1 Introduction

Mostly the invasive microbial infections have been taken among hospitals and medical examination tools [1,2,3]. Many medical devices need to be supplied with electrical conductor accessories that should be directly contacted to the patients [4]. Hence, the accumulations of pathogens which wide-speared in hospitals and medical communities made these devices as carriers of causative agents for many diseases [5]. Antimicrobial conductive polymeric materials have a good challenge in this field [6, 7]. However, the biocompatibility of many electrical soft polymers is lacked [8]. In this area, electrically conductive hydrogels and their composites are considered promising approach that combine in their composition a superabsorbent hydrogel, and an electrical conducting polymers such as poly(3,4-ethylenedioxythiophene), polyaniline, polypyrrole, or metal with electrical properties [9]. Electrical conducting hydrogel is synthesized by polymerization of conductive polymeric monomer on a hydrogel matrix, resulting in superior change of their electrical properties [10, 11]. They are featured by their ability to serve as an interface between the conducting electrode (electronic transporting phase) and the electrolyte (ionic-transporting phase) [12]. This outstanding feature of electrical conducting polymers together with their colloidal stability, low production cost, and the ease of preparation permitted their wide use in many potential applications including generally energy storage devices such as biofuel cells and supercapacitors [13, 14], dye sensitive solar cells, rechargeable lithium batteries, biosensors, bioelectronics, molecular diagnostics, and fabrication of materials with electro catalytic properties [15,16,17]. However, in medical operations during the diagnosis or monitoring, the used instruments are directly connected to the patients; this may cause many infection transmissions. The use of cellulose-based conductive hydrogels helped to overcome these drawbacks [18]. On the other hand, the cellulosic hydrogel acts as a biocompatible biopolymer [4, 19,20,21,22] and improves the electrical behavior of conductive polymers. In context, superabsorbent hydrogel is a hydrophilic, homo- or co-polymer with linear or branched, three-dimensional microporous-network structure, with the proper degree of cross-linking that is able to absorb a large amount of aqueous solution but hardly removed even under regular pressure as compared to other regular absorbing surfaces [23]. The presence of ionic groups in the network of the hydrogel was found to enhance its swelling ability, while the existence of nonionic groups improves the hydrogel salt tolerance [24]. According to the above reported findings, poly(2-acrylamido-2-methylpropanesulfonic acid) and polyacrylamide-based hydrogels demonstrated efficient electrical conductive properties enabling their application in the manufacture of many electronics [25], bio electrodes [26], and muscle actuators [27]. Furthermore, Bao et al. [24] employed acrylic acid, AMPS, and acrylamide for the preparation of carboxymethyl cellulose-g-poly(AA-co-AM-co-AMPS)/montmorillonite as a superabsorbent hydrogel composite by the graft copolymerization of acrylic acid, acrylamide, and AMPS onto carboxymethyl cellulose/montmorillonite composite [28].

Electrode polarization, i.e., establishment of an electrical double layer, it is well known to produce a giant capacitance, due to the very small separation distance (in order of 1 nm) between the two charged layers. It is a great target to overcome the deficit in separation of the double layer’s capacitance from the bulk one. Because the formed double layer at each electrode of the capacitor is in series with the bulk one, the total capacitance should have very high values. Earlier we reported the synthesis, and structure characterization, as well as mechanical properties and the electric conductivity evaluation for the hydrogel prepared from graft copolymerization of carboxymethyl cellulose with acrylamide and acrylamido-2-methyl-1-propane sulfonic acid with and without AgNPs and AgNPs@polypyrrole [28]. In the present work, we investigate the antimicrobial activity, biocompatibility, and dielectric properties of the conducting hydrogel (grafted carboxymethyl cellulose-g-poly (acrylamide-co-acrylamido-2-methyl-1-propane sulfonic acid) loaded AgNPs@polypyrrole) in comparative with the dielectric properties of ionic liquid [BMIM-BF4], take advantage of the mechanical properties of hydrogel over the liquid nature of the ionic liquid.

