Investigation of Effect of Geometry and Surface Treatment on the Electro kinetics of GC Microelectrodes

“The fact that we live at the bottom of a deep gravity well, on the surface of a gas covered planet going around a nuclear fireball 90 million miles away and think this to be normal is obviously some indication of how skewed our perspective tends to be.”
― Douglas Adams

ABSTRACT OF THE THESIS

In this research, the electrical and electrochemical characterization of geometrical and surface treatment effects on the behavior of Glassy Carbon (GC) microelectrodes have been studied. we report on the electrochemical tunability of patternable Glassy Carbon microelectrode probe structures for potential applications in bio-electrical signal recording and stimulation. The structures, made from lithographically patterned negative photoresist and subsequently pyrolyzed, have excellent electrochemical properties and electrochemical stability in biological fluids.

Driven by the need of decreasing the damages and risks associated with neuroprosthetic implants, the present work focuses the electrochemical properties of glassy carbon such as Impedance and charge density before and after plasma etching as well as in different structures of glassy carbon electrodes including pillars and disks. Analyzing these parameters helps study the changes in the impedance as well as the amount of charge delivered by the material in a biological solution simulating the extracellular fluid.

Another important contribution of this study is the investigation of the stability of the electrochemical properties of the material with change in the values of the parameters of the equivalent circuit for Electrochemical Impedance Spectroscopy. This research aims to depict the superiority of glassy carbon for neuroprosthetic devices, replacing current materials widely used in the fabrication of neural probes.

