Bio-Medical Materials and Engineering 24 (2014) 1827–1835 DOI 10.3233/BME-140993 IOS Press

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Structural characterization of platinum foil for neural stimulating electrodes Polona Peˇclin a , Milan Bizjak b , Samo Ribariˇc c and Janez Rozman a,∗ a

Centre for Implantable Technology and Sensors, ITIS d. o. o. Ljubljana, Ljubljana, Republic of Slovenia b Department of Materials and Metallurgy, Faculty of Natural Sciences and Engineering, University of Ljubljana, Ljubljana, Republic of Slovenia c Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Ljubljana, Republic of Slovenia Received 26 February 2013 Accepted 26 March 2014 Abstract. OBJECTIVE: The objective of this study was to investigate the structural properties of a cold-rolled platinum foil used to manufacture multi-electrode spiral nerve cuffs. METHODS: To attain this objective, 0.03-mm-thick cold-rolled platinum foil strips with 99.99 wt% purity were used. The resistivity measurements were made using a 4-point probe technique in which the strips were subjected to dynamic annealing in an argon atmosphere. The stored energy of platinum was recorded in an argon atmosphere using differential scanning calorimetry (DSC). Finally, the microstructure of the strips was investigated by optical microscopy. RESULTS: In the resistivity measurements, a small change is observed at ∼280◦ C. This change could be explained as the partial recovery elicited by the decrease of dislocation density. Above 500◦ C, a significant decrease in resistivity was recorded, and the decrease reached a maximum at ∼750◦ C. These results are consistent with the recrystallization trend detected in DSC, namely the DSC measurement detected very weak heat release during recrystallization, which was actually accumulated during the cold-working. This exothermal peak occurred in the temperature range 380–800◦ C. Keywords: Nerve stimulation, platinum electrodes, annealing, recrystallization, microstructure, resistivity, optical microscopy, differential scanning calorimetry (DSC)

1. Introduction All the applications of neuroprostheses require electrodes with high spatial and nerve fibre-type selectivity, low impedance for recording, and safe reversible charge injection for stimulation. Thus, an understanding of the electrochemical mechanisms underlying the process of neural stimulation in the human body, and those in the recording electrodes is important for developing chronically implanted devices, particularly those employing many electrodes [1,2]. *

Address for correspondence: Janez Rozman, Centre for Implantable Technology and Sensors, ITIS d. o. o. Ljubljana, Lepi pot 11, 1000 Ljubljana, Republic of Slovenia. Tel.: +386 41 415 268; E-mail: [email protected] 0959-2989/14/$27.50 © 2014 – IOS Press and the authors. All rights reserved

