Electrospinning: Materials, Processing, and Applications

251 Pages·2012·3.266 MB·English

by  Prof. Dr. Joachim H. Wendorff, Prof. Dr. Seema Agarwal, Prof. Dr. Andreas Greiner(auth.)

Joachim H. Wendorff,
Seema Agarwal, and
Andreas Greiner
Electrospinning
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Joachim H. Wendorff, Seema Agarwal, and Andreas Greiner
Electrospinning
Materials, Processing, and Applications
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35032 Marburg other items may inadvertently be inaccurate.
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V
Contents
Preface IX
1 Introduction 1
1.1 Fibers – Key Functional Elements in Technology and Nature 1
1.2 Some Background Information 4
1.2.1 Structure of Crystalline and Amorphous Materials 4
1.2.2 Chain Conformation and Structures of Polymer Materials 7
1.3 Processing of Polymer Materials towards Fibers – Fiber Extrusion 10
1.4 Routes to More Advanced Fibers – Mimicking Nature 12
1.5 Electrospinning 16
1.5.1 A Simple Experimental Approach 16
1.5.2 The Hidden Physical Complexity 20
1.5.3 Short History of Electrospinning – The Routes Go Back More Than Two
Centuries 22
1.6 Electrospinning – Important Facts to Remember 24
References 26
2 Nature of the Electrospinning Process – Experimental Observations and
Theoretical Analysis 29
2.1 Experimental Setups 29
2.2 Experimental Observations on Fiber Formation 33
2.2.1 Droplet Formation at the Tip of the Die 34
2.2.2 The Straight Path of the Jet 36
2.2.3 The Looping Part of the Jet – Onset of Bending Instabilities 40
2.2.4 The Deposition Process 42
2.3 Theoretical Analysis of the Nature of the Electrospinning
Processes 45
2.3.1 Droplet Deformation and Onset of Jetting 46
2.3.2 Linear Path of the Jet 52
2.3.3 The Path of the Jet Submitted to Bending 55
2.3.4 Deposition on a Substrate 59
2.3.5 Onset of Further Instabilities 60
2.3.6 Branching Processes 66
VI Contents
2.4 Nature of the Electrospinning Process – Important Facts to
Remember 66
References 67
3 Nanofi ber Properties 69
3.1 Parameters Controlling Nanofi ber Formation 69
3.2 Short Account on Methods of Analysis for the Structure of Electrospun
Nanofi bers 72
3.2.1 Analysis of Fiber Geometry and Topology 72
3.2.2 Analysis of the Internal Fiber Morphology 76
3.3 Control of Nanofi ber Diameters 78
3.3.1 The Feeding Rate 78
3.3.2 Concentration of Polymer Spinning Solution 80
3.3.3 Electrical Parameters: Applied Voltage, Electrode Distance,
Conductivity 83
3.3.4 Additional Factors 84
3.3.5 Mixtures of Solvents/Nonsolvents 84
3.4 Shape of the Fibers 84
3.5 Nanofi ber Topologies, Porous Fibers 88
3.6 Nanofi ber Trajectories in the Deposition Plane 92
3.7 Internal Morphology of Electrospun Nanofi bers 96
3.7.1 Amorphous Polymers 97
3.7.2 Partial Crystalline Nanofi bers 99
3.8 Mechanical Properties of Single Nanofi bers 101
3.9 Nanofi ber Properties – Important Facts to Remember 103
References 103
4 Nonwovens Composed of Electrospun Nanofi bers 105
4.1 Nanofi ber Nonwovens – Functional Elements for Technical
Applications 105
4.2 Methods of Analysis for Properties on Nonwovens 106
4.2.1 Nonwoven Architecture and Total Porosity 106
4.2.2 Analysis of Pore Sizes 107
4.2.3 Internal Specifi c Surface and Permeation Coeffi cient 108
4.2.4 Permeation Coeffi cient 109
4.3 Fiber Arrangements in Nonwovens 109
4.4 Heterogeneous Nonwovens 112
4.5 Porosity and Pore Structures – Theoretical Modeling and Experimental
Analysis 113
4.5.1 Prediction of Monte Carlo Simulations 114
4.5.2 Experimental Results 117
4.5.3 Nonwoven with Unimodal Fiber Diameter Distribution 117
4.5.4 Comparisons of Experimental Results with Theoretical
Predictions 122
4.6 Mechanical Properties of Nonwovens 124
Contents VII
4.7 Nonwovens Composed of Electrospun Nanofi bers – Important Facts to
Remember 125
References 126
5 Electrospinning – Some Technical Aspects 127
5.1 Technical Setups for Creating Jets 127
5.1.1 Some Technical Details 129
5.1.1.1 The Die 129
