mai l : s.ramkumar@ltu.edu

offers enormous scope for both fundamental and applied research. Functionalized nanofibers can be defined as nanofibers with specific foreign materials for adding special functionalities and capabil ities to nanofibers so that they can be put to a myriad of applications hitherto not possible. There are a number of materials that can add functionality to nanofibers, which can be chosen based on end-use appl ications. These materials can be metals and metal oxides at nano scale, biological materials such as enzymes, drugs and even carbon nanotubes (singlewalled and multi walled). These value-added nanofibers can be used effectively in chemical process industries (catalysts, fi lters and sensors), protective clothings, tissue engineering and biomaterials, drug del ivery, etc.

Functionalized nanofibers can be eas i ly fabricated by the electrospinning technique that uses high voltage electric field to produce electrical ly charged jets from polymer solution or melts, which on drying by means of evaporation of the solvent produce nanofibers. The highly charged fibers are field directed towards the opposi tively charged col lector, which can be a flat surface or a rotating drum to collect the fibers. The additional step to fabricate functionalized nanofiber i s the addition of functional materials to the parent solution. It is to be remembered that the functional materials are added into the nanofiber structure so that the products could have both the properties of nanofibers (porosity, high surface to volume ratio, diameters in the nanoscale, etc . ) and the functional properties of those doping materials. The process, materials and their

42 I"JDIAN J . FIBRE TEXT. RES., MARCH 2006

Fig. I - Elcctrospinning set-up

Fig. 2 - SEM photograph of polyethylene oxide nanofibers embedded with magnesium oxide

characteristics should be chosen in such a way that the inherent characteristics of the basic unit and additives are mostly preserved in the final product so as to meet the end-use requirements. This requires a thorough investigation and optimization and therefore offers a new platform for research in the nanofiber area. A typical example i s the addition of metal oxides to the base fiber matrix for giving catalytic activities. Figures 2 & 3 show a nanofiber matrix with metal oxide embedded in i t . Transmission emission microscopy shows that the metal oxide gets embedded into individual fibers in the nanofiber matrix (Fig. 4).

Electrospinning technique also lends opportunity to incorporate carbon nanotubes to provide strength and desirable �lectrical properties. The extraordinary strength of nanotubes associated with remarkable cherrucal and physical properties at nano scale opens up a broad range of appl i cations for the nanotubes .

Fig. 3 - SEM photograph of polyethylene oxide nanofibers embedded with titanium oxide

Fig. 4 - TEM photograph of a single polyethylene oxide nanofiber embedded with magnesium oxide

HUSSAIN & RAM KUMAR: FUNCTIONALIZED NANOFIBERS FOR ADVANCED APPLICATIONS 43

Singlewalled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs) are the two kinds of carbon nanotubes currently used in several applications. SWNT is a hollow cylinder of a graphite sheet whereas a MWNT is a group of coaxial SWNTs (Ref. 5) . Efforts have been made to embed carbon nanotubes into electrospun nanofibers to improve the properties of nanofibers, which have been discussed in some detail in this paper. Properties of nanofibers and nanotubes are described in Tables 1 and 2.

Conductivity of a nanofibrous structure can be achieved by blending the nonconducting and conducting polymers. Norris et al. lo worked on polyethylene oxide (PEO) to add conductivity in its nanofibrous form and they were able to blend PEO and polyaniline doped camphorsulfonic acid followed by spinning using the electrospinning process. They used four-point probe method to measure the conductivity of electrospun nonwoven fiber mat and the conductivity was tested for different ratio of polyaniline and PEO. IO They found that the conductivity of the nanofiber mat was slightly lower than that of the cast film, which might be due to the high porous structure of nanofibrous mats.

2 Hybrid Nanofibers with Desired Properties Nanofibers have marked differences in their

thermal and mechanical properties compared to regular fibers and bulk polymers.6.9 Therefore, i t is absolutely required to i mprove the thermal and mechanical properties of nanofibers for their real life applications. Several efforts have been made to reinforce the nanofibers with some other polymers or chemicals so that these fibers become mechanically stable.

