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Laser patterning of platinum electrodes for safe neurostimulation

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Neural Eng. 11 056017 (http://iopscience.iop.org/1741-2552/11/5/056017) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: This content was downloaded on 26/04/2017 at 12:30 Please note that terms and conditions apply.

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Journal of Neural Engineering J. Neural Eng. 11 (2014) 056017 (17pp)


Laser patterning of platinum electrodes for safe neurostimulation R A Green1, P B Matteucci1, C W D Dodds1, J Palmer1, W F Dueck2, R T Hassarati1, P J Byrnes-Preston1, N H Lovell1 and G J Suaning1 1 2

Graduate School of Biomedical Engineering, University of New South Wales, Sydney 2052, Australia Cochlear Ltd, 1 University Avenue, Macquarie University, NSW 2109, Australia

E-mail: [email protected] Received 6 February 2014, revised 30 June 2014 Accepted for publication 8 July 2014 Published 4 September 2014 Abstract

Objective. Laser surface modification of platinum (Pt) electrodes was investigated for use in neuroprosthetics. Surface modification was applied to increase the surface area of the electrode and improve its ability to transfer charge within safe electrochemical stimulation limits. Approach. Electrode arrays were laser micromachined to produce Pt electrodes with smooth surfaces, which were then modified with four laser patterning techniques to produce surface structures which were nanosecond patterned, square profile, triangular profile and roughened on the micron scale through structured laser interference patterning (SLIP). Improvements in charge transfer were shown through electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and biphasic stimulation at clinically relevant levels. A new method was investigated and validated which enabled the assessment of in vivo electrochemically safe charge injection limits. Main results. All of the modified surfaces provided electrical advantage over the smooth Pt. The SLIP surface provided the greatest benefit both in vitro and in vivo, and this surface was the only type which had injection limits above the threshold for neural stimulation, at a level shown to produce a response in the feline visual cortex when using an electrode array implanted in the suprachoroidal space of the eye. This surface was found to be stable when stimulated with more than 150 million clinically relevant pulses in physiological saline. Significance. Critical to the assessment of implant devices is accurate determination of safe usage limits in an in vivo environment. Laser patterning, in particular SLIP, is a superior technique for improving the performance of implant electrodes without altering the interfacial electrode chemistry through coating. Future work will require chronic in vivo assessment of these electrode patterns. Keywords: platinum electrodes, laser micromachining, neuroprosthesis, in vivo, injection limit, surface patterning 1. Introduction

stimulating electrodes are used to inject a charge into the extracellular space. This facilitates nerve activation as the transmembrane potential becomes depolarized [1]. However, where cellular currents are carried using ions, metallic electrodes use electrons. As a result most metallic electrodes, including platinum (Pt) the most common material used for electrostimulation, induce a change in the ionic environment predominantly by using a driving current to form a double layer of electrons and ions at the interfacial surface of the electrode [1]. Charge injection capacity is an important property to consider when determining the usefulness of an electrode. In order for stimulation to occur, a base threshold must be

Nerve cells have a polarized membrane resulting from active transport of ionic charge, which results in a cell membrane potential of around −70 mV at rest. In order for the nerve to activate, the membrane must become depolarized. This is caused by an influx of ions, generating an action potential, which in the case of nerve fibers, propagates by causing adjacent sections of the fiber to become depolarized. Electronic devices can induce a similar processby altering the extracellular potential and are used in neuroprosthetic devices to provide activation of cells where the native input has been lost as a result of disease or injury. In these devices, 1741-2560/14/056017+17$33.00


© 2014 IOP Publishing Ltd Printed in the UK

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The basis of electrochemical safety of electrodes is a range of voltage across which no irreversible chemical reactions can occur. For most electrode types including Pt, this is defined by the oxidation and reduction of water, and is consequently known as the water window. If a voltage is produced at an electrode interface that falls within this range, it is unlikely that potentially harmful chemical reactions will be initiated. However, if the voltage exceeds this window either in the negative or positive phase, hydrolysis reactions can occur and produce harmful by-products such as hydrogen gas in the negative phase, oxygen gas in the positive phase and acidic ions which can be produced in either phase from various Pt complexes [13, 14].Voltage driven dissolution of the electrode can also occur, releasing metallic ions into the tissue environment [3, 4]. It has been shown that to stimulate visual percepts in visually impaired patients using an epiretinally placed electrode, a charge density between 48 and 357 μC cm−2 is required [15, 16], but the electrochemical injection limit of Pt in vitro has been reported as ranging from 20 to 150 μC cm−2 [2, 11]. This is concerning even without the consideration that the electrochemical injection limit will be reduced when in the implant environment. At an estimate, based on the increase in electrode impedance when implanted (compared to in vitro testing in saline), the injection limit in vivo may be reduced to around 30% of the saline value in the initial stages following implantation [12]. While the impact of the in vivo environment on electrode electrochemical impedance is not well understood, it is clear that improving the charge transfer properties of Pt electrodes will be beneficial to both electrode stability and neural tissue survival during chronic stimulation. The voltage produced across a smooth metal electrode, for a particular charge density, is inversely proportional to the surface area. This relationship may vary for roughened or coated electrodes and is affected by higher charge densities at the edge of microelectrodes, but in general, larger electrodes are able to inject more charge before exceeding the electrochemically safe limits. However, larger electrodes consume greater space and as a result, limit the spatial selectivity or resolution of a device produced from such electrodes. Assuming close apposition with target neural tissue, a greater number of electrodes enables the delivery of a higher resolution signal, which is expected to translate to an improvement in sound perception for cochlear implant users and increased visual acuity for visual prosthesis recipients. Due to the space limitations within organs such as the cochlea and eye, increasing the number of electrodes without reducing the size of the electrodes is not possible. However, reducing the size of an electrode will significantly reduce the charge which can be delivered while still preventing harmful chemical reactions. An alternative way to produce smaller electrodes or conversely increase the safety margin of existing electrodes, is to increase the electrode surface area [17]. Increases in the electrode charge transfer area can be achieved through surface modification or coatings. Coating technologies such as IrOx, platinum black and conductive polymers (CPs) have been extensively studied in vitro and have been shown to greatly increase the charge injection

