Microsyst Technol DOI 10.1007/s00542-016-2893-4

TECHNICAL PAPER

An FPC based flexible dry electrode with stacked double‑micro‑domes array for wearable biopotential recording system Ke Lin1,2 · Xinan Wang1,2 · Xing Zhang1,2 · Bo Wang1 · Jipan Huang1 · Feng Huang1 

Received: 15 February 2016 / Accepted: 23 February 2016 © Springer-Verlag Berlin Heidelberg 2016

Abstract  To overcome drawbacks of the conventional wet Ag/AgCl electrodes, this paper proposed a novel flexible dry electrode with stacked double-micro-domes array for wearable biopotential recording system. By utilizing flexible printed circuit (FPC) substrate and fabrication technologies, we designed a unique structure of a small dome stacking on top of a large dome. Experiments results showed that the proposed electrode could partially disrupt the stratum corneum of the skin without harm: on one hand this allowed to increase the electrode–skin contacting area; on the other hand it ensured the electrode anchoring firmly on the skin surface. And the key specifications of the dry electrode, such as electrode–skin impedance (ESCI) and the signal–noise-ratio (SNR), were shown to be comparable with these of a standard wet Ag/AgCl electrode. Finally, to verify the effectiveness of the proposed electrode in the practical application, a prototype wearable ECG recording system was developed. The measured ECG waveform proved that the proposed electrode was much more flexible than the standard wet Ag/AgCl electrode and still retained good signal quality for long run with super skin compliance and comfort.

* Bo Wang [email protected] 1

Key Laboratory of Microsystem, Peking University Shenzhen Graduate School, Lishui Road 2199, 518055 Shenzhen, China

2

Institute of Microelectronics, Peking University, Yiheyuan Road 5, 100871 Beijing, China





1 Introduction With the growing concern about human health in the aging society, the focus of the healthcare services is moving from treatment to prevention in daily monitoring. In order to prevent diseases in advance, a wide range of vital signals need to be recorded at real-time for a long term. Among these vital signals, biopotentials acquired from skin surface are very important, including the signals such as electrocardiography (ECG), electroencephalography (EEG), and electromyography (EMG). For recording these biopotentials, the conventional wet Ag/AgCl electrodes are widely used as front sensors in com-mon clinical application. Though standard wet Ag/AgCl electrodes with the presence of electrolyte gel can provide excellent signal quality, they also have several drawbacks. For example, skin needs to be abraded as preparation, the gel could cause irritation after long-term usage and when gel dries out with time signal quality might degrade (Searle and Kirkup 2000). To eliminate these problems, various types of dry electrodes without electrolyte gel have been developed as an alternative biopotential acquisition front sensor. There are basically two methodologies, intrusive and non-intrusive dry electrodes. The intrusive dry electrodes usually uses micro- needles array to pierce through outer stratum corneum skin layer (Chen et al. 2013; Forvi et al. 2012; Griss et al. 2001; Yu et al. 2009), which could acquire good signal quality and dispense the necessity of skin preparation. Nonetheless, the intrusive micro-needles array dry electrodes mostly adopt micromechanical techniques such as deep reactive ion etching (DRIE) (Griss et al. 2001; Yu et al. 2009) and isotopic etching processes (Chen et al. 2013; Forvi et al. 2012) on bulk silicon substrate, which

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brings new issues such as high fabrication cost by using expensive equipment, violation of biomedical safety regulation by using high-risk chemical solution and uncomfortableness on skin due to rigid substrate (Chi et al. 2010). For the non-intrusive dry electrodes, one solution is to utilize flexible conductive materials as electrode (Gargiulo et al. 2008; Ishijima 1993; Lin et al. 2011; Yoo et al. 2009). Dry electrodes based on polymer (Yoo et al. 2009), rubber (Gargiulo et al. 2008), fabric (Ishijima 1993) or foam (Lin et al. 2011) have been evaluated by different researchers. These electrodes are comfortable and usable in certain aspect of biopotential recording application, but their high electrode– skin contact impedance (ESCI) leaded to signal quality deterioration (Chi et al. 2010). Another solution to realize non-intrusive dry electrode is active non-contact dry electrode (Chi and Cauwenberghs 2009), which do not require ohmic contact with the skin and biopotential signals are capacitively coupled to a buffer amplifier with a very high input impedance (Chi and Cauwenberghs 2009). However, such kind of dry electrodes are bulky in size and high cost due to additional electronic components and batteries as power source (Yu et al. 2009). In this paper, a novel biopotential recording dry electrode based on flexible printed circuit (FPC) substrate and fabrication technologies was presented. This electrode utilized characteristics of FPC substrates, which was flexible and thin film in morphology, to provide good skin compliance with comfort. Besides, with cost efficient, common used, mass-productive FPC fabrication processes, a stacked double-micro-domes array structure was formed on the electrode, which could lower the ESCI and improve signal quality by increasing the electrode–skin contacting area, partially disrupting the stratum corneum and anchoring the electrode on the skin surface firmly. The developed FPC based flexible dry electrode with stacked double-microdomes array brought a new insight of practical commercial application of dry electrode in real-time long-term biopotential recording and monitoring for disease prevention in healthcare services.

