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Investigating the Interfacial Properties of ElectrochemicallyRoughened Platinum Electrodes for Neural Stimulation Alexander Weremfo, Paul Carter, David Brynn Hibbert, and Chuan Zhao Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504876n • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Investigating the Interfacial Properties of

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Electrochemically-Roughened Platinum

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Electrodes for Neural Stimulation

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Alexander Weremfo,1 Paul Carter,2* D. Brynn Hibbert,1* and Chuan Zhao1* 1

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School of Chemistry, UNSW Australia, Sydney, NSW 2052, Australia

Cochlear Ltd., 1 University Avenue, Macquarie University, NSW 2109, Australia

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ABSTRACT: Platinum electrodes have been electrochemically roughened (roughness factors up

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to 430) and evaluated for use as neural stimulation electrodes. The roughened electrodes show

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superior interfacial properties with increasing surface roughness. The roughened electrode (fR =

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250) have charge injection limit of 1.0 mC cm-2 (400 µs) which is superior to that of titanium

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nitride (0.87 mC cm-2) but comparable to that of carbon nanotubes (1.0-1.6 mC cm-2). The

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surface roughness can also be optimized for different neural stimulation applications based on

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the available charge density at a particular pulse width of stimulation. The roughened platinum

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electrodes

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electrochemical stability under continuous voltage cycling, indicating the potential of this

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biological interface to be safe and stable.

demonstrated good

mechanical

stability under harsh ultrasonication

and

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KEYWORDS: Electrochemical roughening, neural stimulation, charge injection limit, platinum,

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Cochlear implant

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INTRODUCTION

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Chronic neural stimulation by bioelectrodes has been used over the past decades to restore lost or

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impaired neurological functions and this has shown great potential for auditory,1 visual2 and

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somatosensory3 prostheses. A wide range of biomaterials have been investigated for use as

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stimulating electrodes in neural prostheses. Platinum is the most commonly used material

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because of its ability to resist corrosion and to inject charge by reversible reactions4 without

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inducing damage to the surrounding tissues. The charge injection limit, defined as the maximum

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charge density (per geometric area) that can be applied before hydrolysis and evolution of gases,

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has been determined to range from 50 - 150 µC cm-2 for smooth platinum.5 Beyond the limit

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electrode corrosion and tissue damage can occur. For next-generation prostheses such as retinal

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prostheses where a large number of electrodes are required to improve the selectivity and spatial

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resolution of the device, the size of the individual electrodes must be reduced. However, by

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decreasing the size of the electrode for a given injected charge the charge density increases and

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may exceed the charge injection limit of smooth platinum. As individual electrodes in retinal

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arrays can have significant smaller area than those in present-generation cochlear implants this

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restricts the usefulness of smooth platinum as a stimulating microelectrode material.6

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To improve the performance of platinum while decreasing geometric size, one approach is to

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modify the surface to increase the effective surface area of the electrode within the available

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geometrical area. High-surface-area platinum-black can be formed by electrodeposition from

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chloroplatinic acid.7 Though traditional platinum black has excellent electrical properties due to

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its high surface area, it cannot be used for chronic neural stimulation because of ready loss of

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material, and therefore surface area, due to abrasion.8-9 Iridium oxide (IrOx) has been considered

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as an alternative coating material for electrodes due to its ability to permit significantly higher

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levels of charge injection without electrode dissolution or electrolysis of water.10-11 However,

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IrOx has poor adhesion to underlying substrates, and is susceptible to delamination under

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prolonged stimulation due to its low structural and chemical stability.12-13 Conducting polymers,

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particularly poly (3,4-ethylene-dioxythiophene) (PEDOT), are also promising interfacial

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materials as a result of their orderly and well defined chemical structure with good conductivity,

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thermal stability14 as well as biocompatibility.15 However, structural defects like cracking and

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delamination have been reported16-17 which leads to further detachment of the coating, thus

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affecting the function of the electrode. While new versions of conductive hydrogels have shown

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promise with regard to long term mechanical stability, these materials have yet to be proven in

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the in vivo environment.18

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It is known that the charge storage capacity of platinum can be increased by more than an order

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of magnitude by electrochemical roughening via repeatedly forming a hydrated platinum oxide

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layer on the surface which is later reduced.19 The degree of surface roughness can be controlled

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by the duration of the oxidation-reduction cycle (ORC) for a given surface area of the polished

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electrode, wave-frequency (or sweep rate) and switching potential limits.19-20 The resultant

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roughened electrode with high surface area has demonstrated its potential for neural

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stimulation.21 However, no detailed studies have been carried out to characterize and optimize

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their interfacial properties for neural stimulation.

