**Abstract**

 The electroencephalography (EEG) is the way that the brain communicates with a computer and electronic devices also referred to as a mind-machine interface (MMI) or a brain-machine interface (BMI). Human organs such as the brain, heart and muscles producing μV to mV biopotential which can be measured through electroencephalography (EEG), electrocardiography (ECG) and electromyography (EMG) respectively. With the advancement in electronics, microelectromechanical manufacturing and biocompatible material, controlling of electronic actuating devices has become an area of great research interest. The EEG signal is captured by an electrode which is either classified as a dry and wet electrode. This study focuses on a new emerging field of dry EEG electrode design with a carbon nanotube (CNT) as the sensing material. Clinical research has proved that CNT does not have cause any skin allergic and irritation effects when compared to wet electrodes, which use a conductive gel that has an adverse effect on the skin in the long-term. The multiwall CNT is used with conductive polymers with a variation in electrode design aspects such as shape, size, thickness, microneedle array and amount of CNT in composition with the conductive polymer. In this article, the quality of the acquired EEG signal is discussed based on variations of the above-mentioned design aspects.

**Keywords:** electroencephalography, carbon nanotube, conductive polymer, EEG electrodes, microelectromechanical system

### **1. Introduction**

There is rapid growth in medical electronic devices in the wearable health monitoring system such as wheelchair control, robotic prosthetic arm, brain disease, virtual reality and augmented reality etc. [1–4]. In medical diagnosis, biopotentials, such as an electroencephalogram (EEG) from brain, electromyogram (EMG) from muscles, and electrocardiogram (ECG) from heart are commonly used techniques to measure the electrical activity also termed as electrophysiology. Currently, the signal is captured from the outer layer of skin with the help of hydrolytic gel and an Ag/AgCl electrode. The hydrolytic gel becomes dehydrated on long-term measurement which adds noise as well as deteriorates signal quality [5, 6]. Moreover, this conductive gel can cause skin irritation and allergic effects. To overcome this issue, the need arose to develop a dry EEG electrode having long-term signal acquisition

*A Review on Carbon Nanotube-Based Dry Electrode for EEG Recording DOI: 10.5772/intechopen.81083* 

**Figure 1.** 

*Various EEG electrodes (a) spring loaded dry electrode [9], (b) bristle type dry electrode [10], (c) nano contact type dry electrode [11], (d) dry sensor electrode [12].* 

stability with minimum noise. Earlier, some dry electrode were proposed with sensing material such as gold, silver, Ti/TiN, iridium oxide (IrO), platinum, tin, stainless steel etc. [7, 8]. The focus of researchers was on the design of the sensing part of the EEG electrode (**Figure 1**) [9–12]. To match the scalp skin curve and to minimize the effect of motion artifacts, screen-printed electrodes were also proposed and results showed that they were less prone to motion artifacts and noise [13, 14]. It was found that the carbon nanotube does not cause any skin irritation or allergic effects [15]. The carbon nanotube is a nanomaterial because of its excellent physical and chemical property and is now popular in medical field [16, 17]. Mostly, polydimethylsiloxane (PDMS), epoxy-based PTF silver ink (XCMS-015), polypyrrole (PPy) polythiophene (PTh) etc., is used to create the conductive matrix along with CNT. CNTs are also used for the coating material to improve the electrical conductivity [18, 19]. There are several CNT based electrode designs such as a micro-needle array, ENOBIO, CNT/PDMS, vertical CNT, polypyrrole (PPy) conductive polymer coating of dry patterned vertical CNT (pvCNT), self-adhesive and capacitive, and skin like CNT [4, 20–25]. In this article, the design, fabrication and response of this type of electrode will be discussed.

#### **2. Electrode design**

#### **2.1 Electrode standard**

International Federation of Clinical Neurophysiology (IFCN) [26] has standardized 16 channels as a minimum requirement for satisfactory results.

To acquire signal, the electrode-position has a letter F, T, C, P and O, which stand for frontal, temporal, central, parietal, and occipital lobes, respectively as per 10–20 system [26]. The electrode should be provided with reference and electronics grounding. Furthermore, the digital sampling rate should be 200 samples/s, minimum resolution of at least 12 bits and must have minimum steps of 0.5 mV,

minimum electrode impedances below 5 kΩ. Any other noise in the recording should be less than 1.5 mV peak-to-peak and 0.5 mV root-mean-square at any frequency from 0.5–100 Hz including 50–60 Hz [26].

#### **2.2 Coating type dry electrode design**

 The electrode with application of the multiwall carbon nanotube (MWCNT) was tested on a pig and then human with an aim to eliminate the application of skin preparation, conductive gel and improve wearability [20, 27]. This electrode penetrates inside the outer layer of skin called the Stratum Corneum (SC). Here, two designs were proposed: first, non-polarized with a silver/silver chloride (Ag/ AgCl) coating and, second, polarizable electrode without coating the MWCNT with a diameter of ca. 50 nm and length 20–30 μm. A vertical aligned CNT on Parylene-C polymer substrate was used to acquire a signal from the crayfish nerve cord. The electrode was fabricated with the chemical vapor deposition technique and proposed in [28]. Similar to conventional electrode design and ready to plug with existing EEG device, a CNT/PDMS wearable monitoring device was proposed in [21]. The MWCNT with 1–25 μm length had a 93% purity. Ultra-flexible micro-ECoG poly (ethylenedioxythiophene) PEDOT-CNT coated electrodes were introduced by [18]. In this technique, the signal was acquired from the below the scalp skin and this classifies as an invasive method of EEG recording. In this experiment, a 4 μm thick polyimide was deposited on a silicon wafer. A 20 nm thick titanium layer was added onto polyimide then 200 nm thick gold (Au) file placed over titanium. Finally, a 20 nm thick chromium (Cr) layer was used to cover the underlying metal. Through the lithography process, the 100 μm × 100 μm pad was obtained. For a hairy scalp, a self-adhesive and capacitive carbon nanotube based electrode design proposed [4]. The electrode consists of mainly three layers namely solderable metal Ti/Au, PDMS polymer as a middle ring and CNT/aPDMS sensing layer. The high conductive CNT/ aPDMS sensing material has an adhesive property as well as being flexible in shape. It is adjusted on the hair and decreases the electrode-skin impedance.

