In Vivo Charge Injection Limits Increased after ‘Unsafe’ Stimulation

Suzan Meijs

, Søren Sørensen

, Kristian Rechendorff

and Nico Rijkhoff

Center for Sensory-Motor Interaction, Aalborg University, Fredrik Bajersvej 7D, Aalborg, Denmark

Danish Technological Institute, Kongsvangs Alle 29, Århus, Denmark

Keywords: Electrical Stimulation, Charge Injection, Implantable Neural Prosthesis.

Abstract: The effect of unsafe stimulation on charge injection limits (Q

inj

) and pulsing capacitance (C

pulse

) was inves-

tigated. Four stimulation protocols were applied: 20 mA – 200 and 400 Hz, 50 mA – 200 and 400 Hz. In-

creasing Q

inj

and C

pulse

were observed for all stimulation protocols. Corrosion was not observed with any of

the stimulation protocols and no tissue damage was observed for the 20 mA – 200 Hz stimulation group.

This indicates that the ‘safe potential window’ may not be applicable in vivo, as no damage was done stimu-

lating with 20 mA at 200 Hz, while damage was done using the same current at 400 Hz.

1 INTRODUCTION

The performance of stimulation electrodes can be

characterized by their charge injection limits (Q

inj

The amount of charge that can be injected safely

without causing electrode degradation or tissue

damage depends on the electrode material, the elec-

trolyte and the stimulation waveform. Typically, the

potential limits for safe stimulation are determined

under near steady-state conditions using the cyclic

voltammogram (CV). The CV reveals how the elec-

trode material interacts with the electrolyte or the

tissue at stepwise in- and decreasing potentials. The

safe potential window typically is defined by the

potentials at which water reduction and oxidation

occurs. (Cogan, 2008)

The safe potential window and Q

inj

are typically

reported under in vitro conditions, in inorganic

saline solution. Both the safe potential window

(Meijs, submitted), as well as Q

inj

(Kane, 2013; Wei

and Grill, 2009; Meijs, submitted) differ under in

vivo circumstances. Furthermore, Q

inj

and electrode

polarization change during the course of the

implanted period (Kane, 2013; Lempka, 2009;

Meijs, submitted).

In order to investigate the reliability of the in

vivo charge injection limits, electrical stimulation

was performed for 6 hours in anesthetized animals

using charges that exceeeded Q

inj

. Six hours of

stimulation at 200 Hz using a 20 mA current caused

no tissue or electrode damage during a pilot study

and this was therefore used as the least intense

stimulation paradigm. Three other stimulation

paradigms were added by doubling the frequency

and increasing the current to the maximum

stimulator output (50 mA). During stimulation

voltage transients (VT) were recorded and charge

injection limits were measured before, during and

after the stimulation period.

2 METHODS

Four pigs were implanted with 4 porous TiN work-

ing and 4 pseudo-reference electrodes of the same

material each. The work was carried out according

to Danish legislation (ethical approval license nr:

2014-15-0201-00268).

2.1 Electrode Fabrication

TiN coatings were deposited on Ti6Al4V electrode

pins (6 mm

) and Ti disks (1000 mm

) by reactive

magnetron sputtering on a CC800/9 SiNOx coating

unit (CemeCon AG, Germany). The electrodes were

mounted on a rotating stage, which carried out a

three-fold planetary rotation. Sputtering was done

from four Ti targets (88 x 500 mm

) with 99.5%

purity in a Ar/N

mixture atmosphere. The purity of

both gases was 99.999%. An ETFE coated 35N LT

wire (Heraeus, Switzerland) was crimped to the

hollow end of the electrode pins. The pins were

insulated using a PEEK body with silicone tines,

which were glued to the pins. The electrodes were

Meijs, S., Sørensen, S., Rechendorff, K. and Rijkhoff, N..

In Vivo Charge Injection Limits Increased after ’Unsafe’ Stimulation.

In Proceedings of the 3rd International Congress on Neurotechnology, Electronics and Informatics (NEUROTECHNIX 2015), pages 101-105

ISBN: 978-989-758-161-8

101

cleaned thoroughly before e-beam sterilization.

2.2 Surgical Procedure

The animals were first sedated and then anesthetized

with a bolus injection of propofol. Anesthesia was

maintained using propofol infusion. The electrodes

were implanted in tight pockets in the subcutaneous

adipose tissue on the back. The percutaneous elec-

trode wires were encased in surgical tape, which was

sutured to the skin. The electrodes were not used for

one month until the pigs were anesthetized again

using sevofluran to perform electrical stimulation.

