bioengineering Article
Evaluating Major Electrode Types for Idle Biological Signal Measurements for Modern Medical Technology Anas Albulbul Department of Research and Development, Global Innovative Medical Technologies (GIMT), Ottawa, ON K1G 5L1, Canada; [email protected]; Tel.: +1-613-663-8611 Academic Editor: Gou-Jen Wang Received: 10 June 2016; Accepted: 21 August 2016; Published: 24 August 2016
Abstract: Biological signals such as electrocardiogram (ECG) and electromyography (EMG) that can be measured at home can reveal vital information about the patient’s health. In today modern technology, the measured ECG or EMG signals at home can be monitored by medical staff from long distance through the use of internet. Biopotential electrodes are crucial in monitoring ECG, EMG, etc., signals. Applying the right type of electrode that lasts for a long time and assists in recording high signal quality is desirable in medical devices industry. Three types of electrodes (Silver/Silver Chloride (Ag/AgCl) electrodes, Orbital electrodes and Stainless steel electrodes) were tested to identify the most appropriate one for recording biological signals. The evaluation was based on determining the electrode circuit model components and having high capacitance value or high capacitor value of electrode circuit model (Cd ) and low electrode-skin impedance value or low resistor value of electrode circuit model (Rd ). The results revealed that Ag/AgCl is the best type of electrodes, followed by Orbital electrodes. Stainless steel electrodes had performed poorly. However, Orbital electrodes material can last longer than Ag/AgCl and hence perform similar to Ag/AgCl electrodes, which can be idle for monitoring biological signals at home without the need for medical staff to replace the electrodes in a short period of time. Keywords: biological signals; electrodes; electrode-skin impedance; noise
1. Introduction The implications of smart devices at home and the development of medical technologies have improved the healthcare home devices. Monitoring the patient’s health condition at home has become crucial in the current modern world. Biological signals, such as electrocardiogram (ECG), electromyogram (EMG), and electroencephalogram (EEG), are rich in medical information. Biopotential electrodes are designed to assist in measuring and recording biological signals. Biopotential electrodes have the ability to transduce bioelectric activity within the body (ionic current) into electrical current that can be measured and recorded [1,2]. The performance of non-invasive electrodes in detecting biological signals is highly dependent on electrode-skin impedance [3,4]. High electrode-skin impedance would result in poor biological signal quality, low signal amplitude and low signal to noise ratio [2,5]. Selecting the proper type of electrodes that can result in having low electrode-skin impedance and can last longer for recording is important for bio-signal measurements. The main problem of conducting bio-signal measurements at home is the choice of an appropriate bioptential electrode that can last long time and need minimal preparation work for recording bio-signal measurements.
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The main objective of this research paper is to compare the performance of the most common non-invasive biopotential electrodes to benefit the medical industry in choosing the most appropriate type of electrodes for clinical measurements at home. 1.1. Biopotential Electrodes Ideal non-polarizable electrodes permit the charges to pass through the electrode-skin interface without hindrance [5]. In non-polarizable electrodes, reduction/oxidation reactions occur at the electrode-skin interface, exchanging charge carriers from ions to electrons and vice versa [5–8]. These reactions are electrochemically reversible in non-polarizable electrodes [5]. The electrolyte gel is used with non-polarizable electrodes to facilitate the electrochemical reactions and to reduce electrode-skin interface impedance [4,5,9]. Stainless steel electrodes are classified as polarizable electrodes [1]. They are one of the most common polarizable electrodes used in modern wireless sensor technologies for monitoring biological signals (e.g., chairs, shirts) [10,11]. Ag/AgCl electrodes are classified as non-polarizable electrodes and considered as the universal electrodes in clinical measurements (e.g., ECG, EMG and EEG) [1]. They are associated with low electrode-skin impedance, low noise and low motion artifact [12]. 1.2. Electrode-Skin Impedance Electrode-skin impedance plays a major role in biological signal quality. High electrode-skin impedance influences negatively biological signal quality since it is associated with low signal-to-noise ratio [13]. High electrode-skin impedance causes poor detection of biopotentials at the electrodes sites because it forms a strong barrier for the biopotentials to cross it [1]. High electrode-skin impedance could be linked with low mobility of ions across the highly resistant skin layer (stratum corneum) that is in contact with electrodes and low electron/ion exchange at electrodes sites [2,5]. Thus, that could cause weak conductivity between the electrodes and the skin and would reduce the biological signal amplitude (low signal to noise ratio). A mismatch in impedance between the electrodes at the skin surface during recording a biological signal would reduce the common mode rejection ratio of the recording system, increase common mode interference (e.g., power line noise) and decrease the signal-to-noise ratio [5]. Electrode-skin impedance varies from one person to another and from one part of the body to another. For example, when Rosell et al. measured the electrode-skin impedance at different parts of the body for ten subjects using Ag/AgCl electrodes, they found a high electrode-skin impedance of around 1 MΩ at 1 Hz at the leg site, and around 100 kΩ at the forehead site [4]. Non-polarizable electrodes are likely to have lower electrode-skin impedances in comparison to polarizable electrodes [14,15]. 1.3. Properties of Ag/AgCl Electrodes Surface Ag/AgCl electrodes are the most common and favoured electrodes in clinical measurements for recording biological signals such as ECG, EMG and EEG [16]. One of the main advantages of using Ag/AgCl electrodes is the low noise level it generates during biological signals recording [16]. Ag/AgCl electrodes generate lower electrode-skin interface impedance and lower electrode-skin interface impedance value than stainless steel electrodes [16–18]. They are also considered as non-polarizable electrodes; the non-polarizable nature of Ag/AgCl electrodes allows the charges to cross the electrode-electrolyte interface unlike stainless steel electrodes [7,17–19]. 1.4. Properties of Orbital Electrodes Dry polarizable Orbital electrodes are made to last longer than the common clinical wet electrodes such as Ag/AgCl [20,21]. An orbital electrode’s coat is made of a mixture of metals: silver/silver chloride, aluminum, gold/gold chloride, nickel and titanium [21]. The Orbital Research Inc. stated that
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stated that the main advantages of applying Orbital electrodes are the elimination for the need of skin the main advantages of applying Orbital electrodes are the elimination for the need of skin preparation preparation and for an electrolyte gel application during the biological signal recording period [21]. and for an electrolyte gel application during the biological signal recording period [21]. The shape of the Orbital electrode makes it more in contact with the skin than is the case with The shape of the Orbital electrode makes it more in contact with the skin than is the case with regular flat stainless steel or surface Ag/AgCl electrodes. This is due to the presence of pins (spikes) regular flat stainless steel or surface Ag/AgCl electrodes. This is due to the presence of pins (spikes) with a height of approximately 150 μm, which allow the Orbital electrode to penetrate deeper into with a height of approximately 150 µm, which allow the Orbital electrode to penetrate deeper into the stratum corneum layer that dominates the skin’s surface and thus facilitates the pathways for the stratum corneum layer that dominates the skin’s surface and thus facilitates the pathways for biopotential through the skin to the Orbital electrode (Figure 1) [20,21]. Stratum corneum has a high biopotential through the skin to the Orbital electrode (Figure 1) [20,21]. Stratum corneum has a resistance to biopotentials and to electrical current due to the presence of dead skin cells [2,16]. The high resistance to biopotentials and to electrical current due to the presence of dead skin cells [2,16]. application of Orbital electrode can overcome this problem by the presence of pins [20,21]. The application of Orbital electrode can overcome this problem by the presence of pins [20,21].
Figure 1. Orbital electrode’s electrode’s penetration Figure 1. Orbital penetration into into the the skin skin layers layers during during bio-signal bio-signal recording. recording.