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Because installation of such a multi-electrode system onto a peripheral nerve also causes mechanical injuries, the designer must consider and optimize both aforementioned issues. To minimize the mechanically caused injuries, such as abrasion or compressive injuries, and fitness difficulties in the application of multi-electrode systems, flexible substrates and appropriate metal materials have been developed. Multi-electrode cuffs have been used for electrical stimulation of peripheral nerves for more than 35 years [3–5]. In the past two decades, however, particular attention is being paid to vagus nerve stimulation, a technique that is used to treat many autonomous nervous system disorders. Selective nerve stimulation for the treatment of impaired function of internal organs and glands, as well as neurological disorders, is usually delivered from a group of three electrodes to the vagus nerve. The selectivity of stimulation is exclusively dependent on localized charge delivery to specific populations of nerve fibres. However, charge delivery is influenced by the electrode–tissue interface, where charge carriers transduce from electrons in the metal electrode to ions in the tissue. As a result, electrodes for selective nerve stimulation face electrochemically harsh working conditions. At the same time, because they are in contact with nerve tissue, it is imperative that the electrodes remain nontoxic and nonreactive with the surrounding environment. This means that any harmful reactions should not occur while injecting the required amount of a charge [6,7]. For selective nerve stimulation, different materials that support charge injection by capacitive and faradaic mechanisms are available. Other criteria that must be considered when choosing the material for electrodes that make electrical contact with neural tissue are the mechanical characteristics of the material [8]. Pure platinum is commonly used as stimulating electrode material because it can effectively supply high-density electrical charge to activate neural tissue [9]. As with most metals with a face-centred cubic structure, platinum is a highly ductile metal [10]. Moreover, high-purity platinum is nontoxic, insoluble in mineral and organic acids, and does not corrode or tarnish. In addition to its chemical inertness, platinum has many physical properties that are of great value for their use in the technology of implantable stimulating electrodes [11]. These include general properties, mechanical properties and physical properties. Platinum is used in the form of a pure metal because impurities and alloying elements may adversely affect both its working characteristics and its stability against corrosion in physiological media. Note that the mechanical characteristics of platinum also strongly depend on its impurity content. In the electrical stimulation of nerves, platinum injects charge using both faradaic reactions and double-layer charging [6,9,12]. However, in selective nerve stimulation with a high charge density, metallic dissolution products, hydrogen and oxygen gas bubbles, oxidized organic and inorganic species, and pH shifts causing irreversible changes in the tissue proteins, could occur [13]. Electrodes in selective nerve stimulation should also function without facing degradation over a prolonged time period (20 years). Multi-electrode cuffs have been used for electrical stimulation of peripheral nerves for more than 35 years [3–5]. Although the limits of charge-injection are based on avoiding the electrolysis of water, dissolution can in certain circumstances also occur at lower charge densities [14]. The principal approach for controlling the interface voltage in selective nerve stimulation is using a charge-balanced biphasic current stimulus with two phases of equal but opposite charges [15,16]. Mechanical fitness is another important issue in the design of multi-electrode systems. Low strength of high-purity platinum (99.93 wt%), has been accepted in stimulating electrodes despite being a significant disadvantage. However, to optimise the technical parameters for thermal and mechanical processing of

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platinum, its rheological characteristics, including its deformation resistance, must be characterized [17– 19]. Resistivity is one of the fundamental electrical properties of a particular material responsible for its electrical resistance [20]. Resistance can be calculated using both the resistivity of the material from which a particular object is made and the shape and geometry of the object. To determine the maximum long-term charge density that can be delivered to the nerve without harming the neural tissue or to degrade the electrode, the resistivity of the selective nerve stimulation electrode material must be well known. The resistivity of a material changes with temperature. For many materials, the change is a simple linear function of temperature. However, the resistivity of a metal is slightly higher when cold-worked than when in an annealed state [21,22]. The recrystallization temperature of 99.93 wt% platinum, decrease with increasing degree of deformation. The degree of deformation can be calculated as Λ = 2 ln(b0 /b1 ), or as % of reduction ε = (b0 − b1 )/b0 (where b0 and b1 represent strip thickness before and after cold-rolling). Depending on the degree of deformation, recrystallization temperature can be in the temperature range between 400–1000◦ C [23–25]. Minimizing the annealing temperature on the basis of the degree of deformation in the metal is technologically important. When platinum is annealed at temperatures higher than the distinctive recrystallization temperature, collective recrystallization occurs, which adversely affects the plastic characteristics of the platinum. The hardness and tensile strength increase owing to the cold-work strengthening is lost and mechanical properties return to its initial state. In spite of the low strength of pure platinum, multi-electrode stimulating systems with greater strength can be made. This may be achieved through plastic deformation and partial annealing by choosing appropriate annealing regimes. Metallographic analysis can be used to analyse any failure and determine the microstructure of the platinum in order to set up optimum working cycles. The most convenient method to reveal the microstructure of cold-rolled and/or thermally treated platinum foil used for stimulating electrodes is electrolytic etching [26,27]. The present work addresses the effects of cold-rolling on the structural properties of a platinum foil used to manufacture electrodes to be installed in multi-electrode spiral cuffs, which are used for selective nerve stimulation. More precisely, the article presents the temperature-dependent resistive properties of a cold-rolled platinum foil, as well as quantitative and qualitative information about the physical and chemical changes that occur during cold-rolling, which involve endothermic or exothermic processes or changes in heat capacity.

