A Comparative Study of the Electrodes Gels’Electrical Properties in the Measurement Issues of Intrabody Communication

Fengjie Lin,Ziliang Wei,Jiejie Yang,Yangrong Wen,Yueming Gao,Sio Hang Pun,Mang I Vai,Min Du

Abstract:Intrabody communication (IBC) technology is becoming progressively more standardized due to its low power consumption and high security features incorporated into the third physical layer of the IEEE 802.15.6 standard.Even then,there are still many challenges in normalizing the measurement issues of IBC.A major concern that should not be overlooked is the electrodes in the IBC,especially the popular use of gel electrodes.In the channel measurements,gel electrodes are commonly employed to improve the signal-to-noise ratio and prevent electrodes from falling off.In this paper,a comparative study of the electrical properties of gel was investigated during the measurement of human channel characteristics and to clarify the differences of them.Firstly,the basis of electrostatic field pole plate measurements and electromagnetic theory were introduced to interpretate how the relative permittivity and conductivity of different gels will influence the measurement results.Then the in vivo experiments with different gel or dry electrodes were performed to compare the differences induced by the gel.The results indicate that the influence of the gel on the human channel measurement is mainly concentrated below 400 kHz (the attenuation is reduced by 16.7 dB on average),and the stability of the permittivity and conductivity of the gel has a direct impact on the stability of its measurement of the human channel.This result may provide a meaningful reference for the standardization of electrode usage in IBC.

Keywords:intrabody communication;channel characteristic measurement;gel electrodes;permittivity;conductivity

1 Introduction

With the continuous development of medical and engineering convergence,intrabody communication (IBC) technology has become an essential aid in wearable and implantable application scenarios as a low-power,low-cost,and short-range communication technology [1].Most of the existing IBC research has focused on channel modeling methods and transceiver design implementation.Research on IBC electrodes has been less involved.

In most of the IBC systems,electrodes can be regarded as antennas in wireless communication systems [2] and play a key role in IBCs,the importance of which cannot be overstated.In existing IBC channel studies,the electrodes used mainly include various types of metallic dry electrodes (aluminum,copper,stainless steel,etc.)[3−5],silver/silver chloride electrodes containing gel [6−8],and other physiotherapy gel electrodes[9−12].Metal dry electrodes have a single composition,simple structure,and known dielectric and conductive properties,and therefore produce relatively little influence and uncertainty on measurements [13].Callejon et al.[14] compared the effect of three different electrode types (aluminum,copper,and silver/silver chloride electrodes containing gel) on the attenuation of the IBC channel path and confirmed that it does not affect the IBC channel too much as long as good conductor materials are used.

Common electrophysiological electrodes such as ECG,EMG or physiotherapy gel electrodes are widely used in IBC channel measurement studies due to their good adhesion and resistance to polarization. Although these electrodes are certified by the corresponding national standards[15−17],their direct application in the field of IBC channel measurement with higher signal frequency and wider frequency band will bring some unpredictable results due to their own specific application scenarios,such as electrophysiological signal measurement,or electrical signal excitation with low signal frequency [18−19].

Researchers have also noticed this problem and have extensively compared and explored the effects of metallic dry electrodes versus gel electrodes.Gel electrodes have shown better performance despite their smallest size.The literature[20] suggests that the phenomenon may arise due to the enhanced electrical conductivity and adhesion of the gel to the skin,thus injecting a higher current for the same voltage and thus leading to a higher received signal level.However,very few studies have explored the conductivity or dielectric properties of gel to support this conjecture.Okamoto et al.[17] investigated the electrical resistance properties between four electrodes (stainless steel,titanium,dry and silver/silver chloride electrodes containing gel)and skin and their results showed that electrodes with lower skin resistance properties could improve IBC transmission,but still could not show the specific effects and underlying causes of the electrical properties of the gel itself on the IBC channel characteristics.

In summary,many studies,although,have found uncertainty in the effect of gel electrodes on IBC channel measurements,most of the current research has focused on the differences between gel and dry electrodes,or the differences in the effect of the two electrodes on IBC channel attenuation.However,few studies have shown the specific cause of the effect and the magnitude of the effect.In addition,researchers prefer to consider the gel as a good conductor and ignore its inherent dielectric and conductive properties on the IBC channel characteristics.Therefore,a detailed investigation of the dielectric and conductive properties of the gel itself is presented in this paper to provide an in-depth analysis of its influence on the properties of IBCs and the possible underlying mechanisms to provide a more rigorous reference for measurement problems in IBCs.