The stiffness and strength of nanofibers can be increased by impregnating a suitable material. Bergshoef et at. I I fabricated electrospun nylon-4, 6 impregnated with epoxy resin, which was achieved by simply dipping the nanofiber mats into the liquid resin. The result from the tensile tests was found to be encouraging as both the stiffness and strength of the nanofiber composite were much higher than those of the matrix films.

Conductivity is an important property for a material, and conducting nanofibers could find applications in the area of polymer electronics.

Property

Air and moisture transport

Crystal l in ity

Glass transition temperature (Tg), peak crystall ization temperature (Tc) and melting temperature (Till) Tensile breaking strength, elongation­at-break and Young's modulus.

Property

Mechanical

Chemical and electrochemical

Thermal

Electrical

Optical and optoelectronic

Table I - Properties of nanofibcrs

Observation

Comparable with the properties of textiles and membranes currently used. in protective clothing system

Reduced crystal l ine order observed for nanofibers

Decrease in glass transition temperature (Tg) and peak crystal l ization temperature (TJ

Significant reduction i n tensile strength. No significant change in Young's modulus for electrospun pellethane compared to cast fi lms of pellethane. However, peak tensile strength was found to be reduced by 40% and 60% (observed for peak stress).

Table 2 - Properties of nanotubes5

Description

Nanofiber

Electrospun estane6

Poly-L-lactide (PLLA), poly(meta­phenylcne isophthalamide), poly(glycolide) and polyacrylonitrile7.s

Polyethylene terephthalate (PET) and polyethylene naphthalate9 (PEN)

Pellethane6

Nanotubes are reported to have the highest Young's modulus (over I Tpa) and tensile strength l)f (over 1 00 Gpa).

Molecular adsorption, doping and charge transfer on nanotubes are observed due to high speci fic surface and (J-1t rehybridization ..

Highest thermal conductivity.

Defect-free nanotube either semiconducting or metall ic with quantized conductance and this i s due to electron confinement along the tube circumference.

Optical applications are possible due to direct band gap and one-dimensional band structure of the nanotubes with wavelength ranging between 300 nm and 3000 nrri.

44 I NDIAN 1. FIBRE TEXT. RES., MARCH 2006

In order to achieve higher tensi Ie strength and elastic modulus, collagen and PEO were mixed in 1 : 1 weight ratio and the nanofibers within the diameter range of 1 00- 1 50 nm were produced from a 2 wt % solution of col lagen-PEO at a flow rate of 1 00 !!I/min (Ref. 1 2) . These composite nanofibers were found to have tensile strength of 370 kPa with an elastic modulus of 1 2 MPa. Different weight ratios of collagen and PEO ( 10: 1 , 5 : 1 and 1 :2) were also tested and it has been concluded that the maximization of intermolecular interactions between the PEO and col lagen components occurred at the weight ratio of 1 : 1 , as superior mechanical properties were obtained at this mixing ratio.

A blend of chitosan and si lk fibroin was used to improve the mechanical and physical properties of brittle si lk fibroin fi lm. Park et al. 1 3 used si lk fibroin/chi tosan blends having 30% chitosan content and electrospun into continuous fibrous structure. These nanofibers were found to have smaller diameters with narrower diameter distributions than pure s i lk fibroin nanofibers. D iameters of nanofiber gradually decreases up to 100 nm as the chitosan content increases. This is due to the increase in solution conductivity by the addition of chitosan.

On the other hand, reinforcement can also be achieved by the incorporation of nanofiber into epoxy and rubber matrices. Kim and Reneker l 4 worked on to improve the mechanical properties of these matrices using electrospun nanofibers. They used aromatic polybenzimidazole (PBI) nanofibers as reinforcement and the epoxy was toughened due to electrospun nanofibers. This reinforcing effect was higher than that of epoxy composite made with PB I fibri ls . They also reported that the Young' s modulus of nanofiber reinforced styrene-butadiene rubber (SBR) is ten times higher than that of unfi l led SBR.

