CNT/PDMS Composite Flexible Dry Electrodes for Long ... - IEEE Xplore

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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 59, NO. 5, MAY 2012

CNT/PDMS Composite Flexible Dry Electrodes for Long-Term ECG Monitoring Ha-Chul Jung, Jin-Hee Moon, Dong-Hyun Baek, Jae-Hee Lee, Yoon-Young Choi, Joung-Sook Hong∗ , and Sang-Hoon Lee∗ , Member, IEEE

Abstract—We fabricated a carbon nanotube (CNT)/ polydimethylsiloxane (PDMS) composite-based dry ECG electrode that can be readily connected to conventional ECG devices, and showed its long-term wearable monitoring capability and robustness to motion and sweat. While the dispersion of CNTs in PDMS is challenging, we optimized the process to disperse untreated CNTs within PDMS by mechanical force only. The electrical and mechanical characteristics of the CNT/PDMS electrode were tested according to the concentration of CNTs and its thickness. The performances of ECG electrodes were evaluated by using 36 types of electrodes which were fabricated with different concentrations of CNTs, and with a differing diameter and thickness. The ECG signals were obtained by using electrodes of diverse sizes to observe the effects of motion and sweat, and the proposed electrode was shown to be robust to both factors. The CNT concentration and diameter of the electrodes were critical parameters in obtaining high-quality ECG signals. The electrode was shown to be biocompatible from the cytotoxicity test. A seven-day continuous wearability test showed that the quality of the ECG signal did not degrade over time, and skin reactions such as itching or erythema were not observed. This electrode could be used for the long-term measurement of other electrical biosignals for ubiquitous health monitoring including EMG, EEG, and ERG. Index Terms—Biocompatible, carbon nanotube (CNT)/ polydimethylsiloxane (PDMS) composite electrode, ECG electrode, long-term monitoring .

Manuscript received July 11, 2011; revised November 4, 2011 and February 6, 2012; accepted February 18, 2012. Date of publication March 7, 2012; date of current version April 20, 2012. This work was supported by the Korea Healthcare Technology R&D Project, Ministry for Health, Welfare & Family Affairs, Korea, under Grant A092052, the Public Welfare & Safety Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology under Grant 20110020943, and the Strategic Technology Development Program of the Ministry of Knowledge Economy under Grant 10031779. H.-C. Jung and J.-H. Moon contributed equally to this work. Asterisk indicates co-corresponding authors. H.-C. Jung, J.-H. Moon, and Y.-Y. Choi are with Department of Biomedical Engineering, College of Health Science, Korea University, Seoul 136-703, Korea (e-mail: [email protected]; [email protected]; [email protected]). D.-H. Baek is with the School of Electrical Engineering, Korea University, Seoul 136-703, Korea, and also with the Department of Biomedical Engineering, College of Health Science, Korea University, Seoul 136-703, Korea (e-mail: [email protected]). J.-H. Lee is with the Department of Chemical Engineering, Soongsil University, Seoul 156-743, Korea (e-mail: [email protected]). ∗ J.-S. Hong is with the Department of Chemical Engineering, Soongsil University, Seoul 156-743, Korea (e-mail: [email protected]). ∗ S.-H. Lee is with the Department of Biomedical Engineering, College of Health Science, Korea University, Seoul 136-703, Korea (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TBME.2012.2190288

