Study of electrochemical behaviour and corrosion
resistance of materials for pacemaker lead
applications
Andreas Örnberg
Licentiate thesis
Division of Corrosion Science
Department of Chemistry
School of Chemical Science and Engineering
Royal Institute of Technology
SE-100 44 Stockholm, Sweden
Stockholm 2007
This licentiate thesis will, with the permission of Kungliga Tekniska Högskolan,
Stockholm, be presented and defended at a public licentiate seminar on Wednesday 19th
of December, 2007 at 13.00.
TRITA-CHE-Report 2007:79
ISSN 1654-1081
ISBN 978-91-7178-809-2
ii
Abstract
For patients suffering bradycardia, i.e., too slow heart rhythm, the common treatment
is having a pacemaker implanted. The pacemaker system consists of the pacemaker and a
pacing lead. The pacing lead is connected to the pacemaker and at the other end there is a
stimulation electrode. The most common conductor material is a cobalt-based super alloy
(MP35N® or 35N LT®), with the main constituents Ni, Co, Cr and Mo. The pacemaker
electrode is often made of a substrate material with a rough surface coating. The substrate
materials are predominantly platinum/iridium alloy and titanium. The material choice is
of great importance for the performance and stability during long-term service. Excellent
corrosion resistance is required to minimize elution of metal ions in the human body.
In this thesis, the electrochemical behaviour and corrosion resistance of the Co-based
alloys and Ta (as electrode substrate), in a phosphate buffer saline (PBS) solution with
and without addition of H2O2, was investigated by means of potentiodynamic
polarization, cyclic voltammetry, electrochemical impedance spectroscopy and simulated
pacemaker pulsing. The metal release from the Co-based alloy during the passivation
treatment and exposure in the synthetic biological media was measured by using
inductive coupled plasma - atomic emission spectroscopy (ICP-AES). Moreover, surface
composition was analyzed by using x-ray photoelectron spectroscopy.
The results show that the chemical passivation of Co-based alloy 35N LT® increased
the corrosion resistance and reduced Co release significantly, even in more hostile
environment, i.e. PBS with addition of H2O2. The increased corrosion resistance is due to
the Cr enrichment in the surface layer. The reduced Co release is due to a preferential
dissolution of Co from the surface oxide layer during the chemical passivation. The
electrochemical investigation of uncoated and rough TiN coated Ta show that uncoated
Ta is not suitable electrode material due to formation of a highly resistive surface oxide
film. Whereas the rough TiN coated Ta exhibits desirable electrochemical performance
for pacemaker electrodes. The addition of H2O2 in the PBS has a large influence on the
electrochemical behaviour of Ta, but the influence is small on the rough TiN coated Ta.
Keywords: Co-base alloy, 35N LT, MP35N, Ta, TiN coating, electrochemical behaviour,
corrosion resistance, metal release, passivation treatment, pacemaker electrode, H2O2.
iii
iv
Preface
The following papers are included in this thesis:
I
“Corrosion resistance, chemical passivation and metal release of 35N LT and
MP35N for biomedical material application”
A. Ornberg, J. Pan, M. Herstedt and C. Leygraf
Journal of the Electrochemical Society, 154 (9) C546-C551 (2007)
II
“Electrochemical study of tantalum as substrate for pacemaker electrodes”
A. Ornberg, J. Pan and C. Leygraf
Submitted to Journal of the Electrochemical Society (November 2007)
I have performed all of the experimental work related to this licentiate project, except for
the XPS measurements and the ICP-AES measurements. I have taken active part in the
analysis of the results, and in the discussion. I also contributed to the outline and writing
of the papers.
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vi
Contents
1
Introduction _______________________________________________________ 1
1.1
Pacemaker___________________________________________________________ 1
1.2
Pacemaker lead ______________________________________________________ 2
1.3
Electrochemical behaviour, corrosion resistance and metal release ____________ 4
1.3.1
1.3.2
1.3.3
2
3
Pitting corrosion __________________________________________________________ 5
Crevice corrosion _________________________________________________________ 6
Galvanic corrosion ________________________________________________________ 6
1.4
Biological environment ________________________________________________ 7
1.5
Metal-ion induced oxidation (MIO) ______________________________________ 8
Experimental _____________________________________________________ 10
2.1
Investigated materials ________________________________________________ 10
2.2
Electrochemical cell and electrolyte _____________________________________ 10
2.3
Electrochemical impedance spectroscopy (EIS) ___________________________ 11
2.4
Potentiodynamic polarization __________________________________________ 11
2.5
Cyclic voltammetry __________________________________________________ 13
2.6
X-ray photoelectron spectroscopy ______________________________________ 13
2.7
Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) ________ 14
Summary of results ________________________________________________ 15
3.1
Why is the electrochemical behaviour of a material important when used in
biomaterial applications? _____________________________________________ 15
3.2
Can passivation treatment improve the corrosion resistance of cobalt-based alloys
such as 35N LT®? (Paper I)____________________________________________ 16
3.3
Is the chemical passivation treatment beneficial with respect to the metal-ion
induced oxidation of polyurethane materials? (Paper I) ____________________ 18
3.4
Does the addition of hydrogen peroxide to the PBS electrolyte influence the
electrochemical properties of cobalt-based alloys? (Paper I)_________________ 20
3.5
Can tantalum be used as pacing and sensing electrode material in pacemaker
applications? (Paper II) _______________________________________________ 22
3.6
Is the electrochemical behaviour of tantalum affected by the addition of hydrogen
peroxide? (Paper II)__________________________________________________ 23
3.7
Is tantalum a good substrate for the titanium nitride coating used in pacemaker
electrodes? (Paper II)_________________________________________________ 26
4
Main conclusions __________________________________________________ 30
5
Acknowledgements_________________________________________________ 31
6
References _______________________________________________________ 33
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1
Introduction
Biomaterial is a rather new concept. Just 50 years ago the word “biomaterial” was
not used. However, external prosthetics such as artificial limbs and bone fixation devices
have been used throughout history to ease the everyday life of people that have lost a
body part. The first known prosthetic, an iron hand with flexible finger joints, dates back
to 1504 (Lavine 2002). Fifty years ago the understanding of biocompatibility was
inadequate. The major breakthrough for biomaterials came after the World War II, and
since then scientists achieved understanding of biocompatibility. Metallic material
implants such as titanium showed satisfactory results in the early 1940s. Titanium went
from being used in military applications to peacetime uses (Rathner 2004a). Biomaterials
are limited due to the ever-present issues relating to biocompatibility. The
biocompatibility is defined as: “The ability of a material to perform with an appropriate
host response in a specific application.” (Williams 1987). One of the many aspects that
influence biocompatibility of metallic materials is controlled by the chemical, or more
specifically the electrochemical interaction that results in the release of metallic ions into
the tissue, and the toxicology of these released substances. (Williams 1981). Therefore
the electrochemical evaluation of the biomaterial in relevant artificial biological solution
is of great importance.
