review on electrospinning for nanofiber design and

MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM
22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO
REVIEW ON ELECTROSPINNING FOR NANOFIBER DESIGN AND
MANUFACTURING
Jorge Alberto Wadgymar Gutiérreza, José Israel Martínez Lópezb, Alex Elías Zúñigac, Héctor R. Siller Carrillod,
Ciro A. Rodríguez Gonzáleze
a, c
Department of Mechanical Engineering, Tecnológico de Monterrey, Eugenio Garza Sada 2501 Sur, C.P. 64849
Monterrey, NL, México
b,d, e
Center for Innovation in Design and Technology, Tecnológico de Monterrey, Eugenio Garza Sada 2501 Sur, C.P.
64849 Monterrey, NL, México
[email protected], [email protected], [email protected],
[email protected], [email protected]
RESUMEN.
ABSTRACT.
El método de electrohilado ha sido reconocido
como una técnica eficiente para la fabricación de
nanofibras poliméricas. Gran cantidad de
polímeros han sido electrohilados exitosamente en
finas fibras, las cuales tienen potenciales
aplicaciones en el desarrollo de nanocompuestos,
ingeniería del tejido, industria textil, industria
militar, cosméticos y otras aplicaciones
industriales. En este artículo se presenta un estado
del arte del método de electrohilado para la
fabricación de nanofibras, los parámetros que
influyen en este proceso para la obtención de
fibras, algunos materiales que han sido
electrohilados con potenciales aplicaciones en la
industria, modelación y caracterización de
nanofibras. Finalmente, se presenta una breve
descripción de los avances en el diseño y la
manufactura de nanofibras en el Tecnológico de
Monterrey.
Electrospinning has been recognized as an
efficient technique for the fabrication of polymer
nanofibers. Several polymers have been
successfully electrospun into fine fibers, which
have potential applications in nanocomposite
development, tissue engineering, textile and
military industry, cosmetics and other industrial
applications. In this paper is presented a state-ofthe-art of electrospinning for nanofiber
fabrication, the parameters that have an influence
on this process, and some materials that have been
electrspun with potential applications in industry,
modeling and characterization of nanofibers.
Finally, a brief description of the advances on
design and manufacturing of nanofibers at
Tecnológico de Monterrey is presented.
Nomenclature
Vc
H
L
R
γ
P
σzz
R0
λ3
c1, c2
critical voltage
distance between the capillary tip and the
collecting screen
length of the capillary tube
radius of the tube
surface tension of the fluid
total axial tensile force
true axial tensile stress
current fiber radius after stretching
principal stretch of the material element
in the axial direction
two independent material constants
ISBN: 978-607-95309-3-8
R A, R B
τrr, τzz,
τθθ
f(R)
Ri
τ
b
M
m
T
initial interior and exterior radio of the
fiber respectively
shear stress components
material incompressibility
initial radii centered at a point
Cauchy stress-softened tensor
softening parameter
maximum strain intensity
strain intensity
Cauchy stress tensor
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Introduction
Electrospinning, a broadly used technology for
electrostatic fiber formation which utilizes
electrical forces to produce polymer fibers with
diameters ranging from 2 nm to several
micrometers using polymer solutions of both
natural and synthetic polymers has seen a
tremendous increase in research and commercial