1.5. 1.5. Properties Properties of of Stainless Stainless Steel Steel Electrodes Electrodes Dry steel electrodes electrodes are are classified classified as as polarizable polarizable electrodes electrodes [7,22]. [7,22]. Dry electrodes electrodes such such as as stainless stainless steel The The research research performed performed by by Ragheb Ragheb and and Geddes Geddes was was based based on on measuring measuring the the electrode-electrolyte electrode-electrolyte interface interface impedance impedance at at frequencies frequencies range range from from 11 Hz Hz to to 11 MHz MHz [7]. [7]. The The results results showed showed that that stainless stainless steel electrode had high impedance in a range of 30–75 kΩ at low frequency range 100 Hz [7]. Stainless steel electrode had high impedance in a range of 30–75 kΩ at low frequency range 100 Hz [7]. steel electrodes would generate electrode-skin interface interface impedance than the than otherthe types of Stainless steel electrodes would higher generate higher electrode-skin impedance other electrodes [7]. Furthermore, polarizable electrodes such as surface stainless steel electrodes can be types of electrodes [7]. Furthermore, polarizable electrodes such as surface stainless steel electrodes reused due to their resistance to corrosion [1]. can be reused due to their resistance to corrosion [1]. 1.6. 1.6. Measuring Measuring the the Electrode-Skin Electrode-Skin Impedance Impedance An An equivalent equivalent circuit circuit model model can can be be used used to to better better understand understand the the interactions interactions between between aa surface surface electrode and the skin. Warburg was known to be the first to propose an equivalent electrode-electrolyte electrode and the skin. Warburg was known to be the first to propose an equivalent interface circuit model [23]. Feates et model al. had[23]. identified components of the equivalent electrode electrode-electrolyte interface circuit Feates the et al. had identified the components of the circuit model by analyzing the conductivity nature of biological tissues [24]. Their work helped in equivalent electrode circuit model by analyzing the conductivity nature of biological tissues [24]. estimating values of capacitors resistors in the electrode-skin In addition, their study Their workthe helped in estimating theand values of capacitors and resistorsmodel. in the electrode-skin model. In provided more details on the effect of skin capacitance, impedance and electrolyte gel or sweat on the addition, their study provided more details on the effect of skin capacitance, impedance and electrode-skin electrolyte gel impedance. or sweat on the electrode-skin impedance. 2. Materials and Methods 2. Materials and Methods A bioimpedance measurement system is used to measure the electrode-skin impedance in A bioimpedance measurement system is used to measure the electrode-skin impedance in response to different frequencies and to an applied alternating electrical current in accordance with the response to different frequencies and to an applied alternating electrical current in accordance with safety standards. the safety standards. 2.1. Measurement Devices 2.1. Measurement Devices The bioimpedance measurement system used in the study consists of a personal computer (PC) measurement system used in the study consists of a personal computer (PC) (Dell The 390, bioimpedance Processor 3.0 GHz, Pentium 2, Win XP), frequency response analyzer (FRA) (Model # 1255B, (Dell 390, Processor 3.0 GHz, Pentium 2, Win XP), frequency response analyzer (FRA) (Model # 1255B, Solartron Analytical, Farnborough, UK) and an impedance interface device (Model # 1294A, Solartron Analytical, Farnborough, UK).
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Solartron Analytical, Bioengineering 2016, 3, 20 Farnborough, UK) and an impedance interface device (Model # 1294A, Solartron 4 of 10 Analytical, Farnborough, UK). Impedance was measured from 1 Hz to 1 MHz (10 points per decade), averaging 20 cycles per frequency, alternating electrical current of 100 frequency, with withapplying applyinganan alternating electrical current of µA 100root μAmean root square mean supply square current. supply The applied alternating electrical current 100 µA is in accordance with the safety standards. A value of current. The applied alternating electrical current 100 μA is in accordance with the safety standards. 100 µA isofa 100 lowμA ACiscurrent value that may not harm human [5]. body [5]. A value a low AC current value that maythe not harm body the human 2.2. Measurements 2.2. Measurements Each impedance measurement measurement took took approximately approximately 66 min min to to complete. complete. Two Two electrodes electrodes from Each impedance from the the same type were placed on the ventral side of the right forearm, spaced 7 cm apart, with the distal same type were placed on the ventral side of the right forearm, spaced 7 cm apart, with the distal electrode thethe wrist. The measurements were done without performing skin electrode approximately approximately11 11cm cmfrom from wrist. The measurements were done without performing preparation at theat electrodes sites and immediately after placing the electrodes. Five human skin preparation the electrodes sitesperformed and performed immediately after placing the electrodes. Five subjects were participated in the study (Table 1). This study was reviewed and approved by Carleton human subjects were participated in the study (Table 1). This study was reviewed and approved by University Research Research Ethics Committee, approval approval # 12-0350#and it wasand carried following the rules Carleton University Ethics Committee, 12-0350 it wasout carried out following of Declaration of Helsinki of 1975. of All1975. subjects gave their informed consentconsent for inclusion before thethe rules of the Declaration of Helsinki All subjects gave their informed for inclusion they participated in the study. before they participated in the study. Table 1. Information of of subjects subjects participated Table 1. Information participated in in the the study. study.
Height Weight SubjectSubjectHeight (cm)(cm)Weight (kg) (kg) 1 2 3 4 5
1 2 3 4 5
163 174 172 168 170
163 174 172 168 170
68 78 80 65 65
68 78 80 65 65
Age Age 25 25 28 28 29 29 29 29 27 27
GenderGender Male Male Male Male Male Male Male Male Male Male
2.3. Electrodes 2.3. Electrodes Different types were were applied in this Different surface surface electrode electrode types applied in this study. study. The The applied applied electrodes electrodes used used were were pregelled wet surface silver/silver chloride (Ag/AgCl) electrodes (Model # FT002, MVAP II, Medical pregelled wet surface silver/silver chloride (Ag/AgCl) electrodes (Model # FT002, MVAP II, Medical Supplies Inc., Newbury Newbury Park, Park, CA, CA, USA); USA); that that have have a Supplies Inc., a diameter diameter of of 11 cm cm (Figure (Figure 2). 2). Both Both dry dry surface surface Orbital electrodes (Model # ORI F6T, Orbital Research Inc., Cleveland, OH, USA), which have Orbital electrodes (Model # ORI F6T, Orbital Research Inc., Cleveland, OH, USA), which have aa an an effective 150 μm µm length effective diameter diameter of of 1.6 1.6 cm cm and and pins pins (spikes) (spikes) of of aa 150 length (Figure (Figure 3) 3) and and dry dry surface surface stainless stainless steel EL12, Liberating Liberating Technologies, Technologies, Inc. (LTI), Holliston, steel (ST) (ST) electrodes electrodes (Model (Model # # EL12, Inc. (LTI), Holliston, MA, MA, USA) USA) which which have a diameter of 1.42 cm and a height of 0.32 cm were applied (Figure 4). An adhesive tape have a diameter of 1.42 cm and a height of 0.32 cm were applied (Figure 4). An adhesive tape was was attached and Stainless SteelSteel electrodes to be firmly to the skin. attached totoOrbital Orbital and Stainless electrodes to be attached firmly attached to Ag/AgCl the skin. electrodes Ag/AgCl had an adhesive by thetape manufacture. electrodes had antape adhesive by the manufacture.
(a)
(b)
Figure 2. 2. (a) Figure (a) Ag/AgCl Ag/AgClelectrode electrode (electrode’s (electrode’s snap snap side); side); (b) (b) Ag/AgCl Ag/AgClelectrode electrode (electrode’s (electrode’s skin skin side). side).
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(a) (a)
(b) (b)
Figure (electrode’s snap snap side). Figure 3. 3. (a) (a) Orbital Orbital electrode electrode (electrode’s (electrode’s kin kin side); side); (b) (b) Orbital Orbital electrode electrode (electrode’s Figure 3. (a) Orbital electrode (electrode’s kin side); (b) Orbital electrode (electrode’s snap side). side). (a) (b) Figure 3. (a) Orbital electrode (electrode’s kin side); (b) Orbital electrode (electrode’s snap side).