It has been shown by Deitzel et al. 1 5 that the surface concentration of a particular constituent in the electrospun nanofibers can be made significantly higher by electrospinning process. E lectrospun nanofiber having specific surface chemistry was first investigated by Deitzel et aL 15 who used electrospinning for copolymers of polymethyl­methacrylate (PMMA) and tetrahydropertluorooctyle acrylate (TAN) from a mixed solvent of toluene and dimethyl formamide. The atomic percentage of fluorine i n the near surface region of the electrospun fibers was found to be approxi mately double than that of a bulk sample of the copolymer. X-ray

photoelectron spectroscopy (XPS) analysis was used to quantify the atomic percentage of fluorine and the results showed surface segregation of the fluorinated species in the electrospun fibers.

3 Tissue Engineering According to Langer and Vicanti, tissue

engineering is an interdiscipl inary field that applies the principles of engineering and the l ife sciences toward the development of biological substitutes that restore, maintain or i mprove tissue function. 16

Nanofibers, being porous with a wide range distribution of pore diameters, can be considered as engineered scaffolds that could find application in tissue engineering. Mechanical stabil ity, biocompatibi l ity, cell proliferation and ceP-matrix interactions are the critical factors to evaluate the performance of an engineered scaffold.

Natural tissue can be weakened or lost by injury, diseases, etc . and therefore artificial supports are required to heal wounds and repair damaged tissues and decaying bone structure. Nanofiber scaffolds can play a significant role as a degradable i mplant. However. the i mplants should have enough mechanical and biological stabi l i ty, which depend on the selection of materials, structure of the scaffolds and cel l-nanofiber interactions. The first generation of biodegradable nanofibers was mainly l i mited to their fabrication and characterization techniques. Currently, researchers have started investigating the interaction between cell and nanofiber matrices. Usually, a cell prol iferation assay is used and the cel lular morphology and its i nteraction with nanofibers are studied in vitro. Table 3 summarizes the research activities on biodegradable polymer nanofibers and i t i s evident that these nanfibers have been tested with different cell l ines such as fibroblasts, mesenchymal stem cel l , nerve cell , chondrocytes, smooth muscle cel l , endothelial cell , etc.

Recently, the focus has shifted to adding cell adhesive molecules that would i ncrease the cel l attachment capaci ty o f a n engineered scaffold. A joint research by National University of Singapore and John Hopkins University of USA used first cel l­adhesive molecules that are covalently attached to an electrospun scaffold. This research used poly(caprolactone-co-ethyl ethylene phosphate) (PCLEEP) copolymer in acetone (2 1 .5 wt%) to fabricate PCLEEP nanofibrous mesh.43 The PCLEEP scaffold was then c leaned with 70% ethanol and then poly acrylic acid was grafted onto the scaffold surface

HUSSAIN & RAM KUMAR: FUNCTIONALIZED NANOFIBERS FOR ADVANCED APPLICATIONS 45

by photo-polymerization process. Galactose conjugation onto p loyacrylic acid grafted scaffolds was achieved in which the conjugation reaction required 36 h for its completion . In order to evaluate the adhesion capacity of the scaffold, freshly i solated rat hepatocytes were seeded onto the scaffold. It was observed that the hepatocytes cell attachments were significantly higher in galactosylated nanofiber scaffold than in unmodified one.

Casper et al.44 have functionalized poly(ethylene glycol) [PEG] nanofiber with low molecular weight heparin (LMWH) by electrospinning process for its possible use in drug del ivery, ti ssue engineering and wound repair applications. Field emission scanning electron microscopy (FESEM), energy-dispersive x­ray analysis (ED X), UV -VIS spectroscopy and

multiphoton microscopy were used to characterize the electrospun matrices. It has been observed from the electron micrographs that the incorporation of heparin into the electrospun fibers does not affect the surface morphology or fiber diameters. Based on toluidine blue assays of heparin , LMWH can be impregnated into an electrospun matrix in the range 3 .5-85 flg/mg of electrospun fibers and the multi photon microscopy confirmed that the incorporation of PEG-LMWH into the matrix permits retention of the heparin for at least 1 4 days. It has been observed that there were improvements in the binding of basic fibroblast growth factor to the electrospun fibers of PEG­LMWH over LMWH alone. These results show that the functionalization of electrospun scaffolds could play a v i tal role in the application of biomaterials.