attention over the past decade. This growth in
interest can be shown in the increase over 1000%
of the number of publications in less of ten years
in articles regarding this process (Fig. 1) and in
the development of an industry focused on new
commercial products that take advantages of the
properties of the nanofibers produced with
electrospinning.
No. of Publications
800
separation, and electrospinning, which are briefly
described in Table 1.
Electrospinning generates continuous, uniform,
long fibers with diameters down to the nanoscale
dimension. The advantages of the electrospinning
technology make it suitable for small quantity
production for laboratory research use and mass
production for industrial use. Electrospinning
seems to be the only method which can be further
developed for mass production of one-by-one
continuous nanofibers from various polymers. In
this paper, different setups for nanofiber
fabrication are shown; materials that have been
electrospun for different applications, parameters
that influence the process, modeling and
characterization of nanofibers are described.
Process
600
400
200
0
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
Publication Year
Fig. 1 Number of scientific publications of electrpspinning per year
in the last decade.
(Source: ISI Web of Knowledge from Elsevier)
This process offers unique capabilities for
producing novel natural nanofibers and fabrics
with controllable pore structure. Electrospun
fibers have been successfully applied in various
fields, such as, nanocatalysis, tissue engineering
scaffolds,
protective
clothing,
filtration,
biomedical, pharmaceutical, optical electronics,
healthcare, biotechnology, defense and security,
and environmental engineering. A diagram in Fig.
2 shows the different potential applications of
electrospun nanofibers.
In the electrospinning process a high voltage is
used to create an electrically charged jet of
polymer solution or melt out of the pipette. One
electrode is placed into the spinning solution/melt
and the other attached to the collector. The
electric field is subjected to the end of the
capillary tube that contains the solution fluid held
by its surface tension. This induces a charge on
the surface of the liquid. Before reaching the
collecting screen, the solution jet evaporates or
solidifies, and is collected as an interconnected
web of small fibers.
Dong et al. (1) has shown the different kinds of
electrospinning set-ups (Fig. 3). By using these
different setups, electrospinning can produce
different nanofibrous structures with various twoor three-dimensional shapes, including aligned
nanofibers, nanofibrous yarn, tubular structures,
and core-shell nanofibers.
Nanofibrous structure can be generated using
mainly three methods: self-assembly, phase
ISBN: 978-607-95309-3-8
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Filtration
Energy generation
Nanocomposites
NANOFIBER
APPLICATIONS
Cosmetics
Nano-sensors
Tissue Engineering
Scaffolding
Biomedical
Protective clothing
Fig 2. Applications of electrospun nanofibers (References: (2) and (3)).
Process
Scalable
Convenient to
process
Control on fiber
dimension
Fiber diameters
Types of
materials
Advantages
Disadvantages
Phase Separation
Solvent extraction from
gelated polymer solution
to form nanofibers
Self-Assembly
Molecules organize and
arrange themeselves into
an ordered structure
Electrospinning
Uses static electricity to
draw fibers from polymer
solution
X