(a) (a)
(b) (b)
Figure 4. (a) Stainless steel electrode (electrode’s skin side); (b) Stainless steel electrode (electrode’s (a) Figure 4. (a) Stainless steel steel electrode electrode (electrode’s (electrode’s skin side); (b) (b) Stainless Stainless steel steel electrode electrode (electrode’s (electrode’s Figure 4. (a) Stainless skin side); (b) snap side). snap side). snap Figure side). 4. (a) Stainless steel electrode (electrode’s skin side); (b) Stainless steel electrode (electrode’s snap side). 2.4. Equivalent Circuit Model for the Electrode-Skin Impedance 2.4. Equivalent Circuit Model for the Electrode-Skin Impedance 2.4. Equivalent Circuit Model for the Electrode-Skin Impedance 2.4. Equivalent Circuit measurements Model for the Electrode-Skin Impedanceby applying two electrodes on the ventral The bioimpedance were performed The bioimpedance measurements were performed by applying twodiagram electrodes on the ventral side The of the right forearm spaced 7 cmwere apart. The simplified schematic forthe theventral electrodes bioimpedance measurements performed bybyapplying two side The bioimpedance measurements were performed applying twoelectrodes electrodes on on the ventral side of the right forearm spaced 7 cm apart. The simplified schematic diagram for the electrodes system used in the study is presented in Figure 5. of theside right forearm spaced 7 cm apart. The simplified schematic diagram for the electrodes system of the right forearm spaced 7 cm apart. The simplified schematic diagram for the electrodes system used in the study is presented in Figure 5. used system in the study presented in Figurein 5.Figure 5. used inisthe study is presented
Figure 5. The simplified schematicdiagram diagram for system used in the study. Figure 5. The The simplified schematic for the theelectrodes electrodes system used in the the study. Figure 5. simplified schematic diagram for the electrodes system used in study. Figure 5. The simplified schematic diagram for the electrodes system used in the study. In order to determine the impedance for a single electrode from two electrodes used in the In order to determine the impedance a single electrode two electrodes used the order determine the impedance forfor a single electrode fromfrom twoiselectrodes used in theifin study, study, theto total impedance value is divided by two [19,22]. This approach considered reasonable In order to determine the impedance for a single electrode from two electrodes used in the study, the total impedance value is divided by twosize, [19,22]. Thismaterial, approach is considered reasonable if the total impedance value divided by two [19,22]. This approach is considered reasonable if the two the two electrodes are isthe same (e.g., identical identical produced from the same study, theelectrodes total impedance is(e.g., divided by twosize, [19,22]. This approach isproduced considered reasonable if the two areelectrode thevalue same identical identical material, from thebesame manufacture). The circuit components values for the first electrode are same assumed to electrodes are the same (e.g., identical size, identical material, produced from the manufacture). the two electrodes are the same (e.g., identical size, identical material, produced from the same identical with second electrode (Cd =for Cd1the =C d2 , Relectrode d = Rfor d1 = the Rare d2, first and Relectrode s = Rs1to = be Rs2 ).identical The half-cell manufacture). Thethe electrode circuit components values are assumed to the be The electrode circuit components values first assumed with manufacture). The electrode circuit components values for the first electrode are assumed to be) potential (E hc ) represents the potential difference between the skin or electrolyte (gel or sweat) and identical with the(C second electrode d = Cd1= =RC Rd =RsR= d1 R = s1Rd2 s = half-cell Rs1 = Rs2).potential The half-cell second electrode = Cd2 , R(C Rd1 , ,and =, Rand The (Ehc s2 ). R d = Cd1 d = d2d2 identical with theassecond electrode (C d =reside Cd1 = between Cd2, Rd the = Rthe d1 = Rd2, and Rs =[25]. Rs1 =The Rs2capacitance ). The half-cell the electrode a result of potential the ions that electrode and skin potential (Ethe hc) potential represents the difference skin sweat) and represents difference between the skinbetween or electrolyte (gelor orelectrolyte sweat) and(gel theor electrode as a that accommodates thethe charges that difference are locatedbetween between the the skin electrode and skin double layer is and potential (E hc) represents potential or electrolyte (gel or sweat) the electrode as athat result of the ions that between the [25]. electrode and skin [25]. capacitance result of the ions reside between the reside electrode and skin The capacitance thatThe accommodates represented Cd [25]. may occur tothe the electrode charges transfer between and the electrode as abyresult of The the resistance ions that that reside between and skin [25]. the Theskin capacitance
that accommodates the charges that the are electrode located between electrode skin double is the charges that are located between and skinthe double layer and is represented by layer Cd [25]. that accommodates theThe charges that are between electrode andbetween skin double layer is represented by Cd [25]. resistance thatlocated may occur to thethe charges transfer the skin and represented by Cd [25]. The resistance that may occur to the charges transfer between the skin and
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The resistance that may occur charges between the and electrode is represented by electrode is represented by to Rdthe [22]. The transfer series resistance (Rsskin ) represents the resistance of the R [22]. The series resistance (R ) represents the resistance of the electrolyte gel and sweat [22]. d electrolyte gel and sweat [22]. s The value is is generally The tissues tissues resistance resistance to to the theapplied appliedcurrent currentisisrepresented representedbybyRtissues Rtissues.. RRtissues tissues value generally small relative to the impedance value of the electrode-skin interface. The impedance value for healthy small relative to the impedance value of the electrode-skin interface. The impedance value for human arm’s tissue is found to be less than 500 Ω [9]; in contrast the impedance value for electrode-skin healthy human arm’s tissue is found to be less than 500 Ω [9]; in contrast the impedance value for interface can be larger than 1 MΩ thisThus, study,inRthis is assumed be negligible tissues electrode-skin interface can be larger[21]. thanThus, 1 MΩin[21]. study, Rtissues istoassumed to be (i.e., R = 0). When estimating R values, any contributions from R are included in s tissues tissuesRtissues are includedthe negligible (i.e., Rtissues = 0). When estimating Rs values, any contributions from in R s estimate. the Rs estimate. The formula (1) (1) is is the the impedance impedance for for electrode-skin electrode-skin interface interface for for aa single single electrode. electrode. The following following formula Figure 6 is a result of a simplification of the circuit of Figure 5. Figure 6 is a result of a simplification of the circuit of Figure 5. s + Ze Z=e =RR s+
Rd 1 + j2πf Cd Rd
(1) (1)
where f is the frequency (Hz). where f is the frequency (Hz).
Figure 6. Equivalent circuit model for electrode-skin interface. Figure 6. Equivalent circuit model for electrode-skin interface.
In this a least squares nonlinear curve curve fitting fitting methodmethod is applied MATLAB thisstudy, study, a least squares nonlinear is using applied using (MATLAB MATLAB version 7.7,version R2008b,7.7, MathWorks Inc., Natick, Inc., MA, Natick, USA, 2008) estimate model (MATLAB R2008b, MathWorks MA,toUSA, 2008)the toelectrode estimate circuit the electrode components values. The R electrode circuit model components will be determinedwill based circuit model(Rcomponents d, Cd and s) values. The electrode circuit model components be d , Cd and Rs )(R on Bode plotbased that represents a functionimpedance of frequency electrode-skin interface for [1]. determined on Bode impedance plot that as represents as for a function of frequency Least squares nonlinear fitting determines the optimized bestdetermines fit for impedance model based electrode-skin interface curve [1]. Least squares nonlinear curve fitting the optimized best on fit Bode plot, in terms of total square difference from the of measured impedance values. for impedance model based on Bode plot, in terms total square difference from the measured impedance values. 3. Results and Discussion
3. Results and Discussion The estimated average values for the electrode circuit model components (Rd , Cd , and Rs ) for Ag/AgCl, Orbital and Stainless Steelfor electrodes are available in Tables 2–4 respectively. electrode The estimated average values the electrode circuit model components (Rd, CThe d, and Rs) for circuit model components values were estimated by applying least mean squares curve fitting method Ag/AgCl, Orbital and Stainless Steel electrodes are available in Tables 2–4 respectively. The using MATLAB The estimated electrode circuit model values for subject using orbital electrode circuit program. model components values were estimated by applying least mean 2squares curve electrode is presented in Figure 7program. as an exemplary Bode plot. fitting method using MATLAB The estimated electrode circuit model values for subject 2 The main trend for R values of Ag/AgCl electrodes is lower d using orbital electrode is presented in Figure 7 as an exemplary Bodevalues plot. in comparison to Orbital or Stainless Steel electrodes. High of RdAg/AgCl value implies that the electrode-skin High The main trend for Rd values electrodes is lower values in impedance comparisonistohigh. Orbital or biological signal quality requires low R value; hence choosing the type of electrode that competes with d Stainless Steel electrodes. High Rd value implies that the electrode-skin impedance is high. High other typessignal in having a low Rd value for medical devices The value of resistance biological quality requires lowisRdesirable d value; hence choosing the industry. type of electrode that competes to ionic current that occur in the body for the biological signal can determine the quality of the signal with other types in having a low Rd value is desirable for medical devices industry. The value of being recorded [14,17,26]. The existence of gel at the Ag/AgCl electrodes would produce low R d and resistance to ionic current that occur in the body for the biological signal can determine the quality of R The existence of[14,17,26]. pins or spikes Orbitalofelectrodes support the strong attachment s values. the signal being recorded The on existence gel at thewould Ag/AgCl electrodes would produce of electrodes to skin and overcome the effect of highly resistant skin layer (stratum corneum). Low low Rd and Rs values. The existence of pins or spikes on Orbital electrodes would support the strong R values were obtained for Orbital electrodes that are lower than stainless steel electrodes but a bit d attachment of electrodes to skin and overcome the effect of highly resistant skin layer (stratum higher thanLow Ag/AgCl electrodes 2–4for and Figureelectrodes 8A). The materials that thethan Orbital electrodes corneum). Rd values were (Tables obtained Orbital that are lower stainless steel are made from are considered more durable than Ag/AgCl electrodes [21,26]. Therefore, Orbital electrodes but a bit higher than Ag/AgCl electrodes (Tables 2–4 and Figure 8A). The materials that electrodes last for aare longer of time. the Orbitalcan electrodes madeperiod from are considered more durable than Ag/AgCl electrodes [21,26]. Therefore, Orbital electrodes can last for a longer period of time.
