Poly[3,4-ethylene dioxythiophene (EDOT)-co-1,3,5-tri[2-(3,4-ethylene dioxythienyl)]-benzene (EPh)] copolymers (PEDOT-co-EPh): Optical, electrochemical and mechanical properties

Article abstract of DOI:10.1039/c5tb00053j

PEDOT-co-EPh copolymers with systematic variations in composition were prepared by electrochemical polymerization from mixed monomer solutions in acetonitrile. The EPh monomer is a trifunctional crosslinking agent with three EDOTs around a central benzene ring. With increasing EPh content, the color of the copolymers changed from blue to yellow to red due to decreased absorption in the near infrared (IR) spectrum and increased absorption in the visible spectrum. The surface morphology changed from rough and nanofibrillar to more smooth with rounded bumps. The electrical transport properties dramatically decreased with increasing EPh content, resulting in coatings that either substantially lowered the impedance of the electrode (at the lowest EPh content), leave the impedance nearly unchanged (near 1% EPh), or significantly increase the impedance (at 1% and above). The mechanical properties of the films were substantially improved with EPh content, with the 0.5% EPh films showing an estimated 5× improvement in modulus measured by AFM nanoindentation. The PEDOT-co-EPh copolymer films were all shown to be non-cytotoxic toward and promote the neurite outgrowth of PC12 cells. Given these results, we expect that the films of most interest for neural interface applications will be those with improved mechanical properties that maintain the improved charge transport performance (with 1% EPh and below). This journal is

Full text of DOI:10.1039/c5tb00053j

View Article Online
View Journal
Journal of
Materials Chemistry B
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. Ouyang, C.
Kuo, B. Farrell, S. Pathak, B. Wei, J. Qu and D. Martin, J. Mater. Chem. B, 2015, DOI:
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/materialsB

Page 1 of 13
Journal of Materials Chemistry B
View Article Online
DOI: 10.1039/C5TB00053J
Journal of Materials Chemistry B
RSCPublishing
ARTICLE
Poly[3,4-ethylene dioxythiophene (EDOT) –co– 1,3,5-
tri[2-(3,4-ethylene dioxythienyl)]-benzene (EPh)]
copolymers (PEDOT-co-EPh): optical,
Cite this: DOI: 10.1039/x0xx00000x
electrochemical and mechanical properties
Received 00th January 2014,
Accepted 00th January 2014
Liangqi Ouyang,a,# Chin-chen Kuo a,#, Brendan Farrell b, Sheevangi Pathak , Bin
c
a
a,b, *
Wei a, Jing Qu and David C. Martin
DOI: 10.1039/x0xx00000x
www.rsc.org/
PEDOT-co-EPh copolymers with systematic variations in composition were prepared by
electrochemical polymerization from mixed monomer solutions in acetonitrile. The EPh
monomer is a trifunctional crosslinking agent with three EDOTs around a central benzene ring.
With increasing EPh content, the color of the copolymers changed from blue to yellow to red
due to decreased absorption in the near infrared (IR) spectrum and increased absorption in the
visible spectrum
. The surface morphology changed from rough and nanofibrillar to more
smooth with rounded bumps. The electrical transport properties dramatically decreased with
increasing EPh content, resulting in coatings that either substantially lowered the impedance
of the electrode (at the lowest EPh content), leave the impedance nearly unchanged (near 1%
EPh), or significantly increase the impedance (at 1% and above). The mechanical properties
of the films were substantially improved with EPh content, with the 0.5% EPh films
showing an estimated 5x improvement in modulus measured by AFM nanoindentation. The
PEDOT-co-EPh copolymer films were all shown to be non-cytotoxic toward and promote
the neurite outgrowth of PC12 cells. Given these results, we expect that the films of most
interest for neural interface applications will be those with improved mechanical properties
that maintain the improved charge transport performance (with 1% EPh and below).
monomer solution, and the monomer is electrochemically oxidized
onto the anode. This method has proven to be particularly useful for
Introduction
Conducting polymers such as the functionalized
the coating of neural microelectrodes. When one of the most stable
conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT),
is deposited, it forms bumpy and nano-fibrous morphologies that
dramatically increase the effective surface area of the electrodes11.
After coating with PEDOT, the impedance of the electrode in the
biologically important frequency range of 1 kHz and lower is
significantly decreased, typically by two to three orders of
magnitude 12. The charge storage capacity (CSC) of PEDOT-coated
electrodes is also greatly enhanced. Much faster and lower voltage
responses are observed when delivering stimulation charge through
polythiophene poly(3,4-ethylene dioxythiophene) (PEDOT) have
many useful qualities including versatile organic chemistry, simple
processing methods, excellent biocompatibility and the ability to
switch between different oxidation states. They are also known to
facilitate both ion and electron transport. All of these characteristics
make them attractive materials for tissue engineering 1,2, biosensing3–
5
6,7
and neural interfaces
.
In these applications, conducting
polymers are intended to directly interface with electroactive cells
such as neural tissue. Electric charge can be delivered to stimulate
8
tissue or to modulate cell growth1,9. Cellular activities can also be
PEDOT 13
.
recorded with high levels of sensitivity 10
.
Another advantage of employing conducting polymers as
Conducting polymers can be readily synthesized through
direct interfacing materials with tissues is that these are relatively
electrochemical polymerization. Typically, current is delivered into a
This journal is © The Royal Society of Chemistry 2013
J. Name., 2013, 00, 1-3 | 1

Journal of Materials Chemistry B
Page 2 of 13
ARTICLE
VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C5TB00053J
soft materials. The typical young’s modulus of PEDOT is around
this material had improved electrochemical stability. Despite these
developments, the search for conducting polymer materials that are
more mechanically robust remains an ongoing topic of interest.
In order to create more mechanically robust PEDOT
2 GPa, which is significantly lower than that of inorganic metals or
14
semiconductors (~100 GPa)
.
By using various polyanions as
dopants, the stiffness of the polymer coating can be further
decreased 15,16. The soft and flexible polymer surface allows for
better contact with the cells. It has been demonstrated that highly
conformal neural electrodes can be made with PEDOT as the
conducting material 17. Using a softer electrode material decreases
coatings for the neural interfacing applications, we examined a
strategy for chemically crosslinking the PEDOT into a network
structure by introducing
a
branched comonomer during
electrochemical synthesis. A conjugated 3-armed EDOT derivative,
the mechanical properties mismatch between the hard electrode and 1,3,5-tri[2-(3,4-ethylenedioxythienyl)]-benzene (EPh) 31,32, which
the soft tissue 18, which is expected to be beneficial for mitigating the consists of a benzene core surrounded by three EDOT moieties
chronic foreign body response 19,20. Recently, we showed that
(Scheme 1), was used as the crosslinking agent. We felt that this
crosslinker would improve mechanical properties of the PEDOT,
and might be also able to preserve charge transport properties
because of the potential for extended electron conjugation through
the benzene ring in the core of the molecule.
