This document discusses testing silver chloride and stainless steel as biomaterials for electrodes used in functional electrical stimulation (FES) systems. It aims to identify which material experiences less corrosion over time in an ionic solution mimicking biological fluids, and which stimulation patterns best extend electrode life. An experimental plan is proposed involving in vitro and potential future in vivo tests comparing the materials' conductivity and mass changes when implanted and subjected to various stimulation patterns over 9 months. The goal is to improve FES electrode technology by finding durable materials and stimulation methods that reduce corrosion and risks for patients needing repeated surgeries.

1. Silver Chloride Vs. Stainless Steel as a Biomaterial Tested with Electrical
Stimulation Patterns for Use of Biosensor/Electrodes in FES Systems
Functional Electrical stimulation is a therapy technique native to Biomedical
Engineering that allows people with lost extremity or muscle control to regain and
improve motor skill through electrical stimulation techniques delivered to muscles.
Those affected often have had serious accidents, spinal cord injuries, or some sort of
paralysis and are looking to improve their situation with the implementation of systems
that can either be implemented invasively or non-invasively.
The main issue currently with FES systems and also neuroprosthesis in general
is that the percutaneous electrodes consist of metals that can conduct a charge and
electrically stimulate the muscle well, but do not have a long life expectancy. This
promotes problems for patients being that they must get more surgery or procedures to
once again implant these electrodes, as opposed to having ones that can endure the
length of the therapy for long term. Most corrode and affect the surrounding biological
environment to a point where removal is essential to prevent inflammation and
discomfort due to chemical differences in the material and tissue, and the types of
stimulation given to the affected site [3].
Specific Aims
Aim 1: Test silver chloride against stainless steel as the better biopotential
electrode for corrosion in an aqueous solution that mimics the ionic environment
of biological fluids.
The biological interaction between material and body is important to ensure
biocompatibility and decrease inflammation. The main premise is to understand if
corrosion due to chemical properties in the extracellular area is the leading cause of
wear, and how silver chloride reacts as a plausible material when introduced to a highly
ionic environment. Experimental testing will encompass in vitro and in vivo methods
[17]. These will analyze silver chloride and one “control” material, stainless steel. We
are hypothesizing that the silver chloride will see less corrosion and less calcium
deposition than the stainless steel over a period of time. Set-up and experimentation will
be discussed in detail later, but will include the placement of each material in an
aqueous ionic solution for a period of 9 months.
Aim 2: Identify specific stimulation pattern that best allows extension of electrode
life expectancy; biphasic versus monophasic and compensated versus balanced
current.
The charge of the material-tissue interface is not always a net of 0, and current
magnitudes are not always equal either [4]. We hypothesize that the biphasic
compensated current will be best because it helps the flow of charge through the
electrode and surrounding tissue to get a net charge of 0. It has greater charge injection
and more natural type of stimulation [4]. This should emphasize less electrical activity
and less corrosion. Polarization ensues when we only have 1 direction of current,
therefore a negative and positive charge rhythm should prove best [4]. The change in
charge carriers from electrons to ions as well as the exchange of electrons over the
interface will be kept in mind when understanding the experimental importance [21].

2. Experimentation will involve 4 different types of stimulation patterns tested via each
electrode material, including the biphasic compensated.
Background
A standard FES system is comprised of a portable power source (rechargeable
battery), command microprocessor/control unit, stimulator, lead wires, electrodes and
sensors [1]. The invasive alternative is much more complex; often an electrical
transducer will be implemented to target specific natural biosensors within the body, or
a synthetic biosensor will be implemented to activate the surrounding tissue itself.
Usually peripheral nerves surrounding the muscle is where the electrode will attach to to
thus activate the muscle. In addition, it is possible for direct muscle contact, or the nerve
to be wrapped with an electrode cuff [1].
This cuff either surrounds the outermost
neural tissue layer or selects a small cluster
of neurons [5]. With invasive systems, the
electrodes, lead wires and stimulator are
places within the body at the contact site,
where the battery and transducer are
outside [1].
Corrosion
Corrosion is one aspect of biomaterial
breakdown. Stimulation of the electrodes comes via basic electrical currents and their
properties of polarization. The biological environment introduces a slew of ions residing
in the extracelluar fluid that contributes to the attack of any sort of foreign material with a
purpose of conducting a charge. Therefore, metals are more susceptible to corrosion
than other materials [2]. Corrosion principles of gaining or losing electrons, also known
as reduction and oxidation, with respective sites of the reactions taking place at the
cathode and anode. Depending on the site and inflammation intensity, biological
microstructures can reduce or increase this passive layer and affect these materials.
The presence of proteins, cells and bacteria all influence the chemical interaction
between material and body [2]. Once a foreign object is introduced into the body, an
inflammatory response is triggered to maintain homeostasis [7]. This is characterized by
redness, swelling, heat, and pain, or a mixture of all. Acute inflammation is detected by
the introduction of neutrophils, followed by chronic inflammation and macrophage influx
[16]. At this time, pH usually declines and oxidation (loss of electrons) ensues at the site
[2].
Electrode surface coatings are often used to promote cell growth and biointegration.
Often, metals are good electricity conductors, do not store charge, and grow their own
semiconducting films that can polarize charges from oxidation methods. [11]. With
metals that form these layers, corrosion is often slowed and ion transfer declines [3].
Biological Environment and Stimulation
Stimulation of muscles occurs through action potentials, which are initiated by
electrical activity that is a depolarization and then a repolarization of a negative resting
membrane potential. A nerve cell sends an action potential to a fiber, the potential
propagates along the length of the fiber as Ca^2+ is released which initiates muscle
contraction [12]. These contractions are what makes our muscles move and complete
Figure 1: Neural electrode cuff [5]

