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

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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.

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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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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 Pitting corrosion __________________________________________________________ 5 1.3.2 Crevice corrosion _________________________________________________________ 6 1.3.3 Galvanic corrosion ________________________________________________________ 6

1.4 Biological environment ________________________________________________ 7 1.5 Metal-ion induced oxidation (MIO) ______________________________________ 8

2 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

3 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.

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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.

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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).

0

1i

Rp ∝ (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).

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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).

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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):

22222 2 OOHHOO +→++ +−− (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).

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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).

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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?

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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

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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.

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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.

Transpassive region E (V)

log i (A/cm2)

Active region

Passive region

ip

E corr (OCP)

E Repass

Start

End 1

End 2

E Breakdown

Figure 2. Schematic plot of a potentiodynamic polarisation curve.

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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.

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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.

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3 Summary of results

3.1 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.

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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.

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100

101

102

103

104

105

106 -90

-60

-30

010-3 10-2 10-1 100 101 102 103 104

Passivated 35N LT

Unpassivated 35N LT

Impe

danc

e m

odul

us [Ω

.cm

2 ]

Phas

e an

gle

[deg

]

Frequency [Hz]

Figure 3. Bode plots of 35N LT the in PBS, ○) passivated, and ■) unpassivated.

570575580585590595600

Inte

nsity

(a.u

.)

Binding energy (eV)

0 s

4 s

10 s

20 s

30 s

40 s

50 s

Cr 2p Ox Mea)

Unpassivated

570575580585590595600

Inte

nsity

(a.u

.)

Binding energy (eV)

0 s

4 s

10 s

20 s

30 s

40 s

50 s

Cr 2pb)

Passivated

Ox Me

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

a)

Ni

Mo Cr

Co Mo

Cr

b)

Co

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

805 800 7 95 790 785 780 775 770

Inte

nsity

(a.u

.)

Binding energy (eV) 600 595 590 585 580 575 570 Binding energy (eV)

Unpassivated

passivated

Co 2p Ox Me

Me

Inte

nsity

(a.u

.)

Unpassivated

passivated

Cr 2p Ox

Ox

Me

Me

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

100

101

102

103

104

105

106 -90

-60

-30

010-3 10-2 10-1 100 101 102 103 104

Passivated sample without H2O

2

Passivated sample with H2O

2

Impe

danc

e m

odul

us [Ω

.cm

2 ]

Phas

e an

gle

[deg

]

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.

101

102

103

104

105

106

107

108 -90

-60

-30

010-3 10-2 10-1 100 101 102 103 104

Before CV

After CV

Impe

danc

e m

odul

us [Ω

.cm

2 ]

Phas

e an

gle

[deg

]

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.

a)

-0.01

-0.008

-0.006

-0.004

-0.002

0

-1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0

Cur

rent

den

sity

[A/c

m2 ]

Potential [ V. vs Ag/AgCl]

-0.00035

-0.00030

-0.00025

-0.00020

-0.00015

-0.00010

-0.00005

0.00000

0.00005

-1.6 -1.5 -1.4 -1.3 -1.2

Cur

rent

den

sity

[A/c

m2 ]

Potential [ V. vs Ag/AgCl]

b)

C1

C2

C5, 7, 10

C3

C1 C3 C5 C9 C10

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.

101

102

103

104

105

106

107

108 -90

-60

-30

010-3 10-2 10-1 100 101 102 103 104

After CV PBSAfter CV PBS with H

2O

2

Impe

danc

e m

odul

us [Ω

.cm

2 ]

Phas

e an

gle

[deg

]

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 platinum-

iridium 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 TiO2-

like 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

-0.02

-0.01

0

0.01

0.02

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Cur

rent

den

sity

[A/c

m2 ]

Potential [ V vs. Ag/AgCl]

C1

C2

C5

C9, 10

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

10

102

103

104 -90

-60

-30

010-3 10-2 10-1 100 101 102 103 104

Before CV

After CV

Impe

danc

e m

odul

us [Ω

.cm

2 ]

Phas

e an

gle

[deg

]

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

101

102

103

104 -90

-60

-30

010-3 10-2 10-1 100 101 102 103 104

Before pace pulsing

120 h pace pulsing

Impe

danc

e m

odul

us [Ω

.cm

2 ]

Phas

e an

gle

[deg

]

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

Main conclusions 4

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.

H2O2 from inflammatory cells influence the electrochemical behaviour and corrosion of biomaterials.

Biological Environment

Chemical passivation of the Co-based alloy increases its corrosion resistance, and significantly reduces the cobalt release, which may have implications in reduction of metal-ion induced oxidation of the polymer insulation.

Conductor and insulation

Uncoated Ta is not a suitable material for pacemaker electrodes. However, Ta is a suitable substratematerial for the rough TiN coated electrodes.

Electrode

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 e-

mails. 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

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