2 Materials and Experimental

2.1 Materials

Acrylamide (AM) and sodium carboxymethyl cellulose (CMC) medium viscosity, 98.5% with DS = 0.70–0.85, were purchased from Fluka. 2-Acrylamido-2-methyl-1-propane sulfonic acid (AMPS) was supplied by Alfa Aesar Chemicals. Potassium persulfate (KPS), N,N′-methylenebisacrylamide (MBA), Sodium dodecyl sulfate (SDS), and silver nitrate (AgNO3) were purchased from Sigma-Aldrich. Pyrrole was purchased from Science lab.com, Inc, Houston, Texes, USA. The ionic liquid used here for comparison is 1-butyl-3-methylimidazolium tetrafluoroborate–[BMIM-BF4] which was purchased from solvent innovation GmbH and Iolitec GmbH. All reagents, components of microbiological media and tissue culture media were purchased from modern Lab Co, India in an analysis grad without any purification required.

2.2 Experimental

2.2.1 Hydrogel Preparation

Carboxymethyl cellulose-g-poly (acrylamide-co-acrylamido-2-methyl-1-propane sulfonic acid) hydrogels were prepared via graft copolymerization and cross-linking of acrylamido-methyl propane sulfonic acid and acrylamide into carboxymethyl cellulose as described in our previous work and labeled after as (G1). The loading of silver nanoparticles into G1 gives G2 (G1/AgNPs). The polymerization of pyrrole in the presence of grafted carboxymethyl cellulose using AgNO3 as oxidizing agent to give silver nanoparticles@Polypyrrole nanocomposites (G3 and G4, the pyrrole: G1 was 1:1 and 2:1 wt/wt, respectively) was the same as in our previous works [11, 28].

2.2.2 Broad Band Dielectric Spectroscopy (BDS)

The dielectric measurements were taken using a high-resolution Alpha-analyzer from NOVOCONTROL Technologies GmbH & Co. KG in temperatures range 173–323 K and frequency window of 10−1–107 Hz combined with a Quatro temperature controller ensuring absolute thermal stability better than ± 0.5 °C. These two temperatures were selected for comparison of the effect of sliver and polypyrrole around room and electronics liberating heat temperatures. The sample for these measurements is sandwiched between two stainless steel electrodes. The electrodes are separated by a distance of 8.6 mm by mean of Teflon ring as spacer, and the plate diameter is 18.3 mm. It is employed to investigate the complex dielectrics function 𝜀∗ (𝜔, 𝑇) = 𝜀′(𝜔, 𝑇) − 𝜀″ (𝜔, 𝑇), where 𝜀′ is the permittivity and 𝜀″ is the dielectric loss. It is related to the complex conductivity function 𝜎∗(𝜔, 𝑇) = 𝜎′ (𝜔, 𝑇) + 𝜎″ (𝜔, 𝑇) since 𝜎∗(𝜔, 𝑇) = 𝑖𝜔𝜀𝑜𝜀∗(𝜔, 𝑇), implying that 𝜎′ = 𝜀𝑜𝜔𝜀″ and 𝜎″ = 𝜀𝑜𝜔𝜀′ (𝜀𝑜 being the vacuum permittivity).

2.2.3 Surface Area Investigation

Nitrogen adsorption–desorption measurements were taken at 77 K on (a Nova Touch LX4 Quantachrome, USA) to determine the Brunauer–Emmett–Teller (BET) isotherm. Before measurements, samples were kept in a desiccator until testing. Samples were cooled with liquid nitrogen and analyzed by measuring the volume of gas (N2) adsorbed at specific pressures. The pore volume was taken from the adsorption branch of the isotherm at P/Po = 0.995 assuming a complete pore saturation.

2.2.4 Bioactivity Studies

2.2.4.1 Antimicrobial Activity

The antimicrobial activity of the prepared hydrogels was examined against some selected strains representing the majority of the pathogenic groups; (1) Gram-negative bacteria: Escherichia coli (NCTC-10416) and Pseudomonas aeruginosa (NCID-9016), (2) Gram-positive bacteria: Bacillus subtilis (NCID-3610); (3) unicellular fungi: Candida albicans (NCCLS-11) and (4) filamentous fungi Aspergillus niger (ATCC-22342). The antimicrobial activities of the hydrogels were investigated through the agar well diffusion according to our previous studies [4]. The tested bacterial strains were incubated in nutrient broth medium at 37 °C and 180 rpm for 24 h, and fungal strains were incubated in potato dextrose broth media at 27 °C for 72 h. All the microbial suspensions were swapped over the medium. Dried samples of hydrogels were sterilized at 121 °C for 15 min and then suspended with sterilized deionized water at a concentration of 50 mg/mL. Wells of 5 mm diameter were made on agar media by using cork-borer; each well was loaded with 100 µL of tested samples. The plates were incubated at 37 °C for 24 h in nutrient agar media for bacteria and for fungal strains were cultivated in potato dextrose agar media at 27 °C for 72 h. The inhibition zone was observed in triplicate as mean value with standard division.

