particles clustering and more homogeneous distribution
of reinforcement nanoparticles in the steel matrix were
obtained. It was found that the concentration and the size
of particles have an impact on the distribution of the
reinforcement within the matrix. When the weight
percent is increased above 1.0 it starts to affect the
particles’ distribution, with the concentration ratio de-
creasing towards the bottom of the cast ingot. In this case
also the size of the particles plays a role: a larger particle
size leading to an increased degree of incorporated
particles in the steel matrix.
In this study, an innovative pre-dispersion approach
for more the effective addition of ultrafine particles and
nanoparticles into a steel melt through a conventional
casting route was designed. It is based on mixing ultra-
fine particles and nanoparticles powder with dispersion
media.
Acknowledgment
This work was done in the frame of the research
programs P2-0050, which are financed by the Slovenian
Research Agency. The authors would also like to
acknowledge help from Miroslav Pe~ar, in`., from Insti-
tute of Metals and Technology for the AES analysis.
6 REFERENCES
1 R. Casati, M. Vedani, Metal Matrix Composites Reinforced by
Nano-Particles – A Review, Metals (Basel), 4 (2014) 1, 65–83,
doi:10.3390/met4010065
2 S. H. Lee, J. J. Park, S. M. Hong, B. S. Han, M. K. Lee, C. K. Rhee,
Fabrication of cast carbon steel with ultrafine TiC particles. Trans
Nonferrous Met Soc China (English Ed.) 21 (2011), 54–57,
doi:10.1016/S1003-6326(11)61060-1
3 Y. Q. Liu, H. T. Cong, W. Wang, C. H. Sun, H. M. Cheng, AlN nano-
particle-reinforced nanocrystalline Al matrix composites: Fabrication
and mechanical properties. Met.Sic.Eng.A, 505 (2009), 151–156,
doi:10.1016/j.msea.2008.12.045
4 Z. Zhang, L. D. Chen, Consideration of Orowan strengthening effect
in particulate-reinforced metal matrix nanocomposites: A model for
predicting their yield strength. Scripta Mater., 54 (2006), 1321–1326,
doi:10.1016/j.scriptamat.2005.12.017
5 J. Llorca, Fatigue of particle-and whisker reinforced metal-matrix
composites. Prog Mater Sci., 47 (2002), 283–353, doi:10.1016/
S0079-6425(00)00006-2
6 B. N. Chawla, Y. Shen, Mechanical Behavior of Particle Reinforced
Metal Matrix Composites **. Adv Eng Mater., 3 (2001) 6, 357–370,
doi:10.1002/1527-2648(200106)3:6<357::AID-ADEM357>3.3.CO;2-9
7 Z. Ni, Y. Sun, F. Xue, J. Bai, Y. Lu, Microstructure and properties of
austenitic stainless steel reinforced with in situ TiC particulate,Mater.
Des., 32 (2011) 3, 1462–1467, doi:10.1016/j.matdes.2010.08.047
8 F. Akhtar, Ceramic reinforced high modulus steel composites:
processing, microstructure and properties,Can. Metall. Q., 53 (2014)
3, 253–263, doi: 10.1179/1879139514Y.0000000135
9 S-Y.Cho, J-H. Lee, Anisotropy of wetting of molten Fe on Al2O3
single crystal. Korean J Mater Res., 18 (2008) 1, 18–21, doi:10.3740/
MRSK.2008.18.1.018
10 R. V. Väinölä, L. E. K. Holappa, P. H. J. Karvonen, Modern steel-
making technology for special steels, Journal of Materials Processing
Technology, 53 (1995), 453–465, doi:10.1016/0924-0136(95)
02002-4
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
980 Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
981–988
SURFACE CHARACTERIZATION OF PLATINUM STIMULATING
ELECTRODES USING AN ELECTROCHEMICAL SCANNING
METHOD
KARAKTERIZACIJA POVR[INE PLATINASTIH STIMULACIJSKIH
ELEKTROD S POMO^JO ELEKTROKEMIJSKE VRSTI^NE
METODE
Andra` Mehle1, Janez Rozman2,3, Martin [ala4, Samo Ribari~3, Polona Pe~lin2
1Sensum d. o. o., Tehnolo{ki park 21, 1000 Ljubljana, Slovenia
2Centre for Implantable Technology and Sensors, ITIS d. o. o. Ljubljana, Lepi pot 11, 1000 Ljubljana, Slovenia
3Institute of Pathophysiology, Medical Faculty, University of Ljubljana, Zalo{ka 4, 1000 Ljubljana, Slovenia
4Analytical Chemistry Laboratory (L04), National Institute of Chemistry, Hajdrihova 19, Ljubljana, Slovenia
janez.rozman@guest.arnes.si
Prejem rokopisa – received: 2017-04-25; sprejem za objavo – accepted for publication: 2017-06-28
doi:10.17222/mit.2017.044
The purpose of this article is to investigate the electrochemical performance of platinum stimulating nerve electrodes (WE1 and
WE2) with different surface structures to define which one is able to produce a higher neural activation function during nerve
stimulation than that achieved by conventional electrodes. The purpose is also to present a method that enables the
electrochemical scanning of stimulating electrode surfaces. The surface of WE1 was modified using rough sand paper, while the
surface of WE2 was modified using fine sand paper. The potential at the different roughened surfaces in the sodium phosphate
mixture, when excited with specific current pulses, was measured against a Ag/AgCl reference electrode. Voltage transients
were recorded to determine the polarization across the electrode-electrolyte interface. The results indicate that the surface of
WE1 could deliver more current to the nerve tissue and more activation for a fixed input voltage than WE2. Namely, it is shown
that the mean |Zpol| of WE1 was lower than that for WE2 (237.1 
 vs. 251 
). Accordingly, the platinum electrode that was
superficially modified using rough sand paper is more suitable for safe and efficient nerve stimulation than the electrode that
was superficially modified using fine sand paper.
Keywords: platinum, polarization, interfaces, potential parameters
Namen ~lanka je preiskati elektrokemijske lastnosti platinastih elektrod WE1 in WE2 za stimulacijo `ivca, ki imata razli~ni
povr{inski strukturi, s ciljem dolo~iti katera od njiju je sposobna povzro~iti ve~jo aktivacijsko funkcijo vlaken med stimulacijo
glede na konvencionalno povr{ino. Namen je tudi predstaviti metodo, ki omogo~a elektrokemijsko vrsti~no preiskavo povr{in
stimulacijskih elektrod. Povr{ina WE1 je bila obdelana z grobim brusnim papirjem medtem, ko je bila povr{ina WE2 obdelana s
finim brusnim papirjem. Potencial razli~no grobih povr{in v fiziolo{ki raztopini pri vzdra`enju s specifi~nimi stimulacijskimi
impulzi je bil merjen glede na Ag/AgCl referen~no elektrodo. Napetostni prehodni pojavi so bili zajeti z namenom dolo~itve
polarizacije na prehodu med elektrodo in elektrolitom. Rezultati ka`ejo, da lahko WE1 dovede na `ivec ve~ toka in s tem dose`e
ve~jo aktivacijo pri konstantni napetosti kot WE2. Izkazuje se namre~, da je polarizacijska upornost |Zpol| pri WE1 manj{a kot
pri WE2 (237.1 
 proti 251 
). Potemtakem je elektroda, ki je bila bru{ena z grobim brusnim papirjem, bolj primerna za varno
in u~inkovito stimulacijo `ivca, kot elektroda, ki je bila bru{ena s finim brusnim papirjem.
