A review in electrospinning

8/10/2019 A review in electrospinning

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Formation of fibers by electrospinning

Gregory C. Rutledge , Sergey V. Fridrikh

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Received 20 December 2006; accepted 14 April 2007

Available online 22 August 2007

Abstract

Electrostatic fiber formation, also known as electrospinning, has emerged in recent years as the popular choice for producing continuousthreads, fiber arrays and nonwoven fabrics with fiber diameters below 1 m for a wide range of materials, from biopolymers to ceramics. It

benefits from ease of implementation and generality of use. Here, we review some of the basic aspects of the electrospinning process, as it is

widely practiced in academic laboratories. For purposes of organization, the process is decomposed into five operational components: fluid

charging, formation of the cone-jet, thinning of the steady jet, onset and growth of jet instabilities that give rise to diameter reduction into the

submicron regime, and collection of the fibers into useful forms. Dependence of the jetting phenomenon on operating variables is discussed.

Continuum level models of the jet thinning and jet instability are also summarized and put in some context.

2007 Elsevier B.V. All rights reserved.

Keywords: Electrospinning; Nanofiber; Electrohydrodynamics

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1384

2. Brief description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1385

3. Fluid charging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1385

4. Cone-jet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1385

5. Slender thinning jet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1386

6. Jet instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1388

7. Fiber collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1390

8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1390

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1390

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1390

1. Introduction

Electrospinning is a term used to describe a class of fiber

forming processes by which electrostatic forces are employed to

control the production of fibers. It is closely related to the more

established technology of electrospraying, which generally refers

to processes in which electrostatic forces are used to control the

formation of droplets. Spinningin this context is a textile term

that derives from the early use of spinning wheels to form yarns

from natural fiber staples like cotton and is commonly used to

identify fiber-forming processes for synthetic fibers as well. In

both electrospinning and electrospraying, the role of the

electrostatic forces is to supplement or replace the conventional

mechanical forces (e.g. hydrostatic, pneumatic) used to form the

jet and to reduce the size of the fibers or droplets, hence the term

electrohydrodynamic jetting. Electrospraying was described in

the technical literature by Zeleny as early as 1914 [1], while

electrospinning first appears in the patent literature in 1902 [2].

Electrospraying has enjoyed a rich history in the intervening

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Corresponding author. Tel.: +1 617 253 0171.

E-mail address:[email protected](G.C. Rutledge).

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century, with commercial applications in ink jet printing, mass

spectrometry, and fuel injection, to name a few. Electrospinning

on the other hand has developed more slowly. Notable early

studies are those by Baumgarten [3], and by Larrondo and St.

John Manley [4] but apparently neither received subsequent

follow-up. In the 1990's Reneker and co-workers [5,6], drew

attention to electrospinning as a means to produce small diameter,continuous filaments. In contrast to conventional synthetic fiber

forming processes such as those used for high speed spinning of

nylon or polyester, where continuous fibers ranging from 10 to

500 m are produced, electrospinning readily leads to the

formation of seemingly continuous fibers with diameters ranging

from0.01 to 10m. The literature on electrohydrodynamic jetting

and atomization for electrosprays is extensive, and relevant to

electrospinning as well [79]. This brief review is intended to

provide an overview of the electrospinning process, a subjective

assessment of the primary operational variables as they are

understood at the current time, and some of the mathematical

modeling that has been instrumental in developing our under-standing of the process. Due to its brevity and the rapid growth of

the field in the past few years, it is necessarily incomplete, as our

understanding of the technology continues to improve with the

benefit of on-going research.

2. Brief description

In its simplest incarnation, the process of electrospinning is

relatively easy to implement. In the laboratory, a glass pipette is

filled with a solution comprising the spin dopeand mounted at

an angle sufficient to prevent discharge of the fluid from the

pipette under its own weight. A wire electrode inserted into the

fluid reservoir in the pipette and charged to a relatively highvoltage, typically around 10 kV, provides the necessary charging

potential. Under such conditions, the fluid itself charges to a high

potential (positive or negative, depending on polarity of the

voltage generator). The typical spin dope has sufficient

conductivity for the induced charge to relax to a free surface or

interface on a time scale short compared to the experimental time

scale, but it otherwise acts as a dielectric. The term leaky

dielectric is descriptive of such fluids [10]. The repulsion

between charges at the free surface then works against surface

tension and fluid elasticity to deform the droplet into a conicalshape, called a Taylor cone after the pioneering work of G.I.

