Measurement Issues in Galvanic Intrabody Communication: Influence of Experimental Setup.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TBME.2015.2444916, IEEE Transactions on Biomedical Engineering

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Measurement Issues in Galvanic Intrabody Communication: Influence of Experimental Setup M. Amparo Callejón, Javier Reina-Tosina, Senior Member, IEEE, David Naranjo-Hernández, Member, IEEE, and Laura M. Roa, Fellow, IEEE.

Abstract—Significance: The need for increasingly energyefficient and miniaturized bio-devices for ubiquitous health monitoring has paved the way for considerable advances in the investigation of techniques such as intrabody communication (IBC), which uses human tissues as a transmission medium. However, IBC still poses technical challenges regarding the measurement of the actual gain through the human body. The heterogeneity of experimental setups and conditions used together with the inherent uncertainty caused by the human body make the measurement process even more difficult. Goal: The objective of this work, focused on galvanic coupling IBC, is to study the influence of different measurement equipments and conditions on the IBC channel. Methods: different experimental setups have been proposed in order to analyze key issues such as grounding, load resistance, type of measurement device and effect of cables. In order to avoid the uncertainty caused by the human body, an IBC electric circuit phantom mimicking both human bioimpedance and gain has been designed. Given the low-frequency operation of galvanic coupling, a frequency range between 10 kHz and 1 MHz has been selected. Results: the correspondence between simulated and experimental results obtained with the electric phantom have allowed us to discriminate the effects caused by the measurement equipment. Conclusion: this study has helped us obtain useful considerations about optimal setups for galvanic-type IBC as well as to identify some of the main causes of discrepancy in the literature. Index Terms—Electric circuit phantom, experimental setup, gain, galvanic coupling, grounding, intrabody communication, load resistance.

I. I NTRODUCTION HE new paradigms of personalized medicine and e-health systems pursue the ubiquitous monitoring of the user’s condition for a preventive care [1]. This is accomplished through the deployment of wireless body area networks, which

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M. Amparo Callejón is with Biomedical Engineering Group, University of Seville and CIBER de Bioingenier´ıa, Biomateriales y Nanomedicina (CIBERBBN), Seville 41092, Spain (e-mail: [email protected]) J. Reina-Tosina is with Dept. Signal Theory and Communications, University of Seville and CIBER-BBN, Seville 41092, Spain (e-mail: [email protected]) D. Naranjo-Hernández and L. M. Roa are with CIBER-BBN and Biomedical Engineering Group, University of Seville, Seville 41092, Spain (e-mail: [email protected], [email protected]) Manuscript received February 18, 2015; revised May 15, 2015; accepted June 9, 2015. This work was supported in part by Consejer´ıa de Econom´ıa, Innovación y Ciencia, Government of Andaluc´ıa, under Grants P08-TIC04069 and P10-TIC-6214, in part by Fondo de Investigaciones Sanitarias, Instituto de Salud Carlos III, under Grant PI11/00111, and in part by CIBERBBN under Grant PLADEBACT. CIBER-BBN is an initiative funded by the VI National R&D&i Plan 2008-2011, Iniciativa Ingenio 2010, Consolider Program. CIBER Actions are funded by Instituto de Salud Carlos III with assistance from the European Regional Development Fund. c 2015 IEEE. Personal use of this material is permitted. Copyright However, permission to use this material for any other purposes must be obtained from the IEEE by sending an email to [email protected].

connect different biosensors located in, on or near the human body [2]. In this scenario, the use of human tissues as a transmission medium for electric signals, originally designated as intrabody communication (IBC) [3], has raised a great interest during recent years [4]. Since body tissues show high conductivity, low-power signals can be transmitted at lower frequencies without the need for using antennas, making consumption reduction and size minimization possible [5]. In addition, the signal is mainly confined to the human body surface, presenting greater robustness against external interferences. These advantages have led IBC to be included in the IEEE 802.15.6 standard as a third physical layer designated as human body communication (HBC) [6]. The influence of different parameters on IBC/HBC performance, such as channel length, frequency or coupling type have been widely studied, leading to power-saving IBC transceivers [5], [7]–[9]. However, while it is undeniable that IBC research has considerably advanced over the past few years, it is also true that there are still some technical challenges regarding the experimental characterization of the human body channel. In fact, there exist discrepancies observed between experimental results reported by different authors, often making it difficult for any firm comparison to be drawn. For instance, while some experimental results showed a maximum gain between 10 and 100 kHz for galvanic coupling type [10], [11], other results presented just the opposite response, with gain curves increasing with frequency [12]. Furthermore, a difference of up to 20 dB in magnitude can be observed among results reported in [10], [12] and [13]. In a previous work, the authors carried out an exhaustive study analyzing key IBC parameters such as channel length, interelectrode distance, frequency range, subject, part of the body and anthropometrical characteristics, evidencing that these have a considerable effect on IBC gain and might be one of the possible causes of the discrepancies observed [14]. However, there is another important source of discrepancy that has not been covered in depth in the IBC literature and which is indeed related to the effects caused by the measurement equipment. Different measurement devices such as signal generators and oscilloscopes [11], [13], [14], spectrum and network analyzers [15], [16] and battery-powered transceivers [17], have been used interchangeably. However, each of these devices has its own electronic characteristics, which have to be taken into account. For example, oscilloscopes usually have a high input resistance in the range of megohms, while network analyzers present 50-Ω matched ports, which may considerably affect channel gain, and therefore, should be carefully considered.