Klju~ne besede: platina, polarizacija, prehodi, parametri potenciala
1 INTRODUCTION
In recent decades, considerable scientific and techno-
logical efforts have been devoted to understanding and
characterizing the interface between a stimulating elec-
trode and its surrounding medium. In this region, a trans-
duction of charge carriers occurs from electrons in the
metal electrode to ions in the tissue, which is exception-
ally important in determining how the electrodes respond
to charge injection. To be specific, characterizing the
electrode-tissue interface is crucial to determining safe
charge delivery to the nerve.1–3
In implantable prosthetic devices, electrodes are the
interfaces between the electronic circuitry and nerve
tissue and can be used for neural stimulation and/or
neural signal recording. In the past few years, implanted
electrodes have been used extensively for efficient stimu-
lation of peripheral nervous systems. Although much
effort has been made to find optimal anatomical targets
for different nerve-stimulation techniques, little work has
been done to improve the efficiency of nerve stimulation
using analytically driven designs and configurations of
the stimulating electrodes.
The electrode geometry itself plays a significant role
in controlling the activation of neuron populations.4 In
this connection, the electrode geometry can affect the
impedance, spatial distribution of the electric field in the
tissue, and consequently the pattern of neural excitation.
One approach to enhance the efficiency of neural stimu-
lation is to increase the irregularity of the surface current
profile, which can be quantified by defining a metric
known as the topological edginess.5
Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988 981
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 62-4-023.7:669.231.6:621.35 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)981(2017)
In this relation, adequately optimized electrode
geometries and surfaces that increase the variation of the
current density on the electrode surface also increase the
efficiency of the neural stimulation.5,6 This is in accord-
ance with the suppositions of E. N. Warman et al.7 and
F. Rattay8; they showed that electrode-induced neural
excitation could be predicted using the driving function
of a neuron, which is directly proportional to the second
spatial derivative of the extracellular potential and thus,
the spatial derivative of the current density in the tissue.
Therefore, electrode geometries with a greater roughness
and more sharp edges can increase the stimulation
efficiency by increasing the spatial derivative of the
current density, and therefore the driving function.
However, specific neural stimulation applications
sometimes require high electrode charge densities that
may lead to high energy transfer and elicit chemical
reactions that involve changes in the electrode properties
and even corrosion. In this relation, A. Hung et al.9 des-
cribed a pulse-clamp technique that could be used to
accurately quantify the roughness modification in an
electrode used in selective nerve stimulation applications
that demand both a large amount of charge injection and
a small electrode size. However, to ensure safe and rever-
sible charge injection for stimulation, all applications
require low-impedance electrodes. In this relation, the
medium surrounding the stimulating electrodes is
exceptionally important to determine how they respond
to charge injection. Consequently, for the development
of chronically implanted multi-electrode devices, an
understanding of the electrochemical mechanisms
underlying the behavior of neural stimulation electrodes
is important.
When platinum is used as the material for stimulating
electrodes, it injects charge by both reversible Faradaic
reactions and double-layer (DL) charging. However,
reversible Faradaic reactions predominate under most
stimulating conditions.10–12 Furthermore, changes in the
electrolyte composition adjacent to the electrode and the
finite rate of Faradaic reactions can lead to irreversible
processes that cause electrode degradation or tissue
injury.13–16
In most in-vitro experiments, the electrodes were
submerged in a certain electrolyte to simulate a specific
physiological medium.17,18 For in-vitro experiments, volt-
age transient (VT) measurements are used to estimate the
charge-injection limit, which defines the quantity of
charge that can be injected in a current-controlled
stimulation pulse by electrochemical reversible processes
only. More precisely, VTs are analyzed to determine the
maximum negative polarization (Emc) and maximum
positive polarization (Ema) across the electrode-electro-
lyte interface. These potential extremes are then com-
pared to the established maximum potentials, beyond
which it is considered unsafe to polarize the electrode
(typically the water electrolysis potential window).19,20
Determining the optimum surface roughness for
stimulating electrodes is a challenging topic. In recent
years, a variety of mechanical adaptations, such as
adaptations to the geometry and surface roughness of the
electrodes, to contribute to neuro-prostheses designs
have been investigated and implemented.5,6 Namely, the
geometry of an electrode significantly affects the shape
of the generated electric field, which in turn affects the
current density produced by the electrode.
The efficiency of a stimulating electrode is charac-
terized by its ability to activate a certain volume of
neural tissue with lower voltage and power require-
ments.5,6 Electrode designs involving a greater amount of
sharp peaks and edges will have higher current density
variations than electrodes with flat or rounded edges (the
higher current density variations will increase the sti-
mulation efficiency). Besides, such an electrode activates
a significantly larger number of axons with a lower
threshold than electrodes with smooth or rounded
edges.21 In this regard, L. Golestanirad et al.5
demonstrated that the feasibility of increasing the
stimulation efficiency using modified fractal geometries
is still beyond the levels already reported in the
literature. Related experimental studies have shown that
neural stimulation can be improved by creating a rougher
surface; however, the electrodes are covered with a layer
of material with a high porosity.6,21 Nevertheless, the
electrode functionality can also be increased by
depositing conducting porous polymers with
incorporated cell adhesion peptides, proteins, and
anti-inflammatory drugs.22,23 Most of these impro-
vements could reduce the fibrous tissue encapsulation
thickness because of tissue in-growth. However, smooth
surfaces would make it more difficult to initiate
corrosion.
There is a need to accurately predict the neural
activation as a function of the stimulation parameters and
electrode design; thus, we evaluate the effects of the
electrode surface conditions on the electrochemical
performance of the platinum electrode for selective nerve
stimulation in vitro.
For this purpose, a method for the in-vitro evaluation
of platinum electrodes for use in neural stimulation
applications, was developed. The present work focuses
on modifying the geometry of the rectangular platinum
electrode to produce a higher neural activation function
than that achieved by conventional electrodes.8 In this
relation, the aim is to increase the surface current
irregularity while maintaining the amount of total current
delivered to the tissue. We present the preliminary results
for these electrodes with mechanically modified surfa-
ces. Our hypothesis is that such a modified surface could
increase the irregularity of the current density profile on
the electrode surface, and consequently in the adjacent
nerve tissue. For this purpose, a method that enables an
in-vitro assessment of the electrochemical performance
of stimulating electrodes using modified electrode geo-
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
982 Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
metries was developed.24,25 This article investigates the
electrochemical performance of two stimulating electro-
des with different surface structures obtained by treating
the surface with smooth and rough sand paper.
2 EXPERIMENTAL PART
2.1 Electrochemical cell
The body of the electrochemical cell (Figure 1) was
machined from bulk polyamide (nylon) using a milling
machine. The dimensions and design of the cell were
optimized to enable easy manipulation of the electrical
connections and three electrodes. For the measurements,
a 0.03-mm-thick silicone strip embedding tested electro-
des (working electrode number one (WE1) and working
electrode number two (WE2)), was adhered onto the top
of the cylinder made of polyamide (Novilon) using the
medical-grade silicone adhesive. Then, the cylinder was
mounted at the bottom of the chamber using a medi-
cal-grade silicone adhesive (Med RTV Adhesive,
Implant Grade 40064, Applied Silicone Corporation).