I. INTRODUCTION ECENT progress in technology has enabled the continuous monitoring of personal health regardless of a patient’s location [1]–[6]. The rapid improvement of smart phone technology is predicted to enhance the quality of ubiquitous (U)-health care by providing high levels of patient autonomy. To achieve convenient health monitoring, the development of a long-term wearable bioelectrode is critical. Electrocardiography is a key biosignal that requires constant monitoring, and the bioelectrode most commonly used to monitor ECG is a gel-type silver/silver chloride (Ag/AgCl) electrode [7]. Although such electrodes have been widely used, they are limited in their longterm use because they can irritate the skin. The gel also dries over time, causing a dramatic decrease in the signal quality [8]. Additionally, signal noise is enhanced by the space charge layer at the interface between the skin and the electrode paste, and sweat is another source of signal degradation [9]. As replacements of conventional Ag/AgCl electrodes for long-term monitoring applications, dry surface electrodes have been considered because they do not require an electrolyte layer [10]–[14]. Although several dry electrodes have been developed using metallic materials, they have been limited in their practical use due to high electrode-to-skin impedance, poor biocompatibility, and variations in the contact area during motion. The fabrication of soft polymer-based ECG dry electrodes for trouble-free long-term wearability has previously been described [15]–[18]. Our previous electrodes were patterned on the polydimethylsiloxane (PDMS) substrate using metal deposition and patterning based on silicon processes, and they were packaged with a wristband to facilitate easy wearing. Although previous researches overcome several problems associated with dry electrodes, the fabrication process was complicated and required a clean room and metal deposition facilities. In addition, the incompatibility between PDMS and the metal tended to cause failures in stable bonding of the thin metal layers and motion artifact was higher than commercial Ag/AgCl electrodes. Fernandes et al. proposed fabrication of an ECG electrode encapsulated in a nickel-PDMS composite substrate to enable the inductive measuring of ECG signals. The signals were stably measured, but the fabrication process required skilled manual labor (for example, stacking and bonding of PDMS thin layers on either side of a copper sheet), and the ECG signal did not clearly capture the PQRST waves. In addition, the biocompatibility of the nickel composite was not fully proved. In this paper, we propose a carbon nanotube (CNT)/PDMS composite-based ECG electrode that can be readily connected to the conventional ECG device, and showed its use for long-term

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JUNG et al.: CNT/PDMS COMPOSITE FLEXIBLE DRY ELECTRODESFOR LONG-TERM ECG MONITORING

wearable monitoring capability and robustness to environments including motion and sweat. PDMS has good elastic properties, is flexible, and is optically transparent [19]. PDMS has, recently, become a popular choice for biomedical applications for its nontoxicity, high gas, and water permeability [20], [21], and is amenable to a variety of fabrication methods. CNTs show excellent mechanical, electrical, and thermal properties and are inert in the context of biomedical applications. For these reasons, CNTs have been widely used in industrial and biomedical applications. Therefore, CNT/PDMS composites, which combine the characteristics of both materials, may have great potential in the biomedical field as previous research has shown [22], [23]. However, the homogeneous dispersal of CNTs in thick PDMS solution has posed a tremendous challenge. For the dispersion, the surface of CNTs should be functionalized by physical or chemical treatment such as ultrasonification, and plasma or acid treatment to improve compatibility with medium. Such treatment not only requires additional filtration steps but also deteriorates its property. Here, we optimized the dispersion process of untreated CNTs within PDMS by mechanical force only. A two-step dispersal method was employed, which is a typical fabrication method of CNT/polymer composites for a few polar polymers such as polycarbonate [24]. In the first step, CNTs of a high concentration were dispersed under high shear flow conditions. The concentrated CNT dispersion was diluted to the desired CNT concentration during the second dispersion step. To achieve a homogeneous dispersion, the CNT dispersion process was performed over a long period of time [25], [26]. The fabrication of ECG electrodes was simple and straightforward, and their biocompatibility was evaluated in vitro by culturing human epithelial cells on the electrodes and by continuous wearing of the electrode on a wrist for a week. The electrical and mechanical properties of the electrode were tested, and the CNT effects of the materials were investigated. To determine the optimal CNT composition, the size and thickness of the electrode were varied. Based on these parameters, 36 types of electrodes were fabricated and their performances were evaluated. The ECG signals obtained by using CNT/PDMS electrodes of various sizes and concentrations were measured under static, motion, and sweating conditions. During seven days of wearing, the ECG signal was measured daily to investigate the quality of the biosignal over time.

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Fig. 1. Schematic diagram showing the fabrication process of CNT/PDMS composite electrodes. (a) Circular acryl plate and the snap were positioned on petri dish. (b) PDMS was poured over the circular acryl plate and snap and cured for 2 h at 80 ◦ C. (c) Thermally cured PDMS including circular acryl plate and snap was separated from the petri dish. (d) Circular acryl plate was removed. (e) CNT/PDMS solution was poured over the area where the circular acryl plate was removed and cured for 2 h at 80 ◦ C. (f) Separation of CNT/PDMS electrode. (g) Schematic of CNT/PDMS electrodes connected to the snap clip.

