Microsyst Technol DOI 10.1007/s00542-015-2490-y

TECHNICAL PAPER

A flexible dry electrode based on APTES‑anchored PDMS substrate for portable ECG acquisition system Ying Meng1 · Zhenbo Li1 · Jiapin Chen1 

Received: 6 March 2015 / Accepted: 11 March 2015 © Springer-Verlag Berlin Heidelberg 2015

Abstract  We developed a simple and low-cost flexible dry electrode with micro domes by depositing metal film directly on (3-aminopropyl)triethoxysilane (APTES)anchored polydimethylsiloxane (PDMS) substrate for portable electrocardiogram (ECG) acquisition system. However, the adhesion between metal and PDMS was poor, and metal film on PDMS always exhibited wrinkles. To overcome these difficulties, before depositing Cu, PDMS substrate was treated by APTES aqueous solution. Then, we evaluated the metal film through the microscopic photographs, the surface roughness measurements and the adhesion tests. On the base of the deposition technique improvement, a PDMS-based dry electrode for ECG monitoring was fabricated. We studied the performance of the flexible dry electrode and the results showed the fabricated electrode produced good ECG signals with distinct P, QRS, and T waves. In addition, the fabricated flexible dry electrode with micro domes showed lower skin-contact impedance and could obtain ECG signals with higher SNR than the flat dry electrode.

1 Introduction As living standards rise, people begin to give more consideration to self-health and have an eager desire of preventing * Jiapin Chen [email protected] 1



Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Dongchuan Road 800, 200240 Shanghai, People’s Republic of China

diseases in advance. And the appearance of portable monitoring facilities satisfy the needs of people on daily health care and disease prevention (Chen et al. 2013; Cho et al. 2011). These portable monitoring facilities allow easily monitoring and visualizing vital signals at home, thus leading to cost reductions in health care services. The portable facilities monitoring vital signs, such as blood pressure, electrocardiography and electroencephalography usually consist of the front sensor, data acquiring/ processing circuits, signal transmission and host interface. The commonly used front sensor for ECG monitoring is standard Ag/AgCl electrode with electrolyte gel (Hassanin et al. 2012). The electrolyte gel works as a conductive layer between the skin and an electrode to keep small skin–electrode impedance and stable reception of the signal. However, the electrolyte gel is unpleasant and may result in dermatitis. Besides, electrolyte gel dries out with time, which could result in degradation of signal quality. To overcome the drawbacks, various kinds of dry electrodes have been fabricated as an alternative to the wet electrode, such as P-FCB electrodes, micro-needle electrodes, CNT/ PDMS composite electrodes, polyimide–PDMS electrodes and textile-based electrodes (Byun et al. 2013; Moon et al. 2010; Lee et al. 2012; Yoo et al. 2009; Yu et al. 2009; Wang et al. 2012). Among these, flexible and stretchable dry electrode, which is deformable into curvilinear shape to enable functionalities that are impossible to achieve by hard and rigid electronics, has attracted more attention. The key point for fabricating flexible electronics lies in flexible substrate and conductive layer. Conventional flexible materials tend to have low charge carrier density, thus exhibiting low conductivity. And good metal conductors rely on strong metallic bonds to form crystalline structures, making them hard to stretch or compress. Recently, conductive polymer (conductive material mixed with polymer),

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a flexible substrate with conductivity, has been developed to fabricate flexible electronics (Yoo et al. 2009; Hii and Chung 2011). Nevertheless, several problems should be considered. On one hand, the polymer is sticky and it is difficult to achieve the even mixing. On the other hand, the conductivity of the conductive polymer might not as good as metal conductor. In this paper, we introduced a flexible dry electrode with micro domes by directly depositing metal film on PDMS substrate. PDMS, a polymer biomaterial, has been widely used in many fields including biomedical devices, microstructure fabrication and microfluidic chips for its good elastic properties, flexibility, biocompatibility and optical transparency (Yu et al. 2009; Baek et al. 2008; Yu and Bulović 2007). However, previous researches indicated that metals deposited on PDMS exhibited wrinkles and cracks frequently (Watanabe and Mizukami 2012; Lee et al. 2009). Additionally, the adhesion between metal and PDMS substrate was poor (Thompson and Abate 2013). To overcome these difficulties, we modified PDMS substrate with APTES before depositing metal film. Optical photographs, the surface roughness measurements and adhesion tests were used to evaluate metal films on PDMS. Afterwards, we characterized the fabricated dry electrode by skin–electrode impedance and SNR of the obtained ECG signals. Finally, a portable ECG acquisition system was designed to monitoring ECG signals in real time.