PEDOT could even be directly deposited into living rat hippocampus
as a potential strategy for creating locally conductive pathways
between implanted electrodes and the surrounding neural tissue 21
.
However, depending on the chemistry and method of
preparation, conducting polymer films can be relatively fragile.
PEDOT films have been found to crack or delaminate from solid
substrates under mechanical deformation. Sterilization and chronic
stimulation can comprise the film integrity 22,23. When interfacing
Crosslinked conjugated polymer networks have been
investigated for engineering organic semiconductors with isotropic
33
charge transport properties
or for creating stable microporous
structures 34. However, the potential application of crosslinked
conjugated polymer networks as neural interfacing materials had yet
with peripheral nerves, electrodes are likely to be subjected to
24
to be studied.
Here, we report studies of the optical,
particularly severe strains due to chronic muscle movements
.
electrochemical, morphological, and mechanical properties of
PEDOT-co-EPh copolymers with systematic variations in EPh
content. We found that as the EPh content increased, the mechanical
stiffness and strength of the copolymer films increased, but the
electrochemical transport properties decreased. Adding EPh shifted
the UV-vis absorption properties of the PEDOT-co-EPh copolymers
from the near infrared into the UV, making it possible to tune their
color across the visible spectrum. The wetting angles of the
PEDOT-co-EPh films did not change significantly with EPh content,
and all of the copolymer films remained non-cytotoxic for PC-12
cells in vitro. The optimal films for neural interface applications are
expected to be those that show significantly improved mechanical
performance but still maintain high electrical transport properties,
corresponding to EPh additions of around 1% or less.
Cracking and delamination of the conducting polymer coating may
not only decrease the performance of the material itself, but could
also leave behind fractions of the film in the tissue. Although a
number of studies have demonstrated that PEDOT itself is not
cytotoxic 25, the impact of potential fragments of PEDOT remains to
be studied in detail. It is therefore of interest to more carefully
examine the mechanical performance of PEDOT, and to develop
potential methods for improving its stiffness, strength, and
durability.
Several strategies have been reported for improving the
mechanical durability of PEDOT films. For example, composite
materials were made by blending or dispersing PEDOT into PDMS
26
27
or PVA
matrices. These soft polymer matrices provided
mechanical support to PEDOT. However, as the content of these
non-conducting matrix materials increased, the conductivity of the Experimental
composites decreased dramatically. PEDOT was also deposited into
Chemical synthesis
a hydrogel network, where the soft and durable hydrogel served as a
mechanical buffer. Inganas and coworkers chemically crosslinked
the polyelectrolyte dopant PSS and found that the stability of
PEDOT:PSS was substantially improved 28,29. Another method was
The synthesis of 1,3,5-tri[2-(3,4-ethylenedioxythienyl)]-
benzene (EPh) was adopted from Idzik et al 32,35. As shown in
Scheme 1, first, 2 g EDOT (Sigma-Aldrich) was dissolved in
to introduce an EDOT derivative consisting of two EDOT units 160 mL dry tetrahydrofuran (THF, Fisher Scientific) and then cooled
linked through ethylene oxide units 30. CV sweeps confirmed that
down to -78 oC. After equilibrium, 10 mL 1.6M n-butyl lithium
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 3 of 13
Journal Name
Journal of Materials Chemistry B
View Article Online
ARTICLE
DOI: 10.1039/C5TB00053J
(n-BuLi, Fisher Scientific) was added into the system dropwise.
After reacting for 30 minutes, the temperature was increased
to -47 oC and 5.95 g Tribultyl tin chloride (ClSn(Bu)3, Sigma-
Aldrich) was added into the solution dropwise and then the mixture
was allowed to react for 24 hours. THF was removed by rotary
evaporation, and the crude product was then dissolved in hexanes.
The resultant mixture was then filtered and the filtrate was collected
and the solvent was removed to yield slightly yellowish oil. EDOT-
SnBu3 was used without further purification.
Scheme 1 Synthesis route of 1,3,5-Tri[2-(3,4-ethylenedioxythienyl)]-
benzene (EPh)
Electrochemical deposition
Electrochemical depositions were performed in a glass cell
installed with a platinum wire as the counter electrode. An Autolab
PGstat12 Potentiostat/ Galvanostat (EcoChemie) was employed as
the power source. For copolymerization, mixed monomer solutions
were prepared by varying the feed ratio of EPh in the EDOT/EPh
mixture. The feed ratio was described as molar concentration of EPh
in the total amount of monomer. For example, 0.1% EPh feed ratio
was prepared by mixing EPh: EDOT monomers at 1: 999. For all the
solutions, the concentration was kept at 0.02 M in acetonitrile with
0.02 M tetrabutyl ammonium perchlorate as counterion.
Figure 1 (a) PEDOT and (b) EPh crosslinked PEDOT.
Pt microeletrode (0.02 cm2, Bioanalytical Systems, Inc, for EIS
and CV tests) were used as the working electrode for the impedance
and CV measurements of electrochemically deposited polymer films.
For UV-Vis-NIR spectroscopy studies, polymer films were
electrochemically coated onto ITO glass slides (4~8 Ω/cm, 0.5 cm2,
Delta Technologies). Typically, 80 µA/cm2 of constant current were
employed for galvanostatic depositions. The reference was calibrated
with 0.01 M ferrocene in acetonitrile with 0.02 M TBAP as
supporting electrolyte and compared with Ag/AgCl reference
electrode (Fisher Scientific). The ferrocene E1/2 was measured as
0.11 V versus Ag/AgNO3 and 0.48 V versus Ag/AgCl. Thus, for the
potentials measured in organic solvents with Ag/AgNO3 as a non-
aqueous reference electrode, +0.37 V was added into its value for
better comparison with those measured in aqueous solutions against
Ag/AgCl reference electrode. The scan rate was 50 mV/s.