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(Yoo and Hoof 2012). Typically, for a conventional Ag/ AgCl electrode, its equivalent electrical model is shown in Fig. 1a (Yoo and Hoof 2012). In this model, paralleled Cdl and Rcr represent the electrode/gel interface impedance, RGel is the gel resistance, paralleled CSP and RSP describe the epidermal impedance and RTissue is the underlying tissue resistance. Because of relatively non-conductive property of the stratum corneum in epidermis, RSP is normally in the 106 Ω order and is still in the 105 Ω order even with the presence of gel (Chi et al. 2010). Therefore, most of the biopotential signal would be coupled through CSP to electrode (Yoo and Hoof 2012). Thus, the total impedance of this model can be expressed as (Meng et al. 2015)

Z ′ = 1/(s/ρd + jωεs) and its modulus is

  ′ Z  = ρd/(S 1 + (ωερ)2 ) where S is the electrode–electrolyte interface area, ρ and ε are the resistivity and dielectric of the epidermis respectively, d is the epidermis thickness, ω is the signal frequency. According to the analysis above, increasing the electrode contacting area and reducing the stratum corneum (less than 40 μm thick) are effective ways to decrease

2 Materials and methods 2.1 Analysis and design of the dry electrode An electro-chemical electrode–electrolyte interface is established when a metal biopotential electrode comes in contact with human body’s tissues or gel as electrolyte. As a result, the ionic current from ion–electron exchange activity in certain tissues (e.g., myocardia) can be transferred into electronic current to metal electrodes (Griss et al. 2001). This interface could be described as a series of parallel RC structure in electrical circuit simplification

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Fig. 1  Comparison of the conventional Ag/AgCl electrode with the stacked double-micro-domes array dry electrode. a The equivalent circuit model of conventional Ag/AgCl electrode. b The equivalent circuit model of stacked double-micro-domes array dry electrode

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electrode–electrolyte interface impedance and enhance signal coupling strength, which results in improvements of electrode performance. So, we proposed a flexible stacked double-micro-domes array structure (approximately 80 μm in height) as metallic biopotential dry electrode. Its flexibility provided good attachment with the skin surface, and the stacked double-micro-domes array structure contributed to increase the interface contacting area as well as partially disrupt the stratum corneum and anchor the electrode on the skin surface by punching into stratum corneum without penetrating it as what micro-needles do. The equivalent electrical model of flexible stacked double-micro-domes array dry electrode was shown in Fig. 1b. The stacked double-micro-domes array structured dry electrode, shown in Fig. 2a, was directly fabricated on the top side of an FPC substrate, which was a 20 μm-thick polyimide (PI) layer laminated with 10 μm-thick copper films on its both sides. This FPC substrate gives dry electrodes excellent property of skin-compliance even when the attached skin surface was bended, stretched or curved. By rearranging the conventional FPC fabrication process in innovative sequence, the designed stacked doublemicro-dome, which was 100 μm in diameter and 80 μm in height, with the formation of 350 μm-pithed array, could be planted on the substrate. To reduce polarization potential in the electrode–electrolyte interface and improve biocompatibility (Griss et al. 2001), the stacked double-microdomes array structure was coated with a silver layer. With the chosen materials and designed process, the proposed dry electrode not only met the required performance but also could be batch fabricated in a very low cost (about 10 US cents per piece). Unlike other dry electrodes, which needed an additional soldered wire connection for signal out-routing, the FPC based dry electrode formed its wire connection readily in the same fabrication process environment as its core structure. As shown in Fig. 2b, a snap-fastener loop for standard