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On the other hand, baluns have been commonly used when earth-grounded equipment is utilized [14], [15], [18], [19], even though, except for a few works focused on capacitive coupling [20], [21], a comprehensive study of their effects has yet to be conducted. Some authors have alternatively proposed the use of battery-powered transceivers [17], [22], [23]. However, they are often constrained to a single or a reduced range of frequencies, thus not permitting the wideband characterization of the IBC channel. In fact, possibly due to electronic limitations inherent to the experimental setups used, there is a diversity of frequency ranges for IBC research, thus making the establishment of an optimum frequency band even more difficult. The main objective of this paper is to study the influence of different measurement schemes on the gain measured. Therefore, a set of experimental setups for IBC galvanic coupling is proposed with the aim of analyzing key issues such as the differences when using baluns or battery powered devices; the effect of the load resistance; the use of several types of measurement equipment (signal generator, oscilloscope, network and spectrum analyzer, etc.) and the effects caused by different cables and connections. It must be noticed that an added problem concerning the IBC measurement is the inherent source of uncertainty introduced by the human body [24]–[27], whose influence on the experimental characterization of IBC gain is not completely known. This fact, together with the difficulty of reproducing identical experimental conditions over several measurement sets (e.g. over different days) [11], [14], make it especially difficult to distinguish the effects caused by the measurement equipment from those caused by the human body itself. In order to solve this inconvenience, a simplified IBC electric circuit phantom mimicking both the human bioimpedance and gain properties has been specifically designed in this work. An added value of this approach is that, since the experimental gain obtained with this phantom can be easily corroborated with computational simulations based on the equivalent circuit model, this serves as a pattern in order to analyze and detect the diverse effects and artifacts introduced by the measurement equipment, avoiding the uncertainty introduced by the human body. Finally, a comparative analysis of the gain obtained for different setups using this phantom will allow us to draw important conclusions in order to discriminate optimum measurement conditions for galvanic-type IBC. This paper is organized as follows: Section II describes the main characteristics of the IBC electric circuit phantom used in this work. Subsequently, Section III presents the experimental setups proposed by scanning technical issues such as grounding, load resistance and type of measurement device. Finally, Section IV shows the results found and Section V summarizes the conclusions of this paper.

Fig. 1. a) Simplified circuit model emulating transverse and longitudinal current flows in IBC galvanic coupling. b) Debye model for transverse and longitudinal impedances. c) Diagram of the resultant IBC circuit model. d) Implemented IBC electric circuit phantom.

objective is not to obtain an electric model that faithfully reproduces IBC phenomenon, but instead one serving as a simplified prototype with which the validity of different measurement setups can be analyzed, avoiding the uncertainty introduced by the human body. A. Simplified IBC Circuit Model Since the operation mode of the galvanic coupling technique is mainly based on the coupling of electric currents through human tissues, e.g. injected on the upper limbs as suggested in the cylinder-like sketch of Fig. 1a, the frequency range of this study has been established from 10 kHz up to 1 MHz, where the quasi-static approximation of the electric field is valid and inductive and wave propagation effects can be negligible [11], [19]. In this frequency range, a useful, intuitive and simple way to emulate the electric properties of the human body is that based on tissue biompedance. Fig. 1a shows the proposed circuit model consisting of four complex impedances: while Za and Zc model the transverse current flow between the same electrode pair, Zb and Zd emulate the longitudinal current flow through the transmitter (TX) to the receiver (RX) site. For the sake of simplicity and given the four-electrode scheme of galvanic coupling, a symmetric and reciprocal circuit network has been considered (i.e. Za = Zc and Zb = Zd ). As was found in [30], the arm tissue bioimpedance presents a dominant dispersion up to 1 MHz, therefore, Za and Zb have in turn been represented through a single-dispersion Debye model (see Fig. 1b) consisting of an extracellular resistance Rext , an intracellular resistance Rint and a membrane capacitance Cm , according to the following equation:

II. IBC E LECTRIC C IRCUIT P HANTOM The IBC electric circuit phantom proposed in this work is based on a simplified electric circuit model emulating galvanic current transmission through two electrode pairs located on the human body. The reason for such simplicity is that our primary

Z{a,b} =

1 jωCm,{t,l} ) 1 Rint,{t,l} + jωCm,{t,l}

Rext,{t,l} (Rint,{t,l} + Rext,{t,l} +

(1)

where suffixes t and l refer to transverse and longitudinal flows, respectively. The full model is shown in Fig. 1c. It



´ et al.: MEASUREMENT ISSUES IN GALVANIC INTRABODY COMMUNICATION CALLEJON

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Gv (dB) = 20log10

|VRX | , |VT X |

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Re(Zin ) , (3) Re(Zin ) + RL where VT X is related to the voltage source, Vs , to RL , Rs and to the input impedance Zin as: Gp (dB) = Gv + 10log10

Zin RL . (4) Zin RL + Rs (Zin + RL ) The input impedance seen at the TX site, Zin takes the form of the shunt association of the transverse impedance Za and the loop impedance of the circuit model: VT X = Vs

Za (Zb + Zc + Zd ) , Za + Zb + Zc + Zd and the voltage at the RX site can be expressed as: Zin =

Fig. 2. Equivalent circuit model with internal resistances of the signal generator (Rs ) and measurement device (RL ) a) when the voltage measured is VT X and b) when the voltage measured is VRX .