1.1
Pacemaker
One medical device application where biomaterials are employed today is in
pacemaker systems. In October 1958 in Stockholm, Sweden, there was a major break
through in cardiac pacing. Dr Åke Senning implanted the world’s first implantable
pacemaker system, which was designed by Dr. Rune Elmqvist (Lindgren et al. 1992,
Larsson et al. 2003). Since then there has been an enormous development of the pacing
technology, which has resulted in better pacemaker systems, smaller size and enhanced
performance. A pacemaker is a battery powered implantable device that electrically
stimulates the heart to contract and pump blood throughout the body in cases where the
heart’s electrical system is not working properly, i.e. a too slow heart rhythm. When a
1
patient has an abnormally slow heart rhythm, he or she will be diagnosed with
bradycardia, and a pacemaker may be implanted to help the patient. The pacemaker
system includes a pulse generator and a pacing lead. The pulse generator case is made of
commercially pure titanium and holds the electronic module containing sensing,
stimulation circuit and controlling unit. The titanium case is hermetically sealed. The
pacing lead consists of a connector covered by insulation, which is to connect the pacing
lead to the pulse generator. At the other end of the lead there is an electrode enabling
artificial cardiac stimulation. This electrode is fixated to the heart tissue by passive or
active fixation, i.e. tines or a helix. During pacing it is preferred that the stimulation
pulses are transferred through interfacial charging-discharging processes (Norlin et al.
2002). For a successful stimulation the electrode needs to exhibit capacitive
characteristics with enough charging capacity to overcome the so-called pacing threshold
(Norlin et al. 2002). Reversible reactions are normally not associated with undesired
reactions products, and therefore the reversible reactions generally are considered safe
with respect to the surrounding tissue (Norlin 2005). Irreversible reactions should be
minimized since they may cause degradation of the electrode or undesired reactions
products. The choices of electrode material are extremely important to get the desirable
charge transfer properties.
1.2
Pacemaker lead
When designing pacemaker lead the material choice of the components is of great
importance. The most common conductor material is a cobalt-based alloy called
MP35N® or 35N LT®, a super alloy with the main constituents Ni, Co, Cr and Mo. This
is the most common one, but the conductor can also be made from titanium or tantalum
(Schmidt 1997). A conductor can be a solid wire or a drawn filled tube (DFT) and is
typically wound into a coil. The cobalt-based alloy exhibits outstanding fatigue and
corrosion resistance properties (Reese et al. 1991). The superior corrosion resistance and
fatigue properties make the material an excellent material as conductor in pacemaker lead
applications since chemical and mechanical stabilities are essential. Fig. 1 shows a
pacemaker lead with active fixation and a pacemaker.
2
Figure 1. Pacemaker lead with active fixation and pacemaker
The pacemaker electrode often consists of a substrate material with a surface coating
(Schmidt 1997). The substrate materials are predominantly platinum/iridium alloy and
titanium. These substrate materials are often coated with a variety of electrically
conductive materials such as, iridium oxide (IROX), porous platinum (platinum black)
and titanium nitride (TiN) (Schmidt 1997). Platinum, titanium and TiN coated electrodes
have been thoroughly investigated electrochemically by (Norlin et al. 2005), and the
results showed that reversible oxidation and reduction reactions occurred on platinum and
rough TiN electrodes. Oxide growth and partial reduction occurred on titanium and
smooth TiN. Moreover, a pronounced hydrogen adsorption on titanium was evident
(Norlin et al. 2005). Another material, tantalum, appears to have all the necessary
properties that a potential electrode substrate material needs. Tantalum has good
biocompatibility, excellent mechanical properties and chemical resistance (Black 1994).
The clinical use of tantalum dates back to 1940s (Black 1994). Furthermore, when the
medical doctor performs the implantation, it is helpful to follow the movements and the
placement of the pacemaker lead. The high density of tantalum (16.654 g/cm3) makes the
material easily visible under x-ray that is employed during the implantation.
3
1.3
Electrochemical behaviour, corrosion resistance and metal release
Materials for medical applications encounter a hostile environment in the human
body due to the combination of the physiological fluid, inflammatory response, and body
temperature etc., when implanted. Under these conditions the long term stability and
reliability of the materials are of great importance. The chemical stability or more exactly
the electrochemical stability can be evaluated by investigating, e.g. the corrosion
resistance (Aziz-Kerrzo et al. 2001) and the metal release in relevant electrolyte (Castle et
al. 1990). The electrochemical investigations are mainly performed in vitro, however,
they can provide a good understanding of the corrosion processes and help to predict the
in vivo behaviour. Corrosion resistance is one of main parameters that determine the
biocompatibility of metallic biomaterials considered for medical use (Williams 1981).
Excellent corrosion resistance is required to minimize elution of toxic metal ions in the
human body. The corrosion resistance of a metallic material can be measured by means
of electrochemical methods, such as potentiodynamic polarization, electrochemical
impedance spectroscopy (EIS) and cyclic voltammetry (CV).
When using EIS for the evaluation of a material’s corrosion resistance, a parameter
obtained at low frequencies is the polarisation resistance (Rp), that is related to the
corrosion current via formula (1) (Barsoukov and Macdonald 2005).
Rp ∝
1
i0
(1)
Where i0 is the corrosion current density. The polarization (corrosion) resistance can
be determined from the EIS measurement. From the corrosion current density the
corrosion rate can be calculated.
The corrosion resistance of stainless steels and other alloys can be improved by
passivation treatments in acidic solutions (Hultquist et al. 1980, Noh et al. 2000, Asami et
4
al. 1979, Salvago et al. 1994, Salvago et al. 1987). The passivation treatment is described
in detail in paper I.
To study the oxidation-reduction of a material in an electrolyte, CV is a powerful
technique that involves application of a time varying potential to an electrochemical cell
and simultaneous measurement of the resulting current (Prutchi 2005). The current peaks
in the CV curves provide information about the potentials at which specific
electrochemical reactions occur, the influence of potential sweep rate on the peak current
(reaction rate), and the influence of formation of insulating oxides on the current level,
etc. This technique is extremely useful when evaluating candidate materials for
pacemaker electrodes.
