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 AimsAim 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].

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]. CorrosionCorrosion 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]

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 SteelStainless 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 PatternsThe 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 DesignThe 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 corrosionMaterials 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

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 RisksRisk 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

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 AnalysisData 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]

References

[1] Sinkjaer, T., M. Haugland, A. Inman, M. Hansen, and KD Nielsen. "Biopotentials as Command and Feedback Signals in Functional Electrical Stimulation Systems." Biopotentials as Command and Feedback Signals in Functional Electrical Stimulation Systems. PubMed.gov, 25 Jan. 2003. Web. 29 Nov. 2015.

[2] Temenoff, J. S., and Antonios G. Mikos. Biomaterials: The Intersection of Biology and Materials Science. Upper Saddle River, NJ: Pearson/Prentice Hall, 2008. Print.

[3] Zhong, Yinghui, Xiaojun Yu, Ryan Gilbert, and Ravi V. Bellamkonda. "Result Filters." National Center for Biotechnology Information. U.S. National Library of Medicine, Nov.-Dec. 2001. Web. 29 Nov. 2015. Stabilizing electrode-host interfaces: a tissue engineering approach

[4] "Chapter 5: Biopotential Electrodes." Medical Instrumentation Application and Design. Ed. John G. Webster. 4th ed. N.p.: John Wiley & Sons, 2010. N. pag. Print.

[5] http://fescenter.org/about-fes-center/fes-center-facilities/technical-development-laboratory/ , 11/28/2015

[6] Webster, John G. "Telehealth and Mobile Health." Google Books. Ed. Halit Eren. CRC Press, n.d. Web. 13 Dec. 2015.

[7] Zhao, Ruogang. Implantation & Inflammation. New York: UBLearns, Aug. 2015. University Professor notes on inflammatory responses.

[8] Handa, Y. "Development of Percutaneous Intramuscular Electrode for Multichannel FES System." Europe PubMed Central. Europe PubMed Central, July 1989. Web. 6 Dec. 2015.

[9] Kralj, Alojz R., and Tadej Badj. "Functional Electrical Stimulation." Google Books. CRC Press Inc., n.d. Web. 13 Dec. 2015.

[10] Schiller, Jürgen, and Daniel Huster. "New Methods to Study the Composition and Structure of the Extracellular Matrix in Natural and Bioengineered Tissues." Biomatter. Landes Bioscience, 1 July 2012. Web. 1 Dec. 2015.

[11] Brandstetter, C. Wolf, and D. Sharnweber. "Biological Interactions with Surface Charge in Biomaterials." Google Books. RSC Publishing, n.d. Web. 13 Dec. 2015.

[12] Lippmann, Julian. BME 301 Lecture 4a Physiology of Muscle and EMG. New York: DropBox, Aug 2015. University Professor notes on EMG and muscle physiology.

[13] http://www.hhs.gov/ocr/privacy/ , 10 Dec. 2015.

[14] Hisaichi, Ohnabe, and Douglas A. Hobson. "An Introduction to Rehabilitation Engineering." Google Books. Ed. Rory A. Cooper. CRC Press, 2007. Web. 1 Dec. 2015.

[15] Cooper, Geoffrey M. "Cell Walls and the Extracellular Matrix." Cell Walls and the Extracellular Matrix. U.S. National Library of Medicine, 2000. Web. 12 Dec. 2015.

[16] Schultz, Gregory S., Glenn Ladwig, and Anette Wysocki. "Extracellular Matrix: Review of Its Roles in Acute and Chronic Wounds." Extracellular Matrix: Review of Its Roles in Acute and Chronic Wounds. World Wide Wounds, Aug. 2005. Web. 10 Dec. 2015.

[17] http://www.encyclopedia.com/doc/1G2-3404000449.html , 11 Dec. 2015.

[18] Splinter, Robert. "Handbook of Physics in Medicine and Biology." Google Books. CRC Press, 2010. Web. 8 Dec. 2015.

[19] Mitchell, Brian S. "An Introduction to Materials Engineering and Science for Chemical and Materials Engineers." Google Books. John Wiley & Sons Inc., n.d. Web. 2 Dec. 2015.

[20] Sato, Takuya R. Bioelectrodes. Sato Takuya R, assignee. Patent US 3982529 A. 28 Sept. 1976. Print.

[21] Hoffman, Allan S., Frederick J. Schoen, and Jack E. Lemons. "Biomaterials Science." Google Books. Ed. Buddy D. Ratner. Academic Press, n.d. Web. 2 Dec. 2015.

[22] http://www.restorative-therapies.com, 11 Nov. 2015.