produce good electrical conductance. Here, CNTs were hydrodynamically dispersed without chemical modification of CNT surfaces. To achieve an effective dispersion by mechanical force only, CNTs were dispersed via a two-step dispersion procedure. In the first step, we prepared a highly concentrated CNT dispersion containing 4.5wt% CNTs using a milling machine (EXAKT 50, EXAKT Technologies Inc., Oklahoma City, OK). In the second step, the highly concentrated CNT dispersion was diluted to 1, 1.5, 2, and 4.5wt% CNT, respectively, under shear flow (250 r/min, diameter of cylindrical stirrer = 5.5 cm, and gap between the beaker and cylindrical stirrer = 1 mm) for a long time. During the second step, the dispersion time under the flow conditions is critical and we changed this time between 5 (1wt%) and 15 h (4.5wt%) depending on the concentration of CNT. During an intended dispersion process under a uniform flow condition, CNTs were expected to be homogeneously dispersed in PDMS due to the application of hydrodynamic energy stronger than the interaction force between CNTs.

II. MATERIALS AND METHODS A. CNT Dispersion in PDMS

B. Fabrication of CNT/PDMS Composite Electrodes

To fabricate the ECG electrodes, CNTs were dispersed in PDMS. CNTs (Ctube100, length 1–25 μm, multiwall CNT, purity 93%) used in this study were supplied by CNT Company, Ltd., Incheon, Korea. CNTs were manufactured by chemical vapor deposition, and the purity exceeded 90%. The density of PDMS (Sylgard184, Dow Corning Company, Midland, MI) was 1030 kg/m3 , and the viscosity was 6.2 Pa·s. Since CNTs have a large surface area (>500 m2 /g), strong van der Waals interactions are present between CNTs. These interactions make CNTs form aggregates easily, which should be disentangled to

The fabrication of CNT/PDMS composite electrodes involved two steps. The first step was the fabrication of a PDMS master mold shown as follows. Circular acrylic plate of diverse sizes (diameter: 20, 30, 40 mm and thickness: 1, 2, 3 mm) and a steel snap, which is straightforwardly connectable to conventional ECG snap clips, were prepared. These components were aligned with each other and were fixed on the bottom surfaces of the petri dish using double-sided tape [see Fig. 1(a)]. PDMS precursor (10:1 mixture of liquid prepolymer (Sylgard 184A, Dow Corning Company) and crosslinking agent (Sylgard 184B,

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Dow corning, Company) were poured and thermally cured at 80 ◦ C for 2 h [see Fig. 1(b)] [27]. After curing, the PDMS master mold was separated carefully from the petri dish [see Fig. 1(c)]. The circular acryl plate was removed from the PDMS mold [see Fig. 1(d)]. The second step involves the fabrication of CNT/PDMS composite electrodes via the replication process. The prepared CNT/PDMS composite was poured onto the PDMS master mold and thermally cured at 80 ◦ C for 2 h [see Fig. 1(e)] [28]. The completed electrode was retrieved from the master mold [see Fig. 1(f)]. To make separation easier, the PDMS master mold was immersed in methanol for 5 min. The methanol smeared into small gaps between the PDMS mold and polymerized CNT/PDMS, making separation easier. Fig. 1(g) shows a schematic diagram of the complete electrode connected to a snap clip. C. Characterization of the Electrical and Mechanical Properties The composition of CNT and the thickness of the electrodes may be critical in determining the electrical properties. We measured electrode-skin contact impedance as a function of the composition of CNT in PDMS and electrode thickness. The contact impedance between skin and electrode was analyzed, and two electrodes are placed in the forearm 9 cm apart for measurement. They are fixed by using a compression bandage without the skin preparation to improve conductivity. A voltage of 50 mV is applied from 10 to 100 kHz with the impedance analyzer. In contrast to commercial ECG electrodes, our CNT/PDMS electrodes are flexible and stretchable. Therefore, we measured the electrical conductivity according to extensional strain (%) to investigate the sustainability of electrical performance of the CNT/PDMS composite under the stretched condition [29]. For this measurement, a CNT/PDMS strip with dimensions of 20 mm × 20 mm × 1 mm was prepared and mounted on a translation stage (SURUGA SEIKI Company, Ltd., Shizuoka, Japan, MM stage). The CNT/PDMS composite specimens were stretched at a constant speed of 1 mm/s from 1 to 9 mm (5–45% strain) [29], [30], and the surface resistance was measured in situ using the four-point probe. A four-point probe was connected to a source meter (Keithley 2400) and multimeter (Agilent 34401), respectively, and the electrical conductance was calculated [31]. The compositions of CNT/PDMS composite for electrode samples were 1.0, 1.5, 2.0, and 4.5wt%, respectively. The mechanical properties of the electrodes were measured from stress– strain curve using a universal testing machine (UTM, WL2100) [32]–[34]. Samples for the mechanical test were prepared by following the ASTM D638 test shape. The test samples were stretched at a speed of 5 mm/min with a 20-N extension force. D. Short- and Long-Term ECG Measurements To acquire ECG signals using the CNT/PDMS composite electrode, electrodes were placed on the forearms and left leg of each person. The ECG signal was amplified using an ECG amplifier (BIOPAC Systems, Santa Barbara, CA, ECG 100 C), and recorded using the data acquisition system (BIOPAC systems MP150). The electrodes (total 36 types: four composition


