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Synthetic Metals 160 (2010) 1107–1114

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

olyterthiophene as an electrostimulated controlled drug release material ofherapeutic levels of dexamethasone

race Stevenson, Simon E. Moulton, Peter C. Innis, Gordon G. Wallace ∗

RC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, NSW 2522, Australia

r t i c l e i n f o

rticle history:eceived 17 December 2009eceived in revised form 12 February 2010ccepted 23 February 2010vailable online 31 March 2010

eywords:olyterthiopheneontrolled releaseexamethasone

a b s t r a c t

Polyterthiophene has been investigated as a substrate for the controlled release of dexamethasone, asynthetic glucocorticoid anti-inflammatory drug, which is widely used to help reduce inflammation inthe central nervous system. Dexamethasone was incorporated into the polymer as an anionic dopantduring electrochemical polymerisation from water–acetonitrile solutions. Optimal polymer synthesisconditions were established to yield reproducible dexamethasone release profiles into a simulated phys-iological receiving solution. A homogeneous morphology of the polyterthiophene substrate with minimalextraneous features was observed to be critical for achieving reproducibility of release. Release profileswere established using a range of electrochemical stimulation protocols over a 24 h time period. Theamount of dexamethasone released from the polyterthiophene under all electrostimulation protocols

2

lectrostimulatededox

was at therapeutically relevant levels, with a maximum release of ∼80 �g/cm achieved when the poly-mer film was in a reduced state. The oxidation state of the polyterthiophene was found to be critical forcontrolled release of the dexamethasone. Polyterthiophene doped with dexamethasone was observedto auto-reduce when placed into the receiving solution. As a consequence, no significant difference wasobserved between the unstimulated (auto-reduced) polymer and the electrochemically reduced poly-terthiophene. By electrochemically holding the polyterthiophene in the oxidised state, the rate of release

gnific

of dexamethasone was si

. Introduction

The concept of controlled drug release has been widely inves-igated since it was introduced in a patent in 1953 [1]. Devicesesigned for drug delivery are important for improving theharmacological profile of therapeutic compounds [2]. The devel-pment of new materials utilising nanotechnology has been showno have a significant impact upon advancing the development ofontrolled release systems, especially those for targeted deliv-ry [2]. Systems have been developed to give either sustained orelayed release as well as site-specific or targeted delivery [3].esearch efforts have resulted in a range of drug delivery formu-

ations including microparticles [4], nanoparticles [5] and implantevices [6].

Polymers have been widely researched for controlled releaseevices. Biodegradable polymers such as poly(lactic acid) (PLA),oly(glycolic acid) (PGA) and copolymers of the two (PLGA)ave been used for controlled release devices [7]. Other poly-

∗ Corresponding author at: Intelligent Polymer Research Institute, University ofollongong, Wollongong, NSW 2522, Australia. Tel.: +61 2 4221 3127;

ax: +61 2 4298 3114.E-mail address: g.wallace@uow.edu.au (G.G. Wallace).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.02.035

antly impeded with ∼40 �g/cm2 released over a 24 h period.© 2010 Elsevier B.V. All rights reserved.

meric substrates for controlled release include dendritic polymers,polymeric micelles and liposomes [2]. More recently, conduct-ing electroactive polymers (CEPs) have received a great deal ofinterest for biomedical applications [8–12]. Significantly, CPs areelectroactive materials which possess redox properties that allowthem to be reversibly switched from the oxidised conductingstate to the reduced non-conducting state. During this switch-ing process counter ions (or ‘dopants’) will move in and out ofthe polymer in order to create charge balance within the poly-mer (Scheme 1) [13]. Significantly, drugs can be used as dopantmolecules.