In the developed technique, only the working elec-
trode (WE) uncovered face was investigated, while the
other surfaces were insulated from the electrolyte. The
auxiliary electrode (AE) with a large geometrical area
(~600 mm2) was created by adhering a platinum ribbon
to the inner walls of the cell (approximately 20 mm from
the WEs and the reference electrode (RE)). The RE was
a simple open-ended standard Ag/AgCl glass RE,
REF-10 (UNISENSE A/S, Denmark) with an outside tip
diameter of 10 μm.
For our measurements, the WE is large enough and
the standard glass RE small enough that the surface of
the investigated stimulating electrode could be divided
into the number of locations where measurements could
be performed.
The voltage drop between the RE and WE (actually
arising from the series resistance, including the solution
resistance between the WE and the RE plus the electrical
resistance of the lead wire) was minimized by posi-
tioning the RE approximately 0.05 mm from the WE.
For this purpose, a 3D micro-manipulator was installed
on the chamber.
Prior to experimentation, the WE surface was cleaned
by rubbing in a letter-I pattern for a period of 30 s using
a polishing cotton tip. Afterwards, the WE surface was
rinsed thoroughly with ethanol, then de-ionized with
distilled water and air-dried. Finally, the chamber was
filled with a phosphate-buffered saline (PBS) solution, as
summarized in Table 1.
Table 1: PBS solution composition
Component
Mass
concentration
(g/L)
Molar
concentration
(mM)
Molar mass
(g/mol)
NaCl 7.36 126 58.4425
Na2HPO4 11.5 81 141.959
NaH2PO4 · H2O 3.04 22 137.992
pH = 7.27, T = 21.4 °C
The measured VT was amplified using one channel
of the high-performance differential amplifier (Teledyne
LeCroy DA1855A-PR2). However, the voltage drop
across the precision serial resistor at the stimulator
output (10 
) was measured using a custom-designed
differential amplifier with the gain (A) set at A = 10. A
dual-channel digital oscilloscope (TekScope, Tektronix)
was used to monitor both the VT and the drop across the
precision serial resistor. The VT measurements between
one of the WEs and the AE were performed at room tem-
perature in the PBS solutions (schematically shown in
Figure 2). The data was collected at 200 kHz using a
USB 2.0 interface with a high-performance data-acqui-
sition system (DEWE-43, Dewesoft, Slovenia) and
Dewesoft 7.0.2 acquisition software developed by the
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988 983
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Schematic diagram of the measuring setup comprising the
following units: a) electrochemical cell, reference electrode (RE),
working electrode (WE), and auxiliary electrode (AE), b) differential
amplifier, c) data-acquisition system, d) differential amplifier, e) pre-
cision serial resistor (RM), f) precision custom-designed stimulator, g)
PC and h) oscilloscope
Figure 1: a) Electrochemical cell, b) perspective 3D illustration of the
stimulating spiral nerve cuff and c) working electrode
In this relation, adequately optimized electrode
geometries and surfaces that increase the variation of the
current density on the electrode surface also increase the
efficiency of the neural stimulation.5,6 This is in accord-
ance with the suppositions of E. N. Warman et al.7 and
F. Rattay8; they showed that electrode-induced neural
excitation could be predicted using the driving function
of a neuron, which is directly proportional to the second
spatial derivative of the extracellular potential and thus,
the spatial derivative of the current density in the tissue.
Therefore, electrode geometries with a greater roughness
and more sharp edges can increase the stimulation
efficiency by increasing the spatial derivative of the
current density, and therefore the driving function.
However, specific neural stimulation applications
sometimes require high electrode charge densities that
may lead to high energy transfer and elicit chemical
reactions that involve changes in the electrode properties
and even corrosion. In this relation, A. Hung et al.9 des-
cribed a pulse-clamp technique that could be used to
accurately quantify the roughness modification in an
electrode used in selective nerve stimulation applications
that demand both a large amount of charge injection and
a small electrode size. However, to ensure safe and rever-
sible charge injection for stimulation, all applications
require low-impedance electrodes. In this relation, the
medium surrounding the stimulating electrodes is
exceptionally important to determine how they respond
to charge injection. Consequently, for the development
of chronically implanted multi-electrode devices, an
understanding of the electrochemical mechanisms
underlying the behavior of neural stimulation electrodes
is important.
When platinum is used as the material for stimulating
electrodes, it injects charge by both reversible Faradaic
reactions and double-layer (DL) charging. However,
reversible Faradaic reactions predominate under most
stimulating conditions.10–12 Furthermore, changes in the
electrolyte composition adjacent to the electrode and the
finite rate of Faradaic reactions can lead to irreversible
processes that cause electrode degradation or tissue
injury.13–16
In most in-vitro experiments, the electrodes were
submerged in a certain electrolyte to simulate a specific
physiological medium.17,18 For in-vitro experiments, volt-
age transient (VT) measurements are used to estimate the
charge-injection limit, which defines the quantity of
charge that can be injected in a current-controlled
stimulation pulse by electrochemical reversible processes
only. More precisely, VTs are analyzed to determine the
maximum negative polarization (Emc) and maximum
positive polarization (Ema) across the electrode-electro-
lyte interface. These potential extremes are then com-
pared to the established maximum potentials, beyond
which it is considered unsafe to polarize the electrode
(typically the water electrolysis potential window).19,20
Determining the optimum surface roughness for
stimulating electrodes is a challenging topic. In recent
years, a variety of mechanical adaptations, such as
adaptations to the geometry and surface roughness of the
electrodes, to contribute to neuro-prostheses designs
have been investigated and implemented.5,6 Namely, the
geometry of an electrode significantly affects the shape
of the generated electric field, which in turn affects the
current density produced by the electrode.
The efficiency of a stimulating electrode is charac-
terized by its ability to activate a certain volume of
neural tissue with lower voltage and power require-
ments.5,6 Electrode designs involving a greater amount of
sharp peaks and edges will have higher current density
variations than electrodes with flat or rounded edges (the
higher current density variations will increase the sti-
mulation efficiency). Besides, such an electrode activates
a significantly larger number of axons with a lower
threshold than electrodes with smooth or rounded
edges.21 In this regard, L. Golestanirad et al.5
demonstrated that the feasibility of increasing the
stimulation efficiency using modified fractal geometries
is still beyond the levels already reported in the
literature. Related experimental studies have shown that
neural stimulation can be improved by creating a rougher
surface; however, the electrodes are covered with a layer
of material with a high porosity.6,21 Nevertheless, the
electrode functionality can also be increased by
depositing conducting porous polymers with
incorporated cell adhesion peptides, proteins, and
anti-inflammatory drugs.22,23 Most of these impro-
vements could reduce the fibrous tissue encapsulation
thickness because of tissue in-growth. However, smooth
surfaces would make it more difficult to initiate
corrosion.
There is a need to accurately predict the neural
activation as a function of the stimulation parameters and
electrode design; thus, we evaluate the effects of the
electrode surface conditions on the electrochemical
performance of the platinum electrode for selective nerve
stimulation in vitro.