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Figure 7. Experimental electrode-skinimpedance impedance frequency response (Subject, Figure 7. Experimentalresults results for for Orbital Orbital electrode-skin frequency response (Subject, Figure 7. Experimental results for Orbital electrode-skin impedance frequency response (Subject, Orbital-S2) andand thethe model electrodecircuit circuit components values are located the top Orbital-S2) modelplot. plot.Estimated Estimated electrode components values are located at theat top Orbital-S2) and the model plot. Estimated electrode circuit components values are located at the top of of Figure. the Figure. of the the Figure.
(A)
(A)
(B)
(B)
(C) Figure 8. Pregelled Ag/AgCl, Orbital and Stainless Steel electrode’s circuit model components average Log values with standard deviation for all the tested subjects; (A) Rd; (B) Cd and (C) Rs.
(C) Figure 8.8.Pregelled Pregelled Ag/AgCl, Orbital Stainless electrode’s circuit components model components Figure Ag/AgCl, Orbital andand Stainless Steel Steel electrode’s circuit model average average Log values with standard deviation for all the tested subjects; (A) R d; (B) Cd and (C) Rs. Log values with standard deviation for all the tested subjects; (A) Rd ; (B) Cd and (C) Rs .
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The differences in electrodes’ areas were considered in reporting the electrode circuit model components values (Rd , Cd and Rs ) for the three tested electrodes as reported in Tables 2–4. Rd mean value (215.82 kΩ/cm2 ) of Ag/AgCl electrodes is somewhat close to Rd mean value of Orbital electrodes (187.13 kΩ/cm2 ) with respect to surface area. However, it is much smaller than the Rd mean value (2130.98 kΩ/cm2 ) of Stainless Steel electrodes. The differences in Rd values of the same type of electrode among subjects are due to the difference of skin type of subjects (dry or oily), sweat secretion level and concentration of skin’s hair at electrodes sites. Recording biological signals at high Cd values is translated to better biological signal quality [1]. The measurements made by Ag/AgCl electrodes resulted in having higher Cd values in comparison to Orbital or Stainless Steel electrodes (Table 3 and Figure 7B). Orbital electrodes had reported high Cd values. The measured Cd values for Stainless Steel electrodes are far lower than Ag/AgCl or Orbital electrodes due to the nature of polarizable electrodes in accumulating charges at the electrode-skin sites. Table 2. Ag/AgCl, Orbital and stainless steel electrodes’ circuit component Rd average values (kΩ) for all the tested subjects. Electrode Type
Mean Values (kΩ)
kΩ/cm2
Ag/AgCl Orbital Stainless Steel
215.82 299.4 3289.4
215.82 187.13 2130.98
Table 3. Ag/AgCl, Orbital and stainless steel electrodes’ circuit component Cd average values (nF) for all the tested subjects. Electrode Type
Mean Values (nF)
kΩ/cm2
Ag/AgCl Orbital Stainless Steel
18.9 9.3 4.9
18.9 5.2 3.45
Recording biological signals at low Rs values is translated to better biological signal quality [5]. The existence of gel at the Ag/AgCl electrodes generated low Rs values (Table 4 and Figure 8C) [27]. In addition, the existence of pins or spikes in Orbital electrodes and the formation of sweat generated low Rs values that were close to Ag/AgCl electrodes’ Rs values. High Rs values for stainless steel electrodes resulted from the absence of electrolyte gel and were related to sweat formation. Table 4. Ag/AgCl, Orbital and stainless steel electrodes’ circuit component Rs average values (Ω) for all the tested subjects. Electrode Type
Mean Values (Ω)
Ω/cm2
Ag/AgCl Orbital Stainless Steel
399.7 626.8 856.4
399.7 391.8 121.1
4. Conclusions It can be concluded that pregelled Ag/AgCl electrodes would perform better than Orbital or stainless steel electrodes. Applying Ag/AgCl electrodes had resulted in having the lowest Rd or electrode-skin impedance values with a mean value of 215.82 (kΩ). However, Orbital electrodes that have pins in their structures had helped in generating compatible low electrode-skin impedance Rd values with a mean value of 299.4 (kΩ). Applying Stainless Steel electrodes resulted in having the highest Rd values with a mean value of 3289.4 (kΩ). Ag/AgCl electrodes had obtained the highest
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capacitance value (Cd ) followed by Orbital electrodes and Stainless Steel electrodes. The effect of differences in electrodes’ surface areas was considered. The existence of pins in Orbital electrodes and the formation of sweat generated low Rs values that were close to Ag/AgCl electrodes’ Rs values despite the existence of electrolyte gel on Ag/AgCl electrodes. This can be due to the existence of pins that would assist in eliminating the skin’s hair effect and strengthen the attachment to the skin. Stainless steel electrodes had resulted in high Rs values due to differences in material and shape. Due to the deterioration material of Ag/AgCl electrodes with time as a result of interaction with sweat, Orbital electrodes would be the most appropriate electrodes in our opinion for long time use for monitoring biological signals at home. Author Contributions: I would like to thank Prof. Adrian Chan for his advices and Carleton University for assistance in doing this study. I am grateful to all the volunteers who participated in this study and offered their services with patience and interest. Special thanks to GIMT for its support. Conflicts of Interest: The author declares no conflict of interest.
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Grimnes, S.; Martinsen, Ø.G. Bioimpedance and Bioelectricity Basics, 2nd ed.; Elsevier Ltd.: San Diego, CA, USA, 2008; pp. 40–270. Yamamoto, Y.; Yamamoto, T. Dispersion and correlation of the parameters for skin impedance. Med. Biol. Eng. Comput. 1978, 16, 592–594. [CrossRef] [PubMed] Mcadams, E.T.; Jossinet, J.; Lackermeier, A.; Risacher, F. Factors affecting electrode-gel-skin interface impedance in electrical impedance tomography. Med. Biol. Eng. Comput. 1996, 34, 397–408. [CrossRef] [PubMed] Rosell, J.; Colominas, J.; Riu, P.; Pallas-Areny, R.; Webster, J. Skin Impedance from 1 Hz to 1 MHz. IEEE Trans. Biomed. Eng. 1988, 35, 649–651. [CrossRef] [PubMed] Webster, J.G. Medical Instrumentation Application and Design, 4th ed.; John Wiley & Sons: New York, NY, USA, 2010; pp. 189–235. Godin, D.T.; Parker, P.A.; Scott, R.N. Noise characteristics of stainless-steel surface electrodes. Med. Biol. Eng. Comput. 1991, 29, 585–590. [CrossRef] [PubMed] Ragheb, T.; Geddes, L.A. The polarization impedance of common electrode metals operated at low current density. Ann. Biomed. Eng. 1991, 19, 151–163. [CrossRef] [PubMed] Mcadams, E. Biomedical electrodes for biopotential monitoring and electrostimulation. In Bio-Medical CMOS ICs Integrated Circuits and Systems; Springer: New York, NY, USA, 2010; pp. 31–124. Grimnes, S. Impedance measurement of individual skin surface electrodes. Med. Biol. Eng. Comput. 1983, 21, 750–755. [CrossRef] [PubMed] Pacelli, M.; Loriga, G.; Taccini, N.; Paradiso, R. Sensing fabrics for monitoring physiological and biomechanical variables: E-textile solutions. In Proceedings of the 3rd IEEE-EMBS International Summer School and Symposium on Medical Devices and Biosensors, Cambridge, MA, USA, 4–6 September 2006; MIT: Boston, MA, USA, 2006; pp. 1–4. Bifulco, P.; Gargiulo, G.; Romano, M.; Fratini, A.; Cesarelli, M. Bluetooth portable device for continuous ecg and patient motion monitoring during daily life. In Proceedings of the 11th Mediterranean Conference on Medical and Biomedical Engineering and Computing, 2007 IFMBE Proceedings, Ljubljana, Slovenia, 26–30 June 2007; pp. 369–372. Tallgren, P.; Vanhatalo, S.; Kaila, K.; Voipio, J. Evaluation of commercially available electrodes and gels for recording of slow EEG potentials. Clin. Neurophysiol. 2005, 116, 799–806. [CrossRef] [PubMed] Tam, H.; Webster, J.G. Minimizing electrode motion artifact by skin abrasion. IEEE Trans. Biomed. Eng. 1977, BME-24, 134–139. [CrossRef] [PubMed] O’connell, D.N.; Tursky, B. Silver-silver chloride sponge electrodes for skin potential recording. Am. J. Psychol. 1960, 73, 302–304. [CrossRef] Searle, A.; Kirkup, L. A direct comparison of wet, dry and insulating bioelectric recording electrodes. Physiol. Meas. 2000, 21, 271–283. [CrossRef] [PubMed]
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Mcadams, E. Bioelectrodes. In Encyclopedia of Medical Devices and Instrumentation; Wiley-Interscience: Hoboken, NJ, USA, 2006; Volume 1, pp. 120–166. Das, D.P.; Webster, J.G. Defibrillation recovery curves for different electrode materials. IEEE Trans. Biomed. Eng. 1980, BME-27, 230–233. [CrossRef] [PubMed] Geddes, L.A.; Roeder, R. Measurement of the direct-current (Faradic) resistance of the electrode-electrolyte interface for commonly used electrode materials. Ann. Biomed. Eng. 2001, 29, 181–186. [CrossRef] [PubMed] Mcadams, E.T.; Henry, P.; Anderson, J.M.; Jossinet, J. Optimal electrolytic chloriding of silver ink electrodes for use in electrical impedance tomography. Clin. Phys. Physiol. Meas. 1992, 13, 19–23. [CrossRef] [PubMed] Griss, P.; EnokssGriss, P.; Enoksson, P.; Tolvanen-Laakso, H.; Merilainen, P.; Ollmar, S.; Stemme, G. Spiked biopotential electrodes. In Proceedings of the IEEE Thirteenth Annual International Conference on Micro Electro Mechanical Systems, Miyazaki, Japan, 23–27 Jaunary 2000; pp. 323–328. Schmidt, R.N.; Lisy, F.J.; Skebe, G.G.; Prince, T.S. Dry Physiological Recording Electrode. U.S. Patent 6,785,569, 31 August 2004. Geddes, L.A.; Valentinuzzi, M.E. Temporal changes in electrode impedance while recording the electrocardiogram with “dry” electrodes. Ann. Biomed. Eng. 1973, 1, 356–367. [CrossRef] [PubMed] Warburg, E. About the behaviour of so-called “impolarizable electrodes” in the present of alternating current. Ann. Phys. Chem. 1899, 67, 493–499. [CrossRef] Feates, F.S.; Ives, D.J.G.; Pryor, J.H. Alternating current bridge for measurement of electrolytic conductance. J. Electrochem. Soc. 1956, 103, 580–585. [CrossRef] Eggins, B.R. Skin contact electrodes for medical applications. Analyst 1993, 118, 439–442. [CrossRef] [PubMed] Meziane, N.; Webster, J.G.; Attari, M.; Nimunkar, A.J. Dry electrodes for electrocardiography. Physiol. Meas. 2013, 34, R47–R69. [CrossRef] [PubMed] Albulbul, A.; Chan, A.D.C. Electrode-skin impedance changes due to an externally applied force. In Proceedings of the 2012 IEEE International Symposium on Medical Measurements and Applications Proceedings, Budapest, Hungary, 18–19 May 2012. © 2016 by the author; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