Figure 1. Summarized safety limits for constant current, biphasic,

charge-balanced stimulation with Pt electrodes based on publications by Robblee et al and Shannon [1–3], with no phase duration or electrolyte specific limitations. The diagonal line describes a limit above which neural tissue damage can occur. The horizontal lines represent various electrochemical limits reported from in vitro studies on Pt electrodes.

reached otherwise the nerve will not activate. However, the voltage developed from the injected charge must remain within safe limits. A common issue in the literature describing safe injection limits of implant electrodes is the assumption that platinum (Pt), an often used electrode material for tissue stimulation, has a single charge density value, below which stimulation can be considered ‘safe’. There are multiple facets to safe charge injection, including the stability of the electrode chemistry, the preservation of the surrounding tissue environment and the electrode configuration used for stimulation. Pioneering work performed by Rose, Robblee and McHardy [2–4] in electrochemical safety has yielded a series of values which are typically used to describe the electrochemically safe limits. Additional work in modelling current density by Rubinstein et al [5] and Suesserman et al [6] was used to understand how current density varied with electrode configuration. In parallel, research by McCreery [7, 8] has determined a series of values which correlate to tissue integrity under chronic stimulation. Of particular note, Shannon [9], brought the latter two areas of tissue damage and electrode configuration together to devise a mathematical relationship of the variables and relate them to the damage results. This enabled Shannon to describe a region in which safe charge density can be related to electrode geometries. However, while these three fields of research determined a basis for safe stimulation, summarized in figure 1, there is a disconnect between the electrochemical studies, performed in vitro, and the tissue damage studies which rely on empirical in vivo data, most of which relate only to cortical tissue—that is, neural tissue in immediate contact with electrode surfaces. Additionally, all of these studies are limited in that only a single phase duration has been used in any one study to establish these relationships, where studies by Green et al [10–12] have demonstrated a distinct phase dependent behavior in medical electrodes fabricated from a variety of materials. 2

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capacity of microelectrodes [12, 18–21]. However, these technologies are often limited by their mechanical properties, which have included brittleness and delamination. While platinum grey, some IrOx and CP variants have been shown to be both stable and electrically superior to Pt [12, 22, 23], unanswered questions over the long-term stability of these alternatives present a need for simple modification of Pt electrodes which will enable scaling of electrode geometries to produce high-density arrays without additional coatings. Surface modification can be achieved using electrochemical and physical means. Electrochemical methods are difficult to control, but have been shown by Tycozincksi et al to decrease the interfacial voltage of the electrode, improving the charge transfer properties [24]. Physical methods involve using a laser to etch or melt the surface of the electrode [25]. Green et al found that the real surface area of an electrode was increased by 2.6 times using an Nd:YAG laser with nanosecond pulse widths to roughen the electrode surface [11]. The safe charge injection limit was increased by 3.7 times at 0.5 ms phase length [11]. Using similar laser melt methods Schuettler found a 4.5 times increase in the surface of the electrode and a decrease in polarization voltage by 37% [21]. However, Green et al also found that the surface achieved by melt processing imparted from the relatively long pulse duration required for roughening increased surface bound oxides of Pt [25], preventing the full electrode area from being utilized to transduce charge [11]. The results from physical modification using laser techniques are promising, but it is expected that substantial improvements can be made through the use of lasers with shorter pulse widths. It is proposed that lasers which produce fast pulses and ablate the material at the surface without inducing melt processes may enable increases in safe charge injection using Pt electrodes without requiring coating technologies or altering the electrode chemistry. As such 24electrode arrays have been produced with multiple surface types using picosecond lasers, shown in figure 2. Control smooth and nanosecond laser modified electrodes were compared to three new picosecond laser patterns which produced a square profile, triangular profile and roughening on the micron scale through structured laser interference patterning (SLIP). The square and triangular patterns create predictable surface structures that can be compared with the less-predictable nanosecond laser-roughening. Additionally, the differences between square and triangular structuring provides a means of comparing known patterns with each other, enabling insight to how the geometric surface area relates to the charge limitations. The SLIP pattern combines both a geometric laser path, being a scaled down version of the triangular pattern, but combines this with a plasmon interference technique to generate a surface with higher roughness. These electrodes were characterized using a variety of electroanalytical techniques to determine which of the surface types affords the greatest improvement in electrical properties for application in neuroprosthetic devices. An important component of this research was to investigate in vivo assessment of electrode charge injection limits, to

Figure 2. Laser micromachined 24 electrode array in hexagonal

(hex) configuration. In the center row the electrodes, from the left, are: nanosecond, smooth, square patterned, smooth, SLIP and triangular patterned.

enable an in situ assessment of the suitability of Pt electrodes for delivering threshold level stimulations to neural tissue.

2. Methods 2.1. Fabrication of electrode arrays

Electrode arrays were fabricated as per previous publications [26, 27]. Briefly, electrodes were fabricated from laser micromachined Pt foils insulated in poly(dimethyl siloxane) (PDMS). The PDMS was spun onto a supporting microscope slide with a polyimide tape release layer. Following curing of the lower PDMS layer a Pt foil was laminated onto the slide and the Pt tracks and electrode sites were structured. This CAD guided laser micromachining process was designed to achieve electrode sites having a nominal diameter of 380 μm. An overlying PDMS layer was spun onto the supporting slide and cured to insulate the whole structure. Opening of the PDMS above the electrode sites was performed using an excimer laser (Atlex 300 SI, ATL, Germany) with an ArF gas mixture to produce a 193 nm beam. The voltage was set to 15 kV and the frequency to 100 Hz. The beam was passed through a mask to irradiate a 50 μm circle on the target area of PDMS. The irradiated circle was directed in a circular path 335 μm in diameter at 50 μm s−1 to ablate the outline of the electrode site. This was repeated until the ablation depth was sufficient to expose the underlying Pt foil (typically 3–4 passes). The central disc of silicone was then removed manually. Nanosecond Nd:YAG laser (GenesisMarker, ACI, Germany) roughening was performed using a 1064 nm beam with a spot size of 25 μm. Laser power was set to 12% with a pulse repetition frequency of 700 Hz and exposure duration of 1 μs. The pulse width was approximately 15 ns. The beam was deflected at a speed of 5 mm s−1 across the electrode surface in a series of parallel lines spaced 30 μm apart. 3

