Superabsorbent Polymer Electrode for Transcranial Direct Current

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Direct Current Stimulation. Herschel ... promising results for applications such as transcranial direct .... connected in series to a multimeter (Fluke 289) and to the.
Superabsorbent Polymer Electrode for Transcranial Direct Current Stimulation Herschel B. Caytak 1,2, Izmail Batkin 1, Abeye Mekonnen 1, Daniel Shapiro 1,2, Scott Hassoun1,2, Glenn Li 2, Hilmi R. Dajani1, Miodrag Bolic 1, 1 School of Electrical Engineering and Computer Science (EECS), University of Ottawa, Ottawa, Canada 2 NorDocs Technologies Inc, 1-2280 Carling Avenue, Ottawa, Ontario, Abstract— Super absorbent polymer (SAP) is examined as a viable substitute of carbon for transcutaneous stimulation electrode design. Parameters such as fluid release under pressure, current homogeneity, resistance and drying rate were considered. Results showed that SAP electrodes had reduced fluid release under pressure and lower resistance when compared to the sponge electrode. Overall SAP electrodes had lower current homogeneity and less stable drying behavior when compared to the sponge electrode, although SAP current homogeneity was significantly improved by inserting an SAP electrode in a sponge cloth pocket. Keywords—Semi Dry; Superabsorbent Polymer; Transcranial Direct Current Stimulation; Homogeneity;

propose an electrode design based on highly absorptive SAP crystals [10] designed to provide a reservoir of fluid for controlled hydration of the electrode tissue interface. Utilization of SAP gel as the conductive medium of the electrode potentially allows for greater electrode flexibility and conformability to stimulation sites. In addition to high water retention, SAP electrodes should provide a highly conductive interface for transcutaneous stimulation as well as a homogeneous distribution of current. Potential advantages of this design may include enhanced user comfort, stable electrode skin coupling, and increased maximum number of hydration and drying cycles. II.

I.

INTRODUCTION

Standard electrode designs for transcutaneous stimulation utilize saline soaked sponges to provide stable electrical contact to the skin [1]. Cutaneous application of wet sponges may result in leaking of fluid, which can reduce treatment efficacy and cause user discomfort. In addition the conductive carbon electrodes inserted in the sponges may reduce overall electrode elasticity. Studies on hydrogel based electrodes have shown promising results for applications such as transcranial direct current stimulation (tDCS) [1] and Electrical Impedance Spectroscopy [2]. Usage of hydrogels may however result in user discomfort due to the sticky residue remaining after removal of the electrode. Superabsorbent polymer (SAP) crystals have very high absorptive properties hence act as reservoir of fluid. Moreover, they can provide a more flexible medium for transcutaneous stimulation. For recording of EEG without skin preparation, SAP based electrodes have shown a promise [3]. The present work investigates SAP based electrodes as a viable alternative to standard carbon in sponge pocket based electrodes for transcutaneous stimulation, particularly transcranial Direct Current Stimulation (tDCS). tDCS has demonstrated therapeutic benefit for a large number of motor and neuropsychiatric conditions [4-7], user discomfort especially due to the inhomogeneity of the stimulation current [8,9] may be a factor in inhibiting widespread public acceptance of transcutaneous stimulation. We therefore We would like to acknowledge the generous support of our sponsors, NorDocs Technologies Inc., FedDev Ontario (grant SME4SME) and NSERC (Engage grant). Manuscript received March 24, 2013. Corresponding author: Herschel B. Caytak (e-mail: [email protected]).

MATERIALS AND METHODS

A. Electrode Materials and Design Materials used in electrode construction were 18 mm diameter Prym brass alloy press fasteners, conductive aluminum foil, water impermeable plastic, cotton fabric and SAP (Watersorb ®) crystals between 200 and 800 microns in size. The SAP chemical basis was comprised of cross-linked acrylamide, acrylic acid copolymer, potassium salt and ammonium salt [3]. Maximum absorption when in contact with aqueous solutions under free swelling conditions was recorded at 250ml/g. 5 ring electrodes were built, of 5 cm diameter or approximately 20 cm² cross sectional contact area. The diameter of the conductive fabric layer was varied for each electrode for dimensions of 1.8, 2, 3, 4 and 5 cm to test the effect of the conductive layer size on current homogeneity. These electrodes were labeled as E1, E2, E3, E4, and E5, respectively. The plastic and conductive fabric layer, defined as L1 and L2 (Fig. 1) were joined by insertion between the spiked base and top of the fastener followed by clamping the teeth of the base to the underside of the top ring. A bottom cotton layer, L3, was completely sewed on to L1 and L2. Subsequently, 0.5 grams of SAP crystals - enough to turn the fluid/SAP solution into semi solid (jelly) state were inserted into the cotton layer. A 5 cm diameter ring sponge was also experimentally tested to compare sponge and SAP electrical properties. In addition an SAP electrode was inserted in a sponge cloth material (Chiffon Wizcloth ®) to investigate the effect of combining SAP and sponge.