2.3 Electrical Stimulation

Biphasic, charge balanced 200 µs square pulses were

applied, cathodic first with an inter-phase interval of

40 µs during which no current was applied. Stimula-

tion was performed for 6 hours, which was divided

into 3 2-hour sessions. Before, between and after

these sessions, Q

inj

were determined for each elec-

trode. Four stimulation paradigms were applied:

 20 mA, 200 Hz

 20 mA, 400 Hz

 50 mA, 200 Hz

 50 mA, 400 Hz

Electrical stimulation was performed using DS5

(Digitimer, UK), which was shorted between the

pulses. VT were recorded using an oscilloscope. Q

inj

were measured using the VersaSTAT 3 potentio-

galvanostat (Princeton Applied Research, USA).

The pulsing capacitance (C

pulse

) was computed

using the slope (





of the VT:

stim

= C

pulse





(1)

Where I

stim

is the stimulation current. Q

inj

was calcu-

lated using the current at which the safe potential

limits (-0.6 and 0.9 V) were reached (I

max











 



(2)

Where t is the duration of the stimulation pulse (200

µs) and A is the geometrical surface area of the

electrodes (6 mm

). When voltage excursions ex-

ceeded machine limits (±10 V), I

max

was extrapolat-

ed from the highest current using a linear relation.





 



 





 









(3)

Where V

and I

were the measured potential and

current, respectively, and V

and I

were the ex-

trapolated potential and current. When V

reached

the potential limits, I

was used as I

max

in (2). This

method provided accurate results using data for

which I

max

was measured.

2.4 SEM/EDX

SEM (Nova 600, FEI Company) images were rec-

orded at various magnifications to investigate the

surface structure of the electrodes. EDX (EDAX,

AMETEK) spectra were made to determine if there

the surface chemistry of the electrodes changed.

3 RESULTS

During the measurements the shorting part of the

setup broke down, and the last two stimulation ses-

sions could not be done with one of the electrodes in

the 20 mA – 400 Hz group.

The average Q

inj

of all implanted electrodes be-

fore stimulation was 12.3 ± 1.4 µC/cm

. After 2hrs

of stimulation Q

inj

was increased for all of the indi-

vidual electrodes (Fig. 1) and the largest increase

was observed for the group 4.

Figure 1: Q

inj

was increased after the first stimulation

session. It then remained stable for each electrode group.

Figure 2: C

pulse

was increased after the first stimulation

session. It then remained stable for each electrode group.

NEUROTECHNIX 2015 - International Congress on Neurotechnology, Electronics and Informatics

102

Figure 3: After 2 hrs of stimulation, the slopes of electrode

groups were decreased with increased charge injection,

due to the increased C

pulse

The average C

pulse

of all implanted electrodes be-

fore stimulation was 24 ± 2 µF/cm

. Fig. 2 shows an

increase of the average C

pulse

of all electrodes by

approximately the same relative amount as Q

inj

. Fig.

3 shows that the slopes of the VTs were decreased

due to the higher C

pulse

after 2 hours of stimulation.

Analysis of the VT during the stimulation ses-

sions showed that C

pulse

increased after the first 30-

60 minutes of the first stimulation session, but it did

not increase during the second or third session (fig.

4). Furthermore, C

pulse

was increased for all stimula-

tion groups as compared to C

pulse

derived using a

safe stimulation current (fig. 2). C

pulse

was also sig-

nificantly higher at 50 mA than at 20 mA.

SEM showed that the electrode surfaces were in-

tact after 6 hours of intense stimulation. Similar

levels of oxide were observed on all electrodes using

EDX.

4 DISCUSSION

Although Q

inj

increased from the initial level de-

pending on the stimulation current and frequency,

the charge injection was always higher than Q

inj

Above this theoretical limit, tissue and/or electrode

damage is expected (Cogan, 2008). Neither of these

was observed for the electrodes stimulated at 20 mA

– 200 Hz. Increasing levels of tissue damage were,

however, observed with increasing charge injection

in accordance with findings of Mortimer (1980).

Tissue damage was due to the accumulation of a

reaction product to detrimental concentrations or pH

changes beyond the buffering capacity of the tissue,

as heat damage, the mass action theory and corro-

sion were ruled out (Shannon, 1992; Merrill, 2005).

Figure 5: SEM image of an electrode from group 3.