VRX = RL I3 , can be noticed that the circuit diagram proposed in this model presents some similarities with the lattice structure proposed in [13] and [15], [17]. However, without loss of generality, we have omitted the diagonal paths and the electrode-skin interface, given the aforementioned simplification objectives. Finally, a picture of the implemented IBC electric circuit phantom can be seen in Fig. 1d. B. IBC Gain Definition There is still no consensus regarding the characterization of the IBC channel in terms of gain. One expression commonly used in the literature is that based on the ratio between the voltages at the RX and TX sites, Gv . From an experimental viewpoint, in order to quantify these voltages, the measurement device used will inevitably have an effect on the gain measured. This effect needs to be somehow emulated by the model, and therefore, both the output resistance of the signal generator, Rs , and the load resistance of the measurement equipment, RL , have been considered. The equivalent circuit model obtained is shown in Fig. 2. Notice that this configuration corresponds to the case in which an oscilloscope probe is placed at the TX and RX ports to calculate the transmitted and received voltages, VT X and VRX , respectively. However, other devices frequently used by authors, such as vector network analyzers (VNA) and spectrum analyzers, make use of a tracking generator configuration to perform a transmission measurement in terms of a power gain, Gp . The correspondence between the voltage gain measured with oscilloscopes and that obtained through VNA and spectrum analyzers has not always been carefully considered, and might be another cause of discrepancy. Given the fact that an impedance mismatch could exist between the load resistance of the measurement device and that of the human body, Gv can considerably differ from Gp . Even when these considerations seem to be obvious, they will be relevant when different measurement schemes are compared in the following sections. According to Fig. 2, both voltage and power gain can be derived by applying Kirchhoff’s Laws as:

(5)

(6)

where I3 is the output current (see Fig. 2). Equations (2)-(6) are obtained from a loop analysis of the circuit depicted in Fig. 2 and they were simulated using Matlab. C. Identification of Circuit Model Parameters Once the circuit model was theoretically conceived, its parameters were sought in order to approximately emulate two different specifications for IBC galvanic coupling, such as the human arm impedance modeled through Zin and gain. 1) Transverse Parameters Rext,t , Rint,t and Cm,t : Regarding the transverse path, impedance Zin seen between the TX electrodes was determined according to previous results both found in the literature [15], [28], [29] and simulated by the authors in [30]. Specifically, these results have shown that this bioimpedance has a decreasing characteristic with frequency, presenting values in the range of kilo-ohms at low frequencies as kilohertz and values near hundreds of ohms at higher frequencies in the range of megahertz. According to Fig. 1c, impedance Zin is primarily influenced by the transverse parameters of the model Rext,t , Rint,t and Cm,t . This implies that term Zb in (5) was considered to be predominant over Za , i.e. Zin ≈ Za . According to the Debye model, the resistance at zero frequency, R0 , is equivalent to the extracellular resistance Rext,t , while the resistance at infinite frequency R∞ , is equal to the shunt association of the extracellular and intracellular resistances (i.e., R∞ = Rint,t kRext,t ). In this way, parameters Rext,t and Rint,t determine the values to which the bioimpedance tends respectively at low and high frequencies. On the other hand, parameter Cm,t is related to the time constant of Za and therefore controls its dynamics over frequency [31]. Subsequently, their values were chosen in order to emulate approximately a frequency response similar to that found in the literature. In this way, Rext,t was chosen as 2 kΩ and Rint,t as 1 kΩ, which means that R0 = Rext,t = 2 kΩ and R∞ = Rint,t kRext,t ≃ 600 Ω. Finally, regarding Cm,t , a value of 470 pF was selected.



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TABLE I PARAMETERS OF D EBYE M ODEL FOR THE IBC E LECTRIC C IRCUIT P HANTOM Parameter Extracellular resistance Intracellular resistance Membrane capacitance

Transversal Path

Longitudinal Path

Rext,t = 2 kΩ Rint,t = 1 kΩ Cm,t = 470 pF

Rext,l = 20 kΩ Rint,l = 10 kΩ Cm,l = 330 pF

2) Longitudinal Parameters Rext,l , Rint,l and Cm,l : The IBC galvanic signal is attenuated as it passes through the longitudinal path between the TX and RX electrodes. Therefore, the longitudinal parameters of the model were determined in order to approximately emulate some of the gain characteristics found in previous results reported by the authors in [14]. We have observed that the range of losses obtained is related to the difference in magnitude between the resistive parameters in the longitudinal path with respect to the transverse path. This means that the range of losses can be modeled through a constant k1 in such a way that Rext,l = k1 Rext,t and Rint,l = k1 Rint,t . In this way, in order to emulate previous experimental results of gain in the range of −30 and −20 dB, a value of 10 was assigned to k1 . On the other hand, another important outcome reported in [14] was a peak in gain between 10 and 100 kHz. We have now found that this peak appears when the capacitance Cm,l in the longitudinal path is selected as k2 times the capacitance in the transverse path (Cml = k2 Cmt ), in such a way that constant k2 modulates the frequency in which this peak is observed. Therefore, in order to obtain this peak in the frequency range desired, a constant k2 near 0.7 was selected, which in turn led to a value of Cm,l equal to 330 pF. Finally, all these values are summarized in Table I. III. E XPERIMENTAL S ETUPS P ROPOSED With the objective of analyzing issues such as grounding, load resistance, effect of cables and type of measurement device, a variety of experimental setups have been proposed in this work. They have all been tested by measuring the known gain of an IBC electric circuit phantom previously designed with this purpose. A. Grounding When earth-grounded equipment is used, a cabled path between the internal grounds of the measurement devices appears, leading to erroneous measurements of IBC gain. Therefore, a grounding strategy is needed in order to avoid this undesired effect. In the literature, baluns have often been used to isolate both the TX and RX ports from the common internal ground. However, to the authors’ knowledge, an exhaustive analysis of their effects on galvanic coupling performance has never been conducted so far. In addition, a comparison between this and other alternatives, such as the use of portable devices, has neither been carried out. Fig. 3 and 4 show the different setups proposed in this work in order to further analyze different grounding schemes: Setup A comprises

Fig. 3. Experimental setups and equivalent circuit diagrams proposed for the study of different grounding strategies using earth-grounded equipment.