The potentiodynamic polarisation, where the current density is monitored over a
preset potential range, gives information about the passivity characteristics and the
susceptibility to pitting, crevice and general corrosion of the material. The
potentiodynamic polarisation is an effective method to evaluate the passivity
characteristics and the susceptibility to pitting, crevice and galvanic corrosion in relevant
electrolytes. Bellow follows a brief description of the corrosion types.
1.3.1 Pitting corrosion
Pitting corrosion occurs on metals and alloys relying on a passive surface film for its
corrosion resistance in environments containing halides, e.g. Cl- (Bardal 2003). The
pitting corrosion arises due to access of so-called aggressive anions (Strehblow 2002). It
involves the formation of a local anode separated from the cathode where electrochemical
reactions take place. At the cathodic site, i.e. the passivated surface, cathodic reactions
occur, and at the anodic site, i.e. the “pit”, anodic reactions take place. The metal or alloy
is the conductor for electrons moving between the sites. Metallic and non-metallic
inclusions often play a decisive role in the initiation of pitting corrosion. As mentioned
above the presence of aggressive anions is necessary for breakdown of passivity and
stable pit growth (Strehblow 2002).
5
1.3.2 Crevice corrosion
Crevice corrosion occurs in regions of restricted flow, i.e. limited mass transport,
resulting in exhaustion of reactants, e.g. dissolved oxygen (Strehblow 2002). The oxygen
depletion and the accumulation of corrosion products lead to formation of a small
galvanic cell, which enhance the anodic reaction (corrosion) inside the crevice. Crevice
corrosion occurs similar to pitting corrosion mainly on metals or alloys depending on a
passive surface film for their corrosion resistance.
1.3.3 Galvanic corrosion
When a metallic contact is created between a more noble metal and a less noble
metal, the corrosion rate will increase on the latter one while decrease on the former. The
less noble metal or alloy will become the anode and the noble one will become the
cathode (Oldfield 1988, Jones 1996). However an electrolytic connection between the
metals and/or alloys is necessary, creating a closed electrical circuit, for galvanic
corrosion to occur (Baboian 1995a).
Metal release rates of alloys should also be considered when evaluating the corrosion
resistance. Metal release rates can give information about selective dissolution (Castle et
al. 1990), which leads to an elemental enrichment on the alloy surface. For stainless
steels, the selective dissolution results in chromium enriched surface (Castle et al. 1990,
Castle et al. 2004, Asami et al. 1979, Hultquist et al. 1984, Herting et al. 2007).
Enrichment of chromium in the surface film is favourable for the corrosion resistance of
stainless steels (Hong et al. 1996, Noh et al 2000, Shih et al. 2004), resulting in an
enhanced corrosion resistance of stainless steels (Bou-Saleh et al. 2007, Shahryari et al.
2007). Recently selective dissolution and chromium enrichment in the surface film have
also been observed for cobalt-based alloys like MP35N® and 35N LT®. An improved
corrosion resistance was obtained on the MP35N® and 35N LT® owing to the chromium
enrichment of the passive surface film (Ornberg et al 2007).
6
1.4
Biological environment
The biological environment is a very complicated aqueous system, containing ions,
molecules, proteins, cells, etc., and the environment may change due to inflammatory
response. As the first step, phosphate buffer saline (PBS) solution is commonly used to
evaluate the electrochemical behaviour of biomaterials in simulated physiological
solution. The PBS is a standard physiological solution with similar ion concentration of
blood. It contains sodium chloride, sodium phosphate and potassium phosphate, with the
formulation shown in table 1. The simple and consistent formula makes it an excellent
solution for comparative purpose when performing electrochemical investigations. The
pH of PBS is similar to that of blood and the buffer helps to maintain a stable pH.
Table 1. Formulation of PBS in mg/L
____________________________
NaCl
8000
KCl
200
Na2HPO4
1150
KH2PO4
200
_____________________________
Implanting a biomaterial or medical device into human body usually involves injury
to the tissue. Inflammation, wound healing and the foreign body response are always to
some extent associated with tissue injured by surgical implantation. During the
inflammatory response one characteristic is that the inflammatory cells release
superoxide and hydrogen peroxide according to (Tengvall et al. 1989):
−
−
O2 + O2 + 2 H + → H 2 O2 + O2
(2)
This suggests that the electrochemical behaviour under these conditions, i.e. in the
presence of hydrogen peroxide, is of interest for metallic materials considered for
medical device applications. The corrosion resistance of titanium has been shown to
decrease in the presence of H2O2 (Pan et al. 1996), due to an enhanced corrosion attack
by H2O2 (Pan et al. 1994).
7
1.5
Metal-ion induced oxidation (MIO)
When implants or medical devices are placed into living tissue, the tissue will
modify the implants or medical devices. This may be represented by oxidation of metals
or degradation of polymers (Williams 1976). One example of polymer degradation
relevant to pacemaker applications is the metal-ion induced oxidation (MIO). The MIO is
a process of oxidative degradation of the polyurethane insulation and involves a reaction
of the polymer with oxygen that is believed to be released by hydrogen peroxide, which
comes from inflammatory cells on the outer surface of the lead (Mond 2000). The
oxidative degradation of polyurethane is catalyzed by corrosion products from the alloy
(Stokes et al. 1995). The pacemaker lead is a long-term implant, some body fluid might
leak into the pacemaker lead during the long-term service. This may cause corrosion of
the metal surface and start the degradation of the polymer insulation (Sung and Fraker
1987). The most likely catalyst for MIO of polyurethane is cobalt ions released from the
MP35N conductor (Ward et al. 2006). The oxidation is initiated at the soft segment
ether’s α carbon of the polyurethane (Ward et al. 2005). The softer the polyurethane is
the more susceptible it is to MIO. So far MIO has only been reported clinically for
polyurethane pacemaker leads (Ratner et al. 2004b). Even though polyurethane has been
shown to be sensitive to MIO (Stokes 1990, Wiggins et al. 2001, Ward et al. 2005), it has
been used as insulation for pacemaker leads since 1977 (Stokes et al. 1995). This can be
explained by the methods to minimize MIO, the cobalt bearing alloy conductors have
been coated with, e.g., platinum coating (Mond 2000).
8
The following questions have been formulated within this licentiate thesis to increase the
knowledge about the electrochemical behaviour of surface modified materials (for
conductor and for electrode substrate) in biological environment. The detailed results and
discussion can be found in Paper I and Paper II.
1. Why is the electrochemical behaviour of a material important when used in
biomaterial applications?
2. Can passivation treatment improve the corrosion resistance of cobalt-based alloys,
such as 35N LT?
3. Is the passivation treatment beneficial with respect to the metal ion induced
oxidation of polyurethane materials?