Taylor[11]. Beyond a critical charge density, this cone is unstable

and a jet of fluid is emitted from the tip of the cone. This charged

jet then seeks a path to ground. As it does so, the fluid filament is

accelerated and stretched and may experience any of a number of

instabilities, the relative importance of which depends on

numerous variables, including the properties of the fluid, the

electric field environment and the dynamical behavior of the

jetting phenomenon. A suitable collector electrode is used to

direct this path to ground. Thus, the electrospinning process may

be broken down into several operational components: (i) charging

of the fluid, (ii) formation of the cone-jet, (iii) thinning of the jet inthe presence of an electric field, (iv) instability of the jet, and (v)

collection of the jet (or its solidified fibers) on an appropriate

target. A simple schematic of the process is shown inFig. 1.

3. Fluid charging

In electrospinning, generation of charge within the fluid

usually occurs by virtue of contact with and flow across an

electrode held at high (positive or negative) potential, referred to

as induction charging[12]. Depending on the nature of the fluid

and the polarity of the applied potential, free electrons, ions or

ion pairs may be generated as charge carriers in the fluid; the

generation of charge carriers can be very sensitive to solutionimpurities. The formation of ions or ion pairs by induction

results in the formation of an electrical double layer. In the

absence of flow, the double layer thickness is determined by the

ion mobility in the fluid; in the presence of flow, ions may be

convected away from the electrode and the double layer

continually replenished. Charging of the fluid in electrospinning

is typically field-limited, with the break-down field strength in

dry air being on the order of 30 kV/cm between flat plates.

Inductive charging is generally suitable for fluids with

conductivities of the order of 102 S/m. For nonconductive

fluids such as hydrocarbons and polymer melts, charge may be

injected directly into the fluid, as is done in electrostaticthrusters, by the use of two electrodes, one having a needle-like

geometry. An early example of this was reported by Kim and

Turnbull for electrospraying [13]. For electrospinning, Kelly

has reported the production of fibers using a charge injection

device originally developed for fuel atomization[14]. Relative-

ly high throughputs were achieved in the latter case, but the

resulting jet deviates from the conventional cone-jet/thinning jet

mode described in detail below.

4. Cone-jet

Cloupeau and Prunet-Froche [15] have described several

functioning modes of operation for electrohydrodynamic jets

Fig. 1. Schematic of the electrospinning process. (a) high voltage power supply;

(b) charging device; (c) high potential electrode (e.g. flat plate); (d) collector

electrode (e.g. flat plate); (e) current measurement device; (f) fluid reservoir; (g)flow rate control; (h) cone; (i) thinning jet; (j) instability region.

1385G.C. Rutledge, S.V. Fridrikh / Advanced Drug Delivery Reviews 59 (2007) 13841391


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leading ultimately to the formation of charged droplets or aerosol.

For the case of electrospinning, the cone-jet regime is of particular

note since it represents the initial stage of formation of the fiber.

Available information on charge injection devices suggests that

these often form what Cloupeau and Prunet-Foche callramified

jets. At sufficiently high voltages, multiple jetting from the

spinneret may also be observed in the conventional electrospin-ning process [8,16], but is not discussed further here. Tracer

particles have been used to reveal an axisymmetric circulation

flow within the cone, suggesting that the stable jet forms by

shearingfluid off the surface of the cone[9]. Ganan-Calvo[17]

has developed an asymptotic solution for the cone-jet configu-

ration that provides expressions for the electric current, cone-jet

shape, and charge distribution within the jet. Yan et al.[18]have

modeled the steady jet equations of the cone-jet numerically as a

2-dimensional free surface flow.

5. Slender thinning jet

As it emits from the cone, the fluid jet forms a slender,

continuous liquid filament. The fluid is charged and accelerates

in the presence of the electric field created by the high potential

of the spinneret and by the charged fluid itself. Fig. 2. shows

images of several representative jets for different flow rates ( Q)

and applied electric fields (E). Numerous attempts have been

made to decipher trends in electrospun jet operation as functions

of fluid and operating parameters under experimental control.

