Wireless potential difference electrocardiogram constituted by two electrode-pairs wearing comfort

Recently proposed is a newly Electrocardiogram (ECG) with separate powered electrode-pairs without common ground between two pairs instead of the traditional ECG with a reference electrode and the necessity of common ground creating wearing uncomfortable. Even the novel two-electrode-pair ECG can highly improve the wearing comfort, it must become wireless by overcoming wireless difference problem. This study introduces two analog transceivers of two amplitude modulation (AM) frequencies combined with two front-end amplifiers, two electrode-pairs, and one difference amplifier to implement two-electrode-pair wireless ECG. The experimental results demonstrate that the wireless difference is made possible, i.e., ECG can be constructed after wireless transmission. Two-electrode-pair wireless ECG can greatly improve wearing or measurement comfort without many wires around the body, even Lead I is clear with the acceptable signal-to-noise ratio and only clear QRS complex waves of Lead II and III are detectable for wearing comfort in the health applications.


Introduction
As the progress of humans to the 21st century, the more people care about their health and the more attention pays to the ubiquitous wireless biopotential acquisition system. Non-invasive and portable wireless ECG is one of the best ways to monitor heart activity. In order to achieve the purpose of carrying a wireless electrocardiogram, it has become extremely important research scope. The convenience and the comfort of a wireless ECG acquisition system are useful in many situations even at home. When designing a portable ECG system and also having a wireless transmission function, it is necessary to have a driven right leg circuit(DRL) or a reference electrode in order to reduce common-mode noise and have a reference potential for both electrodes [1]. Thus, the body is covered with wires and causes inconvenience to the user. A clinical ECG system pastes electrodes in precise positions on the hands, legs or chests. It requires nursing personnel with specialized medical background to accurately position the electrode in the correct position. Therefore, many researchers have actively studied the convenience of a wireless electrocardiogram, trying to make wireless ECG more convenient and comfortable. A chest lead system using three electrodes usually proposed, the communication method can be Zigbee [2] or Bluetooth [3]. In order to allow portable ECG to be used on curved skin, Winokur et al. [4] proposed L shaped flexible printed circuit board (PCB), which made the portable ECG free to use on the chest but required 5 electrodes.
Non-contact capacitive electrodes technology using adaptive network topology protocol [5] and Bluetooth [6] communication could avoid skin irritation caused by contact with wet electrodes, but DRL was also required. Bluetooth was used to wirelessly transfer ECG data to mobile phones based on the technique of non-contact capacitive electrodes. A Bluetooth wireless ECG adopted concentric ring electrodes with DRL (driven right leg circuit) for common references [7]. The medical cloud service was carried out by uploading the data to the health monitoring system of the web server [8]. The capacitive electrodes were adopted in a single-arm [9] to greatly improve wearable convenience, but DRL was required for both medical cloud services and single-arm applications. A capacitor -1 -electrode designed as a two-electrode ECG was innovative in the development of the two electrodes ECG system [10]. However, two electrodes placed on the left and right wrists must be connected to the host through wires for common ground. A dry foam electrode wearable ECG system was proposed to send an alert to the medical server via Short Message Service (SMS) [11], in which it still required DRL(driven right leg circuit). A DRL was also required for wireless ECG with a dry flexible electrode [12]. Architecture using AC coupling to design a wireless ECG requires four electrodes [13,14], in which three of the electrodes were used to detect the ECG signals of lead I, II, and III with the fourth electrode for DRL. Most commercial two-electrode heart rate sensors on the market adopt AC coupling architecture but common ground is still used [15,16]. A two-electrode wireless ECG was developed by using ZigBee transmission with common ground needed [17].
In the process of transmitting signals of the wireless ECG, the most important is how to effectively reduce the transmission power [18]. The Holter monitor [19] recording heart activity for consecutive 24 hours involves the use of DRL(driven right leg circuit) electrode and wiring over the patient's body limiting the patient's regular activity. The novel wearable clothing provided excellent convenience without any wire connection [20][21][22][23]. It is convenient to wear on special personnel such as soldiers and firefighters, but the general people may not prefer and afford the wearable clothing, because the wearable clothing requires special materials and techniques to design the wires, electrodes, and modules inside the articles of clothing. A wireless ECG was fabricated by three-stage amplitude modulation but with DRL required [24].
For wearing comfort, our previous work [25] proposed a two-electrode-pair ECG without common ground between two measurement electrode-pairs to save wiring over around the body. Each electrode pair was supplied with isolated power but its own ground must be attached on the skin as a ground electrode near the measurement electrode to form an electrode pair. Both ground electrodes without any wire connected together were attached on the skin to share the body fluid (named as body electrode reservoir) via skin and tissue impedance as common ground. Both measurement signals were read out and differentially amplified to form ECG with power and ground separated from two electrode-pairs. This design did create the potential of wearing comfort without any wire all over the body with each electrode-pair attached on the skin by using its own power source and ground. Nonetheless, it still needs wires to connect both amplified biopotential to get potential difference for ECG formation. This study further cuts the connection wires by implementing wireless modules to transmit the amplified potentials and form ECG by the difference between the wirelessly received potential entitled as wireless potential difference. The experimental results show that wireless potential difference does work and two-electrode-pair wireless ECG is made possible for wearing comfort in the health applications.