Polypyrrole (PPy) is the most extensively researched con-ducting polymer for drug release due to its biocompatibility[14], pH stability and the fact that it can be synthesised fromaqueous solution [15]. PPy has been utilised in applications suchas biosensing [16] and release of therapeutic compounds suchas glutamate [17], neurotrophic factor [18] and dexamethasone[3,19]. Conducting polymers derived from terthiophene (TTh) arealso of great interest due to their electrochemical reversibility

and relative ease of synthesis [20]. PTTh has been shown tohave a more ordered structure than materials synthesised fromthe thiophene monomer [21]. Importantly, the terthiophenemonomer has a lower oxidation potential than thiophene andbithiophene monomers, which makes electrochemical growth

1108 G. Stevenson et al. / Synthetic Metals 160 (2010) 1107–1114

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cheme 1. (a) Dexamethasone disodium phosphate (Dex2−Na2+); (b) synthesis oxidation.

f conducting polymer films more facile and less prone to irre-ersible oxidative damage during polymerisation (over-oxidation)22].

Other polythiophene analogues, such as poly(3,4-thylenedioxythiophene) (PEDOT), have been investigated foriomedical applications, for example in neural prosthetic devices23,24]. PEDOT has also been used in a drug release system inonjunction with poly(lactide-co-glycolide) (PLGA) [25]. In thisase electrical stimulation of the PEDOT was used to control releasef the drug from PLGA fibres; the conducting polymer was notoped with the drug and was therefore not the release substrate.

The morphology of the substrate will also influence the abilityf the polymer to interact with the environment and release incor-orated drugs. PTTh has been shown to have a nodular morphologyhich greatly increases the surface area of a polymer film [3]. Intro-uction of a microstructured morphology has also been achievedy using a PTTh pre-layer which has been shown to increase theelease of drug from the overlaying polypyrrole film [3].

In this study we investigate the use of polyterthiophenePTTh) as a substrate for the controlled release of dexamethasoneisodium phosphate (Dex2−), a synthetic glucocorticoid anti-

nflammatory drug which can reduce inflammation in the centralervous system [19]. In the system investigated we have incorpo-ated Dex2− directly into the polymer matrix as a dopant duringhe electrosynthesis of the polymer from terthiophene monomeresulting in a highly microstructured surface. The release of therug from the polymer is then investigated both with and withoutlectrical stimulation.

. Experimental

.1. Reagents

2,2′,5′,2′′-Terthiophene (TTh, >99%), dexamethasone 21-hosphate disodium salt (Dex2−, ≥98%) and tetrabutylammoniumerchlorate (purum grade) were purchased from Sigma–Aldrich.odium chloride (analytical reagent) was purchased from Ajax,nd acetonitrile (Scharlau, HPLC grade) was purchased from Crowncientific. All chemicals were used as received. Phosphate buffered

aline (PBS) was prepared using standard PBS tablets (Merck,ermany) dissolved in MilliQ water (18 M�) containing sodiumhloride (0.15 M). The pH of PBS was 7.4. Gold-coated Mylar wasurchased from CP Films Inc. (USA) and was rinsed with MilliQater then methanol and dried before use.

terthiophene with the incorporation of the dopant anion Dex2− upon monomer

2.2. Polymer electrosynthesis

Polyterthiophene–dexamethasone (PTTh–Dex2−) films wereelectrosynthesised galvanostatically on gold-coated Mylar(masked to an area of 0.9 cm2) at a range of current densitiesand growth times, using an eDAQ (Australia) potentiostat con-trolled by eDAQ ChartTM software (v.5.2.18). The films weregrown from 20 mM terthiophene and 5 mM dexamethasone in1.5 mL of 15% (v/v) MilliQ water in acetonitrile. A mixed solventsystem was necessary to obtain optimal solubility of the monomerand the drug. All monomer/Dex2− solutions were purged withnitrogen before use. The electrochemical cell was a 3.5 mL glasscuvette with a three-electrode system made up of Ag/Ag+ (0.01 MAgNO3 in 0.1 M TBAP) reference, platinum counter and gold-coated Mylar working electrode. Films were rinsed for 10 min inacetonitrile and then MilliQ water after growth and allowed toair dry.

2.3. Characterisation

2.3.1. Cyclic voltammetryElectrochemical characterisation was carried out using cyclic

voltammetry with an eDAQ potentiostat system controlled byeDAQ EChem software (v.2.0.7). A three-electrode system was usedwith Ag/AgCl reference electrode, platinum counter electrode andgold Mylar coated with PTTh–Dex2− as the working electrode. Stud-ies were carried out in a 10 mL electrolyte solution containing PBS.Potential scans were carried out between 0 V and 0.75 V at a scanrate of 50 mV/s.