Within certain limits, for a given fluid there exist ranges of

driving voltage (V) and flow rate over which the electrospinning

can be maintained stably for long periods. The electrospinning

process constitutes an electrical circuit, so one can measure as

well the current (I) flowing through this circuit. Numerousinvestigators have reported currentI increasing with voltage V

in a nonlinear fashion[16,1923]. Theron et al. and Demir et al.

fit their data to power laws, IVx, with values ofx =2.17 and

2.7, respectively, while Deitzel et al. and Shin et al. attributed

the nonlinear behavior to the onset of fluid instabilities which

they observed to occur at a critical voltage. Shin et al. [22]and

Demir et al. [16] also showed that for sufficiently large flow

rates, currentIis roughly proportional to flow rate Q, indicative

of a volumetric charge density,I/Q, independent of flow rate. At

low flow rates,I/Qtends to increase asIbecomes less sensitive

to Q [24]. Theron et al., on the other hand, reported IQx,

1.04bb0.26, for a range of fluids, which they attributed to

ion mobility-limited charging of the fluid in their equipment

configuration[23].

Mathematical description of the thinning liquid jet can be

formulated within the context of conventional electrohydrody-

namics[17]. One such exposition is that presented by Hohman

et al.[25]for Newtonian fluids, which makes use of the slender

body approximation to write perturbative expansions in the

aspect ratio of the jet for the relevant jet characteristics, such as

diameter (h), velocity (v), surface charge density () and local

electric field strength (E), in terms of radial (r) and axial (z)

coordinates. These are substituted into the equations for

conservation of mass (for an incompressible fluid), conserva-tion of charge, differential momentum balance and the

electrostatic field arising from a line of charge whose potential

is expressed by Coulomb's law. The resulting equations are then

truncated after the leading order terms, leading to a relatively

simple 1D model for the slender thinning jet. The relevant fluid

properties, all of which can be characterized independently of

the electrohydrodynamic model, are viscosity (), density (),

surface tension (), conductivity (K) and dielectric constant ().

Operational parameters for the electrospinning process, within

this model, are flow rate (Q), electric current (I), and applied

field (E

), the last of which arises as a consequence of the

applied voltage (V) and inter-electrode distance (D), E

= V/D.

The model was shown to describe well the jet shape for lowconductivity fluids. For high conductivity fluids, convergence

of the equations to a solution was found to be laborious, a

problem associated with specifying the (unknown) initial

surface charge density on the fluid jet exiting the spinneret.

Alternatively, Hartman [26] proposed a different boundary

Fig. 2. Cone-jets of 2% solutions of polyethylene oxide (MW=920 k g/mol) in water. D =45 cm in all cases. (a)Q = 0.02 ml/min,E=0.282 kV/cm; (b)Q =0.10 ml/min,E=0.344 kV/cm; (c) Q = 0.50 ml/min,E= 0.533 kV/cm; (d) Q = 1.00 ml/min, E=0.716 kV/cm. Images courtesy of Dr. Jian Yu.

1386 G.C. Rutledge, S.V. Fridrikh / Advanced Drug Delivery Reviews 59 (2007) 13841391


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condition with specified initial slope (dh/dz)0 at the nozzle.

Feng [27] subsequently reformulated the slender body model

for Newtonian jets using an approximation for the electric field

equation that, fortuitously, circumvents the convergence

problems of Hohman et al. The resulting steady state equations

can then be written as follows[27]:

h2v 1 1

Eh2 Pehvr 1 2

vv0 1

Fr

3

Re

1

h2 gh2v0 0

1

We

h0

h2

X rr0 bEE0 2Er

h

3

E El

ln v rh

0

b

2 Eh

2 00 4

where the relevant dimensionless groups (indicative of the

relative strengths of different terms) are:

electric Peclet number charge convection=conduction:

Pe2

P

e v0

Kh05

Froude number: inertia=gravity Fr v20gh0

6

Reynolds numberinertia=viscosity: Re

qv0h0

g 7

Weber number:inertia=surface tension Weqv20h0

g8

Aspect ratio: v D

h09

Electrostatic force parameter electrostatic=inertia

XP

eE20qv20

10

Charge induction beP

e1 11

and the corresponding characteristic values employed are h0forlength,v0 Q= ph

20

for velocity, E0 I= ph

20K

for electric

field and 0=E0for charge density.is the dielectric constant

of the outer fluid (typically, air in conventional electrospinning).