Evaluating Major Electrode Types for Idle Biological Signal Measurements for Modern Medical Technology Anas Albulbul Department of Research and Development, Global Innovative Medical Technologies (GIMT), Ottawa, ON K1G 5L1, Canada; [email protected]; Tel.: +1-613-663-8611 Academic Editor: Gou-Jen Wang Received: 10 June 2016; Accepted: 21 August 2016; Published: 24 August 2016
Abstract: Biological signals such as electrocardiogram (ECG) and electromyography (EMG) that can be measured at home can reveal vital information about the patient’s health. In today modern technology, the measured ECG or EMG signals at home can be monitored by medical staff from long distance through the use of internet. Biopotential electrodes are crucial in monitoring ECG, EMG, etc., signals. Applying the right type of electrode that lasts for a long time and assists in recording high signal quality is desirable in medical devices industry. Three types of electrodes (Silver/Silver Chloride (Ag/AgCl) electrodes, Orbital electrodes and Stainless steel electrodes) were tested to identify the most appropriate one for recording biological signals. The evaluation was based on determining the electrode circuit model components and having high capacitance value or high capacitor value of electrode circuit model (Cd ) and low electrode-skin impedance value or low resistor value of electrode circuit model (Rd ). The results revealed that Ag/AgCl is the best type of electrodes, followed by Orbital electrodes. Stainless steel electrodes had performed poorly. However, Orbital electrodes material can last longer than Ag/AgCl and hence perform similar to Ag/AgCl electrodes, which can be idle for monitoring biological signals at home without the need for medical staff to replace the electrodes in a short period of time. Keywords: biological signals; electrodes; electrode-skin impedance; noise
1. Introduction The implications of smart devices at home and the development of medical technologies have improved the healthcare home devices. Monitoring the patient’s health condition at home has become crucial in the current modern world. Biological signals, such as electrocardiogram (ECG), electromyogram (EMG), and electroencephalogram (EEG), are rich in medical information. Biopotential electrodes are designed to assist in measuring and recording biological signals. Biopotential electrodes have the ability to transduce bioelectric activity within the body (ionic current) into electrical current that can be measured and recorded [1,2]. The performance of non-invasive electrodes in detecting biological signals is highly dependent on electrode-skin impedance [3,4]. High electrode-skin impedance would result in poor biological signal quality, low signal amplitude and low signal to noise ratio [2,5]. Selecting the proper type of electrodes that can result in having low electrode-skin impedance and can last longer for recording is important for bio-signal measurements. The main problem of conducting bio-signal measurements at home is the choice of an appropriate bioptential electrode that can last long time and need minimal preparation work for recording bio-signal measurements.
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The main objective of this research paper is to compare the performance of the most common non-invasive biopotential electrodes to benefit the medical industry in choosing the most appropriate type of electrodes for clinical measurements at home. 1.1. Biopotential Electrodes Ideal non-polarizable electrodes permit the charges to pass through the electrode-skin interface without hindrance [5]. In non-polarizable electrodes, reduction/oxidation reactions occur at the electrode-skin interface, exchanging charge carriers from ions to electrons and vice versa [5–8]. These reactions are electrochemically reversible in non-polarizable electrodes [5]. The electrolyte gel is used with non-polarizable electrodes to facilitate the electrochemical reactions and to reduce electrode-skin interface impedance [4,5,9]. Stainless steel electrodes are classified as polarizable electrodes [1]. They are one of the most common polarizable electrodes used in modern wireless sensor technologies for monitoring biological signals (e.g., chairs, shirts) [10,11]. Ag/AgCl electrodes are classified as non-polarizable electrodes and considered as the universal electrodes in clinical measurements (e.g., ECG, EMG and EEG) [1]. They are associated with low electrode-skin impedance, low noise and low motion artifact [12]. 1.2. Electrode-Skin Impedance Electrode-skin impedance plays a major role in biological signal quality. High electrode-skin impedance influences negatively biological signal quality since it is associated with low signal-to-noise ratio [13]. High electrode-skin impedance causes poor detection of biopotentials at the electrodes sites because it forms a strong barrier for the biopotentials to cross it [1]. High electrode-skin impedance could be linked with low mobility of ions across the highly resistant skin layer (stratum corneum) that is in contact with electrodes and low electron/ion exchange at electrodes sites [2,5]. Thus, that could cause weak conductivity between the electrodes and the skin and would reduce the biological signal amplitude (low signal to noise ratio). A mismatch in impedance between the electrodes at the skin surface during recording a biological signal would reduce the common mode rejection ratio of the recording system, increase common mode interference (e.g., power line noise) and decrease the signal-to-noise ratio [5]. Electrode-skin impedance varies from one person to another and from one part of the body to another. For example, when Rosell et al. measured the electrode-skin impedance at different parts of the body for ten subjects using Ag/AgCl electrodes, they found a high electrode-skin impedance of around 1 MΩ at 1 Hz at the leg site, and around 100 kΩ at the forehead site [4]. Non-polarizable electrodes are likely to have lower electrode-skin impedances in comparison to polarizable electrodes [14,15]. 1.3. Properties of Ag/AgCl Electrodes Surface Ag/AgCl electrodes are the most common and favoured electrodes in clinical measurements for recording biological signals such as ECG, EMG and EEG [16]. One of the main advantages of using Ag/AgCl electrodes is the low noise level it generates during biological signals recording [16]. Ag/AgCl electrodes generate lower electrode-skin interface impedance and lower electrode-skin interface impedance value than stainless steel electrodes [16–18]. They are also considered as non-polarizable electrodes; the non-polarizable nature of Ag/AgCl electrodes allows the charges to cross the electrode-electrolyte interface unlike stainless steel electrodes [7,17–19]. 1.4. Properties of Orbital Electrodes Dry polarizable Orbital electrodes are made to last longer than the common clinical wet electrodes such as Ag/AgCl [20,21]. An orbital electrode’s coat is made of a mixture of metals: silver/silver chloride, aluminum, gold/gold chloride, nickel and titanium [21]. The Orbital Research Inc. stated that
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stated that the main advantages of applying Orbital electrodes are the elimination for the need of skin the main advantages of applying Orbital electrodes are the elimination for the need of skin preparation preparation and for an electrolyte gel application during the biological signal recording period [21]. and for an electrolyte gel application during the biological signal recording period [21]. The shape of the Orbital electrode makes it more in contact with the skin than is the case with The shape of the Orbital electrode makes it more in contact with the skin than is the case with regular flat stainless steel or surface Ag/AgCl electrodes. This is due to the presence of pins (spikes) regular flat stainless steel or surface Ag/AgCl electrodes. This is due to the presence of pins (spikes) with a height of approximately 150 μm, which allow the Orbital electrode to penetrate deeper into with a height of approximately 150 µm, which allow the Orbital electrode to penetrate deeper into the stratum corneum layer that dominates the skin’s surface and thus facilitates the pathways for the stratum corneum layer that dominates the skin’s surface and thus facilitates the pathways for biopotential through the skin to the Orbital electrode (Figure 1) [20,21]. Stratum corneum has a high biopotential through the skin to the Orbital electrode (Figure 1) [20,21]. Stratum corneum has a resistance to biopotentials and to electrical current due to the presence of dead skin cells [2,16]. The high resistance to biopotentials and to electrical current due to the presence of dead skin cells [2,16]. application of Orbital electrode can overcome this problem by the presence of pins [20,21]. The application of Orbital electrode can overcome this problem by the presence of pins [20,21].