Methods and materials 2.1 Systematic description of wireless potential differential ECG
The general wireless ECG is made by either the bipolar limb leads (figure 1(a)) or the chest lead (figure 1(b)) with DRL (driven right leg circuit) or reference electrodes [26]. And, importantly, common ground is needed and an ECG is formed by potential difference before wireless transmission.
This study proposes wireless potential differential ECG with two electrode-pairs, two front-end amplifiers, two wireless transceivers of two amplitude modulation (AM) frequencies (500 MHz and -2 - 1 MHz), and one instrumentation amplifier for potential difference to form Lead I ECG shown in figure 2. Each electrode-pair with one measurement and one ground electrodes shares the separated power supply and ground (GND 1 and GND 2) with one front-end amplifier and one transmitter module. Two receivers get two measurement potentials wirelessly differenced by one instrumentation amplifier (IA) to form Lead I ECG with the third power supply and ground (GND 3). In total, three sets of power supply and ground are adopted without any wire to create common ground for the testification of wireless potential differential ECG. In order to avoid co-channel interference, it is necessary to use the AM transmitters of different frequencies for wireless transmission.  Figure 3 shows a circuit diagram of wireless potential differential ECG to understand how GND 1 and GND 2 are connected to the body electrode reservoir (GND B) as reference ground via skin and tissues [25,27,28]. E es is half-cell potential. R el and C el represent the impedance of the electrode-electrolyte interface and the polarization effect. R g is the equivalent resistance of the conductive gel between the electrode and the skin. E ss is the potential difference produced by the difference in ion concentration between the stratum corneum and the conductive gel. R ep and C ep are the resistance and capacitor of the epidermal layer. R ds is resistance in the dermis and subcutaneous layers [29]. An adult R tissue impedance from the sternum to the calf was approximately 200 Ω [30], and the impedance of the electrode and the skin equivalent circuit is about 1 MΩ [29,31].
-3 - Based on the circuit diagram in figure 3, the electrocardiac potential V ECG can be modeled as one potential V ECG 2 at one measurement electrode subtracting the other potential −V ECG 2 at the other measurement electrode. The output voltage at the front-end amplifier should be

The input voltage of the instrumentation amplifier can be obtained as
2 − ε 2 in addition to ε 1M or ε 500 K noise introduced from the air interference. As a result, the output voltage v O of the instrumentation amplifier can be expressed in eq. (2.1).
If K 1 = K 2 = K, eq. (2.1) can be simplified as follows: where A IA : the gain of instrumentation amplifier K 1 = A 1 × A 1M and K 2 = A 2 × A 500K A 1 : the gain of the front-end amplifier 1 A 2 : the gain of the front-end amplifier 2 A 1M : the gain of 1 MHz AM transmitter and receiver A 500K : the gain of 500 KHz AM transmitter and receiver ε 1 (t): the noise from GND 1 to GND B ε 2 (t): the noise from GND 2 to GND B ε 1M : the noise induced by 1 MHz wireless transmission ε 500K : the noise induced by 500 KHz wireless transmission ε 1 (t) = V t1 + GND B, V t1 : the potential between GND 1 and GND B ε 2 (t) = V t2 + GND B, V t2 : the potential between GND 2 and GND B -4 -