TABLE OF CONTENTS

PAGE

ABSTRACT……………………………………………………..vi

LIST OF TABLES…………………………………………………ix

LIST OF FIGURES…………………………………………………x

ACKNOWLEDGEMENTS…………………………………………..xi

CHAPTER

1. introduction……………………………………………………1

1.1 Neural Prostheses………………………………………..2

1.2 Motivation for This Study………………………………………………………………………5

1.3 Organization of This Thesis…………………………………………………………………….5

2. LITERATURE SURVEY………………………………………….7

2.1 Background…………………………………………….7

2.2 Challenges in Current Neuroprosthetic Devices …………………………….. ………..8

2.3 Microelectrode Arrays……………………………………………………………………………9

2.4 Types of Microelectrode Arrays………………………………………………………………9

2.4.1 Metal Microelectrode Arrays…………………………………………………………11

2.4.2 Carbon Microelectrode Arrays………………………………………………………15

2.4.2.1 Glassy Carbon Microelectrode Arrays………………………………16

2.5 General Causes of Failure of Microelectrode Arrays………………………………..18

3. Fabrication and characterization methods………………………………19

3.1 Glassy Carbon based Microelectrode Arrays…………………….19

3.1.1 Types of Glassy Carbon Microelectrode Arrays………………………………19

3.1.2 Fabrication of Glassy Carbon Microelectrode Arrays……………………….21

3.1.2.1 Pyrolysis Process – Formation of GC………………………………..26

3.2 Characterization of the GC Microelectrode Arrays………………………………………..28

3.2.1 Electrochemical Characterization…………………………………………………..29

3.2.1.1 Cyclic Voltammetry……………………………………………………….29

3.2.1.2 Charge Injection Capacity……………………………………………….30

3.2.1.3 Electrochemical Impedance Spectroscopy…………………………33

3.2.2 Electrical Characterization…………………………………………………………….35

3.2.2.1 Equivalent Circuit Analysis……………………………………………..35

3.2.2.2 Nyquist Plot…………………………………………………………………..37

4. In vitro testing techniques…………………………………………40

4.1 Electrochemical Cell…………………………………….40

4.2 Surface Modification…………………………………….40

4.3 Cyclic Voltammetry……………………………………..41

4.4 Charge Injection Capacity…………………………………42

4.4.1 Plasma Etching…………………………………………………………………………….43

4.5 Electrochemical Impedance Spectroscopy………………………43

4.5.1 Curve Fitting for  EIS…………………………………………………………………..44

4.5.2 Modified Randles Equivalent Circuit……………………………………………..46

4.5.3 MATLAB Modeling and Varying of Experimental Values……………….47

4.5.3.1 MATLAB Modeling for Randles Equivalent Circuit ………….48

4.5.3.2 MATLAB Modeling for Modified Randles Equivalent Circuit ………………………………………………………………………………………………..55

5. Conclusion……………………………………………………60

5.1 Discussion…………………………………………….60

5.2 Future Studies………………………………………….62

REFERENCES……………………………………………………63

list of tables

PAGE

Table 2.1. Different types of Electrode Arrays and its Comparison……………….10

Table 2.2. Different types of Metal Electrode Arrays and its Comparison…………..12

Table 2.3  Reversible charge storage capacity and other parameters in electrode material

selection…………………………………………………………………………………………………..13

Table 4.1. Physical meaning of the CPE co-efficient Q………………………………………………….43

Table 4.2. Values obtained after curve fitting of the experimental Nyquist plot, through Modulab…………………………………………………………………………………………………………………..45

Table 4.3. Curve fitting values obtained using Modulab for the above circuit…………………..46

list of figures

PAGE

Figure 1.1 Relationship between Neural Interface Technology and nervous system plasticity.2

Figure 1.2 Overview of a BCI System…………………………………………………………………………..3

Figure 1.3 Aspects during design and development of Active Implantable Medical  devices. 4

Figure 2.1 Photograph of the Utah Electrode Array (UEA)……………………………………………11

Figure 2.2 The phases of a Graphite crystal…………………………………………………………………16

Figure 2.3. Structural model for glassy carbon proposed by Jenkins-Kawamura

Source : Jenkins, Gwyn Morgan, and Kiyoshi Kawamura………………………………………………18

Figure 3.1. SEM image of Pillars with 700 µm × 700µm dimension ……………………………….20

Figure 3.2. Electrode used for testing the pillars for their electrochemical properties…………20

Figure 3.3. SEM image of PesKa electrode…………………………………………………………………..21

Figure 3.4. Peska electrode Generation 2 180 µm diameter…………………………………………….21

Figure 3.5. Structures obtained after Step 3…………………………………………………………………..22

Figure 3.6. Hirox 3D Image of structures shown in Figure 3.5………………………………………..22

Figure. 3.7 The complete fabrication process of GC MEAs……………………………………………23

Figure 3.8. Lithography and pyrolysis process for fabricating GC electrodes from a negative tone photoresist………………………………………………………………………………………………………..26

Figure 3.9. Representation of Process of carbonization during pyrolysis………………………….28

Figure 3.10. – Triangular excitation potential……………………………………………………………….30

Figure 3.11. – A CV depicting various electrochemical reactions occurring on an electrode surface……………………………………………………………………………………………………………………..30

Figure 3.12. Solartron Analytical Model 1070E……………………………………………………………31

Figure 3.13. Representation of charge injection mechanisms………………………………………….32

Figure 3.14 Voltage transient in response to a biphasic, symmetric current pulse………………33

Figure 3.15 Electrochemical Test-Set up………………………………………………………………………34

Figure 3.16 Sinusoidal Periodic alternating voltage……………………………………………………….34

Figure 3.17. The Randles model used for fitting the EIS data…………………………………………36

Figure 3.18. Nyquist plot of the EC in Figure 3.17………………………………………………………..37

Figure 3.19. Nyquist plot for Cdl (solid line) and CPE (dotted line)………………………………..39

Figure 3.20. Randles equivalent circuit with Cdl replaced by CPE………………………………….39

Figure 4.1. Water window obtained using CV for GC electrode is -1V to +1V…………………41

Figure 4.2. CV before and after plasma etching is shown below……………………………………..41

Figure 4.3. Charge density measurements before and after plasma etching………………………42

Figure 4.4 Curve fitting obtained through EC analysis of the experimental nyquist plot (Red) through Modulab and its EC values…………………………………………………………………………….44

Figure 4.5. Representation of modified Randles circuit for the small semi-circle………………45