ratios, three diameters, and three thicknesses) were fabricated and their performance evaluated by measuring the ECG signals. The electrodes were attached using a compression bandage (3M Coban Tape). To reduce moisture artifacts, tests were performed immediately after fixing on the body; moisture was removed prior to beginning each test. Five electrodes per each electrode type were prepared to observe the variation among the same type of electrodes. The motion artifacts, a significant concern in high fidelity signal measurements, were investigated by obtaining ECG signals during treadmill walking at a speed of 3 or 5 km/h. To minimize the noise generated by the movement, the cables were fixed onto the examinees’ arms and legs using a compression bandage. This study progressed after approval from the Institutional Review Board of Korea University (IRB No: KU-IRB-12-02A-1). E. Cytotoxicity and Skin Compatibility Tests The cytotoxicity of the materials was tested by culturing skin fibroblast cells on the PDMS/CNT composite electrode surfaces. The cultured CCD-986sk cells were obtained from the Korean Cell Line Bank (Seoul, Korea). The cells were washed with phosphate buffered saline (pH 7.4), and were detached from the flask using the Tryp-LE Express buffer (Invitrogen Corporation, Carlsbad, CA). They were counted using a hemocytometer, and 1 × 105 cells were seeded on the CNT/PDMS electrode surface. Cells were cultured for a week in Dulbecco’s modified Eagle medium (Gibco) with high glucose supplementation, 10% fetal bovine serum, 25-mM 4-(2-hydroxyethyl)-1piperazineethanesulfonic acid, and 25-mM sodium. The viability and morphology of the skin fibroblast cells cultured on the PDMS/CNT composite electrode surface were analyzed using the live/dead assay kit (Invitrogen). The skin compatibility test was conducted for three examinees using a 2 cm × 2 cm × 1 cm CNT/PDMS (2.0wt%) sheet. The CNT/PDMS sheet was attached on the forearm for seven days with clinically usable air permeable tape (Himom band, JW Pharmaceutical Corporation, Seoul, Korea), and skin reactions, including itching and erythema, were investigated. III. RESULTS CNT/PDMS dispersion by the two-step process yielded CNT/PDMS prepolymer dispersions with excellent properties, and the mixing ratio of CNT and PDMS could be controlled as desired. The well-dispersed CNT/PDMS composite was used to fabricate electrodes. Fig. 2 shows nine different kinds of electrodes. From left to right, the diameters increased, while from bottom to top, the thickness increased. The size and thickness were well controlled according to the proposed replication method. The electrode-skin contact impedance was measured using 2-, 3-, 4-cm diameter and 3-mm thickness CNT/PDMS electrodes from each composition rate of CNT in PDMS (1, 1.5, 2, and 4.5wt%) and plotted as log |Z| against log f in Fig. 3. The value of the contact impedance decreases strongly with increasing frequency and is affected by the diameter. The concentration of the CNT ratio at the 4.5 and 2.0wt% showed almost similar


Fig. 2. Images of the nine fabricated CNT/PDMS electrodes and the Ag/AgCl electrode. From left to right, the diameter of the electrodes increased, and from bottom to top, the thickness of the electrode increased.

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Fig. 4. Electric conductance as a function of strain (%) and CNT concentration (1, 1.5, 2, and 4.5wt%).