3. tasks. The Extracellular matrix, like any tissue fluid, is comprised of proteins, DNA, lipids
and polysaccharides, as well as a majority of water. Its charge varies, as it is composed
of calcium phosphate, NaCl, potassium, and small amounts of other elements and ions
[15].
Silver Chloride(AgCl)
The silver chloride electrode is a metal coated with a layer of slightly soluble ionic
compounds of silver chloride with appropriate anions. The entire electrode is dipped in
an electrolyte with high anion concentrations, giving it low solubility in water. Oxidation
of silver ions at the electrode surface to ions in the solution at the interface and the
combination of Ag+ ions with already existing Cl- ions in solution are two of the
processes that take place [4]. Silver chloride has been used before in other experiments
with invasive electrodes as well, and proved to be successful [20].
Stainless Steel
Stainless steel electrodes are known to be reusable when implanted, very stable and
cheap [6]. Being a metal, stainless steel is a conductor. 316 SUS L is a stainless steel
alloy that has had previous experimental success as an invasive electrode material,
because of its resistance to pitting/intrergranular corrosion and cracking [8].
Electrode Stimulation Patterns
The waveform sent through the skin can define action potentials and ultimately its non-
linear patterns can be assessed for better stimulation results and electrode lifespan
increase [4]. FES is implemented by sending pulsations of electrical signals through a
muscle via an associated nerve, and the duration, intensity, and pattern of these signals
can alter the electrode properties. These electrodes are ultimately biopotential
electrodes; electrodes that serve as a transducer to change the ionic natural current of
the body into an electric current served by the electrodes and leads. In this experiment,
4 types of stimulation patterns will be tested. These will be combinations of fast/slow
rise times, and mono/biphasic stimuli [4]. Since the current across an electrolyte-
electrode interface is not consistently 0 at any given point in time, as well as the
magnitudes, which means that the current and stimulation times are truly what are
providing for our change in net charge. Charge patterns can include basic mono or
biphasic rectangular pulses in accordance also with trapezoidal, sine or decaying pulses
[4]. Kept in mind must be tissue impedance; the deeper the area targeted, the more
mass the electrical current must travel through [14].
Experimental Design
The experimental design is directed at our two aims;
Testing silver chloride and stainless steel in an ionic aqueous solution for
corrosion over a time period, and Testing 4 different simulation patterns in
conjunction with the silver chloride and stainless steel in an aqueous solution to
detect the best pattern that allows for minimal corrosion
Materials
3 electrodes will always be subject to 1 test; e.g., Aim 1 will have 3 separate silver
chloride electrode tests as well as 3 stainless steel electrodes for accuracy and
consistency of trends in analyzing data. Aim 2 will assign 3 electrodes of the same
material to each stimulation pattern. (Total of 30.) Lead wires for each electrode, an
external control unit and an implantable stimulator will be acquired [14]. Subjects will be