Body surface potential mapping (BSPM) analysis
As a heart beats, it incites the depolarization and repolarization of the cardiomyocytes, which creates a potential change on the surface of the skin called as body surface potential [32]. Body surface potential mapping (BSPM) shown in figure 4 is the potential distribution on the surface of the chest skin at the given moment of a cardiac cycle [32], in which two electrode pairs ((E 1 , GND 1) and (E 2 , GND 2)) are located.

Device implementation
The proposed system is divided into two parts: a measurement and transmitter circuit as well as a receiver and ECG formation circuit shown in figure 5. In the measurement and transmitter circuit (figure 5a), the body surface potential E 1 or E 2 is amplified by a front-end amplifier TL082 and then transmitted to the air via 1 MHz or 500 KHz AM modulator MC1496 with an AM ring antenna. TL082 comprises a unity-gain buffer and a noninverting amplifier to ensure high input impedance -5 -required for the measurement [6]. In the receiver and ECG formation circuit (figure 5b), the received signal ( E 1 or E 2 ) is demodulated by an envelope detector and a low pass filter (LPF) is used for noise reduction before entering the instrumentation amplifier (IA, AD620). ECG signal is created by the differential amplification in the IA and then filtered by a bandpass filter (0.5~100 Hz). Figure 6 presents the implementation of wireless potential differential ECG device with two electrode-pairs: (a) measurement and transmitter and (b) receiver and ECG formation. In figure 6a, the transmitter circuit (Board 1-b or 2-b) and measurement circuit with an electrode pair (Board 1-a or 2-a) are placed on the top and bottom of the measurement and transmitter box separately. The measurement and transmitter box was made by a 3D printer. In figure 6b, the receiver circuit is implemented in the left half of Board 3 and followed with the ECG formation circuit in the right half of Board 3.    is made by the proposed method while the one with almost no noise (dotted line) is served as the control signal measured by a commercial device (MP36, BIOPAC inc.) with regular DRL (driven right leg circuit) and common ground. It can be seen that the noise of the proposed system is even large but its Q, R, S waves can be seen clearly and have the similar wave patterns as those in the control signal. Signal-to-noise ratio(SNR) of Lead I is 24.86 dB. In figures 9 and 10, the noise is too big to identifying the ECG waveforms of Leads II and III by the proposed method except the clear QRS waves. This is because LL lead is far away from the heart, so the body surface potential is quite weak against the noise. SNR of Leads II and III are 22.13 dB and 21.18 dB, respectively.

Discussion & conclusion
With electrode-pair sharing the common body electrode reservoir, wireless potential difference is made possible to form ECG after wireless transmission. Similar to our previous work [20], the proposed wireless potential differential ECG obtains a clear Lead I with acceptable signal-to-noise ratio (SNR) and clear QRS waves of Leads II and III. The surface potential on the chest is significant in comparison to noise, yielding a clear Lead I ECG, while the surface potential on the ankle is quite weak to give a clear Lead II or III ECG. This limits the applications of the proposed wireless potential differential ECG even it creates wearing comfort without any wire connection around the body. For homecare or exercise control, heart rate is a major monitoring parameter by wearing the chest belt or grabbing two handles for wired connection while riding a bike. For a wrist pulse -8 -signal measurement by a wearable watch, it is difficult to get the accurate heart rate and heart rate variability even it is quite comfortable to be worn.
The proposed wireless potential differential ECG can provide an accurate Lead I with acceptable SNR without any wire around the body for easy wearing comfort. It is expected that the next research should focus on the increase of SNR and arrhythmia detection with many subjects for practical applications, such as homecare or exercise health control. For instance, a digital wireless transmission instead of analog wireless transmission can largely decrease the noise from the air but it is introduced the unwanted delay that may interfere the wireless difference function.