Microsyst Technol

Fig. 1  The equivalent circuit model of dry electrode

2 Materials and methods 2.1 Design of the dry electrode ECG can be required by placing electrodes on a person’s skin, and the coupling between electrode and skin could be described as a resistor and a capacitor in parallel (Chen et al. 2011). The skin–electrode contact impendence Z′ is expressed as

Z ′ = 1/(S/ρd + jωεS) where ω is the signal source, S the area of electrode, d the distance between two electrodes, ε the skin dielectric constant and ρ the skin electrical resistivity. Then, the modulus of Z′ can be written as

  ′ Z  = ρd/(S 1 + (ωερ)2 ). According to the above equation, the contact impedance is proportional to the contact area of the electrode (Fig. 1). In general, higher contact impedance of the electrode might lead to more noise which made the feature extraction

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Fig. 2  Design of the dry electrode. a The schematic drawing of the dry electrode. b The dimensions of the dry electrode

of ECG signals more difficult. Therefore, an electrode with relatively small contact impedance would be more popular. We herein fabricated a flexible dry electrode based on PDMS with micro domes for ECG monitoring, as shown in Fig. 2a, b. The proposed flexible electrode fits the skin more closely and the designed dome array on the electrode contributes to the increase of the contact area. Compared to the flexible electrode, a rigid dry electrode which is difficult to bend generates an air layer between the electrode and skin due to the untight contact. This air layer produces an additional capacitance Cp′ in series with Cp, which reduces the total capacitance. Thus, the fabricated flexible dry electrode could obtain relatively small skin–electrode and higher quality ECG signals. The dimensions of the electrode was 0.8 cm (L) × 0.8 cm (W) × 0.2 mm (H). The height of the dome was 30 μm, the diameter of the dome was 40 μm and the distance between two domes was 40 μm.

Microsyst Technol

Fig. 3  The fabrication process of the dry electrode

2.2 Fabrication process The fabrication sequence of the flexible dry electrode with micro domes was shown in Fig. 3. The process was started with exposing 30 μm AZ50XT photoresist to form an array of cylinders. Then, the resist cylinders were melted at a temperature of 120 °C in an oven to form dome-like structures. Afterwards, the structure was transferred through double-PDMS-molding steps into an array of domes in PDMS substrate. Then, the PDMS with original domes was etched in an oxygen plasma at 12 W for 3 min (PDC-32G Plasma Cleaner/Sterilizer, Harrick Scientific, Ossining, NY) and immersed in 1 % (v/v) APTES (99 %, SigmaAldrich, St. Louis, MO) aqueous solution. After 20 min, the PDMS substrate was removed from APTES and Cu was deposited. Then, a thin Nickel layer was electroplated (2  μm) and the front side of the electrode was sputtered platinum to prevent allergies. In this paper, pseudo-flip-chip bonding technology was introduced for wire connection. We connected the wire to the electrode by using solder and conductive adhesives. The details were shown in Fig. 4a. Before depositing Cu, a small hole at the edge of the PDMS substrate was punctured and a short solder was introduced. Then, conductive adhesive was used to fill the hole and Cu was deposited. Afterwards, copper wire was soldered with joint at the back of the electrode and Nickel was plated. To enhance its appearance and improve its applicability, we encapsulated the dry electrode by referring traditional Ag/AgCl electrode. The electrode was fixed in the center of a medical non-woven plaster and wire was connected at the other side. The detailed explanation followed below (see Fig. 4b). First, a medical plaster (non-woven), an adhesive tape and a thin PDMS film with a small hole were prepared. Then, three rectangle shapes were cut at the center of the medical plaster and a square shape of the attached paper

at the center was cut and torn down. Afterwards, the thin PDMS film and the back side of the dry electrode without metal layer were exposed to oxygen plasma for 40 s. The exposed film and electrode were placed on two different sides of the medical plaster. The dry electrode was put at the side with glue and the wire was fitted through the rectangle shape at the center. Finally, the adhesive plaster was stick to the wire side. 2.3 Measurement of skin–electrode contact impendence The impedances of standard wet Ag/AgCl electrode, flat dry electrode and dry electrode with micro domes were measured on a person’s forearm by an RLC meter (QuadTech 7400 Precision RLC meter, Maynard, MD). The measurement voltage of 2 V was provided with a frequency range of 10–1000 Hz. The electrodes were placed adjacent to each other on a person’s forearm with a distance of 5 cm. All the impedance measurements were carried out after removal of sweater and replicated five times, and got the average value. 2.4 ECG measurement using the portable ECG acquisition system The portable ECG acquisition system developed in this paper comprised flexible dry electrodes, low-noise sampling and processing electronics including Texas Instruments ADS1299 and Arduino UNO microcontroller board, Bluetooth HC-06 communication module and Android user Interface (see Fig. 5). ECG signals recorded by the dry electrode was firstly digitized by ADS1299 and denoised by Arduino microcontroller. Then, ECG data streaming was send by Bluetooth HC-06 and received by an android mobile phone. On the phone, it displayed received data. Since the Android system

