INTRODUCTION:

The carbon-based nanomaterials like carbon nanotubes (CNTs), Graphene, Graphene oxides (Go) and Reduced Graphene Oxide (rGo) etc. needs a special attention for various applications due to their versatile nature¹. Devices like electrocardiogram (ECG), electroencephalogram (EEG) are being used to record the biomedical vital parameters through the surface electrode as an interface with the living tissue. These surface electrodes have undergone a substantial development, even though the basic standard is the “wet gel “Ag/AgCl electrodes. Alternating “Dry” type of surface electrodes provide a potentially greater advantage like long duration usage, low irritation and allergic reaction etc.

These carbon-based materials are in research and yet to be unlashed for its potential application in biomedical devices². Nevertheless, the advent of interdisciplinary areas like polymer sciences, nanofabricated metal³, non-metals and ceramic sciences increased the horizon of possibilities. CNT based⁴, Graphene based^5,6, rGo based⁷ as well as nano-metal fabricated structures based have been studied for both “wet and “dry’ use of electrodes⁸. These electrodes have shown a mixed response and to be replaced in need-based manner in biomedical devices and also allied fields. Compare to the above nonmetal nano-materials used in electrodes, the Super p carbon black could play a major role in biomedical devices and accessories based on its conductivity.

The present research work has focused on the electrode fabrication with a conventional slurry-based gel cast method by using Super p carbon black (SPCB), produced from partial oxidation of petrochemical precursors. The conventional slurry-based gel cast was formed with polymer Ethylene Co Vinyl Acetate (EVA). SPCB (TIMCAL) was mixed with polymer Ethylene Co Vinyl Acetate (EVA) with different mass ratios in a solvent to form the pellet. These pellets could be used directly on the surface of the electrode as inbuilt in a dry form to avoid the un-comfortableness of the patient and longevity of the electrode. The electrode material was characterized by in-situ non- destructive conductivity study using the Electrochemical Impedance Spectroscopic (EIS) method. This process could be used as a surface electrode material in biomedical devices like ECG, EEG and EMG etc.

MATERIALS AND METHODS:

The polymer Ethylene Co Vinyl Acetate (EVA) has wide application in various industries and especially receiving importance in drug delivery applications⁹. The polymer EVA from (Sigma-Aldrich, Poly (ethylene-co-vinyl acetate)-340502 vinyl acetate 40 wt.%, 190°C/2.16 kg, melt index 57g/110 minutes) was purchased and used as a base. On the other hand, Super p carbon black (SPCB) (purchased from Thermo Fischer Scientific (Alfa Aesar) H30253, density 160+/-20 kg/m3 sulfate Ash≤0.10%) exhibits an excellent electrical conductivity¹⁰. Due to its exclusive properties, it has been widely used in electrodes to improve the electronic conductivity. The research study has used EVA to functionalize Super p carbon black to form the electrode, which can be used in ECG to fetch a better signal. Finally, the study was compared with commercially available Ag/AgCl surface electrode (3M, Red Dot) and EVA with Super p carbon black at different ratio along with the Prinstine EVA pellet, whereas Prinstine EVA pellet was taken as blank sample and Ag/AgCl surface electrode as reference sample.

Electrode fabrication:

The electrode was fabricated with a conventional slurry-based gel cast method. Commercially available SPCB (TIMCAL) was mixed with polymer Ethylene Co Vinyl Acetate (EVA) with different mass ratios of 10% and 5% wt./wt. in a solvent. The preparation of Super p carbon black was functionalized with EVA as electrode with Tetra Hydro Furan (THF) anhydrous as solvent (Sigma Aldrich -401757-density 0.889g/ml at 25°C).

Gel cast method to functionalize EVA:

The Gel cast method was done by using polymer Ethylene Co Vinyl Acetate (EVA), anhydrous solvent Tetra Hydro Furan (THF) with Super p carbon black (SPCB). First the EVA was mixed with THF followed by continuous magnetic stirring to dissolve EVA completely¹⁸. Then the Super p carbon black (SPCB) was mixed with the above solution and continuous stirring was done to get homogeneous slurry in the form of sol. The EVA was used as a binder to the electrode material SPCB. The in vitro comparative conductivity study was done in two methods with different mass ratios. In the first method the electrode materials were casted by using 5% wt Super p carbon black (SPCB) and in the second method the materials were casted by using 10%wt SPCB¹¹ functionalized with EVA. Then the slurry was poured onto a copper foil and kept overnight at room temperature for initial drying to form a gel. Further the material was transferred to a vacuum at 70ºC for one day to dry.

Formation of Electrode pellet:

Once the electrode material become dry with different mass ratios, it was processed through the conventional punching device to form the pellets of different shapes based on the uses. These punched pellets were used for EIS measurements. The electrode thus casted was measured dimensionally to get different level of thickness. Now the electrodes pellets are ready for any size-based applications.