Fig. 5. Relation of tensile stress and strain of CNT/PDMS electrodes according to the CNT concentration (1, 1.5, 2, and 4.5wt%).

Fig. 3. Electrode-skin contact impedance as a function of the electrode thickness and CNT concentration (1, 1.5, 2, and 4.5wt%).

contact impedance, indicating that CNT ratio is saturated from 2.0 to 4.5wt%. Fig. 4 shows the variation of electrical conductivity under a stretching condition. The CNT/PDMS specimens (dimensions of 20 mm × 20 mm × 1 mm) were stretched from 5% to 45% at constant extension velocity, and in the case of the 1.5 and 2.0wt% composites, the electrical conductivity was found to decrease as the specimen was stretched. However, the 1.0 and 4.5wt% composites did not show clear changes in conductivity, even when they were stretched by 45%. The tensile stress–strain curves for the CNT/PDMS composites as a function of composition are shown in Fig. 5. They indicated that Young’s modulus (E) of the CNT/PDMS composite was related to the CNT wt%, and as the fraction of CNT increased, E increased. Yet, at 4.5wt% CNT, the yield stress was low and E decreased, indicating that the CNT concentration threshold, from a mechanical perspective, was within 2–4.5wt%. A total of 36 types of CNT/PDMS electrodes were prepared and ECG signals were measured using these electrodes. The typical ECG waves from each electrode are illustrated in Fig. 6. As a control, signals from commercial Ag/AgCl electrodes (HeartRode, Hurev Company, Ltd., Wonju, Korea) were measured at the

same positions. The signals were affected by the variation of CNT wt%, diameter, and thickness. The signal amplitude of the 1wt% CNT/PDMS composite was very low. As the CNT wt% increased, higher signals were detected, which indicates that the CNT concentration is critical to measuring a high-quality signal. The diameter of the electrode was also a significant factor for obtaining ECGs with good quality. As expected, a larger electrode measured ECG signals better than a smaller electrode did except at 1.0wt% CNT composition. A 4-mm electrode measured the highest quality of ECG signals. In contrast to the significant effects of the CNT wt% and diameter, the effect of thickness was low, consistent with the conductivity tests. The commercial ECG electrode provided a control ECG signal, and we compared the signal from CNT/PDMS electrodes to that from an Ag/AgCl electrode by measuring the signal-to-noise ratio (SNR). Among the signals, the SNR of 2wt% 4-cm diameter and 2-mm thickness electrode (45.8 dB) showed almost similar values to that of a commercial electrode (44.5 dB). To gauge the presence of motion artifacts, ECG was measured during a treadmill exercise. The treadmill speed was set to 0, 3, and 5 km/h, and the Ag/AgCl electrode and the 1.5 and 4.5wt% CNT/PDMS electrode (thickness: 3 mm) were used to gather ECG signals as illustrated in Fig. 7. The top ECG signals shown in Fig. 7(a)–(c) are the signals from the resting state, which were noiseless and almost identical. In the ECG signals during slow walking (3 km/h), low baseline noise was observed. The

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Fig. 6. ECG signals measured using 36 types of CNT/PDMS electrodes with the Ag/AgCl electrode used as reference. The control parameter is CNT concentration: (a) 1.0, (b) 1.5, (c) 2.0, and (d) 4.5wt%. For each case, the thickness and diameter of CNT/PDMS electrodes were modulated as follows: thickness (1, 2, and 3 mm) and diameter (20, 30, and 40 mm). (e) ECG signal of the Ag/AgCl electrodes.

Fig. 7. ECG signals measured from three types of electrodes (Ag/AgCl, 1.5wt%, and 4.5wt% CNT/PDMS) to measure the motion effect. The graphs show (a) Ag/AgCl, (b) 4.5wt%, and (c) 1.5wt% CNT/PDMS electrodes. The top is the rest state, the middle is the slow walking state (3 km/h), and the bottom is the fast walking state (5 km/h).