Figure 1. Orbital electrode’s electrode’s penetration Figure 1. Orbital penetration into into the the skin skin layers layers during during bio-signal bio-signal recording. recording.
1.5. 1.5. Properties Properties of of Stainless Stainless Steel Steel Electrodes Electrodes Dry steel electrodes electrodes are are classified classified as as polarizable polarizable electrodes electrodes [7,22]. [7,22]. Dry electrodes electrodes such such as as stainless stainless steel The The research research performed performed by by Ragheb Ragheb and and Geddes Geddes was was based based on on measuring measuring the the electrode-electrolyte electrode-electrolyte interface interface impedance impedance at at frequencies frequencies range range from from 11 Hz Hz to to 11 MHz MHz [7]. [7]. The The results results showed showed that that stainless stainless steel electrode had high impedance in a range of 30–75 kΩ at low frequency range 100 Hz [7]. Stainless steel electrode had high impedance in a range of 30–75 kΩ at low frequency range 100 Hz [7]. steel electrodes would generate electrode-skin interface interface impedance than the than otherthe types of Stainless steel electrodes would higher generate higher electrode-skin impedance other electrodes [7]. Furthermore, polarizable electrodes such as surface stainless steel electrodes can be types of electrodes [7]. Furthermore, polarizable electrodes such as surface stainless steel electrodes reused due to their resistance to corrosion [1]. can be reused due to their resistance to corrosion [1]. 1.6. 1.6. Measuring Measuring the the Electrode-Skin Electrode-Skin Impedance Impedance An An equivalent equivalent circuit circuit model model can can be be used used to to better better understand understand the the interactions interactions between between aa surface surface electrode and the skin. Warburg was known to be the first to propose an equivalent electrode-electrolyte electrode and the skin. Warburg was known to be the first to propose an equivalent interface circuit model [23]. Feates et model al. had[23]. identified components of the equivalent electrode electrode-electrolyte interface circuit Feates the et al. had identified the components of the circuit model by analyzing the conductivity nature of biological tissues [24]. Their work helped in equivalent electrode circuit model by analyzing the conductivity nature of biological tissues [24]. estimating values of capacitors resistors in the electrode-skin In addition, their study Their workthe helped in estimating theand values of capacitors and resistorsmodel. in the electrode-skin model. In provided more details on the effect of skin capacitance, impedance and electrolyte gel or sweat on the addition, their study provided more details on the effect of skin capacitance, impedance and electrode-skin electrolyte gel impedance. or sweat on the electrode-skin impedance. 2. Materials and Methods 2. Materials and Methods A bioimpedance measurement system is used to measure the electrode-skin impedance in A bioimpedance measurement system is used to measure the electrode-skin impedance in response to different frequencies and to an applied alternating electrical current in accordance with the response to different frequencies and to an applied alternating electrical current in accordance with safety standards. the safety standards. 2.1. Measurement Devices 2.1. Measurement Devices The bioimpedance measurement system used in the study consists of a personal computer (PC) measurement system used in the study consists of a personal computer (PC) (Dell The 390, bioimpedance Processor 3.0 GHz, Pentium 2, Win XP), frequency response analyzer (FRA) (Model # 1255B, (Dell 390, Processor 3.0 GHz, Pentium 2, Win XP), frequency response analyzer (FRA) (Model # 1255B, Solartron Analytical, Farnborough, UK) and an impedance interface device (Model # 1294A, Solartron Analytical, Farnborough, UK).
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Solartron Analytical, Bioengineering 2016, 3, 20 Farnborough, UK) and an impedance interface device (Model # 1294A, Solartron 4 of 10 Analytical, Farnborough, UK). Impedance was measured from 1 Hz to 1 MHz (10 points per decade), averaging 20 cycles per frequency, alternating electrical current of 100 frequency, with withapplying applyinganan alternating electrical current of µA 100root μAmean root square mean supply square current. supply The applied alternating electrical current 100 µA is in accordance with the safety standards. A value of current. The applied alternating electrical current 100 μA is in accordance with the safety standards. 100 µA isofa 100 lowμA ACiscurrent value that may not harm human [5]. body [5]. A value a low AC current value that maythe not harm body the human 2.2. Measurements 2.2. Measurements Each impedance measurement measurement took took approximately approximately 66 min min to to complete. complete. Two Two electrodes electrodes from Each impedance from the the same type were placed on the ventral side of the right forearm, spaced 7 cm apart, with the distal same type were placed on the ventral side of the right forearm, spaced 7 cm apart, with the distal electrode thethe wrist. The measurements were done without performing skin electrode approximately approximately11 11cm cmfrom from wrist. The measurements were done without performing preparation at theat electrodes sites and immediately after placing the electrodes. Five human skin preparation the electrodes sitesperformed and performed immediately after placing the electrodes. Five subjects were participated in the study (Table 1). This study was reviewed and approved by Carleton human subjects were participated in the study (Table 1). This study was reviewed and approved by University Research Research Ethics Committee, approval approval # 12-0350#and it wasand carried following the rules Carleton University Ethics Committee, 12-0350 it wasout carried out following of Declaration of Helsinki of 1975. of All1975. subjects gave their informed consentconsent for inclusion before thethe rules of the Declaration of Helsinki All subjects gave their informed for inclusion they participated in the study. before they participated in the study. Table 1. Information of of subjects subjects participated Table 1. Information participated in in the the study. study.
Height Weight SubjectSubjectHeight (cm)(cm)Weight (kg) (kg) 1 2 3 4 5
1 2 3 4 5
163 174 172 168 170
163 174 172 168 170
68 78 80 65 65
68 78 80 65 65
Age Age 25 25 28 28 29 29 29 29 27 27
GenderGender Male Male Male Male Male Male Male Male Male Male
2.3. Electrodes 2.3. Electrodes Different types were were applied in this Different surface surface electrode electrode types applied in this study. study. The The applied applied electrodes electrodes used used were were pregelled wet surface silver/silver chloride (Ag/AgCl) electrodes (Model # FT002, MVAP II, Medical pregelled wet surface silver/silver chloride (Ag/AgCl) electrodes (Model # FT002, MVAP II, Medical Supplies Inc., Newbury Newbury Park, Park, CA, CA, USA); USA); that that have have a Supplies Inc., a diameter diameter of of 11 cm cm (Figure (Figure 2). 2). Both Both dry dry surface surface Orbital electrodes (Model # ORI F6T, Orbital Research Inc., Cleveland, OH, USA), which have Orbital electrodes (Model # ORI F6T, Orbital Research Inc., Cleveland, OH, USA), which have aa an an effective 150 μm µm length effective diameter diameter of of 1.6 1.6 cm cm and and pins pins (spikes) (spikes) of of aa 150 length (Figure (Figure 3) 3) and and dry dry surface surface stainless stainless steel EL12, Liberating Liberating Technologies, Technologies, Inc. (LTI), Holliston, steel (ST) (ST) electrodes electrodes (Model (Model # # EL12, Inc. (LTI), Holliston, MA, MA, USA) USA) which which have a diameter of 1.42 cm and a height of 0.32 cm were applied (Figure 4). An adhesive tape have a diameter of 1.42 cm and a height of 0.32 cm were applied (Figure 4). An adhesive tape was was attached and Stainless SteelSteel electrodes to be firmly to the skin. attached totoOrbital Orbital and Stainless electrodes to be attached firmly attached to Ag/AgCl the skin. electrodes Ag/AgCl had an adhesive by thetape manufacture. electrodes had antape adhesive by the manufacture.