Figure 4.6. Curve fitting for the above circuit shown in Figure 4.5…………………………………46

Figure 4.7. Varying Solution Resistance, Rs………………………………………………………………..48

Figure 4.8. Varying Charge Transfer Resistance, Rct……………………………………………………50

Figure 4.9. Varying Constant Phase Element, CPE……………………………………………………….52

Figure 4.10. Varying Warburg Constant, W…………………………………………………………………54

Figure 4.11. Varying additional Capacitance, CB…………………………………………………………56

Figure 4.12. Varying additional Resistance, RB…………………………………………………………..58

Figure 5.1. Electrode surface made of conducting and insulating areas can be modeled as a resistor in parallel with a capacitor……………………………………………………………………………..60

acknowledgements

This page is optional. Insert your acknowledgements text here (except for students in Biology who will place this section after the text and before the reference list).

chapter 1

introduction

The most complex part of the human anatomy is the human brain. It consists of  neurons and glial cells which are in a ratio of 1:1 according to a new article with glial cells approximated to be lower in counts than a 100 billion [http://onlinelibrary.wiley.com/doi/10.1002/cne.24040/abstract]. Inspite of the immense development in the field of neuroscience, we fail to understand one cell[1]. The latest Innovation in Neuroscience and related technologies, for the Research of the Brain, has even caught the eyes of the political parties[2]. The main focus of Neuroscience is to interpret how a human body forms out of a small mass. Every invention leads to a better understanding of this especially with new technologies available to us ranging from computer coding, ultrasound imaging, medical resonance imaging and much more [1]. “Medical implants are devices or tissues that are placed inside or on the surface of the body of which some are prosthetics, intended to replace missing body parts while other implants deliver medication, monitor body functions, or provide support to organs and tissues” [3]. “Neural implants are technical systems that are mainly used to stimulate parts and structures of the nervous system with the aid of implanted electrical circuitry or record the electrical activity of nerve cells” [4]. A demonstration that the two concepts of Neural interface technology and neuro-plasticity, work in conjunction to ameliorate the standard of living for disabled individuals. “Neuro-plasticity is a foundation for neurologic rehabilitation” (Figure. 1.1) [5]. Thus, the individuals with disability can better use their devices that have a neural interface [5].

Figure 1.1 The symbiotic relationship between neural interface technology and nervous system plasticity.
Source: Wang, Wei, et al. “Neural interface technology for rehabilitation: exploiting and promoting neuroplasticity.” Physical medicine and rehabilitation clinics of North America 21.1 (2010): 157-178.

(“The symbiotic relationship between neural interface technology and nervous system plasticity. Their close interaction leads to increased efficacy of neural interface devices and improved functional recovery of the nervous system, which eventually leads to better quality of life. The neural interface technology is exemplified by a photo of a microelectrode array for cortical surface recording. A schematic of a three-layer neural network models nervous system plasticity. When a group of neurons in the intermediate layer were damaged as a result of neurologic disorders (red “×”) such as stroke, the input layer neurons (green dots) develop or strengthen connections (dashed lines) with spared neurons (blue dots) in the intermediate layer promoting functional recovery”.)[5]

1.1 Neural prostheses

A neural prostheses is a device or technique used to amplify or substitute lost or missing function in individuals who have nervous disorder[6]. The approaches in this category can be vastly divided as: applied current inhibits or stimulates the neurons, or their activity is recorded to intercept motor intention [7]. Stimulation can be opted for its curative potency, as demonstrated in deep brain stimulation to ameliorate the symptoms of Parkinson’s disease or to communicate input to the nervous system (for example by transforming sound to neural input with cochlear prosthetics) [7]. Epidural stimulation of the lumbar spinal cord predominantly stimulates large sensory axons in the dorsal roots which induce muscle reflex responses [8].

