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Porous structures with their high surface areas have found

applications in many different areas. Nanofibers, with their large

surface-to-volume ratio, have the potential for use in various

applications where high porosity is desirable. A porous structure

made out of nanofibers is a dynamic system where the pore size

and shape can change, unlike conventional rigid porous structures.

Nanofibers can also be linked to form a rigid structure if required.

Perhaps the most versatile process for producing nanofibers with

relatively high productivity is electrospinning. Porous, nanofiber

meshes made by electrospinning have been identified for use in

numerous applications (Fig. 1).

Electrospinning nanofibersThere are several methods of producing nanofibers, from high-volume

production methods such as melt fibrillation1, island-in-sea2, and gas

jet3 techniques, to highly precise methods like nanolithography4,5 and

self-assembly6-9. However, their usefulness is limited by combinations

of restricted material ranges, possible fiber assembly, cost, and

production rate. Here, electrospinning has an advantage with its

comparative low cost and relatively high production rate. Micron size

Nanofibers are able to form a highly porous mesh and their large

surface-to-volume ratio improves performance for many applications.

Electrospinning has the unique ability to produce nanofibers of differentmaterials in various fibrous assemblies. The relatively high production

rate and simplicity of the setup makes electrospinning highly attractive

to both academia and industry. A variety of nanofibers can be made for

applications in energy storage, healthcare, biotechnology, environmental

engineering, and defense and security.

Seeram Ramakrishna1,2,3,*, Kazutoshi Fujihara3, Wee-Eong Teo1, Thomas Yong3, Zuwei Ma1, and Ramakrishna Ramaseshan1

1Nanoscience and Nanotechnology Initiative, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore

2Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore3Division of Bioengineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore

*E-mail:[email protected]

ISSN:1369 7021 Elsevier Ltd 2006M ARCH 20 06 | VOLUME 9 | NUM BER 30

Electrospun nanofibers:solving global issues

Defense & security

Environmentalengineering &biotechnology

Membranes & filters

Chemical & biological protectionSensors

Energy

Solar cells & fuel cells

APPLICATIONS

Healthcare

Tissue engineering &tissue repairDrug delivery

Fig. 1 Potential applications of electrospun fibers.
mailto:[email protected]:[email protected]:[email protected]


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The ability to form porous fibers through electrospinning means that

the surface area of the fiber mesh can be increased tremendously. Phase

separation is proposed as the main mechanism behind the formation of

porous fibers. When more volatile solvents are used, solvent-rich regions

begin to form during electrospinning that transform into pores14.

Another method of producing porous nanofibers is the spinning of ablend of two different polymers. One of the polymers is removed after

fiber formation by dissolution in a solvent in which the other polymer is

insoluble15.

Since stretching of the solution arises from repulsive charges, the

electrospinning jet path is very chaotic and only nonwoven meshes are

produced using a typical setup. Nevertheless, more ordered assemblies

that allow the porosity of the mesh to be controlled have been produced

through clever manipulation of the setup and solution composition.

Several methods have been developed that yield aligned fibers with

various degrees of order16-19 and fiber directions20,21 for two- and three-

dimensional assemblies22-26 (Fig. 6). Such assemblies are usually

achieved through control of the electric field between the tip of the

spinneret and the collector, use of a dynamic collector such as a rotating

mandrel, or a combination of both. Li et al.20 used a pair of parallel

conducting electrodes to create an electric field such that the

electrospun fibers are preferentially aligned across the gap in between

the electrodes. Boland et al.16 used a rotating drum at a speed of

1000 rpm to collect aligned fibers. To fabricate a tubular scaffold,

electrospun fibers can be deposited on a rotating tube and the deposited

fiber layer subsequently extracted from the tube. Fiber alignment can be

controlled using auxiliary electrodes to create an electric field profile that

influences the flight of the electrospinning jet (Fig. 7).