4. recruited based on a common affected extremity and common underlying conditions.
For example, 12 patients of similar age group, with paretic right arm control due to
spinal cord injury. Subjects will fill out a questionnaire based on lifestyle habits to better
ascertain if there are any outliers after data is analyzed. The solution will be comprised
of an ionic aqueous mixture containing Ag+, Cl-, Na+, K+ and calcium phosphate. It is
very difficult to mimic the composition of the ECM and biological fluids, so the ion-
conducting ability and electrical conductivity of the fluid will be best generalized as a
constant in this experiment with the mixture of the above group of ions [19].
Potential Risks
Risk will be mitigated by ensuring the in vitro experiment is fully safe and operable to be
incorporated with human subjects. Safety values of 2.0 uC/mm^2 for intramuscular
electrodes [12]. Subjects will complete a waiver form that briefs them on the knowledge
that a non-biological material will be implanted in their body for up to 9 months, and that
they may be subject to allergic reaction or inflammation. At any time, they can opt out of
the experiment if they feel their health is at risk. In addition, if worsening of the control of
the extremity occurs, the subject will be released from the experiment. Subject
confidentiality and HIPPA regulations for privacy will take place [13]. Note: only if a
research method deems safe, reliable, and well tested, it will move to in vivo human
testing. This is a hypothetical experimental progression of events in the sense that the
in vitro testing was successful and approved to be used in humans. Clinical
experimentation only comes after regulatory requirements and fabrication consistency
has been met [5].
Procedure of Aim 1: Silver Chloride vs. Stainless Steel
In Vitro Method: Three 1-beaker size ionic aqueous solutions will be tested that
mimic the biological environment. A silver chloride sample will be placed in one set,
while stainless steel in the other. (These materials have long time usage as
bioelectrodes because of their reusability and ability to withstand corrosion [6]. They will
serve as our controls to compare the success of silver chloride to.) The samples will be
left in the solution for 9 months, to accurately depict the length of a therapy session in
clinical trials. Assessments will be made every 2 weeks and images will be taken of the
sample to see if any physical corrosion can be seen, as well as calcium deposits. In
addition, the pH of the solution will be taken every two weeks to ensure the faux
biological environment is stable, as well as the conductivity of the material and the
strength of the signal received to ensure that it is not losing its electrical property to
conduct a current as it (or does not) corrode with time. Controls here are the amount of
solution, the chemical composition of solution, the concentration of the solution, the size
and surface area of the electrode (generally small enough to be delivered via in vivo
with a hypodermic needle) which will be <1 cm^2 [18].
In Vivo Method: The silver chloride and stainless steel electrodes will be implanted
into the subjects, all on the same affected extremity, and will be left for a 9 month
course of FES rehabilitation. Conductivity will be assessed every two weeks as well as
the strength of the signal being received in the same way as stated in the in vitro trial.
Subjects will go through clinical trials of active and passive therapy techniques;
grasping, releasing, extending, flexing and motor control exercises governed by
clinicians with the help of ergometers or rehabilitation devices [22]. This will always

5. occur for the in vivo methodology. Clinicians will monitor progress as 3 weekly sessions
for 45 minutes each with 15 minute sub-sessions and 3 5-minute breaks over the
course of 9 months. Therapy will progressively get more difficult as stimulation of
muscles will become less intense because of the dependence slowly weaning off of the
need for aid in motor control from the FES system. Note that no charge will be sent to
the electrodes in this trial, but the analysis of the corrosion of the materials without
stimulation to prove which lasts longest is being attained.
Procedure of Aim 2: Stimulation Pattern Testing
In Vitro Method: This trial will be exactly the same as the in vitro method of Aim 1
except this time stimulation patterns will be sent through the aqueous solution. 4
different patterns will be tested; a monophasic constant current, biphasic constant
current, a biphasic balanced constant current, and a biphasic constant current. 4 tests
per material, 3 electrodes per material. In a prior study, 20-25 Hz with a .3ms duration of
pulses was successfully used for a
parapelegic patient [9]. We will be
adopting these constants in this study
throughout as well. Stimulation will be
applied for the same duration intervals as
stated above.
In Vivo Method: Once again, the aspects
of the in vivo trial are the same as Aim 1
except the stimulation patterns will be
included this time (same used patterns
as the in vitro trial.) Subjects will be doing
therapy for their extremity that will include
the same rehabilitation devices and techniques used [22]. A wire lead for stimulation will
be attached to the electrode and leave through the skin. The system for electrical
stimulation will consist of an internal receiver or stimulator, an outer control interface
and the percutaneous electrodes [14].
Data Analysis
Data will be recorded in spreadsheets over the time period. Data analysis will include:
mass decrease for Aim 1; the corrosion will essentially eat away at the electrode,
decreasing its mass. In addition, an increase in mass could also pose issues. Excess
chloride/calcium from the solution due to excessive negative feedback would signal high
corrosion. In addition, percent decrease in conductivity over the time span, as well as
which stimulation pattern provided to the best combination of mass consistency.
Significance
Nanoelectronics such as biopotential stimulation electrodes have a growing field and
precedence in the biomedical engineering community.[9] Understanding that the
identification of a material which slows or halts corrosion in conjunction with a
stimulation pattern that does as well will allow patients less surgery, risk of infection,
money spent and overall trouble with their progress towards normalcy. Improved patient
morale, recovery rate, and control over lost muscles are essentially the ultimate goal in
aiding those with FES.
Figure 2: From top left to right; monophasic constant current,
biphasic constant current, biphasic balanced constant current,
biphasic compensated constant current, tested.[4]

6. References
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