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In the present study, the effect of surface roughness on the in vitro performance of

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electrochemically-roughened platinum as a stimulating electrode is investigated. The surface of a

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platinum electrode was electrochemically roughened in a controlled manner to achieve particular

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roughness factors. The surface morphology and electrochemical performance, including the

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electrochemical impedance, charge storage capacity and charge injection limit have been

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characterized as a function of roughness factor. In addition, the mechanical and electrochemical

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stability of the roughened surface under strong ultrasonication and continuous voltage cycling

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are reported.

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EXPERIMENTAL

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Reagents

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All aqueous solutions of chemicals including sulphuric acid (98 %), sodium chloride (>99.5 %),

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potassium chloride (>99 %), sodium phosphate monobasic (98 %) and sodium phosphate dibasic

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(>98.5 %), purchased from Sigma Aldrich (Australia), were prepared with milliQ water (18 MΩ

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cm-1, Millipore, Australia)

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Procedure for electrochemical roughening

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A platinum wire of diameter of 0.5 mm was embedded in a Teflon insulator and the surface of

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the wire opened via mechanical polishing using sandpaper followed by alumina powder to obtain

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a mirror finish. The electrode was electrochemically cleaned prior to roughening using cyclic

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voltammetry at 0.5 V s-1 between –0.2 and +1.25 V vs Ag|AgCl|sat KCl in 0.5 M sulfuric acid

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solution. The surface of platinum was then electrochemically roughened using a repetitive square

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wave potential cycle. A square wave of 1 kHz with lower and upper potentials of –0.4 and +2.4

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V vs Ag|Ag2SO4 |sat K2SO4 was applied to the electrode in 0.5 M sulfuric acid solution for 15 s

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to 5 min after which the potential was maintained at –0.4 V until oxides on the surface were

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completely reduced. The reference electrode was constructed with silver sulfate, rather than

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silver chloride to avoid possible oxidation of leaked chloride, which affects oxide formation. The

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Ag|Ag2SO4 reference electrode was calibrated against Ag|AgCl|sat KCl, which gave a voltage of

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+0.25 V in 0.5 M H2SO4 solution. To allow comparison with other published work all potentials

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reported herein are referenced to Ag|AgCl|sat KCl. After roughening, the electrode was again

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cleaned electrochemically in a fresh 0.5 M sulfuric acid solution until reproducible cyclic

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voltammograms of platinum were obtained. The surface roughness (fR) was calculated through

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the relationship

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fR =

QH σ H,ideal Ageom

(1)

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where QH is the measured charge of hydrogen desorption and Ageom is the geometric area of the

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electrode. σ H,ideal is the surface density of charge associated with monolayer adsorption of

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hydrogen which has been reported as 210 µC cm-2.22

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Characterization of surface morphology

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The surface morphology of the electrochemically roughened platinum surface was

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characterized by Atomic Force Microscopy (AFM) NanoScope (Veeco Instruments Inc.,

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Australia) in tapping mode using silicon nitride tips (OTESPA 42 N/m). The surface roughness

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and the lateral dimensions of the different nano-rough surface features were analysed using

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Gwyddion SPM data analysis software.

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Electrochemical characterization

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Electrochemical characterization of roughened platinum electrodes was conducted in a three-

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electrode system with Ag|AgCl|sat’d KCl as a reference electrode and platinum counter

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electrode. Cyclic voltammetry measurements were performed using a CHI660D potentiostat (CH

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instruments). Cyclic voltammograms obtained with the electrodes between potential limits of –

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0.25 and –0.15 V, where no faradaic reactions occur, in N2-purged phosphate buffer saline (PBS)

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(pH 7.4) at different scan rates (10, 20, 30, 40 and 50 mV s-1), were used to measure the double

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layer capacitance (Cdl) of the electrodes. The Cdl was calculated as I/ν where I is current at –0.2

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V (A), and ν is the scan rate (V s-1).