X
X


X
X

50 – 500 nm
Organic, Biopolymers
7 – 100 nm
Organic
 Minimum equipment
required
 Batch-to batch
consistency
 Tailorable mechanical
properties and pore size
 Limited to specific
polymer
 Matrix directly
fabricated
 Good for obtaining small
nanofibers
3 nm – 1 µm
Organic, Inorganic,
Metallic, Biopolymers
 Cost effective
 Long continuous fibers
 Production of aligned
fibers
 Tailorable mechanical
properties, size and shape
 Jet instability
 Large nanometer to
micron scale fibers
 Use of organic solvents
 Complex process
 Limited to a few
polymers
Table 1. Comparison of three different methods of nanofiber fabrication (adapted from (1), (4) and (5))
ISBN: 978-607-95309-3-8
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A
B
C
D
E
Fig. 3 Schematic diagrams of different electrospinning set-ups. (A) Standard electrospinning setup. (B) Aligned electrospinning.
(C) Nanofibrous yarn. (D)Tubular structure. (E) Core-shell nanofiber.(Taken from (1)).
Other Electrospinning Setups
Multi-jet
Several modifications in the general setup of the
electrospinning process have been made in order
to improve the efficiency of this process. Some
researchers have tried to increase productivity and
covering area by creating arrays of nozzles with or
without additional electrodes. Despite this
technique results are contradictory; the general
consideration is that the mutual interactions
between jets and the additions of elements make
this technique becomes more difficult. (Theron et
al., 2005 (6)).
2002 (8)). Meanwhile, this setup offers a
continuously production with high production
capacity (1 g min-1 m-1) (He et al., 2008 (9)).
Fig. 4 NanospiderTM technology
(Taken from www.elmarco.com)
Nanospider (Needless electrospinning)
Based in the form free surface electrospinning of
Yarin (Yarin et al., 2004 (7)) there is a patented
alternative called NanospiderTM (Fig. 4), a
polymer solution is for spinning supplied into the
electric field using a surface of a rotating charged
cylindrical electrode. No syringes, capillaries or
nozzles are needed. The production rate of a
conventional electrospinning system is in the
range of 10–100 mg fiber per min (Tsai et al,
ISBN: 978-607-95309-3-8
Electrospinning Parameters
Many parameters can influence the transformation
of polymer solutions into nanofibers.
The parameters can be classified into process
parameters, solution parameters and ambient
parameters.
 Process Parameters
 Solution Parameters
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 Ambient Parameters
A review about the effects of parameters on
electrospinning is presented in (2) and (3). A
summary of this review is presented in this article
and Table 2 shows a brief description of the
variables affected by each parameter.
Process Parameters
Applied Voltage
There is a little dispute about the behaviour of
applied voltage in the electrospinning process.
Some investigations have showed that there is not
much effect of electric field on the fiber diameter;
other researchers have suggested that higher
voltages facilitate the formation of a larger
diameter fiber. Other authors have reported that an
increase in the applied voltage favours the
narrowing of fiber diameter. In most cases, a
higher voltage causes a reduction in the fiber
diameter.. At a higher voltage there is also greater
probability of beads formation. The voltage
influences fiber diameter, but the level of
significance varies with the polymer solution
concentration and on the distance between the tip
and the collector (N. Bhardwaj, S.C. Kundu, 2010
(3)).
Flow rate
It has been observed that the fiber diameter and
the pore diameter increase with an increase in the
polymer flow rate in the case of polystyrene (PS)
fibers. A lower feed rate is more desirable as the
solvent will get enough time for evaporation. If
the flow rate is high, beaded fibers are obtained
due to the unavailability of proper drying time
before reaching the collector.
Types of collectors
Generally, aluminum foil is used as a collector but
due to the need for aligned fibers for various
applications, other collectors such as, conductive
paper, conductive cloth, wire mesh, pin, parallel
or gridded bar, rotating rod, rotating wheel, liquid
non solvent such as methanol coagulation bath
and others are also common types of collectors
nowadays. The fiber alignment is determined by
the type of the target/collector and its rotation
speed.
ISBN: 978-607-95309-3-8
Distance tip-collector
It has been found that a minimum distance is
required to give the fibers sufficient time to dry
before reaching the collector, otherwise with
distances that are either too close or too far, beads
have been observed.
Needle diameter
X. M. Mo et al. (10) found that the formation of
beads was influenced by the needle diameter.
With larger needle diameters beads and clogging
were observed meanwhile with a thicker needle
diameter no beads or clogging were found.
Solution Parameters
Concentration
It has been found that at low solution
concentrations, beads are obtained meanwhile at
higher concentrations results in fewer beads but
larger nanofiber diameters (Z.-M. Huang et al. (2),
N. Bhardwaj, S.C. Kundu 2010 (3)).
X. M. Mo et al. (10) found that a low