(a)
(b)
Figure 2. 2. (a) Figure (a) Ag/AgCl Ag/AgClelectrode electrode (electrode’s (electrode’s snap snap side); side); (b) (b) Ag/AgCl Ag/AgClelectrode electrode (electrode’s (electrode’s skin skin side). side).
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(a) (a)
(b) (b)
Figure (electrode’s snap snap side). Figure 3. 3. (a) (a) Orbital Orbital electrode electrode (electrode’s (electrode’s kin kin side); side); (b) (b) Orbital Orbital electrode electrode (electrode’s Figure 3. (a) Orbital electrode (electrode’s kin side); (b) Orbital electrode (electrode’s snap side). side). (a) (b) Figure 3. (a) Orbital electrode (electrode’s kin side); (b) Orbital electrode (electrode’s snap side).
(a) (a)
(b) (b)
Figure 4. (a) Stainless steel electrode (electrode’s skin side); (b) Stainless steel electrode (electrode’s (a) Figure 4. (a) Stainless steel steel electrode electrode (electrode’s (electrode’s skin side); (b) (b) Stainless Stainless steel steel electrode electrode (electrode’s (electrode’s Figure 4. (a) Stainless skin side); (b) snap side). snap side). snap Figure side). 4. (a) Stainless steel electrode (electrode’s skin side); (b) Stainless steel electrode (electrode’s snap side). 2.4. Equivalent Circuit Model for the Electrode-Skin Impedance 2.4. Equivalent Circuit Model for the Electrode-Skin Impedance 2.4. Equivalent Circuit Model for the Electrode-Skin Impedance 2.4. Equivalent Circuit measurements Model for the Electrode-Skin Impedanceby applying two electrodes on the ventral The bioimpedance were performed The bioimpedance measurements were performed by applying twodiagram electrodes on the ventral side The of the right forearm spaced 7 cmwere apart. The simplified schematic forthe theventral electrodes bioimpedance measurements performed bybyapplying two side The bioimpedance measurements were performed applying twoelectrodes electrodes on on the ventral side of the right forearm spaced 7 cm apart. The simplified schematic diagram for the electrodes system used in the study is presented in Figure 5. of theside right forearm spaced 7 cm apart. The simplified schematic diagram for the electrodes system of the right forearm spaced 7 cm apart. The simplified schematic diagram for the electrodes system used in the study is presented in Figure 5. used system in the study presented in Figurein 5.Figure 5. used inisthe study is presented
Figure 5. The simplified schematicdiagram diagram for system used in the study. Figure 5. The The simplified schematic for the theelectrodes electrodes system used in the the study. Figure 5. simplified schematic diagram for the electrodes system used in study. Figure 5. The simplified schematic diagram for the electrodes system used in the study. In order to determine the impedance for a single electrode from two electrodes used in the In order to determine the impedance a single electrode two electrodes used the order determine the impedance forfor a single electrode fromfrom twoiselectrodes used in theifin study, study, theto total impedance value is divided by two [19,22]. This approach considered reasonable In order to determine the impedance for a single electrode from two electrodes used in the study, the total impedance value is divided by twosize, [19,22]. Thismaterial, approach is considered reasonable if the total impedance value divided by two [19,22]. This approach is considered reasonable if the two the two electrodes are isthe same (e.g., identical identical produced from the same study, theelectrodes total impedance is(e.g., divided by twosize, [19,22]. This approach isproduced considered reasonable if the two areelectrode thevalue same identical identical material, from thebesame manufacture). The circuit components values for the first electrode are same assumed to electrodes are the same (e.g., identical size, identical material, produced from the manufacture). the two electrodes are the same (e.g., identical size, identical material, produced from the same identical with second electrode (Cd =for Cd1the =C d2 , Relectrode d = Rfor d1 = the Rare d2, first and Relectrode s = Rs1to = be Rs2 ).identical The half-cell manufacture). Thethe electrode circuit components values are assumed to the be The electrode circuit components values first assumed with manufacture). The electrode circuit components values for the first electrode are assumed to be) potential (E hc ) represents the potential difference between the skin or electrolyte (gel or sweat) and identical with the(C second electrode d = Cd1= =RC Rd =RsR= d1 R = s1Rd2 s = half-cell Rs1 = Rs2).potential The half-cell second electrode = Cd2 , R(C Rd1 , ,and =, Rand The (Ehc s2 ). R d = Cd1 d = d2d2 identical with theassecond electrode (C d =reside Cd1 = between Cd2, Rd the = Rthe d1 = Rd2, and Rs =[25]. Rs1 =The Rs2capacitance ). The half-cell the electrode a result of potential the ions that electrode and skin potential (Ethe hc) potential represents the difference skin sweat) and represents difference between the skinbetween or electrolyte (gelor orelectrolyte sweat) and(gel theor electrode as a that accommodates thethe charges that difference are locatedbetween between the the skin electrode and skin double layer is and potential (E hc) represents potential or electrolyte (gel or sweat) the electrode as athat result of the ions that between the [25]. electrode and skin [25]. capacitance result of the ions reside between the reside electrode and skin The capacitance thatThe accommodates represented Cd [25]. may occur tothe the electrode charges transfer between and the electrode as abyresult of The the resistance ions that that reside between and skin [25]. the Theskin capacitance
that accommodates the charges that the are electrode located between electrode skin double is the charges that are located between and skinthe double layer and is represented by layer Cd [25]. that accommodates theThe charges that are between electrode andbetween skin double layer is represented by Cd [25]. resistance thatlocated may occur to thethe charges transfer the skin and represented by Cd [25]. The resistance that may occur to the charges transfer between the skin and
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The resistance that may occur charges between the and electrode is represented by electrode is represented by to Rdthe [22]. The transfer series resistance (Rsskin ) represents the resistance of the R [22]. The series resistance (R ) represents the resistance of the electrolyte gel and sweat [22]. d electrolyte gel and sweat [22]. s The value is is generally The tissues tissues resistance resistance to to the theapplied appliedcurrent currentisisrepresented representedbybyRtissues Rtissues.. RRtissues tissues value generally small relative to the impedance value of the electrode-skin interface. The impedance value for healthy small relative to the impedance value of the electrode-skin interface. The impedance value for human arm’s tissue is found to be less than 500 Ω [9]; in contrast the impedance value for electrode-skin healthy human arm’s tissue is found to be less than 500 Ω [9]; in contrast the impedance value for interface can be larger than 1 MΩ thisThus, study,inRthis is assumed be negligible tissues electrode-skin interface can be larger[21]. thanThus, 1 MΩin[21]. study, Rtissues istoassumed to be (i.e., R = 0). When estimating R values, any contributions from R are included in s tissues tissuesRtissues are includedthe negligible (i.e., Rtissues = 0). When estimating Rs values, any contributions from in R s estimate. the Rs estimate. The formula (1) (1) is is the the impedance impedance for for electrode-skin electrode-skin interface interface for for aa single single electrode. electrode. The following following formula Figure 6 is a result of a simplification of the circuit of Figure 5. Figure 6 is a result of a simplification of the circuit of Figure 5. s + Ze Z=e =RR s+
Rd 1 + j2πf Cd Rd
(1) (1)
where f is the frequency (Hz). where f is the frequency (Hz).
Figure 6. Equivalent circuit model for electrode-skin interface. Figure 6. Equivalent circuit model for electrode-skin interface.