1.8  cm   L1  

Super   Absorbent   Polymer   Placed   Between  L2   and  L3  

connected in series to a multimeter (Fluke 289) and to the ground of the power supply, was used to close the circuit when placed in contact with the test electrode. An insulating material (plastic) was used to enable measurements at the defined electrode regions (Fig. 3).

L2   L3  

5  cm  

Figure 1 The design of the electrode consists of three layers labeled L1, L2 and L3, a 1.8 cm radius bronze alloy press fastener and .5 grams of super absorbent polymer in crystal form. L1 is a waterproof plastic layer, L2 is comprised of conductive aluminum foil and L3 is a water permeable cotton fabric that was sewed to L1 and L2. The super absorbent polymer crystals were inserted between L3 and L2.

Each electrode was divided into 5 equal 1.5 cm diameter circles (Fig. 2) to provide regions for localized current measurements. The central circle, labeled C, was set directly opposite the initial current input through the brass alloy press fastener. The centers of the peripheral circles defined as R1, R2, R3, and R4, were oriented at even 45 degree spacing from the origin at a distance equal to the root of 1.5 cm from the center of C.

R1   1.5  cm   5  cm  

R2  

C  

1.5  cm  

1.5  cm   R3   1.5  cm  

R4   1.5  cm  

Figure 2 The electrode is shown divided into five 1.5 cm radii circles, labeled C, R1, R2, R3, and R4, to directly measure the current flow and resistance at different locations of the electrode surface.

B. Testing Apparatus and Configuration Insulated 22 AWG strand copper wires were utilized as lead wires that could be manually connected and disconnected from the electrode snap connectors. A power supply (Extech Instruments ® Quad Output DC Power Supply) generated a constant 10 volts in series with a 100 ohm resistor to serve as a current source to the test electrode. A second electrode,

Figure 3 A A schematic illustration of the testing apparatus and configuration is shown, comprised of a power source set to a constant 10 volts, connected in series to a 100 ohm resistor and a test electrode. Current passing through the different regions of the testing electrode was measured by pressing the test electrode against a contact electrode through a hole of set dimensions in an insulator. The current flow through the test and contact electrode was measured through a multimeter symbolized by A that was in turn connected to ground to complete the circuit. B A circuit diagram representative of the testing configuration is shown comprised of 10 volts connected in series with a 100 ohm resistor and Rx, the test and contact electrode resistance. The current is measured at A.

C. Experiments Four sets of experiments were performed to measure and compare the electrode properties. The last 3 experiment sets were implemented with electrode wetting in non-ionized tap water and with soaking in a 0.1% NaCl solution. Each measurement was repeated 5 times, after which the mean and standard deviation of the mean was recorded. The level of salinity was chosen to mimic physiological levels of salt concentration allowing for potentially long term stimulation with minimal skin irritation [11]. In addition low levels of salt concentration ensured the SAP gel would retain an absorptive capacity of at least at 40ml/g. 1) Fluid Release under Pressure Experimental testing with a Mark 10 strain gauge showed that approximately 2.5 Newtons of compressive force are applied to the electrode area of approximately 20 cm2 when the electrode is secured to measurement sites on the head with bands or a cap. 20 ml of tap water were used to hydrate each test electrode comprising of an SAP electrode, a carbon sponge electrode and a SAP electrode inserted in a sponge cloth pocket. The strain gauge was attached to a JLW Instruments Test Stand and was used to apply 2.5 Newtons of compressive force to the test electrode. Excess fluid released from the electrodes were examined by implementing a simple test in which parameters are chosen arbitrarily. 3 paper towels of a total combined thickness of 0.7 mm were placed under the test electrode. After 10 seconds of applied compressive force to the test electrode, the pressure was released and the diameter of the wet spot of the paper layer in contact with the electrode as well