For this purpose, a method for the in-vitro evaluation
of platinum electrodes for use in neural stimulation
applications, was developed. The present work focuses
on modifying the geometry of the rectangular platinum
electrode to produce a higher neural activation function
than that achieved by conventional electrodes.8 In this
relation, the aim is to increase the surface current
irregularity while maintaining the amount of total current
delivered to the tissue. We present the preliminary results
for these electrodes with mechanically modified surfa-
ces. Our hypothesis is that such a modified surface could
increase the irregularity of the current density profile on
the electrode surface, and consequently in the adjacent
nerve tissue. For this purpose, a method that enables an
in-vitro assessment of the electrochemical performance
of stimulating electrodes using modified electrode geo-
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
982 Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
metries was developed.24,25 This article investigates the
electrochemical performance of two stimulating electro-
des with different surface structures obtained by treating
the surface with smooth and rough sand paper.
2 EXPERIMENTAL PART
2.1 Electrochemical cell
The body of the electrochemical cell (Figure 1) was
machined from bulk polyamide (nylon) using a milling
machine. The dimensions and design of the cell were
optimized to enable easy manipulation of the electrical
connections and three electrodes. For the measurements,
a 0.03-mm-thick silicone strip embedding tested electro-
des (working electrode number one (WE1) and working
electrode number two (WE2)), was adhered onto the top
of the cylinder made of polyamide (Novilon) using the
medical-grade silicone adhesive. Then, the cylinder was
mounted at the bottom of the chamber using a medi-
cal-grade silicone adhesive (Med RTV Adhesive,
Implant Grade 40064, Applied Silicone Corporation).
In the developed technique, only the working elec-
trode (WE) uncovered face was investigated, while the
other surfaces were insulated from the electrolyte. The
auxiliary electrode (AE) with a large geometrical area
(~600 mm2) was created by adhering a platinum ribbon
to the inner walls of the cell (approximately 20 mm from
the WEs and the reference electrode (RE)). The RE was
a simple open-ended standard Ag/AgCl glass RE,
REF-10 (UNISENSE A/S, Denmark) with an outside tip
diameter of 10 μm.
For our measurements, the WE is large enough and
the standard glass RE small enough that the surface of
the investigated stimulating electrode could be divided
into the number of locations where measurements could
be performed.
The voltage drop between the RE and WE (actually
arising from the series resistance, including the solution
resistance between the WE and the RE plus the electrical
resistance of the lead wire) was minimized by posi-
tioning the RE approximately 0.05 mm from the WE.
For this purpose, a 3D micro-manipulator was installed
on the chamber.
Prior to experimentation, the WE surface was cleaned
by rubbing in a letter-I pattern for a period of 30 s using
a polishing cotton tip. Afterwards, the WE surface was
rinsed thoroughly with ethanol, then de-ionized with
distilled water and air-dried. Finally, the chamber was
filled with a phosphate-buffered saline (PBS) solution, as
summarized in Table 1.
Table 1: PBS solution composition
Component
Mass
concentration
(g/L)
Molar
concentration
(mM)
Molar mass
(g/mol)
NaCl 7.36 126 58.4425
Na2HPO4 11.5 81 141.959
NaH2PO4 · H2O 3.04 22 137.992
pH = 7.27, T = 21.4 °C
The measured VT was amplified using one channel
of the high-performance differential amplifier (Teledyne
LeCroy DA1855A-PR2). However, the voltage drop
across the precision serial resistor at the stimulator
output (10 
) was measured using a custom-designed
differential amplifier with the gain (A) set at A = 10. A
dual-channel digital oscilloscope (TekScope, Tektronix)
was used to monitor both the VT and the drop across the
precision serial resistor. The VT measurements between
one of the WEs and the AE were performed at room tem-
perature in the PBS solutions (schematically shown in
Figure 2). The data was collected at 200 kHz using a
USB 2.0 interface with a high-performance data-acqui-
sition system (DEWE-43, Dewesoft, Slovenia) and
Dewesoft 7.0.2 acquisition software developed by the
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988 983
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Schematic diagram of the measuring setup comprising the
following units: a) electrochemical cell, reference electrode (RE),
working electrode (WE), and auxiliary electrode (AE), b) differential
amplifier, c) data-acquisition system, d) differential amplifier, e) pre-
cision serial resistor (RM), f) precision custom-designed stimulator, g)
PC and h) oscilloscope
Figure 1: a) Electrochemical cell, b) perspective 3D illustration of the
stimulating spiral nerve cuff and c) working electrode
same company and stored on a portable computer
(LenovoT420, China).
2.2 Crafting the working electrodes
Crafting the WEs involved several steps: 1) cutting
0.66-mm-wide strips from the 0.03-mm-thick platinum
foil using a sharp blade, 2) cutting two 6-mm-long indi-
vidual strips, 3) folding the obtained strips into thirds,
creating U-shaped forms, and 4) inserting the 2-mm-
wide strip made of the silicone sheet between the
U-shaped structure to form a sandwich structure. The
final dimensions of the electrode surfaces exposed to the
PBS were width=0.66 mm, length=3mm, and surface
area=2 mm2.
The interconnection between the WEs and the lead
wires was performed using a simple construction
custom-designed micro-spot-welder based on a thyristor
and powered using a large capacitor discharge by highly
experienced and well-trained personnel. An energy of
approximately 6 Ws (defined experimentally) provided
reproducible results for the proposed application of
welding the lead wire (AS 631, Cooner Wire, USA)
within the sandwich structure. The welding energy for
both electrodes is defined in Table 2. This table also
provides information for the two different types of sand
paper (Waterproof Silica Carbide Paper FEPA P#500 and
FEPA P#4000, Struers ApS, Denmark) that were used to
increase the real surface of the WEs. Specifically, the
surface of WE1 was enlarged using rough sand paper
(FEPA P#500) while WE2 was enlarged using fine-
grained sand paper (FEPA P#4000). In this relation, each
of the WE surfaces was treated by grinding in a letter-I
pattern for a period of 10 s using an appointed sand
paper and vertical force of approximately 5 N.
In Figure 1, WE1 is situated at the right-hand side of
the cell and WE2 at the left-hand side of the cell.
Table 2: Welding energy and sand paper used in the experiments
WE Chargingcurrent (A)
Charging
voltage (V)
Welding
energy (Ws) Sand paper
1 5 2 6 FEPA P#500
2 5 2 6 FEPAP#4000
2.3 VT measurements
VTs were measured using a biphasic, quasi-trape-
zoidal current stimulating pulse waveform (pulse) with
the intensity of the cathodic phase (ic), which was pre-
viously tested in selective nerve stimulation on an iso-
lated left porcine vagus nerve (not shown in this paper)26,
delivered from the precision custom-designed stimulator
between an appointed WE and the AE. The relevant
parameters (ic = -4.0 mA; width of the cathodic phase,
tc = 155 μs; width of the cathodic exponential decay,
texp = 100 μs; time constant of the exponential decay,
exp = 45 μs; intensity of the anodic phase, ia = 0.45 mA;
width of the anodic phase, ta = 490 μs; charge density
within the cathodic phase, qc = -800.00 μC/cm2; and
charge density within the anodic phase, qa = 849.00
μC/cm2) were pre-set by the stimulator. For this pulse,
Emc and Ema were measured across the electrode-electro-
lyte interface. These potentials were then tested to
determine whether any of them exceeded the values con-
fining the water electrolysis window, which were defined
using cyclic voltammetry (not shown in this paper;
[-0.60 V + 0.85 V] measured in PBS).27–29
There were several characteristic voltages and
potentials that contributed to the entire voltage drop (dV)
and were accounted for in the calculations of Emc and
Ema: polarization across the electrode-electrolyte inter-
face (dEp), potential of the WE at the onset of the pulse
(Eipp), and access voltage (Va; drop across the electrolyte
resistance plus over-potential terms). Emc is defined by
Equation (1):30
Emc = Eipp + dEp = Eipp + (dV – Va) (1)
VTs above each of the two WEs were measured from
various locations on the surface of the WE while the
above-mentioned pulses were applied between the WE
and CE. The scanning displacements, transverse accord-
ing to each of the two WEs, were controlled optically
using a precise micrometre dial, while at the same time,
they were measured using precise linear potentiometer
(20 kΩ) connected to the aforementioned high-perfor-
mance data-acquisition system. During the scanning, the
tip of the RE was situated as close as possible to the
WEs using a micrometre enabling precise vertical dis-
placement.