For the synthesis of EPh, 0.63 g EDOT-SnBu3, 0.460 g
Tetrakis(triphenylphosphine) palladium(0) (PdPPh3, Sigma-Aldrich)
and 2.846
g 1,3,5-tribromobenzene tetrakis(triphenylphosphine)
palladium(0) (PdP(Ph3) was dissolved in 150 mL anhydrous toluene
under argon protection. The solution was stirred and refluxed under
o
110 C for 48 hours. After being cooled to room temperature, water
was added into the reaction mixture. The mixture was extracted with
CHCl3 for 3 times. The organic layer was collected and washed 3
times with saturated NaCl solution and then DI water. It was then
dried against anhydrous magnesium sulfate. The solvent was
removed by rotary evaporation. The product was purified through a
chromatographic column with hexane:ethyl acetate as eluent. The
1
final product was a yellow solid. (400 MHz, CDCl3): H NMR δ =
7.92 (s, 3H); 6.34 (s, 3H); 4.36~4.34 (m, 6H); 4.30~4.28 (m, 6H).
13C NMR δ = 142.19; 138.42; 133.68; 122.22; 117.25; 97.80; 64.83;
64.50.
Characterization
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3

Journal of Materials Chemistry B
Page 4 of 13
VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5TB00053J
All the electrochemical impedance spectroscopy (EIS) and well-defined curvature features were employed. By analysing
cyclic voltammetry (CV) characterizations of polymer films were the image generated from imaging the TiO2 standards, the
performed on the same Autolab used for depositions. The tests were accurate tip radius R was obtained. Since the tip may wear out
performed in 1x monomer-free phosphate buffered saline (PBS) after scanning, the calibration was performed before each
solution with an Ag/AgCl glass body electrode (Fisher Scientific) as experiment. Three scans were taken at different spots for each
the reference and a platinum foil as the counter electrode. The scan sample. The modulus mapping was made by indenting 512 ×
rate was 100 mV/s for CV. For EIS measurement, a 0.01 V bias 512 points over a 2 µm × 2 µm image area.
versus Ag/AgCl was applied. The frequency range scanned was from
1-100k Hz.
In vitro cell culture tests: All the polymer films were prepared by
Optical and fluorescence microscopy was performed on a depositing onto gold-coated silicon substrates (approximately
Nikon Eclipse LV100 POL microscope equipped with a Nikon 0.2 cm2) at a charge density of 120 mC/cm2. For each group, 3 to 4
Intensilight C-HGFI lamp. The images were collected via a Nikon independent samples were prepared for each test. Prior to cell
DS-Ri1 camera.
UV-Vis spectra were collected on a Shimadzu UV-3600 UV- overnight. After rinsing with PBS 2 times, the samples were
Vis-NIR spectrophotometer. transferred into 50 µg/mg collagen I (from rat tail, Invitrogen) PBS
seeding, the samples were sterilized by immersing into ethanol for
Scanning electron microscope images were collected on a Zeiss solutions and incubated for 4 hrs. After collagen coating, the samples
Auriga 60 CrossBeam focused ion beam (FIB) / scanning electron were rinsed with PBS and transferred into a 24-well culture plate.
microscope (SEM). The acceleration voltage was 3 kV. A secondary Rat pheochromocytoma cells expressing green fluorescent protein
electron detector was employed.
(PC-12-Turbo-GFP, Marin Pharm) were seeded onto the samples at
a density of 2×104 per well through a 22 gauge needle to dispense
Mechanical Measurements: The quantitative nanomechanical the cells. The cells were cultured in growth medium (RPMI 1640
(QNM) modulus was calculated by PeakForce QNMTM AFM mode (Corning Cellgro) plus 5% fetal calf serum (FCS), 10% horse serum
with Nanoscope Dimension 3100 software on a Bioscope Catalyst (HS), 2 mM L-alanyl-L-glutamine, 1% Pen-Strep and 1% non
(Bruker Nano/ Veeco) AFM. All samples were measured at room essential amino acids (NEAA) overnight before switching to
temperature in the dry state. A TAP150A probe (force constant of differentiation medium (RPMI 1640 with 150 ng/mL NGF, 1% FCS
5 N/m) was used to indent the sample surface to a depth of about 1- and 1% HS) 36. For cell viability tests, the cell numbers were
2 nm. Firstly, taking the adhesion between the tip and the surface calculated before switching to differentiation medium. For neurite
into account, the reduced Young’s modulus Er was calculated from
length measurements, after day 4, the differentiation media was
replaced with freshly prepared new differentiation medium.
3(ꢀ!"# − ꢀ
!"!
ꢀ! =
Cell viability tests: The cell culture medium was replaced
4 ꢀꢀ!
with 0.25% trypsin solution (with 3.32 mM EDTA, Corning)
o
and incubated at 37 C for 3 to 5 minutes to detach cells. The
where Ftip and Fadh are the , R is the tip radius, d is the
deflection. The actual Young’s modulus can be calculated from
trypsin solution with cells was transferred to a 15 mL centrifuge
tube. 5 mL fresh cell culture medium was added. The cell
suspension was centrifuged at 200 RPM for 5 mins at room
temperature. The medium was then pipetted out. 1 mL of new
medium was added and the cell pellet was dispersed into the
medium. 10 µL of this cell suspension was withdrawn and
mixed with 10 µL Trypan Blue solution (0.4%, Invitrogen),
which only stains the dead cells. After mixing, the mixture was
ꢀ
(ꢀ − ꢀꢀꢀ ) (ꢀ − ꢀꢀꢀ )
=
+
ꢀꢀ
ꢀꢀ
ꢀꢀ
where ES is the sample modulus, EI is the indenter modulus, vs
and vI are the Poisson ratio of the sample and the indenter,
respectively. In our work EI >> ES, so we can neglect the
second term of this equation. The force constant K was
calibrated by using thermal tuning with each measurement. To
obtain an estimate of the probe radius R, TiO2 standards with
pipetted onto a cell counting chamber slide (CountessTM
,
Invitrogen). The slide was placed into a Countess Automated
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 5 of 13
Journal Name
Journal of Materials Chemistry B
View Article Online
ARTICLE
DOI: 10.1039/C5TB00053J
Cell Counter (Invitrogen). The cell viability was given as the Compared to EDOT, the CV sweeping in EPh solution showed a
percentage of live cells in the total cell numbers. For each significantly decreased current response. Except for a redox peak at
sample, 5 to 10 independent measurements were performed.