Polymer

Table 3 - Nanofibers as biomaterials in tissue engineering

Description

Collagen (Type I) and polyethylene oxide1 2

Col lagen 17

Poly(D,l-lactide-co-glycolide) 18 (PlGA)

Poly(etnylene-co-vinyl alcohol) fibers l9

Fi bri nogen20

POlY(i;-caprolactonef (PCl)

PlGA and PlA-PEG block copolymers blcnded with DNA22

Collagen23 (Type I I )

Poly(l_-Iactid-co-f:-caprolactone) [ P(llA-Cl)] (75:25) copolymer nanofibrous scaffold24

Silk fibroin25

POlY(I;-caprolactone) scatTolds26

Silk fibroin27 (SF)

Collagen and elastin28

Elastomeric poly(ester urethane)urea combined with Type I cOl lagen29

Superior mechanical propeI1ies were observed for col lagen-PEO blends at the weight ratio of I : I .

Collagen nanofibers were shown to be a scaffold as this matrix promoted cell growth.

Cell s were observed to maintain phenotypic shape and the orientation of the nanofiber guided the cel lular growth.

Electrospun fibers have bccn shown to support the culturing of smooth muscle cells and fibroblasts.

Fibrinogen nanowebs find applications as a tissue-engineering scaffold, wound dressing substance and hemostatic bandage.

The cell -polymer constructs were cultured with osteogenic supplements under dynamic culture conditions for up to 4 weeks. SEM showed that the surfaces of the cel l -polymer constructs were covered with cel l multilayers in 4 weeks.

This work represents the first successful demonstration of plasmid DNA incorporation into a polymer scaffold using electrospinning.

This study was aimed for carti lage tissue engineering and the scaffolds were found compatible with the condrocytes that produce and maintain the cartilagenous matrix.

A favorable interaction between this scaffold with human coronary artery smooth muscle cel l s (SMCs) was demonstrated.

Adhesion, spreading and proliferation of human bone marrow stromal cells (BMSCs) on these s i lk matrices were studied.

Nanofibrous PCl acts as a biologically preferred scaffold/substrate for prol iferation and maintenance of the chondrocytic phenotype.

The cell c:Jlture was performed for normal human keratinocytes and fibroblasts on the SF nanofibers and this matrix was found to be a good scaffold.

Research results provide prel iminary data on the development of electrospun scaffold from collagen and elastin for a three layer vascular construct.

Collagen incorporation significantly enhanced the smooth muscle cell adhesion onto electrospun scaffolds. The adhesion of smooth muscle cell s onto electrospun sc"ffolds was found to be enhanced by i ncorporating col lagen.

-Con/do

46 INDIAN 1. FIBRE TEXT. RES., MARCH 2006

Table 3 - Nanofibers as biomaterials in tissue engineering - Contd.

Polymer

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)3o (PHBV)

Poly(lactic-co-glycolic acid) (pLGA) and chitin3l

Poly(L-lactide-co- E-caprolactone) [P(LLA-CL)] (75:25) copolyme�2

POIY(E-caprolactone)33 (PCL)

Poly(glycolic aCid)34 (pGA)

Poly(lactide-co-glycolide)35

Poly(L-lactic-co+caprolactone) [P(LLA-CL)] (75:25) copolyme�6

Gelatin fibers and gelatinIPCL composite fibrous scaffolds37

Poly(L-lactic aCid)38 (PLLA)

Aligned poly(L-lactic acid) (PLLA) nano/micro fibrous scaffolds for neural tissue engineerin�9

Poly(L-lactic acid-co-succinic acid-co- l A-butane diol)4o (PLASB)

PLA and PLAlbeta-tertiary calcium phosphate (TCP) scaffold4l

POIY(E-caprolactone)42 (PCL)

Description

The electrospun PHBV nanofabric provides an attractive structure for the attachment and growth of chondrocytes as cell culture surfaces for tissue engineering.

The PLGA-chitin composite matrix was observed to be better matrix than the PLGA matrix according to cell adhesion and spreading for normal human keratinocytes.

Smooth muscle cells and endothelial maintain their phenotype shape on this scaffold and this suggests its potential use as a blood vessel substitute.

The nanofiber scaffold was comparable with cell aggregates or pellets (widely used protocols for chondrogenesis) in terms of chondrogenesis.

This improves the soft tissue biocompatibility of PGA through acid pretreatment.