 Another research group designed CNT and adhesive PDMS matrix and recorded alpha rhythm steady state to evoke potential and auditory steady state response. They proposed an electrode with three layers—bottom aPDMS layer comes in contact with skin while CNT/aPDMS is the middle layer and Au/Ti/polyimide is the upper layer [24]. A research group [23] proposed a novel dry electrode where a vertical MWCNT was coated by polypyrrole (PPy) conductive polymer. This coated pillar was placed over a stainless steel foil of ϕ10mm. The CNTs with 1–1.5 mm growth taken at the space interval of 50, 100 and 200 μm.

#### **2.3 Conductive polymer**

Most of the proposed electrodes for EEG, ECoG, ECG etc., use Parylene-C, polydimethylsiloxane (PDMS), poly-ethylenedioxythiophene (PEDOT), polyimide and polypyrrole (PPy) as conductive polymers [4, 18, 21, 23, 24, 28]. This is mainly due to the flexibility, high conductivity and biocompatibility and these materials have been used for vision prosthesis [29], neural prosthetic and neural interface [30, 31], and microneedle type sensing for neural interface.

#### **3. Electrode response discussion**

The alpha waves (8–12 Hz) and β waves (15–30 Hz) were recorded at position Fp2 with limited trails and they found less noisy signal compared to a wet electrode.

#### *A Review on Carbon Nanotube-Based Dry Electrode for EEG Recording DOI: 10.5772/intechopen.81083*

 A flexible CNTs electrode [28] received signal from a crayfish nerve cord with a 1 kHz frequency at 11.07 kΩ impedance which falls under the typical EEG recording. The result shows that CNTs and polymer matrix-based electrodes adhered along the bio-tissue for long-term recording. The research group noticed that CNT based electrodes had low impedance values when compared to Au or Pt based micro fabricated electrodes as well as smaller plastic capacitance. CNT/PDMS electrodes [21] with variation in CNT/PDMS composition, electrode thickness and diameter for a long-term wearable application were designed with samples 1, 1.5, 2 and 4.5 wt% CNTs in composition and 1, 2 and 3 mm variation in thickness while 20, 30, 40 mm variation in diameter. It was concluded through an electrical conductivity test that the conductivity of the electrode depends on the CNTs concentration. The proposed design was adjustable to current EEG setup by replacing the wet electrode. The results noticed that there was a negligible effect of electrode thickness on quality of ECG signal. Furthermore, the signal quality was increased with high wt% of CNTs.

 The commercially available Ag/AgCl electrode is prone to motion artifact due to high skin-electrode impedance and this can be avoided using CNTs based electrodes. Further, another research group did a study on the effect of the vertically aligned CNTs electrode [22] on ECG signal and found similar results. Here, the CNTs were vertically deposited on the stainless steel material with 50, 100, 200 and 500 μm spacing. It has been noted that the impedance remained stable over the period of time in the pvCNT type electrode. The same design was improved by coating the electrode with the electrically conductive polymer (PPy) to increase the bonding strength with a substrate. To record the EEG signal from the hairy scalp is a challenging task as the presence of hair and air will increase the skin-electrode impedance and will result in degradation of signal quality. For this challenge, a self-adhesive and capacitive electrode design proposed in [4]. The alpha and steadystate evoke potential acquired by placing a dry electrode close to a wet electrode, the reference and ground electrodes placed at A2 and Fpz location respectively. The electrode was fabricated with 2.5 mm height and 3 mm diameter, on applying gentle pressure, its CNT/aPDMS layer filled the air gap and comes in contact with the skin even in the presence of hairs. Signal to noise ratio was recorded 3.71 ± 0.17 dB, the same aspect was noted 2.79 ± 0.13 dB in aPDMS without CNT. The signal quality was found to be good when compared with a wet electrode including an effect of motion artifacts.

## **4. Conclusion**

 All the proposed designs and results of CNTs indicate that it can be used as a substitute for a wet electrode to avoid the skin irritation and allergic effects without compromising signal quality. Through CNT/PDMS, ECG electrode signals were successfully taken for 7 days without degradation, which also shows that CNTs do not cause allergy and irritation on the skin like wet electrode [21]. This electrode can be reused after cleaning with alcohol, which reduces the cost as compared to wet electrodes are disposed once gel drys or evaporates. Also, Ag/AgCl electrodes signal quality deteriorates as sweat affects the gel. Moreover, through the cytotoxicity test it was found that this design was suitable for long-term ECG monitoring due to biocompatibility. The micro-ECOG design [18] proved that electro-chemically PEDOT-CNT nanocoatings are suitable for *in vivo* application to record signal from rat somatosensory cortexes via multi-whisker deflections. Furthermore, Platinum (Pt) electrodes coated with bioactive PEDOT [29] coatings can be applied to both vision prostheses and a wide range of other neuroprosthetic implants, which also

#### *Proceedings of the 4th International Conference on Innovations in Automation...*

 help to study biocompatibility of conductive polymers. Most of these studies were carried out on volunteers below 35 years of age, a future scope is to test it on elderly people. These results show that in future there is scope for further experiments on CNTs and conductive polymer matrix application for human neural compatibility and commercialization.