Fig. 2  Schematic of the FPB based flexible dry electrode with the stacked double-micro-domes array. a Stacked double-micro-domes array structure. b Bare dry electrode with wire connection. c Dry electrode packaged as offset ECG electrode

ECG-lead snap-fastener was extended from the stacked double-micro-domes array structure through an integrated electrical connection on the FPC substrate. The bare dry electrode was designed to be packaged in the formation of offset ECG electrode, as in Fig. 2c, which could minimize the baseline drift by reducing electrode–skin contacting area shift resulted from lead wire dragging. The wire connection and package design provided a clear demonstration of easy integration, high reliability and good compatibility for our proposed dry electrode structure. 2.2 Fabrication procedure The making of the flexible dry electrode, which was compatible with conventional wet Ag/AgCl electrodes, included three parts: the formation of wire connection, the fabrication of stacked double-micro-domes array and the assembling of dry electrode for packaging. Before the stacked double-micro-domes array was fabricated on the front side of this FPC substrate, a standard PCB line implementation process was introduced on the back side of the FPC substrate first in order to form electrical connection from the stacked double-micro-domes array to the ECG-lead snap-fastener. The details were shown in Fig. 3.

Fig. 3  The formation of wire connection

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A 2 mm-diameter hole for snap-fastener was punched 2 cm away from the edge of the stacked double-microdomes array. Then, A 20 μm-thick dry film photoresist was laminated on the both side of FPC substrate. By exposed in UV (350–410 nm) and developed in K2CO3 solution, the pattern of stacked double-micro-domes array area was transferred, as well as the connection line and snapfastener loop. Afterwards, an etching solution was adopted to remove the revealed copper, and the photoresist masks were striped to form the electrical connection. This process allowed a configurable way for biopotential signal routing and integration with a PCB system. The fabrication process of the stacked double-microdomes array and the formation of through-holes for wire connection were shown in Fig. 4. First, several holes with 100  μm diameter were drilled, equally surrounded the expected area of the stacked double-micro-domes array. Then, a 40 μm-thick 1st dry film photoresist was laminated on the front side of this FPC substrate, at the same time a 20 μm-thick dry film photoresist was laminated on the back side for protection. And the dry film photoresist was exposed in UV by laser directing imaging (LDI) technology and developed in K2CO3 solution to transfer the designed patterns and form the cylindrical grooves with 100  μm diameter for both the lower micro-domes array

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and the surrounded through-holes. Then, the drilled FPC substrate with exposed dry film photoresist was immersed into acid electrolyte solution as cathode together with copper as anode in the electroplating bath. After 120 min applied with a 1.2 A/m2-density DC current, an array with 50  μm-height 100 μm-diameter lower micro-domes was electroplated on this FPC substrate and the copper coated through-holes were formed. Then, a chemical micro-etching process was applied to improve the surface roughness of the lower micro-domes array to enhance the adhesion strength with the following dry film photoresist and the upper micro-domes array. Afterwards, a 30 μm-thick 2nd dry film photoresist was laminated and exposed to form a batch of new cylindrical grooves with 50 μm diameter aligned with the lower micro-domes only. Likely, the 2nd copper electroplating process was adopted and an array with 30 μm-height 50 μm-diameter upper micro-domes was stacked exactly on the lower micro-domes array. Then, the two-layer photoresist masks were striped and another chemical micro-etching process was applied to remove impurities on the surface of stacked double-micro-domes array. At last, a 300 nm-thick silver thin film was chemical-vapor-deposited (CVD) on its surface to provide sterilization and improve biomedical compatibility (Griss et al. 2001). After cut from the FPC substrate, the bare dry electrode was about to be assembled with a standard conventional Ag/AgCl electrode adhesive non-woven fabric and ECGlead snap-fastener. The completed ready-to-use compatible ECG dry electrode is shown in Fig. 5. 2.3 Design of wearable biopotential recording system Based on the above described flexible dry electrode, a functional verification system was designed. This system was developed for wearable ECG recording application, it comprised an ECG analog front end (AFE) together with a high-precision ADC included in Texas Instruments ADS1191 and a Bluetooth system on chip (SoC) with an

Fig. 4  The formation of stacked double-micro-domes array

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Fig. 5  The assembling of dry electrode for packaging

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Fig. 6  The concept of wearable ECG recording system. a Block diagram of wearable ECG recording system. b Bow-tie shaped ECG recording patch