+1.1 V that increase after each CV cycle, there was not much
electroactivity in the EPh solution within the scan range. In the
Neurite outgrowth measurements: The samples were transferred mixed monomer solutions, although the CV curves showed similar
onto a glass slide that had PBS as the mounting medium after 4 days wide capacitance that was contributed by the EDOT moieties there
of exposure in differentiation medium. A cover glass was mounted was an overall decrease of current response after each CV cycle,
onto the slide. The fluorescence images were collected under the especially in the negative potential range compared to that in neat
Nikon Eclipse LV100 POL microscope. For each group, 3 to 4 EDOT solution. This decreased electroactivity suggested that the
samples were prepared and for each sample, 10 to 12 images on less electroactive EPh, even at relatively low concentration (2%),
different spots of the film were taken. All the images were loaded could be simultaneously oxidized with EDOT and incorporated into
into FIJI software. The length of neurites was measured by drawing the film, as evident from the CV curve.
a line from the neurite end to the junction between neurite cell
bodies using FIJI software.
Statistics: The statistical analysis was performed with the computer
program Minitab 17. The null hypothesis was that the cell viability
for the different materials were all equal. The alternate hypothesis
was at least one of the means of cell viability for the different
materials were not equal. This was tested by performing four One-
Way Analysis of Variance (ANOVA) tests: one ANOVA test for
each day of data recording (Day 2, Day 5, Day 8, Day 11). The
response of the ANOVA was cell viability and the factor was the
material. In addition, a Dunnett method of multiple comparisons
was performed with PEDOT as the control group level. All were
performed at 95% confidence level (α=0.05).
Figure 2 First 10 cycles of CV recorded during potentiodynamic
polymerizations in monomer solutions (versus Ag/AgCl). Red arrows
indicate expanded CV curves after each cycle.
Results and Discussion
Electrochemical polymerization of EDOT and EPh copolymers
(PEDOT-co-EPh)
Optical properties of PEDOT-co-EPh
EPh monomer was synthesized following Idzik et al’s
procedure 32. The oxidation potential of EPh and EDOT in
PEDOT-co-EPh copolymer films were obtained by
applying an oxidizing current (80 µA/cm2, potential recorded around
acetonitrile was found to be at +0.85 V and +1.25 V versus SCE,
+2.4 V) in a series of mixed monomer solutions. The galvanostatic
respectively.
It was suggested that the copolymer can be
deposition method was chosen as it gives better control of the film
deposition. It was found that the film color changed systemically as
the amount of EPh in the solution increased. As shown in Figure 3,
the electrochemically deposited neat PEDOT had a characteristic
sky-blue color due to its partially oxidized state. The film deposited
at 0.1% EPh showed a slightly yellowish color. The yellow tone
became much clearer at 0.2% EPh while at 1% the film turned into a
golden color. Further increasing the EPh feed ratio led to a reddish
color in the resultant films. At 20%, the golden color became
electrochemically synthesized by applying a voltage that was higher
than both of the monomers in a mixed monomer solution 37. Figure 2
shows cyclic voltammetry (CV) scans in the monomer solutions.
Above +1.2 V (vs Ag/AgCl), EPh, EDOT and the monomer mixtures
(1% and 2% EPh feed ratio) were irreversibly oxidized. The
integrated area under the CV curve expanded after each scan,
confirming the formation of an electroactive film. Typically, the CV
curves in EDOT solution during polymerization only showed a wide
capacitive response as
a result of overlapping redox peaks.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5

Journal of Materials Chemistry B
Page 6 of 13
VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5TB00053J
Figure 3 Optical micrographs of P(EDOT-co-EPh) films (transmitted light). Scale bars represent 100 μm.
pinkish. The homopolymer PEPh had a deep red color. The UV-Vis- absorption at 500 nm increased. A decrease of absorption at longer
NIR spectra of the galvanostatically deposited PEDOT-co-EPh wavelengths was also found.
polymer films, formed with the same total deposition charge
At higher EPh feed ratios, there was a further blue-shift of
(72 mC/cm2) showed that the copolymerization shifted the wavelength of the absorption maxima. Additional peaks around
absorption maximum to shorter wavelengths. As shown in Figure 4, 450 nm also started to appear. For example, 10% EPh feed ratio
the neat PEDOT films had a low and broad absorption from visible resulted in twin peaks at 432 nm and 500 nm in the spectrum. The
light to near IR. Adding only 0.1% EPh did not significantly change absorption around 450 nm increased with further EPh concentration
the absorption properties. However, at 0.2% EPh, broad peaks from in the feed. At 20%, the relative height of ~450 nm peak to its
500 and 760 nm started to emerge in the spectra. This explains the 500 nm counterpart greatly increased. For the PEPh homopolymer
yellowish tone on the overall blue film. At 0.5% EPh, a new films, the 500 nm absorption became a shoulder peak.
absorption peak at ~650 nm was found, accompanied by a shoulder
The λmax of the films deposited in different EPh feed ratio is
around 500 nm. It also had a high absorption from 900 to 1400 nm, summarized in Figure 4b. It was clear that λmax decreased quickly at
peaking around 1100 nm. Increasing the EPh feed ratio to 1%, the very low EPh feed ratios. The absorption maximum blue shifted
Figure 4 (a) UV-vis-NIR absorption of the PEDOT-co-EPh copolymers; (b) λmax versus composition; (c) normalized absorption
in near-infrared (900-1600 nm) and visible (400-600 nm) wavelength range versus composition.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 7 of 13
Journal Name
Journal of Materials Chemistry B
View Article Online
ARTICLE
DOI: 10.1039/C5TB00053J
from over 1000 nm for 0.1% EPh to around 500 nm for 1% EPh.
As is usual with electrochemical deposition, the film thickness
The absorption at longer wavelength (900 to1600 nm) also decreased could be readily controlled with the deposition charge. At higher
quickly at less than 1% EPh, while the absorption at shorter deposition charges, thicker films were formed, as is evident from the
wavelengths (400 to 600 nm) increased with increasing EPh feed increased optical absorption. Figures 5 show the UV-vis-NIR spectra
(Figure 4c). It was thus possible to tune the absorption maximum of of the PEDOT and PEPh as a function of deposition charge. The
the films by tuning the EPh feed ratio from 0.1% to 1%.
broad absorption of PEDOT starting from the visible to IR range
generally increased with increasing deposition charge. The
characteristic λmax of PEPh was found to be between 400 nm and
600 nm. An absorption around 430 nm increased with deposition
charge. At low deposition charge (0.4 mC/cm2), a broad absorption
peak of PEPh homopolymer appeared around 500 ~600 nm. With
increasing deposition charge, two peaks, around 450 and 550
respectively, could be identified. The peak around 450 increased
with the increasing deposition charge. It became the main λmax at
higher deposition charges while the ~550 peak became a shoulder
peak.