With such versatility, electrospun fibers are being explored for use in

many different applications. Currently, most tests use nonwoven fiber

meshes made out of smooth fibers. Ceramic nanofibers derived from

nonwoven electrospun fiber meshes have opened up new areas of

opportunities. Besides nonwoven meshes, testing of other fibrous

assemblies for potential applications has been limited. Nevertheless, theversatility of electrospun fibers can be seen in the established results

and on-going research in major areas like healthcare, biotechnology and

environmental engineering, defense and security, and energy storage

and generation.

Healthcare applicationsCurrent medical practice is based almost entirely on treatment regimes.

However, it is envisaged that medicine in the future will be based

heavily on early detection and prevention before disease manifestation.

Together with nanotechnology, new treatment modalities will emerge

that will significantly reduce medical costs.

MARCH 20 0 6 | VOLUME 9 | NUM BER 32

REVIEW FEATURE Electrospun nanofibers: solving global issues

Published application

Issued patent

50

45

35

40

30

25

20

15

10

5

0Before2000

2000 2001 2002 2003 2004

Year

Number

Fig. 5 Number of filed patents and patent applications in the US.

Others 12

Japan 8

Korea 23Europe 21

USA 63China 16

Fig. 4 Distribution of universities working on electrospinning around the world.


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With recent developments in electrospinning, both synthetic and

natural polymers can be produced as nanofibers with diameters ranging

from tens to hundreds of nanometers with controlled morphology and

function. The potential of these electrospun nanofibers in human

healthcare applications is promising, for example in tissue/organ repair

and regeneration, as vectors to deliver drugs and therapeutics, as

biocompatible and biodegradable medical implant devices, in medical

diagnostics and instrumentation, as protective fabrics against

environmental and infectious agents in hospitals and general

surroundings, and in cosmetic and dental applications.

Tissue/organ repair and regeneration are new avenues for potential

treatment, circumventing the need for donor tissues and organs in

transplantation and reconstructive surgery. In this approach, a scaffold is

usually required that can be fabricated from either natural or synthetic

polymers by many processing techniques including electrospinning and

phase separation.

The biocompatibility of the scaffold is usually tested ex vivobyculturing organ-specific cells on the scaffold and monitoring cell growth

and proliferation. An animal model is used to study the biocompatibility

of the scaffold in a biological system before the scaffold is introduced

into patients for tissue-regeneration applications.

Nanofiber scaffolds are well suited to tissue engineering as the

scaffold can be fabricated and shaped to fill anatomical defects; its

architecture can be designed to provide the mechanical properties

necessary to support cell growth, proliferation, differentiation, and

MARCH 20 06 | VOLUME 9 | NUMBER 3

Electrospun nanofibers: solving global issues REVIEW FEATURE

Fig. 7 Controlling fiber alignment on a tubular scaffold through mechanical rotation and modification of the electric field.

Fig. 6 Two- and three-dimensional structures made of electrospun fibers.


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motility; and it can be engineered to provide growth factors, drugs,

therapeutics, and genes to stimulate tissue regeneration. An inherent

property of nanofibers is that they mimic the extracellular matrices

(ECM) of tissues and organs. The ECM is a complex composite of fibrous

proteins such as collagen and fibronectin, glycoproteins, proteoglycans,

soluble proteins such as growth factors, and other bioactive molecules

that support cell adhesion and growth. Studies of cell-nanofiberinteractions have shown that cells adhere and proliferate well when

cultured on polymer nanofibers27-29.

One of our aims is to fabricate electrospun polymer nanofiber

scaffolds for engineering blood vessels, nerves, skin, and bone. We have

demonstrated that human coronary artery smooth muscle cells cultured

on synthetic nanofibrous scaffolds of the copolymer poly(L-lactic

acid)/poly(-caprolactone), or PLLA/PCL, show normal morphology andgood proliferation. The cells organize along the aligned nanofibers in a

directional manner typified by the orientation of the cytoskeletalprotein -actin (Fig. 8), suggesting that nanofiber orientation can impart

a functional development on the cells30.