Fig. 4. Experimental setup and equivalent circuit diagram proposed for the study of different IBC grounding strategies using battery-powered equipment.

four alternatives (A1-A4), all of them making use of earthgrounded equipment, specifically, a 33500B signal generator to inject the signal and an MSO6032A digital oscilloscope to measure the voltage waveform, both of them of Agilent Technologies Inc. Alternatively, Setup B was implemented using battery-powered devices. More detailed characteristics of these setups are summarized as follows: •

Setup A: Setup A1 (♯earth-grounded) was conceived to analyze and quantify the effect of not using a grounding strategy in IBC gain. Therefore, a common internal ground is shared between the signal generator and the oscilloscope in this case. Regarding the electric circuit model, this implies that setup A1 corresponds to a scheme in which Zb 6= Zd , and in fact Zd has been shorted. Setups A2, A3 and A4 were implemented in order to study different configurations using baluns. In Setup A2 (♯balunTX), the balun has been placed only at the TX site, while in Setup A3 (♯balunRX), it is located at the RX site. In the case of Setup A4 (♯balunTX&RX), they have been introduced at both ports. Given the frequency range




of study, a pair of PT4 balun transformers of Oxford Electrical Products presenting a bandwidth of 2 MHz were used. • Setup B (♯battery-powered) was implemented in order to eliminate the influence of the external ground at both the TX and RX sites without the need for using baluns. Specifically, a portable signal transmission generator was designed and implemented with this purpose. The basis of this experimental instrument was a frequency direct digital synthesizer (DDS) chip (AD9834 of Analog Devices), which was configured through a microcontroller (PIC18LF2431 of Microchip Corp.) to generate the transmitted signals. In order to provide a constant voltage level at the output of the DDS, an amplifier stage based on operational amplifiers was added. Regarding the voltage measurement, a portable oscilloscope Handyscope HS3 of TiePie Engineering connected to a battery-powered laptop was used. For the sake of clarification, the cables and connections used for each setup are shown in Fig. 3 and 4. In addition, an input signal voltage of 3 Vpp , which was well below the safety recommendations provided in [32], [33], was applied. B. Cables and Connections Cables and connections have an unavoidable effect on the measurement process. Their discontinuities are usually prone to attenuating and radiating at higher frequencies, limiting the range of study of the IBC channel for galvanic coupling [27]. It is therefore desirable that their effect be minimized as much as possible and optimum cables be used in terms of IBC performance. However, it is not uncommon for the effect produced by the cables to combine with that produced by the human body, thus making it difficult to distinguish between the effects caused by each. As already explained, we have taken advantage of using an electric circuit phantom with a known response in order to validate the effect of different types of cables and connectors. Specifically, an oscilloscope probe (10073C Probe of Agilent), a coaxial cable and customized short insulated single conductor wires were tested at the RX site of Setup A1. The reason why this setup was chosen in this case is that since it does not use baluns, the frequency constraint (2 MHz) associated with these is removed and the performance of cables can be tested at higher frequencies, which is an interesting condition in order to discriminate from which frequency they start to introduce artifacts in the measurement. Therefore, in this special case, the frequency of study ranged from 10 kHz to 10 MHz. In addition, two different load resistances of 50 Ω and 1 MΩ were considered. C. Load Resistance Another open question in the experimental characterization of the IBC channel concerns the input resistance of the measurement devices and transceivers used. In the literature, experimental setups using both 1 MΩ and 50 Ω have indistinctly been reported [14], [15], even when this could considerably affect the measured gain. It might be noticed that in the specific case of galvanic IBC, the impedance seen

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Fig. 5. Setups C and D using a spectrum analyzer and a VNA, respectively, both with 50-Ω matched ports. Each setup was in turn configured using different grounding strategies, i.e. without baluns, with a balun only placed at the TX site and with two baluns at both the TX and RX sites. These three alternatives have been designed with labels 1), 2) and 3).

before and after the measurement device is that presented by the human body through the electrode interface, which is indeed dependent on frequency [28], [30]. Therefore, using 50-Ω equipment to characterize the IBC channel might not be the most suitable choice. Taking these considerations into account, it can be concluded that this is an important issue that needs to be carefully addressed in order to obtain an optimum signal coupling and avoid an undesired impedance mismatch. In the case of the digital oscilloscope used in this work, RL is a configurable parameter that can be chosen by the user between two different values of 50 and 1 MΩ. In order to shed some light onto this issue, Setup A2 was used to study the effect of the load resistance on the gain measured, considering both voltage and power gain definitions. D. Spectrum and Vector Network Analyzers RF spectrum analyzers and VNAs have often been used in the literature to characterize the IBC channel, especially in the case of capacitive coupling, but also for galvanic coupling [15]. For this last technique, other experimental setups using signal generators and oscilloscopes have also been proposed [11], [13], [14]. One main difference between them lies in the fact that VNAs present 50-Ω matched ports and oscilloscopes usually exhibit higher load resistances of 1 MΩ. In addition, gain is usually computed through a voltage and a power ratio, respectively. In order to compare results, the gain of the proposed IBC electric circuit phantom has been experimentally obtained using a spectrum analyzer and a VNA, allowing us to compare the performance derived when an oscilloscope is used. In addition, since the spectrum analyzer and the VNA used in this work are earth-grounded devices, different grounding strategies with and without baluns have also been tested. Specifically, Fig. 5 shows the setups used: Setup C (♯spectrumanalyzer) uses the tracking generator mode of an FSL Spectrum Analyzer of Rohde & Schwarz, while Setup D