Surface Morphology of PEDOT-co-EPh films
The copolymerization significantly changed the surface
morphology of PEDOT films. As shown in Figure 6, when using
perchlorate as dopant, the PEDOT film had a fuzzy, fibrous surface
composed of ~100 nm polymer islands interconnected by numerous
nanofibers. It was found that only 0.1% EPh in the monomer feed
significantly changed the surface morphology. The nanofibers
completely disappeared from the surface. Instead, the polymer
islands packed together and fused onto the underlying layer. At 1%
EPh, the texture of film became very smooth. No fine structures
could be found on the surface except for sparsely distributed
~200 nm particles. At 10% EPh in the feed, the film started to
demonstrate a bumpy morphology. At 20%, the surface was
composed of ~500 nm large bumps which in turn were formed by
clusters of ~50 nm small particles. This morphology was similar to
the surface morphology of the PEPh homopolymer.
Another observation was the buckling of the polymer
surface. The polymer films were all deposited onto ITO substrates
from acetonitrile solutions and then dried in air before being
subjected to morphological observations in the vacuum of the SEM.
Acetonitrile is known to cause modest swelling of PEDOT38. On a
substrate that has no specific interactions with the film, the swelling
and drying can lead to film buckling. The buckling pattern was thus
a function of the extension of swelling (the interactions between the
solvent molecules) and the mechanical properties of the film itself.
For PEDOT, dense buckling patterns were formed on the surface.
Figure 5 UV-Vis-NIR spectra of polymers at different
deposition charges. Top: PEPh; Bottom: PEDOT. The insert is
a zoom in of PEPh UV-vis absorption at very low deposition
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

Journal of Materials Chemistry B
Page 8 of 13
VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5TB00053J
The ridges in the buckled film were about 5 um wide. It was clear (EIS). With its electronic and ionic conductivity and large effective
that copolymerizing with EPh substantially decreased the extent of surface area, PEDOT coatings have been shown to greatly decrease
buckling seen in the SEM. From 0.1% to 10% EPh feed ratio, the the system impedance of metal electrodes. As shown in Figure 7, at
buckling was significantly reduced. At 20% EPh and above the films frequencies below 1 kHz, a decrease of ~2 orders of magnitude
remained smooth and flat, with no buckling. This transition is could be found with PEDOT coatings.
evidently related to the increase in mechanical properties and
decrease in solvent swelling due to crosslinking.
The EPh-crosslinked PEDOT, however, became much less
conducting than the PEDOT homopolymer. With only 0.1% EPh
monomer in the feed, the copolymer film showed similar impedance
spectra as the PEDOT film at the same deposition charge density. By
increasing the EPh feed ratio to 0.2%, a slight increase in impedance
Figure 7 (a) Impedance amplitude of PEDOT and PEDOT-co-EPh
copolymers as a function of frequency; (b) impedance amplitude at 1
Hz and 1 kHz for the PEDOT-co-EPh copolymers as a function of EPh
content; dashed lines indicate the impedance amplitude of bare Pt
electrode at 1 Hz (red) and 1 kHz (black), respectively. (Deposition
charge: 288 mC/cm2)
Figure 6 SEM images of P(EDOT-co-EPh) films as a function of
EPH feed ratio; A column: lower magnification (scale bar
represents 50 μm); B column: higher magnification (scale bar
represents 5 μm)
Increased impedance of PEDOT-co-EPh
magnitudes was found. With this conducting polymer coating, the
impedances at lower frequencies (1 Hz to 100 Hz) were still much
The charge transport properties of the PEDOT-co-EPh polymer
films were characterized by electrochemical impedance spectroscopy
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 9 of 13
Journal Name
Journal of Materials Chemistry B
View Article Online
ARTICLE
DOI: 10.1039/C5TB00053J
lower than the bare electrode. However, it showed slightly higher detach from ITO substrate as an intact whole. However, without any
impedances than the bare electrode at higher frequencies.
supporting matrix, these films were still too brittle for tensile tests.
When the EPh feed ratio was increased to 1%, the impedance
In order to evaluate the effects of crosslinking on the
benefit from the conducting polymer coating was diminished. At 1% mechanical properties of PEDOT films, a PeakForce QNMTM AFM
EPh feed ratio, the impedance magnitude of the coated electrode was method was adopted. The films were indented at a nanometer depth
similar to that of the bare electrode. A slight increase of impedance
was found at higher frequency ranges (from 1 kHz to 100 kHz).
With further increases in the EPh feed ratio, the copolymer film
became even more insulating. Above 2%, the impedance of the
coating was higher than that of the bare Pt. Similar insulating effects
were found for 10%, 20% and 50% EPh films (Figure 7).
Figure 7b shows the impedance magnitudes at 1 Hz and 1 kHz
for different EPh feed ratios. The initially low impedance increased
substantially with increasing EPh feed ratio, becoming higher than
the bare electrode for EPh concentrations above 1%. The maximum
was found at 10% and then plateaued for higher EPh feed ratios.
For the purpose of neural recording and stimulations, it is
desirable to maintain low impedance at the electrode/tissue interface.
Previously, it was found that the low impedance of conducting
polymer coated electrodes could be correlated with the high effective
surface areas of its fuzzy morphology 39. Here two factors could be
Figure 8 AFM QNM modulus distribution. (a) AFM image of the
related to the decreased conductivity of the polymer coatings. From
the morphology, it was likely that the relatively dense surface from
crosslinking would not support sufficient ion transport through the
interface, resulting in higher impedance. On the other hand, from the
UV-vis-NIR absorption, it was found that the copolymerization
decreased the effective conjugation length of the polymer film. It
was thus possible that the EPh unit served as a defect to the
conjugated backbone 40,41. With shorter conjugation chain length, the
material became less conducting.
polymer films (2 µm by 2 µm); (b) AFM QNM modulus distribution
of PEDOT, 0.1% EPh and 0.5% EPh
(1 ~ 2 nm) with an AFM tip. The typical force constant of the tip
was 5 N/m with a frequency of 150 kHz. The tip radius ranged from
2~12 nm depending on manufacturing conditions. The precise tip
radius was calibrated with a TiO2 standard before each set of
measurements. The elastic modulus of the materials was calculated
from the force curves using the Derjaguin-Muller-Toporov (DMT)
model 42–44. Figure 8 shows the topological map and modulus
histogram of PEDOT, 0.1% and 0.5% EPh feed ratios. The latter two
compositions were compared with PEDOT since they maintained the
relatively high conductivity needed for neural probe coatings. It was
found that the indentation modulus calculated from QNMTM mode
with the DMT model for PEDOT was 0.75 ± 0.16 GPa. The film
deposited at 0.1% EPh had a modulus of 1.35 ± 0.48 GPa. However,
at 0.5% EPh, the modulus became 4.9 ± 2.1 GPa, which was ~5
times larger than the PEDOT film.