In this a least squares nonlinear curve curve fitting fitting methodmethod is applied MATLAB thisstudy, study, a least squares nonlinear is using applied using (MATLAB MATLAB version 7.7,version R2008b,7.7, MathWorks Inc., Natick, Inc., MA, Natick, USA, 2008) estimate model (MATLAB R2008b, MathWorks MA,toUSA, 2008)the toelectrode estimate circuit the electrode components values. The R electrode circuit model components will be determinedwill based circuit model(Rcomponents d, Cd and s) values. The electrode circuit model components be d , Cd and Rs )(R on Bode plotbased that represents a functionimpedance of frequency electrode-skin interface for [1]. determined on Bode impedance plot that as represents as for a function of frequency Least squares nonlinear fitting determines the optimized bestdetermines fit for impedance model based electrode-skin interface curve [1]. Least squares nonlinear curve fitting the optimized best on fit Bode plot, in terms of total square difference from the of measured impedance values. for impedance model based on Bode plot, in terms total square difference from the measured impedance values. 3. Results and Discussion
3. Results and Discussion The estimated average values for the electrode circuit model components (Rd , Cd , and Rs ) for Ag/AgCl, Orbital and Stainless Steelfor electrodes are available in Tables 2–4 respectively. electrode The estimated average values the electrode circuit model components (Rd, CThe d, and Rs) for circuit model components values were estimated by applying least mean squares curve fitting method Ag/AgCl, Orbital and Stainless Steel electrodes are available in Tables 2–4 respectively. The using MATLAB The estimated electrode circuit model values for subject using orbital electrode circuit program. model components values were estimated by applying least mean 2squares curve electrode is presented in Figure 7program. as an exemplary Bode plot. fitting method using MATLAB The estimated electrode circuit model values for subject 2 The main trend for R values of Ag/AgCl electrodes is lower d using orbital electrode is presented in Figure 7 as an exemplary Bodevalues plot. in comparison to Orbital or Stainless Steel electrodes. High of RdAg/AgCl value implies that the electrode-skin High The main trend for Rd values electrodes is lower values in impedance comparisonistohigh. Orbital or biological signal quality requires low R value; hence choosing the type of electrode that competes with d Stainless Steel electrodes. High Rd value implies that the electrode-skin impedance is high. High other typessignal in having a low Rd value for medical devices The value of resistance biological quality requires lowisRdesirable d value; hence choosing the industry. type of electrode that competes to ionic current that occur in the body for the biological signal can determine the quality of the signal with other types in having a low Rd value is desirable for medical devices industry. The value of being recorded [14,17,26]. The existence of gel at the Ag/AgCl electrodes would produce low R d and resistance to ionic current that occur in the body for the biological signal can determine the quality of R The existence of[14,17,26]. pins or spikes Orbitalofelectrodes support the strong attachment s values. the signal being recorded The on existence gel at thewould Ag/AgCl electrodes would produce of electrodes to skin and overcome the effect of highly resistant skin layer (stratum corneum). Low low Rd and Rs values. The existence of pins or spikes on Orbital electrodes would support the strong R values were obtained for Orbital electrodes that are lower than stainless steel electrodes but a bit d attachment of electrodes to skin and overcome the effect of highly resistant skin layer (stratum higher thanLow Ag/AgCl electrodes 2–4for and Figureelectrodes 8A). The materials that thethan Orbital electrodes corneum). Rd values were (Tables obtained Orbital that are lower stainless steel are made from are considered more durable than Ag/AgCl electrodes [21,26]. Therefore, Orbital electrodes but a bit higher than Ag/AgCl electrodes (Tables 2–4 and Figure 8A). The materials that electrodes last for aare longer of time. the Orbitalcan electrodes madeperiod from are considered more durable than Ag/AgCl electrodes [21,26]. Therefore, Orbital electrodes can last for a longer period of time.
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Figure 7. Experimental electrode-skinimpedance impedance frequency response (Subject, Figure 7. Experimentalresults results for for Orbital Orbital electrode-skin frequency response (Subject, Figure 7. Experimental results for Orbital electrode-skin impedance frequency response (Subject, Orbital-S2) andand thethe model electrodecircuit circuit components values are located the top Orbital-S2) modelplot. plot.Estimated Estimated electrode components values are located at theat top Orbital-S2) and the model plot. Estimated electrode circuit components values are located at the top of of Figure. the Figure. of the the Figure.
(A)
(A)
(B)
(B)
(C) Figure 8. Pregelled Ag/AgCl, Orbital and Stainless Steel electrode’s circuit model components average Log values with standard deviation for all the tested subjects; (A) Rd; (B) Cd and (C) Rs.
(C) Figure 8.8.Pregelled Pregelled Ag/AgCl, Orbital Stainless electrode’s circuit components model components Figure Ag/AgCl, Orbital andand Stainless Steel Steel electrode’s circuit model average average Log values with standard deviation for all the tested subjects; (A) R d; (B) Cd and (C) Rs. Log values with standard deviation for all the tested subjects; (A) Rd ; (B) Cd and (C) Rs .
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The differences in electrodes’ areas were considered in reporting the electrode circuit model components values (Rd , Cd and Rs ) for the three tested electrodes as reported in Tables 2–4. Rd mean value (215.82 kΩ/cm2 ) of Ag/AgCl electrodes is somewhat close to Rd mean value of Orbital electrodes (187.13 kΩ/cm2 ) with respect to surface area. However, it is much smaller than the Rd mean value (2130.98 kΩ/cm2 ) of Stainless Steel electrodes. The differences in Rd values of the same type of electrode among subjects are due to the difference of skin type of subjects (dry or oily), sweat secretion level and concentration of skin’s hair at electrodes sites. Recording biological signals at high Cd values is translated to better biological signal quality [1]. The measurements made by Ag/AgCl electrodes resulted in having higher Cd values in comparison to Orbital or Stainless Steel electrodes (Table 3 and Figure 7B). Orbital electrodes had reported high Cd values. The measured Cd values for Stainless Steel electrodes are far lower than Ag/AgCl or Orbital electrodes due to the nature of polarizable electrodes in accumulating charges at the electrode-skin sites. Table 2. Ag/AgCl, Orbital and stainless steel electrodes’ circuit component Rd average values (kΩ) for all the tested subjects. Electrode Type
Mean Values (kΩ)
kΩ/cm2
Ag/AgCl Orbital Stainless Steel
215.82 299.4 3289.4
215.82 187.13 2130.98
Table 3. Ag/AgCl, Orbital and stainless steel electrodes’ circuit component Cd average values (nF) for all the tested subjects. Electrode Type
Mean Values (nF)
kΩ/cm2
Ag/AgCl Orbital Stainless Steel
18.9 9.3 4.9
18.9 5.2 3.45
Recording biological signals at low Rs values is translated to better biological signal quality [5]. The existence of gel at the Ag/AgCl electrodes generated low Rs values (Table 4 and Figure 8C) [27]. In addition, the existence of pins or spikes in Orbital electrodes and the formation of sweat generated low Rs values that were close to Ag/AgCl electrodes’ Rs values. High Rs values for stainless steel electrodes resulted from the absence of electrolyte gel and were related to sweat formation. Table 4. Ag/AgCl, Orbital and stainless steel electrodes’ circuit component Rs average values (Ω) for all the tested subjects. Electrode Type
Mean Values (Ω)
Ω/cm2
Ag/AgCl Orbital Stainless Steel
399.7 626.8 856.4
399.7 391.8 121.1
4. Conclusions It can be concluded that pregelled Ag/AgCl electrodes would perform better than Orbital or stainless steel electrodes. Applying Ag/AgCl electrodes had resulted in having the lowest Rd or electrode-skin impedance values with a mean value of 215.82 (kΩ). However, Orbital electrodes that have pins in their structures had helped in generating compatible low electrode-skin impedance Rd values with a mean value of 299.4 (kΩ). Applying Stainless Steel electrodes resulted in having the highest Rd values with a mean value of 3289.4 (kΩ). Ag/AgCl electrodes had obtained the highest
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capacitance value (Cd ) followed by Orbital electrodes and Stainless Steel electrodes. The effect of differences in electrodes’ surface areas was considered. The existence of pins in Orbital electrodes and the formation of sweat generated low Rs values that were close to Ag/AgCl electrodes’ Rs values despite the existence of electrolyte gel on Ag/AgCl electrodes. This can be due to the existence of pins that would assist in eliminating the skin’s hair effect and strengthen the attachment to the skin. Stainless steel electrodes had resulted in high Rs values due to differences in material and shape. Due to the deterioration material of Ag/AgCl electrodes with time as a result of interaction with sweat, Orbital electrodes would be the most appropriate electrodes in our opinion for long time use for monitoring biological signals at home. Author Contributions: I would like to thank Prof. Adrian Chan for his advices and Carleton University for assistance in doing this study. I am grateful to all the volunteers who participated in this study and offered their services with patience and interest. Special thanks to GIMT for its support. Conflicts of Interest: The author declares no conflict of interest.
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