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ORGANIC BIOELECTRONICS
Electrochemical Devices based on Conjugated Polymers
Joakim Isaksson
Norrköping, 2007
Organic Bioelectronics
Electrochemical Devices based on Conjugated Polymers
Joakim Isaksson
Linköping Studies in Science and Technology. Dissertations, No. 1128
Copyright ©, 2007, Joakim Isaksson, unless otherwise noted
Printed by LiU-Tryck, Linköping, Sweden, 2007
ISBN: 978-91-85831-03-6
ISSN: 0345-7524
Cover by Joakim Isaksson: Ion Pump with Cell (front) and Wettability Switch (back)
“It’s a one-day experiment…
…OK, OK, maybe two days…”
Professor Magnus Berggren
ABSTRACT
Since the Nobel Prize awarded discovery that some polymers or “plastics” can be
made electronically conducting, the scientific field of organic electronics has arisen.
The use of conducting polymers in electronic devices is appealing, because the
materials can be processed from a liquid phase or from a solution, much like ordinary
non-conducting plastics. This gives the opportunity to utilize printing technologies
and to manufacture electronics “roll-to-roll” on flexible substrates, ultimately at very
low costs. Even more intriguing are the possibilities to achieve completely novel
functionalities in combination with the inherent compatibility of these materials with
biological species. Therefore, organic electronics can merge with biology and
medicine to create organic bioelectronics, i.e. organic electronic devices that interact
directly with biological samples or are used for other biological applications.
This thesis aims at giving a background to the field of organic bioelectronics and
focuses on how electrochemical devices may be utilized in biological applications. An
organic electronic wettability switch that can be used for lab-on-a-chip applications
and control of cell growth as well as an electrochemical ion pump, which can regulate
cell communication and serve as an efficient drug delivery device, are introduced.
Furthermore, the underlying electrochemical structures that are the basis for the
above mentioned devices, electrochemical display pixels etc. are discussed in detail.
In summary, the work contributes to the understanding of electrochemical polymer
electronics and, by presenting new bioelectronic inventions, builds a foundation for
future projects and discoveries.
I
POPULÄRVETENSKAPLIG SAMMANFATTNING
Efter upptäckten 1977, vilken år 2000 belönades med Nobelpriset i Kemi, att vissa
polymerer eller ”plaster” kan göras elektriskt ledande, har forskningsområdet
organisk elektronik vuxit fram. Organisk elektronik handlar om att använda ledande
organiska polymerer för att bygga elektroniska komponenter, vilket är lockande
eftersom materialen i regel kan hanteras i flytande form ungefär som vanliga plaster.
Detta innebär att elektronik kan tillverkas från rulle till rulle med tryckteknologier på
plastfolie eller papper till väldigt låga kostnader. Ännu mer intressant är möjligheten
att med ledande polymerer få helt nya funktioner som inte är tänkbara med klassisk
elektronik och dessutom uppvisar många ledande polymersystem väldigt god
kompatibilitet med biologiska material. Sammantaget innebär detta att organisk
elektronik kan integreras med biologi och medicin för att skapa organisk
bioelektronik, dvs. organiska elektronikkomponenter som direkt interagerar med
biologiska material eller mer generellt används för biologiska applikationer.
Målet med avhandlingen är att ge en bakgrund till forskningsområdet organisk
bioelektronik, med fokus på elektrokemiska komponenter. Här introduceras en
organisk elektronisk ”vätbarhetsswitch” med tillämpningar på analyschip och för
kontroll av celltillväxt samt en elektrokemisk ”jonpump” för elektroniskt styrd
cellkommunikation och administrering av biologiska substanser. Vidare diskuteras
de grundläggande elektrokemiska strukturer som är basen för ovan nämnda
komponenter, elektrokemiska displaypixlar, etc. Sammanfattningsvis bidrar arbetet
till förståelsen för elektrokemisk polymerelektronik och bygger med de nya
bioelektroniska komponenterna en grund för framtida forskningsprojekt och
upptäckter.
II
FOREWORD AND ACKNOWLEDGEMENTS
With a recent degree in engineering biology, directed towards biomaterials, and an
additional semester in the biomedical research school, the field of organic
electronics was a new and exciting challenge for me. Although it took some time to
really get to know the world of polarons, cleanroom practices and cut-and-paste
electronics, the enthusiasm and drive of the Organic Electronics group immediately
swept me away. I quickly realized that (when you learn to prioritize) it is very
stimulating to work in a group where the time required for new project ideas is
always more than the available man-hours and where out-of-the-box thinking is
highly valued. During the course of my Ph D studies, the focus slowly shifted from
printed organic electronics towards organic bioelectronics. Actually, one of the
main intentions with the engineering biology study program is to educate people in
physics and math, as well as in chemistry and biology, in order to function as a link
between the engineering and medical communities. Therefore it was very satisfying
to take part in the initiation of the ion pump project, in close cooperation with cell
biologists at Karolinska Institutet. The project has been the perfect example of truly
multidisciplinary research, where the initial hurdles can be quite large but the final
results and the road to get there are very rewarding. I am pleased to see that the
results of the research efforts, summarized in this thesis, have generated new
projects and opportunities for the future, and I will always take pride in my tiny
contribution to the development of organic bioelectronics. I hope that the reader of
this thesis will find the contents interesting and inspiring, regardless of whether he
or she is working with research on related topics or is someone who simply wants to
have a glance at what I have been up to for five years.
With that said, I could of course not have done the work on my own and there are
several people who have my sincere gratitude and deserve credit for the thesis
becoming a reality. I would therefore like to thank all of you, who have helped me,
with large or small things, at work or in private, during my time at LiU
Norrköping. More specifically, I would like acknowledge the following people:
III
Min handledare, Professor Magnus Berggren, som har gett mig chansen att
arbeta i gruppen Organisk Elektronik och varit en outtömlig källa av inspiration
och idéer. En obotlig optimist och en cynisk skeptiker matchar varandra ganska
bra! Tack för att du har gett mig förtroende och ansvar samtidigt som du har
stöttat när det behövts.
My co-supervisor, Nate Robinson, who always had the time to help and never
was afraid to ask the difficult questions. Thanks for everything that you taught me,
from scientific experimental designs to “secret” management skills. We do not
always agree, but I usually count on you being right.
Agneta Richter-Dahlfors, som tillsammans med Magnus har gjort det möjligt
för mig att få medverka i integrationen mellan biologi och organisk elektronik. Tack
för ett väldigt givande samarbete (jag vet nu bättre än någonsin att ett
cellexperiment inte är något man ”snyter ur näsan”).
Min motsvarighet på ”andra sidan”, Peter Kjäll, som ihärdigt har odlat celler på
alla komponenter som jag skickat upp och trollat fram fantastiska resultat. Tack för
många trevliga dagar på MTC, även de tillfällen när alla experiment har gått åt
*******. Att hålla skärpan efter lunch i ett mörkt mikroskoprum med en
DeLuxeBurgare i magen är en utmaning utöver det vanliga...
David Nilsson, min mentor och jonpumpskamrat. Jag har för länge sedan tappat
räkningen på alla prover vi analyserade, men jag saknar våra AAS-dagar.
Sophie Lindesvik, som har koll på allt och lite till om allt och lite till som rör
universitetet. Tack för just allt och lite till, inklusive att den här avhandlingen finns.
Payman Tehrani, min ständigt glada labpartner i displayprojektet. Tack för
trevliga timmar i labbet och för att du (oftast) höll reda på alla WM-polymerer.
Övriga medförfattare, inte minst XPS-perfektionisten Calle Tengstedt och
yrvädret Linda Robinson.
Klas Tybrandt, min ”efterträdare” som direkt blev självgående och drivande i
jonpumpsprojektet och därmed gav mig tid att skriva den här avhandlingen.
IV
Daniel Simon and Xavier Crispin, for valuable comments on the contents of this
thesis
The entire group of Organic Electronics. Thanks all of you for personal
friendships, scientific team-spirit, long coffee breaks and stimulating discussions.
Acreo, för tillhandahållande av material, hjälp i labbet (Anurak med flera),
patentering etc.
Familj och vänner som har hjälpt mig glömma jobbet och därmed ladda
batterierna emellanåt:
Oscar, som har blivit en god vän på jobbet och tillsammans med Anna, Oliver och
Alva förgyllt och förenklat tiden utanför detsamma. Tack för värme, generositet,
middagar, barnpassning (dygnet runt) och vänskap i allmänhet.
Johan och Jenny. Tack för midsommar, nyår, schackspel, Lagavulin och annat
som helt enkelt gör livet lite bättre.
Mamma och Pappa, som har onyanserat höga tankar om mig men som ständigt
finns där. Storasyster, för att du alltid har varit just Storasyster.
Sist, kanske minst, men också utan konkurrens viktigast i mitt liv och därmed en
stor anledning till att jag är där och den jag är idag; min lilla familj. Johanna,
min älskling och bäste vän, som tar hand om mig, alltid tror på mig och betyder
mer än vad jag kan sätta ord på. Tack för allt. Moa och Viktor, som sätter färg på
min vardag och ger perspektiv på andra utmaningar i livet. Tack för att ni finns.
Joakim Isaksson, Norrköping 2007-08-30
The work was funded by The Swedish Foundation for Strategic Research,
The Royal Swedish Academy of Sciences and The Swedish Research Council.
V
LIST OF INCLUDED PAPERS
Paper 1
A Solid-state Organic Electronic Wettability Switch
Joakim Isaksson, Carl Tengstedt, Mats Fahlman, Nathaniel D. Robinson and Magnus Berggren
Advanced Materials 16, 316-320 (2004).
Contribution:
All experimental work, except the photoelectron spectroscopy. Wrote the first draft of
the manuscript and was involved in the final editing and submission of the manuscript
in cooperation with the co-authors.
Paper 2
Electronic Modulation of an Electrochemically Induced Wettability Gradient to Control Water
Movement on a Polyaniline Surface
Joakim Isaksson, Nathaniel D. Robinson and Magnus Berggren
Thin Solid Films 515, 2003-2008 (2006)
Contribution:
All experimental work. Wrote most of the manuscript and coordinated the final editing
and submission of the manuscript in cooperation with the co-authors.
Paper 3
Electrochemical Control of Surface Wettability of poly(3-alkylthiophenes)
Linda Robinson, Joakim Isaksson, Nathaniel D. Robinson and Magnus Berggren
Surface Science 600, L148-L152 (2006)
Contribution:
Small part of the experimental work. Coordinated the final editing and submission of
the manuscript in cooperation with the co-authors.
VI
Paper 4
Evaluation of Active Materials Designed for use in Printable Electrochromic Polymer Displays
Payman Tehrani, Joakim Isaksson, Wendimagegn Mammo, Mats R. Andersson,
Nathaniel D. Robinson and Magnus Berggren
Thin Solid Films 515, 2485-2492 (2006)
Contribution:
All the experimental work (not including polymer synthesis) together with P. Tehrani.
Was involved in the final editing of the manuscript in cooperation with the co-authors
Paper 5
Electronic Control of Ca2+ Signaling in Neuronal Cells using an Organic Electronic Ion Pump
Joakim Isaksson#, Peter Kjäll#, David Nilsson, Nathaniel D. Robinson, Magnus Berggren and
Agneta Richter-Dahlfors
Nature Materials 6, 673-679 (2007)
Contribution:
Significant part of the experimental work. Wrote a large part of the manuscript draft
(not including the biology sections) and coordinated the final editing and submission of
the manuscript in cooperation with the co-authors.
#
Shared authorship
Paper 6
Electronically Controlled pH Gradients and Proton Oscillations
Joakim Isaksson, David Nilsson, Peter Kjäll, Nathaniel D. Robinson, Agneta Richter-Dahlfors and
Magnus Berggren
Submitted
Contribution:
All the experimental work. Wrote most of the manuscript and coordinated the final
editing and submission of the manuscript in cooperation with the co-authors.
VII
RELATED WORK NOT INCLUDED IN THE THESIS
Electrochemical Wettability Switches Gate Aqueous Liquids in Microfluidic Systems
Linda Robinson, Anders Hentzell, Nathaniel D. Robinson, Joakim Isaksson and Magnus Berggren
Lab on a Chip 6, 1277-1278 (2006)
The results from the paper above where acknowledged as a “Research Highlight” in Nature
(2 November 2006) with the following paragraphs:
MICROFLUIDICS
Go with the flow
Lab Chip 6, 1277–1278 (2006)
The controlled flow of liquids in microscopic
channels on ‘microfluidic’ chips could permit
chemical analysis of tiny samples as well as
more efficient industrial chemical synthesis.
But how can the fluid traffic be directed down
the right channels?
Microscopic valves and gates are
cumbersome, so Nathaniel Robinson and his
colleagues at Linköping University in
Sweden propose to do away with all moving
parts.
They made channels with floors whose
wettability can be controlled electrically,
using conducting polymers that have
different surface properties when
electrochemically oxidized or reduced. Water
injected into such a system flows
preferentially along the oxidized channels
(pictured right).
Patents
Wettability Switch (WO/2005/053836)
Electrically Controlled Ion Transport Device (Patent pending)
VIII
TABLE OF CONTENTS
1. INTRODUCTION
1
2. CONDUCTING POLYMERS
5
2.1 Conjugated polymers
5
2.2 Doping of conjugated polymers
8
2.3 Optical properties of conjugated polymers
12
2.4 Examples of commonly used conjugated polymers
15
2.5 Ionic conduction in polymer films
20
3. BASIC ELECTROCHEMICAL STRUCTURES
21
3.1 Reduction and oxidation of conducting polymers
21
3.2 “Structure 1”
24
3.3 “Structure 2”
25
3.4 “Structure 3”
27
4. SURFACE ENERGY – WETTABILITY
29
4.1 Surface tension in theory
30
4.2 Contact angle measurements
31
4.3 Other techniques to study surface tension
36
5. CELL COMMUNICATION
39
5.1 Communication between and inside cells
39
5.2 Calcium signaling
42
6. ORGANIC BIOELECTRONICS
47
6.1 Sensors
47
6.2 Microfluidics
49
6.3 Actuators
52
6.4 Delivery systems
53
7. ELECTROCHEMICAL DEVICES IN PAPERS 1-6
7.1 Electrochemical wettability switch
55
55
7.2 PEDOT:PSS displays with improved optical contrast
64
7.3 Organic electrochemical ion pump
67
8. CONCLUDING DISCUSSION
77
REFERENCES
79
PAPERS 1-6
NOMENCLATURE
β
Parameter for determination of pendant drop shape
φ
Hanging drop turning angle (rad)
γ
Surface energy, surface tension (N/m)
γd
Dispersive part or surface energy (N/m)
γp
Polar part of surface energy (N/m)
γlv
Liquid-vapor surface tension (N/m)
γsl
Solid-liquid interfacial energy (N/m)
γsv
Solid-vapor surface energy (N/m)
λ
π-bond
Wavelength (m)
Bond between overlapping p-orbitals
θ
True contact angle (degrees)
θ’
Measured contact angle (degrees)
∆ρ
σ-bond
Difference in density (kg/m3)
A
Area (m2)
Bond between overlapping s-orbitals
A-
Anion, here with the charge -1
A, B, C, D
Electrode labels
a. u.