Post surgery-induced abdominal adhesion was found to be reduced for this electrospun material.

This scaffold was intended for blood vessels and tubular scaffold with suitable fiber alignment was achieved using a rotating mandrel.

The gelatin-PCL composite scaffolds were found to be a good candidate for bone-marrow stromal cell (BMSC) culture.

This matrix was found to be promising for neural stem cell (NSC) differentiation and neurites out-growth, and promotes the adhesion of NSC as well.

The differentiation rate for NSC was higher for PLLA aligned nanofibers than that for microfibers.

Mouse fibroblast cells were used for cell prol iferation and it was then compared with the bulk PLASB films.

The hydrophilicity, proliferation and adhesion of cells were found to be better for PLAlbeta-TCP composite compared to pure PLA scaffold.

This nanofiber scaffold was tested for its ability to support and maintain multilineage differentiation of bone marrow-derived human mesenchymal stem cells (hMSCs) in vitro and it was found to be a very promising scaffold.

Most recently, nanofiber membrane, fabricated with polycaprolactone (PCL) and PCUCaC03 has been introduced as guided bone regeneration (GBR) material by Fujihara et al.45 GBR membranes prevent invasions by non-functional scar tissues and expedite the bone growth. The authors used MTS assay [3-( 4,5-dimethylthiazol-2-yl)-5 -(3-carboxymethoxy­phenyl)-2-( 4-sulfophenyl)-2H-tetrazolium, inner salt] to examine the fibroblasts attachment and their proliferation on composite nanofibrous membrane. In MTS assay, metabolically active cells produce a dye, which is absorbed at 490 nm wavelength. Two types of GBR membranes (PCL-rich and CaC03-rich) were tested and both of them showed good attachment and cell proliferation. The PCL-rich membrane showed higher absorbance intensity than CaC03-rich membrane.

gained tremendous momentum over the past few years. By improving their mechanical stability and finding novel ways to incorporate functional materials, it would make functionalized nanofibers as potential candidates for highly efficient biomaterials, such as drug delivery vehicles, tissue scaffolds, etc.

4 Drug Delivery and Nanofibers There is a high potential for nanofibers to be a

carrier of drugs to the specific sites. In order to incorporate drugs into the nanofiber matrix, a drug must be encapsulated into the nanofibrous structure. There are a few crucial parameters for drug delivery, such as burst release of drugs and the order of drug release kinetics. This particular area of research is at a infancy and a few researchers have started working on drug delivery via nanofibrous structures. A summary of these activities is provided in Table 4.

As is evident from the aforementioned research activities, biodegradable nanofibrous structures have

•

HUSSAIN & RAMKUMAR: FUNCTIONALIZED NANOFIBERS FOR ADVANCED APPLICATIONS 47

Table 4 - Summary of recent works on drug delivery by nanofibers

Drug Polymer Observation

Tetracycline hydrochloride Poly (lactic acid) (PLA). poly (ethylene-co-vinyl acetate) (PEV A) or PLA : PEV A ( 50:50) (ref. 46)

Drug release profiles from the electrospun mats were compared to a commercially available drug.

Hydrophilic antibiotic drug [Mefoxin (R). cefoxitin sodium]

Poly(lactide-co-glycolide)47 (PLGA) Sustained release of drug has been observed and there was no loss of nanofibrous structure.

Antibiotic (Cefazolin) Poly (Iactide-co-glycolide)48 (PLAGA) PLAGA nanofibers have been observed to have potential to work as antibiotic delivery systems for the treatment of wounds.

Pac • .Jlxel. doxorubicin hydrochloride an�oxorubicin

Itraconazole (Poorly water soluble drug)

Poly (L-lactide)49 (PLLA)

Water soluble polymerO

Release of tetracycline hydrochloride from electro spun PEVA [poly(ethylene-co-vinylacetate)], PLA [poly(lactic acid)] and a 50:50 blend of these two has been explored by Kenawy et al.46 It has been found that the electrospun PEVA and 50:50 PLA and PEVA mats give relative smooth release of drug over 5 days. Electrospinning was carried out using 14% (w/v) solution of PEV A, PLA and their 50:50 blend in chloroform. As tetracycline i s not soluble in chloroform, it was solubilized in a small amount of methanol and added to the polymer solution. The resulting solution containing polymer and drug was then electro spun and collected on a rotating drum to produce a sheet of 100-250 !-lm thickness. Release of the drug was monitored in pure water by UV -VIS followed by elemental analysis.