ARM Cortex M0 processer from Nordic nRF51822. The block diagram of this system was shown in Fig. 6a. The acquired ECG signal from two flexible dry electrodes was low-noise amplified and filtered in the ECG AFE, then sampled at 125 Hz by a 16 bits sigma-delta ADC. The processer read sampled data from the ADC and sent it through the Bluetooth radio module to the mobile devices such as a smart phone or a tablet. Moreover, the flexible dry electrodes were integrated in our recording system as a patch. Similar to the offset ECG electrode packaging method, the two flexible dry electrodes was assembled in a bow-tie shaped adhesive non-woven patch. And their wire connections were routed to a rigid PCB with circuits on it, as shown in Fig. 6b. This configuration of system structure guaranteed the good combination of wearing comfortability and easy-integration of the proposed flexible dry electrode.

3 Experimental setups and methods 3.1 Measurement of electrode–skin contact impedance Keeping the ESCI as low as possible is a crucial objective for biopotential electrode, which ensures good biopotential signal acquiring performance (Griss et al. 2001). The ESCI measurement setup was shown in Fig. 7. A pair of proposed dry electrodes was placed on the chest in a distance of 20 cm, and the two dry electrodes were connected to a potentiostat/galvanostat (PARSTAT 2273, Princeton Applied Research) for electro-chemical interface impedance analysis over a range of frequencies from 0.1 Hz to 2 kHz, which covers the major part of interested ECG spectrum. Besides the proposed dry electrodes, standard wet Ag/AgCl offset-electrodes (YB55-O, Tianrun Medical Instruments) with the identical electrode shape

Fig. 7  The ESCI measurement setup for electrode

were also tested under the same measurements setup as a comparison. The measurement was performed 15 min later after both types of electrodes were settled on body. The ESCI analysis results were given in impedance spectrum with amplitude and phase charts. The amplitude of impedance spectra indicated how the resistive characterization varied with the signal frequency changes, and the phase of impedance spectra showed the corresponding capacitive characterization of the electrode. 3.2 Measurement of biopotential recording and SNR To verify the feasibility of proposed dry electrodes in the biopotential monitoring application, a measurement of ECG recording was performed. The measurement platform was based on an ECG recording device (ADS1191ECG-FE Demonstration Kit from Texas Instruments), which sampled single-lead ECG signal and was then analyzed on a PC software. For the proposed stacked double-dome dry electrodes, a qualified recorded ECG waveform needed to be measured to prove the performance optimization. So, the test results of ECG measurement by proposed dry electrodes were compared with those by conventional Ag/AgCl electrodes (YB55-O, Tianrun Medical Instruments) and flat sliver-coated dry electrodes without stacked double-microdomes array structure. The comparison could reveal how effective the proposed stacked double-micro-domes array structure was to contribute a comparative signal quality without the gel. In all the measurements, the ECG recording device was set to 1kSPS sampling rate with 12 amplification gain level, and the tested electrodes were placed on the up-left chest with 10 cm space in between. In order to reduce the interference of power-line into the minimum level, a right-leg driver electrode was placed in the middle of the two signal electrodes (Spinelli et al. 1999) and the data was proceeded by a 4th order 50 Hz notch filter in Matlab. In addition to time-domain ECG waveform comparison, an estimated SNR based on signal spectrum analysis

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(Zhang et al. 2015) was also introduced in the measurement results analysis. Since it was impossible to calculate the real SNR of an ECG signal as generally there was no way to know what a “clean” ECG was, some assumptions were made in this measurement to help achieve an estimated comparison result. First, it could be presumed that ECG signals were basically the same when they were acquired on the same person in a short term and the “clean” ECG signal was in the 5–40 Hz segment, where the rest of band was just noise; Second, electrode was assumed to introduce white noise in the intrinsic noise level of the ECG recording device, and better electrode acquired more signal power while brought less noise. Based on the above assumptions, the original ECG waveforms acquired from different electrode were computed to their spectral power in the band from DC to 500 Hz, then the corresponding SNR could be calculated as

SNR = 10 log

P=

f =0Hz 

Fig. 8  The fabricated dry electrode. a Cross section of stacked double-micro-domes structure. b Stacked double-micro-domes array on dry electrode

PS PS = 10 log PN P − PS

2 VRMS,f

f =500Hz

P=

f =5Hz 

2 VRMS,f

f =40Hz

where P was the acquired signal power, PS was the “clean” ECG signal power, PN was the noise power and VRMS,f was the effective voltage at frequency f. With the estimated SNR, the proposed electrode performance could be quantified to show how good its ability to acquire biopotential signal was.