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Electrochemical impedance spectroscopy (EIS) measurements were performed using a Solatron

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1250 frequency response analyser. EIS spectra were acquired in PBS at open circuit potential

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with a ± 10 mV amplitude ac signal over a frequency range from 100 KHz – 0.1 Hz.

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The charge injection limit was determined by potential transient responses where charge

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balanced biphasic pulses of varied stimulation parameters (pulse widths and current amplitudes)

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were applied using a homemade stimulator. The stimulator was capacitively coupled and

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electrode shorting was applied between pulses to maintain charge balance. The electrode

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potential excursion between the stimulating electrode and the reference electrode was monitored

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on a digital oscilloscope (TPS 3034C, Tektronix). Figure 1A shows a typical input biphasic

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current pulse (0.3 mA, 400 µs and 10 µs delay). The potential excursion response (Figure 1B) to

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the current pulse shows an initial, rapid change in potential, known as the access voltage (Va),

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due to the ohmic resistance of the electrolyte, followed by a slowly rising polarization voltage

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(Vpol), which is due to the charging of the electrode/electrolyte interface. The Vpol was calculated

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by subtracting the Va from the maximum negative voltage in the transient (Vt). The polarization

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voltage of phase one of the biphasic pulse (i.e. the cathodic phase) is used to determine the

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charge injection limit. The charge injected that polarizes the electrode interface to the potential

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for water reduction (Vpol = −0.60 V) is used to define the charge injection limit throughout this

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paper.5 The charge injection limit was obtained by continuously increasing the current amplitude

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until the polarization voltage reached –0.6 V.

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RESULTS AND DISCUSSION

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Electrochemical roughening and surface morphology

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The platinum electrode surface was roughened by repeated oxidation and reduction cycling using

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high frequency voltage pulses, which facilitated the exchange of surface oxygen with the bulk

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platinum atoms. This process exposes the inner layers of platinum to further oxidation, and

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complete reduction of the oxide results in an irregular arrangement of platinum atoms creating

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nanostructured surfaces.20 The surface roughness was controlled in our experiment by varying

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the duration of the roughening time (15 – 300 s) while the other parameters were fixed. Figure

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2A shows voltammograms of platinum electrodes obtained at different roughening times. The

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roughness factor increased with increasing roughening time (Fig. 2B). The method gave

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reproducible results, with relative standard deviations of roughness factor, measured from 5

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repeats, in the range 7 % to 14 %.

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The nano-rough surface features and enhancement of surface roughness were observed and

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confirmed by AFM. Figure 3 shows representative AFM images of nanoscale surface

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morphology of the smooth and the electrochemically roughened platinum surfaces. The

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morphology of the surface changed significantly as the duration of the roughening cycle

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increased. The surface was dominated by nodular nanostructures (30 – 40 nm size) at moderately

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roughened surface (tR = 3 min, fR = 156). However as the time increases (tR = 5 min, fR = 430)

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the nanostructures aggregated forming large “cauliflower-like” structures with micropores

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occurring between the aggregated structures in addition to the interstices of the neighbouring

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individual particles. RMS roughness was determined by measuring the variations in peak-to-

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valley height across the surface. As shown in Table 1, the RMS roughness value increases as

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roughening time increased.

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Electrochemical characterization of roughened Pt electrodes

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Charge storage capacity. The charge storage capacity of an electrode, which comprises the

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charge stored in the double layer and reversible faradaic reactions, is a key indicator for the

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performance of a stimulating electrode material. Capacitive charge injection is the ideal

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mechanism for neural stimulation as no chemical change is induced to either the electrode or the

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tissue.23 The charge storage capacity of the electrodes was determined by measuring the double

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layer capacitance (Cdl) of the electrode. Normalizing the measured Cdl with the geometric area of

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the electrode, the corresponding specific Cdl increased nearly linearly as surface roughness

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increases (Figure 4). For example, for fR= 156 Cdl = 8.9 ± 0.5 mF cm-2, which is about 44 times

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greater than that of a smooth platinum electrode (0.2 ± 0.1 mF cm-2), is greater than that of 3D

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nanoporous platinum (5.1 mF cm-2),7 and is even comparable to carbon nanotubes (10 mF cm-

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attributed to the increased effective surface area. The exceptionally high Cdl value makes the

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roughened platinum electrode favourable for neural stimulation as most charge will be delivered

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by capacitive means.