Verreck et al. 50 have carried out research on the characterization of drug nanofibers and studied the perfonnance of a poorly water soluble drug (itraconazole) dispersed on water soluble polymer­based nanofibers. Scanning electron microscopy showed the fiber diameter of 1 -4 !-lm and 300-500 nm depending on the applied voltage. Differential scanning calorimetry measurements found that the melting endotherm for itraconazole was not present. This was due to the formation of an amorphous solid dispersion . The dissolution rate of this composite material was assessed by USP II apparatus at 100 rpm. 'l'hey examined the dissolution rate for three different ways of drug releasing mechanism, namely (i) direct application of nanofiber composite, (ii) folded in sinker, and (iii) folded in gelatin capsule.

The burst release of the drugs could be avoided. The drug release profile nearly follows zero-order kinetic.

Three different ways of drug releasing mechanism have been reported: (i) direct application of the nanofiber composite. (ii) folded in sinker and (iii) folded in gelatin capsule.

When the nanoweb material is folded and placed in a sinker or hard gelatin capsule, longer dissolution time is generated. The authors also examined the effect of dissolution rate on other parameters, such as drug/polymer ratio and the diameter of the electro spun . fibers. They concluded that the electrospinning technique could be applicable for controlled drug delivery of poorly water-soluble

. drugs. Hydrophilic antibiotic drug such as mexofin®

(cefoxitin sodium) has been successfully incorporated to elctrospun poly(lactic-co-glycolide) by Kim et al. 47. They demonstrated a sustained release of this drug from the nanofibrous scaffolds, keeping their structure and bioactivity intact. Staphylococcus aureus cultures were investigated to determine the antimicrobial functionality possessed by the nanofibrous structure. The maximum release of drug was obtained after 1 h of incubation in water at 37 °e. On the other hand, the growth of Staphylococcus au reus was inhibited by more than ninety per cent.

Magnetic particles are often used in the drug delivery carrier for tracking the drug carrier, and magnetite (Fe304) is a good candidate for this application. Nanocrystalline magnetite nanoparticles, having the diameter in the range of 5-10 nm, have been incorporated into the nanofibers of poly(hydroxyethylmethacrylate) and poly-L-Iactide using the electrospinning process. 51 Tan et al.51 prepared the aqueous solution of magnetite particles and used that with the polymeric solution to fabricate the supermagnetic polymer nanofibers. Their results

48 INDIAN J. FIBRE TEXT. RES . MARCH 2006

show that the nanofibers containing up to 35 % magnetite particles demonstrate super-magnetism at room temperature.

Research to-date indicates the potential of functionalized nanofibers to serve as biodegradable gauzes and drug delivery vehicles.

5 Composite Nanofibers for Applications in Chemical Industries Polymeric nanofibers find applications in chemical

and process industries in different areas such as catalysis, sens ing, physical and chemical adsorption processes, etc.

A catalyst is usually acceptable for large-scale appl ication when it provides easy recycling and continuous operations without affecting the purity of products. There is a need for improving the catalytic efficiency and the stabi l ity of enzymes used in the processes so that these processes can compete with the traditional large-scale chemical processes. Immobi l ized enzymes offer easy catalyst recycl ing, feasible continuous operations, and simple product purification and binding of enzymes to the polymer nanofibers. Therefore, its use in the industrial processes overcomes the l i mitations of nanosized materials such as their dispersion in reaction solutions and the subsequent recovery. Jia et at. 52 fabricated functional ized nanofibers of polystyrene, which was impregnated with the enzyme alpha-chymotrypsin . These nanofibers show high activities in both aqueous and organic media, which is really outstanding as most of the previously studied enzymes show high activities in either. aqueous or organic media.