4 Results and discussions 4.1 Flexible dry electrode morphology The fabricated flexible dry electrode was shown in Fig. 8. The stacked double-micro-domes structure was clearly visible on the substrate of the electrode as shown in Fig. 8a, b, the dome-shaped top surface was formed because of electrolyte surface tension during electroplating process. Figure 9 gave images of a bare flexible dry electrode with and without wire connection and an offset ECG electrode pack-aged from it. In benefit of easy formation property of the dry electrode fabrication process, the packaged flexible dry electrode showed good compatibility and good reliability during the experiments.

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Fig. 9  The assembled dry electrode. a Bare dry electrode. b Packaged offset ECG electrode

4.2 Electrode–skin contact impedance of flexible dry electrode The ESCI of an electrode has fundamental influence on the quality of biopotential signals it acquires. As shown in Fig. 10a, compared with standard wet Ag/AgCl electrode, the impendence amplitude of the proposed flexible dry electrode was a little higher than Ag/AgCl electrode at

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4.3 Biopotential recording and SNR

Fig. 10  Comparison of impedance spectroscopy for standard wet Ag/AgCl Electrode and proposed flexible dry electrode. a Impedance amplitude vs. frequency. b Impedance phase vs. frequency

Table 1  Impedance and phase comparison of different electrodes Frequency Proposed flexible dry electrode

Standard wet Ag/AgCl electrode

Impedance (kΩ) Phase (°) Impedance (kΩ) Phase (°) 0.1 Hz 100 Hz

210 88

8.1 38.7

115 63

13.1 75.5

1 MHz

19

72.1

25

80.0

the frequencies below 100 Hz. And from 100 Hz on, the proposed dry electrode had a similar impendence amplitude with the Ag/AgCl electrode, which was benefited from larger contact area of the stacked double-micro-domes array structure even with-out conductive gel. Moreover, as shown in Fig. 10b, lower phase value of the proposed flexible dry electrode represented its less capacitive impedance than Ag/AgCl electrode, which indicates that the interface between it and skin was more resistive and the stratum corneum was partially disrupting. Table 1 summarized different impedance amplitude and phase value at several specific frequency of the pro-posed flexible and standard wet Ag/AgCl electrode.

The test results of ECG recording measurement compared among standard Ag/AgCl electrode, proposed flexible dry electrode and flat sliver-coated dry electrode were shown in Fig. 11. For the characteristic of ECG signal, from Fig.  11b, the proposed flexible dry electrode had successfully revealed clear P-wave, QRS-complex and T-wave of the ECG signal structure. This gave a possibility for practice medical application for doctors and nurses to make potential disease diagnoses. From the time-domain ECG waveform comparison, it was clear to observe that the noise introduced from the pro-posed flexible dry electrode was almost the same as the noise from the standard Ag/AgCl electrode, and has much better noise performance than the flat sliver-coated dry electrode. The same conclusion could be drawn from the signal spectral power comparison, while the spectral noise floors of both flexible dry electrode and standard Ag/AgCl electrode were nearly equally low, the flat sliver-coated dry electrode’s was a lot higher than the former two’s. A quantized result of SNR, calculated from each signal spectral power, was shown in Fig. 12, which confirmed the results of noise performance comparison from visual observation of wave-forms and spectral power graphics. The results proved that the stacked double-micro-domes structure on our proposed flexible fry electrode helped get clear ECG signal and achieve good SNR by lower ESCI, thanks to the proposed techniques of increasing the interface contacting area, partially disrupting the stratum corneum and anchoring the electrode on the skin surface firmly. 4.4 ECG recording by wearable biopotential recording system The wearable biopotential recording prototype system was successfully fabricated and tested. The prototype system appearance and its test set-up which was worn on the upleft chest with 10 cm space in between were depicted in Fig.  13. A raw ECG data and its filtered data by a 50 Hz notch filter and a bandpass filter with a 0.1–60 Hz bandwidth, processed and plotted on the PC client program, were shown in Fig. 14. In the measured results, the recorded ECG waveform was quite similar to that of Limb Lead-I in a standard medical ECG monitor, which meant this could be used for diagnostic purpose in practice. Besides, the raw ECG waveform was so clear that its filtered waveform almost had no changes with it. This was benefited from the proposed flexible dry electrode which made the system has few power line and motion artifacts interfere by getting rid

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Fig. 11  Comparison of ECG waveforms and spectral powers for standard wet Ag/AgCl electrode, proposed flexible dry electrode and flat silvercoated dry electrode. a Standard wet Ag/AgCl electrode. b Proposed flexible dry electrode. c Flat sliver-coated dry electrode

Fig. 12  SNR calculated from acquired ECG signal of different electrodes

Fig. 14  Plotted ECG data waveform on the PC client program

5 Conclusion

Fig. 13  Fabricated wearable ECG recording prototype system

of long waving lead wires. Hence, the designed wearable ECG recording system was promising for wearable medical monitoring application.