2.3.2. Fourier transform Infrared spectroscopySolid-state Fourier transform infrared spectroscopy (FTIR) was

carried out on a Shimadzu IRPrestige-21 spectrometer by specu-lar reflectance with a Spec30 attachment (Pike Technologies). FTIRspectra of films were obtained before and after release to establishthe presence of incorporated Dex2− within the films.

2.3.3. UV–vis spectroscopy of PTTh–Dex2− filmsSolid-state UV–vis spectra of films were obtained using a Shi-

madzu UV-3600 UV–vis-NIR spectrophotometer with integratedsphere attachment controlled via UVProbe (v.2.10) software. Spec-tra of films were recorded between 280 and 2600 nm.

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G. Stevenson et al. / Synthe

.3.4. Scanning electron microscopyScanning electron microscopy was performed using a JEOL7500

eld emission scanning electron microscope (FESEM) at typicaloltages of 5 kV. SEM was used to analyse the morphology ofhe polymer films. Energy dispersive spectroscopy (EDS) analy-is was carried out using a JEOL hyper-minicup EDS. EDS wassed to qualitatively analyse the presence of Dex2− in the polymerlms.

.3.5. Raman spectroscopyRaman spectra were obtained on a Jobin Yvon Horiba HR800

aman spectrometer with LabSpec software. A laser wavelength of85 nm was used.

.3.6. Release of dexamethasone from PTTh–Dex2− filmsTwo electrical stimulation protocols were investigated in this

tudy; constant potentials of 0 V and +0.6 V vs. Ag/AgCl referencend square wave pulsed potential from 0 V to +0.6 V at 1 Hz. Eachelease protocol was continuously applied for the duration of theelease experiment (24 h). All stimulated release experiments werearried out in a three-electrode system with Ag/AgCl referencelectrode and platinum counter electrode, with the working elec-rode being PTTh–Dex2− film on gold Mylar. The electrolyte usedor release was PBS (3 mL). Release conditions were controlled byDAQ (Australia) potentiostat and monitored using eDAQ ChartTM

.5.2.18 software. Release of Dex2− was also monitored when nolectrical stimulation was applied to the film (passive release) foromparison. All release experiments were carried out in tripli-ate.

.3.7. Detection of dexamethasoneDexamethasone release was detected using UV–vis spec-

roscopy. Spectra were recorded using a Shimadzu UV-1601pectrophotometer with UVProbe (v.2.10) software. UV data wasollected using 3.5 mL quartz cuvettes (1.0 cm path length) pur-hased from Starna Pty Ltd. (Australia). A calibration curve ofex2− at the �max (242 nm) was produced to calculate themount of Dex2− release in these studies. The release exper-ment was set up in the UV spectrometer to allow in situ

onitoring of the Dex2− release. Real-time release profiles werebtained using the kinetics function of the software to takebsorbance readings at 30 s intervals at Dex2− �max (242 nm). Theelease solution was gently stirred for the duration of all releasexperiments to assist the diffusion of the released drug at thelectrode surface through the bulk solution. At the end of eachelease experiment a complete UV spectrum (190–800 nm) of theelease solution was obtained in order to confirm the absencef interferences at the Dex2− �max (data not shown). Statisticalnalysis of the cumulative release of Dex2− from unstimulatednd stimulated polymer films was made using a standard t-est.

. Results and discussion

.1. Synthesis of PTTh–Dex2−

In previous work, polyterthiophene (PTTh) was synthesisedrom acetonitrile solutions [26]. In this study this solvent proved toe unsuitable as the dexamethasone disodium phosphate (Dex2−)opant was insoluble in this solvent. Dex2− has been success-ully used as a dopant in polypyrrole synthesised under aqueous

onditions [19]. Importantly, it has been reported that polybithio-hene can be polymerised from a mixed acetonitrile/water solventystem [27]. In this study a range of concentrations of water incetonitrile were tested until both the monomer and the dopantere solubilised. The optimal solvent system suitable for the Dex2−

tals 160 (2010) 1107–1114 1109

dopant and terthiophene monomer was found to be 15% (v/v)water/acetonitrile solutions.