Conductivity study of Electrode pellets:

To study in ECG and EEG the electrode was mechanically compacted with a roller compressor and cut into circular pieces as pellets. These pellets were tested for their conductivity by using electrochemical impedance spectroscopy. The Electro Chemical Impedance Spectroscope (Novo Control Tech.de, Alpha-AT Mainframe & Test High version -3µHz to 40MHz.) was used for this non-destructive analysis. The EIS method was employed for polymer based in vitro conductivity measurement¹² using the in-frequency Nyquest Plot and the inference was drawn¹³.

In Vitro Study for EIS analysis:

The in vitro comparative conductivity study was done with Prinstine EVA pellet, 5% wt Super p carbon black pellet, 10% wt Super p carbon black pellet and commercially available Ag/AgCl surface electrode (3M, Red Dot). Before using the Ag/AgCl surface electrode for the comparative study the Ag/AgCl electrode was extruded to separate the metallic silver part alongside removing the gel and suitably washing it with polar solvent for residual removal. The Prinstine EVA has taken as blank whereas Ag/AgCl electrode was taken as reference to do the comparative analysis. The comparative thickness of super p carbon black pellets was used as samples for EIS analysis.

RESULTS AND DISCUSSION:

Electrode fabrication:

The electrode was fabricated by conventional gel-cast method at different ratios of SPCB in 10% and 5% wt/wt. with a solvent and the result was recorded by measuring with Screw Gauge to determine the dimensional thickness of the electrode (Table 1). The data was compared with the reference value and blank sample.

Table 1: Comparative analysis of Fabricated Electrodes thickness with Reference and Blank

Samples	Dimensional thickness (mm)	Inference
Prinstine EVA	0.8	Blank
5%wt SPCB-EVA	0.9	Same with 10%wtSPCB-EVA
10%wt SPCB-EVA	0.9	Same with 5%wtSPCB-EVA
Ag/AgCl	1.0	Reference

Gel cast method to functionalize EVA:

The gel cast method was done by using EVA, anhydrous solvent THF with SPCB. The EVA was dissolved with anhydrous THF within an hour with the help of continuous magnetic stirring. The solution was divided into three parts equally. One part of the solution was used as Prinstine EVA for using as blank sample. To the second part of the solution 5%wt SPCB and to the third part 10%wt SPCB was mixed separately and kept over stirrer to get homogeneous slurry. Then the slurry was kept overnight at room temperature for initial drying to form a gel followed by a vacuum at 70ºC for one day to dry (Fig 1). The EVA was used which was explained ⁹ for simplistic model that enables predictions of release rates of EVA film with high accuracy. They have also demonstrated the polymeric structures of the EVA film in the Franz cell experiments and the extruded coaxial fibers in the release study. Since EVA is a transparent thermoplastic copolymer where VA units are randomly distributed throughout the ethylene polymer and ranges between 1 and 40 wt.% to find the crystallinity, melting point, stiffness and polarity ¹⁴.

Prinstine EVA

5%wtSPCB EVA

10%wt SPCB-EVA

Fig 1: Preparation of solutions by gel-cast method

Formation of Electrode pellet:

The electrode pellet formation process using SPCB material was elaborated¹⁵ using binder PVDF for LIB work by in situ scanning Electrochemical Microscopy study. In our work the dry electrode was processed through the punching device to form the pellets for EIS measurements. The shape was rounded and the diameter was recorded as 0.96cm. The casted electrode was measured dimensionally with thickness of 0.9mm (Fig 2).

Prinstine EVA

5%wt SPCB-EVA

10%wt SPCB-EVA

Fig 2: Analysis of Electrodes Pellets.

Conductivity study of Electrode pellets:

For a non-destructive diagnostic technique of studying both the bulk transport properties of a material and the electrochemical reactions at interface of electrodes, Electrochemical impedance spectroscopy (EIS) method is among the first¹⁶. The electrode pellets were tested for their conductivity by using electrochemical impedance spectroscopy. The EIS method was employed and the conductivity was measured by using the in-frequency Nyquest Plot. The highest conductivity was found in 10%wt SPCB-EVA whereas the lowest conductivity in Prinstine EVA. Further the conductivity was compared with the standard sample Ag/AgCl, which is much closure with the 10%wt SPCB-EVA sample (Table 2). The Nyquest Plot inference was drawn (Fig 3).

Table 2: Comparative analysis of Conductivity study of Electrode pellets

Samples	Conductivity	Inference
Prinstine EVA	0.0203	Blank
5%wtSPCB-EVA	1.93	Average Conductivity
10%wtSPCB-EVA	2.14	Better Conductivity
Ag/AgCl	2.13	Reference

Fig 3: Analysis of Nyquest Plot of Combined spectra

In vitro Study for EIS analysis:

Based on the advantage of using EIS that too during operation of electrode is to separate a complicated electrochemical processes and reactions into elementary processes based on differences in time constants¹³, providing insights into it. This complicity also involves the in vitro comparative conductivity study with EIS analysis (Table 3). Here in, the data of 5%wt Super p carbon black pellet, 10%wt SPCB pellet was compared with Prinstine EVA pellet and commercially available Ag/AgCl surface electrode.