Mechanical Properties
The addition of EPh clearly improved the mechanical cohesion
of the PEDOT films. As discussed before, the copolymerization had
shown effects on the buckling behaviour of the electrochemically
deposited films. Macroscopically, much more uniform and robust
films were observed with higher EPh feed ratios. We found that
above 10% EPh feed, the thicker copolymer films tended to easily
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 9

Journal of Materials Chemistry B
Page 10 of 13
VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5TB00053J
Figure 9 Influence of PEDOT-co-EPh composition on PC-12 viability and neurite outgrowth in vitro.(a) PC-12 cell viability cultured on gold
and various polymer-coated substrates on days 2, 5, 8 and 11. (b) Representative fluorescence images of differentiated cells. Scale bars
indicate 200 µm (c) Mean neurite lengths of the PC-12 cells cultured on polymer films and corresponding neurite length distribution.
It should be noted that mechanical properties of neat PEDOT the two AFM PeakForce measurements. In Hassarati et al’s paper 45
,
films are known to be influenced by factors such as dopant and the calibration of tips was performed on a relatively soft PDMS
humidity. Green et al measured the indentation modulus of PEDOT standard (with a modulus around 1 MPa). The relative calibration
films with different small molecule dopants 16. It was found that method forces the value of ratio K/R0.5 to be correct at a given
larger dopants lead to higher modulus. The indentation modulus for indentation depth. This assumes limited modulus difference between
electrochemically polymerized PEDOT film doped with perchlorate standards and the samples to be measured. As the modulus of
was measured as 1.29 ± 0.72 GPa 16. However, using a similar AFM PEDOT was expected to be around hundreds of MPa to several GPa
PeakForce method, the same group reported that the average range, we found that the modulus varies dramatically between
modulus of perchlorate doped PEDOT (PEDOT: ClO4) film was groups when using this soft standard for calibration. In order to
68.0 MPa 45. The authors suggested that the indentation method measure this large range of modulus in a consistent manner, a
suffered from the large tip radius (100 µm), which would likely have universal calibration method was adopted in this research. The force
contributed to the overestimation of the stiffness 45. However, in this constant of the cantilever was calibrated through thermal tuning and
research, the modulus measured from 2~20 nm tip nanoindentation the precise tip radius was measured through analysing standard TiO2
was also found to be in the GPa range. One possible factor for this samples.
discrepancy is the different calibration methods employed between
10 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 11 of 13
Journal Name
Journal of Materials Chemistry B
View Article Online
ARTICLE
DOI: 10.1039/C5TB00053J
The relative humidity (rH) can also have a significant influence
Conclusions
on the stiffness of the PEDOT films. Lang et al 46 found that for cast
PEDOT:PSS (Baytron P) films, the Young’s modulus varied from
0.9 ± 0.2 GPa at 55% rH to 2.8 ± 0.5 GPa at 23% rH. However it
should be noted that in this PEDOT:PSS dispersion, excessive PSS
was required to stabilize the electrolyte complex 47. Typically, the
weight ratio for PSS to PEDOT can vary from 2.5:1 to 6:1. Also,
PSS is highly hygroscopic. The dependence of the Young´s modulus
on humidity can thus be largely attributed to the hydration kinetics
of PSS 48. The counterion concentration in these electrochemically
polymerized PEDOT films, on the other hand, is substantially lower
49. Thus, variation of stiffness with humidity is expected to be
smaller. It was found that the modulus of hydrated PEDOT:ClO4
A three-armed EDOT crosslinker with a benzene core, EPh, was
electrochemically copolymerized with EDOT in mixed monomer
solutions to create heavily branched or crosslinked PEDOT-co-EPh
copolymer coatings on metal electrodes. It was found that
copolymerization with EPh dramatically changed the optical
absorption of the PEDOT-co-EPh films. The absorption at longer
wavelengths (900 to 1600 nm) decreased quickly with increasing
EPh feed ratio, while new absorptions between 400 to 600 nm
increased at the same time. As a result, the film color gradually
changed from sky blue for PEDOT to pinkish and red for PEPh. The
crosslinking also led to a denser surface morphology of the resultant
films, as seen in SEM. Electrochemical impedance spectroscopy
showed that the copolymerization had a negative effect on the
impedance of the coatings. At the lowest EPh feeds (less than
0.02%), the PEDOT-co-EPh films had comparable impedances to
neat PEDOT coatings. However, the impedance quickly increased
with higher EPh feed ratios. Macroscopically, more coherent film
formation was observed at higher EPh feed ratios, suggesting
improved mechanical strength. This observation was confirmed by
AFM nanoindentation tests, where at 0.5% EPh, the film had 5x
larger QNMTM indentation modulus than PEDOT. Lastly, in vitro
cell culture tests showed that the PEDOT-co-EPh films exhibited
excellent biocompatibility to PC-12 cell lines, with limited
cytotoxicity and the ability to support neurite outgrowth. Our results
suggest that for neural interface applications, the materials expected
to be of most interest will be PEDOT-co-EPh copolymers with less
than 1% EPh. These materials retain the ability to facilitate charge
transport between the electrode and tissue, yet also have
substantially improved mechanical properties.
films decreased for 38% while PEDOT:PSS over 90% 45
.
Biocompatibility
The excellent biocompatibility of PEDOT has been well
documented25,50. However, it was unknown if the changes in
chemical composition of PEDOT-co-EPh copolymers as well as the
surface stiffness would change the biocompatibility of the films. For
this purpose, PC-12 cell lines were employed as an in vitro model to
test the biocompability of PEDOT-co-EPh films with different EPh
compositions. The cell viability on day 2, 5, 8 and 11 after seeding
was tested. It was found that all of the samples well supported the
cell growth. As shown in Figure 9 (a), the viability across all days
remained above 80%. A slight drop of viability on day 8 for 0.2%
EPh feed ratio (p < 0.05) and on day 11 for 1% EPh (p < 0.001) was
found, but it was still remained above 70%.