There is a growing interest on metal nanopartic les and they hold real promise for their use in catalysis, chemical sensing, etc. Li et at. 53 recently showed that the gold particles could be selectively deposited on electrospun anatase nanofibers by util izing the photocatalytic fe:lture of titania. This approach provides a simple route to fabricate metal-decorated titania nanofibers that have applications in catalysis and chemical sensing. Demir et al.54 fabricated electrospun poly (acryloni trile-co-acrylic acid) fibers impregnated with catalytic palladium nanoparticles. The catalytic activity of palladium nanoparticles in electrospun mats was determined by selective hydrogenation of dehydrolinalool in toluene at 90°C and the fabricated nanofibers showed 4.5 times higher . catalytic activity than that of the current PdJAh03 catalyst.

Recently, nanofibers have been used as urea biosensors and gas sensors. Urease acts as a catalyst in the hydrolysis of urea to ammonia and carbon dioxide and due to its specificity, urease can be used in urea detection. Nanocomposite fibers of urease and polyvinylpyrrolidone (PVP) were prepared using the electrospinning process by Sawicka et a155. The immobil ized enzyme (urease) maintained its reactivity within the nanofibrous structure and ' t: . membrane catalytical ly acted in different concentratiL-,.. �a solution. '''''''0\ u·-

In another work, Ding et al.56 introduce&vJ �vel gas sensor composed of nanofibrous membranes dnd quartz crystal m icrobalance (QCM). The homogeneous blend of cross-l inkable poly(acrylic acid) and polyvinyl alcohol (PV A) were electrospun on to the surface of QCM which showed a much higher ammonia gas sensitivity than that of continuous fi lm coated QCM sensor.

Nanocrystal l ine metal oxides are another group of particles that can be deposited on the nanofibers. Due to their chemical and electrical properties, there i s a great interest for metal oxide nanofibers. Drew et al.57

fabricated titanium dioxide (Ti02) and tin oxide (Sn02) coated polyacrylonitril e nanofibers. They fol lowed the i mmersion technique as nanofibers were immersed i n an aqueous solution of metal hal ide salts and halogen scavengers at room temperature to apply the metal oxide on to the surface of nanofibers. Scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy were used for the characterization of nanofibers. The metal oxide coated nanofibers could find applications in catalysis, sensing and photovoltaic conversions.

Inftead of using metal oxides as coating materials on the nanofiber surface, metal oxide dispersed polymers are electrospun from single solution . Hussain et al.58 used nanocrystall ine magnesium oxides with polyethylene oxide to develop composite nanofibers. Sufficient care has been taken in the mixing part of metal oxides and the polymer solution, as the objective i s to disperse the metal oxide particles in the solution. They were successful in dispersing the metal oxides on to the . nanofiber surface. The performance . of the nanofibers was tested by the adsorption properties of nanofiber composites using thermogravimetric analyzer.

Spinel l ithium manganese oxide (LiMn204) and nicke l cobaltite ( NiCo204) , wel l known as electrode materials, have been used by Yang et al. 59•60 i n the

HUSSAIN & RAMKUMAR: FUNCTIONALIZED NANOFIBERS FOR ADVANCED APPLICATIONS 49

electrospinning process with poly(vinyl)alcohol. They used calcinations technique in both cases and it has been reported that the crystall ine phase and morphology of the fibers were largely influenced by the calcination temperature. These materials are expected to i mprove the performance of electrocatalytic properties of the electrodes due to their high surface area.

6 Functionalized Nanofibers in Protective Clothing Although there i s a plethora of l iterature on the

process-property relationships of nanofibers, l imi ted information is available on the fundamental studies of nanofibers used in chemical protective clothing. Heidi Gibson and Phil Gibson of the US Army are carrying out research on the electrospinning of nanofibers for chemical protection. Publ ished research from the US Army and other researchers has focused predominantly on the filtration and transport studies. Gibson et at. 6 studied the transport properties of electrospun fiber mats and they conclucted that the nanofiber layers give very less resistance to the moisture vapor diffusion transport. Tsai et al. 61 studied the nonwoven fibers of polyethylene-oxide (PEO), polycarbonate (PC) and polyurethane (PU) usmg different fiber charging methods l ike electrostatic spinning, corona charging and tribocharging, and inferred that the electro spun fibers have higher filtration efficiency than other nonwoven webs. PU and PC were found to have higher charge retention capacities than electrospun PEO fibers.61 Gibson et at. 62 have analyzed the possibility of using thin nanofiber layers over the conventionally used nonwoven fi ltration media for protective clothi ng. Polyurethane and nylon 6 nanowebs were applied over open cell foams and carbon beads and then tested for airflow resistance. They concluded that the airflow resistance, filtration efficiency and the pOle sizes of nonwoven filter media could be easily altered by coating with the l ight weight electrospun nanofibers.62