as the third bottom layer was measured and normalized to the electrode diameter. 2) Electrode Current Homogeneity The current homogeneity of the SAP electrodes E1-E5, the sponge electrode, and a SAP electrode in a sponge cloth pocket was measured. Current magnitude was measured by placing the measurement regions of the test electrode in contact with a second electrode (Fig. 3). Measurement was taken at the center and four peripheral locations of the electrode. Homogeneity was evaluated as the ratio of the average peripheral current measured at R1-R4 to the central current measured at C [12]. This test was designed to examine the effectiveness of the electrode medium of evenly dispersing the current from the core to the periphery. In addition, average current density has been shown to increase at the edges of electrodes [13, 14].

B. Electrode Current Homogeneity Figure 4 A and B show the current magnitude of the SAP, sponge and SAP in sponge pocket electrodes when soaked in tap water and 0.1 % NaCl saline. Figures 5 shows the homogeneity of the electrodes measured as the ratio of the average peripheral current measured at R1-R4, to the current measured at C, the center region.

3) Electrode Mean Resistance Mean electrode resistance of 5 measurements was measured by placing the test electrode in contact with the contact electrode, recording the current flowing through the electrodes, and calculating resistance. 4) Electrode Drying Rate A 2 hour test was performed to compare drying rates of a SAP electrode, the circular sponge electrode and a SAP electrode in a sponge cloth pocket. Since tDCS stimulation is not recommended for more than an hour duration [15], 2 hours was arbitrarily chosen as the maximum conceivable duration of tDCS stimulation. Two of each type of test electrodes were hydrated with 20 ml of fluid and then placed in contact with each other under a compressive force of 2.5 Newtons to measure changes in electrode resistance over time. The current was measured every five minutes over a two hour period and then converted into resistance values in Ohms. III.

RESULTS

A. Fluid Release under Pressure Table 1 compares the results of fluid release under pressure of SAP, sponge and SAP in sponge cloth pocket electrodes. Results show that the sponge electrode released the greatest amount of fluid resulting in a wet spot 133% the electrode diameter, whereas the SAP electrode in a sponge cloth pocket released the least amount of fluid.

Figure 4 A The mean current of 5 measurements taken at every measurement region of the SAP, sponge and SAP in sponge cloth pocket (far right) hydrated with 20 ml of tap water is shown. The errors bars show the standard deviation of the mean B The mean current of 5 measurements taken at every measurement region of the SAP, sponge and SAP in sponge cloth pocket (far right) hydrated with a 20 ml saline solution with a concentration of 0.1% NaCl is shown

TABLE I PRESSURIZED FLUID RELEASE OF SAP AND SPONGE ELETRODES Electrode Type Sponge

Contact Layer Wet Spot Normalized to Electrode Diameter [%] 133

Third Layer Wet Spot Normalized to Electrode Diameter [%] 133

Description

SAP

100

80

Third layer semi dry

SAP in Sponge Cloth

80

46

Contact layer semi dry

Soaked through all layers Figure 5 Homogeneity of the current distribution, measured as the ratio of average peripheral current to central current, is shown (left) for SAP electrodes E1 – E5, the sponge electrode, and SAP electrode E1 placed in a sponge cloth pocket – E1 SP. Measurements were taken after electrode were hydrated with 20 ml of tap water. The same set of measurements were repeated and recorded after hydrating the electrodes with a 20 ml 0.1% NaCl saline solution (right).

C. Electrode Mean Resistance Table 2 shows the mean resistance and standard deviation of the mean of the SAP, sponge and SAP in a sponge pocket. TABLE 2 MEAN SAP AND SPONGE ELECTRODE RESISTANCE

[Ω]

E1

E2

E3

E4

E5

Sponge

E1 in Sponge Cloth

Tap Water

55.1

52.1

51.6

35

59.2

106.8

164.5

SD of the Mean

2.1

3.6

4.8

2.8

2.9

1.5

6.8

0.1% NaCl

61.1

79.9

185.8

103.8

157.7

236.1

102.9

SD of the Mean

3.6

5.8

17.7

9.9

16.3

21.6

3.2

D. Electrode Drying Rate Figure 6 demonstrates the drying rate of SAP, sponge, and SAP in sponge cloth electrodes, measured as change in electrode resistance over time. The slope of the graph is an indication of drying rate and drying rate stability.