Arbitrary units
AB
Electrolyte that covers electrodes A and B
C+
Cation, here with charge +1
D
Drain contact
DBSA
Dodecylbenzene sulfonic acid
dH2O
de-ionized water
E
Energy (eV)
e-
Electron
Eox
Oxidation potential (V)
F
Force (N)
FURA-2 AM
Intracellular fluorescent probe for Ca2+ detection
g
Gravitational constant (m/s2)
G
Gate contact
HCN-2
Neuronal cell line
HOMO
Highest Occupied Molecular Orbital
IBC
Current between electrodes B and C
[K+]
Concentration (M), in this case of potassium ions
l
Length (m)
∆L*
LUMO
Change in luminance (lightness), optical contrast
M+
Cation, here with charge +1
n
Number of replicates
OTFT
Organic Thin Film Transistor
Ox
Oxidized
P-
n-doped polymer, here with charge -1
P+
p-doped polymer, here with charge +1
P0
Neutral polymer
P3AT
poly(3-alkyl thiophene)
P3BT
poly(3-butyl thiophene)
P3HT
poly(3-hexyl thiophene)
P3OT
poly(3-octyl thiophene)
Lowest Unoccupied Molecular Orbital
PANI
Polyaniline
PBFI
Fluorescent probe for detection of K+
PEDOT
poly(3,4-ethylene dioxythiophene)
PSS
poly(styrene sulfonate)
r
Surface roughness
R0
Radius of curvature at the apex of a pendant drop (m)
Red
Reduced
s
Dimensionless curvature length of a pendant drop
S
Source contact
V
Potential (V)
VAB
Potential between electrodes A and B (V)
VOCCs
Voltage Operated Calcium Channels
w
Width (m)
W
Work (Nm)
Generic variables, like x, y and z are used in different contexts throughout the text.
CHAPTER 1
1. INTRODUCTION
In the year 2000, the Nobel Prize in chemistry was awarded to Alan
J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa for “the
discovery and development of electrically conductive polymers”.1
23 years earlier, these three gentlemen had discovered that
polymers, what we generally call plastics, could not only be used as
insulating materials but also be made electronically conducting.2
Until their surprising breakthrough, polymers were considered as
important materials because they are easy to process from solution
and
their
mechanical
properties
can
be
tailor-made
during
synthesis, but intrinsically conducting polymers sounded like
something of a paradox. Some polymers were already known to have
semiconducting properties but Heeger, MacDiarmid and Shirakawa
found that the materials can be “doped” to achieve metallic
conduction. The idea of making electronic devices with the same
process, flexibility and, perhaps more importantly, at the same cost
as e.g. a plastic bag is of course very appealing and the discovery has
opened up a completely new field of science. Since the polymers have
a carbon base, in equivalence to the materials in living organisms,
the use of intrinsically conducting polymers in electronic devices is
often called organic electronics.
1
Chapter 1: Introduction
Since it is generally possible to dissolve or emulsify polymers in water or organic
solvents and thereby handle the materials as liquids, common printing
techniques can be used for manufacturing of electronic components.3-5 Instead
of printing different colors, several conducting polymers can be used as inks in
the printing press. There are of course a number of practical issues to deal with
but roll-to-roll printing of electronics on flexible substrates such as paper or
insulating plastic foil is already a reality. The speed and performance of such
printed devices is not comparable to what is achieved in the traditional silicon
industry but since a printing press can process up to several 100 meters per
minute, the cost for each device is close to nothing. This, in combination with
the environmental hazards of tedious silicon processing and the fact that many
conducting polymers are biocompatible,6,7 make organic electronics even more
interesting.
Examples of organic electronic applications include light emitting diodes,8-10
solar cells,11-13 thin film transistors,14,15 electrochemical transistors and logic
circuits16-18 and sensors.19-21 Apart from the electronic conductivity itself,
conducting polymers have a number of other exceptional properties, as
compared to inorganic materials, which make it possible to create completely
new types of devices. One way of using the unique properties of conjugated
materials and to achieve novel functionalities is to focus the attention towards
biology. The application of organic electronics on biology and medicine is called
organic bioelectronics, which is the topic of this thesis.
The aim of the thesis is to briefly describe the scientific background of organic
bioelectronics, followed by a more detailed explanation of the functionality and
characteristics of newly developed devices. To start off, the upcoming chapter
gives an introduction to the physics and chemistry of conjugated polymers,
exemplified by a few polymer systems as well as some of their applications.
Thereafter, fundamental electrochemical structures that utilize these materials
are described. Two chapters are dedicated to give a brief background on the
topics of surface science and cell communication, which are vital to
understanding the bioelectronic devices presented at the end of the thesis. A
review chapter on the field of organic bioelectronics aims at showing the
2
Chapter 1: Introduction
importance and potential of this research area, as well as the almost perfect
match between organic electronics and life sciences. This puts the
electrochemical bioelectronic devices in a context and also pinpoints some of
the challenges that remain to be solved. The thesis is concluded with a detailed
description of the electrochemical bioelectronic inventions that comprise the
base for the accompanying papers, followed by a concluding discussion and a
glance towards future bioelectronic projects and applications.
Papers 1-3 describe the organic electronic wettability switch, which gives the
possibility to electronically control the wettability of a surface, create surface
energy gradients etc. The device is interesting for bioelectronic applications like
microfluidic channels and control of cell-growth but could also have novel uses
in e.g. printing technologies. Paper 4 discusses printable organic electrochromic
displays with a contrast-enhancing layer. Papers 5 & 6 present the organic
electronic ion pump, used for cell communication studies and potentially also
for drug delivery applications. The ion pump utilizes the ion conductivity of a
conjugated polymer-polyelectrolyte system to electrophoretically transport ionic
species from a source electrolyte and deliver them to cells that live on the
surface of the polymer in a second electrolyte.
The work is based on the development of all-organic, electrochemical and
presumably printable devices with very similar structures and materials but
completely different applications. The papers and the new devices comprise a
small, but hopefully important, contribution to the scientific fields of organic
electronics in general and, with the exception of the electrochromic display,
organic bioelectronics in particular.
3
4
CHAPTER 2
2. CONDUCTING POLYMERS
A polymer (poly = many in Greek) is a material that consists of many
repeated units, called monomers (mono = one). Natural polymers
are for instance DNA, proteins, cellulose etc., but polymeric
materials can also be synthesized. Since the number of possible
polymer designs is close to infinite, the chemical and physical
properties can more or less be tailor-made for a specific application.
Most polymers or their monomers are soluble in water or organic
solvents. This means that the material can be handled and processed
as a liquid and therefore simple manufacturing techniques such as
moulding, casting, spin coating, screen printing etc. can be used to
create structures and shapes before or after polymerisation. The
class of polymers that can conduct electricity is referred to as
(intrinsically) conducting polymers.
2.1 Conjugated polymers
What is the difference between the polymer in a plastic bag and one that can
conduct electricity like metals? A common material in regular plastic bags is
polyethylene, which is a very good insulator. In polyethylene, each carbon binds
to two other carbons and to two hydrogen atoms (Figure 1a). The physical
chemist would say that the carbon is sp3-hybridized, i.e. hybridization between
the s-, px-, py and pz-orbitals yields four equivalent hybrid orbitals, and forms
four σ(sigma)-bonds to the surrounding atoms. Every carbon valence electron is
5
Chapter 2: Conducting Polymers
then localized in such a bonding molecular orbital and can therefore not
transport current.
Polyacetylene (Figure 1b) has a very similar chemical structure but in this case
each carbon only binds to three other atoms. The carbon is now sp2-hybridized
and forms three σ-bonds. However, since carbon has four valence electrons,
each carbon atom has one electron left in a non-hybridized p-orbital. These
atomic p-orbitals are oriented perpendicular to the polymer backbone, and
overlap in a delocalized electron cloud along the polymer chain to form
molecular π(pi)-bonds. The result is a conjugated molecular structure with
alternating single and double bonds between carbon atoms.
Figure 1. Chemical structures of a. Polyethylene and b. Polyacetylene
with shorthand notations below.
The most stable form of polyacetylene (trans-polyacetylene), drawn in Figure 2
with shorthand notation, where each vertex represents a carbon atom with
hydrogen atoms, has a so-called degenerate ground state. I.e. there is no
difference in energy if the positions of the double bonds and the single bonds
are interchanged. This means that the electrons in the π-bonds can be found
between any two carbon atoms and thus move along the polymer chain. The
longer the molecule, the smaller is the energy gap and for very long molecules,
energy bands are formed.
6
Chapter 2: Conducting Polymers
If all bond lengths between carbon atoms were the same, polyacetylene would
behave like a one-dimensional metal. However, this is not the case, because of
so-called Peierls distortion, i.e. it is more energetically favorable for the
molecule to have an alternating bond configuration. Therefore, an energy gap
appears in the band structure, like for inorganic semiconductors. The highest
occupied molecular orbital (HOMO) defines the high energy edge of the valence
band and the lowest unoccupied molecular orbital (LUMO) is the bottom of the
conduction band.
Figure 2. Polyacetylene has a degenerate ground state, i.e. the two
single-double bond alternation schemes have the same total energy.
The presence of a band gap means that energy has to be given to the system in
order to excite electrons from the valence band to the conduction band and pure
polyacetylene therefore has semiconducting properties instead of metal
behavior. Due to the conjugation (delocalization of π-electrons), the bandgap is
still very small (1.5 eV) compared to polyethylene (>8 eV), which is an insulator.
Most other conjugated polymers (e.g. polythiophene, see 2.4.1) have a nondegenerate structure, i.e. there is only one energy ground state. In this case the
single and double bonds cannot be interchanged without a cost in energy, due to
distortion of the molecular structure, and the band gap of a polymer with a nondegenerate ground state is slightly larger than in a degenerate conjugated
molecule. Once again, thanks to the conjugation, the band gap is small enough
to yield semiconducting properties.
7
Chapter 2: Conducting Polymers
2.2 Doping of conjugated polymers
Pure polyacetylene is a semiconducting material, but the number of thermally
excited charge carriers and their mobilities are limited. Therefore, although the
conductivity is higher than in polyethylene, it is far from that of a metal. Heeger,
MacDiarmid and Shirakawa discovered that charge carriers can be introduced
in polyacetylene by chemical doping of the material, which results in a
conductivity increase by several orders of magnitude. This discovery earned
them the Nobel Prize in chemistry and revolutionized the field of conjugated
polymers. A comparison of conductivities for different organic and inorganic
materials is shown in Figure 3 (from Ref. 22).
Figure 3. Conductivity of a few organic and inorganic materials.
Adapted from Ref. 22.
Positive doping of a conjugated polymer means that an electron is removed
from the valence band (addition of a positively charged “hole”) and negative
doping denotes when an electron is added to the conduction band. Although the
8
Chapter 2: Conducting Polymers
physics is different, this way to create mobile charge carriers in an organic
semiconductor can be compared to p- and n-doping, respectively, in inorganic
semiconductors like Si. However, a high percentage of dopant is needed to
increase the conductivity in the polymer compared to only a few ppm in Si.
Conjugated polymers can be doped in a number of different ways, such as
chemical and electrochemical doping, charge injection doping, photo doping
and acid-base doping.22,23
For degenerate systems, like trans-polyacetylene, a geometrical defect in the
polymer chain can create a modification of the bond length alternation pattern,
as shown in Figure 4. The domain “wall” between the two types of bond length
alternation is called a soliton and is characterized by a new energy state in the
middle of the band gap. The “extra” charge resides in this state, which is
localized over a relatively short distance (X = 10 to 20 C-C bonds in Figure 4). If
the bond order is shifted without the addition of an external charge, a neutral
soliton with spin = ½ is created, as shown in Figure 4a. Solitons are present in
trans-polyacetylene because of defects or simply on chains containing an odd
number of carbon atoms. Although they have no charge, the solitons can be
detected experimentally because of their spin.
When an electron is added to the polymer chain through n-doping, existing
solitons are energetically favorable to charge and a negative soliton with no spin
appears (Figure 4b). If an electron is instead subtracted from the polymer, a
positive soliton without spin is created as seen in Figure 4c.24,25 The energy
states described above refer to the idealized case of an isolated polymer chain.
In reality, interactions between chains must be considered and if the charged
states stem from chemical or electrochemical doping, a counter ion will be
ionically coupled to the polymer chain in order to maintain charge neutrality
(see also 3.1). This ion will contribute to the geometrical distortion of the
molecule and localize the soliton.
Most conjugated polymers are, however, not degenerate, i.e. they have one
unique energy ground state. When a non-degenerate conjugated polymer is
doped, the geometric distortion induces a state of higher energy. The structure
9
Chapter 2: Conducting Polymers
distortion coupled to the introduced charge is called a polaron, which can move
along the polymer chain and thereby participate in the electronic conductivity of
the material.26,27 Polarons can also be created in non-degenerate systems, such
as trans-polyacetylene, but only at higher doping levels when all solitons have
been charged.
Figure 4. Solitons in trans-polyacetylene. a. Neutral. b. Negative. c.