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Fig. 6. Experimental results for voltage gain measured with the proposed circuit phantom (solid line) and on the human arm (markers) for two different load resistances of 1 MΩ and 50 Ω, respectively. Measurement testbed corresponds to setup A2.

Fig. 7. Comparison between computational simulations (solid lines) and experimental results (markers) obtained with the IBC electric circuit phantom for setups A and B using diverse grounding strategies. The load resistance considered in these experiments was RL =1 MΩ.

(♯VNA) uses a 8712ES VNA of Hewlett-Packard, both with 50-Ω input and output ports. Due to the operation range of this device, which starts from 300 kHz, a pair of FTB-1-1*C15+ baluns of Minicircuits Inc., with a frequency range from 0.2 to 500 MHz, were chosen. An available power of 0 dBm was configured and a pair of coaxial cables with SMA connectors were used. In addition, a response calibration was carried out in order to normalize the insertion losses of the baluns and the coaxial cables used.

not only was it chosen with the aim of validating the simulated results of voltage gain obtained with the electric circuit phantom, but also with the objective of analyzing the effect of the load resistance RL . The experimental results for the circuit phantom and the human arm are compared in Fig. 6. Notice that in spite of the simplicity of the proposed circuit, there is a reasonable correspondence with the on-body measurements, notwithstanding the dispersion found between different days, which is within the range found in previous works [11], [14]. Therefore, these results show the applicability of the phantom proposed in order to evaluate different types of experimental setups, as can be seen in the following subsections.

IV. R ESULTS

AND

D ISCUSSION

In this section, a set of on-body measurements are first presented with the aim of validating the IBC electric circuit phantom. Once validated, the results obtained for the measurement setups proposed using this phantom are shown and compared with computational simulations with the equivalent circuit model to reveal the actual influence of the measurement configurations and equipments on IBC gain. For the sake of clarity, results have been organized according to the following subsections: A. On-body Measurements With the aim of validating the IBC electric circuit phantom proposed, a set of experiments was carried out on the human arm. A female volunteer with anthropometrical characteristics such as: height (1.57 m), weight (50 kg), arm length (50 cm) and arm diameter (4.3 cm), was recruited for this study. Four square copper electrodes with an area of 2cm×2cm were placed on the left upper arm of the subject with a channel length of 5 cm and an inter-electrode distance of 12 cm. In order to avoid any discrepancy associated with these parameters, this configuration was the same for all the experiments performed. Specifically, seven measurements over different days were obtained and both the average and standard deviation were calculated in order to provide a higher statistical significance. Regarding Setup A2 used in this case,

B. Grounding The results for the experimental setups A and B, which use different grounding strategies, can be seen in Fig. 7. The two solid lines represent the computational simulation responses of voltage gain according to (2), one when there is a common ground, and another when this ground has been isolated. In this case, the load resistance RL of MSO6032A digital oscilloscope was configured as 1 MΩ in Setups A1-A4. A first result is that the frequency characteristic of both responses follow the same trend, except for a constant difference in magnitude of about 5 dB, leading to a more optimistic result of gain when a coupled ground exists. This result was expected since a cabled path between the TX and RX ports appears in this case. In addition, it can be seen that setups A1, A2 and B obtained experimental results that successfully fitted the computational simulations in the frequency range studied, thus confirming their validity. Therefore, the use of a single balun at the TX site (Setup A2) is a good alternative in order to isolate the internal ground (GND) of the earth-grounded equipment, leading to similar results to those obtained with battery-powered devices (Setup B). However, as can be seen in Fig. 7, setups A3 and A4 present a divergence with respect to simulated results up to 70 kHz, with a maximum error near 18 dB at 10 kHz. A common characteristic of these two setups




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The results obtained for Setup A1 using three different types of cables and connections is summarized in Fig. 8. It can be seen that their performance is satisfactory up to 1 MHz, and that a discrepancy about 10 dB starts to be noticed from 4 MHz, suggesting that great part of the injected signal is dramatically attenuated from this frequency. This effect is even higher for the coaxial cable used when a load resistance of 1 MΩ is selected, reaching a maximum deviation of 20 dB at 20 MHz. These results evidence the importance of testing the performance of the cables used before making IBC invivo measurements, for example with a circuit phantom such as that proposed in this work, with the aim of avoiding any confusion with some other effects associated with the IBC channel itself. In conclusion, we have corroborated that up to the frequency limit of 1 MHz, previously selected in order to comply with the quasi-static approximation assumed, the cables and connectors used behave well and offer results that satisfactorily agree with computational simulations.