2 Method

The electrical properties of the gel primarily cover the conductivity and permittivity,where the permittivity can be measured by the corresponding equipment,while the conductivity requires certain calculations.In this section,an explanation of the relevant electrical properties’ parameters and the calculation of the conductivity based on electromagnetic theory are presented firstly.Then the experimental methods of human channel measurement and gel electrical property measurement are described separately.

2.1 Electrical Properties of Dielectrics

If a material can store electric energy when subjected to an external electric field,it is called“dielectric”,and the permittivity is an important data to characterize the dielectric properties.When the medium is applied with an electric field,it will generate induced charge and weaken the electric field:the molecules of the medium rearrange due to the Coulomb force in the electric field. This rearrangement is equivalent to the formation of an electric field component opposite to the external electric field in the molecules inside the medium,resulting in the weakening of the electric field intensity inside the medium compared with the outside,that is,polarization.Permittivity represents the polarization degree of dielectric,that is,the binding capacity to charge.The greater the permittivity,the stronger the binding capacity to charge.At different frequencies,the polarization properties of dielectrics will be different,so the permittivity will also change.

The permittivity describes the interaction between the material and the electric field.The ratio of the reduction of the electric field in the medium to the original applied electric field (in vacuum) is the relative permittivity,which is often expressed in the complex number:

Permittivity ε is the product of the relative permittivity εrand absolute permittivity in vacuum ε0:

The imaginary part of the permittivity is called the loss factor,which indicates how much electric energy in the material is dissipated to the external electric field,including the effects of dielectric loss and conductivity.The vector diagram is used to represent the complex permittivity,and the vector sum forms an angle δ with the real axis,which is called the loss angle.

The tangent of dielectric loss angle is expressed as the energy consumed to obtain a given stored charge,that is,the relative“loss”of the material,which is usually expressed by the ratio of the imaginary part to the real part of the permittivity:

Electric conductivity is a parameter used to describe the difficulty of charge flow in a material.It is the reciprocal of resistivity and is commonly represented by σ.

In the experiment,the permittivity of the medium can be calculated according to the capacitance value:

where Cpis the equivalent shunt capacitance,t is the thickness of the material,and A is the area of the electrode.

Therefore,the relative permittivity of the experimental material can be calculated by measuring the capacitance and using the following formula:

where d is the diameter of the electrode.

The imaginary part of the permittivity,i.e.the loss factor,can be measured directly,and the loss tangent can be calculated according to the values of the real part and the imaginary part.

In homogeneous lossy media,according to Maxwell’s equation:

the permittivity of lossy medium can be described as

Hence the tangent value of loss angle can be calculated as

So conductivity of medium σ can be written as

2.2 Experimental Materials and Setup

With the aim of making the experiment more general and universal while controlling variables as much as possible,we used three different types of gel electrodes produced by the same company as the research objects (as shown in Fig.1),namely,physiotherapy electrode LT-1-C,ECG electrode LT-301 and EMG electrode (Shanghai Litu Medical Appliances Co,Ltd.).The relevant parameters of the three electrodes and their gels are shown in Tab.1,where we cut the physiotherapy electrodes into circles in order to possibly keep the gel size and shape consistent for the three electrodes.In the subsequent experiments,we mainly used the gel itself as the variable to be controlled,and therefore,we uncovered the gel in the original electrodes to facilitate the analysis.For better differentiation,we have referred to the original electrode as the“gel electrode”and the electrode after removal of the gel as the“dry electrode”.

Fig.1 The three different types of gel electrodes used in the experiments and the electrodes’ front(a),back(b) and gel(c)

Tab.1 Parameters of electrodes and their gels

2.3 Comparative Experiments on Human Channel

In order to investigate the effect of gel on the IBC channel attenuation measurement,we conducted arm channel characteristics measurements with three gel electrodes and three dry electrodes,and the measurement setup is shown in Fig.2 . The measurement scheme used the classical four-electrode method,with the distance between the electrode pairs at the transmitter end being 4 cm,the distance between the electrode pairs at the receiver end remaining the same as theirs,and the distance between the transmitter and receiver ends (channel length)being 6 cm.Each experiment was performed under the same arm site of the same subject (male,24 years old,180 cm,75 kg) to minimize the error due to the different channels.Due to the lack of gel adhesion when using dry electrodes for measurement,pressure-sensitive tape was used for corresponding fixation to avoid air capacitance between the electrodes and the skin.