After switching to differentiation medium, the outgrowth of
neurites was measured. As shown in Figure 9(b) , it was found that
the PC12 cells grew well on these substrates, with a large percentage
of cells expressing interconnected neurites. The mean neurite length
was found to be between 150 to 250 µm. Figure 9 (c) shows the
neurite distributions of the cells cultured on different polymer
substrates. Most neurites were around 100~200 µm. However,
neurites that grew over 800 µm were also observed. These results
demonstrate that the EPh crosslinked PEDOT showed no significant
issues with cytotoxicity toward PC-12 cells. With the ability to
deliver charge, these polymer substrates could also potentially be
Acknowledgements
This research was supported in part by the National Institutes of
Health through EUREKA grant 1RO1EB010892, by the Defense
Advanced Research Projects Agency (DARPA) grant N66001-11-C-
4190, and by the University of Delaware.
employed to guide cell growth through electrical stimulation 51
.
Notes and references
1.
Schmidt, C., Shastri, V., Vacanti, J. & Langer, R. Stimulation of
neurite outgrowth using an electrically. Proc. Natl. Acad. Sci. U. S.
A. 94, 8948–8953 (1997).
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 11

Journal of Materials Chemistry B
Page 12 of 13
VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5TB00053J
2.
Gomez, N. & Schmidt, C. E. Nerve growth factor-immobilized
polypyrroleꢀ: Bioactive electrically conducting polymer for
enhanced neurite extension. J. Biomed. Mater. Res. A 81a, 135–149
(2006).
17.
18.
Khodagholy, D. et al. Highly conformable conducting polymer
electrodes for in vivo recordings. Adv. Mater. 23, H268–72 (2011).
Nguyen, J. K. et al. Mechanically-compliant intracortical implants
reduce the neuroinflammatory response. J. Neural Eng. 11, 056014
(2014).
3.
4.
Oh, W.-K., Kwon, O. S. & Jang, J. Conducting Polymer
Nanomaterials for Biomedical Applications: Cellular Interfacing
and Biosensing. Polym. Rev. 53, 407–442 (2013).
19.
20.
Harris, J. P. et al. Mechanically adaptive intracortical implants
improve the proximity of neuronal cell bodies. J. Neural Eng. 8,
066011 (2011).
Kim, D.-H., Wiler, J. a, Anderson, D. J., Kipke, D. R. & Martin, D.
C. Conducting polymers on hydrogel-coated neural electrode
provide sensitive neural recordings in auditory cortex. Acta
Biomater. 6, 57–62 (2010).
Potter, K. a, Buck, A. C., Self, W. K. & Capadona, J. R. Stab injury
and device implantation within the brain results in inversely
multiphasic neuroinflammatory and neurodegenerative responses.
J. Neural Eng. 9, 046020 (2012).
5.
Gerard, M., Chaubey, A. & Malhotra, B. D. Application of
conducting polymers to biosensors. Biosens. Bioelectron. 17, 345–
59 (2002).
21.
Ouyang, L., Shaw, C. L., Kuo, C.-C., Griffin, A. L. & Martin, D. C.
In vivo polymerization of poly(3,4-ethylenedioxythiophene) in the
living rat hippocampus does not cause a significant loss of
performance in a delayed alternation task. J. Neural Eng. 11,
026005 (2014).
6.
Kim, D.-H. et al. in Indwelling Neural Implants: Strategies for
Contending with the In-Vivo Environment (ed. Reichert, W. M.)
(Taylor and Francis, 2007).
7.
Fattahi, P., Yang, G., Kim, G. & Abidian, M. R. A review of
organic and inorganic biomaterials for neural interfaces. Adv.
Mater. 26, 1846–85 (2014).
22.
23.
24.
25.
26.
27.
28.
29.
30.
Cui, X. T. & Zhou, D. D. Poly (3,4-Ethylenedioxythiophene) for
Chronic Neural Stimulation. IEEE Trans. neural Syst. Rehabil.
Eng. 15, 502–508 (2007).
8.
Kozai, T. D. Y. et al. Nanotechnology and Neuroscience: Nano-
electronic, Photonic and Mechanical Neuronal Interfacing. 71–131
(Springer New York, 2014). doi:10.1007/978-1-4899-8038-0
Green, R. et al. Electrochemical stability of poly(ethylene
dioxythiophene) electrodes. in the 5th IEEE/EMBS conference on
Neural Engineering 5, 566–569 (2011).
9.
Saltó, C. et al. Control of neural stem cell adhesion and density by
an electronic polymer surface switch. Langmuir 24, 14133–8
(2008).
Kung, T. A. et al. Regenerative Peripheral Nerve Interface
Viability and Signal Transduction with an Implanted Electrode.
Plast. Reconstr. Surg. 133, (2014).
10.
11.
Ludwig, K. a et al. Poly(3,4-ethylenedioxythiophene) (PEDOT)
polymer coatings facilitate smaller neural recording electrodes. J.
Neural Eng. 8, 014001 (2011).
Asplund, M. et al. Toxicity evaluation of PEDOT/biomolecular
composites intended for neural communication electrodes. Biomed.
Mater. 4, 045009 (2009).
Cui, X. & Martin, D. C. Electrochemical deposition and
characterization of poly(3,4-ethylenedioxythiophene) on neural
microelectrode arrays. Sensors Actuators B Chem. 89, 92–102
(2003).
Teng, C., Lu, X., Zhu, Y., Wan, M. & Jiang, L. Polymer in situ
embedding for highly flexible, stretchable and water stable
PEDOT:PSS composite conductors. RSC Adv. 3, 7219 (2013).
Chen, C. et al. Mechanical characterizations of cast Poly(3,4-
ethylenedioxythiophene):Poly(styrenesulfonate)/Polyvinyl Alcohol
thin films. Synth. Met. 161, 2259–2267 (2011).
12.
13.
Cui, X. & Martin, D. C. Fuzzy gold electrodes for lowering
impedance and improving adhesion with electrodeposited
conducting polymer ® lms. Sensors actuators A 103, 384–394
(2003).
Press, A. et al. Conducting Polymer Hydrogels as 3D Electrodesꢀ:
Applications for Supercapacitors ** By Soumyadeb Ghosh * and
Olle Inganäs. 1214–1218 (1999).