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spectroscopy (XPS). Additionally, optical profilometry was used to quantify the surface roughness. Samples were visualized using SEM at 10 kV and 270 x magnification (JEOL Neoscope). XPS was carried out (Kratos XSAM800 XPS, Japan) and peaks were analysed to determine the elemental constitution of the electrode surfaces following laser patterning. Curve fitting was performed (Eclipse Datasystem V2.1, USA) and where curves were found to contain more than one chemical state of a given species, the PeakFit function was used to elicit the different chemical states. The intensity ratio of the doublet was maintained at 2:1 and the energy separation was maintained at 1.18 eV. A GTK1-M Contour light interference profilometer (Bruker, USA) with 20 x magnification and Vision 64 software was used to generate a surface profile. The data was masked to eliminate the insulation material and enable calculation of the electrode site absolute surface area.

Figure 3. Laser scanning patterns for structuring Pt with (A) square pattern and (B) triangular pattern.

Three roughening patterns were produced using a picosecond, mode-locked laser (Duetto, Time-BandWidth, Switzerland). The laser was fitted with a second harmonic generator (SHG) to produce a 532 nm beam with a spot size of 11 μm. For all patterns, a pulse repetition frequency of 340 kHz was used and the pulse width was approximately 11 ps. A square hatch pattern, depicted in figure 3(a), was produced by scanning the beam across the electrode surface at 250 mm s−1 in a series of parallel lines spaced 20 μm apart, then repeating the process with a second series of lines arranged at 90° to the first. A triangular hatch pattern, figure 3(b), was produced in a similar method, but using three series of lines spaced 60° apart. For both of these patterns, laser power was set at 80% of maximum (12 W). For the square pattern, eight repetitions were required to achieve the desired depth. For the triangular pattern, five repetitions were needed. When ablating metals with ultra-short pulsed lasers, ripples can be seen along the edge of the ablated area, with a spatial frequency near to the wavelength of the laser [28]. A contributing factor to the creation of these ripples is the interference between the incident light and induced surface plasmons propagating across the metal surface, an effect known as surface plasmon interference (SPI) [28, 29]. To produce the third structured laser interference pattern (SLIP) electrodes, the contribution of SPI to the surface morphology is maximized by reducing the laser power down to a point where ablation only occurs at points of constructive interference. With sustained irradiation, the surface continues to change from the initial rippled pattern and a pitted or porous surface structure can be achieved. This was performed by decreasing the laser power to 32% while increasing the number of repetitions to 1000. The scanning pattern was similar to the triangular pattern outlined above, but with the line spacing reduced to 8 μm to produce overlap between adjacent tracks, and ensure that the entire surface was irradiated. Post-fabrication the arrays were wiped with 80% ethanol in DI water using a clean room, particulate free cloth. Electrode arrays were visualized under scanning electron microscopy (SEM) and chemically assessed by x-ray photoelectron

2.2. Baseline electrical characterization

A three-electrode cell was used to perform cyclic voltammetry (CV) and ascertain changes in charge storage capacity (CSC). Testing was performed using an eDAQ potentiostat in combination with eChem software (eDAQ Pty, Australia). The linear voltage sweep was set to range within the limits of the water window, between −0.6 and 0.8 V versus an isolated Ag/AgCl reference with a large, low impedance Pt counter electrode. The electrolyte was Dulbecco’s phosphate buffered saline (DPBS). A sweep rate of 150 mV s−1 was applied for a total of 50 cycles, before CSC was calculated by integrating the current response with respect to time. Four electrodes of each surface type were constructed on each array and three arrays were used in total. All arrays were submersed in 80% ethanol for 20 mins prior to electrical studies. Electrochemical impedance spectroscopy (EIS) was performed on an eDAQ system coupled with the ZMan software (eDAQ Pty, Australia) in the same three-electrode cell used for CV. Samples were analyzed by application of a 50 mV sinusoid across a range of frequencies tested from 1 Hz to 100 kHz. Bode plots were produced of the impedance magnitude and phase response. 2.3. In vitro electrochemical charge injection limit

Many neuroprosthetic devices use current-controlled biphasic pulses to stimulate neural tissue. In the resulting potential transient produced from biphasic stimulation, the Va (access voltage) is the instantaneous voltage change when a biphasic pulse is applied or removed. The Emc (maximum cathodic voltage) is found at the termination of this voltage, where the transient potential begins to decay asymptotically towards the open circuit voltage in the absence of active electrode shorting or current reversal, shown in figure 4. The electrochemical limit used to define safe charge injection for cathodic first, biphasic current injection, is when Emc is at the reduction potential for water. This is nominally at −0.83 V versus the standard hydrogen electrode, or as more often reported −0.61 V versus Ag/AgCl. In this study the charge injection 4

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guidelines for animal experimentation. Adult cats (n = 3) were anesthetized and implanted with electrode arrays in the suprachoroidal space as detailed previously in Wong et al [32]. Potential transients were recorded to confirm electrodes were functioning and in contact with the tissue. The in vivo charge injection limit was determined using adjacent electrodes on the array as the reference electrode, as specified above, with the potential for water reduction taken as −0.8 V. Studies by Hibbert et al suggest that the proteins present in the biological environment do not shift the water window, but rather reduce the impact of chemical response by slowing the reduction/oxidation reaction [33]. A large, low impedance Pt counter electrode (>3 mm2 ball electrode) was placed subconjunctivally, within the orbit. Measurements were made in conjunction with other visual percept investigations, presented elsewhere [34, 35], and hence these studies were performed at 36 h post implantation.