Fig.2 Experimental setup of human channel characteristics measurement and electrode configuration

The measurement instrument chosen was a vector network analyzer (E5061B,Keysight),which measures in the frequency range of 5 Hz to 3 GHz.In this work,the measurement frequency range explored was 10 kHz–10 MHz and the power of the input signal was 0 dBm.Both the transmitting electrodes and the receiving electrodes are connected to the vector network analyzer through a pair of baluns to reduce the effect of common ground.Multiple measurements were averaged to reduce the measurement error.

2.4 Permittivity Comparison Experiment

According to the parallel pole plate measurement method under electrostatic field,we measured the dielectric properties of three different gels. The size and thickness of the three gels could be found in Tab.1.The measurement instrument selected was an impedance analyzer(E4990A,Keysight) with a measurement frequency range of 1 kHz–30 MHz and an applicable voltage range of ±42 V.The measurement fixture model 16451B was used to connect to the impedance analyzer for measuring dielectric materials up to 15 MHz.The 16451B was supplied with a fixture assembly,four interchangeable protective electrodes and accessories.The measurement setup was shown in Fig.3 .The frequency range of the impedance analyzer was set to be consistent with the IBC channel experiment (10 kHz–10 MHz),and the number of measurement points is 101.The electrode-B in the fixture assembly was selected for measurement,with an inner diameter of 5 mm,and the diameter range of the object to be measured was 10–56 mm,and the thickness range was 0.1–10 mm.The calibration and measurements were performed step by step according to the corresponding procedures. To minimize the error caused by the environment,all experiments were performed at the same temperature and humidity.Multiple measurements were performed on the same gel and averaged.

Fig.3 Measurement settings of gel permittivity where the enlarged portion is the measurement fixture

3 Results

For the sake of clarity,the results were presented and analyzed in three parts in this section.The experiments were measured several times to confirm their reproducibility,thus verifying the validity of the measurement method and the experimental results.The effects of gel on the IBC channel characteristics were first summarized and analyzed and discussed,then the relative permittivity of three different gels and their interrelationships are compared,and finally the conductivity of the three gels and their trends were compared based on the analysis.

3.1 Effect of Gel on IBC Channel Characteristics

Fig.4 shows the arm channel attenuation characteristics of the physiotherapy electrode,ECG electrode and EMG electrode in the original gelcontaining case and the de-gelled case,respectively.In contrast,the effects of all three electrodes on the channel characteristics of IBC before and after debonding showed a similar shape,i.e.,the channel attenuation of the gel electrode was smaller than that of the dry electrode in the low frequency range (f <400 kHz),followed by an opposite trend and smaller difference in the high frequency range (f >400 kHz).In addition,the channel attenuation of the three gel electrodes showed a decreasing and then increasing bend in the low frequency range,while the channel attenuation of the three dry electrodes showed a nearly linear and smooth trend in the whole test frequency band.The reason for this phenomenon may be that the dielectric or conductive properties of the gel itself change with frequency in the low frequency range.Taking the ECG electrode as an example,the difference in channel attenuation between the gel electrode and the dry electrode in the low-frequency range reaches 16.7 dB on average,and the difference in the extreme point can reach 24.8 dB,while the average difference in the high-frequency range is only 2.7 dB.This result can show that the gel itself has a more obvious positive effect in the low frequency range during the process of IBC channel characteristics measurement,while in the high frequency range there is some negative effect but the difference is small.Such an effect may be closely related to the dielectric and conductive properties of the gel itself.

Fig.4 The arm channel attenuation of three different electrodes in the original gel-containing case and the degelled case:(a) physiotherapy electrode;(b)ECG electrode ;(c) EMG electrode

3.2 Measurement Data of Different Gels

Fig.5 illustrates the capacitance values and loss angle tangents of different gels over the measurement frequency range. The capacitance values and loss angle tangent values of the three electrode gels exhibit a relatively consistent trend.The capacitance value of different gels decreases continuously with the increase of frequency.And the loss angle tangent value increases and then decreases with the increase of frequency,except that the frequency at which it starts to decrease is different.The data in this section are raw data obtained from impedance analyzer measurements which will be used to calculate the relative permittivity and conductivity of the gels.