This area of study, known as neuroprosthetics, has sought to create devices, known as “brain-computer interfaces” (BCIs), that acquire brain signals and translate them into machine commands such that they reflect the intentions of the user [9]. Capturing motor intention and executing the desired movement form the basis of brain-controlled interfaces (BCI), a subset of neural prosthetics used to decode intention in order to restore motor ability or communication to impaired individuals [7]. BCI systems enable a new real-time interaction between the user and the outside world where signals that indicate the brain activity of the user are translated into an output and  the user receives feedback on this output, which in turn affects the user’s brain activity and influences subsequent output [10]. Therefore, if a person uses a BCI to control a neuroprosthetic arm, the position of the arm after each movement will influence the person’s intent for the next movement and affect the brain signals that encode that intent [10]. Figure 1 shows the main components of a BCI system which can be applied to BCIs that either substitute for or enhance neuromuscular output.[10]Figure 1.2 Overview of a BCI system.
Source: Daly, Janis J., and Jonathan R. Wolpaw. “Brain–computer interfaces in neurological rehabilitation.” The Lancet Neurology 7.11 (2008): 1032-1043.

But before neural prosthetics can advance, engineers will be called on to make innovative use of materials to design and fabricate devices that allow sustained electronic functioning in the harsh environment of the human body, without causing tissue infection and other serious adverse conditions [11]. Research efforts have focused on enhancing the performance of various types of materials used in neural prosthetics, in addition to developing interface technologies that enable the micro devices to be safely implanted in human tissue for long periods [11]. Different injuries require different treatments and neural prosthetics. The type of injury needs to be examined first and then the determination of the treatment should be done. For treatments requiring neural prosthetics, the type and location of placement of the device will differ. Additionally, the material used affects the functioning, reaction and life span of the neural prostheses. Thus, various aspects should be taken into account while the design and development of a neural prostheses; an outline of which is shown in the Figure. 2 [12].

Figure 1.3 Aspects during design and development of Active Implantable Medical  devices.

Source: Stieglitz, Thomas. “Manufacturing, assembling and packaging of miniaturized implants for neural prostheses and brain-machine interfaces.” SPIE Europe Microtechnologies for the New Millennium. International Society for Optics and Photonics, 2009.

1.2 MOTIVATION FOR THIS STUDY

The motivation of this research is the need to develop a neural prostheses with good electrochemical properties that promote longevity, tissue growth and better performance of the neural prostheses. The neural microelectrodes made from lithographically patterned and subsequently pyrolyzed polymer precursor  have excellent electrochemical stability in ionic solutions and respond well to chemical surface property modifications. In addition, lithographically patternable GC (Glassy Carbon)offers a unique tailorability functionality that enables fabrication of electrodes with a range of mechanical, electrical, and electrochemical properties that closely match the behavior of soft tissues [13]. The important contribution of this study was the electrical and electrochemical characterization of the neural microelectrodes related to the geometrical and surface treatment effects respectively. The electrical analysis was conducted by studying the Impedance of electrodes with different dimensions, surface plasma treatment and materials using Randle’s equivalent circuit analysis as well as equivalent MatLab modeling. The important contribution of these tests was to determine surface behavior and modifications with change in dimensions, material and surface treatment. Additionally, the electrochemical effects were explored. Firstly, Characterization of the electrochemical behavior of micro fabricated GC electrodes for different structures to study the effect of shape and size of the electrode on the surface electrochemical properties. Secondly, evaluation of surface modification of GC electrodes before and after oxygen plasma treatment, to compare the effect of plasma treatment on the surface properties of GC electrodes. Lastly, potentiometric measurements of GC electrodes for charge injection capacity before and after plasma treatment to see the change in the charge density as well as the charge injection.

1.3 ORGANIZATION OF THIS THESIS

This thesis consists of 6 chapters. The first chapter provides an introduction to the neural prosthetics and the motivation for this Research. The second chapter provides an overview of the previous studies on neural prosthetics, current modifications and related test set up. It not only summarizes fabrication methods for glassy carbon electrode but also, compares different structures of glassy carbon electrode. The third Chapter discusses briefly about the testing, circuit designing and MatLab modeling that played a crucial role in forming the basis of this study. Importantly, this chapter also describes the surface treatment test methodologies. The fourth chapter presents the results of the electrical and electrochemical testing of different neural prosthetics. This chapter also discusses the circuit modeling obtain through Modulab as well as MatLab. Lastly, the fifth chapter concludes the study with a discussion of the findings, and considers some of the many possible future directions that this research could be directed and applied to.