) . The significantly increased Cdl of the electrochemically roughened platinum electrodes is

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Interfacial Impedance. Low interfacial impedance is required for neural stimulation to minimize

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electrode polarization and to decrease the power consumption of neural prostheses. Figure 5

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shows the impedance spectra in Bode format for both smooth and roughened platinum surfaces

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in PBS solution. The interfacial impedances of the roughened platinum surfaces are significantly

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less than the smooth one, and the impedance decreases with increasing surface roughness. At 1

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kHz, which is the biologically relevant frequency for neural activity, the impedance of the

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roughened Pt (fR = 96) was more than two orders of magnitude less than that of a smooth surface.

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Similar impedance results have been reported for electrodeposited platinum-iridium alloys,25 and

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poly(3,4-ethylenedioxythiophene) (PEDOT).26 The impedance spectra were fitted using an

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equivalent circuit model, a constant phase element in parallel with an interfacial resistance in

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series with electrolyte resistance, to model the nanoporous interface.25,

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element (ZCPE = 1/Q(iω)α where 0 ≤ α ≤ 1) was used instead of pure capacitance to compensate

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for the non-ideal capacitive response of the interface.

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The results of spectra fitting are summarized in Table 2. Since both smooth and roughened

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surfaces exhibited a near ideal capacitive behaviour with α close to 1, Q is called capacitance in

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this paper. As shown in Table 2, the interfacial capacitance of the roughened surface increases

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whiles the polarization resistance, which relates to the interfacial charge transfer resistance,

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decreases with increasing roughness factor. Since capacitive impedance is inversely proportional

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to the electrode capacitance, the observed decrease in impedance is attributed to increased

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capacitance as a result of the increased effective surface area.

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A constant phase

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Charge injection limit. The charge injection limit, which is the important parameter for the safe

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use of stimulating electrodes, was measured using increasing biphasic current pulses. Figure 6A

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shows the voltage excursions recorded for smooth electrode to increasing current pulses at 400

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µs duration and 10 µs interpulse delay. The polarization voltage increases with increasing the

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current pulse amplitude and the charge injected when Vpol = −0.6 V was used to determine the

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charge injection limit. Figure 6B shows the potential response of both smooth (fR = 2) and

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roughened (fR = 24) platinum electrodes to a current pulse (0.3 mA current amplitude, 400 µs

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pulse width and 10µs interpulse delay).

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The charge injection limit increases by several fold as the surface roughness increases (Figure 7).

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The highly roughened, nanostructured platinum electrodes have significantly increased

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capacitance with low impedance, which all contribute to high charge storage capacity and

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therefore high charge injection limit. At fR = 250, the charge injection limit of the roughened

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platinum electrode is 1.0 mC cm-2, which is superior to that of titanium nitride (0.87 mC cm-2)28

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but comparable to carbon nanotubes (1-1.6 mC cm-2).29

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The effect of stimulation pulse width on the charge injection limit of the roughened surfaces was

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further investigated. As shown in Figure 7, the charge injection limit of the smooth surface

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increases marginally while that of the roughened surfaces increases several fold with increasing

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pulse width of stimulation. This is due to the large charge storage capacity of the roughened

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surfaces which becomes accessible as the duration of the pulse increases. At high rates of

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stimulation (i.e. short pulses) access to the available charge is limited by pore resistance which

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reduces the charge injection limit.23,

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electrode could be determined based on the available charge density at a particular pulse width.

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The optimum roughness factors for the platinum electrodes in this study were determined for

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different neural stimulation applications at the pulse width used for the specific neural

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stimulation application. For Cochlear implants using shorter pulse durations (25 – 50 µs),32 fR

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around 23 is the optimum as further increases in roughness have no significant effect on the

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charge injection limit. For retinal prostheses that require longer pulse widths (200 µs)33 the

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optimum fR is 156. This indicates that for high stimulation rates, less roughness is preferred since

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the available charge is limited by pore resistance. The ability to vary surface roughness thus

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provides the opportunity to fabricate implantable electrodes with properties that can be tailored

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for specific neural stimulation application.