Positive. X denotes the localization of the soliton
One example of a simple non-degenerate conjugated polymer is poly(paraphenylvinylene) (PPV). Doping of this polymer changes the bond alternation
and creates a geometrically distorted region with quinoid structure instead of
the regular aromatic, as shown in Figure 5. This also leads to the appearance of
new energy levels inside the band gap. Upon further doping, it is sometimes
energetically favorable to form bipolarons, which are doubly charged and
spinless, instead of two separate polarons. In a simplified picture, the charges in
10
Chapter 2: Conducting Polymers
the bipolaron repel each other because of Coulomb forces but, at the same time,
the separation of two polarons costs elastic energy since the quinoid
configuration is a higher energy state than the aromatic. This situation can be
compared to the equilibrium state of a spring and the bipolaron will therefore be
localized over few rings, as shown in Figure 5.
Figure 5. Formation of a negative polaron and bipolaron in poly(paraphenylvinylene).
11
Chapter 2: Conducting Polymers
The energy levels of the polarons and bipolarons are, to a large extent,
correlated to the HOMO and LUMO of the neutral quinoid form and the energy
difference between these levels is smaller than in the aromatic case. The new
levels formed inside the bandgap upon doping create new allowed optical
transitions at lower energies (see also 2.3). As in the soliton case, the formation
of polarons and bipolarons described above is idealized and correspond to one
isolated molecular chain, without charge neutralizing counter ions that localize
the polaronic charge carriers.
2.3 Optical properties of conjugated polymers
Conjugated polymers have small optical bandgaps and many of the materials
therefore absorb light in the visible region (wavelengths, λ = 400-800 nm; or
energy, E = 1.5-3 eV). Consequently, conducting polymers are often colorful
materials, which also means that if the electronic structure and associated
allowed optical transitions are modified, e.g. upon doping, we can follow the
color transition with our eyes when the absorption of light changes.
2.3.1 Absorption and emission
A material can absorb a photon, i.e. a quantum of light, if the photon energy
matches the difference between the energy of the excited state and the ground
state (Figure 6). If that is the case, the molecule receives the photon energy and
is excited to a higher energy state, with increased atomic separation. From this
state the molecule can return to the energy ground state via radiative or nonradiative processes. In a radiative decay, a new photon is created and light is
thereby emitted (spontaneous emission). The light emission is quenched if a
non-radiative decay allows the molecule to come back to the ground state by
donating the excitation energy to surrounding molecules via collisions
(phonons) or energy transfer. Since some of the energy is always lost in
vibrations, the energy of the emitted light is lower than that of the absorbed
light (red-shifted wavelength), as shown schematically in Figure 6.
12
Chapter 2: Conducting Polymers
Figure 6. Absorption and emission. a. Schematic energy diagram
showing two electronic states with vibrational levels. The energy “wells”
show the relationship between energy and the reaction coordinate. The
incoming photon is absorbed by the molecule, which is then excited. After
vibrational relaxation, the molecule can return to the electronic ground
state by emission of a new photon with lower energy. b. Sketch of
absorption and emission spectrum with visible vibrational peaks. The
emitted light has a lower energy and the wavelength is thereby higher
(red-shifted).
13
Chapter 2: Conducting Polymers
Organic light emitting diodes also utilize the radiative decay of an excited state
to emit light, but in this case, excited electron-hole pairs, referred to as excitons,
are created by injection of charges with an applied electric field. Organic solar
cells work the other way around. Incoming photons excite the molecules and,
when the excited charges are separated, power can be collected at the
electrodes.
2.3.2 Electrochromism in conjugated polymers
Since optical absorption corresponds to differences in energy states, the
absorbance spectrum of a material is a signature of the electronic and
vibrational levels (Figure 6). When a conjugated polymer is doped, new energy
levels are created inside the band gap (as seen in Figure 5) and new optical
transitions are thereby possible. Many conjugated polymers can be
electrochemically doped, with a resulting change not only in conductivity but
also in color. Such a material, which alters color upon electrochemical
switching, is called electrochromic.28-30 Examples of applications for organic
electrochromic
materials
are
flexible
thin
electrochemical
displays,16
autodimming windows31 etc.
The use of organic materials for electrochromic applications has several
advantages, besides the previously mentioned ease of manufacturing. Addition
of side chains to conjugated polymers or modification of the effective
conjugation length alters the energy level configuration and thereby the color of
the material.32,33 It is therefore possible, to some extent, to design materials with
the right color absorption to match the demand of a specific application. Since
electrochemical switching of conjugated polymers also alters the electronic
conduction properties, organic electrochromic pixels may be integrated with
electrochemical transistors17 and logic18 to build truly all-organic matrixaddressed displays.16 Examples of common electrochromic polymers are given
below and simple organic electrochemical structures are described in more
detail in 3.2-3.4.
14
Chapter 2: Conducting Polymers
2.4 Examples of commonly used conjugated polymers
The research field of conjugated polymers has accelerated rapidly since the
discovery of doped polyacetylene, and a number of different polymers and their
derivatives have been synthesized in order to achieve materials with tailor-made
properties. Polyacetylene itself is of course the ”original” conducting polymer
but difficulties with processing and poor stability in ambient atmosphere limits
the use of the material in real applications. Other groups of polymers are
therefore today utilized much more frequently in organic electronics. A few
materials of interest for the work in this thesis will be presented below.
2.4.1 Polythiophenes
Polythiophene and derivatives thereof have become popular conjugated
polymers for applications such as light emitting diodes,8,10,34 field-effect
transistors,14,35,36 solar cells,37,38 memory applications39,40 and capacitors.41 The
first synthesis of a material similar to polythiophene was reported already at the
end of the 19th century but it was not until the 1980s that the well-defined
polymeric material was presented.42
Figure 7. a. polythiophene. b. poly(3-alkylthiophene), R denotes an alkyl
chain. c. Example of the regioregular configuration, which preserves the
conjugation in the material.
Polythiophenes (Figure 7a) are insoluble in organic solvents due to the rigid
polymer backbone and side-chains have therefore been added to the material.
15
Chapter 2: Conducting Polymers
Common polythiophene derivatives are molecules with an alkyl chain on the 3position of the thiophene ring as shown in Figure 7b. Poly(3-hexylthiophene)
(six carbons in the alkyl side chain) has proven to be a material with both good
solubility in organic solvents and nice film-forming properties. Today, pure and
highly
regioregular
head-to-tail
poly(alkylthiophenes)
(Figure
7c)
are
commercially available and can be used to form conjugated and well ordered
semiconducting thin layers by e.g. spin casting. Poly(3-alkylthiophenes) can be
utilized as the active materials in the electrochemical wettability switch,
described in 7.1 and Papers 1-3.
2.4.2 PEDOT:PSS
One polythiophene derivative has become especially important in organic
electronics, particularly for electrochemical devices. During the late 1980s,
Bayer AG in Germany developed poly(3,4- ethylene dioxythiophene) or PEDOT
(Figure 8a). This p-doped polymer in combination with the charge balancing
counter-ion poly(styrene sulfonate) (PSS-) (Figure 8b) forms a water soluble
(emulsion) polymer-polyelectrolyte system (PEDOT:PSS) with nice filmforming properties, high conductivity and tremendous stability in the p-doped
(oxidized) state.32
Figure 8. a. poly(3,4-ethylene dioxythiophene) (PEDOT). b. poly(styrene
sulfonate) (PSS)
16
Chapter 2: Conducting Polymers
Applications incorporating PEDOT:PSS range from solid state devices like light
emitting diodes43,44 and solar cells,45,46 where the polymer works as the anode or
hole
injecting
electrochromic
sensors,21,47,48
layer,
to
displays,16,31
electrochemically
electrochemical
bio-electrodes49,50
active
structures,
transistors
and
such
as
logics,17,18
etc. The mechanical and chemical stability of
PEDOT:PSS films in combination with good electrochromic properties and high
conductivity make the material an excellent base for all-organic electrochemical
devices. A film of PEDOT:PSS can be reversibly switched between the oxidized
and neutral state several times. PEDOT:PSS is almost transparent in the
oxidized doped state and dark blue in the neutral semiconducting state, as seen
from the absorbance spectrum in Figure 9. Electrochromic displays with
PEDOT:PSS on paper and plastic are discussed in detail in 7.2 and Paper 4. The
PEDOT:PSS electrochemical ion pump is presented in 7.3 and Papers 5 & 6
Figure 9. Absorbance spectrum of PEDOT:PSS film in the neutral and
oxidized (p-doped) state.
2.4.3 Polyaniline
Another essential conjugated polymer is polyaniline (PANI). The first samples
of polyaniline, also called aniline black, were prepared in the early 19th century
but the material was usually an unwanted deposit on the anode during
electrolysis with aniline. In 1910, Green and Woodhead51 managed to control the
17
Chapter 2: Conducting Polymers
synthesis of PANI and also started to characterize the polymer. Due to the cheap
raw material, stable conducting forms, strong electrochromism and ease of
processing, PANI has since then become a popular material in both industry
and science.42,52
PANI is a complex conjugated polymer, since the material has a number of
intrinsic oxidation states and several routes of doping. Fully reduced PANI is
transparent and called leucoemeraldine, with a chemical structure as shown in
Figure 10a. The dark blue semi-oxidized (~50%) state is named emeraldine base
(Figure 10b) and the violet blue fully oxidized material is termed pernigraniline
base (Figure 10c). Contrary to most other polyaromatics, none of these states
are electronically conducting, not even the fully oxidized pernigraniline form.
Instead, PANI becomes conducting when the semi-oxidized state is protonated
and a green emeraldine salt is formed. This highly conducting doped form,
shown in Figure 10d, can actually be reached through two completely different
pathways. If the emeraldine base is treated with e.g. hydrochloric acid, protonic
acid doping occurs as protonation of the imine nitrogen atoms (⎯N==) creates
positively charged protonated imines (⎯NH+⎯), balanced by negative ions
from the acid. This is so called non-redox doping, but in another route, chemical
or electrochemical doping of the reduced leucoemeraldine base can also be
utilized to obtain the very same conducting salt.22,42,52-54
The nonconductive emeraldine base is soluble in N-methyl-pyrrolidone but the
conducting salt, protonated with hydrochloric acid, is practically insoluble in
organic solvents. One way to achieve a soluble (emulsion) conducting PANI salt,
without incorporating covalent side chains, is to dope the material with
surfactant alkyl sulfonic acids that donate protons to the polymer.55,56 This way
of preparing the p-doped material also results in a slightly different chemical
composition. Apart from the larger surfactant counter ions, which of course
affect solubility and film formation, protonated amine units (⎯NH2+⎯) have
also been detected along with the imine dittos in these systems.52,57,58 The effect
of the surfactant counter ion on the surface properties of such a system is a key
element in the wettability switch, described in 7.1 and Papers 1-3
18
Chapter 2: Conducting Polymers
Figure 10. Polyaniline (PANI) oxidation states. a. Leucoemeraldine. b.
Emeraldine base. c. Pernigraniline. d. Protonated emeraldine salt (charge
balancing counter ions not shown). The dots represent unpaired electrons
(radicals).
The electrochromic properties of PANI makes it a suitable candidate for multicolor electrochromic displays.28 Other areas of application include antistatic
coatings
and
corrosion
protection,59,60
biomaterials,61,62
light
emitting
diodes,63,64 sensors,65,66 field effect transistors67 and batteries.68
19
Chapter 2: Conducting Polymers
2.5 Ionic conduction in polymer films
A very useful property of many conjugated polymer systems is the ability to
conduct not only electrons but also ions, e.g. when electrochemical doping or
undoping of the polymer is accompanied by mass transport of ions in or out of
the material. This provides the possibility to electrochemically switch the entire
bulk of a conjugated polymer film, but the ionic movement per se and the effect
that it has on the volume and conductivity of the polymer can also be directly
used in applications such as artificial muscles,69-71 sensors47,48 and drug release
devices.72-74
Polymer-polyelectrolyte systems with a surplus of counter ions in the film can
have ion conductivities that are equal to or even larger than that of aqueous
solutions.75,76 PEDOT:PSS is an example of such a porous, phase separated
material, with electronically conducting conjugated polymer islands surrounded
by insulating PSS.75-78 Ions from an aqueous solution enter the polymer film
hydrated and the ion conductivity in the material will therefore increase, once it
is swelled with water. The transport mechanism of ions in the film can be either
electromigration or diffusion, depending on the ion gradient and electric field
that act on the species.79 In this case, the ion conduction primarily occurs in the
(hygroscopic) PSS phase and is thus effectively independent of the oxidation
state of the PEDOT. There are, however, examples of conjugated polymer coated
membranes with electrochemically switchable ion conductivity80,81 and even
protein transport properties.82
From a bioelectronics perspective, the fact that conjugated polymers can
conduct ions and work as ion-to-electron transducers48 (and vice versa) is of
course very intriguing and can be used in e.g. organic neural electrodes.83,84 In
biological systems, local potentials across membranes are created and sustained
by differences in ionic concentrations and ions like Ca2+ are versatile cell
signaling substances that govern many important processes in our bodies
(Chapter 5).85-87 Electronically controlled transport of such ions through the
polymer film and communication with cells on top of the polymer layer is the
principle of the organic electronic ion pump, described in 7.3 and Papers 5 & 6.
20
CHAPTER 3
3. BASIC ELECTROCHEMICAL STRUCTURES
Since many conjugated polymers can be doped and undoped by
electrochemical reactions
and thereby change their intrinsic
properties, such as color, conductivity and wettability, these
materials are well suited as the working material in different
electrochemical devices. The devices presented in this thesis, and
many devices before that,16,17,21,88,89 all originate from similar simple
electrochemical structures that can be easily manufactured on
flexible substrates with “printable” materials. The work towards
achieving such all-printed electrochemical devices is an outcome of
the collaboration between the group of Organic Electronics90 and
Acreo AB.91 The function of these fundamental base structures will
therefore be described in this chapter.