concentration of the P(LLA-CL) copolymer
solution (less than 3wt%) caused the formation of
beads in the electrospun nanofibers. Also the
concentration affected the fiber diameter, by
increasing the concentration the larger the fiber
diameter.
Molecular weight
This parameter has an important effect on some
properties such as viscosity and surface tension. It
has been observed that a low molecular weight
solution tends to form beads instead of fibers and
a high molecular weight solution tends to form
fibers with larger average diameter (N. Bhardwaj,
S.C. Kundu 2010 (3)).
Viscosity
The solution viscosity is an important parameter
that influences the fiber size and morphology
during electrospinning. With low viscosity there is
no continuous fiber formation and with high
viscosity difficulty in the jet ejection from
polymer solution is presented. Researchers have
reported maximum spinning viscosities ranging
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from 1 to 215 poise (N. Bhardwaj, S.C. Kundu
2010 (3)).
Surface tension
The formation of droplets, bead and fibers
depends on the surface tension of solution and a
lower surface tension of the spinning solution
helps electrospinning to occur at a lower electric
field (N. Bhardwaj, S.C. Kundu 2010 (3)).
Ambient parameters
Parameter
Voltage
Flow rate
Type of collector
Tip- Collector distance
Apart from solution and processing parameters,
there are also ambient parameters that include
humidity, temperature etc. Researchers have
found that with increase in temperature, there is a
yield of fibers with decreased fiber diameter. The
variation in humidity while spinning polystyrene
solutions has been studied and shows that by
increasing humidity there is an appearance of
small circular pores on the surface of the fibers;
further increasing the humidity leads to the pores
coalescing.
Variables affected
Process variables
Fiber diameter
Rate of evaporation of solvent
Jet velocity
Material transfer rate
Fiber alignment parallelism
Whole process
Solution variables
Jet velocity
Concentration
Molecular weight
Whole process
Fiber diameter
Viscosity
Whole process
Surface tension
Minimum electric field
An increase typically causes
Reduction (-)
Increase (+)
Increase (+)
Increase (+)
Increase (+)
(using rotating device)
A minimum distance is necessary
Reduction(-)
Low: formation of beads;
High: interruption of fibers formation.
Increase (+)
Low: difficulties in fiber formation;
High: not allow the jet formation
Reduction (-)
Table 2. Typical effect of process parameters in electrospinning.
Characterization
Geometrical Characterization
Geometric properties of nanofibers such as fiber
diameter, diameter distribution, fiber orientation
and fiber morphology can be characterized using:
 Scanning electron microscopy (SEM)
 Field emission scanning electron microscopy
(FESEM)
 Transmission electron microscopy (TEM)
 Atomic force microscopy (AFM)
availability of accurate testing apparatus, few
papers
discuss
about
the
experimental
characterization for the mechanical properties of a
single nanofiber.
Three testing apparatus are available to measure
tensile and bending properties of a single
nanofiber: 1.Cantilever technique, 2.AFM-based
nanoindentation system 3.Nano tensile tester.
Fig. 5 shows a schematic diagram of the
cantilever technique for tensile testing of single
nanofibers.
Mechanical characterization
Single Nanofiber
Due to its extremely small diameter, the handling
difficulty to extract a single fiber from the
electrospun nanofiber web and also non-
ISBN: 978-607-95309-3-8
Fig. 5 Cantilever techinique for tensile testing of single
nanofibers (Taken from (5)).
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solution through a millimeter diameter capillary
tube into solid fibers.
A bending test of single nanofibers can be
performed using AFM-based nanoindentation
system, as shown in Fig. 6.
Fig. 6 AFM-based nanodindentation system
for bending test (Taken from (5)).
A tensile test of continuous fibers from the range
of few millimeters to several centimeters in length
can be performed by a commercial nano tensile
testing system. Tan and Lim (11) conduct a tensile
test of a single polymer nanofiber using a nano
tensile tester (Nano Bionix System, MTS, USA).
Nanofiber Membrane
Mechanical tests of nanofibrous nonwoven
membranes can be performed using conventional
testing techniques. Xu et al. (12) obtained the
typical stress-strain curve (Fig. 8) of P(LLA-CL)
nanofibrous scaffold under tensile loading. Li et
al. (13) obtained a stress-strain curve of PLGA
electrospun nanofibers, which were found to be
suitable for soft tissue.
When the applied electrostatic forces overcome
the fluid surface tension, the electrified fluid
forms a jet out of the capillary tip towards a
grounded collecting screen.
Jet initiation
The formation of fine threads from viscous liquid
drops in an electric field is due to the maximum
instability of the liquid surface induced by the
electrical forces. An issue related with the
initiation of the jet is the strength of the
electrostatic field required. Taylor also showed
that the critical voltage Vc (expressed in kilovolts)
at which the maximum jet fluid instability
develops is given by Eq. 1
2
H  2L