Chapter 2

Literature survey

This chapter introduces the need and techniques used for neural prostheses. It gives a background summary of neuroprosthetic devices along with the challenges and failures currently faced by different neural prostheses. Additionally, it briefly describes microelectrode arrays (MEA) and its types. This chapter focuses on characterization of different metal and carbon electrodes with an emphasis on the use of Glassy carbon in Neural Prostheses.

2.1 BACKGROUND

The past decade has seen significant advances in various invasive and non-invasive stimulation therapies that provide increased relief of symptoms with fewer serious side effects, such as transcranial magnetic stimulation (TMS), deep brain stimulation (DBS) and vagal nerve stimulation (VNS) [14]. The dominant invasive treatment for brain stimulation, a therapy increasingly used for neurological and psychiatric disease, is deep brain stimulation (DBS) in which an electrode is surgically implanted deep in the brain and used to deliver electrical pulses at high frequency (generally 120–160 Hz) [15]. Studies to date show that humans and animals can learn to use electroencephalographic activity (EEG) recorded from the scalp, electrocorticographic activity (ECoG) recorded from the cortical surface, or signals recorded within the cortex (neuronal action potentials or local field potentials (LFPs)) to control the movements of a cursor or other device in one or two dimensions [16]. More recently, implantable electrode-array based techniques such as epi- and subdural electrocorticography (ECoG) and finely spaced ECoG electrodes (μECoG) have become a major research area [17]. Over roughly the past decade, neural prostheses based on intracortical electrode arrays have also been investigated fairly extensively due to their ability to record action potentials from individual neurons, as well as local field potentials from small clusters of neurons since each intracortical electrode penetrates one to several millimeters into the brain (typically cerebral cortex), bringing the electrode tip into close proximity of the neurons of interest, resulting in a higher performance as these individual neurons are widely believed to be the fundamental information encoding units in the nervous system [17].
2.2 CHALLENGES IN CURRENT NEURoprosthetic Devices

Neuroprosthetic devices have shown significant clinical effectiveness and a great potential as an alternative treatment in patients suffering from neural impairment and disorders, such as deafness, Parkinson’s disease, intractable pain, paralysis and epilepsy [18,19]. At present, neural prostheses possess a variety of challenges ranging from size to performance capabilities over longer durations [20]. Some of the neuroprosthetic devices require large numbers of microelectrodes arrayed on a substrate with small geometry size in order to excite a group of neurons selectively and efficiently [20]. This has been a great challenge, because microelectrodes must sustain particularly high current density pulses through the electrode tissue interface without causing irreversible Faradic reactions and tissue damage [21], requirements generally satisfied by increasing the electrode size [22]. Undesirable electrochemical change not only damages the electrodes but also causes abnormalities in neural function and cell structure [23]. In order to allow long-term operation, the surface of the microelectrodes should be able to facilitate charge transport and minimize inflammatory reaction and gliosis [22].

An ideal recording electrode should be as small as possible to reduce perturbation of the local electric field and minimize tissue damage during insertion but, at the same time, should have low impedance to enhance signal quality during recordings [24]. While electrode miniaturization results in high selectivity, a negative side effect is that the charge density requirements of the electrode material are increased [25]. Consequently, performance of chronic microstimulation-based devices is limited due to both the lack of materials that safely and reliably deliver appropriate charge and the insulating effect of tissue reactions [25].

2.3 mICROELECTRODE ARRAYS

Following the revolutionary theoretical proposition of Donald Hebb’s hyposthesis which states that the neural assemblies form the basic functional unit of operation of the mammalian central nervous system; the possibility of recording the extracellular electrical activity of populations of single neurons, distributed across multiple structures that define a neural circuit, became one of the ‘holy grails’ of systems neurophysiology [26]. Interfacing electronics directly with the human nervous system holds considerable promise for allowing closed-loop control of neural prostheses by disabled patients [27]. While multielectrode recordings have become routine [28] in animal neurophysiology and have been used in humans [27], robust and stable long-term recording and stimulation remain a challenge [27]. To obtain reliable and successful chronic signals, the electrode must be biocompatible, have low impedance, and high charge injection density [27].