Subscript 0 is used to indicate values taken at the nozzle.

Gravitational and aerodynamic contributions were analyzed by

Reneker et al. and determined to be inconsequential for the

steady electrospun jet[28].

In general, the results of Hohman et al. and of Feng indicate

that the shape of the jet is strongly dependent upon the evolution

of surface charge density and local electric field. Concurrent

with the initial rapid reduction in jet diameter, the latter two

quantities rise quickly to maximum values as charges relax to

the jet surface and surface advection current becomes more

important relative to bulk conduction current. The characteristic

length over which the initial dramatic thinning of the jet takes

place can be identified with the axial distance where advectionand conduction currents are equal. Analysis of these equations

(Fridrikh, unpublished) leads to the following relation for this

nozzle regime length L:

L5 K4Q7q3 ln v 2

8p2El

I5P

e 2 12

Beyond this characteristic length, the jet thins more slowly.

Sufficiently far from the nozzle (circa 30h0), the jet approaches

the asymptotic regime where all terms except electrostatic and

inertial must eventually die out. In this asymptotic regime, the

following scaling relations for jet diameter, charge density and

local electric field[29,30]have been derived:

h Q3q

2p2El

I

14

z14 13

r I3q

32p2El

Q

14

z14 14

EEl

IQq

2El

12 lnv

4pP

e

z

32 15

The scaling expression for jet diameter was first derived by

Kiricheneko et al.[29]and more generally for pseudoplastic and

Fig. 3. Schematic illustration of perturbations associated with several of the

lowest order instabilities, distinguished by their azimuthal wave number, s. Top

views illustrate cross-sections of the jet at maximum amplitudes of (oscillatory)

perturbation, with bold and dashed contours representing different positions

along the jet length. Bottom views illustrate changes in shape and centerline

down the length of the jet.+/ are used to indicate regions of positive or negative

deviation from the unperturbed jet shape. Perturbations are exaggerated beyond

the linear instability regime. (a) unperturbed cylindrical fluid element; (b)

varicose (s =0) instability; (c) whipping (s =1) instability (also called bending

or kink instability in the literature); (d) splitting (s =2) instability. Growth of

the varicose instability leads to equal sized droplets; growth of the splitting

instability leads to two equal sized sub-jets. Higher order (sN2) instabilities are

also conceivable.



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dilatant fluids by Spivak and Dzenis [31]. It was confirmed

experimentally by Shin et al.[22].

In between the nozzle regime and asymptotic regime,

viscosity may play a role in determining the rate of thinning

of the jet. A balance between electrostatic and viscous terms

(Fridrikh, unpublished) suggests the following scaling in this

regime:

h 6gQ2

pEl

I

12

z1 16

Feng [27] and more recently Carroll and Joo [32] have

furthermore extended the slender jet model to include

viscoelastic effects. Here too, the primary role of extensional

thickening is to alter the shape of the jet in the intermediate

regime of the thinning jet, resulting in sharply thinning cone-jets

such as that shown in Fig. 2a. Recent experimental studies of

model Boger fluids by Yu et al. confirm the rapid initial thinning

of the cone-jet, for viscoelastic fluids, and suggest theemergence of a new regime where the scaling of diameter

with distance along the jet increases from the inverse 1/4 power,

typical of Newtonian fluids, towards the inverse 1/2 power for

fluids with strong extensional thickening[33].

Although illuminating, these analytical solutions for the steady

jet behavior are limited by the breakdown of the slender jet

approximation in the nozzle regime, where the cone-jet occurs.

Numerical methods such as that employed by Yan et al. [18]for

the cone-jet can be matched to the slender thinning jet solutions.

6. Jet instability

In principle, the cone-jet operation is sufficient to draw outcontinuous fibers to very small diameter by electrospinning.