2
Vc  4
ln
 1.5 0.117R 
2  R

L
(1)
Modeling of single nanofiber
Some studies have been focused on the
development of a model that describes the
behavior of polymer nanofibers under tension. Wu
and Dzenis (14) studied the size effect on the
elastic behavior of solid and hollow polymer
nanofibers subjected to uniaxial stretching. In
their work, a one-dimensional nonlinear elastic
model was developed, considering the coupling
effect of surface tension and fiber radius on the
tensile response of these nanofibers.
Considering a thin solid cylindrical polymer fiber
subjected to uniaxial stretching (Fig. 9a) as
hyperelastic isotropic material, Wu and Dzenis
(14) obtained the relation for the total axial tensile
force P (Eq. 2) and for the true axial tensile stress
σzz (Eq. 3).
Fig. 8 Stress-strain curve of electrospun P(LLA-CL)
nanfibrous scaffold (Taken from(12)).
Modeling and Simulation

c  R 
1 
P  R02  3  2  c1  2   0
3 
3 
3

Modeling of Electrospinning Process
 zz 
It is necessary to understand how the
electrospinning process transforms the fluid
ISBN: 978-607-95309-3-8


c 
P
1 
  3  2  c1  2 
2
3 
R0 
3 

(2)
(3)
R0  3
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Organic
PA 6
PA 6/12
PAI
PAA
PUR
PES
PVA
And for a thin hollow cylindrical fiber subjected
to uniaxial stretching (Fig. 9b), the relations for
the total axial tensile force P (Eq. 4) and for the
true axial tensile stress σzz (Eq. 5) are:


P  π RAB 2 τ zz  τ rr  τ θθ Rf(R) Rf'(R)  f(R) dR
R


 πγ R A f(RA )  RB f(RB )
 zz 





R
R B 2 zz   rr    Rf ( R ) Rf ' ( R )  f ( R ) dR
A
R B f ( R B )  R A f ( R A )
2
R A f ( R A )  RB f ( RB )
2
2
RB f ( RB )
 RA f (RA )

(4)
 
2
(5)