As is evident, substantial amount of earlier research activities have focused on the filtration and transport properties of nanofibers.62 Only most recently, research focus has shifted to self-detoxifying nanofibers.58.63 Incorporation of reactive species into chemical protective clothing was studied to decontami nate G-type agents, VX and mustard (HD).63 Polyoxometalates (POMs) were adsorbed on to activated carbon and nano metal oxides (MgO and Ah03), and the chloroethyl ethylsulfide (CEES) has been used to evaluate their performances. The best

performance has been observed for POMs adsorbed on microporous carbons. It has also been observed that the POMs adsorbed onto nano magnesium oxide particles are more reactive than POM or nano magnesium oxide. The authors also studied the effectiveness of i sodosobenzoate-�-cyclodextrin (IBA-�-CD) to hydrolyze the soman analogue (dimebu) and found that the reaction rates are close for the electrospun fibers i mpregnated with IBA-�CD and IBA-�CD alone.

Nanocrystall ine metal oxides have created a new opportunity for decontaminating hazardous substances. When the size of the metal oxides reaches nano scale, surface reactivity i ncreases due to high surface concentrations of reactive edges and defect sites. This high surface reactivity along with high surface area allows their use for the effective decontamination of chemical warfare agents and related toxic substances. Scientists are currently working on self detoxifying nanofibers in which metal oxides are randomly dispersed on to the nanofiber matrices. They used nanocrystall ine magnesium oxide which has been reported to react with organophosphorus compounds at room temperature by dissociative chemisorption.64

7 Carbon Nanotube-Nanofiber Composites Singlewalled carbon nanotubes (SWCNT) and

multi walled carbon nanotubes (MWCNT) are at the center of attraction in nanotechnology due to their unique electronic and mechanical properties. As nanofibers are not strong enough by their strengths, nanotubes could be dispersed and aligned into nanofibers to develop stronger composites.

Fabrication of eJctrospun poly(ethylene oxide) nanofibers embedded with MWCNT was successfully achieved by the electrospinning process.65 The dispersion of MWCNT in solution is highly required to have them aligned and separated into the nanofiber matrix, which was ach ieved in water by using amphiphiles. It has been seen from the transmission electron m icroscopy (TEM) i mages that most of the MWCNT' s were aligned along the fiber axis. The authors also reported the twisted, bent and other irregular structure of MWCNTs in the composites.

Composite nanofibers of polyacrylonitrile nanofibers and MWCNT's were also reported by Hou et at.66 MWCNTs were oriented along the axes of the nanofibers. Results showed that the tensile modulus of the reinforced PAN nanofibers increased by 144%

50 INDIAN J. FIBRE TEXT. RES., MARCH 2006

(20 % MWCNTs by weight) and at 5% MWCNT, tensile strength was increased by 75%.

Ko et ai.67 demonstrated that the electrospinning process is capable of placing SWCNTs into a continuous nanoscale composite fibril . This material shows superior mechanical properties due to reinforcement and alignment of the nanotubes in the composites. They also reported that this composite is carbonized at 1 100 °C and SWCNT/carbon yarns are formed in this way. It has been shown that SWCNT structure has been maintained after the carbonization process.

8 Conclusions Since the 1990s, research on the electrospinning of

nanofibers has gained a tremendous momentum due to the potential of nanofibers for use in different high­tech applications. There has been a major upsurge in the number of research publications in this field focusing primarily on the process, structure and mechanics of the process. However, to our best knowledge there has been no succinct review article on function ali zed nanofibers that find applications in tissue engineering, drug delivery and protective clothing. This review article has endeavored to focus on the research activities on nanofibers with added functionalities. It is hoped that this overview article will serve as a useful tool for future research on nanofibers for hitherto unexplored applications.

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