Figure 4. Schematic of biphasic stimulus potential transient. The

metrics used in charge injection limit assessment include Va (the access voltage) and Emc (the maximum cathodic voltage) found at the termination of Va. The total voltage across the cathodic phase is Vt, which is used in latter studies on electrode stability.

2.5. Electrode stability

The stability of electrode surfaces was explored through electrical ageing with 150 million continuous stimulations applied versus a monopolar return bathed in DPBS. A custom 24-channel stimulator was used to apply sequential 0.4 ms, 250 μA pulses [36, 37] to 15 electrodes on each array. These parameters were determined in previous in vivo studies to elicit a cortical response in the feline visual cortex [12, 32]. The inter-phase delay was 20 μs and inter-stimulus delay was 0.5 ms, with each electrode being sequentially stimulated.The arrays were maintained at 37 °C for 35 days. The voltage transient was captured daily on an isolated oscilloscope (Tektronix, USA) and the total cathodic voltage (Vt in figure 4) across the electrode interface was used as the primary metric. The DPBS was refreshed routinely to avoid any increase in ion concentration due to evaporation. All arrays were checked daily for shorting by contact testing with a multi-meter and also through analysis of the biphasic potential transient for changes which suggested a sudden increase in available surface area consistent with a short. No shorts were found during the duration of these studies. SEM imaging and quantitative optical profilometry was performed before and after the study to compare surface profiles following stimulation.

limit was determined in two ways: firstly using a conventional isolated Ag/AgCl reference electrode, and then repeated using a Pt microelectrode which was part of the array and adjacent to the test electrode as the reference. This second technique was developed to enable in vivo assessment of electrochemical charge injection limits. Pt electrodes have been shown to be suitable as reference electrodes for assessing the water window [30], demonstrating stability across the relevant temperature range with repeatable hysteresis [31]. In vitro CV (from −1.5 V to 1.5 V at 50 mV s−1 in DPBS) was used to determine the difference in the water window resulting from the change in reference electrode, see figure 5. The counter electrode in both arrangements was a large, low impedance Pt electrode (9 mm2) placed at a distance more than ten times that of the displacement between the reference and working electrode, to eliminate contamination of the reference recording. These two reference electrode arrangements were then used to determine the in vitro electrochemical charge injection limits resulting from biphasic stimuli, to further assess the accuracy of this technique prior to application in vivo. In vitro studies were performed in DPBS with a custom-built biphasic stimulator which delivered current-controlled chargebalanced biphasic pulses. The interphase delay was set at 0.01 ms and limits were assessed for phase durations ranging from 0.1–0.8 ms in 0.1 ms increments. The current was initially applied at 10 μA and was automatically increased in 1 μA increments using in-house software until Emc reached the −0.6 V and −0.8 V for the Ag/AgCl and Pt references, respectively.

3. Results 3.1. Fabrication of electrode arrays

The electrodes were produced such that each surface type was represented on a single electrode array, with even distribution of each surface type across the array. This was designed such that there was no spatial bias across the electrode array when implanted, which could lead to different surfaces being subjected to lower or higher tissue impedances related to local anatomic geographical variability and other dynamics such as clotting and inflammation. The SEM images pictured in figure 6 show the different structures which have been

2.4. In vivo electrochemical charge injection limit

All experiments were conducted as a sub-project within a larger study with prior approval from the Animal Care and Ethics Committee at the University of New South Wales, and in accordance with the Australian Research Council 5

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Figure 5. Cyclic voltammetry of Pt electrodes using a Ag/AgCl reference electrode (dashed line) and a Pt reference electrode (solid line)

assessed in DPBS. The dotted lines show the shift in the water window associated with water reduction and hydrogen gas production.

Figure 6. Light and SEM images showing the representative surfaces for each laser pattern on the Pt microelectrodes, being (A) smooth, (B) nanosecond laser roughened, (C) square, (D) triangular and (E) SLIP.

produced at the electrode surface. The nanosecond laser roughened electrode surface was the most difficult to produce as alignment between the two lasers was prone to errors which led to an off-centered pattern. As such this pattern was confined to the central region of the electrode, with some

smooth areas in the border region. The effects of surface roughening with this technique would accordingly be less prominent than in previous studies [11, 21]. XPS analysis of the Pt electrode surfaces confirmed that nanosecond roughening imparted greater oxide content than 6

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Table 1. XPS analysis of electrode surface composition focusing on carbon associated with oxide layer formation, oxygen and platinum peaks. Values represent average % of surface composition.

Binding energy and chemistry Element

Peak (eV)

Pt4f7 A 71.2 Pt4f7 C 72.6 Pt4f7 B 74.6 C1s A 284.3 C1s B 286.5 C1s C 288.0 C1s D 288.6 N1s A 398.2 N1s B 400.2 O1s A 531.1 O1s B 533 Ratio of total Pt/O Ratio of total Pt/PtO

Peak area (% of total composition)

Relevant chemistry






Pure Pt PtO Highly oxidized PtO Carbon associated with bound oxygen

17.11 6.3 1.94 29.96 11.08 3.85 3.86 1.29 6.56 14 4.04 1.41 2.08

8.82a 5.14 1.49 46.25a 19.38 1.38 0.88 1.14 0.87 8.43 6.22a 1.05 1.33

20.5 9.97 2.65 34.43 9.08 2.91 1.56 2.46 1.84 10.22 4.39 2.27 1.62

16.28 7.63 2.06 30.11 12.19 5.65 1.68 2.14 4.92 13.89 3.45 1.50 1.68

23.37 8.59 2.86 36.73 3.99 3.67 1.06 2.58 2.41 11.35 3.39 2.36 2.04

O bound as Pt(OH)2 O bound as PtO2


Note that the nanosecond surface has a significantly lower pure Pt signal (p < 0.05) combined with an increase in both C1s and O1s associated with bound PtO.