1.5wt% CNT/PDMS composite electrode exhibited large noise fluctuations, and the P and T waves were not clearly observed. In contrast, the Ag/AgCl and 4.5wt% CNT/PDMS composite electrodes provided almost qualitatively comparable signals. The lowest ECG signals in Fig. 7 were measured during faster walking (5 km/h). More noise was observed under these conditions than under the normal walking state, and QRS and T waves were detected at signals from Ag/AgCl and 4.5wt% CNT/PDMS composite electrodes. These results illuminate the feasibility of robust ECG measurements using CNT/PDMS composite electrodes under normal conditions. The effects of sweat were also tested, and, as shown in Fig. 8, no critical differences among signals were observed in the presence of sweat, even though the heart rate increased after exercise. The ECG was measured

for 5–7 days since wearing, and degradation of the ECG signal over time was not observed as shown in Fig. 9, indicating the superior characteristics as a long-term monitoring electrode. The cytotoxicity was tested by culturing skin fibroblast cells (CCD-986sk) on the surface of four types of electrodes having different CNT compositions to examine whether the electrodes released toxic materials. The cells were cultured for one week, and the viability was tested. As shown in Fig. 10(a), most cells on the 1.0, 1.5, 2.0, and 4.5wt% CNT/PDMS composite samples were alive and were spread uniformly on the electrode surface. Their viability on all electrodes exceeded 95%, indicating that the CNT/PDMS composite did not affect cell growth. Skin compatibility tests were conducted over seven days of continuous wearing of the electrodes on the arms of ten examinees


Fig. 8. ECG signals to investigate sweat effect. (a) ECG signal measured before exercise (no sweat). (b) ECG signal measured after exercise (with sweat).

Fig. 9. ECG signals measured on 0, 5, 6, 7 days after wearing electrode; no significant degradation of signal was observed.

Fig. 10. (a) Results of the live/dead assay for cells cultured on the four types of CNT/PDMS electrode. The cell viability on all electrodes exceeded 95%. (b) Skin of examinees’ forearms wearing the CNT/PDMS sheet for seven days. Removal of the electrode revealed that no side effects were observed.

(age: 20–27, gender: 5 men, 5 women). The skin under the electrode was found to be normal and no itching or erythema was observed during and after seven days of wearing [see Fig. 10(a)].

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The nanotubes dispersed in the electrode appeared to contact well with each other and form conducting pathways. The electrical conductance tests indicated that the electrical performance depended on the concentration of CNTs. Extensive measurements of the ECG signals using 36 types of electrodes showed that the composition ratio and diameter were critical parameters for measuring high-quality ECG signals. In contrast, the electrode thickness hardly affected the ECG signal measurement. The electrode fabrication process was relatively simple and compatible with automated mass production processes. The snaps were strongly bonded to the CNT/PDMS composite electrode, and the electrodes were easily connectable to snap clip of conventional ECG machines. The long-term wearability of the electrodes was demonstrated through several tests. Seven days of wearing indicated that the electrode did not cause any reactions on the skin. ECG signals were measured focusing on 5–7 days after wearing, and did not show degradation over time. The signal from day 1 to day 4 was also measured and demonstrated no significant variations in the shape of the ECG signal. For the quantitative comparison of signals, SNRs of ECG signals on days 0, 5, 6, and 7 were measured, and found to be within a ±5% error range. The signal quality was high and the motion artifacts were low for the 4.5wt% electrode. Most dry electrodes have motion artifacts that are more prominent than those of commercial Ag/AgCl electrodes due to the higher contact impedance and inferior contact with the skin. However, our electrode exhibited superior contact with the skin, and the signal under the motion was comparable to that of the Ag/AgCl electrode. These results indicate that the CNT/PDMS composite electrode overcomes the problems that most dry and wet electrodes have, providing superiority in terms of long-term wearability. Another unique feature of the proposed electrode is that it is reusable upon cleaning with alcohol. After seven days of testing, the electrode was cleaned and reapplied, and a change in ECG signal quality was not observed at all. This reusability will contribute to preserving the environment by minimizing the waste of used electrodes. In addition, the electrode performance was not disturbed by sweat. The Ag/AgCl electrode was found to be sensitive to sweat because the sweat can change the properties of the gel. However, the CNT/PDMS composite electrode measured ECG signals robustly in the presence of sweat. The cytotoxicity tests revealed that the electrode material itself was safe for biomedical application. The long-term wearability test showed that the ECG signal does not degrade over time, indicating that the CNT/PDMS electrode was useful for the continuous monitoring of ECG signals. The CNT/PDMS electrodes may be used for the long-term measurement of other electrical biosignals, including EMG, EEG, and ERG.