3.1 Reduction and oxidation of conducting polymers
An electrochemical reaction means that a chemical phenomena is associated
with charge separation of some sort, often in the form of charge transfer.92 Two
or more half-reactions have to take place, one oxidation and one reduction
reaction. In a simple two-electrode electrochemical cell (Figure 11), the
electrodes are separated in space but linked by two conducting paths. The
electrolyte between the electrodes can transport ionic charges and the
conducting wires of course conduct electrons. The electrodes may either work as
inert sources and sinks for electrons transferred to and from species in solution,
21
Chapter 3: Basic Electrochemical Structures
or take part in the electrochemical reaction itself. If the sum of the free energy
changes at both electrodes is negative, energy is released and the cell works as a
battery. If it is positive, a bias voltage can be applied to drive the electrochemical
reaction, as illustrated in Figure 11.
Figure 11. Sketch of a simple electrochemical cell. The anions (A-) are
electrostatically drawn towards the anode (+) and the cations (M+)
towards the cathode (-).
Many conducting polymers can be either n-doped by reduction (addition of
electrons to the polymer) or p-doped by oxidation (withdrawal of electrons from
the polymer), as shown in reaction (1) and (2), respectively. P denotes the
polymer, Mx+ means one or many cations (positively charged) and Ay- stands for
one or many anions (negatively charged).
P 0 + xe− + M x + → P x − M x +
(1)
P 0 + A y − → P y + A y − + ye −
(2)
n-doped conjugated polymers are often not as chemically stable as p-doped
polymers, if oxygen or water are present, and the polymer systems used in
electrochemical devices are therefore often switched between a neutral
(undoped) and an oxidized (p-doped) state. As an example, commercially
available PEDOT:PSS (2.4.2) is originally (as received) conducting and partially
22
Chapter 3: Basic Electrochemical Structures
oxidized. From here the material can either be further oxidized to a more
conducting state or reduced to the semi-conducting neutral polymer. The
reactions are reversible in both directions, which makes the polymer system
suitable for electrochemical devices. The reduction (left to right) and oxidation
(right to left) of PEDOT:PSS can be written as:
PEDOT + : PSS − + e− + M + ↔ PEDOT 0 + PSS − M +
(3)
An electrochemical technique such as voltammetry, i.e. the applied potential is
swept and the current measured, is a common way to characterize conducting
polymers. This technique yields the oxidation potential, i.e. a measure of how
much energy is needed to withdraw electrons from the polymer HOMO level,
and the reduction potential, a similar characteristic (addition of electrons) of the
LUMO. The measurement is performed with a potentiostat and a threeelectrode system.92 The reaction to be studied occurs at the working electrode. A
reference electrode, such as Ag/AgCl, with a well-defined electrochemical
potential is used to keep the working electrode at a constant absolute potential.
To maintain the potential as stable as possible, a third auxiliary electrode
(counter electrode) is used to pass the current. The counter electrode is often
made of Pt and should have a large active area. The potentiostat can also be
used to synthesize polymeric materials on the working electrode by
electropolymerization.93,94 The monomer is then present in the electrolyte and
when a potential is applied, oxidation or reduction of the monomer can create
reactive radicals. Several radicals can connect to form polymeric chains and
since the reaction takes place at the working electrode, the polymeric film will
cover that surface.
Organic electrochemical devices should preferably be self-supporting structures
that contain relatively few layers and, in the best case, are printable. Therefore,
the design of a typical organic electrochemical cell is somewhat different from
that shown in Figure 11. The electrolyte is generally solid or gel-like instead of a
liquid. Conducting polymer materials work as one or several electrochemical
electrodes, where reduction and oxidation takes place, but may also function as
electronic and ionic conductors. The polymers can e.g. be spin-coated on the
23
Chapter 3: Basic Electrochemical Structures
substrate but are, as in the case of PEDOT:PSS, sometimes commercially
available in rolls of pre-coated films on plastic foil or paper. The devices often
have a lateral configuration, i.e. the electrodes are situated in the same plane
with the electrolyte on top or underneath. Patterning of these polymer films is
generally done by simply cutting non-conducting lines or by over-oxidation of
the PEDOT, i.e. electrochemically or chemically destroying the conductivity of
the material.3 In order to achieve smaller features, photolithographic techniques
may also be used to manufacture the devices.
3.2 “Structure 1”
If an electrolyte is cast to cover a stripe of polymer film, made from a conducting
polymer-counter ion system like PEDOT:PSS, the resulting device configuration
is referred to as “Structure 1”. If a voltage is applied along the polymer film, as
shown in Figure 12, a gradient in oxidation state and color will appear.
There are two parallel paths for charge transfer between the electrodes in the
system. The polymer film is electronically conducting, which means that
electrons can move through the material, which then works as a resistor. Since
the polymer is not a perfect electronic conductor, there is always a potential
difference between the two sides of the polymer stripe when a voltage is applied.
Therefore, with the electrolyte on top, the positive side of the polymer will start
to be oxidized and the negative side reduced. As long as the electrochemical
reaction occurs in the polymer film, charges can be transported as ions through
the electrolyte. Reduction of the polymer film drastically increases the
impedance locally and thereby makes it more difficult for current to pass
straight through the “resistor”. More reduction and oxidation will then take
place as the oxidation gradient builds up. This behavior of the “Structure 1” is
responsible for the saturation of drain-source current in the electrochemical
transistor by Nilsson et al.17,95 If the applied potential goes to zero (open-circuit)
the internal potential difference in the polymer will even out and the gradient
disappear.
24
Chapter 3: Basic Electrochemical Structures
Figure 12. Structure 1. a. Side view with electronic and ionic conduction
paths. b. Top view.
3.3 “Structure 2”
If the polymer film of Structure 1 is instead divided into two electrodes by a nonconducting line, “Structure 2” has been created, as shown in Figure 13.
Electrochemically, Structure 2 is a direct equivalent to the electrochemical cell
shown in Figure 11. The two electrodes are electronically isolated and ionic
transfer is the only path for charge transport between them. When a bias voltage
is applied, the negative electrode will start to be reduced and the positive
electrode becomes oxidized. The loss or gain of charges inside the polymer film
is balanced out by migration of ions in the electrolyte and across the electrolytepolymer interface. The electronic current in the wires and the ionic current in
the electrolyte will flow as long as electrochemistry can take place at the
polymer electrodes. If the voltage supply is removed, there is no closed circuit
25
Chapter 3: Basic Electrochemical Structures
for the electrons to spontaneously go back and the electrodes will therefore keep
their respective oxidation states, until chemically affected by e.g. oxygen in the
atmosphere.
With
electrochromic
materials
such
as
PEDOT:PSS
or
polyaniline:dodecylbenzene sulfonic acid (PANI:DBSA), Structure 2 works as a
very simple display pixel and several such structures can, in combination with
electrochemical transistors, build up all-organic matrix-addressable displays.16
Improvement of optical contrast in PEDOT:PSS paper display pixels is
discussed in 7.2 and Paper 4. Structure 2 also represents the basic configuration
of the electrochemical wettability switch (7.1 and Papers 1-3), the
electrochemical ion pump (7.3 and Papers 5 & 6) and works as the gate in the
electrochemical transistor.17
Figure 13. Structure 2. a. Side view with electronic and ionic conduction
paths. b. Top view.
26
Chapter 3: Basic Electrochemical Structures
3.4 “Structure 3”
The device called “Structure 3” is slightly more complicated than the previously
described configurations. In this case, the polymer film is divided into three
separate pieces by non-conducting lines, as shown in Figure 14, but only the
outer two segments work as charge donating/accepting electrodes. When a
voltage is applied between the two electronic contacts, there are several paths
for charge transport. Because of the bias voltage, the two outer electrodes are
oxidized and reduced respectively, and ions are transported between them, in
the same way as seen for Structure 2 (Figure 13).
Figure 14. Structure 3. a. Side view with electronic and ionic conduction
paths. b. Top view.
27
Chapter 3: Basic Electrochemical Structures
Parallel to this charge transport, the middle segment of conjugated polymer is
also capable of conducting electrons if ion to electron transduction can take
place through oxidation on one side and reduction on the other. Therefore, an
electrochemical gradient will be induced in the middle polymer piece of
Structure 3 when the outer electrodes are biased. This electrochemical gradient
only exists as long as electrochemistry occurs at the two electrodes. If the
voltage is disconnected or the electrode material is consumed, the outer pieces
of polymer will stay oxidized and reduced, but the gradient in the middle will
disappear, in the same way as with Structure 1.
The induced oxidation gradient is a result of the local electric field in the
electrolyte and can therefore, as cleverly shown by Said and co-workers,89 be
used to map and visualize electric fields in the electrolyte. When PANI:DBSA is
used as the conducting polymer, the oxidation gradient also serves as a
wettability gradient, which can be used to control the spreading of water
droplets on the polymer surface (7.1 and Papers 1 & 2).
28
CHAPTER 4
4. SURFACE ENERGY – WETTABILITY
The formation and characteristics of interfaces between liquids and
solids depend on the surface properties of the involved materials.
Wetting behavior can therefore provide valuable information in a
number of scientific and industrial areas. For small volumes, surface
effects are even more pronounced and surface science is therefore a
crucial part of micro- and nanotechnology. Examples of application
areas depending on (and utilizing) wettability are e.g.:
‰
Microfluidics – How do small volumes of a liquid move on a surface
or in a channel?
‰
Control of surface treatments and cleaning steps – Is the
surface clean?
‰
Anti-fouling surfaces – Why are the leaves of the lotus plant
always dry?
‰
Biomaterials – How will the biomaterial interact with human
tissue?
‰
Pharmaceuticals – How will the drug particles dissolve and be
absorbed in the body?
‰
Composites – How will the reinforcement material adhere to the
matrix?
‰
Printing technologies – How will the ink adhere and spread on the
paper coating?
29
Chapter 4: Surface Energy - Wettability
4.1 Surface tension in theory
The molecules at the surface of a liquid or a solid behave differently from their
counterparts in the bulk, since they lack neighbors in one direction. The surface
therefore has “free energy” along the interface exposed towards the surrounding
media and it is, neglecting other forces, energetically favorable to decrease the
surface area as much as possible. This is why liquids form spherical (lowest
area/volume ratio) droplets in e.g. air.
The surface energy, γ, is characteristic for a solid or liquid material in the
interface towards a specified gas phase, such as air (with vapor). The surface
energy determines the energy needed to increase the area of the surface, i.e.
high surface energy means strong attraction between the surface molecules of a
material. For liquid-gas interfaces, the surface energy is generally referred to as
the surface tension or interfacial tension. A common example of surface tension
and surface energy is that of a thin rectangular frame of width l, pulled through
a liquid-gas interface, e.g. water-air, as drawn schematically in Figure 15 (from
Ref. 96).
Figure 15. Creation of a thin liquid film by pulling a wire frame through
an air-water interface. Adapted from Ref. 96
30
Chapter 4: Surface Energy - Wettability
A thin film of the liquid may be formed in the frame and a certain force is then
needed to lift the rectangle and thereby increase the surface area (the weight of
the frame and film are not considered here). If γ is the force per unit length to
create the new interface, i.e. the interfacial tension, the force needed to move
the frame and thereby create two new interfaces, one on each side of the liquid
film, can be written as:
F = 2γ l
(4)
If the frame is moved a short distance, dx, the amount of work done is given by:
dW = 2γ ldx
(5)
If the area, A, of the liquid film is considered instead, γ has the units of energy
per unit area (surface energy) and equation (5) can be written as (once again the
factor 2 comes from the two sides of the film):
dW = 2γ dA
(6)
This shows that γ can be thought of as either an interfacial tension or as a
surface energy, the quantities are identical. The SI unit for surface energy /
surface tension is Nm/m2 = N/m. The surface tension can be divided into polar
(permanent dipoles) and dispersive (induced dipoles) parts. Polar materials,
like water, typically have strong bonds between the molecules and therefore
show high surface tensions in air.
4.2 Contact angle measurements
When a small volume of liquid is placed on a solid surface in e.g. air, the
equilibrium state between the two materials and the surrounding vapor
determines the shape of the liquid droplet. In a simplified picture, the free
energy of the solid surface can be seen as a force that will tend to spread the
liquid droplet in order to decrease the solid-vapor interface. On the other hand,
there is a cost in energy to form a new liquid-solid interface and therefore the
31
Chapter 4: Surface Energy - Wettability
interfacial energy between the liquid and solid works as a force in the opposite
direction. Finally, the surface tension of the liquid tries to contract the droplet in
order to decrease the liquid-vapor interfacial area and is therefore a third force,
parallel to the tangent of the droplet. The shape of the droplet can be seen as the
equilibrium between these forces as shown in Figure 16. The solid surface
energy in the surrounding vapor is denoted γsv, the liquid-vapor surface tension
γlv and the solid-liquid interfacial energy γsl.96,97
Figure 16. Surface tensions (γ) and contact angle (θ) when a liquid
droplet is placed on a solid surface in the vapor atmosphere.
The angle between the solid surface and the droplet tangent is called the contact
angle, θ, and is a direct measure of how a liquid wets a solid surface (Figure 16).
The relationship between the surface tensions and the contact angle is expressed
in the Young-Dupré equation, which simply reflects the trigonometric
relationship between the three forces.96,97
cos θ =
γ sv − γ sl
γ lv
(7)
Contact angle measurements are fairly simple in practice, but since the
technique is very surface sensitive, cleanliness is extremely important. The most
common instrument used to determine contact angles is the goniometer. It
usually consists of a sample table, a syringe for dispensing of the liquid and a
microscope lens or a camera to record the contact angles (Figure 17). If the
32
Chapter 4: Surface Energy - Wettability
system has a camera connected to a computer, several images can of course be
recorded over time and contact angle values are then generated from the
acquired images by software. When the liquid drop is created at the tip of the
syringe, which is then lowered down until the drop contacts the solid surface,
the measured contact angle is “static”. If nothing else is stated, contact angle
values refer to static measurements.
Figure 17. Photograph of a goniometer setup. Liquid droplets for contact
angle measurements are applied to the solid surface with the syringe. The
camera takes pictures of the droplets and computer software can then
evaluate the images.