2.4 VT analysis
A set of approximately 160 VTs, each corresponding
to a specific location at the investigated WE obtained by
the displacement x performed by the RE above the WE
was measured. Afterwards, each VT in a set was auto-
matically analysed/processed to determine the aforemen-
tioned characteristic voltages and potentials that con-
tributed to the entire voltage drop dV. Characteristic
voltages and potentials for a certain set (displacement
x) were then calculated as the mean of all the values in
a set. In this regard, the potential of the WE at the onset
of the pulse Epp was estimated as the mean value of 50
measured signal samples before the onset of the pulse.
The points that determine the voltage Va and the potential
Ema of the VT are located at pulse times where the rate of
change of inclination of the signal tangent is high. Thus,
the times tV a and t E ma of those points were estimated by
locating local maxima of the second derivative of the
voltage response. The second derivative of the signals
was estimated by convolving the signals with a digital
filter of the form [-1 0 1] twice. The voltage Va and the
potential Ema for voltage transient V(t) are defined as
shown in Equations (1) and (3):
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
984 Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Va = E V tVipp a− ( ) (2)
E V t Ema ma= ( ) (3)
Once we have the estimations for Va and Ema, the cal-
culation of all the other voltage and potentials shown in
Equations (4), (5) and (6), is straightforward:
ΔV E V t= −ipp tmin ( ) (4)
Δ ΔE V Vp a= − (5)
E V t Vmc t a= +min ( ) (6)
Afterwards, the polarization impedance |Zpol| was
calculated as dEp/ic at location x, where the RE was the
closest to the WEs, i.e., at the middle of the electrodes.
3 RESULTS AND DISCUSSION
Figure 3 shows the stimulation pulse and elicited VT
with the indicated characteristic voltage and potential
elements that contributed to dV, i.e., Eipp, Va, and dEp in
both WEs. These elements were then accounted for in
the calculation of Emc and Ema in each of the two WEs.
Figure 3 shows that the onset of ic elicited the
near-instantaneous VT voltage, where Va could be easily
determined at the instant rise. However, when ic was ter-
minated, the behavior of the potential in the exponential
decay region where ic was exponentially approaching the
lowest value prevented an easy determination of Va. In
this regard, the analysis technique described by Cogan30,
which introduced a short interruption of the stimulation
current between the cathodic and anodic phase of the
pulse, could not be used because of the quasi-trapezoidal
stimulation waveform used in the study.
Emc and Ema, scanned over WE1 and WE2, are de-
monstrated in Figure 4. The figure also shows the values
for Eipp, dV, Va, and dEp.
3.1 Results for WE1
Figure 4 shows that the average dEp in WE1 is 0.59
V. The value of Va is 0.24 V, while the value of Eipp is
-0.11 V. It is difficult to accurately measure Va when ic is
terminated, which is a practical issue when determining
Emc, as shown in Figure 3. However, Ema was determined
with relative ease. As in the VT of the pulse used, Emc
and Ema reached values of -0.7 V and -0.01 V, respec-
tively. Accordingly, Emc slightly exceeded the safe po-
tential limits for water electrolysis, while Ema remained
slightly negative and did not exceed this limit. Figure 4
also shows that WE1 was cathodically limited for the
pulse waveform used; i.e., Emc reached the cathodic limit
before Ema reached the anodic limit of the water electro-
lysis window. dV was 0.83 V across the electrode-elec-
trolyte interface.
3.2 Results for WE2
Figure 4 shows that the average dEp in WE2 was
0.615 V; the value of Va was 0.109 V, and the value of
Eipp is -0.075 V. Again, the practical issue in determining
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988 985
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Stimulation pulse and elicited VT in WEs with the indicated
elements that contributed to the entire voltage drop, dV
Figure 4: Schematic side-view diagram of the WEs and variables,
Eipp, dV, Va, dEp, Emc, and Ema: a) WE1 and b) WE2
same company and stored on a portable computer
(LenovoT420, China).
2.2 Crafting the working electrodes
Crafting the WEs involved several steps: 1) cutting
0.66-mm-wide strips from the 0.03-mm-thick platinum
foil using a sharp blade, 2) cutting two 6-mm-long indi-
vidual strips, 3) folding the obtained strips into thirds,
creating U-shaped forms, and 4) inserting the 2-mm-
wide strip made of the silicone sheet between the
U-shaped structure to form a sandwich structure. The
final dimensions of the electrode surfaces exposed to the
PBS were width=0.66 mm, length=3mm, and surface
area=2 mm2.
The interconnection between the WEs and the lead
wires was performed using a simple construction
custom-designed micro-spot-welder based on a thyristor
and powered using a large capacitor discharge by highly
experienced and well-trained personnel. An energy of
approximately 6 Ws (defined experimentally) provided
reproducible results for the proposed application of
welding the lead wire (AS 631, Cooner Wire, USA)
within the sandwich structure. The welding energy for
both electrodes is defined in Table 2. This table also
provides information for the two different types of sand
paper (Waterproof Silica Carbide Paper FEPA P#500 and
FEPA P#4000, Struers ApS, Denmark) that were used to
increase the real surface of the WEs. Specifically, the
surface of WE1 was enlarged using rough sand paper
(FEPA P#500) while WE2 was enlarged using fine-
grained sand paper (FEPA P#4000). In this relation, each
of the WE surfaces was treated by grinding in a letter-I
pattern for a period of 10 s using an appointed sand
paper and vertical force of approximately 5 N.
In Figure 1, WE1 is situated at the right-hand side of
the cell and WE2 at the left-hand side of the cell.