During electrosynthesis of polyterthiophene (PTTh), Dex2− wasincorporated into the polymer film as a molecular dopant. Initialsynthesis of PTTh–Dex2− films was carried out by cyclic voltam-metry (CV) (data not shown). Terthiophene was found to beginto oxidise at approximately +0.5 V, with subsequent CV scansresulting in an increase in current magnitude, indicating thatelectroactive polymer was being deposited. However, the films pro-duced by the CV method were uneven and lacked morphologicalreproducibility. Galvanostatic growth has been used by others toprepare conducting polymer films doped with therapeutic agentsand was investigated as an alternate deposition method [3,12].It has also been shown that galvanostatic growth produces uni-form conducting films of polypyrrole doped with Dex2− [13]. Bythis method the films produced were evenly deposited across theelectrode surface and were dull black in colour; with a more evenappearance than the CV grown films which varied in colour acrossthe surface.

3.2. Growth current and duration optimisation

To optimise growth current, films were electrosynthesisedat various current densities for 15 min. CV analysis was car-ried out in PBS to observe the electrochemical response inthe medium that would be used for release. The uniformityand reproducibility of the PTTh–Dex2− films were characterisedby SEM. The growth current was varied at 0.05 mA inter-vals between 0.11 mA/cm2 (0.1 mA/0.9 cm2) and 0.44 mA/cm2

(0.4 mA/0.9 cm2). During polymerisation, the current, voltage andcharge generated at the working electrode were monitored. Dur-ing growth (data not shown) the deposition voltage tended toincrease slightly for the entire duration of polymerisation, indi-cating that the film resistivity increased slightly on extendeddeposition. Using this approach an optimal working electrodepotential within the +0.8 to +0.9 V range was selected as it hasbeen shown by others to be optimal for terthiophene oxidation[26].

At a deposition current density of 0.11 mA/cm2, a voltageof approximately +0.6 V was generated at the anode. As thegrowth current was increased, the voltage generated at theworking electrode increased as expected. CV analysis of thepost synthesis PTTh–Dex2− films showed that the 0.11 mA/cm2

films gave a small capacitive current, with no clear redox peaks(Fig. 1). As the growth current was increased to 0.28 mA/cm2

(0.25 mA/0.9 cm2), an increase in capacitance was observed indicat-ing greater polymer deposition with an anode deposition potentialof approximately +0.8 V. Redox peaks also became apparent withan oxidation peak at approximately +0.45 V and reduction peakat approximately +0.52 V. As the growth current was furtherincreased to 0.44 mA/cm2 an anode voltage of approximately1 V was observed. CV analysis of films grown at 0.44 mA/cm2

showed a smaller capacitance than the 0.28 mA/cm2 film, indi-cating that the 0.44 mA/cm2 film has a smaller electroactivesurface area. The higher voltage generated at 0.44 mA/cm2 mayhave caused some over-oxidation of the polymer during growthwhich would have had a detrimental effect on the quality ofthe polymer. However, as redox peaks were still visible in theCV, there was still electroactivity in the polymer.

Differences in the morphologies of the films grown at vary-ing currents were also observed (Fig. 1). All films have a nodular

bulk structure which became denser as the growth current wasincreased. Films grown at 0.28 mA/cm2 appeared more porousthan those grown at 0.44 mA/cm2. The underlying bulk structureof the polymer appeared to be uniform on all films, regardless ofgrowth current. However, extraneous features were also observed

1110 G. Stevenson et al. / Synthetic Metals 160 (2010) 1107–1114

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ig. 1. Cyclic voltammetry of PTTh–Dex2− films grown for 15 min at: 0.11 mA/cm2, 0he arrows on the cyclic voltammogram indicate the direction of the potential scan.11 mA/cm2 (d).

n the films (Fig. 1). The size and number of these appeared toncrease with increasing growth current and were undesirable ashey introduce heterogeneity to the polymer films. From analysisf the results, 0.28 mA/cm2 was chosen as the optimal growth cur-ent as this gave films with reasonable electrochemistry as wells the most reproducible and uniform morphology with minimalxtraneous features.