Current neural electrodes such as microwires [28,29] and microfabricated electrode arrays [27,30] suffer from high initial impedance and low charge-transfer capacity because of their small-feature geometry [31]. Furthermore, cellular reactive responses increase the electrode–tissue impedance due to insertion trauma and the chronic foreign body reaction induced by tethering, micromotion, and device biocompatibility [32,33]. Although several strategies have been conducted to improve the electrical properties and reactive tissue responses of neural electrodes [32,34-36], the long-term efficacy of these devices is still a challenge [27]. Microelectrode arrays (MEA) have been widely applied to study electrogenic cell cultures and tissue slices by extracellular recording and stimulatio [37]. Using a MEA based on thin-film technology leads to the advantage of simultaneous measurement of a large number of electrodes and thus a large number of cells [37].

2.4 Types of mICROELECTRODE ARRAYS

The design of electrode arrays falls into one of following categories: planar, penetrating, cuff, and regenerating electrodes [38].

Table 2.1 Different types of Metal Electrode Arrays and its Comparison

Source: Lacour, Stéphanie P., et al. “Flexible and stretchable micro-electrodes for in vitro and in vivo neural interfaces.” Medical & biological engineering & computing 48.10 (2010): 945-954.

Neural recordings using penetrating microelectrodes have been a mainstay of neurophysiology research for decades [39], as penetrating neural electrodes provide a greater spatial resolution and selectivity than surface-type electrodes, by being able to be in close proximity of neurons [40]. In fact, developments in penetrating electrode technology including Hubel’s tungsten microelectrode and the silicon microelectrode array have been primary enablers of advancements in neurophysiology and systems neuroscience [39]. In the cerebral cortex, penetrating microelectrodes are required to record activity of single neurons as the cells of interest are generally located 1-2 mm below the cortical surface while, electrodes placed on the surface of the cortex are typically used to record field potentials, which are associated with synaptic activity in large groups of neurons, rather than the spiking activity of single cells [39].

Figure 2.1. Photograph of the Utah Electrode Array (UEA)
Source : Kim, Sohee, et al. “Integrated wireless neural interface based on the Utah electrode array.” Biomedical microdevices 11.2 (2009): 453-466.

2.4.1 METAL microelectrode arrays

Different types of metals and metal alloys have been fabricated within the electrodes for neural stimulation including noble metals, such as platinum which have a long history as neural electrodes [41].

Table 2.2 Different types of Metal Electrode Arrays and its Comparison
Source: Wilks, Seth J., et al. “Poly (3, 4-ethylenedioxythiophene) as a micro-neural interface material for electrostimulation.” Frontiers in neuroengineering 2 (2009).

A variety of factors are taken into account while selecting a metal electrode for neural prostheses. An account of some of the parameters accounted for while selection of material for implantation is listed in Table 2.2. The parameters listed might change depending on the mesuring technique and environmental as well as other factors affecting the measured output.

Table 2.3  Reversible charge storage capacity and other parameters in electrode material selection.
Source: Merrill, Daniel R., Marom Bikson, and John GR Jefferys. “Electrical stimulation of excitable tissue: design of efficacious and safe protocols.” Journal of neuroscience methods 141.2 (2005): 171-198.

NOTE: r = real area; g = geometric area; AF = anodic first, charge-balanced; CF = cathodic first, charge-balanced; AB = cathodic first, charge-balanced, with anodic bias.

2.4.2 Carbon mICROELECTRODE ARRAYS

Carbon fibers are widely used to reinforce other materials because of their mechanical properties and their low density [57]. Carbon has many allotropes, such as graphite, diamond, fullerene, and glass-like carbons [58].