If the volume of the liquid droplet is increased while in contact with the surface,
the tangent line moves away from the syringe and the measured angle is
referred to as the “advancing” contact angle. If the volume is instead decreased
in a similar fashion, the contact angle is “receding”. In many cases, there is a
hysteresis between the advancing and receding contact angle values. The
33
Chapter 4: Surface Energy - Wettability
advancing angle is always equal to or larger than the receding and the difference
could originate from e.g. relaxation from contaminated surfaces or liquids,
surface roughness or surface immobility.97 One example of obvious hysteresis is
a water droplet sliding down a dirty windowpane. The front of the droplet is
slightly pinned to the window and therefore forms a large advancing contact
angle as gravity drags the water down. Similarly, if the back of the droplet is
stuck, it will form a small receding angle against the glass.
In many cases, water is used as the standard liquid when measuring contact
angles. A surface that water wets (low contact angle) is called hydrophilic, while
materials with high water contact angles are labelled hydrophobic. From Figure
16 and equation 7, one can see that a high solid surface energy will generally
result in low water contact angles, since γsv is then high and γlv is likely low, and
vice versa. The absolute value of the solid surface energy cannot be measured
directly, because the interfacial tension is also unknown, but it is possible to
estimate with contact angles from several liquids. A series of homologous
liquids, such as alkanes of different length, can be used to create a so-called
Zisman plot. The cosines of the contact angles are plotted versus the surface
tensions of the liquids and give a linear relationship. The “critical surface
tension” corresponding to cos θ = 1 (γsv = γlv) can be extrapolated from the plot
and this value gives a good estimation of the surface energy of the material
studied.96,97
Another approach to estimate the surface energy of a solid surface is to measure
contact angles with two or more liquids, of which one should be mainly polar
and the other non-polar. The intermolecular energy is the sum of the polar and
dispersive component and this relationship also holds for the surface tensions.
The polar and dispersive part of the solid surface energy can therefore be
evaluated by e.g. the geometric mean equation (γp denotes the polar and γd the
dispersive parts of the interfacial tensions):
d
d
γ SL = γ SV + γ LV − 2 ⎡ ( γ LV
γ SV
)+
⎣⎢
(γ
p
LV
Equation (7) and (8) give equation (9):
34
γ SVp ) ⎤
⎦⎥
(8)
Chapter 4: Surface Energy - Wettability
d
d
γ LV (1 + cos θ ) = 2 ⎡ ( γ LV
γ SV
)+
⎣⎢
(γ
p
LV
γ SVp ) ⎤
⎦⎥
(9)
If the contact angles have been measured with at least two liquids, with known
polar and dispersive liquid-vapor interfacial tensions, equation (9) can be used
to calculate the values of the solid surface energy components (two liquids give
two equations for two unknowns).98
Apart from the surface energy of the solid material, the surface topography can
also affect the wetting behavior. A structured surface with high roughness will
decrease the water contact angles compared to a smooth material if the original
contact angle is less than about 90°. The topography of the surface then forms
small capillaries for the water. The liquid can penetrate the micro cavities and
will therefore quickly wet the surface. If the angle is slightly larger than 90°, the
uneven surface will instead result in much higher angles. In this case the water
cannot enter the capillaries and has to form larger angles to the available solid
surface. The effect of surface roughness on contact angle measurements can be
estimated with the simple Wensel equation (10), where r denotes the surface
roughness, defined as the ratio of the true area of the solid to the apparent area,
θ ’ is the measured contact angle and θ is the “true” contact angle.96,97,99
r=
cos θ '
cos θ
(10)
If the surface is very hydrophobic or very rough, air trapping underneath the
drop has to be considered (the Cassie regime) but this state is generally only
meta-stable for water.99 The creation of rough surfaces is a common technique
to achieve superhydrophilic and superhydrophobic surfaces, artificially as well
as in nature. Sun and co-workers100 have e.g. achieved reversible thermal
switching of water contact angles between ~0° and 150°. A famous example of a
superhydrophobic surface in nature is the leaf of the lotus plant (Figure 18).
This ingenious structure barely gets wet when it rains and is also self-cleaning,
since the water forms large contact angles and simply rolls off the rough waxy
surface of the leaves, which have several levels of texture.101,102
35
Chapter 4: Surface Energy - Wettability
Figure 18. Lotus leaf and raindrop. Used with permission from Victoriaadventure 103 and the photographer Byoung Sup Ghill (Copyright ©)
4.3 Other techniques to study surface tension
4.3.1 Pendant drop
The surface tension of a liquid (in e.g. air) or the interfacial tension between
liquids can be calculated directly from the shape of a drop, which hangs from
the syringe in the surrounding media. The shape of the drop (Figure 19) is
mathematically described by three first order differential equations (11-13).104,105
dx
= cos φ
ds
(11)
dz
= sin φ
ds
(12)
dφ
sin φ
= 2+ βz −
ds
x
(13)
β=
36
∆ρ gR02
γ
(14)
Chapter 4: Surface Energy - Wettability
x and z are coordinates of a point on the drop profile divided by R0, the radius of
curvature at the apex, which makes them dimensionless. φ is the turning angle
at that point and s is the dimensionless curvature length from apex to the same
point. The shape of the droplet is characterized by β, which is defined in
equation (14), where ∆ρ is the difference in density between the phases and g is
the gravitational constant.104,105
Figure 19. The pendant drop coordinate system.
From a digital image of the droplet, acquired with e.g. a goniometer, computer
software can quickly solve the system of differential equations numerically to
best fit the droplet shape, with the same methods as when analyzing contact
angles. If the liquid and gas densities are known, the surface tension can then be
extracted. The pendant drop method has the advantage of being independent of
the liquid contact angle to the solid support, as oppose to tensiometry that is
described below, and gives quick accurate values of surface tensions. Evaluating
37
Chapter 4: Surface Energy - Wettability
the surface tension of a liquid before using it for contact angle measurements is
a good way to check for possible contamination.
4.3.2 Tensiometry
A classical technique to measure surface tension and interfacial interactions is
tensiometry.106,107 The principle of tensiometry is the same as in the example
with the rectangular frame described in 4.1, i.e. the force exerted when a solid
probe interacts with a surface or interface is a measure of the involved
interfacial tensions. Apart from the surface tension of the analyzed liquid, the
forces will depend on the size and shape of the probe as well as the contact angle
between the liquid and the probe. Different shapes and types of probes can be
used, but the most common ones are rings and plates. The results from
tensiometric measurements are often very accurate and objective, but the
technique requires a fair amount of the liquid to be analyzed and if the solid is
studied, it has to be of a regular shape with the same surface on all areas that
contact the liquid.
38
CHAPTER 5
5. CELL COMMUNICATION
Cells, in all organisms, need to be able to sense and interact with
their environment in order to survive. In multicellular organisms,
like ourselves, all the cells have to function together at the same time
and a complex system of inter- and intracellular signaling pathways
have therefore evolved. The scientific field of cell signaling is
naturally huge and to accurately describe even a few of the most
important pathways and signaling molecules is far beyond the scope
of this thesis. Therefore, this chapter only briefly illustrates the basic
aspects of cell communication in general terms and exemplifies with
one incredibly important, yet very simple, signaling entity; the
calcium ion.
5.1 Communication between and inside cells
Cells can communicate with each other in a number of different ways, as
outlined in Figure 20. Cells that are not in direct contact communicate by the
release of signaling molecules, which are detected by receptors on (sometimes
inside) the target cells (Figure 20a). Such molecules that bind specifically to
protein receptors are generally referred to as ligands. If the ligands, e.g.
hormones, travel long distances in the organism through the blood stream,
often to another tissue, the signaling is called endocrine. If the signaling
molecules are instead rapidly detected and have their effect in close proximity to
39
Chapter 5: Cell Communication
the signaling cell, the communication is denoted paracrine. If the signaling cell
and the target cell are one and the same, the signaling is called autocrine.108,109
Figure 20. Cell communication methods. a. Release of ligands, detected
by receptors on the target cell. b. Molecules at the surface of the signaling
cell are detected by membrane receptors along the target cell surface. c.
Direct transport of intracellular signaling molecules through gap
junctions. d. Synaptic signaling via transport of an action potential along
the axon, followed by electric or chemical communication with the target
cell. Adapted from Ref. 108 & 109.
Cells that are in direct contact with each other can of course communicate
rapidly and easily by either receptor-ligand signaling (Figure 20b) or directly
through gap junctions (Figure 20c). In the first case, the signaling cell has ligand
molecules in the plasma membrane, which recognize and bind to specific
40
Chapter 5: Cell Communication
surface markers on the target cell. Gap junctions, on the other hand, are protein
tubes that are formed between the plasma membranes of adjacent cells. They
literarily work as hundreds of pipes that let small signaling molecules pass
directly from one cell to the other. The passage can, however, be closed at any
time by a conformational change of the gap proteins, e.g. as a response to high
ionic concentrations or if the neighboring cell dies.108,109
The signaling mechanism in our nervous system is synaptic signaling (Figure
20d), which means that the signal is propagated as changes in the electrical
membrane potential along the axon of the neuronal cell. A stimulus, which
locally shifts the membrane potential (depolarization), activates voltage-gated
Na+ channels that allow Na+ to enter the cell. The influx of positive charges
further depolarizes the membrane, which has a resting potential of about –70
mV, and causes new channels to open. When the membrane potential reaches
an equilibrium value of +50 mV, the Na+ channels are temporarily inactivated
and the potential returns to its resting value. This decrease in potential is also
speeded up by a simultaneous flux of K+ out from the cell, through voltageactivated K+ channels. At the same time, the neighboring part of the membrane
is triggered and the wave of self-amplifying depolarization (action potential)
spreads along the membrane. The inactivation of the Na+ channels lasts for a
few ms after reaching the resting potential, which prevents the signal from
propagating backwards. Since the cells are very long (up to 1 m) the signal can
be transported fast (up to 100 m/s) over extensive distances. The transfer of the
signal between cells (synapse) can be either electrical, i.e. an ion current travels
directly between cells through gap junctions, or chemical via opening of voltageoperated calcium channels (VOCCs, see 5.2), activated by membrane
depolarization, and subsequent release of neurotransmitters that are detected
by receptors on the neighboring target cell.108-110
The propagation of an action potential, where one stimulus triggers a sequence
of events, is one example of how the signals are amplified in cellular
communication. Amplification also takes place inside the cells through so called
second messengers. The extracellular signaling molecule, the first messenger,
starts the production or release of small intracellular molecules, i.e. second
41
Chapter 5: Cell Communication
messengers. Each such second messenger in turn activates new molecules in a
cell signaling cascade, which not only amplifies the signal to create an effective
end result in the target cell, but also allows for improved control of the response
through possible inhibitory feedback loops. Furthermore, signaling molecules in
one cascade can activate other pathways, which may work in parallel to induce
the same end result or diverge into new or even counter-acting cellular
responses. The fact that two completely different signals can use the same
second messenger, highlights the complex nature of cell signaling and the
difficulty in scientifically studying cell communication, e.g. by inhibition of
specific receptors or second messengers. For instance, how can the cell know
which signal that originally activated a specific second messenger if it is active in
several pathways? One example of how this is actually solved in nature is the use
of oscillating signals, which carry information not only in the molecular species
in themselves, but also in both their frequency and amplitude.111,112 This has in
recent years proven to be the case with Ca2+ signaling, which is discussed in the
section below.
5.2 Calcium signaling
One of the most versatile intracellular signaling molecules in our body is the
calcium ion, which in a sense controls almost everything that we do. Our muscle
movement, brain activity and every single heart beat involves Ca2+ signaling. In
addition, this very simple ionic species is involved in fertilization, cell
development and differentiation as well as programmed cell death.86 The key to
such an enormous span of regulated cellular processes from a single signaling
molecule is resolution in time and space. The actual location of concentration
changes inside the cell as well as the frequency and amplitude of Ca2+
oscillations provide the necessary triggering and addressing of the signaling
cascades. The wide dynamic range can be exemplified by neurotransmitter
exocytosis, which is activated within microseconds and, at the other end of the
scale, cell proliferation and fertilization with periodicities of several hours (see
Figure 21).86,87
42
Chapter 5: Cell Communication
Figure 21. Intracellular Ca2+ signaling. Ca2+ pulses are, via second
messengers, triggered by a stimulus and generated from the balance
between ON and OFF reactions. Calcium ions can enter or leave the cell
via specific transporting pathways through the cell membrane and be
released from or stored in intracellular compartments. Organelles like the
mitochondria are involved in changing the dynamics of the signaling
process. Examples of oscillation period times corresponding to cellular
responses are shown at the bottom of the figure. Adapted from Ref. 87.
43
Chapter 5: Cell Communication
As shown schematically in Figure 21, the intracellular Ca2+ level is constantly
determined by a balance between ON reactions, which introduce calcium into
the cell, and OFF reactions that removes the signal. Controlled variations in
these ON and OFF signals, as a response to an external stimulus, create the
necessary Ca2+ pulses to trigger specific signaling systems.87
During the ON reactions, most of the intracellular Ca2+ ions have been
transported from outside the cell, through the plasma membrane. The
extracellular Ca2+ concentration is much higher than the intracellular one,
which creates an electrochemical gradient across the cell membrane that is not
permeable to the ions. If Ca2+-specific entry channels (shown as generic
uniporters in Figure 21) are opened in the plasma membrane, the gradient
drives Ca2+ into the cell, thereby increasing the intracellular concentration. A
commonly studied type of transport channels are the VOCCs (see also 5.1) that
exist in excitable cells, such as muscle cells, glandular epithelial cells and
neurons. VOCCs can generate very rapid actions in e.g. muscle contraction and
synaptic exocytosis but may also be involved in slower events like inflammatory
responses.87,113 Other types of membrane Ca2+ channels include receptoroperated channels and second-messenger-operated channels. A second
important source of Ca2+ is the intracellular stores, primarily in the
endoplasmatic reticulum (or the muscle cell equivalent, sarcoplasmatic
reticulum). Release from these stores is regulated with second messengers, as a
response to external stimuli and, indirectly, by Ca2+ itself. To fine-tune the
calcium signal, in time and space, buffers in the cell are loaded with Ca2+ during
the ON reaction and unloaded during the OFF reaction. At the same time, other
Ca2+-binding proteins work as effectors that activate different calcium
responsive cellular processes.87
In order to create Ca2+ pulses or oscillations with specific frequencies, OFF
reactions must balance out the ON reactions and decrease the intracellular
calcium concentration. Calcium ions are removed from the cytoplasm by various
pumps and exchangers, such as Ca2+-ATPase pumps, Na+/Ca2+ exchangers and
mitochondrial uniporters, which transports the ions out from the cell or into the
internal stores, as shown in Figure 21. These tools are not only important for the
44
Chapter 5: Cell Communication
cell in the event of a signaling cascade, but also to remain homeostasis with a
stable resting level of Ca2+ in the cytosol.87,114
As shown in Figure 21, mitochondria have an active role in the OFF reaction.