4. Does the addition of hydrogen peroxide to the PBS solution influence the
electrochemical properties of cobalt-based alloys?
5. Can tantalum be used as pacing and sensing electrode material in pacemaker
applications?
6. Is the electrochemical behaviour of tantalum affected by the addition of hydrogen
peroxide?
7. Is tantalum a good substrate candidate for the titanium nitride coating used in
pacemaker electrodes?
9
2
Experimental
The primary goal of this thesis work has been to increase knowledge about the
electrochemical behaviour of surface modified metallic materials used in pacemaker lead
applications in biological environment, e.g. phosphate buffer saline solutions. The
materials were mainly considered for the conductor and the substrate for coated
electrodes. Moreover, measurements were performed to determine metal release from
selected materials in artificial physiological solutions. The investigation involves the
following electrochemical and analytical techniques: i) potentiodynamic polarisation and
cyclic voltammetry (CV), ii) electrochemical impedance spectroscopy (EIS), iii) x-ray
photoelectron spectroscopy (XPS), and iv) inductively coupled plasma-atomic emission
spectroscopy (ICP-AES).
2.1
Investigated materials
The materials investigated within this licentiate thesis were the two cobalt-based
alloys MP35N® and 35N LT®, used for the conductor wires of pacemaker leads; and pure
Ta as substrate for rough TiN coated pacemaker electrodes.
The MP35N® samples were supplied by Lake Region in Minnesota, USA, and the
35N LT® by Heraeus in Penthalaz, Switzerland. Before the electrochemical
measurements, all the samples were cleaned in 50% acetone and 50% isopropanol in an
ultra-sonic bath for 12 min.
Pure Ta sheets were purchased from Alfa Aesar, Johnson Matthey. Selected samples
were sent to Heraeus in Hanau, Germany, for rough TiN coating. The uncoated Ta
samples were polished with SiC paper down to grit 1200 and rinsed with deionized water
and cleaned with isopropanol prior to the electrochemical measurements. The rough TiN
coated samples were investigated in as-received condition.
2.2
Electrochemical cell and electrolyte
All electrochemical measurements
were
performed
in
a
three-electrode
electrochemical cell. A saturated Ag/AgCl (+0.197V vs. NHE) electrode was used as
10
reference electrode, and a platinum mesh was used as counter electrode. The electrolyte
was phosphate buffer saline (PBS) solution, and in some cases with the addition of 100
mM H2O2. All measurements were done at room temperature.
2.3
Electrochemical impedance spectroscopy (EIS)
In EIS measurement, a small sinusoidal alternating voltage perturbation is applied to
the electrochemical system under steady state conditions, and the impedance response is
measured as a function of the frequency of the alternating perturbation. The results can be
displayed either as Nyquist plot where the imaginary part is plotted against the real part
of the impedance; or as Bode plot where both logarithm scale of the impedance modulus
and linear scale of the phase angle are plotted against the logarithm of frequency (Bard A.
and Faulkner L. 2001).
EIS is a powerful non-destructive technique (Jayaraj et al. 2004, Brevnov et al. 2004,
Norlin et al 2002), allowing characterization of electrode-electrolyte interfaces, and
investigation of the interfacial electrochemical processes, yielding information such as
charge transfer parameters and the electrochemical double-layer structure (Bard and
Faulkner 2001). When performed at the open-circuit potential, it is particularly useful for
monitoring the electrochemical characteristics and detection of the change of the
electrode materials.
2.4
Potentiodynamic polarisation
Potentiodynamic polarisation is a direct current technique that can give information
on the corrosion rate, passivity and breakdown, and susceptibility to pitting, crevice and
galvanic corrosion of the material. In the measurement, a potentiostat is used to control
the driving force for the electrochemical reactions taking place on the working electrode.
The magnitude of this driving force dictates which electrochemical processes taking
place at the working electrode and the counter electrode, as well as their rate normally
measured as the current density (A/cm2) at the applied potential. In practice, often a
cyclic polarisation is performed at a fixed potential scan rate, e.g. 10 mV/min.
11
In Fig. 2 a schematic plot of a potentiodynamic polarisation curve is shown, with
active, passive and transpassive regions. The open circuit potential (OCP) corresponds to
the corrosion potential Ecorr. Another feature is the potential at which the anodic current
increases drastically with applied potential (the breakdown potential). In general, the
more noble this potential is, the less susceptible the metal is to the initiation of localized
corrosion (Baboian 1995b). Moreover, the repassivation potential is the potential at
which the “hysteresis” loop is completed during the reverse scan (Baboian 1995b). The
“hyseresis”, illustrated as dashed line in Fig. 2, indicates that local corrosion has
occurred, such as pitting corrosion, and at the repassivation potential the initiated pits
stop to grow. However, the increase in the current at a certain anodic potential does not
always have to be a breakdown of the protective oxide layer. Instead an oxidation
reaction (e.g., oxygen evolution) can occur, which may result in an increased oxide
thickness and more protective oxide layer towards anodic polarisation. Consequently, the
current will be decreased on the reverse scan. This is illustrated with the dotted line in
Fig. 2.
ip
E (V)
Transpassive region
End 2
EBreakdown
ERepass
Passive region
End 1
Ecorr (OCP)
Active region
Start
log i (A/cm2)
Figure 2. Schematic plot of a potentiodynamic polarisation curve.
12
2.5
Cyclic voltammetry
During CV cycles, a time varying potential is applied to an electrochemical system
and the current response under the conditions is measured. CV is a method useful for
characterization of reversible and irreversible oxidation – reduction reactions. The current
peaks in the CV curves give information about the potentials at which specific
electrochemical reactions take place, the influence of potential sweep rate on the peak
current (reaction rate), and the influence of formation of insulating oxides on the current
level, etc.
In this work, when studying electrochemical behaviour of electrode materials, the
scanned potential range was set between – 1.5 and 1.5 V vs. reference electrode, and the
scan rate at 100 mV/s. In some cases, additional CV cycles were performed with the
scanned potential range between - 2.5 and 1.5 V. The choice of potential range was
chosen to exceed the potentials at which oxygen evolution and hydrogen evolution
reactions may occur. This range is also within the potential range relevant for pacemaker
pulses (Norlin et al. 2005).
2.6
X-ray photoelectron spectroscopy
XPS technique provides information about the elemental composition of the
outermost surface film (1-5 nm) on metal surfaces. It is one of a number of surface
analyse techniques that irradiates solid surfaces with photons having a specific energy, in
order to excite the emission of photoelectrons. The photoelectrons are emitted if the
energy of the photons is larger than the binding energy of the electrons. Electrons having
lower binding energy than the excited photoelectrons can be analysed in terms of their
kinetic energy, from which the binding energy can be calculated. Every element has its
own characteristic spectrum of binding energies, which are slightly shifted depending on
chemical state so that oxidation valences of the elements also can be resolved (Briggs and
Seah 1994). The kinetic energy of ejected photoelectrons limits the depth from which it
can arise, giving XPS a high surface sensibility and sampling depth of a few nano meters.