the picosecond laser patterns, as shown in table 1. The increased oxygen incorporation is indicated by a decreased signature related to the presence of pure Pt at the 71.2 eV peak, combined with an oxygen peak increase associated with PtO2 formation at 533 eV [38, 39] compared to untreated controls and picosecond patterned surfaces. Supporting this is associated carbon binding which is known to increase due to oxide formation, shown via the peak at 284.3 eV [39]. The percentage content of Pt and O detected at each surface is difficult to directly compare due to the semi-quantitative nature of XPS and the presence of contaminating elements, including nitrogen and additional carbon species. As such two ratios were calculated in table 1 to compare the total detected amount of Pt/O and also Pt/PtO. These results clearly show that the nanosecond roughening has the lowest Pt/O ratio at 1.05 and the lowest Pt/PtO ratio at 1.33, or alternately the lowest amount of pure Pt. Interestingly, the SLIP electrodes have the highest Pt/O ratio (2.36), a level which is also significantly higher than that of the untreated smooth Pt (1.41). However, both the smooth and SLIP electrodes have similar ratios of Pt/PtO at ∼2.0. The optical profilometry measurements yielded a surface index (SI) which indicate the SLIP electrodes have the highest surface area with an SI of 2.9. For these electrodes with a nominal geometric area of 0.11 mm2, the SLIP electrodes have a real surface area of 0.33 mm2. Similarly the nanosecond laser roughened electrodes have a real surface area of 0.26 mm2, the square electrodes are 0.25 mm2and the triangle electrodes are 0.21 mm2. The smooth electrodes were also not perfectly flat, having a SI of 1.15 and a real surface area of 0.13 mm2. The surface index is plotted for each surface type relative to the CSC in figure 7.

demonstrated that there was an increase in CSC, plotted in figure 8, related to the increased electrode surface area. This is shown by the change in CSC from 9.7 mC cm−2 for the smooth surface to 13.5 mC cm−2 for the nanosecond laser roughened electrodes, 14.1 mC cm−2 for the triangular and 17.8 mC cm−2 for the square patterned electrodes. The SLIP technique imparted the greatest increase with a total CSC of 23.0 mC cm−2, being 2.3 times greater than the smooth electrodes. EIS was performed to determine the opposition of a surface to the flow of charge. The impedance response is shown as a Bode plot in figure 9. As expected, at all frequencies modified surfaces imparted a decrease in impedance magnitude. Even at high frequency (10–100 kHz) the impedance magnitude of the smooth surface is almost double that of the SLIP surface. The SLIP surface also presents with a distinct phase shift, which reduces phase lag in comparison to smooth electrodes at frequencies greater than 10 Hz. The nanosecond laser roughened, square and triangular structured electrodes present with very similar impedance responses which reflect their similarity in surface area. 3.3. Charge injection limits

The electrochemical charge injection limit is rarely assessed in the implant environment due to the limitations in forming a three-electrode cell which can adequately assess voltage transients on implanted electrode arrays. In this studyvalidation was obtained in vitro of a three -electrode set up which makes use of the adjacent electrodes on the array as reference electrodes, to facilitate assessment of in vivo injection limits. The comparison of the two methods is presented in figure 10 for two types of electrode surfaces, being the conventional smooth electrodes and the square patterned electrodes. It can be easily seen that the two methods are comparable with the maximum deviation between the two metrics being 9% and the average deviation being 4%, with

3.2. Baseline electrical properties

CV clearly depicted the increasing CSC for patterned Pt relative to the unmodified smooth Pt, shown in figure 7. It was 7

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Figure 7. Example of the typical CV response recorded for each of the Pt surface patterns, in DPBS with large Pt counter and Ag/AgCl

reference electrode.

Figure 8. Charge storage capacity versus surface index of microelectrodes with laser roughened surfaces (n = 12). A linear relationship is

clear for all surfaces except the nanosecond roughened electrodes.

no statistically significant difference across the phase duration spectrum. For simplicity, in vitro electrode injection limits for all arrays are shown in comparison to the in vivo values in figure 11. The different electrode surfaces were shown to produce electrochemical injection limits which concurred with other electrochemical metrics. As expected all surfaces

presented a phase duration dependent result with the injection limit increasing with the phase duration. The SLIP surface produced injection limits in saline which ranged from 130–364 μC cm−2 across the 0.1–0.8 ms phase duration range, compared to the smooth electrodes which varied from 58–98 μC cm−2. The nanosecond laser roughened and square patterned arrays had very similar results of 100–197 μC cm−2 8

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Figure 9. Bode plot of impedance behavior of Pt electrodes with varied surface structure (n = 12).

and119–235 μC cm−2, respectively. The triangular pattern had lower limits ranging from 69–160 μC cm−2 across the same phase durations.

performance across the range of phase durations was consistent with the in vitro data, but with a significant reduction in total charge transduced before reaching the electrochemical potential for water reduction. The percentage reduction in injection limit from in vitro to in vivo presented in table 2, was consistent across all phase durations. The nanosecond laser roughened surface experienced the highest drop in charge injection limit of 63.8 ± 1.9%, yielding a limit range of 33–68 μC cm−2 followed by the SLIP surface with 57.1 ± 3.2% or 51–144 μC cm−2. Surprisingly, the triangular patterned surface experienced the least reduction with a loss

3.4. In vivo electrode characterization

In the feline model in vivo electrochemical charge injection limits were obtained through utilizing adjacent electrodes within the array as reference electrodes. The in vivo charge injection was compared to values obtained in physiological saline in figure 11. The relative relationships of electrode 9