2. Methods A platinum foil was obtained from a platinum ingot of chemical composition 99.99 wt%, Rh 0.01 wt% by cold-rolling. The platinum foil experienced a considerable strain hardening during cold-rolling. To renew the softness and ductility, recrystallization was promoted by subsequent annealing at 700–800◦ C. Finally, the foil was cold-rolled to achieve narrow tolerances and the final thickness of 0.03 mm, using a small degree of deformation (Λ = 0.11, ε = 5.6%). The long-term electrochemical stability of the stimulating electrodes within the multi-electrode spiral nerve cuff, shown schematically in Fig. 1, depends on long-term stability of platinum crystal grains. The electrical resistivity measurements of platinum strips, used for electrodes in spiral nerve cuff, were made with the modified 4-point resistivity measurement technique (also called a Kelvin probe).

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Fig. 1. (a) Perspective illustration of the 99-electrode spiral cuff and an arbitrarily chosen longitudinal row of electrodes. (b) Matrix of 99 electrodes.

For this purpose, a high-temperature furnace that could withstand temperatures up to 900◦ C and a sample holder with four spring-loaded platinum probes was applied. Direct current was applied via two probes (source and sink) that were connected to the current source (Model: 6220, Keithley Instruments, Inc., Cleveland, OH, USA), and the voltage was measured between the other two probes, connected to a nanovoltmeter (Model: 2182A, Keithley, Keithley Instruments, Inc., Cleveland, OH, USA). To maintain a linear pattern of heating, the system included a digital proportional-integral-derivative (PID) controller. The PID controller was interfaced with a computer via an RS232 port. A thermocouple (K-type) was used to measure the furnace temperature, while a Pt–PtRh thermocouple (S-type) was used to measure sample temperature. The temperature data were retrieved using a USB data acquisition module (Model: NI-9219, National Instruments, Corporation, Austin, TX, USA) and a USB data acquisition chassis (Model: NI cDAQ-9174, National Instruments Corporation, Austin, TX, USA). With this configuration, the test current was forced through the test resistor through one set of test leads, while the voltage across the sample was measured through a second set of leads, called sense leads. Because the voltage drop across the sense leads was negligible, the voltage measured by the nanovoltmeter was considered to be essentially the same as the voltage across the resistor. Both instruments worked in tandem; intercommunication was realized via Trigger Link (TL) and an RS232 serial port interfaced with the computer using a data bus IEE488 (GPIB), an interface (Model: NI-GPIB-USB-HS, National Instruments Corporation, Austin, TX, USA), and Lab-view software (Version 10.0, National instruments, National Instruments Corporation, Austin, TX, USA). For determination of the distinctive temperatures at which microstructural transformations occur during heating in-situ electrical resistance measurement has been applied. The strips underwent dynamic annealing in argon atmosphere (5.0) at a temperature range from room temperature to ∼900◦ C, with the heating rate of 5 K/min. The temperature range was initially estimated to be high enough to ensure complete recrystallization [17]. For electrical resistance measurement an electrical measuring current of 2–3 A/mm2 was applied and a potential difference of approximately 10 µV was measured using the aforementioned nanovoltmeter. To obtain information about the physical and chemical changes during the cold-rolling of the foil, differential scanning calorimetry (DSC) was also used. In this technique, the difference in the amount of heat required to increase the temperature of a foil sample and a reference in an argon atmosphere (5.0) is measured as a function of temperature. These measurements provide quantitative and qualitative