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D. Load Resistance Setup A2 was chosen to study the influence of the load resistance RL in the IBC gain, considering two different values of 1 MΩ and 50 Ω. Alternatively, both voltage and power gains, according to (2) and (3), have been computed for the sake of comparison. The results obtained are shown in Fig. 9. For an RL of 50 Ω, Gp is equal to Gv , however, for an RL of 1 MΩ, a difference of about 30 dB can be seen between them. This difference is related to the second term in (3), and will eventually depend on the values of both Zin and RL . It must be noticed that an IBC electric circuit phantom with a known Zin is being used in this case, which allows Gp to be easily computed according to (3). However, when onbody measurements are carried out, the actual bioimpedance

Gain (dB)

C. Cables and Connections

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Gp, 1 MHz

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Fig. 10. Computational simulations of voltage gain Gv (solid lines) and power gain Gp (dashed lines) versus load resistance RL for three different frequencies of 10, 100 and 1000 kHz (red, blue and green, respectively). Experimental results with IBC phantom are represented with markers.

Zin is unknown and has seldom been calculated by authors in their works [15]. In addition, since galvanic coupling is based on electric current conduction phenomena, the highest voltage level is usually sought at the RX site. These arguments support the preference of voltage ratio to define the IBC galvanic channel gain. The results show that the curve obtained with 1 MΩ offers a better value of voltage gain, with a difference in magnitude of about 20 dB in the frequency range studied with respect to that obtained with 50 Ω. An explanation to this is that the impedance of the human body Zin is not near 50 Ω, but is in fact higher, especially at lower frequencies. Therefore, the shunt association of this with a load resistance RL of 50 Ω leads to a lower value of voltage at the RX end, thus decreasing Gv considerably. Furthermore, Gv presents a maximum in the range between 10 and 100 kHz for 1 MΩ, while when 50 Ω is selected, this maximum does not appear and Gv presents an inverse characteristic, increasing with frequency. In Fig. 10, computational simulations of both voltage and



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TBME-00230-2015.R1

been analyzed in this work. The results obtained allowed us to draw useful conclusions towards the design of more accurate measurements setups for IBC galvanic coupling:

−10 −20

•

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Experimental IBC phantom (Gp, Setup C2) Experimental IBC phantom (Gp, Setup C3)

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Experimental IBC phantom (Gp, Setup D1) Experimental IBC phantom (Gp, Setup D2)

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Experimental IBC phantom (Gp, Setup D3) Computational simulation (Gp=Gv, isolated ground, RL=50 Ω) Computational simulation (Gp=Gv, coupled ground, RL=50 Ω)

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6

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Fig. 11. Equivalence between experimental results of power gain Gp obtained with setups C and D (markers) and computational simulations of gain (Gp = Gv in this case since RL = 50 Ω) for both coupled and isolated ground modes (solid lines).

•

•

power gain varying the resistance RL from 10 Ω to 1 MΩ for three different frequencies (10, 100 and 1000 kHz) are presented. Experimental results have also been obtained with the IBC electric circuit phantom and presented in this figure. It can be seen that Gv and Gp tend to the same value for load resistances below the range of tens of ohms. From the perspective of voltage gain, the optimal RL would be one with 1 MΩ. From the perspective of power gain, the optimal RL would be one that approximates the resistive component of Zin at the corresponding frequency. Therefore, taking into account these considerations, the measurement with a load resistance of 50 Ω is neither optimal in terms of voltage nor power gain. E. Spectrum and Vector Network Analyzers The experimental results obtained using RF equipment (spectrum analyzer and VNA), are shown in Fig. 11. As has previously been mentioned, the frequency from which these are valid is about 300 kHz, which is the lower frequency recommended by the manufacturer. Again, the difference found between coupled and isolated ground modes was about 5 dB, similar to the results obtained with oscilloscopes. However, no difference concerning the position of baluns was observed in this case, and both options 2) and 3) shown in Fig. 5 led to equivalent results. Nevertheless, the most interesting conclusion that can be drawn from Fig. 11, is that the experimentally measured gain with RF equipment fitted well the simulations obtained when both VT X and VRX are considered to be calculated using an oscilloscope with a load resistance RL of 50 Ω (notice that Gp = Gv in this case). In this way, a correspondence between these measurement setups, using different devices such as a VNA, a spectrum analyzer and an oscilloscope can be established. V. C ONCLUSION Technical issues such as grounding, effect of cables, load resistance and influence of the measurement equipment have

A grounding strategy is required if earth-grounded equipment is used. Our results suggest that a single balun in the TX site is a good alternative (Setup A2), showing similar results to those obtained with battery-powered devices (Setup B). On the contrary, setups A3 and A4, which use a balun in the RX port, have been discarded due to the deviations found compared with simulations. The importance of testing the frequency from which radiation and attenuation effects associated with cables and connections start to predominate has been evidenced in this work, since this will constrain the operation range of the experimental setups proposed. For a given Zin , Gp tends to Gv for load resistances in the range of tens of ohms. In fact, an equivalence between them using both RF-equipment and oscilloscopes can be established for an RL of 50 Ω. If a voltage gain is to be measured with an oscilloscope, an optimal value is obtained when load resistances in the range of 1 MΩ are selected. However, an optimal value for Gp is only obtained with load resistances that are matched to those presented by the human body. In this sense, the use of RF equipment such as spectrum analyzer and VNA, which have 50 Ω-ports, may not be a good choice, since the measurement with a load resistance of 50 Ω is not optimal neither in terms of voltage nor power gain.