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Microsyst Technol

Fig. 4  Wire connection and encapsulation of the dry electrode. a Pseudo-flip-chip bonding technology for wire connection. b Encapsulation of the dry electrode referring to Ag/AgCl electrode Fig. 5  The schematic drawing of the portable ECG acquisition system

is capable of running application software in the background mode, the application used in this paper has the ability to transfer data during a phone call.

3 Results and discussions 3.1 Characterization of Cu on PDMS Wrinkles and cracks of metals on PDMS substrate have greatly limited the fabrication of PDMS-based flexible electronics. To reduce the wrinkles and cracks, PDMS substrate

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was treated by APTES before depositing Cu. Compared with the flat substrates, it was more likely to exhibit wrinkles and cracks on PDMS with micro structures. To demonstrate the APTES-anchored PDMS substrate was beneficial to the reduction of wrinkles and cracks, we studied Cu films PDMS with micro domes. Figure 6 shows microscope photographs of metal films on native PDMS, O2etched PDMS and APTES-anchored PDMS, respectively. It is seen that APTES-anchored PDMS exhibited dramatically less wrinkles and cracks compared with that on native PDMS. We also measured surface roughness of Cu film on flat PDMS to evaluate the wrinkles using Veeco NT1100

Microsyst Technol

Fig. 6  Metal films on PDMS substrates. a Metal film on native PDMS. b Metal film on O2-etched PDMS. c Metal film on APTES-anchored PDMS

optical profiler (Veeco Instruments Inc, USA), see Fig. 7. The results showed the surface roughness of Cu decreased to 289.43 nm from the original 433.22 nm after PDMS was treated by APTES. This suggested Cu film on APTESanchored PDMS got smoother than that on native PDMS. To test the adhesion between metal and PDMS, the Scotch tape test was conducted using 3 M Scotch-tape (reference number of 600, 3 M) (Yang et al. 2010). The adhesive tape pieces were firmly placed on the Cu film and a pressure of 20 kPa was then applied for 5 min. A glass slide was put between the adhesive tapes and the weight in

order to apply uniform pressure. As the adhesive tape was released from PDMS, a fraction of Cu film was transferred to the adhesive tape depending on the adhesion between Cu and PDMS substrate. Visible damage upon removal of the adhesive tape provided an immediate indication of the extent of film adherence. Figure 8 shows the adhesion results on various kinds of PDMS substrates. The adhesion results show Cu film on native PDMS lift off the most while Cu on APTES-anchored PDMS lift off the least. This implied that the adhesion between metal and PDMS was enhanced when PDMS substrate was pretreated by APTES. 3.2 Fabrication of the dry electrode

Fig. 7  The surface roughness of Cu film on PDMS substrate

The flexible dry electrode successfully fabricated was depicted in Fig. 9a, b showed micro domes on the fabricated electrode. Unlike the conventional wire connection methods (such as wire bonder or soldering at the edge of the electrode) which affected the comfort and signal quality, this paper introduced the pseudo-flip-chip bonding technology to offer a shorter signal path and more rapid communication. The connection between wire and connection site of the electrode was stable and no disconnection occurred during the experiments.

Fig. 8  The adhesion test results

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Microsyst Technol

Fig. 9  The fabricated dry electrode. a The size of the dry electrode. b Micro domes on the dry electrode

Fig. 10  The skin–electrode contact impedance of flat dry electrode, dry electrode with micro domes and standard wet Ag/AgCl electrode

3.3 The skin–electrode contact impedance of the electrode Skin–electrode contact impedance measurement has always been of interest because it influences the reliability of the collected signal. Here, we measured the impedance of the dry electrode with micro domes, and compared that to the standard wet Ag/AgCl electrode. The impedance variations showed the fabricated electrode had higher contact impedance than Ag/AgCl electrode (see Fig. 10). This phenomena has been demonstrated by many researchers and is explained as being a result of free of conductive gel (Hassanin et al. 2012). In addition, we also fabricated flat dry electrode using the same fabrication process mentioned in Sect. 2.2. Both the flat dry electrode and dry electrode with micro domes had the same dimension. The results indicated that the impedance of the fabricated dry electrode with micro domes was smaller than that of the flat one, which means that dry electrode with micro domes was more suitable for ECG measurement. 3.4 ECG measurement using portable ECG acquisition system The portable ECG acquisition system designed in this paper consisted of dry electrodes, a data sampling and processing