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information about physical and chemical changes that involve endothermic or exothermic processes or changes in heat capacity. The thermal analysis was carried out with an STA449C Jupiter from Netzsch, Selb, Germany, which has the accuracy of ±0.25%. Recrystallized platinum was used as the reference. The heating rate was 10 K/min, and the temperature measurement was performed using an S-type thermocouple (Pt–PtRh). Microstructure was examined before and after heat treatment of cold-rolled strips. Samples for microstructure observation were prepared according to the classical metallographic procedure (sectioning, embedding the strip samples in resin, grinding, polishing), and electrolytically etched [26,27] in a saturated electrolytic solution of sodium chloride in concentrated hydrochloric acid (100 cm3 HCl (37%) + 10 g NaCl) with AC power supply of 3–6 V. 3. Results The results of the resistivity measurements are plotted in Fig. 2. At lower temperatures, platinum exhibit practically linear increase of electrical resistance with increasing temperature. At higher temperatures, the change of electrical resistivity becomes nonlinear. This can be seen more precisely in the dR electrical resistivity derivative curve ( ddR T ). As can be seen from the Fig. 2 on dT curve for deformed strip, ◦ first non-linear resistivity change was recorded above 500 C, and a maximum decrease was attained at approximately 750◦ C. This is in accordance with microhardness decrease according to Loginov et al. [17] for the platinum of the same purity at small deformations. It is well known that any kind of crystal lattice distortion, caused by solute atoms, impurities, dislocations, etc., increases electrical resistivity of the metal. Therefore, nonlinear change of electrical resistivity must be related to the microstructural transformations in the material matrix. During cold-working, dislocation density increases. Despite the fact that the increased dislocation density has a weak influence on the electrical resistivity of the metal, it still could be detected by the sensitive method of electrical resistance measurement. Non-linear change in electrical resistance corresponds to nucleation and growth of new crystal grains with lower dislocation density during recrystallization. For the same strip, already dynamically annealed up to 860◦ C, deviation of electrical resistance is recorded at a temperature just above the temperature where non-linear changes

Fig. 2. Electrical resistance curves for recrystallized and deformed Pt strip as a function of temperature. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/BME-140993.)

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in electrical resistance for deformed strip has ended and corresponds to the growth of the already recrystallized grains. The results of the DSC experiment are shown in Fig. 3, in which heat flux is plotted as a function of temperature. The position of the exothermal peak was between 380 and 800◦ C, and it represents the loss of energy released through heating. The intensity of the recrystallization process is weak and it is observed over a relatively large temperature range. The released heat was determined to be only 11.29 J/g. In comparison, the enthalpy of fusion of pure platinum is accepted to be approximately 22.11 kJ/mol or 113.34 J/g according to the data found in [28]. According to the DSC heating curve, the start of heat release is detected at 377.5◦ C and the maximum at 622.5◦ C. The obtained temperature is consistent with the comprehensive study of platinum recovery by Zhang et al. [29] and Raub [30], where pure platinum was recovered after heavy cold-working at approximately 200◦ C. In our case, deformation degree is small, which means that recovery will start at much higher temperature. An optical micrograph (differential interface contrast technique) of cold-rolled and recrystallized strips is shown in Fig. 4(a) and (b), respectively. The grains appear as slightly elongated particles, due to plastic deformation.

Fig. 3. The DSC heating curve of pure cold-rolled platinum foil as a function of temperature.

Fig. 4. (a) Optical micrograph of cold-rolled Pt foil. (b) Optical micrograph of the Pt foil continually annealed with constant heating rate (5 K/min) to 860◦ C. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/BME-140993.)

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After dynamic annealing up to 860◦ C, slightly larger grains are observed, compared to deformed structure. Relatively small grain growth after annealing can be attributed to relatively short annealing time.

4. Discussion In the past few decades, considerable efforts have been devoted to develop neuroprosthetics that interface with the human nervous system using electronic implantable devices. The present work aims to give some explanation of the microstructure of platinum foil used for stimulating electrodes, both in the coldroll-hardened and annealed condition, and to demonstrate the usefulness of the 4-point probe resistivity measurement technique, DSC and optical microscopy for describing them. In this regard, the analysis, design criteria, and structural properties of the platinum foil used for fabricating stimulating electrodes within a multi-electrode spiral cuff for the selective stimulation of peripheral nerves are presented [8,9]. In certain circumstances, significant chemical reactions may occur during electrical stimulation that can destroy a platinum electrode [6–9], thereby depositing toxic metal ions in the body of a patient. Therefore, it is imperative to understand all the problems that may occur when a multi-electrode spiral cuff is used for nerve stimulation and the situations where a stimulating electrode poses the greatest risk to the stimulated nerve [5]. For approximately estimating the electrical resistivity of metals, Matthiessen’s rule is used [31,32]. According to this rule, the total specific resistivity of a metal is equal to the sum of individual specific resistivities as long as each of them corresponds to a separate mechanism. The rule could be applied to metals, in which one component of resistivity, residual resistivity (ρSolute ), is determined by the scattering of electrons on impurities and dislocations; it does not depend on temperature. The other component, ideal resistivity (ρSolvent ), is associated with electron scattering by thermal lattice vibrations and does depend on temperature. Accordingly, the electrical resistivity of alloy can be expressed as: ρAlloy (c, T ) = ρSolvent (T ) + ρSolute (c), where ρSolute is a function of the concentration of impurities and dislocations, c, and ρSolvent is a function of temperature, T . In this study, in which 99.99 wt% platinum cold-rolled foil was tested, the effect of solute atoms in the formation of the secondary phases is very limited. Therefore, the contribution of such secondary phases to ρSolute is also small. Thus, it can be concluded that ρSolute is mainly influenced by the presence of vacancies and dislocations. Previous resistivity studies on pure metals have shown that the effect of a solvent on ρSolvent is much larger than that of the ρSolute component [33]. Therefore, the main contribution to changes in the electrical resistivity of a cold-rolled pure platinum foil during recrystallization could be attributed to the effect of a solvent via ρSolvent . From resistivity measurements, we concluded that the 4-point probe resistivity measurement technique used in this study was adequately sensitive to detect resistivity changes caused by cold-rolling, thereby allowing the recrystallization temperature to be determined. Because of the very weak heat released during the DSC measurement, the DSC method is estimated to be less appropriate for investigating such thin platinum foils.