5.1.2 High-Voltage Supply 130
5.1.3 Counterelectrode 131
5.1.4 Spinning Chamber 131
5.1.5 Multiple-Die Setups 132
5.1.6 Die-Free Multiple-Jet Injection 133
5.2 Designs of Counterelectrode Confi gurations Allowing Preparation of
Nonwovens with a Broad Range of Architecture 137
5.3 Electrospinning – Some Technical Aspects – Important Facts to
Remember 141
References 142
6 Modifi cation of the Electrospinning Technique 143
6.1 Towards Advanced Modes of Fiber Preparation and Deposition 143
6.2 Near-Field Electrospinning – High-Precision Deposition
Electrospinning 144
6.2.1 Deposition of Individual Fibers 148
6.2.2 Deposition of Nonwoven Test Pattern 151
6.2.3 Deposition of Precision Nonwovens Paths 151
6.3 Towards Core–Shell and Hollow Fibers 155
6.3.1 Coelectrospinning/Coaxial Electrospinning 155
6.3.2 Core–Shell Fibers and Hollow Fibers via a Template (TUFT
Approach) 160
6.3.3 Background Information 160
6.4 Modifi cation of the Electrospinning Technique – Important Facts to
Remember 162
References 163
7 Materials Considerations 165
7.1 Introduction 165
7.1.1 Background Information 166
7.1.1.1 Solutions and Dispersions 166
7.1.1.2 Bioerodible and Biostable Polymers 166
7.2 Spinning from Organic Solvents 166
7.3 Spinning of Water-Soluble Polymers 170
7.4 Spinning of Biopolymers 172
7.5 Spinning of Complex Polymer Systems 175
7.5.1 Background Information 175
VIII Contents
7.6 Nanofi bers from Polymer Hybrids, Metals, Oxides 178
7.7 Melt Spinning 180
7.8 Materials Considerations – Important Facts to Remember 182
References 182
8 Technical Applications of Electrospun Nanofi bers 185
8.1 Nanofi bers, Nanofi ber Nonwovens – Elements of Nanotechnology 185
8.2 Filter Applications 187
8.2.1 Basic Processes Controlling Filter Effi ciencies 188
8.2.2 Effect of Nanostructuring 190
8.3 Textile Applications 197
8.4 Catalysis 201
8.5 Nanofi ber Reinforcement 203
8.5.1 Background Information 203
8.5.2 Molecular/Nanofi ber-Based Approach 205
8.6 Surface Modifi cations 206
8.7 Template Applications 210
8.8 Plant Protection via Pheromones 210
8.8.1 Background Information 210
8.8.2 Nanofi ber-Based Approach 212
8.9 Technical Applications – Important Facts to Remember 214
References 215
9 Medicinal Applications for Electrospun Nanofi bers 217
9.1 Nanotechnology and Medicinal Applications in General 217
9.2 Tissue Engineering 218
9.2.1 Background Information 218
9.3 Wound Healing 226
9.4 Transport and Release of Drugs 228
9.4.1 Model Experiment on siRNA Transfection 230
9.4.2 Application in Tumor Therapy 230
9.4.3 Inhalation Therapy 232
9.5 Nanotechnology and Medicinal Applications in General – Important
Facts to Remember 233
9.5.1 Tissue Engineering 233
9.5.2 Wound Healing with Nanofi bers 234
9.5.3 Transport and Release of Drugs/Drug Delivery 234
References 235
Index 237
IX
Preface
Nanofi bers and nonwovens composed of them are unique nanostructures with an
extraordinary potential both in technical areas and in medicinal applications. Filter
applications, functionalization of textiles, fi ber reinforcement, catalysis, drug deliv-
ery, wound healing or tissue engineering are just a few examples of potential
applications. The route towards such nano -o bjects is based primarily on electros-
pinning: a unique technique relying on self - organization via electric charges and
their interactions with an applied fi eld. The mechanism controlling fi ber forma-
tion, the properties of fi bers and nonwovens prepared by this technique, materials
chosen for electrospinning and also application in technical and life science areas
are the topic of this book. Both experimental facts and theoretical models will be
discussed in detail and the same holds for various modifi cations of electrospinning
that have been developed recently. Electrospinning and nanofi bers prepared by
this technique will without any doubt lead to major scientifi c and technical
advances in nanotechnology and it is a principal aim of this book to promote such
advances.