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Therefore the optimum roughness of a stimulating

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Stability of electrochemically roughened platinum surfaces

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To demonstrate the robustness of the roughened platinum electrode, roughened platinum was

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subjected to ultrasonication (80 W, 37 kHz) at room temperature for a period of 60 minutes and

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the change in electrochemical surface area measured.34-35 Ultrasonic agitation can remove

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loosely bonded surface particles thereby showing the mechanical durability of the roughened

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electrode. As shown in figure 8, even for a highly roughened surface (fR = 351) the

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electrochemical surface area retained about 95 % of its initial surface area after 5 minutes of

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harsh ultrasonication, and which only fell to 82 % after 60 minutes. In contrast, an

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electrodeposited platinum black electrode retained only ~50 % of its electrochemical surface area

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after 5 minutes of ultrasonication. This shows that the electrochemically roughened surfaces,

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which are formed from an existing surface, have greater integrity than the deposited particles of

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platinum black. This result indicates the robustness of the electrochemically-roughened surface

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to endure mechanical stress.

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The electrochemical stability of the roughened platinum electrode was further studied in PBS

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under a biphasic stimulation at 1.25 mA with 0.2 ms pulse width (0.13 mC cm-2), which is well

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above the charge injection threshold to elicit neuronal responses.36-37 Stimulation was carried out

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for 14 days at 2000 Hz, equivalent to 2.4 billion stimulations. As shown in Figure 9, the

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polarization voltage during the period of stimulation remained unchanged across the roughened

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platinum electrodes. The cyclic voltammograms obtained before and after 2.4 billion

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stimulations (Figure 10) are almost identical with only slight decrease in the charge storage

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capacity (CSC) from 137.5 ± 2.6 to 128.7 ± 5.8 mC cm-2 after stimulation. However no

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significant difference was observed for the CSC values before and after biphasic stimulation,

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indicating the electrochemical stability of the roughened platinum electrodes for use at a

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biological interface.

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CONCLUSION

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Platinum electrodes have been electrochemically roughened to a maximum roughness factor of

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430 and their electrochemical interfacial properties investigated. In comparison to a smooth

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platinum electrode, the roughened electrodes demonstrated much lower impedance, higher

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charge storage capacity and charge injection limit with increasing roughness. The charge

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injection limit for a roughened platinum electrode with roughness factor of 250 (1.0 mC cm-2)

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was found to be superior to that of titanium nitride (0.87 mC cm-2) but comparable to carbon

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nanotubes (1.0-1.6 mC cm-2). The surface roughness of the roughened platinum electrodes for

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different neural prostheses can be optimized based on the pulse width of the injected charge, with

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optimum roughness factor of 23 and 156 for cochlear implants and retinal prostheses

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respectively. The roughened platinum surfaces also show excellent mechanical and

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electrochemical stability. This study highlights the usefulness of electrochemically roughened

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platinum electrodes for neural stimulation which requires smaller electrode size with high

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resolution.

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AUTHOR INFORMATION

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Corresponding Authors

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*E-mail: [email protected], [email protected], [email protected]

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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This work was funded by the Australian Research Council linkage Grant with Cochlear Ltd.

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(LP100200770). CZ also thanks Australian Research Council for the award of an Australian

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Research Fellow (DP110102569). The authors also thank Abbas Barfidokht for help with the

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AFM measurements and the School of Biomedical Engineering (UNSW) for the home-built

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biphasic stimulator.

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REFERENCES

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1. Wilson, B. S.; Lawson, D. T.; Müller, J. M.; Tyler, R. S.; Kiefer, J., Cochlear Implants: Some Likely Next Steps*. Annual Review of Biomedical Engineering 2003, 5, 207-249. 2. Weiland, J. D.; Liu, W.; Humayun, M. S., Retinal Prosthesis. Annual Review of Biomedical Engineering 2005, 7, 361-401. 3. Fitzsimmons, N. A.; Drake, W.; Hanson, T. L.; Lebedev, M. A.; Nicolelis, M. A. L., Primate Reaching Cued by Multichannel Spatiotemporal Cortical Microstimulation. The Journal of Neuroscience 2007, 27, 5593-5602. 4. Merrill, D. R.; Bikson, M.; Jefferys, J. G. R., Electrical Stimulation of Excitable Tissue: Design of Efficacious and Safe Protocols. Journal of neuroscience methods 2005, 141, 171-198. 5. Rose, T. L.; Robblee, L. S., Electrical Stimulation with Pt Electrodes. Viii. Electrochemically Safe Charge Injection Limits with 0.2 Ms Pulses (Neuronal Application). Biomedical Engineering, IEEE Transactions on 1990, 37, 1118-1120. 6. Zhang, H.; Shih, J.; Zhu, J.; Kotov, N. A., Layered Nanocomposites from Gold Nanoparticles for Neural Prosthetic Devices. Nano Letters 2012, 12, 3391-3398. 7. Park, S.; Song, Y. J.; Boo, H.; Chung, T. D., Nanoporous Pt Microelectrode for Neural Stimulation and Recording: In Vitro Characterization. The Journal of Physical Chemistry C 2010, 114, 8721-8726. 8. Schuettler, M.; Doerge, T.; Wien, S. L.; Becker, S.; Staiger, A.; Hanauer, M.; Kammer, S.; Stieglitz, T., Cytotoxicity of Platinum Black. In 10th Annual Conference of the International Functional Electrical Stimulation Society, IFESS_2005: Montreal, Canada, 2005.