It must be also noticed that some differences obtained are in the range of those found in the literature, thus suggesting that the variety of experimental setups used could be the cause, among others, of the discrepancies observed between results reported by different authors. The need for evaluating the validity of the experimental setups used before carrying out experiments directly on the human body should be highlighted. This would help discriminate the artifacts associated with the measurement equipment, which in turn can lead to misleading IBC results, as has been shown in this work. R EFERENCES [1] Y.-L. Zheng et al.,“Unobtrusive sensing and wearable devices for health informatics,” IEEE Trans. Biomed. Eng., vol. 61, no. 5, pp. 1538-1554, May 2014. [2] S. Movassaghi et al, “Wireless body area networks: a survey,” IEEE Commun. Surveys Tuts., vol.16, no.3, pp.1658-1686, Third Quarter 2014. [3] T. G. Zimmerman, “Personal area networks: near-field intrabody communication,” IBM Systems Journal, vol. 35, no. 3-4, pp. 609-617, 1996. [4] M. Seyedi et al, “A survey on intrabody communications for body area network applications,” IEEE Trans. Biomed. Eng., vol. 60, no. 8, pp. 2067-2079, Aug. 2013. [5] J. Bae et al, “A 0.24-nJ/b wireless body-area-network transceiver with scalable double-FSK modulation,” IEEE J. Solid-State Circuits, vol.47, no.1, pp. 310,322, Jan. 2012. [6] Body area networks (BAN). [Online]. Available: http://www.ieee802.org/15/pub/TG6.html. Last access: June 2015. [7] H. Wang et al, “A 5.4-mW 180-cm transmission distance 2.5-Mb/s advanced techniques-based novel intrabody communication receiver analog front end,” IEEE Trans. Very Large Scale Integr. (VLSI) Syst., vol. PP, no. 99, pp. 1, Jan. 2015.




[8] Ho Chee Keong et al, “High bandwidth efficiency and low power consumption Walsh code implementation methods for body channel communication,” IEEE Trans. Microw. Theory Techn., vol. 62, no. 9, pp. 1867-1878, Sep. 2014. [9] H. Cho, J. Bae and H.-J. Yoo, “A 37.5 body channel communication wake-up receiver with injection-locking ring oscillator for wireless body area Network”, IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 60, no. 5, pp. 1200-1208, May 2013. [10] M. Callejón et al, “Distributed circuit modeling of galvanic and capacitive coupling for intrabody communication,” IEEE Trans. Biomed. Eng., vol. 59, no. 11, pp. 3263- 3269, Nov. 2012. [11] S.H. Pun et al, “Quasi-static modeling of human limb for intra-body communications with experiments,” IEEE Trans. Inf. Technol. Biomed., vol. 15, no. 6, pp. 870-876, Nov. 2011. [12] M.S. Wegmueller et al, “An attempt to model the human body as a communication channel,” IEEE Trans. Biomed. Eng., vol. 54, no. 10, pp. 1851-1857, Oct. 2007. [13] Y. Song et al, “The simulation method of the galvanic coupling intrabody communication with different signal transmission paths,” IEEE Trans. Instrum. Meas., vol. 60, no. 4, pp. 1257-1266, Apr. 2011. [14] M.A. Callejon et al, “A comprehensive study into intrabody communication measurements,” IEEE Trans. Instrum. Meas., vol. 62, no. 9, pp. 2446-2455, Sep. 2013. [15] B. Kibret et al, “Investigation of galvanic-coupled intrabody communication using the human body circuit model,” IEEE J. Biomed. Health Inform., vol. 18, no. 4, pp. 1196-1206, July 2014. [16] X.M. Chen et al, “Study of Channel Characteristics for Galvanic-Type Intra-Body Communication Based on a Transfer Function from a QuasiStatic Field Model,” Sensors, vol. 12, pp. 16433-16450, 2012. [17] M.S. Wegmueller et al, “Signal transmission by galvanic coupling through the human body,” IEEE Trans. Instrum. Meas., vol. 59, no. 4, pp. 963-969, Apr. 2010. [18] Z. Lucev, I. Krois and M. Cifrek, “A capacitive intrabody communication channel from 100 kHz to 100 MHz,” IEEE Trans. Instrum. Meas., vol. 61, no. 12, pp. 3280-3289, Dec. 2012. [19] J. Bae et al, “The signal transmission mechanism on the surface of human body for body channel communication,” IEEE Trans. Microw. Theory Techn., vol. 60, no. 3, pp. 582-593, Mar. 2012. [20] Z. Lucev et al, “On the transformer effects in the measurements of capacitive intrabody communication transmission characteristics using grounded instruments,” in 6th European Conf. Int. Federation for Medical and Biological Engineering, 2015, pp. 260-263. [21] J. Sakai et al, “Balun’s effect on the measurement of transmission characteristics for intrabody communication channel,” in IEEE MTTS Int. Microwave Workshop Series RF and Wireless Technologies for Biomedical and Healthcare Applications (IMWS-BIO), 2013, pp. 1-3. [22] R. Xu et al, “Equation environment coupling and interference on the electric-field intrabody communication channel,” IEEE Trans. Biomed. Eng., vol. 59, no. 7, pp. 2051-2059, July 2012. [23] Nie Zedong et al, “An investigation on dynamic human body communication channel characteristics at 45 MHz in different surrounding environments,” IEEE Antennas Wireless Propag. Lett., vol.13, pp. 309312, Feb. 2014. [24] T. Ogasawara et al, “Human body communication based on magnetic coupling,” IEEE Trans. Antennas Propag., vol. 62, no. 2, pp. 804-813, Feb. 2014. [25] M. Seyedi and D. Lai, “Effect of Limb Joints and Limb Movement on Intrabody Communications for Body Area Network Applications,”Journal of Medical and Biological Engineering, vol.34, no. 3, pp. 276-283, 2014. [26] G. Koutitas, “Multiple human effects in body area networks,” IEEE Antennas Wireless Propag. Lett., vol. 9, no. 5, pp. 1080-1086, May 2007. [27] N. Cho et al, “The human body characteristics as a signal transmission medium for intrabody communication,” IEEE Trans. Microw. Theory Techn., vol. 55, no. 5, pp. 1080-1086, May 2007. [28] D. Muramatsu et al, “Input impedance analysis of wearable antenna and its experimental study with real human body,” in IEEE International Conference on Consumer Electronics (ICCE 2014), 2014, pp. 151-152. [29] J. Rosell et al, “Skin impedance from 1 Hz to 1 MHz,” IEEE Trans. Biomed. Eng., vol. 35, no. 8, pp. 649-651, Aug. 1988. [30] M. Amparo Callejon et al, “Galvanic coupling transmission in intrabody communication: a finite element approach,” IEEE Trans. Biomed. Eng., vol. 61, no. 3, pp. 775-783, Mar. 2014. [31] L. M. Roa et al, “Applications of bioimpedance to end stage renal disease (ESRD)”, Modelling and control of dialysis systems, Studies in computational intelligence, Springer-Verlag, vol. 404, pp. 689-769, 2013.