However, technical advances tend in the majority of cases to pose particular risk
and in fact risk assessment has become a major concern in nanotechnology within
recent decades. One major reason is that nano - objects composed, for instance, of
metals, metal oxides, inorganic compounds, even pure carbon have the tendency
to display properties and functions that deviate strongly from the corresponding
features known for the same materials on a macroscopic scale. Nanoparticles,
nanofi lms, nanorods or nanotubes, for that matter, display specifi c properties that
depend not only on their characteristic dimensions but also on the shape. Quantum
fi lms, quantum dots or quantum wires composed of semiconductor materials are
well - known examples [1] .
Magnetic and semiconductor phenomena may serve to illustrate the fundamen-
tal changes taking place at small scales and to point out in which way the conver-
gence of nanotechnology and medicine can offer novel approaches in medicine.
Ferromagnetic materials, such as cobalt or iron oxide, display the unique phenom-
enon of superparamagnetism if nanoparticles are prepared from such materials.
No permanent magnetization can be achieved in an external magnetic fi eld for such
nanoparticles, yet the induced magnetization is of similar magnitude as in the case
of ferromagnetism. Superparamagnetic nanoparticles serve as drug carriers that
X Preface
can be positioned in the body via external magnetic fi elds and that can be forced to
release drugs in a controlled way by cycling the magnetic fi eld thus causing a tem-
perature increase within the particles. Semiconductor nanoparticles, such as the
ones made from cadmium selenide, show optical adsorption and emission at dis-
tinct wavelengths in the visible range with the wavelength depending at constant
material composition strongly on the size of the nanoparticle.
Such features are the origin of the great potential of nano - objects in various
types of applications. On the other hand, they may be the source of risks for bio-
logical objects, for the health of human beings, and for the environment. Gold
may serve as a good example and this also holds for silver. Such materials are inert
in the bulk state yet become strongly catalytic as nano - objects. Silver nanoparticles
have been used to destroy various types of viruses and gold nanoparticles to cata-
lyze specifi c chemical reactions. So, the interaction of objects with matter of bio-
logical origin may change dramatically as the size is reduced, as the shape is
modifi ed. In fact, a broad range of investigations has considered this topic, studies
on the penetration of carbon nanotubes into the body, the accumulation at various
locations and possible harmful reactions were, and are, the topic of scientifi c
studies but also less scientifi c discussion in the media.
It certainly makes a lot of sense to handle the preparation of nano - objects, the
use of nano- objects and fi nally the disposal of them with utmost care, it makes a
lot of sense to impose corresponding regulations and to follow them strictly similar
to the case of handling of chemical compounds including toxic compounds [2] .
This should in principle also hold for nanofi bers prepared by electrospinning.
However, while major studies have been performed for a broad range of nano -
objects including in particular carbon nanotubes no such major investigations
exist for nanofi bers, particularly for those produced from natural or synthetic
polymers.
On the other hand a sizable number of papers have been published in areas
such as wound healing, drug delivery including gene delivery, tissue engineering
in which the interaction of the nanofi bers with specifi c types of tissue, with stem
cells and specifi c cell lineages, with DNA, etc. have been studied in detail. This
already allows a set of conclusions concerned with health risks. Furthermore, it is
important to point out that nanofi bers produced from polymers do not show in
general quantum effects usually associated with nanotechnology. The reason
simply is fi rst that the nature of the electronic states of organic materials including
polymer materials that control optical and electronic properties does not resemble
the one known for semiconductors or conductors.
Electronic states that are not localized but rather extend throughout the bulk
material are characteristic of such nonorganic materials with the consequence that
modifi cations fi rst of the absolute size and secondly of the geometry of a body
made from them have strong effects on properties, particularly as the sizes
approach the few tens of to a few nanometers scale. Organic materials, on the
other hand, display localized states for electronic excitations, electronic transport
with the states being defi ned by molecular groups such as chromophore groups
or complete molecules. The consequence is that the electronic states are not