Table 3: Comparative EIS analysis for in vitro conductivity Study

Parameters	5%wtSPC-EVA	10%wtSPC-EVA	Ag/AgCl	Prinstine EVA
Conductivity (S/cm(x10-3)	1.93	2.14	2.13	0.0203
Dimensional Thickness (mm)	0.9	0.9	1.0	0.8

Comparative Study of in-vitro conductivity:

The Super p carbon black functionalized EVA electrode has higher conductivity capability. Similar approach for synthesizing conductive membrane substrates using EVA and carbon black demonstrated their potential applications in energy-storage devices¹⁷. The higher relative concentration (10%wt) of Super p carbon black in EVA electrode manifestoes a marked increased conductivity by in vitro EIS studies. The dimensional thickness was same for 5%wt SPC-EVA and 10%wt SPC-EVA, whereas the commercial Ag/AgCl surface electrode is thicker than both the samples. In the other hand the conductivity was better in 10%wt SPC-EVA than another sample (Fig 4).

Fig 4: Comparative analysis of Dimensional Thickness in vitro conductivity Study

The Electrochemical interfacial parameters of different electrodes:

The Nyquist plots of 1) 10% SPCB-EVA, 2) 5% SPCB-EVA, 3) 3M Ag/AgCl and 4) Pristine EVA sample was plotted respectively (Fig 5 to 8). These Nyquist plots help interpretation of impedance data, requires the use of an appropriate model. The Randles equivalent circuit is one of the simplest of possible model to describe the process at the interface in an electrochemical sample system. In our study we use an analyzer software based Randles equivalent circuit commonly used in Electro Chemical Impedance Spectroscopy for interpretation of impedance spectra. This spectral interpretation is interfacial electrochemical reaction in presence of semi-infinite linear diffusion model of electro active particles on a flat electrode structure. The corresponding equation of the faraday impedance is used to derive and estimate the electrical signal conductivity σe. The behavior of range of carbon composite electrodes of different matrix including Super P carbon, for demonstrable interfacial electrical conductivity using EIS method studies for electrochemical capacitor storage applications¹⁹. The signal conductivity of Super p carbon black functionalized EVA electrode system, in its 10%wt of Super p carbon black 2.14x10^-3 S/cm in relative to 5%wt of Super p carbon black in 1.93x10^-3 S/cm and pristine EVA membrane 2.03x10^-5 S/cm has a marked Signal conductivity parameter. This value of conductivity is comparable to the generic Ag/AgCl Surface Electrode membrane 2.13x10^-3 S/cm. The range of signal is demonstrably similar to the 10%wt Super p carbon black functionalized EVA electrode membrane. The role of EIS simulations in interpretation of Nyquist plots for electrodes from EIS method and its close resembles to experimental measurements although these were aimed for EDLC electrodes²⁰, we have used them for our work with SPCB composite electrodes. Few authors^21-30 also evaluated the models of various instruments at different levels.

Fig 5: Nyquist plot of 10% SPCB-EVA sample system

Fig 6: Nyquist plot 5% SPCB-EVA sample system

Fig 7: Nyquist plot Ag/AgCl sample system

Fig 8: Nyquist plot Pristine EVA sample system

The measured signal conductive capability might be largely due to the Super p carbon black possible increased molecular cross link network matrix in the given EVA polymer base. Although the need for study of higher proportional role of Super p carbon black functionalized EVA electrode membrane not intended as the in vitro conductivity was at par to conventional Ag/AgCl electrodes. In future, the role of novel conductive polymer other than EVA needs to be demonstrated.

CONCLUSION:

The Super p carbon black functionalized EVA electrode membrane, its comparable invitro conductivity parameter, provides an amicable alternate to the conventional Ag/AgCl surface electrode. With the need for advanced electro conductive pad wearables, the present model of Super p carbon black functionalized EVA electrode membrane can have future application. Even the “dry” electrodes for biomedical devices like ECG, EEG etc. need an alternate thinking. Future work on its use and study with human subjects is needed. Similarly, the EVA base polymer can be replaced with conductive polymer base for further conductivity improvement.

ACKNOWLEDGEMENT:

The authors would like to thank Sathyabama Institute of Science and Technology, Chennai, India for providing the support to work and also like to thank Prof. N. Sathyanarayan and his team of Department of Physics, Pondicherry University for guiding and providing the facility to complete the work.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

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Received on 31.01.2022 Modified on 19.08.2022

Research J. Pharm. and Tech 2023; 16(7):3183-3188.

DOI: 10.52711/0974-360X.2023.00523