Graphite structure consists of ideally infinite sheets of graphene stacked in parallel sheets of chicken wire [58].  The anisotropy of the graphite crystal is reflected in its chemical behavior as the reaction with gases or vapors occurs preferentially at “active sites”, that is, the end of the basal planes of the crystal which are the zigzag face {101} and the arm chair face {112} as shown in Figure. 2.2, and at defect sites, such as dislocations, vacancies and steps [59]. Reaction with the basal plane surface is far slower; the difference in surface energy accounts for different rate of reaction which means slow at the basal plane (0.11 J/m2) and rapid at edge/prismatic plane (5J/m2) found at the termination of the basal planes or at defects within the basal plane [59]. The chemical reactivity is also affected by porosity, since high porosity leads to large increase surface area with resulting increase in reactivity [59]. The carbon atoms in graphite are all sp2 hybridized, with an intra-planar C-C bond length of 0.142 nm and interplanar spacing (d002) of 0.335 nm [58]. The conductivity of graphite is 4180 W/m.K [59].

In diamond, carbon is sp3 hybridized and tetrahedral, with a C-C bond length of 0.154 nm; usually contains dopants, such as boron, to provide sufficient conductivity for electrochemistry [58]. Diamond has a very low conductivity (σ = 2000 – 2100 W/m.K) and is thereby an electrical insulator [59].

Among the fullerenes, carbon nanotubes (CNTs) are “rolled up” single or multiple layers of graphene sheets, which form single-walled CNTs (SWNTs) or multi-walled CNTs (MWNTs), respectively [58].

Figure 2.2. The phases of a Graphite crystal
Source : Pierson, Hugh O. Handbook of carbon, graphite, diamonds and fullerenes: processing, properties and applications. William Andrew, 2012.

2.4.2.1 GLASSY Carbon mICROELECTRODE ARRAYS

Glassy (or vitreous) carbon is typically a hard solid prepared by heat treatment at elevated temperatures (1000-3000 °C) of polymeric precursors such as copolymer resins of phenolformaldehyde or furfuryl alcohol-phenol [60] and is characterized by isotropy of properties, high purity, strength and hardness, and extremely low permeability to both gases and liquids [61]. The chemical properties are similar to that of carbon, but with a much reduced activity at least in part because of a low porosity [61]. These polymers are used because of their high carbon yield on pyrolysis (the ratio of carbon present after/before carbonization is ∼50%) [60]. Glassy carbon (GC) is isotropic, electrically conducting, and impermeable to gases and has a low coefficient of thermal expansion [62]. Glassy carbon has a lower density (1.3-1.5 g/cm3) than graphite (2.27 g/cm3) or diamond (3.52 g/cm3); this density reflects its porous microstructure; however the voids constituting the pores are not connected which results in the low gas permeability observed for that material [62]. Carbon is thermally stable in a non-oxidizing environment at temperatures higher than 3000 °C [62]. Glassy carbon is also widely used as an electrode material for electrochemical applications [61,62] as well as in extremely corrosive environments because of its high corrosion resistance and inertness under a wide variety of conditions [62]. It resists strong acids and bases and is inert over a wide range of electrical potentials [63]. The surface chemistry of glassy carbon allows tailoring of its interfacial properties (surface energy, electron-transfer kinetics) by physisorption or chemisorption of molecules, polymers, or metals, or by procedures involving electrochemical pretreatments, polishing, or laser activation [63,62]. The ability to functionalize the surface of oxidized carbon further to introduce hydrocarbon or fluorocarbon groups offers a route to materials with very low surface free energies and low potential for stiction [63,62].

In the 1970s, the structure of GC was considered to have a ribbon-like geometry with entangled graphitic planes [56] [Fig. 2.3]. High-resolution transmission electron microscopic (TEM) analysis of commercially available GC showed that carbons derived from pyrolysed polymers contain small isolated crystals of graphite [64]. During carbonization, the C-C bonds of the polymer precursor backbone do not break, thus the graphitic regions cannot fully develop into graphene sheets [64]. The carbon can form graphitic regions of only limited size, with La and Lc in the range of 3-7 nm, and inter-planar spacing (d002) larger than 0.34 nm [64]. The relative amount of trigonal carbon atoms depends on the heat treatment [61].