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9. Maher, M.; Pine, J.; Wright, J.; Tai, Y.-C., The Neurochip: A New Multielectrode Device for Stimulating and Recording from Cultured Neurons. Journal of neuroscience methods 1999, 87, 45-56. 10. Cogan, S. F.; Troyk, P. R.; Ehrlich, J.; Plante, T. D., In Vitro Comparison of the ChargeInjection Limits of Activated Iridium Oxide (Airof) and Platinum-Iridium Microelectrodes. Biomedical Engineering, IEEE Transactions on 2005, 52, 1612-1614. 11. Troyk, P. R.; Detlefsen, D. E.; Cogan, S. F.; Ehrlich, J.; Bak, M.; McCreery, D. B.; Bullara, L.; Schmidt, E., "Safe" Charge-Injection Waveforms for Iridium Oxide (Airof) Microelectrodes. Conf Proc IEEE Eng Med Biol Soc 2004, 6, 4141-4. 12. Cogan, S. F.; Guzelian, A. A.; Agnew, W. F.; Yuen, T. G. H.; McCreery, D. B., OverPulsing Degrades Activated Iridium Oxide Films Used for Intracortical Neural Stimulation. Journal of neuroscience methods 2004, 137, 141-150. 13. Mailley, S. C.; Hyland, M.; Mailley, P.; McLaughlin, J. M.; McAdams, E. T., Electrochemical and Structural Characterizations of Electrodeposited Iridium Oxide Thin-Film Electrodes Applied to Neurostimulating Electrical Signal. Materials Science and Engineering: C 2002, 21, 167-175. 14. Xiao, Y.; Cui, X.; Hancock, J. M.; Bouguettaya, M.; Reynolds, J. R.; Martin, D. C., Electrochemical Polymerization of Poly(Hydroxymethylated-3,4-Ethylenedioxythiophene) (Pedot-Meoh) on Multichannel Neural Probes. Sensors and Actuators B: Chemical 2004, 99, 437-443. 15. Abidian, M. R.; Kim, D. H.; Martin, D. C., Conducting-Polymer Nanotubes for Controlled Drug Release. Advanced Materials 2006, 18, 405-409. 16. Cui, X. T.; Zhou, D. D., Poly (3,4-Ethylenedioxythiophene) for Chronic Neural Stimulation. Neural Systems and Rehabilitation Engineering, IEEE Transactions on 2007, 15, 502-508. 17. Jan, E.; Hendricks, J. L.; Husaini, V.; Richardson-Burns, S. M.; Sereno, A.; Martin, D. C.; Kotov, N. A., Layered Carbon Nanotube-Polyelectrolyte Electrodes Outperform Traditional Neural Interface Materials. Nano Letters 2009, 9, 4012-4018. 18. Green, R. A.; Hassarati, R. T.; Goding, J. A.; Baek, S.; Lovell, N. H.; Martens, P. J.; Poole-Warren, L. A., Conductive Hydrogels: Mechanically Robust Hybrids for Use as Biomaterials. Macromol Biosci 2012, 12, 494-501. 19. Chialvo, A. C.; Triaca, W. E.; Arvia, A. J., Changes in the Electrochemical Response of Noble Metals Produced by Square-Wave Potential Perturbations: A New Technique for the Preparation of Reproducible Electrode Surfaces of Interest in Electrocatalysis. J. Electroanal. Chem. Interfacial Electrochem. 1983, 146, 93-108. 20. Burke, L. D.; Roche, M. B. C., Hydrous Oxide Formation on Platinum—a Useful Route to Controlled Platinization. J. Electroanal. Chem. Interfacial Electrochem. 1984, 164, 315-334. 21. Tykocinski, M.; Duan, Y.; Tabor, B.; Cowan, R. S., Chronic Electrical Stimulation of the Auditory Nerve Using High Surface Area (Hiq) Platinum Electrodes. Hear Res 2001, 159, 5368. 22. Trasatti, S.; Petrii, O. A., Real Surface Area Measurements in Electrochemistry. J. Electroanal. Chem. 1992, 327, 353-376. 23. Cogan, S. F., Neural Stimulation and Recording Electrodes. Ann. Rev. Biomed. Eng. 2008, 10, 275-309.