Wilks, S. J., Richardson-burns, S. M., Hendricks, J. L., Martin, D.
C. & Otto, K. J. Poly ( 3 , 4-ethylenedioxythiophene ) as a micro-
neural interface material for electrostimulation. Front. Neuroeng. 2,
1–8 (2009).
Ghosh, S. & Inganäs, O. Electrochemical Characterization of Poly (
3 , 4-ethylene dioxythiophene ) Based Conducting Hydrogel
Networks. 147, 1872–1877 (2000).
14.
15.
Martin, D. C. et al. The Morphology of Poly(3,4-
Ethylenedioxythiophene). Polym. Rev. 50, 340–384 (2010).
Perepichka, I. F., Besbes, M., Levillain, E., Salle, M. & Roncali, J.
Hydrophilic Oligo ( oxyethylene ) -Derivatized
Optoelectroelectrochemical Properties and Solid-State Chromism.
863–871 (2002).
Molino, P. J. et al. Influence of Biodopants on PEDOT Biomaterial
Polymers: Using QCM-D to Characterize Polymer Interactions
with Proteins and Living Cells. Adv. Mater. Interfaces 1, n/a–n/a
(2014).
31.
32.
Idzik, K. R. et al. Electrochemical and spectral properties of meta-
linked 1,3,5-tris(aryl)benzenes and 2,4,6-tris(aryl)-1-phenoles, and
their polymers. Electrochim. Acta 55, 7419–7426 (2010).
16.
Baek, S., Green, R. a & Poole-Warren, L. Effects of dopants on the
biomechanical properties of conducting polymer films on platinum
electrodes. J. Biomed. Mater. Res. A 102, 2743–54 (2014).
Idzik, K. R., Beckert, R., Golba, S., Ledwon, P. & Lapkowski, M.
Synthesis by Stille cross-coupling procedure and electrochemical
12 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 13 of 13
Journal of Materials Chemistry B
View Article Online
Journal Name
ARTICLE
DOI: 10.1039/C5TB00053J
properties of C3-symmetric oligoarylobenzenes. Tetrahedron Lett.
51, 2396–2399 (2010).
48.
49.
Dupont, S. R., Novoa, F., Voroshazi, E. & Dauskardt, R. H.
Decohesion Kinetics of PEDOT:PSS Conducting Polymer Films.
Adv. Funct. Mater. 24, 1325–1332 (2014).
33.
Roncali, J., Leriche, P. & Cravino, a. From One- to Three-
Dimensional Organic Semiconductors: In Search of the Organic
Silicon? Adv. Mater. 19, 2045–2060 (2007).
Zotti, G. et al. Electrochemical and XPS Studies toward the Role of
Monomeric and Polymeric Sulfonate Counterions in the Synthesis ,
Composition , and Properties of Poly ( 3 , 4-
ethylenedioxythiophene ). Macromolecules 36, 3337–3344 (2003).
34.
35.
Cooper, A. I. Conjugated Microporous Polymers. Adv. Mater. 21,
1291–1295 (2009).
50.
51.
Asplund, M., Nyberg, T. & Inganäs, O. Electroactive polymers for
neural interfaces. Polym. Chem. 1, 1374 (2010).
Zhu, S. S. & Swager, T. M. Conducting Polymetallorotaxanesꢀ:
Metal Ion Mediated Enhancements in Conductivity and Charge
Localization. J. Am. Chem. Soc. 119, 12568–12577 (1997).
Forciniti, L., Ybarra, J., Zaman, M. H. & Schmidt, C. E. Schwann
cell response on polypyrrole substrates upon electrical stimulation.
Acta Biomater. 10, 2423–33 (2014).
36.
37.
Drubin, D. G., Feinstein, S. C., Shooter, E. M. & Kirschner, M. W.
Nerve growth factor-induced neurite outgrowth in PC12 cells
involves the coordinate induction of microtubule assembly and
assembly-promoting factors. J. Cell Biol. 101, 1799–807 (1985).
Yohannes, T., Carlberg, J. C., Inganäs, O. & Solomon, T.
Electrochemical and spectroscopic characteristics of copolymers
electrochemically synthesized from 3-methylthiophene and 3,4-
ethylenedioxy thiophene. Synth. Met. 88, 15–21 (1997).
38.
39.
Zotti, G., Schiavon, G. & Zecchin, S. A Polyconjugated Polymer
with a High Spin Density Polaron State. Chem. Mater. 11, 3624–
3628 (1999).
Cui, X., Hetke, J. F., Wiler, J. a., Anderson, D. J. & Martin, D. C.
Electrochemical deposition and characterization of conducting
polymer polypyrrole/PSS on multichannel neural probes. Sensors
Actuators A Phys. 93, 8–18 (2001).
40.
41.
42.
Ho, H. et al. Colorimetric and Fluorometric Detection of Nucleic
Acids Using Cationic Polythiophene Derivatives. Angew. Chemie
Int. Ed. 41, 1548–1551 (2002).
Leclerc, M. Optical and Electrochemical Transducers Based on
Functionalized Conjugated Polymers. Adv. Mater. 11, 1491–1498
(1999).
Sweers, K., van der Werf, K., Bennink, M. & Subramaniam, V.
Nanomechanical properties of α-synuclein amyloid fibrils: a
comparative study by nanoindentation, harmonic force microscopy,
and Peakforce QNM. Nanoscale Res. Lett. 6, 270 (2011).
43.
44.
45.
Dokukin, M. E. & Sokolov, I. Quantitative mapping of the elastic
modulus of soft materials with HarmoniX and PeakForce QNM
AFM modes. Langmuir 28, 16060–71 (2012).
Landoulsi, J. & Dupres, V. Direct AFM force mapping of surface
nanoscale organization and protein adsorption on an aluminum
substrate. Phys. Chem. Chem. Phys. 15, 8429–40 (2013).
Hassarati, R. T. et al. Stiffness quantification of conductive
polymers for bioelectrodes. J. Polym. Sci. Part B Polym. Phys. 52,
666–675 (2014).
46.
47.
Lang, U., Naujoks, N. & Dual, J. Mechanical characterization of
PEDOT:PSS thin films. Synth. Met. 159, 473–479 (2009).
De Kok, M. M. et al. Modification of PEDOT:PSS as hole
injection layer in polymer LEDs. Phys. Status Solidi 201, 1342–
1359 (2004).
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 13