On collagen-modified nanofibers, human coronary artery endothelial

cells exhibit cobble-stone morphology (Fig. 9a), typical of endothelial

cells cultured on a polystyrene surface with comparable adhesion and

proliferation rates31. On aligned PLLA nanofibers, c17.2 neural cells

adhere, elongate along the fibers, and neurites extend along the

direction of the aligned fibers (Fig. 9b)32. Human dermal fibroblasts

have been demonstrated to grow better on collagen nanofibrous

scaffolds than polystyrene tissue culture surfaces (Fig. 9c)33.

A recent study carried out with human coronary endothelial cells

cultured on nanofibrous scaffolds34 indicates that nanofiber scaffolds

positively promote cell-matrix and cell-cell interactions, with the cells

having a normal phenotypic shape and gene expression. This can be

attributed to the ECM-like properties of the nanofiber scaffolds that

mimic the natural tissue environment.

Further research is required to elucidate the influence of nanofibers

on the biochemical pathways and cellular signaling mechanisms that

regulate cell morphology, growth, proliferation, differentiation,

motility, and genotype. Insight into how natural ECM components

secreted by cells replace the biodegradable polymeric scaffolds is also

needed. This complete understanding of cell-nanofiber scaffold

interactions will pave the way for successful engineering of various

tissues and organs, such as vascular grafts, nerve, skin and bone

regeneration, cornea transplants, skeletal and cardiac muscle

engineering, gastrointestinal and renal/urinary replacement therapy, andeven stem cell expansion and differentiation to specific cells types and

organ regeneration.

In the pharmaceutical and cosmetic industry, nanofibers are

promising tools for controlled delivery of drugs, therapeutics,

molecular medicines, and body-care supplements. For example, DNA

covalently attached to a patterned carbon nanofiber array and

inserted into cells by centrifuging the cells onto the array, does not

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Fig. 9 (a) Metabolic dye CMFDA staining of human coronary endothelial cells cultured on random, collagen-blended nanofibers. (b) Metabolic dye CMFDA stainingof c17.2 neural cells cultured on aligned nanofibers. (Reprinted with permission from32. 2005 Elsevier.) (c) Scanning electron micrograph of human fibroblastscultured on random, pure collagen nanofibers. Metabolic dyes are cell stains that only fluoresce or produce a color in live cells.

(a) (b) (c)

Fig. 8 Human coronary artery smooth muscle cells cultured on alignednanofibers that have been stained for-actin filaments. (Reprinted withpermission from17. 2004 Elsevier.)

(a) (b) (c)


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rejection) without a significant drop in flux performance42. No particles

were found trapped in the membrane, so the membrane could be

effectively recovered upon cleaning. This opens up new avenues of

application of electrospun membranes for the pretreatment of water

prior to reverse osmosis.In our laboratory, nanofiber membranes are also being tested as

affinity (or adsorptive) membranes. Affinity membranes are a broad

class of membranes that selectively capture specific target molecules (or

ligates) by immobilizing a specific capturing agent (or ligand) onto the

membrane surface. In biotechnology, affinity membranes have

applications in protein (such as IgG) purification and toxin (such as

endotoxin) removal from bioproducts. In the environmental industry,

affinity membranes have applications in organic waste removal and

heavy metal removal in water treatment.

To be used as affinity membranes, electrospun nanofibers must

be surface functionalized with ligands. In most cases, the ligand

molecules should be covalently attached on the membrane to preventleaching of the ligands. Cellulose nanofiber membranes have been

surface functionalized with cibacron blue for the purification of

albumin43. Cellulose nanofiber membranes functionalized with

protein A/G (a recombinant 50 449 Da protein from Pierce

Biotechnology that has an increased ability to bind IgG molecules)

shows a high ability to capture IgG molecules with a capacity of

~134 g/cm2, which is higher than that of the commercialized

membrane (~80 g/cm2).