However, the fluids typically used in electrospinning do not

always solidify sufficiently en route to the collector to remain

fibrous after impact on the collector. In practice, as the jet thins,

it ultimately succumbs to one or more fluid instabilities which

distort the jet as they grow. A family of such instabilities exists

and can be analyzed for different conditions of the symmetry

(axisymmetric or nonaxisymmetric) of the growing perturba-

tions of the jet, using linear instability analysis [34,35].Fig. 3

illustrates the perturbations associated with several of the lowest

order instabilities. Although splaying of the jet was proposed in

the 1990's as the primary mode of instability [5], and someevidence has been found for splitting in post mortem

micrographs of fibers[36]and for secondary jetting in images

of jets[21,28](a particularly beautiful example appears in the

book by Frankel[37]), such events are relatively rare; the most

common mode of instability in electrospun jets appears to be the

growth of lateral excursions of the jet, the so-called whipping

mode[3,28,38]. This phenomenon is illustrated inFig. 4for jets

of water and polymer solution; it should be noted that this

instability is not a consequence of viscoelasticity of the fluid

and may occur for Newtonian[25,39]as well as non-Newtonian

fluids. The main competing mode of instability, at least for the

relatively dilute solutions often employed to achieve the

smallest fiber diameters in electrospinning, is that of droplet

break-up, the mode that leads to electrospraying. For the steady

thinning jets described above, Hohman et al.[25]performed the

linear instability analysis for both droplet break-up and

whipping modes and produced operating diagrams for

electrospinning that illustrate the combinations of controllable

parameters (flow rate, Q, and applied field, E

= V/D) under

which a particular fluid will electrospin (whipping) rather thanelectrospray (droplet breakup). The full dispersion relation for

the whipping instability of viscous, charged fluids in a finite

electric field was reported by Hohman et al.[25]. In the vicinity

of the onset of whipping, charge repulsion is relatively strong

compared to electrical shear stress, and surface tension effects

are relatively minor, allowing one to neglect the contributions to

the dispersion relation arising from the electric field, surface

tension and finite conductivity. In this approximation, consid-

erable simplification of the dispersion relation is obtained

[40,41]:

xk4 3qQh2

4p3Reh0 k2

2pr2

P

e 32ln v x2q: 17

where is the growth rate and k is the wave number of the

perturbation. Solving for and differentiating this with respect

tok, one obtains the growth rate and wave number for the most

unstable (i.e. fastest growing) mode:

xmax 9p5r4h0Re

8h2q2QP

e 2 2ln v 3 2

1=318

kmax p3rh0Re

h2qQ 1=3

2pqP

e 2ln v 3

1=6

19

These equations reflect the destabilizing effect of charge

repulsion (strongest for short wave length perturbations) and the

stabilizing effect of viscosity (also strongest for short wave

length perturbations), as well as the damping of the instability

growth rate due to inertia.

With respect to droplet breakup, surface stresses due to

charge-charge repulsion and due to interaction of charges on the

fluid with the applied electric field serve to destabilize the

whipping instability relative to droplet breakup, making the

former the more important mode at high field strength and

charge density. As a caveat, however, it should be borne in mind

that multiple instabilities can grow simultaneously, and even awhipping jet can ultimately decay into droplets, as illustrated by

the water jet inFig. 4. Of particular note, Hohman et al. reported

a second, electrically driven droplet breakup instability that

becomes important at high electric fields in fluids of finite

conductivity, in addition to the conventional, surface tension-

driven instability. Hohman et al. interpreted their equations for

both droplet breakup and whipping instabilities in terms of their

underlying physical mechanisms for a number of limiting cases.

From the equation of motion reported by Hohman [25],

Fridrikh et al. [24] also obtained an expression for the lateral

growth of the jet excursions arising from the whipping

instability far from its onset, deep in the nonlinear regime.

This equation contains terms due to acceleration of the charged



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7/8

presence can have benefits for selected applications, such as

wettability [49]. At least one strategy to overcome this

phenomenon involves stabilization of the jet against the droplet

breakup by introducing a more viscoelastic outer fluid in a

two-fluid co-axial jet configuration[5052]. The resulting core-

shell fiber morphology may be of interest in its own right;

alternatively, either the shell or core may be removed to yieldsolid or hollow fibers, respectively. On a related note, Gupta and

Wilkes [53] have reported a side-by-side configuration for a

two-fluid spinneret.