Once the growth current was optimised, the optimal growthime was investigated. Films were electrosynthesised as previ-usly described at a current of 0.28 mA/cm2 for varying timesetween 5 min and 30 min. Post growth CV studies showed an

ncrease in current response of approximately 0.2 mA between0 min and 20 min films (data not shown) indicating more poly-ers was deposited at the longer growth time. Films grown for

onger than 15 min exhibited a powder-like deposition on the sur-ace, which was undesirable as it affected film reproducibility. SEM

nalysis of the films again showed the presence of extraneouseatures that were prevalent at longer growth times (Fig. 2). Theptimal growth time for release films was determined to be 10 mins this minimised the formation of extraneous features and gavehe most uniform and reproducible films.

ig. 2. Cyclic voltammetry of PTTh–Dex2− film, grown at 0.28 mA/cm2 for 10 min. The CVor 15 and 20 min films were omitted for clarity. The arrows in the cyclic voltammogramrown at 0.28 mA/cm2 for 10 min (b), 15 min (c) and 20 min (d).

A/cm2 and 0.44 mA/cm2 (a). The CVs were recorded in PBS at a scan rate of 50 mV/s.racterisation SEM images for films grown at 0.44 mA/cm2 (b), 0.28 mA/cm2 (c) and

3.3. Characterisation

Prior to controlled release studies, the PTTh–Dex2− films werecharacterised by specular reflectance FTIR and diffuse reflectanceUV–vis spectroscopy. FTIR was employed to qualitatively showthat Dex2− had been incorporated into the polymer films dur-ing electrosynthesis. A characteristic C O peak at ∼1660 cm−1 forDex2− was observed in the spectrum of PTTh–Dex2− film (Fig. 3).This peak was not observed in the standard spectrum of PTTh,obtained of a PTTh film doped with tetrabutylammonium perchlo-rate (TBAP). Peaks corresponding to the PTTh in the film could alsobe determined with C C anti-symmetric stretch at ∼1490 cm−1

and C–H out-of-plane bend at ∼790 cm−1 clearly identifiable. Thesolid-state diffuse reflectance UV–vis spectrum of pre-release filmsconfirmed that the films were in an oxidised (doped) state as char-acteristic �–�* (∼370 nm), �-polaron (∼680 nm) and �-polaron*

(∼1140 nm) bands were observed (Fig. 4).

3.3.1. Release studiesAs determined above, the optimal PTTh–Dex2− polymer used

for the release studies was synthesised galvanostatically at

was recorded in PBS at a scan rate of 50 mV/s. The 5th CV cycle is shown (a). CVsindicate the direction of the potential scan. Characterisation SEM images for films

G. Stevenson et al. / Synthetic Metals 160 (2010) 1107–1114 1111

Fig. 3. FTIR spectra of polyterthiophene standard film doped with TBAP (PTTh Std),polyterthiophene film doped with dexamethasone (PTTh–Dex2−) and dexametha-s 2−

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Fig. 5. (a) Cumulative Dex2− release over initial 2 h of 24 h period from PTTh–Dex2−

films stimulated with constant potential 0 V (�), constant potential +0.6 V (�) andpulsed potential (0 V to +0.6 V at 1 Hz) (�). Passive release from unstimulated films

one disodium phosphate standard (Dex Std). Peaks corresponding to C O stretchn Dex2− were observed at ∼1660 cm−1 in PTTh–Dex2− spectrum (�). Characteristiceaks for PTTh were observed at 1490 cm−1 (C C anti-symmetric stretch) (*) and90 cm−1 (C–H out-of-plane bend) (**) in the PTTh–Dex2− spectrum.

.28 mA/cm2 for 10 min in a 15% (v/v) Milli-Q/acetonitrile sol-ent mix. The polymers were rinsed in Milli-Q water for 10 min toemove any surface bound Dex2− prior to the release experiments.he release of Dex2− from the polymer was investigated under pas-ive (unstimulated) and electrically stimulated conditions in PBSolution.