Water pollution is now becoming a critical global issue. One

important class of inorganic pollutant of great physiological significance

is heavy metals, e.g. Hg, Pb, Cu, and Cd. The distribution of these metals

in the environment is mainly attributed to the release of metal-

containing wastewaters from industries. For example, copper smelters

may release high quantities of Cd, one of the most mobile and toxic

among the trace elements, into nearby waterways44. It is impossible to

eliminate some classes of environmental contaminants completely, such

as metals, by conventional water purification methods. Affinity

membranes will play a critical role in wastewater treatment to remove

(or recycle) heavy metals ions in the future. Polymer nanofibers

functionalized with a ceramic nanomaterial, such as hydrated

alumina/alumina hydroxide and iron oxides, could be suitable materials

for fabrication of affinity membranes for water industry applications.

The polymer nanofiber membrane acts as a carrier of the reactive

nanomaterial that can attract toxic heavy metal ions, such as As, Cr, and

Pb, by adsorption/chemisorption and electrostatic attractionmechanisms.

Compared with heavy metal pollutants, overall water quality is much

more sensitive to organic pollutants. Although such organics are usually

no more than 1% of the pollution in a river, they tend to use up its

dissolved oxygen, making the water unable to sustain life. While the

transformations and pathways of metals in the environment have been

studied to some extent, much less information is available on most

commercial organic products because of their complex structures. Again,

affinity membranes provide an alternative approach for removing

organic molecules from wastewater. For example, -cyclodextrin is a

cyclic oligosaccharide comprising of seven glucose units. It has a stereo-

specific toroidal structure with a hydrophobic interior and hydrophilicexterior that can capture hydrophobic organic molecules from water by

forming an inclusion complex. -cyclodextrin has been introduced

into a poly(methyl methacrylate) nanofiber membrane using a physical

mixing method to develop an affinity membrane for organic waste

removal45.

Electrospun nanofibers have also received great attention for sensor

applications because of their unique high surface area. This is one of the

most desirable properties for improving the sensitivity of

conductometric sensors because a larger surface area will absorb more

of a gas analyte and change the sensors conductivity more significantly.

Nanofibers functionalized with a semiconductor oxide such as MoO3,

SnO2, or TiO2 show an electrical resistance that is sensitive to harmful

chemical gases like ammonia and nitroxide46. Single polypyrrole

nanofibers containing avidin were studied as biosensors for detecting

biotin-labeled biomolecules such as DNA. Specific binding of the

biomolecules to the nanofibers changes the electrical resistance of a

single nanofiber47. A fluorescent polymer, poly(acrylic acid)-poly(pyrene

methanol), or PAA-PM, was used as a sensing material for the detection

of organic and inorganic waste. The fluorescence is quenched by

adsorbed metal ions Fe3+ or Hg2+ or 2,4-dinitrotoluene (DNT) on the

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Fig. 11 An electrospun polysulphone membrane: (a) surface; (b) cross-section; and (c) magnified cross-section images.

(a) (b) (c)


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nanofiber surfaces48. In our laboratory, nylon-6 nanofiber was

functionalized with biotinylated glucose oxidase to develop a novel

biosensor for testing glucose concentration49.

Defense and security applicationsMilitary, firefighter, law enforcement, and medical personnel requirehigh-level protection when dealing with chemical and biological threats

(which include chemicals like nerve agents, mustard gas, blood agents

such as cyanides, and biological toxins such as bacterial spores, viruses,

and rickettsiae) in many environments ranging from combat to urban,

agricultural, and industrial. Current protective clothing is based on full

barrier protection such as hazardous materials (HAZMAT) suits, or

permeable adsorptive protective overgarments such as those used by

the US military. The obvious limitations of these suits are weight and

moisture retention, which prevent the user from donning them for long

periods.

Nanostructures with their small size, large surface area50, and lightweight will improve, by orders of magnitude, our capability to:

Detect chemical and biological warfare agents with sensitivity and

selectivity;

Protect through filtration and destructive decomposition of harmful

toxins; and

Provide site-specific in vivoprophylaxis.

Polymer nanofibers are considered as excellent membrane materials

for this purpose owing to their light weight, high surface area, and

breathable (porous) nature51. The high sensitivity of nanofibers toward

warfare agents makes them excellent candidates as sensing interfaces

for chemical and biological toxins in concentration levels of parts per

billion52. Governments across the world are investing in strengthening

the protection levels offered to soldiers in the battlefield53. Various

methods of modifying nanofiber surfaces to enhance their capture and

decontamination capability of warfare agents are currently under

investigation. One protection method is through chemical surface

modification and attachment of reactive groups such as oximes,

cyclodextrins, and chloramines54,55 that bind and detoxify warfare

agents.