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5. Conclusion Thus, we can conclude that only by combining the knowledge about appropriate stimulation parameters and thermal treatments of cold-rolled platinum foil, we can achieve the optimum fabrication of electrodes for selective nerve stimulation. More precisely, in order to achieve suitable final properties of the platinum foil for fabricating stimulating electrodes, it is crucial to establish the right combination of plastic deformation and annealing treatment [17,21,22,24,25,30] of the foil. It is also important to understand the behaviour of the materials used over longer periods of time, which can sometimes be as long as 20 years. From the measurements of electrical resistivity and thermal effects with DSC, we can conclude that small changes to well-deformed platinum foil are apparent at temperatures above 280◦ C. This means that the cold-work done on the platinum foil during the fabrication of a stimulating electrode will have adequate temperature stability in the low temperature ranges that are also typical for selective nerve stimulations. The most important finding is that the combined results of the resistivity and DSC measurements provide good criteria for selecting materials, and that appropriate thermal and mechanical working processes are required to fabricate stimulating electrodes. In this regard, the directions our further work could take are the following: • further optimization of mechanical fitness of stimulating electrodes as an important issue in the design of multi-electrode systems, • further optimization the technical parameters for thermal and mechanical processing of platinum foil to be used for fabrication of stimulating electrodes, • determination the maximum long-term charge density, that can be delivered to the nerve at simulated physiological conditions using the electrochemical impedance spectroscopy and cyclic voltammetry technique and, • performing more experiments to show group statistics. Acknowledgements Financial support from the Ministry of Education, Science, Culture and Sport, Republic of Slovenia, research programme P3-0171, is gratefully acknowledged. The authors express their sincere thanks to Prof. Jožef Medved and Dr. Grega Klanˇcnik, Department of Materials and Metallurgy, Faculty of Natural Sciences and Engineering, University of Ljubljana, who offered their time and expertise with DSC measurements. References [1] P.F. Johnson and L.L. Hench, An in vitro model for evaluating neural stimulating electrodes, J. Biomed. Mater. Res. 10(6) (1976), 907–928. [2] D.R. Merrill, The electrochemistry of charge injection at the electrode/tissue interface, in: Implantable Neural Prostheses 2, Biological and Medical Physics, Biomedical Engineering, 2010, pp. 85–138. [3] R.B. Stein, D. Charles, L. Davis, J. Jhamandas, A. Mannard and T.R. Nichols, Principles underlying new methods for chronic neural recording, Can. J. Neurol. Sci. 2 (1975), 235–244. [4] G.G. Naples, J.D. Sweeny and J.T. Mortimer (inventors), Implantable cuff, method and manufacture and method of installation, U.S. Patent #4,602,624, issued July 1986.

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Structural characterization of platinum foil for neural stimulating electrodes.

The objective of this study was to investigate the structural properties of a cold-rolled platinum foil used to manufacture multi-electrode spiral ner...
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