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Figure 10. In vitro charge injection limit of smooth and square patterned microelectrodes, determined using an adjacent Pt electrode as a reference compared to using a conventional isolated Ag/AgCl reference electrode (n = 4).

the animal model. The potential transient at the end of the cathodic phase, (Vt in figure 4) was plotted in figure 13 for one array with three electrodes of each surface type. The SLIP electrodes consistently experienced the lowest end of phasevoltage, ranging from 0.54–0.82 V. While there was more variability on the triangular electrodes, the voltage was generally below 2.0 V. The square patterned, nanosecond laser roughened and smooth electrodes all experienced considerable variability in voltage transient across the study period, with the end of phase voltage ranging from 1.0–14.0 V. Across these electrode types the end of cathodic phase voltage more than doubled from the starting voltage at repeated time points throughout the study. It was observed that gassing occurred on these electrode types, resulting in the formation of bubbles (see figure 14), which corresponded with open circuit voltage recordings (14 V device compliance). It was however, noted that the bubble formation also protected the electrode from dissolution, and agitation of the electrolyte to remove the bubbles returned the voltage transient to a normal magnitude which was typical for each surface type. Figure 15 clearly shows the stability of the SLIP and triangular electrodes across the study period. These two surface patterns produced the only electrodes with potential transient measurements which showed a normal distribution across the arrays being studied. This was mainly due to the lack of gassing which caused intermittent high voltage on other electrodes types. In figure 15 it is also shown that over the 150 million stimulations applied in this study there was a small increase in potential. The linear fit of this data clarifies that the gradient of this line for both electrode types was on the order of 1−10 V/stimulation. Assuming this linear trend continues, this translates to a potential increase of 0.1 V per billion stimulations. Statistical analysis comparing the SLIP and triangular surface demonstrated that across the stimulation time period the SLIP electrodes performed significantly better than the triangular electrodes, having a consistently

Table 2. Average drop in charge injection (CI) limit from in vitro to

in vivo, (n = 12). Surface morphology Smooth Nanosecond Square Triangular SLIP

% Drop in CI limit


52.0 63.8 53.1 40.4 57.1

±3.2 ±1.9 ±3.4 ±3.3 ±3.2

of only 40.4 ± 3.3% (35–96 μC cm−2), which is 12% better than smooth electrodes with a charge injection limit of 26–44 μC cm−2 across the phase range of 0.1–0.8 ms. The in vivo injection limit results are plotted in figure 12 alongside threshold values which have been reported in the literature to elicit visual percepts in both animal models and human patients. In the feline model it is clear that the smooth electrodes experience electrochemical limits at values lower than the threshold for visual cortex activation (∼90 μC cm−2), but the SLIP and square patterned electrodes have injection limits which supersede this value at longer phase durations. The SLIP electrodes have an electrochemical injection limit higher than threshold at 0.2 ms and above, where the square electrodes have an injection limit above threshold from 0.5 ms onwards. Of ten patient visual percept thresholds reported in literature [15, 16, 40, 41], only two are within the electrochemical limits of smooth Pt electrodes. It should be noted that the patients in these studies have either electrodes with coatings which enable greater injection capacity [23] or have received only minimal stimulation as part of a short term clinical trial.

3.5. Electrode stability

All electrodes were subjected to 150 million stimulations at levels which were shown to elicit a visual cortex response in 10

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Figure 11. Electrochemically safe charge injection limits determined in saline in vitro compared to those obtained in vivo for Pt electrodes

with surface patterning (n = 12).

Figure 12. In vivo electrochemical charge injection limits plotted with respect to visual percept thresholds obtained from retinal stimulation in both the feline model with suprachoroidal device placement and human patients with epiretinal or subretinal device placement. The diameter of electrodes used in the various human trials is also noted. Feline data was previously reported in [4]. Human data was obtained from publications [5–8].

lower end of phase potential (paired t-test with significance at p < 0.05). The SEM images support the potential transient data, showing that SLIP and triangular patterned electrodes were stable under continuous stimulation. The square electrodes which experienced large increases in voltage were found to have significant surface dissolution as shown in figure 16(C),

which compared Day 1, Day 18 and Day 35. The nanosecond patterned electrodes showed similar loss of material with flattening of some areas of the electrodes (figure16(D)). The smooth electrodes had some evidence of surface pitting, but qualitative observations by SEM do not show definitive dissolution. As a result electrodes were also analyzed by optical profilometry, with results presented in table 3. These results 11

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Figure 13. End of phase one (cathodic phase) voltage on electrodes under continuous biphasic stimulation for 150 million pulses, n = 3. The variation in the voltage is an indirect measure of the stability of the electrode impedance. Results are shown for one array, but the experiment was repeated across four arrays with repeatable results.

confirm not only the significant loss of material from both the nanosecond and square patterned electrodes, but also the increase surface area consistent with pitting of the smooth electrodes. The triangular and SLIP electrodes both experienced a small average loss in surface area, but these were not statistically significant losses. It is important to note that electrodes on which bubbles formed presented as an open circuit and were inherently protected by that bubble from further damage by dissolution. Ultimately, these open circuit electrodes were subjected to fewer stimuli than the electrodes on which bubbles did not form. Repeat studies on three electrode arrays were performed in a stirred medium to simulate shear forces, but recurrent bubble formation continued to hamper attempts to elicit thresholds for electrode dissolution. All arrays (a total of 12 electrodes of each surface type, three on each array) experienced similar results across the studies with gassing and doubling of the voltage experienced by 100% of smooth electrodes, 11% of triangular, 55% of square and 67% of nanosecond laser roughened electrodes under stimulation conditions. Neither gassing nor significant increases in end of phase voltage were experienced by the SLIP electrodes.

Figure 14. Sample image of gas bubbles present on array. These bubbles indicate a breach in the electrochemical charge injection limit for E6 (smooth), E8 (nanosecond) and E15 (nanosecond) on this array.