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module, a Bluetooth transmission module and an android user interface. For data sampling, a medical analog front end ADS1299 which integrated amplifier and also ADC was used (see Fig. 11a). The digitalized ECG was denoised through a high-pass filter, a low-pass filter and a band stop filter, which were designed in an Arduino microcontroller. After filtering, ECG signals were further processed for QRS feature extraction. Then, the ECG data stream was send through Bluetooth HC06 and received and displayed in real time by an android mobile phone (see Fig. 11b). The term “real time” in this paper is used to express that data transfers are achieved without perceivable delays among the devices. The graphical user interface was implemented in Java™ using Android SDK 2.3, which allowed starting and stopping the processing and visualization of the results. Android was used because of its widespread use, open nature and the portability of the code. Additionally, it allowed simple integration of external ECG sensors via Bluetooth. The graphical user interface designed was divided into two working area. The upper area displayed the real-time ECG signal and the current value of different features like heart rate, R–R interval in ms and number of recognized QRS complexes were displayed in the lower area. We used the portable ECG acquisition system mentioned above to record ECG signals with different electrodes and the results showed in Fig. 12. It is obviously that ECG

Microsyst Technol Fig. 11  The portable ECG acquisition system. a The hardware system. b Screenshot showing the main interface of the application

Table 1  SNR of ECG signals with different electrodes Name of the electrode

SNR (dB)

Standard wet Ag/AgCl electrode Dry electrode with micro domes

23.35 21.82

Flat dry electrode

17.93

ranging from 0.05 to 100 Hz and S′ was defined as ECG signal without filtering. Before calculation, the power line interference (50 Hz) was removed from both signals. Table  1 summarizes the resulted SNR of the three different electrodes. Results have revealed that the proposed dry electrode with micro domes hold better signal quality and performance than flat dry electrode.

4 Conclusions Fig. 12  ECG signals recoreded by different electrodes. a ECG signals recorded by Ag/AgCl electrodes. b ECG signals recorded by dry electrode with micro domes. c ECG signals recorded by flat dry electrodes

signal obtained using the fabricated dry electrodes with micro domes was similar to that obtained using wet Ag/ AgCl electrodes. The figure indicated clear observations of the QRS-complex and T-wave cardiac signatures. That is, the fabricated dry electrodes with micro domes record the characteristic ECG peaks relatively effectively. Moreover, we also measured ECG signal with the flat dry electrode and found more noise added on the ECG signal. To further evaluate the performance of the electrodes, ECG signals were analyzed to calculate signal-to-noise ratio (SNR) using the equation SNR = 20 log(S/(S′−S)). Where S was the filtered ECG signal with a frequency

In this paper, we successfully fabricated a flexible dry electrode with micro domes by directly depositing Cu film on PDMS substrate. To enhance the adhesion between metal and PDMS, and reduce wrinkles of metal film, the PDMS substrate was chemically anchored by APTES before depositing metal. We studied Cu film on PDMS substrates and found Cu on APTES- anchored PDMS showed less wrinkles than on native PDMS. In addition, the adhesion between Cu and APTES-anchored PDMS was enhanced significantly. Afterwards, the performance of the fabricated dry electrode with micro domes was evaluated. The impedance measurements showed the dry electrode with micro domes had lower skin–electrode contact impedance than the flat dry electrode. The prepared dry electrode was free of gel and provided ECG signal quality that was comparable to the Ag/AgCl electrode. Besides, ECG signals obtained by the fabricated dry electrode had higher SNR

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than that obtained by the flat one. Moreover, the dry electrode used as a front sensor of the portable ECG acquisition system could successfully recorded ECG signals. And the exploratory research on smoothing the surface of the metal films on PDMS is a guide to fabricate flexible electronics by directly depositing metal films on PDMS substrates. Acknowledgments  We thank the National Natural Science Foundation of PRC (No. 61175100 and No. 51275285), the Research Fund of Medicine and Engineering of Shanghai Jiao Tong University (No. YG2011ZD01) for National Natural Science Foundation of China (No. 51035005), 973 Program (2013CB329401). Conflict of interest  The authors declare that they have no conflict of interest. Ethical standard  All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Informed consent  Informed consent was obtained from all individual participants included in the study.

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