Already at low concentration changes, as a response to the character of the
intracellular calcium signal, mitochondria can rapidly take up Ca2+ through
uniporters to quickly clear the cytosol from the ion. The calcium ions are then
released in a much slower fashion and can subsequently be permanently
removed from the cell by pumps and exchangers.87,114
Apart from variations of the Ca2+ concentration in time, spatial resolution is
crucial for calcium signaling, especially in rapid responses.87,115 As an example,
the individual dendritic spines on neurons work as independent signaling units,
thereby increasing the computational capacity of the cell. Also, calcium signals
localized on one cell can spread from cell to cell and even between different cell
types.87
The knowledge of calcium signaling reveals that it is necessary to consider not
just the signaling entity per se but also the spatial and temporal properties of
the signal, when communicating with cells or studying cell communication. This
is an aspect that makes cell signaling even more complex and a strong
motivation, whether thought of as a scientific tool for cell studies or a delivery
device for future therapeutic purposes, for the organic electronic ion pump,
presented in 7.3 and Papers 5 & 6.
45
46
CHAPTER 6
6. ORGANIC BIOELECTRONICS
Compared
to
metals
and
inorganic
semiconductors,
organic
materials such as conjugated polymers are softer and chemically
much more similar to the materials in our body. This is reflected in
the fact that many polymer systems are highly biocompatible,6,7 i.e.
living cells can easily adhere and proliferate on the material surface.
Also, the potential use of flexible transparent substrates, the
formation of porous layers with large effective surfaces, high
electronic and ionic conductivity, electronic switching of various
physical properties etc. make conjugated polymer devices very
promising candidates for future bioelectronic applications.
6.1 Sensors
The use of conjugated polymers in different biological and chemical sensors has
recently attracted a lot of attention.116,117 One approach is to simply include
different biomolecules in conducting polymer electrodes, which localizes and
controls the spatial resolution of the biologically active molecule and acts as a
transducer, by converting the biochemical signal into an electronic ditto.117,118
Similarly, changes in the color or conductivity of the polymer itself, e.g. as a
response to the chemical nature of the environment, can be used as the
transducer mechanism in chemical sensors.48,65
47
Chapter 6: Organic Bioelectronics
In the search for sensors that are inexpensive and can be miniaturized and
integrated with other electronic components on portable high sensitivity devices
for immediate retrieval of data, organic thin film transistors (OTFTs) are very
attractive alternatives.116,119-121 These devices provide electronic output with in
situ amplification of the signal, as well as compatibility with standard
microelectronic manufacturing techniques.
The OTFT sensors can generally be divided into two types of devices, as depicted
in Figure 22. The first category describes sensors based on organic or polymeric
field effect transistors (Figure 22a), i.e. the conductivity of the transistor
channel is modulated by controlling the charge carrier density in an organic
semiconducting film with an applied electric field across an insulating layer. The
detection principle can here be due to direct analyte-semiconductor interactions
or indirect via specific receptor molecules on the device surface. The sensor
response variables can be changes in the bulk and field-induced conductivity,
threshold voltage or field-effect mobility.20,116,119,122 Examples of such sensors
are chemical gas sensors19,20,120 as well as ionic and biomolecular sensors.123,124
The second category of OTFT sensors is represented by the electrochemical
transistor, as exemplified in Figure 22b. In this case the modulation of the
channel conductivity is due to electrochemical doping/dedoping of the organic
semiconductor, charge balanced by ions from a solid or liquid electrolyte. The
electrochemical transistor is generally slower than its field effect counterpart
but, on the other hand, requires very low voltages (~ 1 V) and is easily integrated
with liquid analytes. Also, these devices are not built up from structures
including any critical dimensions, such as very thin films, and should therefore
be easier to integrate in a roll-to-roll manufacturing process.125 All-organic
lateral transistors17,18 and sensors21 on plastic or paper can be realized by
combining “Structure 1” (channel) and “Structure 2” (gate), described in
Chapter 3. Recent examples of novel sensor concepts utilizing the
electrochemical transistor is a glucose sensor126,127 and an ion-selective sensor
that uses ion channels in a bilayer lipid membrane.128 The response variable in
these cases is the level of modulation from the gate potential.
48
Chapter 6: Organic Bioelectronics
Figure 22. Examples of typical organic thin film transistor sensors. a.
Field-effect transistor for sensing of vapors.19 The gate (G) is a conductor
underneath an insulating oxide. The source (S) and drain (D) are
evaporated gold contacts. Exposure of the semiconducting channel to
different vapors is detected as changes in the source-drain current. b.
Electrochemical transistor for glucose sensing.126,127 The source (S), drain
(D) and active channel are in the same layer of PEDOT:PSS. The gate (G)
is in this case a platinum wire. The analyte solution contains the sample
and the enzyme (here glucose oxidase) for the specific analysis. The
modulation of source-drain current by the gate is determined by the
concentration of analyte in the solution.
6.2 Microfluidics
In order to realize cheap, fast and high sensitivity analysis of e.g. blood or saliva,
for use at home or close to the patient (bed-side) at the hospital, it is necessary
to combine the different sensor concepts with systems that handle small
volumes of liquid. The “lab-on-a-chip” approach aims at having the entire
analysis system on easy-handled miniature single-use chips, but the delivery of
fluid and control of fluid motion is still a major problem, and in many cases
such techniques require large external instrumentation. To be able to create an
independent analysis lab on the microscale, the driving force to control fluid
49
Chapter 6: Organic Bioelectronics
motion must originate from within the integrated device and not from external
pumping equipment.129,130
One way to guide and regulate liquid flow in microfluidic applications is to
create transitions from wetting to non-wetting regions on the surfaces in contact
with the fluid. A liquid on a solid surface will find it energetically favorable to
move from an area with lower wettability (high contact angle) towards higher
wettability (decreased contact angle).131,132 Therefore, several publications on
how to switch the contact angle of a surface have been presented in the last few
years. The external stimuli can be e.g. temperature,133 light,134 electric fields135 or
electrochemistry.136 Also, a number of cleverly integrated systems, using e.g.
pressure driven flow of bubbles,137 microheater arrays138 and electrophoretic
manipulation of drops,139 have been realized and in some cases also
commercialized.
The
use
of
conjugated
polymers
to
regulate
wetting
in
all-organic
electrochemical devices is presented in 7.1 and Papers 1-3. Such an approach is
intriguing, because the devices switch at very low voltages (~ 1 V) and numerous
polymer systems can be used.58,140-143 Taken together, this gives the potential to
integrate these devices with organic electronic sensors and transistors to create
very cheap single-use lab-on-a-chip structures on paper or plastic foils. As a first
proof of concept, the organic electronic wettability switch has been used to
guide water in microfluidic bifurcations.144 Liquid flow in a microfluidic channel
assembled on top of the organic device is shown in Figure 23a and a conceptual
image of an integrated system, where the response from sensor-transistors in
the channel guide the liquid downstream, is presented in Figure 23b.
Causley and co-workers also further developed the concept of the organic
electronic wettability switch for microfluidic applications, but they used the
liquid within the channel as electrolyte instead of having a solid electrolyte layer
underneath.143 The prime target of that particular work was to induce sample
reagent mixing and fluid movement on a microfluidic chip, using very low
addressing power. In another work aiming at creating low power fluid
management systems for integrated microfluidics, the same group utilized
50
Chapter 6: Organic Bioelectronics
conjugated polymer actuators, described in the section below, to develop an
organic microfluidic pump.145
Figure 23. Organic microfluidics. a. Green colored water in a
microfluidic channel on top of an organic wettability switch.144 The right
side has been electrochemically switched to a more hydrophilic state and
consequently the water prefers that path. Top: Picture before water has
entered the channel. Bottom: Picture of water in the channel, where the
path that most of the water takes is indicated with an arrow. b.
Conceptual image of an organic microfluidic system containing
transistor-sensors integrated with wettability switches. This gives the
possibility to analyze the sample with the sensors and also guide different
samples in different paths, depending on the outcome of the sensing.
51
Chapter 6: Organic Bioelectronics
6.3 Actuators
When a conjugated polymer film is electrochemically oxidized or reduced, as
discussed in Chapter 3, the electronic current is accompanied by mass transport
of charge balancing counter ions into or out from the material. These ions,
especially when carrying solvent molecules such as water, can have a relatively
large total volume and thereby significantly alter the volume of the polymer
film, as shown in Figure 24. If part of the film is fixated to an
electromechanically inert material, such as gold, the swelling of the polymer will
cause the entire structure to bend or extend in a certain direction. This is the
principle behind one kind of conjugated polymer actuators; micromuscles.6971,145-147
By using microfabrication techniques, it is possible to create electronically
addressable micro-robots that can manipulate objects on the size of a single
cell.69,146 Also, since the electrochemical devices are biocompatible,6,7 operate at
low voltages and run in liquid solutions, such as cell media or body fluids,
micromuscles are highly interesting for biomedical applications like cell clinics,
blood vessel connectors or drug delivery systems (see section below).70,148
Figure 24. Conjugated polymer actuator. In the simple case illustrated
here, the neutral polymer film contains no ions (left). When a voltage is
applied between the electrodes (right), the polymer is oxidized (positively
charged) and counter ions enter into the film, thereby changing the
volume of the material.
52
Chapter 6: Organic Bioelectronics
6.4 Delivery systems
When a drug is administered as a single dose, e.g. when we take a pill, the time
that the concentration of the active substance in our body is actually within the
therapeutic range is highly limited. The only way to extend this time frame,
during which the concentration is above the minimum effective level, is to
increase the initial dose and, as illustrated in Figure 25, thereby risk toxic effects
from the temporarily very high concentration of drug in the body. If the drug
could instead be released slowly in a controlled fashion (dotted line in Figure
25), the time that a certain amount of substance is effective in the body could be
drastically prolonged. This not only improves the therapeutic effect but is also a
competitive and cost effective advantage on the huge pharmaceutical market.
Examples of drug delivery systems existing today include diffusion controlled
and chemically controlled systems.149
Figure 25. Drug concentration in the body for single dose administration
and controlled release (adapted from Ref. 149). The therapeutic range is
between the minimum effective level and the toxic level.
Instead of only having a constant concentration of the drug in the optimum
range over a long period of time, one step further would be a system capable of
responding quickly to the varying needs of the patient (dash-dotted line in
Figure 25). This gives the possibility to, at any time, adjust the concentration of
53
Chapter 6: Organic Bioelectronics
the drug in the body, e.g. as a response to an integrated or external sensor
output in a feedback system. Once again, when searching for biocompatible,
cheap and flexible electro-responsive devices, organic electronics is one
appealing choice and several electronically regulated drug delivery systems
using conjugated polymers have been presented during the last few years.
Examples of such organic bioelectronic delivery systems include PEDOT:PSS
coated nanotubes,150 mechanical systems using polymer actuator valves70,148,151
as well as conjugated polymer films, in which the drug is the counter ion,
temporarily coupled to the doped polymer and released when the doping level is
changed electronically.72-74 The two latter types are the most common choices
and are associated with slightly different advantages and disadvantages. The use
of actuators as mechanical valves makes it possible to release large or small,
charged or neutral molecules but requires relatively complex structures with
moving parts that may be expensive to manufacture and sensitive to mechanical
failures. The delivery systems based on polymer films loaded with the drug as
counter ions are, on the other hand, much simpler devices. They are, however,
intrinsically limited to charged drugs and often have the problem of undesired
leakage through ion exchange with the surrounding electrolytes.
Although currently designed as an in vitro tool to study cell communication, by
delivering ionic species, the ion pump described in 7.3 and Papers 5 & 6 can also
be considered as an electronically controlled drug delivery system, for future in
vivo use. This device has comparably low leakage and can locally deliver high
doses of the specific chemical, with a very good on/off ratio, but is currently
limited to delivering relatively small charged species.
54
CHAPTER 7
7. ELECTROCHEMICAL DEVICES IN PAPERS 1-6
The unique properties of conjugated polymers give possibilities for
devices with completely new functionalities that would not be
possible to achieve with inorganic metals or semiconductors. Three
electrochemical devices, with very similar structures but completely
different features and applications, are presented in this chapter.
The electrochemical wettability switch and the ion pump are novel
devices
for
primarily
biological
applications,
while
the
electrochromic display with an additional polymer layer is an
improvement
to
already
existing
technology
with
potential
utilization in scientific and industrial areas related to displays. The
chapter is also a summary of the published work that comprises this
thesis and further details about the devices can therefore be found in
Papers 1-6.
7.1 Electrochemical wettability switch
As mentioned in Chapter 4, many physical processes are governed by surface
interactions and it is therefore crucial to have control of the surface properties
in numerous applications. When a conjugated polymer is oxidized or reduced,
the bulk properties, such as color and conductivity, change as a result of the
chemical reorganization in the material, but what happens at the surface? It
turns out that the wettability or water contact angle of the polymer surface will
in fact often be modified along with the oxidation state (see also Papers 1-3).142
55
Chapter 7: Electrochemical Devices in Papers 1-6
In undoped or neutral polymer systems, e.g. polyalkylthiophenes, doping will
induce dipoles at the surface and thereby generally increase the surface tension
of the material (polar materials have high surface tension / low water contact
angles). More complex systems, such as PANI pre-doped with DBSA (Figure
26), where the charge-balancing counter-ion is a surfactant, i.e. a molecule with
one hydrophobic and one hydrophilic end, can actually work the other way
around.