Table 2: Welding energy and sand paper used in the experiments
WE Chargingcurrent (A)
Charging
voltage (V)
Welding
energy (Ws) Sand paper
1 5 2 6 FEPA P#500
2 5 2 6 FEPAP#4000
2.3 VT measurements
VTs were measured using a biphasic, quasi-trape-
zoidal current stimulating pulse waveform (pulse) with
the intensity of the cathodic phase (ic), which was pre-
viously tested in selective nerve stimulation on an iso-
lated left porcine vagus nerve (not shown in this paper)26,
delivered from the precision custom-designed stimulator
between an appointed WE and the AE. The relevant
parameters (ic = -4.0 mA; width of the cathodic phase,
tc = 155 μs; width of the cathodic exponential decay,
texp = 100 μs; time constant of the exponential decay,
exp = 45 μs; intensity of the anodic phase, ia = 0.45 mA;
width of the anodic phase, ta = 490 μs; charge density
within the cathodic phase, qc = -800.00 μC/cm2; and
charge density within the anodic phase, qa = 849.00
μC/cm2) were pre-set by the stimulator. For this pulse,
Emc and Ema were measured across the electrode-electro-
lyte interface. These potentials were then tested to
determine whether any of them exceeded the values con-
fining the water electrolysis window, which were defined
using cyclic voltammetry (not shown in this paper;
[-0.60 V + 0.85 V] measured in PBS).27–29
There were several characteristic voltages and
potentials that contributed to the entire voltage drop (dV)
and were accounted for in the calculations of Emc and
Ema: polarization across the electrode-electrolyte inter-
face (dEp), potential of the WE at the onset of the pulse
(Eipp), and access voltage (Va; drop across the electrolyte
resistance plus over-potential terms). Emc is defined by
Equation (1):30
Emc = Eipp + dEp = Eipp + (dV – Va) (1)
VTs above each of the two WEs were measured from
various locations on the surface of the WE while the
above-mentioned pulses were applied between the WE
and CE. The scanning displacements, transverse accord-
ing to each of the two WEs, were controlled optically
using a precise micrometre dial, while at the same time,
they were measured using precise linear potentiometer
(20 kΩ) connected to the aforementioned high-perfor-
mance data-acquisition system. During the scanning, the
tip of the RE was situated as close as possible to the
WEs using a micrometre enabling precise vertical dis-
placement.
2.4 VT analysis
A set of approximately 160 VTs, each corresponding
to a specific location at the investigated WE obtained by
the displacement x performed by the RE above the WE
was measured. Afterwards, each VT in a set was auto-
matically analysed/processed to determine the aforemen-
tioned characteristic voltages and potentials that con-
tributed to the entire voltage drop dV. Characteristic
voltages and potentials for a certain set (displacement
x) were then calculated as the mean of all the values in
a set. In this regard, the potential of the WE at the onset
of the pulse Epp was estimated as the mean value of 50
measured signal samples before the onset of the pulse.
The points that determine the voltage Va and the potential
Ema of the VT are located at pulse times where the rate of
change of inclination of the signal tangent is high. Thus,
the times tV a and t E ma of those points were estimated by
locating local maxima of the second derivative of the
voltage response. The second derivative of the signals
was estimated by convolving the signals with a digital
filter of the form [-1 0 1] twice. The voltage Va and the
potential Ema for voltage transient V(t) are defined as
shown in Equations (1) and (3):
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
984 Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Va = E V tVipp a− ( ) (2)
E V t Ema ma= ( ) (3)
Once we have the estimations for Va and Ema, the cal-
culation of all the other voltage and potentials shown in
Equations (4), (5) and (6), is straightforward:
ΔV E V t= −ipp tmin ( ) (4)
Δ ΔE V Vp a= − (5)
E V t Vmc t a= +min ( ) (6)
Afterwards, the polarization impedance |Zpol| was
calculated as dEp/ic at location x, where the RE was the
closest to the WEs, i.e., at the middle of the electrodes.
3 RESULTS AND DISCUSSION
Figure 3 shows the stimulation pulse and elicited VT
with the indicated characteristic voltage and potential
elements that contributed to dV, i.e., Eipp, Va, and dEp in
both WEs. These elements were then accounted for in
the calculation of Emc and Ema in each of the two WEs.
Figure 3 shows that the onset of ic elicited the
near-instantaneous VT voltage, where Va could be easily
determined at the instant rise. However, when ic was ter-
minated, the behavior of the potential in the exponential
decay region where ic was exponentially approaching the
lowest value prevented an easy determination of Va. In
this regard, the analysis technique described by Cogan30,
which introduced a short interruption of the stimulation
current between the cathodic and anodic phase of the
pulse, could not be used because of the quasi-trapezoidal
stimulation waveform used in the study.
Emc and Ema, scanned over WE1 and WE2, are de-
monstrated in Figure 4. The figure also shows the values
for Eipp, dV, Va, and dEp.
3.1 Results for WE1
Figure 4 shows that the average dEp in WE1 is 0.59
V. The value of Va is 0.24 V, while the value of Eipp is
-0.11 V. It is difficult to accurately measure Va when ic is
terminated, which is a practical issue when determining
Emc, as shown in Figure 3. However, Ema was determined
with relative ease. As in the VT of the pulse used, Emc
and Ema reached values of -0.7 V and -0.01 V, respec-
tively. Accordingly, Emc slightly exceeded the safe po-
tential limits for water electrolysis, while Ema remained
slightly negative and did not exceed this limit. Figure 4
also shows that WE1 was cathodically limited for the
pulse waveform used; i.e., Emc reached the cathodic limit
before Ema reached the anodic limit of the water electro-
lysis window. dV was 0.83 V across the electrode-elec-
trolyte interface.
3.2 Results for WE2
Figure 4 shows that the average dEp in WE2 was
0.615 V; the value of Va was 0.109 V, and the value of
Eipp is -0.075 V. Again, the practical issue in determining
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988 985
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Stimulation pulse and elicited VT in WEs with the indicated
elements that contributed to the entire voltage drop, dV
Figure 4: Schematic side-view diagram of the WEs and variables,
Eipp, dV, Va, dEp, Emc, and Ema: a) WE1 and b) WE2
Emc (Figure 3) is the difficulty in accurately measuring
Va when ic is terminated. Emc and Ema reached values of
-0.7 V and -0.054 V, respectively as for the VT of the
pulse. Accordingly, Emc slightly exceeded the safe poten-
tial limits for water electrolysis, while Ema (determined
with relative ease) remained slightly negative and did not
exceed this limit. Figure 4 also shows that WE2 was
cathodically limited at the pulse waveform that was used;
i.e., Emc reached the cathodic limit before Ema reached the
anodic limit of the water electrolysis window. dV was
0.72 V across the electrode-electrolyte interface.
Figure 5 shows the voltage changes of the variables
dEp, Emc, and Ema as scanned over WE1 and WE2. The
average variability in Emc and Ema were assumed to in-
crease as the number of sharp peaks and edges produced
on the surface of WE1 increased.31 These results support
the hypothesis, i.e., that the current density would
decrease with a greater number of sharp peaks and edges
on WE1. Finally, Table 3 shows |Zpol| calculated from
dEp/ic at x, where the RE was the closest to the WEs, i.e.,
at the middle of electrodes.
Table 3: Polarization impedance |Zpol| for WE1 and WE2 derived
from two scans
dEp (V)
WE1 WE2
First
scan
Second
scan
Mean
value
First
scan
Second
scan
Mean
value
|Zpol|
(Ω) 238,8 235,4 237,1 256 246 251
The results indicate that the surface of WE1
(modified with rough sand paper) delivers more current
to the nerve tissue and more activation for a fixed input
voltage than WE2 (modified with fine sand paper).
Consequently, as the total delivered current is reduced,
this activation is obtained at a relatively reduced input
power.
Our results are consistent with the results of other
investigators.5 Namely, in the study, the pulse was pre-
defined to retain a near-zero net charge while employing
an asymmetry in the current and pulse widths for the
cathodic and anodic phases.30 By doing so, the Eipp of
both WEs is maintained within a potential range that
avoids WE damage and potential tissue injury. In clinical
practice, the latter is generally more of a concern than
damage to the WE. The relatively large real area of both
WEs obtained by mechanical roughening is largely
responsible for accommodating a certain amount of
charge on the DL prior to initiating the Faradaic reac-
tions. However, WE1 presumably accommodated larger
amounts of charge than WE2. The electrode impedance
is strongly related to the surface area at the interface
between the electrode and the electrolyte.