IV. DISCUSSION CNT/PDMS electrodes of various compositions (1, 1.5, 2, and 4.5wt%), thicknesses (1, 2, and 3 mm), and diameters (20, 30, and 40 mm) were fabricated, and their mechanical and electrical properties were investigated. The performances as long-term wearable electrodes were extensively evaluated. The CNT/PDMS complex was proved to be a good flexible conductor due to the well dispersion of CNT and flexibility of PDMS.

V. CONCLUSION In this paper, we successfully fabricated CNT/PDMS composite electrodes in a straightforward manner and demonstrated that it overcomes limits of conventional dry or wet electrodes. The CNT/PDMS composite electrodes were flexible, biocompatible, and suitable for long-term measurement of the ECG signals. The proposed CNT/PDMS composite electrodes

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measured ECG signals robustly even in the presence of motion or sweat, and was connectable to commercial ECG machine without an adaptor. The signal quality depended on the composition of the CNT/PDMS composite, and on the size of the electrode. The electrode did not generate any noticeable side effects, such as itching or irritation, even after one week of continuous wearing. The signal quality did not decrease over time, in contrast to Ag/AgCl electrodes. The aforementioned results indicate that the composite electrodes are suitable for long-term ECG monitoring due to biocompatibility and robust performance under conditions of motion and sweat. We expect that the proposed CNT/PDMS composite electrodes can be used in daily life to enable ubiquitous mobile health care technology. The reusability of the electrode will contribute to preserving the environment by reducing the waste of used electrodes. ACKNOWLEDGMENT The authors would like to thank Dr. H. H. Ahn of Korea University Hospital for comments about the skin status. REFERENCES [1] J. Yoo, L. Yan, S. Lee, H. Kim, B. Kim, and H.-J. Yoo, “An attachable ECG sensor bandage with planar-fashionable circuit board,” in Proc. Int. Symp. Wearable Comput., Sep. 4–7, 2009, pp. 145–146. [2] K. K. Kim, Y. K. Lim, and K. S. Park, “The electrically noncontacting ECG measurement on the toilet seat using the capacitively-coupled insulated electrodes,” in Proc. 26th Annu. Int. Conf. IEEE Eng. Med. Biol. Soc., Sep. 1–5, 2004, vol. 1, pp. 2375–2378. [3] M. Braecklein, C. Moor, I. Tchoudovski, and A. Bolz, “New system for cardiological home monitoring with integrated alarm function,” in Proc. 2nd OpenECG Workshop, 2004, vol. 51, pp. 51–52. [4] M. Coyle, “Ambulatory cardiopulmonary data capture,” in Proc. 2nd Annu. Int. IEEE-EMB Microtechnol. Med. Biol., 2002, pp. 297–300. [5] A. Karilainen, T. Finnberg, T. Uelzen, K. Dembowski, and J. Muller, “Mobile patient monitoring based on impedance-loaded SAW-sensors,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 51, no. 11, pp. 1464– 1469, Nov. 2004. [6] R. Wijesiriwardana, K. Mitcham, and T. Dias, “Fibre-meshed transducers based real time wearable physiological information monitoring system,” in Proc. 8th Int. Symp. Wearable Comput., 2004, pp. 40–47. [7] J. G. Webster, Medical Instrumentation: Application and Design. New York: Wiley, 1998, pp. 196–215. [8] A. Searle and L. Kirkup, “A direct comparison of wet, dry and insulating bioelectric recording electrodes,” Physiol. Meas., vol. 21, pp. 271–283, 2000. [9] B. A. Taheri, R. T. Knight, and R. L. Smith, “A dry electrode for EEG recording,” Electroencephalogr. Clin. Neurophysiol., vol. 90, no. 5, pp. 376–383, 1994. [10] A. Karilainen, S. Hansen, and J. Muller, “Dry and capacitive electrodes for long-term ECG monitoring,” in Proc. 8th Annu. Workshop Semicond. Adv. Future Electron., Nov. 26, 2005, pp. 155–161. [11] S. Mason. (2005). Dry electrode technology: What exists and what is under development? [Online]. Available: http://www.bci-info.tugraz.at/ Research_Info/research_forums/signals/0002/ [12] G. Ruffini, S. Dunne, E. Farres, J. Marco-Pallares, C. Ray, E. Mendoza, R. Silva, and C. Grau, “A dry electrophysiology electrode using CNT arrays,” Sens. Actuators A, Phys., vol. 132, no. 1, pp. 34–41, 2006. [13] G. Ruffini, S. Dunne, L. Fuentemilla, C. Grau, E. Farres, J. Marco-Pallares, P. C. P. Watts, and S. R. P. Silva, “First human trials of a dry electrophysiology sensor using a carbon nanotube array interface,” Sens. Actuators A, Phys., vol. 144, no. 2, pp. 275–279, 2008. [14] J. Muhlsteff and O. Such, “Dry electrodes for monitoring of vital signs in functional textiles,” in Proc. 26th Annu. Int. Conf. IEEE Eng. Med. Biol. Soc., 2004, pp. 2212–2215. [15] J. Y. Baek, J. H. An, J.-M. Choi, K.-S. Park, and S.-H. Lee, “Flexible polymeric dry electrodes for the long-term monitoring of ECG,” Sens. Actuators A, Phys., vol. 143, no. 2, pp. 423–429, 2008.