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24. Gabay, T.; Ben-David, M.; Kalifa, I.; Sorkin, R.; Abrams, Z. e. R.; Ben-Jacob, E.; Hanein, Y., Electro-Chemical and Biological Properties of Carbon Nanotube Based MultiElectrode Arrays. Nanotechnology 2007, 18, 035201. 25. Petrossians, A.; Whalen, J. J.; Weiland, J. D.; Mansfeld, F., Publisher’s Note: Electrodeposition and Characterization of Thin-Film Platinum-Iridium Alloys for Biological Interfaces [ J. Electrochem. Soc. , 158 , D269 (2011)]. Journal of The Electrochemical Society 2011, 158, S15. 26. Cui, X. T.; Zhou, D. D., Poly (3,4-Ethylenedioxythiophene) for Chronic Neural Stimulation. IEEE transactions on neural systems and rehabilitation engineering : a publication of the IEEE Engineering in Medicine and Biology Society 2007, 15, 502-8. 27. Norlin, A.; Pan, J.; Leygraf , C., Electrochemical Behavior of Stimulation⁄Sensing Materials for Pacemaker Electrode Applications: Iii. Nanoporous and Smooth Carbon Electrodes. Journal of The Electrochemical Society 2005, 152, J110-J116. 28. Weiland, J. D.; Anderson, D. J.; Humayun, M. S., In Vitro Electrical Properties for Iridium Oxide Versus Titanium Nitride Stimulating Electrodes. Biomedical Engineering, IEEE Transactions on 2002, 49, 1574-1579. 29. Wang, K.; Fishman, H. A.; Dai, H.; Harris, J. S., Neural Stimulation with a Carbon Nanotube Microelectrode Array. Nano Letters 2006, 6, 2043-2048. 30. Norlin, A.; Pan, J.; Leygraf, C., Investigation of Electrochemical Behavior of Stimulation/Sensing Materials for Pacemaker Electrode Applications: I. Pt, Ti, and Tin Coated Electrodes. Journal of The Electrochemical Society 2005, 152, J7-J15. 31. Posey, F. A.; Morozumi, T., Theory of Potentiostatic and Galvanostatic Charging of the Double Layer in Porous Electrodes. Journal of The Electrochemical Society 1966, 113, 176-184. 32. Xu, J.; Shepherd, R. K.; Millard, R. E.; Clark, G. M., Chronic Electrical Stimulation of the Auditory Nerve at High Stimulus Rates: A Physiological and Histopathological Study. Hearing Research 1997, 105, 1-29. 33. Hambrecht, F. T., Visual Prostheses Based on Direct Interfaces with the Visual System. Baillieres Clin Neurol 1995, 4, 147-65. 34. Meyer, R. D.; Cogan, S. F.; Nguyen, T. H.; Rauh, R. D., Electrodeposited Iridium Oxide for Neural Stimulation and Recording Electrodes. Neural Systems and Rehabilitation Engineering, IEEE Transactions on 2001, 9, 2-11. 35. Rui, Y.-F.; Liu, J.-Q.; Yang, B.; Li, K.-Y.; Yang, C.-S., Parylene-Based Implantable Platinum-Black Coated Wire Microelectrode for Orbicularis Oculi Muscle Electrical Stimulation. Biomedical microdevices 2012, 14, 367-373. 36. Wong, Y. T.; Chen, S. C.; Seo, J. M.; Morley, J. W.; Lovell, N. H.; Suaning, G. J., Focal Activation of the Feline Retina Via a Suprachoroidal Electrode Array. Vision research 2009, 49, 825-33. 37. McCreery, D. B.; Yuen, T. G. H.; Bullara, L. A., Chronic Microstimulation in the Feline Ventral Cochlear Nucleus: Physiologic and Histologic Effects. Hearing Research 2000, 149, 223-238.