Roughening the surface of both the WEs significantly
decreases the |Zpol| of the WE-electrolyte interface, while
significantly increasing the capacitance of the DL.32
However, the results show that the |Zpol| of the
WE1-electrolyte interface decreased more than that of
the WE2-electrolyte interface and the capacitance of the
DL was reversed. Both WEs presumably produce a uni-
form charge density over each WE, which permits
maximum utilization of the proprietary WE surface.33
Table 3 shows that the mean |Zpol| of WE1 was lower
than that for WE2 (237.1 
 vs. 251 
), which confirmed
our initial presumption.34
One weakness of the developed method is the sen-
sitivity of the VT measurements with respect to the
distance between the WEs and RE. Fortunately, this
weakness could be minimized using the optical system,
enabling precise control of this distance.
The results support our hypothesis that the Eipp of
WE1 at the onset of the predefined pulse (-0.83 V)
settles at a slightly more negative value at the end of the
pulse, indicating that it changes in a negative direction
(refer to the measured VT in Figure 3). Presumably, Emc
slightly breaks the cathodic limit before Ema reaches the
anodic limit of the water electrolysis window because of
the asymmetric predefined pulse. Consequently, the
functionality of WE1 is cathodically limited. This
suggests that Emc slightly exceeds the safe potential
limits of water electrolysis [-0.65 V to +0.85 V] while
the Ema potential does not. Similarly, the Eipp of WE2 at
the onset of the predefined pulse of -0.75 V is also
settled at a slightly more negative value at the end of the
pulse, indicating that it does change less significantly in
a negative direction. The functionality of WE2 is also
cathodicaly limited.
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
986 Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Comparison of dEp (left), Emc (middle), and Ema (right) with respect to x for WE1 and WE2
In this regard, C. Newbold et al.22 showed that the
WE is polarized until another (possibly irreversible)
reaction is recruited to maintain the ic if ic exceeds the
rate at which the counterions for the intended reversible
processes are transported to or from the WE. This can
happen in vivo, where the ic distribution can lead to
much larger potentials at the edges of the WEs, resulting
in possible WE degradation. Considering these results,
the described modification of the surface could
potentially require less input power, while maintaining
the same level of neural activation in vivo.
4 CONCLUSIONS
Superficially modified WE1 (using rough sand paper)
elicited an increased irregularity and decrease of the
current density profile on the surface and consequently
in the adjacent nerve tissue in vivo using current pulses.
Furthermore, superficially modified WE2 (using fine
sand paper) elicited a slightly decreased irregularity and
corresponding increase of the current density profile on
the surface and consequently in the adjacent nerve tissue
in vivo under the same conditions. Thus, the impedance
of WE1 is lower than that of WE2. Accordingly, WE1 is
more suitable for safe stimulation than WE2.
However, these promising observations warrant
extensive in-vivo testing. Future investigations should
focus on the involvement of VT measurements and an
impedance characterization of the platinum WE in the
presence of proteins in vitro and acutely in vivo.
These preliminary findings suggest that the develop-
ment of such scanning electrochemical methods could
potentially provide significant advantages in targeting
specific neural populations, allowing optimized thera-
peutic protocols. This study has contributed to the
further development of multi-electrode spiral cuffs for
the efficient and safe selective stimulation of autono-
mous peripheral nerves and the simultaneous selective
recording of neural responses.
Acknowledgment
This work was financed by research grant P3-0171
from the Slovenian Research Agency (ARRS), Ministry
of Education, Science and Sport, Ljubljana, Republic of
Slovenia.
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World Journal, 1 (2014), 310283
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Emc (Figure 3) is the difficulty in accurately measuring
Va when ic is terminated. Emc and Ema reached values of
-0.7 V and -0.054 V, respectively as for the VT of the
pulse. Accordingly, Emc slightly exceeded the safe poten-
tial limits for water electrolysis, while Ema (determined
with relative ease) remained slightly negative and did not
exceed this limit. Figure 4 also shows that WE2 was
cathodically limited at the pulse waveform that was used;
i.e., Emc reached the cathodic limit before Ema reached the
anodic limit of the water electrolysis window. dV was
0.72 V across the electrode-electrolyte interface.
Figure 5 shows the voltage changes of the variables
dEp, Emc, and Ema as scanned over WE1 and WE2. The
average variability in Emc and Ema were assumed to in-
crease as the number of sharp peaks and edges produced
on the surface of WE1 increased.31 These results support
the hypothesis, i.e., that the current density would
decrease with a greater number of sharp peaks and edges
on WE1. Finally, Table 3 shows |Zpol| calculated from
dEp/ic at x, where the RE was the closest to the WEs, i.e.,
at the middle of electrodes.
Table 3: Polarization impedance |Zpol| for WE1 and WE2 derived
from two scans
dEp (V)
WE1 WE2
First
scan
Second
scan
Mean
value
First
scan
Second
scan
Mean
value
|Zpol|
(Ω) 238,8 235,4 237,1 256 246 251
The results indicate that the surface of WE1
(modified with rough sand paper) delivers more current
to the nerve tissue and more activation for a fixed input
voltage than WE2 (modified with fine sand paper).
Consequently, as the total delivered current is reduced,
this activation is obtained at a relatively reduced input
power.
Our results are consistent with the results of other
investigators.5 Namely, in the study, the pulse was pre-
defined to retain a near-zero net charge while employing
an asymmetry in the current and pulse widths for the
cathodic and anodic phases.30 By doing so, the Eipp of
both WEs is maintained within a potential range that
avoids WE damage and potential tissue injury. In clinical
practice, the latter is generally more of a concern than
damage to the WE. The relatively large real area of both
WEs obtained by mechanical roughening is largely
responsible for accommodating a certain amount of
charge on the DL prior to initiating the Faradaic reac-
tions. However, WE1 presumably accommodated larger
amounts of charge than WE2. The electrode impedance
is strongly related to the surface area at the interface
between the electrode and the electrolyte.
Roughening the surface of both the WEs significantly
decreases the |Zpol| of the WE-electrolyte interface, while
significantly increasing the capacitance of the DL.32
However, the results show that the |Zpol| of the
WE1-electrolyte interface decreased more than that of
the WE2-electrolyte interface and the capacitance of the
DL was reversed. Both WEs presumably produce a uni-
form charge density over each WE, which permits
maximum utilization of the proprietary WE surface.33
Table 3 shows that the mean |Zpol| of WE1 was lower
than that for WE2 (237.1 
 vs. 251 
), which confirmed
our initial presumption.34
One weakness of the developed method is the sen-
sitivity of the VT measurements with respect to the
distance between the WEs and RE. Fortunately, this
weakness could be minimized using the optical system,
enabling precise control of this distance.
The results support our hypothesis that the Eipp of
WE1 at the onset of the predefined pulse (-0.83 V)
settles at a slightly more negative value at the end of the
pulse, indicating that it changes in a negative direction
(refer to the measured VT in Figure 3). Presumably, Emc
slightly breaks the cathodic limit before Ema reaches the
anodic limit of the water electrolysis window because of
the asymmetric predefined pulse. Consequently, the
functionality of WE1 is cathodically limited. This
suggests that Emc slightly exceeds the safe potential
limits of water electrolysis [-0.65 V to +0.85 V] while
the Ema potential does not. Similarly, the Eipp of WE2 at
the onset of the predefined pulse of -0.75 V is also
settled at a slightly more negative value at the end of the
pulse, indicating that it does change less significantly in
a negative direction. The functionality of WE2 is also
cathodicaly limited.