In association with the Defense Science and Technology Agency

(DSTA) in Singapore, our laboratory is working on functionalizing

nanofibers to be used in facemasks for chemical and biowarfare defense

(Fig. 12). The facemask consists of two main components: a high-

efficiency particulate air (HEPA) filtering layer and an activated charcoalbed that adsorbs harmful gases and contaminants.

Nanofiber membranes may be used to replace the activated charcoal

in adsorbing toxins from the atmosphere. Active reagents can be

embedded into the nanofiber membrane by chemical functionalization,

post-spinning modification, or through using nanoparticle polymer

composites (Fig. 13). Preliminary tests using chemical warfare simulators

such as paraoxon and dimethyl methyl phosphonate on the

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Fig. 12 Schematic showing the cross section of a facemask canister used for protection from chemical and biological warfare agents.


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functionalized fibers show evidence of decontamination. Metal

nanoparticles (Ag, MgO, Ni, Ti, etc.), which have proven abilities in

decomposing warfare agents, can also be embedded in the nanofibers.

There are many avenues for future research in nanofibers from the

defense perspective. As well as serving protection and decontamination

functions, nanofiber membranes will also have to provide the durability,

washability, resistance to intrusion of all liquids, and tear strength

required of battledress fabrics.

Energy generation applicationsNatural energy resources such as crude oil, coal, natural gas, and

uranium are a necessity for everyday life. Rapid economic growth inAsia and the subsequent increase in demand for energy mean that the

rate of oil production is no longer adequate. This is evident in the

soaring price of crude oil, which has reached over $60 per barrel 56.

Large volumes of carbon dioxide emitted by the burning of fossil fuels

is also the main culprit in climate change. Thus, there is an urgent need

to identify new sources of energy that are environmentally friendly and

able to replace current supplies. Polymer batteries, fuel cells,

photovoltaic cells, wind power generators, and geothermal power

generators are some possible alternatives.

Given their high porosity and inherent large total surface area,

electrospun nanofiber membranes are being considered for polymer

batteries57-59, photovoltaic cells60-63, and polymer electrolyte

membrane fuel cells (PEMFCs).Polymer batteries have been developed for PC notebooks and cell

phones to replace conventional, bulky lithium batteries. The

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PVDF nanofibrous membranewhich absorbs lithium electrolyte

LiCoO2 cathode

MCMBanode

Fig. 14 Polymer battery assembled by sandwiching PVDF nanofiber membranes between a mesocarbon microbead (MCMB) anode and a LiCoO2 cathode58,59.

Fig. 13 Schematic of the incorporation of functional groups into a polymer nanofiber mesh.


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components of polymer batteries are a carbon anode, a lithium cobalt

oxide cathode, and a polymer gel electrolyte. When a battery is

subjected to charging, Li+ ions are confined in the carbon anode. Ondischarging, the Li+ ions move to cathode. Noteworthy properties of

polymer batteries are less electrolyte leakage, high dimension

flexibility, and high energy density per weight. However, there is still a

need to improve energy density per weight of polymer batteries to

increase their market share. Choi et al.57 and Kim et al.59 have

assembled a new type of polymer battery using poly(vinylidene

fluoride), or PVDF, nanofiber membranes (Fig. 14). The porous

structure of the PVDF nanofiber membrane favors high uptake

(350 wt.%) of lithium electrolyte so that electrolyte leakage is

reduced. These factors make it possible to hold a large quantity of

lithium electrolyte in thinner battery packs. The large surface area of

the nanofibrous network also enhances ion conductivity, thus polymerbatteries comprising nanofiber membranes may improve energy

density per weight as compared with conventional polymer batteries.