Unstimulated release of Dex2− from PTTh films was carried outo observe the release profile for Dex2− when no stimulation waspplied to the polymer films. Electrochemically controlled releasetudies of Dex2− from PTTh–Dex2− polymer films were performedsing constant potentials of 0 and +0.6 V and square wave pulsedotential stimulation of 0 V to +0.6 V at 1 Hz. These potentials werehosen as they correlated to the observed oxidised (+0.6 V) andeduced (0 V) states of the PTTh–Dex2− polymer film (Figs. 1 and 2).he mass of Dex2− released shown, in Fig. 5(a) and (b), was deter-ined by UV–vis quantitative analysis of the Dex2− at 242 nm

�max). UV–vis studies of the release solutions showed spectra typ-cal of Dex2− (data not shown).

Stimulated release profiles were individually compared tonstimulated release via a t-test. The t-test was carried out at 23

ime points across the 24 h period of release. However, in the firsth of controlled release a t-test was performed at 10 min inter-als as that portion of the 24 h profile exhibited rapid changes. Theelease profiles over 24 h for films held at a constant potential of

ig. 4. UV–vis–NIR spectrum of PTTh–Dex2− film. Characteristic �–�* (∼370 nm)�], �–polaron (∼680 nm) [��] and �–polaron* (∼1140 nm) [���] bands werebserved. A glitch in the spectrum at ∼940 nm was due to detector changeover, andas expected as the samples are in solid state.

(�) is included for comparison. Error bars indicate the standard error of the mean(n = 3). (b) Cumulative Dex2− release over 24 h. The mass of Dex2− released wascalculated from the standard Dex2− UV–vis calibration curve at �max = 242 nm.

0 V (i.e. constantly holding the PTTh–Dex2− film in a reduced state)and for unstimulated PTTh–Dex2− films indicated that there wasno statistical difference in the release between the profiles at anyanalysed time point. None of the controlled release profiles showedFickian diffusion when modelled using Peppas equation [28]. Thisindicates that mixed mechanisms are involved in release.

The application of an oxidising potential (+0.6 V) to thePTTh–Dex2− polymer films resulted in a significantly differentrelease profile. The profile indicated that the release of Dex2−

was comparatively slower and more sustained when the polymerwas held in an oxidised state. A t-test analysis indicated a sta-tistically significant difference in release between the stimulatedoxidised (+0.6 V) and unstimulated films at the time points over24 h. Notably, the application of an oxidising potential eliminatesthe burst release as observed for the pulsed, passive and reducedpotential stimulation protocols. The elimination of burst releasecharacteristics in controlled release systems is highly desirable.

Electrochemical reduction of a conducting polymer typicallyresults in the loss of the cationic charge carriers (polarons andbipolarons) from the polymer back bone [29]. A consequence ofthe loss of the cationic characteristic on the polymer backbone willbe the expulsion of anionic dopant molecules from the conduct-ing polymer matrix or ingress of cationic charges from the bathing

solution environment to maintain electro-neutrality. The applica-tion of more negative reducing potentials should therefore increasethe rate of anionic drug release. Conversely, the application of a pos-itive oxidising potential should increase the cationic characteristic

1112 G. Stevenson et al. / Synthetic Me

Fig. 6. Raman spectra of dry PTTh–Dex2− film and film soaked in PBS for 2 mina −1

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nd 19 h. Changes in C C symmetric stretching (1442 cm ) (�), C–C stretching1163 cm−1) (�) and C–S–C deformation (694 cm−1) (�) bands indicated that thexidation state of the polymer has changed upon wetting in solution. Spectra werease line corrected and normalised to the C–H band (1050 cm−1).

f the host polymer matrix and impede or ideally inhibit the loss ofhe anionic drug dopant.

PTTh–Dex2− films stimulated by pulsed potential (0 V to +0.6 Vt 1 Hz) also demonstrated no statistical difference in release com-ared to unstimulated films after the first 10 min. However, releaserofiles suggested that under pulsed stimulation the release of theex2−, after an initial burst release, was more sustained with anpproximately linear Dex2− release from 200 min onwards.