Fig.5 The capacitance values and loss angle tangents of different gels over the measurement frequency range

3.3 Relative Permittivity of Different Gels

According to equation (5) and the measured capacitance values,it is possible to calculate the relative permittivity of different gels.Based on the results of the gel effect on the IBC channel characteristics in the previous section,we first explored the relationship between this effect and the relative permittivity of the gel.The relative permittivity of the gel of the three electrodes used in this work also showed a more consistent trend in general,i.e.,a constant decrease with increasing frequency,differing in their starting values and the rate of change in the low frequency range (f <400 kHz) (as shown in Fig.6).The relative permittivity of the three gels in the high frequency range (f >400 kHz) almost overlap,which may explain the smaller difference in channel attenuation between the gel electrode and the dry electrode in the high frequency range.

Fig.6 Comparison of relative permittivity of three different electrodes

3.4 Conductivity of Different Gels

In this section,we calculated the conductivity of the three gels by correlation theory and explored the connection between them and the IBC channel characteristics.The conductivity of the different gels can be calculated by equation (9) and the measured capacitance and loss angle tangent values.The trends of conductivity changes of the three gels still have many common points (as shown in Fig.7).First,the conductivity of all three gels increased in the low-frequency range(f <400 kHz),and then level off in the high-frequency range (f >400 kHz).Combined with the comparison of the relative permittivity of the gel,it was easy to see that the dielectric and conductive properties of the gel in the low-frequency range were constantly changing in a certain direction,while in the high-frequency range they all reach a stable state.While the low frequency range of the gel electrode has a greater impact on the IBC channel characteristics,the high frequency range had no significant impact. This phenomenon showed that there was a significant correlation between the stability of the dielectric and conductive properties of the gel itself and the effect on the IBC channel characteristics.

Fig.7 Comparison of conductivity of three different electrodes

4 Discussion

In this paper,we show the relevance of the electrical parameter properties of electrode gels in IBC channel measurements,with the main effect being reflected in the low frequency range of the experimental frequency.The influence of the electrical parameter properties is primarily reflected in the change rate of the relative permittivity and conductivity.Although the change rate of relative permittivity has a qualitative correlation with channel attenuation,it is difficult to quantitatively generalize its specific correlation law with the IBC channel characteristics obtained from different electrodes,which may be due to the fact that the information contained in the relative permittivity is not comprehensive enough.On the other hand,since the conductivity includes information on the loss factor in addition to the relative permittivity,its change rate is more closely associated with the change pattern of the IBC channel characteristics.Therefore,the effect of the change rate of conductivity on the IBC channel characteristics in the low frequency range is probably a more dominant factor than the relative permittivity.

In another perspective,the influence of electrical parameter properties can also be understood as the influence of electrode impedance in the measurement of channel characteristics.The relative permittivity reflects to some extent the magnitude of electrode reactance,and the change in conductivity reflects the change in electrode resistance,which is probably the main reason for the association between gel dielectric properties and IBC channel attenuation.However,in addition to the impedance of the electrode itself,the contact impedance may also have some degree of influence on the measurement results. Consequently,in the subsequent work,we will further explore the influence of electrodes on the IBC channel measurement issues in conjunction with contact impedance and develop an equivalent circuit model of electrode impedance and contact impedance.

5 Conclusion

In the existing studies of IBC,the inconsistency in the use of electrodes is an important reason for the uncertainty in the measurement of the IBC channel characteristics.The results obtained in different studies also exhibit highly variable characteristics due to the different electrodes used.In this work,we investigate the effect of widely used gel electrodes on the IBC channel characteristics,and specifically analyze them according to their gel conductivity and dielectric properties. The results show that the permittivity and conductivities of different types of gels differ somewhat.The gel has a high rate of change of permittivity and conductivity in the low frequency range (f <400 kHz),and correspondingly,the gel electrode shows a higher rate of change of channel attenuation than the dry electrode in the same frequency range.On the other hand,in the high frequency range (f >400 kHz),the dielectric conductivity characteristics of the gel all tend to be smooth,and correspondingly,the IBC channel attenuation of the gel electrode also exhibits smoother in the same frequency range compared to the low frequency range.Therefore,there is a significant correlation between the smoothness of the dielectric and conductive properties of the gel and the smoothness of the channel attenuation.In addition,the effect of the gel electrode on the IBC channel characteristics is mainly concentrated in the low frequency range (the attenuation is reduced by 16.7 dB on average),while the dry electrode shows a better performance in the high frequency range.In the future,gel electrode appropriate for high frequency can also be developed to meet the demand for signal-to-noise ratio and adhesion in channel measurements.The investigation of electrode characteristics in this paper is expected to provide a reference for a more realistic channel measurement method in the field of IBC measurements.