2.2.4.2 Cytocompatibility Test

The samples were tested for their cytotoxic effect against Vero cell line. When the Vero cells reached confluence of (75–90%) (usually 24 h), the cell suspension was prepared in complete growth medium (DMEM) supplemented with 50 mg/mL gentamycin according to our previous study [19]. Then the cells in each well were incubated at 37  °C with 100 mL of MTT solution (5 mg/mL) for 4 h. After the end of incubation, the MTT solution was removed; then 100 mL of DMSO was added to each well. The absorbance was detected at 570 nm using a micro-plate ELISA reader (SunRise TECAN, USA). The absorbance of untreated cells was considered as 100%. The results were determined by three independent experiments [19, 29]. After the end of the treatment, the plates were inverted to remove the medium, the wells were washed three times with 100 μL of phosphate buffered saline (PH 7.2) and then the cells were fixed to the plate with 10% formalin for 15 min at room temperature. The fixed cells were then stained with 100 μL of 0.25% crystal violet for 20 min. The stain was removed, and the plates were rinsed using deionized water to remove the excess of stain then allowed to dry. The cellular morphology was observed using an inverted microscope (CKX41; Olympus, Japan) equipped with the digital microscopy camera to capture the images representing the morphological changes compared to control cells as well as the IC50 values of samples were calculated.

3 Results and Discussions

3.1 Electrical and Dielectric Properties

The electrical conductivity and permittivity at two spot frequencies, namely 1 kHz and 100 kHz and two fixed temperatures, namely 303 and 343 K, respectively, of each sample, were determined and are presented in Table 1. Both conductivity and permittivity increased upon the addition of silver nanoparticle and polypyrrole at 1 kHz at the two temperatures. The effect of polypyrrole is more pronounced than that of silver nanoparticles. This behavior could be attributed to the trapping of the small silver ions with water molecules within the hydrogel, while the conductive polymer (Polypyrrole) has its self-conductivity that increases in the presence of water [30]. Similar behavior could be noticed also at the higher frequency except for sample G2. It has lower values of permittivity than the pure hydrogel (G1). This could be explained as that the shift of a local relaxation dynamic to lower frequencies that reduces the permittivity at the higher frequency, which will be discussed in details later.

Table 1 Real part of conductivity in addition to permittivity of the investigated samples at 303 and 343 K at two spot frequencies of 1 and 100 kHz
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The variation of loss tangent (\(\tan \delta = \frac{{\varepsilon^{{\prime \prime }} }}{{\varepsilon^{{\prime }} }}\)) with frequency for the four investigated samples at 303 K is illustrated in Fig. 1. The figure shows a clear relaxation peak around 1 MHz. The remarkable high values of the dissipation factor, in general, reflect the conductivity contribution rather than relaxation dynamics at molecular scale according to the relation: \(\varepsilon^{{\prime \prime }} = \frac{\sigma }{{\varepsilon_{o} \omega }}\).

Fig. 1
figure 1

Frequency dependence of the dissipation factor, tan δ, for the four investigated samples at 303 K

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The peak appeared in G1 shifted toward lower frequency values by the addition of silver and/or polypyrrole, i.e., the dynamic became slower. The sample of highest polypyrrole concentration, sample G4, shows promising results for energy storage applications, due to shift of loss to lower values while permittivity to the higher one.

3.2 Hydrogel-Electrode Polarization Driving Force for Supercapacitor

Energy storage is a vital target especially in combination with solar cell energy production systems. So, it is necessary to manufacture a device that characterizes at least with high-energy storage capacity, fast charging and maintaining a high number of charging–discharging cycles as well. One of the devices that could be appreciable for this aim is the supercapacitor. The variation of the real part of conductivity, σ′, with frequencies at four selected temperatures, namely (173, 193 K) and (273, 293 K), respectively, is depicted as a representative example for each sample in Fig. 2. The frequency dependence of σ′ is characterized on the high frequency range by a plateau-like behavior, and its values are independent of frequency and directly yield the DC conductivity, σdc [31,32,33].