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Figure 15. Plot of average end of cathodic phase voltage for the triangular patterned and SLIP electrodes, showing stable performance across

the study period with significantly lower impedance for the SLIP electrodes compared to the triangular electrodes. Error bars are 1 SD (* p < 0.05; n = 3).

4. Discussion

which utilized only 69% of the real surface area for charge transfer. This is clearly shown in figure 8, where all laser patterns with the exception of the nanosecond pattern have a linear relationship between surface index and CSC. Some clarity can be gained by considering the XPS results which suggest that all Pt surfaces, including the smooth electrodes contain oxygen bound Pt complexes. This is not unexpected as Pt is known to experience some chemisorption of oxygen under passive conditions [42]. It is clear from these results that the nanosecond laser roughening significantly reduces the Pt available for charge transfer, with an almost equal content of Pt and O available at the electrode surface. What is more interesting is that the oxide content of the SLIP electrodes appears to be lower than that of the smooth untreated electrodes. It is proposed that the picosecond ablation technique removes the passivating layer. With reduced oxide content the SLIP electrodes should have a greater surface area available for capacitive charge transfer in addition to the geometric increase imparted by roughening, and lower CV peaks associated with Faradaic reactions due to oxygen and Pt chemical interactions. In these studies it is difficult to determine the contribution of chemistry isolated from the surface area and future studies will assess picosecond laser treatment of smooth electrodes without surface structuring. These findings concur with previous studies which indicated nanosecond laser melt processes incorporate covalently bound oxides within the upper layer of the Pt, preventing efficient double layer charging at these sites [11, 25]. It also supports the hypothesis that shorter picosecond pulse lasers can ablate the Pt without imparting substantial chemical changes to the electrodes which can block charge transfer.

The electrical properties of Pt electrodes are critical to the chronic performance of neuroprosthetic devices. In particular appropriate characterization of electrodes and novel surface profiles are required to assess long-term performance and modes of failure. The most important property is the charge injection limit, and assessment in situ can enable the accurate definition of electrochemical safety limits. In this study it was shown that SLIP of Pt electrodes using picosecond ablation of surface material resulted in an efficient, high charge transfer surface area, which could stimulate tissue at neural activation threshold levels. Key to this study was the demonstration of in vivo charge injection limits, which show the potential discrepancy between current in vitro methods of electrode characterization and the resulting in vivo performance requirements. Of the four patterning techniques SLIP produced the highest increase in surface area, being 2.9 x greater than the nominal geometric surface area. CV was used to determine the baseline CSC produced from these electrodes. Assuming that the surface patterning manipulated the Pt without changing the surface chemistry, it would be expected that the increase in CSC directly correlated with the increase in surface area. However, when the CSC was compared to the surface area, a ratio of CSC/SI expressed as a percentage of the smooth electrode control, it was shown that the laser patterning did affect the available charge transfer area. The triangular pattern used 91% of the increased surface area, the square pattern used 96% and the SLIP technique used 93%. While all picosecond laser patterns experienced significantly higher relative charge transfer than the nanosecond patterning 13

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Figure 16. Surface stability of electrodes across 150 million clinically relevant stimulations delivered over 35 days, with (A) SLIP, (B) triangular, (C) square, (D) nanosecond patterning and (E) smooth Pt electrodes, shown at day 1, day 18 and day 35. Damaged areas are shown in red circles.


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Table 3. Change in surface index for electrodes over 150 million stimulations at 250 μA, 0.4 ms charge balance biphasic stimuli in saline.

Surface index

Before stimulation (Mean +/− SD) After stimulation (Mean +/− SD) % Change






1.15 +/− 0.02 1.26 +/− 0.08 ↑ 9.3%

2.31 +/− 0.16 1.72 +/− 0.10 ↓ 25.5%

1.83 +/− 0.07 1.76 +/− 0.17 ↓ 3.6%

2.20 +/− 0.09 1.66 +/− 0.08 ↓ 24.5%

2.93 +/− 0.17 2.82 +/− 0.11 ↓ 3.9%

The EIS data agreed with the voltammetry findings which suggest the SLIP electrodes provided the most efficient charge transfer and the lowest impedance across all stimulation frequencies. It has been shown that short-burst pulse trains can activate more defined neuronal cell populations and may be an important development in the design of visual prostheses which utilize both on and off retinal ganglion cell (RGC) types [43]. The frequencies of interest range from 10–700 Hz, and across these frequencies the SLIP electrodes provided an 86–82% reduction in impedance. The other surface patterns had comparable impedance spectra, with no statistically significant difference between pattern types, yielding an average 56–66% reduction in impedance across the same frequency range compared to smooth electrodes. The phase lag on the SLIP electrodes is also notable, having a significant shift towards the lower frequencies compared to the other electrode surfaces. Electrochemical analysis of roughened surfaces reported by Pajkossy [44] suggest that these phase shifts are the result of capacitance dispersion. As surfaces are roughened and transition from relatively simple crystalline structures, they develop inhomogeneity in their surface energy, due to the presence of multiple crystalline faces, high density edge distribution, kink sites, and dislocations [44]. This changes the distribution of activation energy and hence the binding efficiency of ions involved in the double layer. As a result a phase shift occurs which can be modelled as a constant phase element [45]. This only occurs on the SLIP electrodes and suggests that the larger geometric structures of the square, triangle and nanosecond laser patterns retain a similar surface energy to the smooth Pt electrodes. Klauke et al [41] have shown that the threshold for stimulation of visual percepts using a retinal implant is not uniquely determined by charge density. For a given charge density, whether a current pulse elicits a percept depends strongly on pulse duration [41]. It is therefore important that electrode arrays intended for use in neuroprosthetic implants are electrically characterized across a wide range of pulse widths. Historically, the literature which seeks to establish electrochemical injection limits for neuroprosthetic electrodes has focused on single or only a few discreet phase widths, commonly 0.2 ms [2, 46] however, as shown in many of the human trials, longer pulse widths, ranging from 0.4 ms to 1 ms, are often required to elicit a visual percept in a blind patient [15, 16]. More importantly, due to patient disease variability, the role of proteins and impact of inflammatory reactions, the ability to assess charge injection limit in situ is critical. The method described in this paper is proposed to