Moreover, by performing angle-resolved measurements or ion sputtering, compositional
depth profiles can be obtained, which show the variations in the composition from the
surface to a certain depth in the material.
13
In this work, the XPS measurements were performed using a PHI 5500 instrument
with monochromatic Al Kα radiation (1486.6 eV), at a base pressure of 5 × 10-10 torr and
a working pressure < 5 × 10-9 torr. Depth profiling was obtained by Ar+ ion sputtering (at
3 kV).
2.7
Inductively coupled plasma-atomic emission spectroscopy (ICP-AES)
In the ICP-AES measurement, the plasma is formed by argon gas flowing through a
radiofrequency field where it is kept in a state of partial ionization, i.e. the gas consists
partly of electrically charged particles. This allows it to reach very high temperatures. At
high temperature, most elements emit light of characteristic wavelengths, which can be
measured and used to determine the concentration. In this work, the total concentration of
dissolved Co, Ni, Cr, and Mo were analyzed by using ICP-AES, and Analytica AB,
Luleå, Sweden, performed the analysis. The detection limit was 0.04 μg/L for Co, 0.2
μg/L for Ni, 0.2 μg/L for Cr and 0.8 μg/L for Mo, respectively.
14
3
3.1
Summary of results
Why is the electrochemical behaviour of a material important when used in
biomaterial applications?
The biocompatibility is defined as “the ability of a material to perform with an
appropriate host response in a specific application.” (Williams 1987). One of several
aspects that influence the biocompatibility of a metallic material is the electrochemical
behaviour in the relevant electrolyte, e.g., phosphate buffer saline (PBS). The
biocompatibility is generally evaluated by the host reaction of the tissue surrounding the
implanted material. The biocompatibility of metallic materials is controlled by the
chemical and the electrochemical interactions that result in the release of metals into the
tissue, and the toxicology of these released substances (Williams 1981). There is no
doubt that corrosion adversely affects the biocompatibility of a material due to the toxic
effect of the corrosion products (Zitter et al. 1987). Corrosion reactions of the metallic
biomaterial with the biological environment are electrochemical in nature. If the
corrosion resistance is high, the release rate of metallic ions is low. Therefore it is
important to evaluate the corrosion resistance and hence the electrochemical behaviour of
the metallic material when used in biomaterial applications.
Evaluation of the corrosion resistance can be performed with different
electrochemical techniques, and one very powerful technique is potentiodynamic
polarisation. From this measurement the current density at different potentials are
obtained, which provide information of the electrochemical behaviour and corrosion
resistance of the biomaterial in the environment. The current density corresponds to the
rate of electrochemical reactions associated with corrosion processes taking place on the
material surface in the electrolyte under polarisation. The corrosion resistance is inversely
proportional to the current density. It follows that a low current density indicates a high
corrosion resistance and consequently a low level of metals release, beneficial for the
biocompatibility. The electrochemical evaluation methods represent fast and low cost
techniques for material screening tests to gain knowledge about the materials
biocompatibility.
15
In pacemaker electrode applications, the electrochemical behaviour of the material is
crucial for the charger transfer processes occurring at the electrode-tissue interface, and
hence the performance of the electrodes. In these cases, other electrochemical techniques
like electrochemical impedance spectroscopy and cyclic voltammetry are also very useful
for the evaluation of the electrochemical behaviour of the biomaterial (Norlin 2005a).
In all, information of the electrochemical behaviour is important for the choice of
materials for pacemaker lead applications.
3.2
Can passivation treatment improve the corrosion resistance of cobalt-based alloys
such as 35N LT®? (Paper I)
The corrosion resistance depends strongly on the surface oxide film on metallic
materials that rely on the oxide film for their passive behaviour. It has been previously
reported by others that the corrosion resistance of stainless steels can be improved by
passivation treatments in acidic media, e.g. nitric acid solutions (Wallinder et al. 1999,
Barbosa 1983). The corrosion resistance of stainless steels is dependent on the chromium
content (Asami and Hashimoto 1979, Herting et al. 2007). By passivation treatment the
chromium content in the oxide film can be increased and hence the corrosion resistance.
Like stainless steels, the 35N LT® forms a protective oxide film owing to its high
chromium content, and the passivation treatment is expected to be beneficial with respect
to the corrosion resistance.
In this work, a chemical passivation treatment of 35N LT was performed in a nitric
acid solution. The corrosion resistance (polarisation resistance) of the samples before and
after the passivation treatment was evaluated by using electrochemical impedance
spectroscopy, and the surface composition of the samples was analyzed by using x-ray
photoelectron spectroscopy. Fig. 3 shows typical EIS spectra obtained in PBS solution.
16
106
-90
104
-60
103
102
-30
101
Phase angle [deg]
2
Impedance modulus [Ω.cm ]
105
Passivated 35N LT
Unpassivated 35N LT
100
10-3
0
10-2
10-1
100
101
102
103
104
Frequency [Hz]
Figure 3. Bode plots of 35N LT the in PBS, ○) passivated, and ■) unpassivated.
Unpassivated
Cr 2p
a)
Passivated
Ox Me
40 s
Intensity (a.u.)
40 s
Intensity (a.u.)
Ox Me
50 s
50 s
30 s
20 s
10 s
30 s
20 s
10 s
4s
4s
0s
600
Cr 2p
b)
0s
595
590
585
580
575
Binding energy (eV)
570
600
595
590
585 580
575
570
Binding energy (eV)
Figure 4. Cr 2p XPS spectra recorded after 0, 4, 10, 20, 30, 40 and 50 s for a)
unpassivated 35N LT and b) passivated 35N LT.
17
The quantitative analysis of the EIS spectra using an equivalent circuit show, that the
passivation treatment resulted in an increase of the polarisation resistance with more than
4 times when exposed to PBS. The results indicate that the passivation treatment improve
the corrosion resistance of 35N LT. This is most likely due to the chromium enrichment
of the surface oxide layer. The XPS data provide evidence for the chromium enrichment
of the surface oxide film due to the passivation treatment. Fig. 4 shows detailed spectra
for Cr 2p for the unpassivated and passivated samples. The quantitative analysis of the
XPS data indicates significantly increased chromium content in the surface oxide film
after the passivation treatment. These results suggest that the higher polarisation
resistance after the passivation treatment is attributed to the chromium enrichment of the
oxide film.