In this study the application of a reducing potential did notignificantly increase the rate of drug release when compared ton unstimulated film. It is has been shown that polythiophenesill undergo auto-reduction upon immersion in solution [30]. Evi-ence of the auto-reduction process was confirmed by Ramanpectroscopy for a dry and wet PTTh–Dex2− film (Fig. 6). Thedvantage of Raman over FTIR spectroscopy is that it is insensitiveo the interferences of the electrolyte solvent and Dex2− dopantnd therefore clearly able to show redox state changes within thepaque, non-transmissive film. When the PTTh–Dex2− was placednto the electrolyte media a clear shift to the reduced state wasoticed as evidenced by changes to the C C symmetric stretching1442 cm−1), C–C stretching (1163 cm−1) and C–S–C deformation694 cm−1) bands. These changes were characteristic of a changen oxidation state of polythiophene as shown by Bazzaoui et al31]. A consequence of this auto-reduction was that even wheno potential stimulation was applied to the PTTh–Dex2− film, thebsence of cationic charge within the matrix resulted in free releasef the Dex2− dopant. Qualitatively, the application of 0 V to forceTTh–Dex2− reduction appeared to give enhanced rate of release,ig. 5 (a) and (b), however statistical analysis of the data scatteretween replicates did not confirm statistical differences. This dif-ers from previous results obtained for polypyrrole, where it wasound that very little drug was released from unstimulated films3,19], indicating that polypyrrole does not undergo spontaneouseduction in solution. Only when the film was electrochemicallyeduced was dopant expelled from the polymer. Even when theolymer was stimulated by cyclic voltammetry, release was stillttributed to the reduction of the polymer film [19].

When the PTTh–Dex2− polymer was forced into the oxidisedtate at +0.6 V the release was significantly reduced with respecto the earlier studies, but not completely inhibited. In the firsth the release was relatively flat and limited presumably to non-

2−

pecifically bound Dex at the PTTh surface. However, over 24 hhis shifted to an approximately linear release rate. Release may beue to a conformational change in the polymer causing an overallelaxation of the structure when it is in an oxidised state [32,33].his relaxation results in a more open structure which may allow

tals 160 (2010) 1107–1114

the receiving solution to penetrate deeper into the film, causing lossof Dex2− molecules which have been loosely trapped in the porouspolymer during growth. Ion exchange may have also contributed torelease when the polymer film was held at an oxidising potential.Dex2− ions may have exchanged with anions from the surroundingbuffer solution, for example phosphate or chlorine ions. As previ-ously stated, the application of an oxidising potential eliminatesthe burst release which was observed for the pulsed, passive andreduced potential stimulation protocols. Burst release has also beenobserved for polypyrrole when in a reduced state [18,34]. Preven-tion of this initial burst can allow release to be sustained for longerperiods, rather than a large quantity of drug being released in thefirst few hours and then very little (if any) further release.

Using pulsed potential stimulation resulted in the PTTh–Dex2−

switching between the relatively more open (oxidised) and (closed)reduced morphologies, therefore stimulating Dex2− release. Therelease profile for pulsed potential stimulation is similar to reducedand unstimulated films. This may be because the film was in areduced state 50% of the time, therefore the initial burst release wasnot inhibited. However, at 24 h release was still gradually increas-ing, unlike the unstimulated and reduced film profiles, suggestingthat release rate had been reduced overall.

The amount of Dex2− released under all protocols (stimulatedand unstimulated) was at therapeutically relevant levels and far inexcess of the quantity of Dex2− released from polypyrrole reportedin previous studies by our group [3]. Previous work has shown thatDex2− is effective at levels of 0.2–0.68 �mol [35]. In this work, up to80 �g/cm2 (approximately 50 �mol) was released from the poly-mer film, compared to polypyrrole which has been shown to releasevalues ranging from 6 �g/cm2 [3] to 16 �g/cm2 [19]. Introductionof a microstructured morphology, by using a polyterthiophene pre-layer, has been shown previously to increase the release of drugfrom a polypyrrole film [3]. In this case we have incorporated thedrug directly into the microstructured surface, removing the needfor a second polymer layer to enhance surface area.