facilitate the assessment of electrochemically safe charge injection limits in situ without the need for additional electrodes. When the adjacent Pt electrode is used as a reference electrode instead of a typical Ag/AgCl electrode, there are some differences that merit consideration. The Pt reference used for in vivo assessment of charge injection limit was smaller than that used by Kasem and Jones to demonstrated the utility of Pt as a reference electrode [31], but similar to that used by O’Mahony et al [30]. Comparison of the Pt reference and Ag/AgCl reference in these studies, shown in figure 5, clearly depicts a shift in the CV signature, but the shape of the response curve is not substantially different for either electrode or plots previously published in literature [31, 47, 48]. The only area where the response of the two reference electrodes is apparently different is in the region where oxidation of Pt occurs. In this part of the CV plot an additional peak appears when Pt is used as the reference. This peak coincides with an oxidation peak commonly shown for CV conducted in acid electrolytes or on Pt with modified surfaces [48, 49]. While this suggests that the Pt reference has a slightly difference response at these positive voltages and may be more strongly influenced by dissolved oxygen, it is more important to note that the CV signature is the same for both reference electrodes in the region of interest for assessing charge injection limits where water is reduced. Future studies will seek to provide further validation for using adjacent Pt electrodes as reference electrodes when assessing in vivo electrochemical charge injection limits.This will be conducted through comparison to measurements made via a salt bridge connected reference electrode at the cortex, using methods published by Cogan et al [50]. It is recognized that further studies are required to ensure that stable measurements can be obtained in vivo over chronic implant periods using this electrode arrangement. However, this method has the potential to enable the safety of electrodes to be directly compared to the stimulation thresholds required to elicit a biological response. It may also provide insight into the factors which effect electrode dissolution in vivo such as confined space creating a saturated micro-environment, the presence of additional redox species, as well as buffer mechanisms which have the capacity to limit the evolution of toxic by-products. In these studies, no Pt electrode could support safe stimulation of the feline retina across the entire phase range tested, from 0.1–0.8 ms, however, the SLIP electrodes were capable of safely delivering the charge required for stimulation when the phase width was above 0.2 ms. When compared to the threshold required for 15

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stimulation of the diseased human retina, only 50% of the patient thresholds reported in literature [15, 16, 40, 41] are lower than the in vivo limit measured in the feline model for the SLIP electrodes. However, it is likely that the chronic biological environment of the diseased human retina will be significantly different to that of an acute feline model. This further highlights the importance of being able to adequately assess in situ electrode safety and understand the mechanisms which lead to tissue or electrode damage. One concern related to the use of roughened electrodes is the stability of these surfaces. It has been proposed that sharp points or the edges of pores where charge density is higher may be prone to dissolution as a result of a localized increase in voltage [51]. It was shown in these studies that the SLIP and triangular patterns did not experience significant variation in surface features or electrical performance under continuous stimulation with biphasic pulses delivered at visual percept threshold levels. When considering this relative to the charge injection limits (in vitro, conducted in saline to be comparable to the test conditions), it was expected that the nanosecond, square, triangular and SLIP electrodes would tolerate this level of chronic stimulation. However, both the square and nanosecond laser roughened electrodes presented with high voltage fluctuations and surface damage. While it has been claimed that the electrochemical charge injection limit is conservative [9, 22, 52], these results suggest otherwise. The SEM of the square electrodes showed an apparent smoothing of the electrode at the outer border region, but the electrode dissolution is not specific to the pattern edges or pores. Similarly, the damage on the nanosecond laser electrodes was in patches, which suggest that the dissolution occurred as a result of a localized defect or gassing. It is predicted that the observed gassing which occurred on these electrodes and raised the interfacial voltage, blocked a substantial portion of the electrode surface. However, where small areas were still exposed to the saline, higher current densities may have occurred on localized areas or patches of the surface, resulting in irreversible chemical reactions and subsequent material loss. It is important to note that these studies were not performed in a relevant biological fluid, but rather physiological saline. As a result, it is difficult to know whether dissolution would be inhibited by protein blocking as suggested in studies by Donaldson and Donaldson [53] and Robblee et al [3]. The mechanisms which cause electrode dissolution in vivo are not fully understood. An in vivo study on cat cortex showed initial Pt dissolution at 100 μC cm−2, but after 9 h further dissolution did not occur [4]. Future studies will seek to establish the critical protein interactions which effect Pt electrode dissolution and their relative impact on roughened or patterned electrodes.

was shown through impedance spectroscopy, CV and biphasic stimulation at clinically relevant levels. A new method was investigated which enabled in vivo assessment of electrochemical charge injection limits. All of the modified surfaces provided advantage over the conventional smooth Pt electrodes, with the SLIP electrodes demonstrating the highest electrochemically safe charge injection limit, being almost three times larger than that of smooth Pt. It was demonstrated that electrode modification was necessary to achieve electrochemically safe charge injection in vivo, above the threshold for neural stimulation in the feline model. The SLIP and triangular patterned electrodes were stable both electrically and physically over 150 million stimulations at clinically relevant levels. Chronic in vivo studies will be an important component of future work to establish the utility of these patterning techniques in the stimulated implant environment.

Acknowledgements The authors acknowledge partial funding from Bionic Vision Australia, a Special Research Initiative of the Australian Research Council.

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5. Conclusions Pt electrodes were patterned through picosecond laser micromachining to produce implant electrodes with increased surface area, without significantly effecting the material composition. Improvement in the capacity for charge transfer 16

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Laser patterning of platinum electrodes for safe neurostimulation.

Laser surface modification of platinum (Pt) electrodes was investigated for use in neuroprosthetics. Surface modification was applied to increase the ...
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