In all, the corrosion resistance can be improved by passivation treatment, owing to
enrichment of chromium content in the surface oxide layer.
3.3
Is the chemical passivation treatment beneficial with respect to the metal-ion
induced oxidation of polyurethane materials? (Paper I)
Metal-ion induced oxidation (MIO) is a process of oxidative degradation that so far
only has been reported clinically for polyether urethanes pacemaker leads (Ratner et al.
2004b). When a pacemaker lead is implanted in the body some body fluid might leak into
the pacemaker lead under long-term usage. The body fluid may cause corrosion of the
metal surface and start the degradation of the polymer insulation (Sung and Fraker 1987).
It has been shown by other authors that the polyurethane insulation is sensitive to MIO
(Stokes 1990, Wiggins et al. 2001, Ward et al. 2005). The oxidative degradation arises
from interaction between the polyurethane, i.e. the soft segment ether, and certain
corrosion products of the metallic conductor, especially Co species (Stokes 1990, Stokes
and McVenes 1995, Wiggins et al. 2001, Ward et al. 2005). The pacemaker industry has
tackled the problem by using thin coatings on the conductor coils, e.g., platinum (Stokes
and McVenes 1995, Karicherla and Jenney 2004).
18
The chemical passivation approach is based on the anticipation that the selective
dissolution known for stainless steels is also applicable for the cobalt-based alloy 35N
LT. In this work, the chemical passivation was performed in 10.5% HNO3 at 35°C for
150 minutes. To obtain information about the dissolution processes occurring during the
chemical passivation, the metal released in the passivation solution was measured by
means of inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The
release rate is significantly higher for Co and Ni than for Cr and Mo. The percentage of
individual metal release rates relative to the total release rate (sum of Co, Ni, Cr and Mo)
is calculated to be 43% for Co, 40% for Ni, 10% for Cr and 7% for Mo. Compared to
their contents in the alloy (35% Co, 35% Ni, 20% Cr and 10% Mo), Fig 5, these results
suggest preferential dissolution of Co and Ni during the chemical passivation in the
acidic solution, which results in a concomitant surface enrichment of Cr and Mo and the
formation of a more stable and protective passive film.
The preferential dissolution of Co is supported by the XPS analysis performed on
unpassivated and passivated samples. The XPS analysis shows that passivated samples
are considerably enriched in Cr-oxide (Fig. 6) and depleted in Co-oxide (Fig. 6), in the
surface oxide film. Moreover, the Cr enrichment and Co depletion in the surface oxide
layer will most likely result in a reduced Co release during exposure to aqueous media,
since in this case the Co release is believed to occur through transport across the surface
oxide film, and the Cr-enriched oxide film acts as an enhanced barrier against the Co
release
b)
a)
Cr
Cr
Mo
Co
Co
Mo
Ni
Ni
Figure 5. a) Shows the elemental contents in the alloy (35% Co, 35% Ni, 20% Cr, 7%
Mo). b) Shows the individual metal release rates relative to the total metal release rate
(43% Co, 40% Ni, 10% Cr, 10% Mo) of 35N LT during passivation in 10.5 % HNO3 at
35 ° C for 150 min.
19
Cr 2p
Co 2p
Ox
Ox
Me
Unpassivated
Me
Intensity (a.u.)
Intensity (a.u.)
Unpassivated
Ox
passivated
Me
Me
passivated
600 595 590 585 580 575 570
Binding energy (eV)
805 800 7 95 790 785 780 775 770
Binding energy (eV)
Figure 6. a) Cr 2p XPS spectra unpassivated and passivated 35N LT, b) Co2p XPS
spectra unpassivated and passivated 35N LT.
In all, the results suggest that the chemical passivation treatment is beneficial with
respect to metal-ion induced oxidation of polyurethane insulation.
3.4
Does the addition of hydrogen peroxide to the PBS electrolyte influence the
electrochemical properties of cobalt-based alloys? (Paper I)
One of the features of an inflammatory response is the release of superoxide and
hydrogen peroxide from inflammatory cells into the extra-cellular space (Tengvall et al.
1989). The corrosion resistance of titanium has been shown to decrease in the presents of
H2O2 (Pan et al. 1996), due to an enhanced corrosion attack by H2O2 (Pan et al. 1994). In
this work, to evaluate the influence of hydrogen peroxide on the electrochemical
properties of cobalt-based alloys, 100 mM H2O2 was added to the PBS electrolyte in the
measurements. Representative results from the electrochemical impedance spectroscopy
(EIS) of 35N LT® in PBS with and without addition of H2O2 are shown in Fig. 7.
20
106
-90
104
-60
103
102
-30
101
Phase angle [deg]
2
Impedance modulus [Ω.cm ]
105
Passivated sample without H 2O2
Passivated sample with H O
2
2
0
10
0
-3
10
10-2
10-1
100
101
102
103
104
Frequency [Hz]
Figure 7. Bode plots of 35N LT in PBS with and without addition of 100 mM H2O2 , ○)
passivated sample without H2O2, and □) passivated with H2O2.
Clearly these results show that addition of the oxidant H2O2 influenced the
electrochemical properties, i.e. leading to a decreased corrosion resistance, of 35N LT® in
the PBS electrolyte. Moreover, the results show that the acidic passivation treatment
improves the polarisation resistance also in the more hostile environment of PBS with
addition of H2O2. Although the improvement is not as pronounced as for the samples in
PBS only, the polarization resistance of the passivated samples in PBS with addition of
H2O2 is increased by approximately 2 times as compared to unpassivated samples.
In all, the addition of the oxidant H2O2 influenced the electrochemical properties, i.e.
leading to a decreased corrosion resistance, of 35N LT® in the PBS electrolyte.
21
3.5
Can tantalum be used as pacing and sensing electrode material in pacemaker
applications? (Paper II)
Tantalum has a long and successful clinical use, which dates back to 1940s (Black
1994). The good biocompatibility in soft and hard tissue is contributing to its success.
Moreover, the excellent mechanical properties and the chemical resistance motivate the
extensive usage of tantalum in diagnostic and implant applications. Its density is also a
favorable property when designing components for pacemaker applications. Due to the
high density (16.654 g/cm3) it will become visible under x-ray, which the physicians use
during implantation to follow the movements and the placement of the pacemaker lead.
Tantalum with surface modification forming tantalum oxide (Ta2O5) has been used in
electrostimulation application, e.g., neural and nervous system (Robblee et al. 1983,
Johnson et al. 1977, Brummer et al. 1975), and as dielectric pacemaker electrodes
(Schaldach 1971).