Most plausible seem changes in the pH, as water

reduction occurs in saline at -0.6 V for TiN, result-

ing in increased levels of OH

. Electrode potentials

were below -1 V for all electrodes at the start of the

first stimulation session. Water reduction is likely to

have contributed to charge transfer during all stimu-

lation protocols, as C

pulse

derived from the voltage

transients during the stimulation sessions (0 min in

fig. 4) was doubled for 20 mA electrodes and more

than tripled for the 50 mA – 400 Hz electrodes as

compared to C

pulse

derived at 1 mA (pre-stim in fig.

2). This is likely due to the transfer of charge via

water reduction, which is a faradic process and does

not increase the electrode potential (Merrill, 2005).

No complete levelling off of the potential was ob-

served, however.

Figure 4: C

pulse

increased during the first stimulation session for electrodes for which tissue damage was observed.

In Vivo Charge Injection Limits Increased after ’Unsafe’ Stimulation

103

Furthermore, the slope of the voltage transients

decreased towards the end of the cathodic phase,

indicating that at increasing cathodic potentials,

water reduction contributes more to the total charge

transfer (Merrill, 2005). This trend was observed

primarily during the first 30-60 minutes for the elec-

trode groups for which damage was observed, but

not for the 20 mA – 200 Hz group. After 30-60

minutes, the decrease in slope was less and it re-

mained stable throughout the rest of the study. This

makes it plausible that tissue damage due to stimula-

tion induced pH changes occurred during the first

30-60 minutes of stimulation, which fits well with

the pH changes observed in saline as a function of

time (Mortimer, 1980).

The increase in capacitance that is observed after

30-60 minutes (fig. 4) for the electrode groups with

tissue damage is likely due to destruction of the

fibrous capsule. This removes the diffusion limita-

tion, which typically limits the charge injection

capacity of implanted electrodes (Cogan, 2008). Q

inj

and C

pulse

derived at safe stimulation levels (fig. 1

and 2) were increased more for electrode with than

without tissue damage.

Electrode damage was not observed, though the

average anodic potentials were above 1 V for all

electrode groups and potentials of more than 2 V

have been observed in all groups, except 20 mA –

400 Hz. Oxidation of the TiN surface into a thin

oxide/oxynitride film occurs at 0.5-0.9 V. These

processes lead to passivation and protect the under-

lying TiN from further oxidation. At higher anodic

potentials (1-1.5 V) oxidation of the the TiN into

hydroxide and/or TiO

occurs. Lastly, at potentials

higher than 2 V, which have been observed in this

study, oxygen evolution takes place accompanied by

oxidation of TiN to TiO

. (Avasarala, 2010) There

are three reasons why we may not have detected

increasing levels of oxide with increasing charge

injection. 1) The oxidation reaction is reversed dur-

ing the cathodic phase (Merrill, 2005). 2) The oxide

has dissolved in the acidic environment (Avasarala,

2010) that was created due to intense electrical stim-

ulation (Merrill, 2005). 3) The oxide levels on all

electrodes are below the detection limit for EDX.

The increase in Q

inj

and C

pulse

when no tissue

damage occurred is in accordance with a decrease in

polarization resistance observed for cochlear im-

plants (Tykocinski, 2005; Newbold, 2014) and a

decrease in complex impedance of deep brain stimu-

lation electrodes before and after stimulation

(Lempka, 2009). Newbold (2014) argues that the

stimulation induced changes are confined to the

electrode tissue interface and that protein ad- and

desorption may be responsible for them, as they saw

no changes in the voltage drop that is due to resistive

properties of the tissue (IR drop). For the 20 mA –

200 Hz group, there were no changes in IR drop and

no tissue damage. For all other electrode groups,

however, tissue damage was observed, as well as a

decrease in IR drop.

All electrodes were capable of the same charge

injection before stimulation was started yet tissue

damage occurred in the 20 mA – 400 Hz group, but

not in the 20 mA – 200 Hz group. This shows that a

safe stimulation protocol for implanted electrodes is

not only established by keeping within a certain

potential window (Merrill, 2005) and that the ‘safe’

window is not necessarily applicable in vivo. Safe

stimulation also depends on the stimulation frequen-

cy, as this limits the time of the tissue to restore the

pH (Ballestrasse, 1985). There may be a safe amount

of charge that can be injected regardless of the fre-

quency. It is, however, difficult to determine this

amount, as a purely linear voltage change was not

observed even within the theoretical ‘safe window’.

It would therefore be interesting to investigate

whether tissue damage occurs using ‘safe’ stimula-

tion currents at very high frequencies.

ACKNOWLEDGEMENTS

The authors would like to thank the staff at the bio-

medical laboratory at Aalborg University Hospital

and Jetske van Breda, Alana Gerhardt and Maria

Alcaida for assistance during the surgeries.

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