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In this work, a flexible dry electrode with stacked doublemicro-domes array had been developed for wearable biopotential recording application. The proposed fabrication process based on FPC substrate provided a cost-effective, high-yield and mass-productive way to make standard and custom flexible dry electrode with good reliability and compatibility. Benefited from the stacked double-microdomes array structure, the flexible dry electrode achieved comparative ESCI and SNR performance with the standard wet Ag/AgCl electrode. After integrated with ECG AFE and wireless communication, a wearable ECG recoding system with flexible dry electrode was capable of helping doctors capture critical ECG data waveform for clinic purpose.

Microsyst Technol Acknowledgments  This work was supported by R&D project of Shenzhen Science and Technology Innovation Committee (Project No. JSGG 2013091840947999) and National Natural Science Foundation of China (Project No. 61471011). Compliance with ethical standards  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.

References Chen Y, Pei W, Chen S, Zhao S, Wang H, Gui Q, Chen H (2013) Fabrication and characterization of surface-modified dry electrode for monitoring biopotential. In: 2013 8th IEEE international conference on nano/micro engineered and molecular systems (NEMS), pp 474–477 Chi YM, Cauwenberghs G (2009) Micropower non-contact EEG electrode with active common-mode noise suppression and input capacitance cancellation. In: Engineering in medicine and biology society. EMBC 2009. Annual International Conference of the IEEE, pp 4218–4221 Chi YM, Jung T-P, Cauwenberghs G (2010) Dry-contact and noncontact biopotential electrodes: methodological review. IEEE Rev Biomed Eng 3:106–119

Forvi E et al (2012) Preliminary technological assessment of microneedles-based dry electrodes for biopotential monitoring in clinical examinations. Sens Actuator A Phys 180:177–186 Gargiulo G, Bifulco P, Calvo RA, Cesarelli M, Jin C, Van Schaik A (2008) Mobile biomedical sensing with dry electrodes. In: International Conference on intelligent sensors, sensor networks and information processing. ISSNIP 2008. IEEE, pp 261–266 Griss P, Enoksson P, Tolvanen-Laakso HK, Meriläinen P, Ollmar S, Stemme G (2001) Micromachined electrodes for biopotential measurements. J Microelectromech Syst 10:10–16 Ishijima M (1993) Monitoring of electrocardiograms in bed without utilizing body surface electrodes. IEEE Trans Biomed Eng 40:593–594 Lin C-T, Liao L-D, Liu Y-H, Wang I-J, Lin B-S, Chang J-Y (2011) Novel dry polymer foam electrodes for long-term EEG measurement. IEEE Trans Biomed Eng 58:1200–1207 Meng Y, Li Z, Chen J (2015) A flexible dry electrode based on APTES-anchored PDMS substrate for portable ECG acquisition system. Microsyst Technol. doi:10.1007/s00542-015-2490-y Searle A, Kirkup L (2000) A direct comparison of wet, dry and insulating bioelectric recording electrodes. Physiol Meas 21:271 Spinelli EM, Martinez NH, Mayosky MA (1999) A transconductance driven-right-leg circuit. IEEE Trans Biomed Eng 46:1466–1470 Yoo HJ, Hoof CV (2012) Bio-medical CMOS ICs. Springer, Berlin Yoo J, Yan L, Lee S, Kim H, Yoo H-J (2009) A wearable ECG acquisition system with compact planar-fashionable circuit boardbased shirt. IEEE T Inf Technol Biomed 13:897–902 Yu L, Tay F, Guo D, Xu L, Yap K (2009) A microfabricated electrode with hollow microneedles for ECG measurement. Sens Actuator A Phys 151:17–22 Zhang Y, Wei S, Long Y, Liu C (2015) Performance analysis of multiscale entropy for the assessment of ECG signal quality. J Electr Comp Eng 2015:31

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