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
986 Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Comparison of dEp (left), Emc (middle), and Ema (right) with respect to x for WE1 and WE2
In this regard, C. Newbold et al.22 showed that the
WE is polarized until another (possibly irreversible)
reaction is recruited to maintain the ic if ic exceeds the
rate at which the counterions for the intended reversible
processes are transported to or from the WE. This can
happen in vivo, where the ic distribution can lead to
much larger potentials at the edges of the WEs, resulting
in possible WE degradation. Considering these results,
the described modification of the surface could
potentially require less input power, while maintaining
the same level of neural activation in vivo.
4 CONCLUSIONS
Superficially modified WE1 (using rough sand paper)
elicited an increased irregularity and decrease of the
current density profile on the surface and consequently
in the adjacent nerve tissue in vivo using current pulses.
Furthermore, superficially modified WE2 (using fine
sand paper) elicited a slightly decreased irregularity and
corresponding increase of the current density profile on
the surface and consequently in the adjacent nerve tissue
in vivo under the same conditions. Thus, the impedance
of WE1 is lower than that of WE2. Accordingly, WE1 is
more suitable for safe stimulation than WE2.
However, these promising observations warrant
extensive in-vivo testing. Future investigations should
focus on the involvement of VT measurements and an
impedance characterization of the platinum WE in the
presence of proteins in vitro and acutely in vivo.
These preliminary findings suggest that the develop-
ment of such scanning electrochemical methods could
potentially provide significant advantages in targeting
specific neural populations, allowing optimized thera-
peutic protocols. This study has contributed to the
further development of multi-electrode spiral cuffs for
the efficient and safe selective stimulation of autono-
mous peripheral nerves and the simultaneous selective
recording of neural responses.
Acknowledgment
This work was financed by research grant P3-0171
from the Slovenian Research Agency (ARRS), Ministry
of Education, Science and Sport, Ljubljana, Republic of
Slovenia.
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988 Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988
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M. S. VIJAYANAND, M. ILANGKUMARAN: OPTIMIZATION OF MICRO-EDM PARAMETERS USING GREY-BASED ...
989–995
OPTIMIZATION OF MICRO-EDM PARAMETERS USING
GREY-BASED FUZZY LOGIC COUPLED WITH THE TAGUCHI
METHOD
OPTIMIZACIJA PARAMETROV MIKROELEKTROEROZIJE
Z UPORABO MEHKE LOGIKE V POVEZAVI S TAGUCHI METODO
Muthiyalu Shanmugam Vijayanand1, Mani Ilangkumaran2
1Paavai Engineering College, Department of Mechanical Engineering, Pachal, 637018 Namakkal, Tamilnadu, India
2K.S.R. College of Technology, Department of Mechatronics Engineering, Tiruchengode, 637018 Namakkal, Tamilnadu, India
msvijayanand08@gmail.com
Prejem rokopisa – received: 2017-04-29; sprejem za objavo – accepted for publication: 2017-07-28
doi:10.17222/mit.2017.048
The correct selection of process parameters for the best performance output of a micro-electro-discharge machining
(Micro-EDM) process is challenging because the performance measures of micro-EDM are non linear. This work aims to solve
and control complex non-linear systems by applying the hybrid grey-based fuzzy logic together with the Taguchi technique in
the field of micro-EDM. Input parameters, namely, the discharge current, pulse-off time and pulse-on time were selected to
obtain the target responses such as the material-removal rate (MRR) and tool-wear rate (TWR). Nine experiments were
performed based on the Taguchi L9 orthogonal array. An analysis of variance was performed to find the significant contribution
of the intervening process parameter in a single performance characteristic using the grey-based fuzzy-logic expert system.
Multi-performance characteristics indexes (MPCIs) were analysed and the results were calculated with good accuracy.
Keywords: ANOVA, fuzzy logic, orthogonal array, grey-based Taguchi technique, electrical-discharge machining, drilling
Pravilna izbira procesnih parametrov za doseganje najbolj{ega izkoristka procesa mehanske obdelave z mikroelektro erozijo
(angl. Micro-EDM) je izziv, ker so procesni parametri mikro-EDM nelinearni. Namen pri~ujo~e raziskave je bil re{evanje in
nadzor kompleksnih nelinearnih sistemov mikro-EDM mehanske obdelave z uporabo hibridne mehke logike (Grey-based fuzzy
logic) v povezavi s Taguchi metodo. Avtorji raziskave so izbrali naslednje vhodne parametre: razelektritveni tok, ~as vklopa in
~as izklopa impulza. Na njihovi osnovi so dobili odgovore na zastavljeni vpra{anji; kak{na je hitrost odstranjevanja materiala in
kak{na je hitrost obrabe orodja. Na podlagi Taguchi L9 ortogonalne matrike so izvedli devet prakti~nih preizkusov mehanske
obdelave z μ-EDM. Izvedli so analizo variance, podprto z ekspertnim sistemom na osnovi mehke logike, da bi ugotovili u~inek
intervencijskega procesnega parametra pri eni sami spremenljivki. Dolo~ili so indekse u~inkovitosti (angl. MPCIs) in izra~unani
rezultati so bili zelo to~ni.
Klju~ne besede: analiza variance (ANOVA), mehka logika, ortogonalna matrika, robustna statisti~na Taguchi metoda, mikro
elektroerozija (μ-EDM), vrtanje
1 INTRODUCTION
In recent technological advancements, the products
are to be lighter, thinner and smaller. Many advantages
arise when a part is miniaturized, such as energy and
space savings, accelerating chemical reactions, attractive
appearance, and cost-effectiveness.1 The Monel 400
alloy is considered as the most promising and the most
commonly used nickel-based alloy because of its excel-
lent corrosion resistance and toughness over a wide
temperature range. The Monel alloy has been extensively
used in the chemical industry, food-processing industry,
heat-exchanger tubes, nuclear reactors, sub marines and
ship propellers.2 The Monel alloy work hardens rapidly
as it undergoes a high strain during machining. This
hardening effect decreases further machining of the
alloys. Therefore, it is very difficult to machine these
alloys using conventional machine tools.3 Several re-
search works4–6 have been carried out and reported on
machining the nickel-based alloys using different
conventional and non-conventional machining methods.
Micro-machining is the most fundamental technology
used for the production of miniaturized parts and com-
ponents.7 Micro-EDM has been known as one of the
indispensible micro-machining techniques with obvious
advantages of machining complex structures with high
aspect ratios, high precision and accuracy irrespective of
workpiece material’s hardness and toughness.8 Micro-
EDM uses electrical discharge between two electrodes,
and the spark from them generates such an extremely
high temperature that the material is removed by vapor
bubble.9
Many studies1–3 were performed previously on ma-
chining nickel-based alloys with EDM and electro-che-
mical machining. P. Kuppan et al.10 investigated the
effect of various process variables of EDM in deep-hole
drilling of Inconel 718. The objective of this study is to
investigate the interaction effects of the process variables
such as peak current, pulse-on time, duty factor, and
electrode speed on machining characteristics. The results
reveal that the material-removal rate is more influenced
Materiali in tehnologije / Materials and technology 51 (2017) 6, 989–995 989
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 621.18.08:621.9.08 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)989(2017)