7.1.1 PANI:DBSA wettability switch
When PANI is in the fully reduced leucoemeraldine state, the DBSA molecules
are not bound to the polymer chain and can show their water-loving parts at the
surface when water is placed on top. This means that the material will be
hydrophilic and give low water contact angles. If, on the other hand, PANI is
oxidized to the doped pernigraniline state, the hydrophilic sulfonic acid groups
of DBSA bind ionically to the polymer backbone and the hydrophobic alkyl
chains are instead exposed at the surface. Therefore the oxidized polymer film
shows a more hydrophobic behavior, see Figure 26 and 27.
Figure 26. Polyaniline salt doped with dodecylbenzene sulfonic acid
(PANI:DBSA). a. PANI:DBSA with x undoped amine groups, y protonated
imines and z protonated amines. b. DBSA chemical structure with the
hydrophilic sulfonic acid group and the hydrophobic 12-carbon alkyl
chain.
56
Chapter 7: Electrochemical Devices in Papers 1-6
Figure 27. The structure 2 electrochemical wettability switch. a.
PANI:DBSA wettability switch. b. Water droplets and associated contact
angles on oxidized and reduced PANI:DBSA.
The electrochemical wettability switch utilizes the control of the oxidation state
of a polymer film via an applied potential. If the oxidation state of the material
also determines the surface wettability, it is thereby possible to electronically
control the latter. The simplest wettability switch is Structure 2 with
PANI:DBSA as the polymer film (Figure 27a). The difference compared to the
Structure 2 in Figure 13 is that, in this case, the electrolyte has instead been
placed underneath the polymer film in order to expose the active surface to the
surrounding environment. When a bias voltage is applied, the negatively
addressed PANI:DBSA electrode will switch to the yellow reduced state and the
positively biased electrode to the dark blue oxidized state. Since both states of
the polymer are conducting enough, the switch is reversible. If water contact
angles are measured on each of the surfaces, it is clear that the applied voltage
has modified the surface wettability (see contact angles in Figure 27b). Since the
57
Chapter 7: Electrochemical Devices in Papers 1-6
oxidation state of the polymer system also changes the color of the material, it is
possible to couple the electrochromic state to the water contact angle of the
surface. The relationship between water contact angle, absolute oxidation
potential, versus a Ag/AgCl reference electrode, and optical absorbance at 700
nm, where the difference in absorbance between oxidation states is large, for a
PANI:DBSA electrochemical wettability switch is shown in Figure 28.
45
grees)
Contact Angle (de
40
35
30
25
20
15
10
5
0
-0.2
0.0
E
ox ( V
0.2
vs.
Ag
/Ag
Cl )
1.0
0.8
)
nm
00
7
0.4
e(
nc
0.2
ba
r
0.0
so
Ab
0.6
0.4
Figure 28. PANI:DBSA contact angle versus oxidation potential and
absorbance at 700nm.
7.1.2 P3AT wettability switch
Similarly to the device structure with PANI:DBSA, it is also possible to make
electrochemical wettability switches that include undoped polyalkylthiophenes
(P3AT) as the active material. As mentioned in the beginning of this chapter,
electrochemical doping of the material creates dipoles, i.e. positive polarons
charge-neutralized by counter ions, in the polymer film, thereby changing the
wettability of the surface. Since the P3AT is initially neutral (pristine) and
cannot easily be reduced, silver paste was used as the negatively addressed
58
Chapter 7: Electrochemical Devices in Papers 1-6
counter electrode, underneath the electrolyte, as shown schematically in Figure
29a.
Figure 29. P3AT wettability switch. a. Device architecture. When the
potential is applied, the positively addressed P3AT is oxidized and the
negatively biased silver is reduced. b. Three different P3ATs with different
side groups (O is octyl, H is hexyl and B is butyl).
Three different P3ATs with varying side-chain lengths, as shown in Figure 29b,
have been tested. The alkyl side-chains have a hydrophobic character and
thereby increase the water contact angle of the neutral polymer film, as
compared to the non-substituted polythiophene. As seen in Figure 30, the
longer the side-chain, the more hydrophobic is the neutral P3AT film. When the
59
Chapter 7: Electrochemical Devices in Papers 1-6
polymer is oxidized, the surface becomes more polar and the water contact
angle decreases. However, the alkyl side-chains also shield the water from the
polar polymer backbone to some extent and the switching effect therefore
depends on the number of carbons in the alkyl group (Figure 30). Shorter sidechains provide less shielding and thereby increase the wettability switching
effect.
Figure 30. P3AT contact angles. a. Static water contact angles of P3OT,
P3HT and P3BT in the pristine (neutral) and electrochemically oxidized
state. b. Difference in contact angle between neutral and oxidized states of
the P3ATs.
60
Chapter 7: Electrochemical Devices in Papers 1-6
Since the initial contact angle and the corresponding wettability switch depends
on the chemical nature of the side groups, it is possible to fine tune the system
to get the desired properties, simply by the choice of substitution chemistry. As
an example, the use of surface morphology to increase the net surface energy
switch effect and create superhydrophobic to superhydrophilic transitions
depends on the contact angle of the smooth surface. Theoretically, in order to
achieve a switch from superhydrophobic to superhydrophilic, the change in
contact angle should occur around 90°.99,100 Thus, in this case, P3HT (102° to
89°) would probably be the best candidate for such an electronic switch.
7.1.3 Electronic modulation of wettability gradients
In equivalence to the device described in 7.1.1, it is also possible to make a
PANI:DBSA Structure 3 wettability switch. When the outer electrodes are
biased, an oxidation, color and wettability (water contact angle) gradient is
induced in the middle polymer section. If a drop of water is placed on the
wettability gradient, it will automatically move or spread towards the reduced,
more hydrophilic side. Hence, it is possible to guide the droplet in the direction
of the electric field, without the use of a microfluid channel or hundreds of volts.
Figure 31. 4-terminal electrochemical wettability switch. Electrodes A
and D form structure 2, while B, C and D constitute Structure 3. The bistable voltage Vad will modulate the oxidation state, color and wettability
in D, while Vbc controls the gradient.
61
Chapter 7: Electrochemical Devices in Papers 1-6
If the polymer film is divided into four electrodes, i.e. a 4-terminal
electrochemical wettability switch, it is possible to modulate the level of an
induced wettability gradient via a bi-stable Structure 2 voltage. As seen in
Figure 31, electrodes A and D form Structure 2, while B, D and C combine to
Structure 3. D is thus common for both substructures and this is where the
gradient will be formed.
When a bias voltage, Vad (~3 V), is applied between A and D, the electrode
chosen to be positive will be oxidized and the negative electrode will thereby be
reduced. If Vad is disconnected and another potential, Vbc (~15 V), is
subsequently applied between B and C (along “Structure 3”), a gradient in
oxidation, color and wettability will be induced along electrode D. Since the A-D
structure is bi-stable, the initially applied Vad voltage will determine the average
level of oxidation in electrode D, even after that the gradient has been created.
The gradient will therefore exhibit different absolute levels, depending on Vad,
for the same Vbc. (Figure 32)
Figure 32. Color gradient at electrode D of a 4-terminal electrochemical
wettability
switch,
modulated
with
different
Vad.
Approximate
corresponding water contact angles are shown on the right-hand side yaxis. Vbc = 15 V
62
Chapter 7: Electrochemical Devices in Papers 1-6
Figure 33. Droplet movement on the surface of electrode D in a 4terminal electrochemical wettability switch, modulated with different Vad.
Vbc = 15 V a. Droplet movement for three Vad (three devices). b.
Photographs of droplet movement.
63
Chapter 7: Electrochemical Devices in Papers 1-6
The wettability characteristics along D is thus also determined by Vad, which
means that spreading and motion of water on the polymer surface can be
modulated by the small bi-stable voltage (Figure 33). If D is initially oxidized
(positive Vad), water droplets placed in the center of the induced gradient will
hardly move at all, but for negative Vad, the water droplet spreads quickly
towards the reduced side and make a net transport of almost 1 mm.
7.2 PEDOT:PSS displays with improved optical contrast
Electrochromic pixels consisting of PEDOT:PSS, as the color-switching
material, on plastic foil or paper are very promising for all-organic printed
displays. Pixel elements (Structure 2, see 3.3) can be combined with
electrochemical transistors to build up active matrix-addressed displays or
simply be individually addressed, such as in a 7-segment device (Figure 34a).
Figure 34. Printable displays. a. Two lateral seven segment displays on
paper, patterned on the same piece of PEDOT:PSS. b. Printing press for
organic electronics with printed test pattern. Both photos copyright ©
Niclas Kindahl, Fotofabriken and Acreo AB.91
64
Chapter 7: Electrochemical Devices in Papers 1-6
PEDOT:PSS is a suitable base-material for simple organic printable displays,
since the pre-coated polymer films are relatively environmentally stable and can
be patterned through various manufacturing processes (see e.g. Ref. 3). It is also
possible to process the polymer material in the form of a solution (emulsion),
which gives additional flexibility when transfering the manufacturing to a real
printing press (Figure 34b).
PEDOT:PSS exhibits fast and reversible electrochromic switching but, as can be
seen from Figure 9, the absorption of the colored reduced state does not cover
the entire visible spectrum (400-800 nm). The lack of absorption around and
below 550 nm, where our eyes are very sensitive, gives a relatively low perceived
optical contrast as compared to many other electrochromic systems. One
possible solution to this problem is to add another polymer film directly on top
of PEDOT:PSS in Structure 2. Apart from proper processability, mechanical
stability and good adhesion to the PEDOT:PSS layer, such a second polymer
needs the following properties in order to improve the function of the displays:
‰
The polymer should absorb light between 400 and 550 nm in the
reduced (neutral) state and be transparent when oxidized.
‰
The polymer film needs good ionic conductivity to promote fast
switching of the display. Electronic conductivity is not as crucial, since
that is well provided by PEDOT:PSS in the lateral direction.
‰
The oxidation potential has to be sufficiently low, in combination with
printable water-based electrolytes, to enable oxidation without risking
over-oxidation (irreversible loss of conductivity in the material).
Several conjugated polymers, synthesized by the group of Mats Andersson at
Chalmers University of Technology in Gothenburg, have been explored as
candidates for the contrast-enhancing layer. The chemical structures are
outlined in Figure 35.
From spectroscopic and electrochemical characterizations (details in Paper 3),
two main design criteria for the second polymer layer can be suggested. First,
65
Chapter 7: Electrochemical Devices in Papers 1-6
oligo(ethylene oxide) side chains seem to increase the ionic conductivity of the
polymer layer. This is expected, since oligo(ethylene oxide) prevents
crystallization of the polymer molecules and is therefore commonly used in solid
polymer electrolytes.152 Second, polymers with ethylenedioxythiophene (EDOT,
as in the PEDOT monomer, Figure 8a) moieties show lower oxidation potentials
and can consequently be reversibly switched in the water-based electrolyte
without over-oxidation.
Polymers III and IV fulfill these criteria and become transparent when
oxidized, which means that they are well-suited for use in the contrastenhancing layers.
O
O
O
O
O
O
S
O
O
n
S
O
I
n
S
S
n
II
O
O
III
O
O
O
O
O
O
O
O
O
n
O
O
O
O
n
V
O
O
O
O
S
S
O
O
O
O
VII
O
S
VIII
S
n
O
O
O
O
N
VI
O
n
S
S
IV
O
N
S
S
O
O
O
O
S
S
O
O
O
n
O
S
S
N
S
N
IX
Figure 35. Conjugated polymers for a contrast-enhancing layer on
PEDOT:PSS displays.
66
n
Chapter 7: Electrochemical Devices in Papers 1-6
Organic flexible displays with polymer III and IV, respectively, deposited on
PEDOT:PSS
have
been
manufactured
and
characterized
with
spectrophotometry. As shown in Figure 36, the optical contrast defined as the
change in luminance ∆L* (details in Paper 3), is almost doubled by the addition
of a second polymer layer.
Figure 36. Optical absorbance, measured in transmittance mode, of
flexible organic PEDOT:PSS displays with and without contrastenhancement layer. ∆L* denotes the optical contrast. a. Only PEDOT:PSS.
b. PEDOT:PSS plus polymer III. c. PEDOT:PSS plus polymer IV.
7.3 Organic electrochemical ion pump
Due to their organic chemical composition and relatively soft mechanical
properties, conjugated polymers generally work well as biocompatible materials
(see Chapter 6). In many cases, our living cells use ionic species, such as
potassium and calcium, in the signaling cascade when communicating internally
and with each other. The internal ionic concentrations often oscillate at specific
frequencies, as a response to external stimuli, and thereby induce several
activities, e.g. gene expression, in the cell (Chapter 5). If the ion transport to and
from the cell could be controlled electronically, with spatial and temporal
resolution, it would be possible not only to study in detail how cells interact with
each other and the surrounding environment but also to stimulate specific
responses.
67
Chapter 7: Electrochemical Devices in Papers 1-6
Figure 37. The ion pump. a. The architecture of the ion pump shown as
separated layers. The four PEDOT:PSS electrodes are labelled A to D.
PEDOT:PSS in the over-oxidised region (pink colour) between B and C
conducts ions but not electrons. b. Schematic top view of the device with
voltages applied. x = 7 mm, y = 12 mm, z = 4 mm or 50 µm and w = 2 mm.
c. Schematic cross-section of the B and C electrodes. P denotes PEDOT. M+
and C+ are cations.
68
Chapter 7: Electrochemical Devices in Papers 1-6
The goal with the organic electrochemical ion pump project has been to do just
that, i.e. use the properties of conjugated polymers to, in a controlled fashion,
deliver ions to cells that grow on the polymer surface. An electrophoretic ion
pump including PEDOT:PSS as the active material has therefore been developed
and studied (details in Papers 5 & 6). In the device, electronic addressing
signals control and drive an ionic current, which enables transfer of ions from a
source electrolyte to a receiving electrolyte separated by a thin film polymer
channel. The four electrodes denoted A-D in Figure 37, are biased with three
voltages (VAB, VBC and VCD). This configuration results in transport on/off ratios
of above 300, delivery of large amounts of ions and exact electronic control of
the released dose.