The discovered size effect on the nanofiber
deformation should be taken into account for
precise prediction of their properties and
mechanical response (14).
(a)
(b)
PAN
PEO
PS
PVDF
PVP
PVP-I
Inorganic*
Metallic
Biopolymer
TiO2
SiO2
Al2O3
ZnO
Li4Ti5O12
ZrO2
MgAl2O4
Pt
Cu
Mn
Gelatin
Chitosan
Collagen
Cellulose
PLA
PCL
* After calcination or post-processing treatment
Table 3. Materials for nanofiber fabrication with
NanospiderTM technology
(Source: www.elmarco.com)
Applications
Electrospun nanofibers are broadly applied in
many applications shown previously in Fig. 1. In
the medical field it has been seen that
electrospinning can bring a suitable substitute to
the natural extracellular matrix, and soft tissues as
skin or cartilage due the features of a wide range
of pore size distribution, high porosity, and high
surface area-to-volume ratio, which are favorable
parameters for cell attachment, growth and
proliferation. (Li et al., 2002 (13)).
Nowadays,
nanofiber
membranes
are
commercially available (Fig. 10) for this kind of
applications showing good properties, which
make them suitable for tissue engineering
applications.
Fig. 9 Coordinate systems for the analysis of (a) solid cylindrical
nanofibers and (b) hollow nanofibers (Taken from (14)).
Electrospun Materials
There are a wide variety of polymers that have
been used to produce electrospun nanofibers,
including natural polymers, synthetic polymers
and blends of both, biopolymers. Besides
polymers, inorganic and metallic materials have
been used to produce nanofibers by
electrospinning. Table 3 shows some materials
that have been processed to produce nanofibers
with the NanospiderTM technology.
ISBN: 978-607-95309-3-8
Fig. 10 Commercial nanofiber membrane (Nanosan®, SNS Nano
Fiber Technology, LLC)
Advances
on
Nanofiber
Design
and
Manufacturing at Tecnológico de Monterrey
Nanofiber Design
The modeling of a single nanofiber considers it as
a hyperelastic isotropic material, which is a good
approach for rubber-like polymers. In the topic of
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rubber-like materials, studies have been
performed in order to describe phenomenological
models
that
complement
the
softening
phenomenon exhibited by rubber-like materials.
The Mullins effect in isotropic, hyperelastic,
incompressible
rubber-like
materials
was
investigated by Zúñiga and Beatty (17)
considering a rubber cord under uniaxial
extension. The uniaxial stress in the elastic stresssoftened material is given by:
  e b
M m
Τ
(6)
Calva (18) proposed a model to predict
analytically the softening phenomenon of rubberlike materials, showing a good prediction when
compared to experimental data for uniaxial
extension. This type of material models are the
basis for the design of nanofiber devices for
applications in tissue engineering and biomedical
fields.
Nanofiber Manufacturing
Tecnológico de Monterrey is working in the
design and fabrication of an electrospinning
apparatus in order to produce nanofibers,
characterize and control the parameters; and
optimizes nanofiber production, trying to get good
properties for applications in tissue engineering
scaffolds.
The main components of a basic electrospinning
apparatus are:
 High Voltage Power Supply
 Syringe Pump
 Ground Collector
The high voltage will be applied with the Gamma
Model ES20P-5W/DAM (Gamma High Voltage
Research). A Single-Syringe Infusion Pump KDS
100 (KD Scientific) was selected to pump the
solution, however, by now instead of using a
commercial syringe pump, a stepper motor and a
ball screw will be used to pump the polymer
solution out of a syringe, and the ground collector
will be an aluminum disc having a radius of about
15 cm. A schematic diagram of the design of our
electrospinning apparatus is shown in Fig. 11.
ISBN: 978-607-95309-3-8
Fig. 11 Schematic diagram of electrospinning
apparatus by Tecnológico de Monterrey
Conclusions
Electrospinning
has
had
a
tremendous
development in the last two decades due to the
unique capabilities to produce nanofibers, which
are lately being of great interest for industry in
many applications such as tissue engineering,
biomedical applications, protective clothing
among others.
Desirable properties of the nanofibers of interest
include primarily their mechanical behavior and
biological characteristics such as biocompatibility.
Different setups have been developed in order to
reach a higher productivity and investigations
changing different parameters have been done in
order to get better properties in nanofibers.
Because of the limitations that electrospinning
has, practical applications of nanofibers are
limited so far. However, recent advances in this
technique are promising to improve the efficiency
of this method, so more applications can be
realized.
Acknowledgments
This work supported by the Research Chairs of
Materials and Nanotechnology and of Intelligent
Machines of the Tecnológico de Monterrey,
Campus Monterrey, and also, to the CONACYT
project: Synthesis and Constitutive Modeling of
Biocompatible Polymers for Microfluidic Devices
# 61061.
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“Degradation
Behaviors
of
Electrospun
Resorbable Polyester Nanofibers”, Tissue Eng
(2009).
(2) Zheng-Ming Huang, Y.-Z. Zhang, M. Kotaki,
S. Ramakrishna. “A review on polymer nanofibers
by electrospinning and their applications in
nanocomposites”, Composites Sci and Tech
(2003).
smooth muscle cell and endothelial
proliferation”, Biomaterials (2004).
cell
(11) E.P.S. Tan, C.T. Lim, “Mechanical
characterization of nanofibers – A review”, Comp
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ISBN: 978-607-95309-3-8
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