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[32] International Commission on Non-Ionizing Radiation Protection, “Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz),” Health Physics, vol. 74, no. 4, pp. 494-522, 1998. [33] International Commission on Non-Ionizing Radiation Protection, “Guidelines for limiting exposure to time-varying electric and magnetic Fields (1 Hz-100 kHz),” Health Physics, vol. 99, no. 6, pp. 818-836, 2010.

M. Amparo Callejón was born in Huelva, Spain. She received the Telecomm. Eng. degree in 2009 from the University of Seville, Spain, where she is currently working toward the Ph.D. degree at the Biomedical Engineering Group. Her current research interests include intrabody communication, body sensor networks and bioelectromagnetics.

Javier Reina-Tosina (S’99-M’06-SM’09) was born in Seville, Spain. He received the Telecomm. Eng. and Doctor degrees from the University of Seville, Spain, in 1996 and 2003, respectively. Since 1997, he has been with the Department of Signal Theory and Communications, University of Seville, where he is currently an Associate Professor. His research interests include the integration of information technologies in biomedicine, intelligent devices for homecare, and bioelectromagnetics.

David Naranjo-Hernández (S’10-M’12) was born in Azuaga (Badajoz), Spain. He received the Telecomm. Eng. and Ph.D. degrees from the University of Seville, Spain, in 2007 and 2014, respectively. Since 2007, he has been a research member of the Biomedical Engineering Group of the University of Seville, Spain. His current research interests include body sensor networks, bioimpedance and bioelectromagnetics.

Laura M. Roa (M’93-SM’96-F’03). She received the Ph.D. degree (cum laude) from the University of Seville, Spain. She is a Full Professor at the University of Seville, where she founded the Biomedical Engineering Research Group and is currently in charge of. Her research interests include multiscale computational modeling, architectures for the integration of social/health services, intelligent devices for ambient assisted living, and bioelectromagnetics. Prof. Roa is a Fellow of the American Institute for Medical and Biological Engineering, the Institute of Electrical and Electronics Engineers, the International Academy for Medical and Biological Engineering and the European Alliance for Medical and Biological Engineering & Science.


Galvanic coupling transmission in intrabody communication: a finite element approach.

Investigation of galvanic-coupled intrabody communication using the human body circuit model.

Setup of galvanic sensors for the monitoring of gilded bronzes.

Cascaded Network Body Channel Model for Intrabody Communication.

Calibration function for an experimental setup for the measurement of irradiance distributions via a transmitting diffuser.

BrainCycles: Experimental Setup for the Combined Measurement of Cortical and Subcortical Activity in Parkinson's Disease Patients during Cycling.

Characterization of Impulse Radio Intrabody Communication System for Wireless Body Area Networks.

Experimental measurement of preferences in health care using best-worst scaling (BWS): theoretical and statistical issues.

In-vitro cell treatment with focused shockwaves-influence of the experimental setup on the sound field and biological reaction.

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The Experimental Demonstration of High Efficiency Interaction-free Measurement for Quantum Counterfactual-like Communication.

Effects of human limb gestures on galvanic coupling intra-body communication for advanced healthcare system.

The Modeling and Simulation of the Galvanic Coupling Intra-Body Communication via Handshake Channel.

Quality of life: conceptual and measurement issues.

Issues in the measurement of quality of life in hemophilia.

Measurement issues in the assessment of physical activity in children.

Issues of therapeutic communication relevant for improving quality of care.

Economic principles in communication: an experimental study.

A setup for simultaneous measurement of electrophysiological and psychometric temporal encoding in the auditory system.

Methodologic issues in the measurement of urinary renin.

Measurement of flood peak effects as a result of soil and land management, with focus on experimental issues and scale.

challenges in assessing aerobic capacity in youth.

Influence of the stimulus parameters of galvanic vestibular stimulation on unilateral spatial neglect.

Exploring ways to manage healthcare professional-patient communication issues.