The left electrolyte (AB) in Figure 37 is a reservoir containing a relatively high
concentration of the ion to be transported into the right electrolyte (CD). The
CD electrolyte contains another cation or can be a more complex solution such
as cell medium. The two electrodes labeled B and C are separated by a watersaturated polymer electrolyte (over-oxidised3 PEDOT:PSS), which conducts ions
but not electrons. This is equivalent to the simple Structure 2, but with a much
“slower” electrolyte that prevents the ions in the liquid electrolytes, whose areas
are defined with the hydrophobic photoresist SU-8, from diffusing and mixing
spontaneously. When applying the potential VBC, which oxidizes B and reduces
C, the cations in the reservoir electrolyte (and hence in the polymer film B) are
transported in the electric field through the polymer electrolyte to the target
electrolyte. As soon as the ions reach the edge of the CD electrolyte, they rapidly
diffuse away from the 200 nm thin polymer film and enter the liquid. Electrode
A and B form yet another Structure 2, biased with VAB, which allows B to be
continuously regenerated. Similarly, electrode D, via VCD, prevents electrode C
from being completely reduced. The following paragraphs describe the
characteristics of the ion pump when transporting potassium ions (K+).
The name of the device implies that it should be possible to pump ions against a
concentration gradient. Figure 38a shows that this is indeed the case. In this
measurement, both the reservoir and the target electrolyte contained 2 mM
potassium acetate, i.e. no additional ions were added to the target electrolyte.
69
Chapter 7: Electrochemical Devices in Papers 1-6
Initially, there is an ion exchange between the electrolytes and the PEDOT:PSS,
which can be seen as a decrease in the potassium concentration after leaving the
solution on the polymer film for 25 min. If potentials are instead applied for the
same length of time, the final concentration of K+ is significantly higher in the
CD electrolyte as compared to the concentration in AB.
Figure 38. K+ transport with the ion pump. a. Comparison of [K+] in AB
and CD when starting with identical electrolyte solutions in both. OFF
indicates that no voltages were applied between any of the electrodes and
ON means VAB = 1 V, VBC = 10 V, VCD = 1 V. Standard deviations are shown
as error bars (n = 4). b. Time resolved transport of K+ between the
electrolytes and of electrons between B and C. (VAB = 1 V, VBC = 10 V, VCD =
1 V). c. Current measured between B and C for three different VBC (VAB = 1
V, VCD = 1 V). d. Corresponding relationship between transported K+ and
total B-C charge after 10 min. The red line shows the linear fit and the
outer blue lines represent the 95% confidence interval. In b-d, the AB
electrolyte was 0.1 M KCl, and CD was 0.1 M Ca(C2H3O2)2 (calcium
acetate).
70
Chapter 7: Electrochemical Devices in Papers 1-6
Figure 38b shows the transport of total charge (integrated B-C current) and K+
between B and C in a device operated for 30 min. The transport rates are
relatively constant during the first 8-10 min of operation, while at later stages,
the current and consequently the ion transport rate decreases rapidly. The
electric field along the polymer electrolyte channel only exists provided that
electrochemical reactions occur in B and C. When the limiting electrode, in this
case A, is consumed, electrode B quickly runs out of oxidation sites and the
electric field disappears. It is evident from Figure 38b that when the current
drops, the ionic transport rate is slightly higher than the measured B-C charge
transport rate. This results from K+ that electromigrate partly through the
polymer electrolyte as long as the electric field is maintained and subsequently
diffuse into the target electrolyte. Locally, however, the concentration will
rapidly saturate or decrease when no electromigration occurs, since diffusion in
the electrolyte removes ions from the line of delivery (see Figure 39a and 41).
The current characteristics for three different VBC is shown in Figure 38c, and
the correlation between the total charge and the amount of transported ions
after 10 min of operation is displayed in Figure 38d. There is a linear
relationship between the amount of K+ transported and the total charge
transfered between B and C after 10 min. It is possible to control the transport
of ions by the choice of applied potentials and, perhaps more importantly, count
the number of delivered ions simply by integrating the B-C current. If no
voltages are applied, the diffusion-mediated leakage of K+ from the reservoir
electrolyte to the target electrolyte is very small, as experiments show that less
than 10 nmol K+ is found in the target electrolyte after 4 h.
The relatively low optical absorbance of PEDOT:PSS thin films in combination
with its biocompatible surface properties (Paper 5) make the material highly
suitable for microscopy-based cell studies. Therefore, we† have tested to grow
neuronal cells on the C (and D) electrodes of the ion pump devices and to locally
open VOCCs (Chapter 5) in the cell membrane, by electronically delivering high
concentrations of K+. If the CD electrolyte with the cells is Ca2+-containing cell
medium, a calcium response will be triggered in the cells when the potentials
are applied. By loading the cells with the fluorescent marker FURA-2 AM, which
†All experiments with cells were performed by Peter Kjäll in
the group of Agneta Richter-Dahlfors at Karolinska Institutet.
71
Chapter 7: Electrochemical Devices in Papers 1-6
is only active when it resides inside the cells, changes in the intracellular
calcium level can be detected with a fluorescent microscope in real-time. The
results of such microscope measurements are summarized in Figure 39.
72
Chapter 7: Electrochemical Devices in Papers 1-6
Figure 39. Microscopy recordings of electronically induced ion fluxes in
electrolyte and cells. a. Time-lapse microscopy of ion pump-mediated K+
transport from AB (0.1 M KCl) to CD electrolytes (0.1 M Ca2+ acetate
containing the K+-sensitive probe PBFI) at VBC = 10 V (continuous line).
Dotted line = dH2O as AB electrolyte, where increased fluorescence at 60
min results from manual addition of 50 mM KCl directly to the CD
electrolyte. Dashed line = activation of the ion pump for 15 min, followed
by a switch to the OFF state causes the fluorescence increase to stop.
Reactivation of the pump 10 min later causes the [K+] increase to continue.
b. Intracellular Ca2+ fluxes in FURA-2 AM-loaded HCN-2 cells in cell
culturing medium on the CD electrode resulting from ion pump delivery of
K+ from the AB electrolyte. Continuous line = intracellular Ca2+ response
when only VBC = 10 V is applied. Dotted line = increased K+ transport rate
(VBC = 10 V, VAB and VCD = 1 V) causes a pronounced Ca2+ response in cells.
Dashed line = inactive ion pump causes no cellular response (OFF state),
while increased fluorescence at 30 min results from manual addition of 50
mM KCl to CD electrolyte. Dashed-dotted line = dH2O as AB electrolyte
causes no Ca2+ response in HCN-2 cells. c. Intracellular Ca2+ fluxes result
from K+-mediated depolarization of the cell membrane. Dashed line = Ca2+
response in FURA-2 AM-loaded cells on the CD electrode results from K+
transported from AB to CD electrolyte (VBC = 10 V, VAB and VCD = 1 V).
Continuous line = experiment in the presence of GdCl3 in the CD
electrolyte. Dotted line = experiment in the presence of nifedipin in the CD
electrolyte. d. Visualization of the pH gradient formed in the CD
electrolyte during pumping of protons (VAB=1 V, VBC = 10 V, VCD = 1 V).
Deep red colour indicates pH ~ 2 and clear yellow is pH ~ 5. e, f. The ion
pump provides spatial control of cellular responses: e. Continuous line =
time-lapse microscopy of intracellular [Ca2+] in FURA-2 AM-loaded HCN2 cells located adjacent to the 4 mm wide barrier. Dotted line = cells
located 1 mm and 2 mm (dashed line) from the barrier. f. Time-lapse
microscopy of intracellular [Ca2+] in FURA-2 AM-loaded HCN-2 cells
located on the 50 µm wide microchannel (continuous line) and cells
located 500 µm from the microchannel (dotted line).
73
Chapter 7: Electrochemical Devices in Papers 1-6
To show that it is really the VOCCs that are activated by the elevated potassium
concentration, these calcium channels can be selectively blocked with Gd3+ or
nifedipin.153,154 If either of these chemicals is present in the CD-electrolyte, no
response is detected when K+ is transported to the cells (Figure 39c).
To be able to target and stimulate single cells or even parts of cells, devices that
provide spatial control of ion fluxes are necessary. The ion pump delivers ions to
the target electrolyte through the polymer electrolyte channel between B and C.
When reaching the edge of the CD electrolyte, ions immediately diffuse from the
200 nm thin film into the solution. During operation of the device, the
concentration of ions becomes very high at the distribution point, while it
decays with increasing distance, because diffusion cannot evenly redistribute
the ions as fast as they are supplied. This effect is qualitatively visualized using
H+ (AB electrolyte = HCl, pH 1; CD electrolyte = KCl, pH 5). The color change of
a pH-sensitive indicator paper placed on top of electrode C in the CD electrolyte
indicates delivery of protons (Figure 39d and 40). A continuous increase of [H+]
close to the delivery line occurs, while diffusion creates a pH gradient along
electrode C. To investigate whether this also is reflected in the ability of K+ to
induce Ca2+ fluxes in cells, Ca2+ imaging of FURA-2 AM loaded cells located at
different distances from the barrier was performed. Cells located in the
immediate vicinity of the barrier respond as expected with increased
intracellular Ca2+ while cells located 1 mm and 2 mm away from the barrier are
unresponsive (Figure 39e). To further increase the spatial resolution and
stimulate individual cells, devices with 50 µm wide (a size that corresponds to
many eukaryotic cell types) B-C channels were manufactured (z = 50 µm in
Figure 37). Figure 39f demonstrates the induced Ca2+ response in a cell located
on such a microchannel, while distantly located cells, approximately 500 µm
away from the delivery point, were unresponsive.
The experiments above show that the ion pump has temporal resolution in the
sense that it can be turned ON and OFF but, as discussed in Chapter 5, many
cellular signals are not static with one level for OFF and another for ON.
Instead, they have an oscillating character, carrying information in both the
74
Chapter 7: Electrochemical Devices in Papers 1-6
amplitude and the frequency. To evaluate the possibility of creating such
oscillating signals with the ion pump, protons were pumped with the device and
detected with pH paper in the CD-electrolyte. By applying short square-wave
pulses and rely on diffusion in the electrolyte to remove protons from the
delivery line during the time between pulses, it is possible to create oscillating
ion signals, as shown in Figure 41 (details in Paper 6).
Figure 40. pH gradient along electrode C. a. Change in pH at constantly
applied potentials (VAB = VCD = 1 V, VBC = 5 V). Protons are delivered faster
than the rate with which they diffuse away from the delivery line. The pH
close to the release line therefore continuously drops at the same time as a
gradient is created along electrode C. b. Examples of photographs of pH
paper at different time scales with the same potentials as above applied.
75
Chapter 7: Electrochemical Devices in Papers 1-6
Figure 41. Electronically controlled oscillating ion signals. Short pulses
can be used to create oscillations in the CD electrolyte close to the release
line. VAB = VCD = 1 V during the pulses, otherwise 0 V. a. 15 s pulses with
VBC = 10 V. b. 10 s pulses with VBC = 5 V.
76
CHAPTER 8
8. CONCLUDING DISCUSSION
The work with the bioelectronic devices presented in this thesis is of
course not ending here, since many opportunities, and hopefully
rewards, remain. The organic electrochemical wettability switch has
the potential to be used in more complex bioelectronic microfluidic
devices and corresponding applications. Also, since cell adhesion (or
protein adhesion) and proliferation on a surface can be highly
dependent on the wetting properties, the wettability switch can be
utilized to electronically control where and when cells are present on
a surface. The organic electronic ion pump has already been
integrated with cells and a future focus in that project is instead to
pump larger and more specific molecules, preferably with improved
resolution in both time and space. Another route to go with the
device is to incorporate it with tissue or even living animals, as a first
step towards true drug delivery applications.
A general take-home-message from the seven preceding chapters is that
conjugated polymers are far more than just electronically conducting plastics.
By using the materials in electrochemical or electronic devices, it is possible to
control physical properties, like e.g. resistivity, shape, surface wettability and
color, by applying a voltage. The number of applications where such organic
electronics can be incorporated is naturally enormous and one important part of
the conjugated polymer evolution is integration with biology and medicine.
Whether used in artificial muscles, neural prosthetics, sensors, drug release
77
Chapter 8: Concluding Discussion
systems or complete lab-on-a-chip solutions, organic electronics has the
potential to further bridge the gap between man-made electronics and living
species from nature.
As stated several times in this thesis, there is a very good match between organic
electronics and biological materials but that is of course only worth something if
people and resources are devoted to utilize the technology. The scientific
communities of physics/chemistry and biology/medicine are getting closer and
closer to each other, as more and more groups see the rewards of common
multidisciplinary efforts and shared knowledge. With such powerful joint forces,
the relatively young scientific field of organic bioelectronics can continue to
grow and be an important part of life science research. The potential to create
considerable scientific and commercial value is huge, both when it comes to
increasing the understanding of fundamental biologic questions as well as to
incorporate organic bioelectronics in the diagnostic and therapeutic health-care
tools of the future.
There are, needless to say, still several general issues regarding e.g. life-time,
stability and operational speed of organic devices, which have to be solved
before any products can enter the market in volume. These are, however, not
specific problems to organic bioelectronics but rather a part of the ongoing
organic electronics development. Therefore, the entire conjugated polymer
community continuously works at finding new ways to improve the
performance and stability of organic electronic devices, and repeatedly
succeeds.
Furthermore,
the
necessary
specifications
for
bioelectronic
applications are in many cases comparably modest. As an example, sensors and
lab-on-a-chip solutions are often meant to be cheap, single-use devices for bedside utilization close to the patient. In this case, the requirements on speed and
life-time are generally low or moderate and can be met by organic electronics.
No doubt, organic bioelectronics is one of the research fields where organic
electronics is not simply a cheaper alternative to existing technologies but
instead leads the way towards new breakthroughs.
78
REFERENCES
REFERENCES
1.
Official Nobel Prize Webpage. (2007). http://nobelprize.org
2.
Chiang, C.K. et al. Electrical conductivity in doped polyacetylene. Phys.
Rev. Lett. 39, 1098-1101 (1977).
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