Most conventional photovoltaic cells use single-crystalline,

polycrystalline, or amorphous Si. It is well known that a single-crystal Si

cell can achieve an energy translation efficiency of ~20%, and this value

is higher than other types of solar cells. However, the biggest

shortcoming for single-crystal Si solar cells is their high manufacturing

cost. There is also a need for a large surface area to obtain sufficient

electrical output.As an alternative, Grtzel and colleagues64 have developed dye-

sensitized solar cells. The principle here is that sensitizing dye molecules

coated onto TiO2 nanoparticles absorb photons and transfer excited

electrons through the conduction band of TiO2 to the cathode.

A nanotopographic TiO2 layer works as the electrode and enhances the

total surface area to achieve a high electrical output. Dye-sensitized

solar cells are less costly to manufacture than Si-based solar cells, but

there are issues that need to be addressed, including reducing

electrolyte leakage and improving the energy conversion efficiency

(generally ~4-10%). With respect to electrolyte leakage, an alternative

solution is to use a viscous polymer gel electrolyte. However, it is

difficult to infuse a viscous gel into a conventional TiO2

nanotopographic layer. Song et al.61-63 have solved this problem by

using TiO2 nanofiber membranes fabricated by electrospinning in

combination with sol-gel processes (Fig. 15). The viscous polymer gel

electrolyte can easily penetrate into the porous nanofiber membrane.

Their assembled TiO2 nanofiber dye-sensitized solar cells are able to

achieve an energy conversion efficiency of 6.2%63.

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e-

H2

O2

H2O

H2O

Polymerelectrolyte

membrane

Anode

2H+

H2 2H

+ + 2e- 1/2 O2 + 2H+ + 2e-

Cathode

Fig. 16 Principle of electricity generation in fuel cells.

Fig. 15 Dye-sensitized solar cells assembled using TiO2 nanofiber membranes61-63.


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Electricity generation in PEMFCs is through the chemical reaction of

hydrogen at the anode and oxygen at the cathode (Fig. 16). Protons are

transmitted through an electrolyte membrane that contains distilled

water, while electrons are transmitted from the anode to the cathode.

The key properties of electrolyte membranes are high proton

conductivity and shielding of electron transport. As the membrane needs

to hold distilled water for proton conductivity, water retention of the

membrane is also important. Nafion (DuPont), a perfluorosulfonic acid

polymer film, has been widely used so far. However, Nafion membranes

are expensive at up to $800/kg. For the same membrane area,

electrospun Nafion fiber membranes require less material than

conventional Nafion fuel cell membranes, thereby reducing cost. Porous

nanofiber membranes are also able to hold distilled water, thus

enhancing proton conductivity. Therefore, such nanofiber membranes

have the potential to be used in PEMFCs.

ConclusionGiven the versatility of electrospinning for generating highly porous

nanofiber meshes made out of different materials, it is no surprise that

it has found possible uses in different fields ranging from healthcare,

biotechnology, and environmental engineering to defense and security,

and energy generation. Electrospinning may be able to produce

microengineered scaffolds for tissue engineering. Improved wound

dressings could be made out of nanofiber meshes impregnated with

drugs. Membranes for water treatment or use in biotechnology could be

made of electrospun fibers. Nanofiber clothing and filters could deal

more effectively with chemical and biological threats. In the future, we

may no longer be dependent on crude oil thanks to more efficient

conversion of other energy sources to electricity. With the ability to

mass-produce nanofibers, electrospinning may well be one of the most

significant nanotechnologies of this century.
http://www.wtec.org/nanoreports/cbre/CBRE_Detection_11_1_02_hires.pdfhttp://www.wtec.org/nanoreports/cbre/CBRE_Detection_11_1_02_hires.pdfhttp://www.sc.doe.gov/bes/reports/files/NCT_rpt.pdfhttp://www.nymex.com/index.aspxhttp://www.nymex.com/index.aspxhttp://www.nymex.com/index.aspxhttp://www.sc.doe.gov/bes/reports/files/NCT_rpt.pdfhttp://www.wtec.org/nanoreports/cbre/CBRE_Detection_11_1_02_hires.pdf

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nano electrospinning