Figure 2.3. Structural model for glassy carbon proposed by Jenkins-Kawamura
Source : Jenkins, Gwyn Morgan, and Kiyoshi Kawamura. Polymeric carbons–carbon fibre, glass and char. Cambridge University Press, 1976.

2.5 GENERAL Causes of failure of mICROELECTRODE ARRAYS

Widespread clinical use of implanted electrodes is hampered by a lack of reliability in chronic recordings, independent of the type of electrodes used [65]. By recording reliability we mean the ability to predictably obtain a high percentage of electrodes with good quality single/multi units for several years after the implant [65]. Two compelling perspective factors for the effects that correlate with the observed degradation of the signal and electrode metrics in chronic neural recordings arise from the series of events that occurs after implantation [66]. First, the ‘biotic’ factor, effects include the inflammatory response, disruption of the blood– brain barrier (BBB), initiation of astrogliosis and recruitment of microglia and macrophages to the insertion site and second are the ‘abiotic’ factors, effects of which include the electrode physical changes (damaged insulation, change in surface area, oxidation/reaction, corrosion) that impact the electrical recording properties of the neural probe following prolonged exposure to brain extracellular fluids and molecules in vivo [66]. Although it is difficult to conclude which one of the two factors solely contributes or leads to the failure of the device. The failure can even be a result of an effect of both the factors.

Chapter 3

FABRICATION AND CHARACTERIZATION Methods

This chapter presents the discussion of the MEMS fabrication of Glassy Carbon (GC) electrode array as well as the theory and reasons for the selection of the chosen test methods for electrical and electrochemical characterization.

3.1 Glassy carbon based
microelectrode arrays

In 1874, Robert Bartholow demonstrated that electrical stimulation of a human brain can result into movements [70]. Since then, different microelectrodes have been used for neural implantation to facilitate brain machine Interface. As discussed in the previous chapter, MEAs are used for this purpose. MEA’s made out of metal possess innate characteristics which limit their electrochemical properties, resulting in low charge injection through capacitive or reversible faradic mechanisms and high impedance which degrades the output signal [68]. Kassegne et. al have reported microelectrodes made from lithographically patterned and subsequently pyrolyzed polymer precursor which have excellent electrochemical stability in ionic solutions and respond well to chemical surface property modifications [69]. Therefore, in the current research, we have used GC electrodes for the purpose of testing and successive implantation.

3.1.1 types of GLASSY CARBON microelectrode arrays

As mentioned previously GC is our material of choice because of its superior mechanical as well as electrochemical properties [69,71]. As a result,  GC electrodes are used for electrochemical analysis in this research. These electrodes are fabricated using MEMS technology. The analysis is carried out using MEAs of GC electrodes. There are two geometries of GC electrodes presented in the current research; the Gandalf pillars and the pesKa. The Gandalf pillars are GC MEAs which have a dimension of 500 µm × 500 µm, height and width respectively. They have been named Gandalf pillars after the famous movie character and their tall cylindrical structure as shown in Figure. a. Figure. b shows the Gandalf pillars fabricated onto a copper foil with the help of Polydimethylsiloxane (PDMS) and epoxy. This was used to test the Gandalf pillars for its electrochemical properties as will be discussed in further sections.

Figure 3.1. SEM image of Pillars with 700 µm × 700µm dimension [69]
Figure 3.2. Electrode used for testing the pillars for their electrochemical properties.

PesKa MEAs, on the other hand, are a new design which facilitated microfabrication of novel design that is more biocompatible, as it is flexible enough to conform to surfaces that require the electrode to bend. It also has breathing holes to facilitate tissue growth and regeneration. Fig. c shows the SEM image of a PesKa electrode. Figure. d shows the PesKa electrode mounted on a PCB and connected to it through a ziff connector. It is named after the initials of the creators.