Energy dispersive spectroscopy (EDS) analysis of the film wasmade to estimate the doping levels of the PTTh–Dex2− films grownunder optimal conditions on two replicate studies. Sulphur andfluorine X-ray peaks, characteristic of PTTh and Dex2−, respec-tively, were used to estimate the PTTh–Dex2− doping ratios andcalculate the theoretical incorporation of Dex2− as a dopant. Anal-ysis of phosphorous, a marker specific to Dex2−, was unsuccessfuldue to weaker EDS signals. All PTTh–Dex2− films were washedwith Milli-Q grade water for 10 min and then dried prior to anal-ysis to minimise the influence of non-doping, surface adsorbedDex2−. The atom mole ratio of S:F was found to be 6:1 (mol%) indi-cating approximately one Dex2− molecule per 6 thiophene rings(i.e. one Dex2− per two terthiophene monomer units). Based onthese findings, a total Dex2− loading of 149 �g/cm2 was estimated(assuming one electron transfer during polymerisation, this esti-mate is the maximum possible mass of Dex2− which could beincorporated). Previous studies have reported Dex2− loading intopolypyrrole of 17.4 �g/cm2 [19] and 86 �g/cm2 [3]. The estimatedloading results correlate well with the controlled release studiesshowing that approximately 53 ± 9%, 47 ± 2% and 47 ± 7% of theavailable Dex2− was released by unstimulated, reductive and pulsepotential stimulation protocols, respectively. Significantly, appli-cation of a potential holding the PTTh–Dex2− in an oxidised statereleased only 26 ± 14% of available Dex2−. This compares to ∼15%[3] and 92% [19] Dex2− release observed for polypyrrole.

An SEM study of the surface morphology of the unstimulated

and stimulated PTTh–Dex films is shown in Fig. 7. At lowermagnifications there appeared to be little difference betweenthe as-grown and post-release films. Post-release SEM images athigher magnification exhibited subtle changes in the film mor-phologies after release. The most marked changes in morphology

G. Stevenson et al. / Synthetic Metals 160 (2010) 1107–1114 1113

n prot

wp

dabpttitrbc

4

eap

Fig. 7. SEM images of PTTh–Dex2− films after release under each stimulatio

ere observed for PTTh–Dex2− films stimulated with pulsedotential.

It has been established that by oxidising PTTh films the release ofrug is significantly slowed; inhibiting the initial burst release andllowing a more linear release profile. Stimulation protocols coulde further varied to use asymmetric pulsing, which could hold theolymer in an oxidised state longer than a reduced state. This, inheory, would allow greater release than is achieved by holdinghe film in an oxidised state, but less than when the film is heldn a reduced state. Altering the pulse duration to vary how longhe polymer was held in oxidised/reduced state should enable theelease profile to be varied. Further to this, investigation into waysy which the auto-reduction of polythiophene could be preventedould allow greater control over drug release.

. Conclusions

Controlled release of dexamethasone disodium phosphate,lectrochemically incorporated into polyterthiophene, is readilychieved by the use of electrochemical stimulation at oxidisingotentials. Dexamethasone successfully dopes polyterthiophene

ocol. Analogous images for an unreleased film are included for comparison.

and releases from the polymer matrix at therapeutically rele-vant levels. The oxidation state of the polyterthiophene is criticalfor controlled release of the dexamethasone. Polyterthiophenedoped with dexamethasone auto-reduces when placed into salinephosphate buffered receiving solution. No statistical differenceis observed between the unstimulated (auto-reduced) polymerand the electrochemically reduced polyterthiophene. Significantly,dexamethasone release is reduced by 50% over 24 h by applying anoxidising +0.6 V potential to the polymer.

Acknowledgements

The authors gratefully acknowledge the continued financial sup-port from the Australian Research Council and Australian ResearchCouncil Federation Fellowship (Wallace). We also thank Mr. TonyRomeo for his invaluable assistance with SEM microscopy.

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