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Ha-Chul Jung received the B.S. degree in electrical engineering from Kwangwoon University, Seoul, Korea, in 2011. He is currently working toward the M.S. degree in the Department of Medical Electronics and Information Engineering, Korea University, Seoul. His current research interests include microelectrode, biological microelectromechanical systems, and neural engineering.


Jin-Hee Moon received the B.S. degree in electronic engineering from Yonsei University, Seoul, Korea, in 2003, and the Ph.D. degree of unified master’s and doctor’s course in biomedical engineering from Seoul National University, Seoul, in 2010. He was involved in the study of artificial kidney and regeneration of dialysate fluid, and developed a device inducing mild hypothermia for cardiac arrest patient. He is currently in the Department of Biomedical Engineering, College of Health Science, Korea University, Seoul. His current research interests focus on microelectrodes, biological microelectromechanical systems, and intrabody area network.

Dong-Hyun Baek received the B.S. degree in physics from Sunmoon University, Asan, Korea, in 2007. He is currently working toward the Ph.D. degree in the School of Electrical Engineering, Korea University, Seoul, Korea. He is also in the Department of Biomedical Engineering, College of Health Science, Korea University. His research interests include microelectrodes, microelectromechanical systems, and device packaging.

Jae-Hee Lee is currently a senior and majoring in chemical engineering at Soongsil University, Seoul, Korea. His research interests include the dispersion of carbon nanotubes in a non-Newtonian fluid.

Yoon-Young Choi received the B.S. degree in biology from Dankook University, Seoul, Korea, in 2008, and the Master’s degree from the Department of Medical Electronics and Information Engineering, Korea University, Seoul. She is currently with the Department of Biomedical Engineering, College of Health Science, Korea University. Her current research interests include lab on a chip, stem cell culture, and tissue engineering.

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Joung-Sook Hong received the B.S. degree in chemical engineering from Cheju National University, Jeju, Korea, in 1995, and the M.S. and Ph.D. degrees in chemical engineering from the Seoul National University, Seoul, Korea, in 1997 and 2005, respectively. She was a Research Associate at the University of Queensland in 2006, a Research Assistant Professor at Korea University in 2007, and a Senior Researcher at Chemical Research Center, Samsung Cheil Industry, in 2008. She is currently a Professor in the Department of Chemical Engineering, Soongsil University, Seoul. Her current interests include the dispersion of carbon nanotube in a non-Newtonian fluid, drop generation and deformation, interfacial rheology, polymer blend, and composite.

Sang-Hoon Lee (M’96) received the B.S. degree in electrical engineering and the M.S. and Ph.D. degrees in biomedical engineering from Seoul National University, Seoul, Korea, in 1983, 1987, and 1992, respectively. From 1985 to 1992, he was a Researcher, and from 1992 to 1998, he was an Instructor and Assistant Professor in the Department of Biomedical Engineering, Seoul National University Hospital. From 1998 to 2006, he was a Professor in the Department of Biomedical Engineering, Dankook University. In 2002, he was a Visiting Scientist in the Department of Biomedical Engineering, University of Wisconsin-Madison. He is currently a Professor in the Department of Biomedical Engineering, College of Health Science, Korea University, Seoul. His current interests include the development of single-cell handling and analysis of microfluidic devices and flexible implantable sensor for biomedical applications.