In this work, EIS and cyclic voltammetry (CV) measurements of tantalum were
performed to investigate its electrochemical behaviour. During the CV cycles, anodic
oxidation may occur at high potentials, which may cause oxide formation on TiN surface.
And, cathodic reactions at low potentials (more negative) may lead to hydrogen ingress
into the surface layer and/or partial reduction of the thin oxide formed on the surface.
Consequently, the electrochemical performance of the electrode material may change.
Representative spectra from the EIS measurements before and after the CV cycles are
shown in Fig. 8. After the CV cycles of tantalum, the polarisation resistance reaches a
high level approximately 10 MΩcm2. It is likely due to formation of an oxide film with
high electrical resistance, as reported in literature that tantalum without any surface
modification can develop an extremely high impedance surface coating which can
insulate the electrode to the point of non-function (Stokes 1996).
Additional experiments were performed on tantalum under pacemaker pulses. The
pacemaker was programmed with extreme settings simulating patient needing stimuli of
(-)7.5 V at base rate 60 min-1 and a pulse width of 0.4 ms. The results from the EIS
measurements also show a very high polarisation resistance (about 10 MΩcm2) after only
120h of pulsing.
22
In all, this indicates that tantalum without surface modifications is not a suitable
material choice for pacing and sensing electrodes, due to formation of a highly resistive
oxide film on the surface.
108
-90
2
Impedance modulus [Ω.cm ]
107
-60
105
104
-30
Phase angle [deg]
106
103
Before CV
102
After CV
101
-3
10
0
10-2
10-1
100
101
102
103
104
Frequency [Hz]
Figure 8. EIS before and after CV of an uncoated tantalum sample in PBS.
3.6
Is the electrochemical behaviour of tantalum affected by the addition of
hydrogen peroxide? (Paper II)
The electrochemical behaviour, e.g., the corrosion resistance, has been shown to
change in the presence of H2O2 in PBS for titanium (Pan et al. 1994) and cobalt based
alloy 35N LT® (Ornberg et al. 2007). Both titanium and the 35N LT® show a lower
polarisation resistance after the introduction of H2O2, which implies that the corrosion
resistance becomes lower. In this work, 100 mM H2O2 is added to simulate the
inflammatory
response
caused
by
inflammatory
cells
releasing
H2O2.
The
23
electrochemical investigation of tantalum in PBS with the addition of H2O2 showed a
similar decrease under these conditions.
The cathodic region from a CV cycles of tantalum in PBS without and with addition
of H2O2 are shown in Fig. 9.
0.00005
a)
2
Current density [A/cm ]
2
Current density [A/cm ]
0.00000
0
-0.00005
-0.00010
-0.00015
-0.00020
-0.00025
-0.00030
-0.00035
-1.6
C10
C9
C5
C3
C1
-1.5
b)
-0.002
-0.004
-0.006
C2
C3
C5, 7, 10
-0.008
C1
-1.4
-1.3
Potential [ V. vs Ag/AgCl]
-1.2
-0.01
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
Potential [ V. vs Ag/AgCl]
Figure 9. CV curves of uncoated Ta exposed to PBS, a) without and b) with addition of
H2O2.
The biggest difference compared to the sample in PBS only is in the cathodic
potential region. The current density obtained in the cathodic region during the first cycle
is significant higher than the next following cycles. After a few cycles the current
densities reach a stable level. This is only observed for the samples in PBS with the
addition of H2O2. This may be explained such that the addition of H2O2 influences the
electrochemical behaviour until the H2O2 is consumed or decomposed. In the anodic
region, the current peaks indicate that oxygen evolution reaction takes place on the
surface of tantalum. Moreover the results suggest that the oxide grows to a more and
more electrically insulating oxide film, as judged from the decreasing current densities
with increasing number of cycles. This is explained more in detail in paper II.
The results from the EIS measurement of tantalum in PBS with and without H2O2
after CV are shown in Fig. 10. The polarisation resistance of tantalum in PBS with the
addition of H2O2 is about 1 MΩcm2 after the CV cycles. Compared to the tantalum
24
samples in PBS without H2O2 after the CV cycles, this is a decrease about 10 times. It can
be speculated that tantalum is similar to titanium, and the H2O2 attacks the native oxide
layer (Pan et al. 1994) that forms on titanium and tantalum, resulting in a decreased
corrosion resistance.
8
10
-90
7
2
Impedance modulus [Ω.cm ]
10
-60
105
4
10
-30
3
10
After CV PBS
After CV PBS with H O
2
10
2
2
1
0
10
10-3
Phase angle [deg]
6
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
Frequency [Hz]
Figure 10. Bode plots of uncoated tantalum in PBS with and without addition of 100 mM
H2O2 , ○) passivated sample without H2O2, and □) passivated with H2O2.
In all, the results from the electrochemical investigation of tantalum show that the
addition of H2O2 affects the electrochemical behaviour and results in a decreased
corrosion resistance.
25
3.7
Is tantalum a good substrate for the titanium nitride coating used in pacemaker
electrodes? (Paper II)
Pacemaker electrodes of today are often constructed of a substrate material coated
with high surface area coating, e.g., titanium nitride with fractal structure (Hubmann et al.
1992). Substrate materials normally used today in pacemaker electrodes are platinumiridium alloy and titanium. Tantalum seems to have all the necessary properties for the
substrate material, e.g., good biocompatibility, excellent mechanical properties and
chemical resistance. In this work, tantalum coated with high surface area titanium nitride
was investigated electrochemically by CV, EIS and pacemaker pulses.
A representative voltammogram between -1.5 to 1.5 V vs. Ag/AgCl is shown in Fig.
11. One feature observed during the CV cycling is that the current density at elevated
anodic potentials decreases with the number of cycles, due to an increased resistance at
the electrode-electrolyte interface. This indicates that some oxidation process of TiN
takes place at anodic potentials, leading to a thin oxide film forms on the surface of TiN.
It was previously reported that TiN can be oxidized at anodic potentials and form an
oxide film on the surface (Rudenja et al. 1999). (Azumi et al. 1998) found that the
oxygen atoms penetrate into the TiN coating and replace nitrogen atoms to form a TiO2like oxide. The TiN most likely oxidises according to formula (3) (Milosev et al. 1997):
2TiN + 2O2 → 2TiO2 +N2
(3)
At low cathodic potentials the current density decreases with the number of cycles.
Electrochemical processes taking place under this potential sweep condition are most
likely related to formation of TiHx within the TiN coating. It is known that the
conductivity is less for TiHx than for TiN because of higher resistive properties (Azumi el
al. 2002). The oxide formation at anodic potentials mentioned above may also contribute
to the decreased current at cathodic potentials due to the resistance of the TiO2-like oxide.