Figure 6 The drying rate of SAP and sponge electrodes measured as change of resistance over time is shown. Results are shown for SAP and sponge when hydrated with 20 ml of tap water and 20 ml of a 0.1 % NaCl solution respectively. Results are shown for SAP in a sponge cloth pocket only hydrated with the saline solution.

IV.

DISCUSSION

Most tDCS applications use a current of between 1 – 2 mA for treating various neuropsychiatric disorders. Thus, the stimulation electrode is required to deliver a minimum of 1 – 2 mA of current. Moreover, the electrode skin-interface needs to stay wet for the entire duration of the tDCS session in order to provide an electrolysis medium. Studies advised that excess fluid from electrodes under pressure is not desirable in tDCs as it may cause patients’ discomfort and cause spreading of current [16]. Electrodes that maintain wet skin surface whilst withholding liquid are considered ideal. Furthermore, the injected current should be uniformly distributed across the electrode-skin interface. The present study considers fluid

release under pressure; current homogeneity; resistance and drying rate as parameters to compare SAP based electrodes to standard carbon based electrodes. Optimal electrode design includes the ability to deliver homogeneous current, while maintaining a stable low resistance electrode skin interface with minimal fluid release. Electrode performance metrics were evaluated based on combined performance on all the examined parameters. In the context of this paper the SAP in sponge performed favorable in 3 out 4 of the parameters examined, everything besides for current stability. The results of the pressurized fluid release test reinforce the motivation to replace the sponge electrode as the primary method of delivering stimulation current. The sponge-based electrode released the largest amount of fluid soaking an area through the paper cloth towels 133% the area of the sponge. The SAP electrode soaked 100% of the area of the sponge on the contact layer but only 80% of the third layer. In addition the third layer was observed to be partly dry. The least fluid release was obtained with a combination of SAP placed in a sponge cloth pocket. Figure 4 A shows the current magnitude measured for each measurement zone of the SAP and that a 0.1 NaCl % solution was used as a hydrating solution. We can observe that overall all the investigated electrodes provided at least a current of 20 mA, sufficient for the vast majority of required stimulation currents. Contrary to what was expected, Figure 4 B shows that current homogeneity and magnitude decreased for all electrodes besides for SAP in a sponge cloth pocket when hydrated with the saline solution. This may be attributed to an unanticipated increase in resistivity due to reuse, since the saline test was conducted on all the electrodes tested prior with tap water besides for the SAP in a sponge cloth pocket. The same observation can be made after analysis of the results presented in Table 2. Figure 5 showed that when the electrodes were hydrated with tap water, the sponge electrode was measured to have the highest current homogeneity, whereas when the saline solution was used, SAP in a sponge cloth pocket provided the highest current homogeneity, a result that is promising for attempts to develop this design into a stable medium for delivery of homogeneous current. Analysis of Figure 5 shows that a smaller size conductive layer in the E1 electrode provided the most homogeneous current when compared to the other SAP electrodes. The decreased homogeneity shown in Figure 5 B may be a result of material degradation caused by reuse. Future experimental work will not involve reuse to allow better-controlled results. Table 2 shows that SAP electrodes had an average lower resistance than sponge electrodes with the exception of the SAP sponge inserted in a sponge cloth pocket. The electrode drying rate as measured by resistance change over time shows that sponge electrodes exhibited the highest stability over the 2 hour test period. An attempt was made to increase the stability of the SAP electrode by inserting it into a sponge pocket, but results showed that besides for increased resistance, no improvement of the instability of SAP electrodes resistance was noted.

V.

CONCLUSION

The use of various forms of SAP electrodes has been explored as viable alternatives to sponge electrodes. Results show that SAP electrodes, especially when inserted in a sponge cloth pocket, release less fluid than comparable sponge based electrodes. A further advantage of SAP electrodes includes lower average resistance, decreasing the voltage required to maintain a set current level. Current homogeneity and stability of drying was generally superior in sponge-based electrodes although we demonstrated that sponge cloth may be used as a high resistance layer to improve current homogeneity. Further work is required to improve the stability of the resistance of SAP based electrodes during prolonged use. REFERENCES

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