7. Fiber collection

Electrospun fibers are charged in flight, as described above,

and travel downfield until impact with a lower potential

electrode, or collector. A particular strength of the electropin-

ning process is that fiber collection may be guided by

manipulation of the electric field lines. The most straightfor-

ward configuration to analyze is the parallel electrodeconfiguration described by Taylor [7] and subsequently

modified for electrospinning by Shin [36] and illustrated in

Fig. 1 earlier. In this configuration, static parallel plate

electrodes are used at both the nozzle and the collector; this

configuration ensures parallel field lines throughout the process

zone, while the field configuration around the nozzle helps to

suppress corona discharge. A similar observation was made

using a ring electrode near the nozzle[19]. For the simplest case

of a static, planar collection electrode, the conventional

electrospinning process is axially symmetric and provides no

preference of direction for the fibers upon collection, resulting

in a nonwoven membrane of unoriented fibers. The integrity of

the nonwoven fabric depends on the random overlapping offibers, interactions between fibers, and welding that may

occur at contact points between fibers if they are still somewhat

fluid when collected. However, the axial symmetry of the

process is easily overcome by a number of strategies, thus

resulting in different degrees of fiber orientation in the final yarn

or membrane product. One such strategy involves use of a

rotating drum[54]or moving belt collector to produce oriented

fiber membranes. A rotating wheel with beveled edge has been

used successfully to collect fibers into an oriented, multifila-

ment thread[55]. Shaped collectors such as pins [56], parallel

bars[57], or even simple wire meshes can be used to direct fiber

collection preferentially, as the charged fibers tend to formbridges between pins or bars when appropriately spaced. In this

manner, small area arrays of highly oriented fibers have been

obtained. Rotating one of a pair of parallel disks or rings can be

used to produce short sections of twisted yarn[58]. For more

complicated collector shapes, build-up of the polymer mem-

brane on portions of the collector can alter the field lines to

ensure conformal coating of the object. Introduction of

secondary ring electrodes can be used to prolong the steady

jet regime and to focus the fiber deposition, as mentioned earlier

[42]. For the rare case where fiber instabilities are completely

absent, oriented fibers can be collected by writing, for

example on the face of a rotating wheel[59]. Conductive liquid

nonsolvents have also been used as the collector, resulting in a

wet-spinning variant of the electrospinning process and

production of fibers from solutions of nonvolatile liquids[6,60].

8. Conclusion

The main attractions of electrospinning to date have been the

ease of implementation on a lab scale and the very small diameterfibers that are readily obtained. As emphasized by Dzenis [61],

electrospinning is a top-down manufacturing process, which

offers certain advantages to fiber formation in terms of cost and

productivity over more complicated bottom-up approaches.

Fibers diameters between 100 nm and 1 m are most typical,

although fibers with an average diameter as small as 30 nm [62]

and as large as 10 m have been reported. This is two to three

orders of magnitude smaller thanconventional, commercial fibers

(10500 m) and the typical human hair (ca. 100 m).

Furthermore, in contrast to other forms of nanofibers (e.g. carbon

nanotubes), electrospun fibers are essentially continuous. This is

supported by both modeling and observations; fiber ends arerarely observed in micrographs of fibers obtained during steady

state operation. The continuous nature of the fibers is potentially

important for occupational health reasons.

Small diameter implies that a large fraction of the material is

near the surface of the fiber, and molecules embedded within the

fiber encounter shorter diffusion path lengths to the fiber

surface. Commensurate with the reduction in diameter of these

fibers, the resulting nonwoven membranes typically have very

high specific surface area, high porosity and small pore size

(pore size being equated here with some characteristic distance

between fibers) compared to other fibrous materials. These

qualities make electrospun materials attractive as candidates for

tissue engineering scaffolds, drug delivery systems, catalyst andenszyme supports, sensors, and other applications in biology,

medicine and controlled release.

Acknowledgements

The authors are grateful to the National Textile Center

(Project #'s M98-D01 and MD01-D22) and the US Army

Institute for Soldier Nanotechnologies at MIT (ARO contract

DAAD-19-02-D0002) for funding and to our several colla-

borators and team members for enlightening discussions.

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