This is described more in detail in paper II.
26
C1
0.02
2
Current density [A/cm ]
C2
C5
0.01
C9, 10
0
-0.01
-0.02
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Potential [ V vs. Ag/AgCl]
Figure 11. Cyclic voltammetry cycles between 1.5 to -1.5 V of a tantalum sample coated
with TiN in PBS.
The EIS results obtained at the open-circuit potential from the tantalum coated with
TiN are shown in Fig. 12. The polarisation resistance after the CV cycles is slightly
higher than that before the CV cycles, the average value before and after CV was
calculated to 12 kΩcm2 and 10 kΩcm2, respectively. Note that these values were obtained
by using the geometrical area, and the large effective area gives apparently a low level of
polarisation resistance. Under the potential sweep conditions, the slight increase in the
polarisation resistance is in agreement with the CV results suggesting some oxide
formation when subjected to the CV cycles. It should be mentioned that the potential
sweep rate of 100 mV/s used for the CV cycles is too low as compared to the situation
during the pacemaker pulses where very fast transient processes are occurring (Norlin et
al. 2005). Therefore the observed increase in the polarisation resistance due to the oxide
formation at the anodic potentials during the CV cycles is not applicable to the real
pacemaker pulsing situations.
27
4
-90
103
-60
2
-30
10
Phase angle [deg]
2
Impedance modulus [Ω.cm ]
10
Before CV
After CV
10
-3
10
0
-2
10
-1
10
10
0
10
1
10
2
3
10
10
4
Frequency [Hz]
Figure 12. EIS before and after CV of a TiN coated tantalum sample in PBS.
The TiN coated tantalum was also subjected to pacemaker pulses, to simulate pacing
conditions on the electrode material a pacemaker was connected to the three-electrode
cell. The EIS results after 120 hours of pacemaker pulsing are shown in Fig. 13 in
comparison with that before pulsing. The polarisation resistance decreases slightly with
the pulsing time. The results from spectra fitting suggest that after 120 h of pacemaker
pulses the Rp value is close to half of the original value. The decrease in the polarisation
resistance is probably due to the cathodic reactions at the low pulsing potentials, which
result in reduction of the initial oxide presented on the TiN surface. This decrease in the
polarisation resistance due to the pacemaker pulsing is probably beneficial with respect to
charge transfer capacity of the electrode.
28
4
-90
2
Before pace pulsing
120 h pace pulsing
3
10
-60
102
-30
1
10
10-3
Phase angle [deg]
Impedance modulus [Ω.cm ]
10
0
10-2
10-1
100
101
102
103
104
Frequency [Hz]
Figure 13. EIS of TiN on tantalum in PBS before and after 120h of pacemaker pulses.
Overall, for tantalum coated with the TiN coating, although some oxide formation
may take place during relatively slow cyclic polarisation, no increase in the polarisation
resistance occurs under the pacemaker pulses.
In all, based on the results from CV, EIS and pacemaker pulses tantalum is a good
candidate substrate material for the TiN coating used in pacemaker electrodes.
29
4
Main conclusions
The electrochemical behaviour of surface modified materials (for pacemaker lead
conductor and electrode substrate) in synthetic biological media, PBS with and without
H2O2 addition, was studied by electrochemical methods including potentiodynamic
polarisation, cyclic voltammetry, electrochemical impedance spectroscopy and simulated
pacemaker pulsing. The surface composition was analyzed by XPS. The metal release
during the passivation treatment and exposure in the synthetic biological media was
measured by ICP-AES. The main conclusions are schematically shown in Fig. 14.
Detailed results and discussion are provided in paper I and paper II.
Biological Environment
H2O2 from inflammatory cells
influence the electrochemical
behaviour and corrosion of
biomaterials.
Conductor and insulation
Chemical passivation of the Cobased alloy increases its corrosion
resistance, and significantly reduces
the cobalt release, which may have
implications in reduction of metalion induced oxidation of the polymer
insulation.
Electrode
Uncoated Ta is not a suitable
material for pacemaker electrodes.
However, Ta is a suitable substrate
material for the rough TiN coated
electrodes.
Figure 14. Schematic main conclusions regarding the pacemaker lead conductor,
electrode material and influence of H2O2 in the biological environment.
30
5
Acknowledgements
First of all I would like to express my sincere gratitude to Professor Christofer
Leygraf for giving me the opportunity of joining this research group, and for providing an
exciting research environment.
Another person I would like to articulate my gratitude to is my supervisor Docent
Jinshan Pan for his unfailing interest in my work and for his generous and experienced
guidance throughout this work.
I am also in dept to Susanne Nilsson, my supervisor at St. Jude Medical AB, for
providing the financial support to this licentiate work, for her personal commitment and
assistance and for believing in me even in times of trouble. Additionally I would like to
acknowledge the invaluable help and support from the other two members in the steering
group, Rolf Hill and Olof Stegfeldt. I would also like to express many warm thanks to
Eva Micski and Anna Norlin Weissenrieder, former members of the steering group who
during the start up phase of the project provided commitment and help with identifying
interesting research topics. Anna Norlin Weissenrieder requires special thanks, since she
was the person who first introduced me to the pacing world during my master thesis, and
has ever since been a great support, professionally and a highly valuable personal friend.
All colleagues, past and present, at the Division of Corrosion Science are
acknowledged because all of you have given me many laughs, valuable support and for
the fanciful Friday coffee breaks. Klara and Gunilla need to be mentioned because they
have been forced to make small errands on my behalf, for which I am greatly thankful.
Docent Inger Odnevall Wallinder is greatly acknowledged for her interest in my work
and her help with all administrative issues.
Thanks to all my colleagues in the Lead Materials and Technology Development
Group at St Jude Medical AB, Susanne, Rolf, Kenneth, Mikael, Myriam, Åsa, Mika,
Marcus and Henrik for your support and interest in my licentiate work. Torbjörn
Andersson is gratefully acknowledged for his interest in my work and the encouraging emails. Marie Herstedt at St Jude Medical AB is gratefully acknowledged for her help
with XPS measurements.
31
I am very grateful to my extended family that have encouraged me during this work.
Most of all I am grateful to my parents for their love, support and encouragement
throughout the years. I am also grateful to my sister and her family for always being
there, giving advice and support. At last but not least I would like to express my sincere
gratitude to my wife and daughter, Katja and Elsa, for their understanding and support
during my licentiate work, and especially during the last months before finishing my
thesis. I love you more than anything in the world, thanks for believing in me!
32
6
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