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TABLE OF CONTENTS GRAPHIC

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Figure 1. (A) A biphasic current waveform with current amplitude (i) of 0.3 µA, 400 µs pulse

3

width and 10 µs interpulse delay. (B) Potential transient response to the current pulse, where Va

4

is the access voltage and Vt is the maximum negative voltage transient.

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Figure 2. (A) Cyclic voltammograms (0.5 V s-1, −0.2 V to 1.25 V) of Pt electrodes after

3

electrochemical roughening for different roughening times (Other conditions: reduction potential

4

= −0.15 V, oxidation potential = +2.65 V, frequency = 1 kHz, electrolyte 0.5 M H2SO4). (B)

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Surface roughness factor as a function of roughening time. Error bars are 95 % confidence

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intervals calculated from measurements of 5 independently-prepared surfaces.

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Figure 3. Representative AFM images showing the morphology and roughness of the different

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platinum surfaces. (A) smooth surface, (B) 3 min roughening and (C) 5 min roughening

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Figure 4. Plot of Cdl vs fR. Error bars ± standard error of the mean (n = 5)

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Figure 5. Bode plots of (A) impedance and (B) phase angle of smooth and roughened Pt surfaces

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in PBS over a frequency (f) range of 10-1 - 105

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Figure 6. Potential transient responses of (A) smooth Pt electrode to increasing current pulses at

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400 µs pulse width and 10 µs interpulse delay (B) smooth and roughened Pt surfaces on injecting

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a biphasic current pulse (0.3 mA, 400 µs/phase and 10 µs delay).

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Figure 7. Measured charge injection limit (Qinj) of platinum electrodes with increasing surface

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roughness (fR) at different pulse width. Error bars ± standard error of the mean of 5

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measurements taken using multiple samples

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Figure 8. Retention of electrochemical surface area of different Pt surfaces as a function of

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ultrasonication time. A0 and At are the electrochemical surface areas before and after specific

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ultrasonication time. Error bars are ± the standard error of the mean (n = 5)

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Figure 9 Interfacial polarization (Vpol) of three individual roughened platinum electrodes (fR =

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~156) over a time period of 14 days under a continuous biphasic stimulation at 1.25 mA with 0.2

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ms phase widths at 2000 Hz

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Figure 10 Cyclic voltammograms of roughened platinum electrode (fR = 156) before and after 14

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days of continuous biphasic stimulation (1.25 mA with 0.2 ms phase widths at 2000 Hz)

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Table 1. The root-mean-square roughness (RMS) values and the roughness factors (fR) of

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platinum electrodes at various roughening time Roughening time

fR

/ min

RMS roughness / nm

0

2 ± 0.2

3.1 ± 1.2

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156 ± 22

10.3 ± 1.9

5

430 ± 50

54.7 ± 9.1

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Table 2. Constant phase element (ZCPE), polarization resistance (Rp) and capacitance dispersion

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(α) values of smooth and roughened Pt surfaces obtained by fitting electrochemical impedance

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spectra to the equivalent circuit model. The standard error of the mean of each value is indicated

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in brackets (n = 4). Surface

fR

ZCPE

Rp

/ mF cm-2

/ KΩ cm2

α

Smooth Pt

2

0.19 (0.01)

14.63 (0.16)

0.872 (0.004)

Rough Pt

14

4.26 (0.42)

1.29 (0.06)

0.857 (0.003)

Rough Pt

23

4.72 (0.26)

1.25 (0.18)

0.865 (0.003)

Rough Pt

42

5.25 (0.28)

0.54 (0.03)

0.88 (0.01)

Rough Pt

96

9.67 (0.77)

0.48 (0.01)

0.884 (0.004)

Rough Pt

156

15.69 (1.37)

0.29 (0.02)

0.930 (0.004)

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Investigating the interfacial properties of electrochemically roughened platinum electrodes for neural stimulation.

Platinum electrodes have been electrochemically roughened (roughness factors up to 430) and evaluated for use as neural stimulation electrodes. The ro...
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