Fig. 2
figure 2

Frequency dependence of σ′ at four selected temperatures (173, 193 and 273, 293 K) for all the prepared samples

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The determined DC conductivity, σdc, for all the samples at the selected temperatures is given in Table 2. Generally, σdc increases remarkably by the addition of the conductive polypyrrole rather than of silver only. This may confirm the trapping of silver nanoparticles within the hydrogel as stated previously [28].

Table 2 DC conductivity (σdc [S/cm]) determined for the investigated samples at four selected temperatures
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Further decrease in frequency shows a one or multistep decrease in σ′. This decrease in conductivity with decreasing frequency is attributed to the establishment of electrode polarization [34,35,36]. This means a formation of an electrical double layer at each electrode surface. The conductivities are thermally activated with low rate in the electrode polarization region by polypyrrole addition.

The dissipation factor, tan δ, for the prepared samples is graphically illustrated against frequency at three selected temperatures as shown in Fig. 3. The decreasing steps in the real part of conductivity are transformed into peaks in tan δ representation. It became well known that BDS probes the electronic response of an electrochemical system by applying an oscillatory potential of small amplitude. The relaxation dynamics of the interfacial structure is shown here as peaks; its maximum position characterizes the relaxation time.

Fig. 3
figure 3

Frequency dependence of tan δ at three selected temperatures as indicated for all the prepared samples

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The dielectric response of a free movable charge carrier is usually dominated by diffusion coefficient and not by molecular dynamics. This diffusive charging process is called electrode polarization that originates the electrochemical double layer (ECDL). Even ECDL originates from diffusion on a macroscopic length scale; the peak-like behavior in tan δ representation can be characterized by the HN-relaxation function in analogy to the description of molecular relaxation. One peak is shown at a certain frequency 3.8 kHz for the lower temperature 193 K as shown in Fig. 3. This frequency fon, onset of the electrode polarization, a steep increase in ε′ corresponding to the maximum in tan δ is detected first. At much lower frequencies, another peak of maximum f ≈ fonset representing another electrode-blocking step is out of our frequency window for the first three samples. It became remarkably faster for the sample G4 of higher concentration of conductive polymer and shifted toward higher frequencies. At the intermediate temperature 253 K, both peaks appear for sample G1 and G2 during the considered frequency window; however, G2 has one additional weak peak between the two commons. Although both of G3 and G4 samples contain silver as G2, this weak peak disappeared in both of the two high conductive samples (G3 and G4). This disappearance could be due to the addition of polypyrrole which makes the dynamic so fast and hence it merged with the higher frequency peak and broadened it. The explanations of this intermediate peak could interpret the gradual disappearance for jumping in DC-conductivity values in vicinity of 273 K (freezing temperature of pure water). The main conclusion here considered three processes of vastly different characteristic time scales in the frequency window. The first one regarding the ion transport normal to the interface happens on milli-second-scale which is the faster. The second is at one tenth second-scale process associated with molecular reorientation of electrode adsorbed cations. The slower dynamic process at minute scale relaxation is unable to observe in our limited frequency window since its peak position is at much lower than 0.1 Hz. It is tentatively assigned to lateral ordering within the first layer.

Analysis of tan δ peaks in order to estimate the electrode polarization onset frequency and calculation the processes’ activation energy can be performed with HN equation.

The relaxation time is a function of temperature according to Arrhenius equation defined by:

$$\nu \left( T \right) = \nu_{o} \exp^{{\left( { - \frac{{E_{\text{A}} }}{{k_{\text{B}} T}}} \right)}}$$
(1)

with \(\nu \left( T \right)\) is the characteristic relaxation frequency at temperature T, \(\nu_{o}\) is the pre-exponential factor, \(k_{\text{B}}\) is the Boltzmann constant and \(E_{\text{A}}\) is the activation energy.

In general, the process of lowest onset frequency moves faster toward higher frequencies by heating more than the other two, and so merged with that appear in the higher frequency range. This coupling effect can be assigned to certain temperatures 295, 275, 270 and 268 K for G1, G2, G3 and G4, respectively. The coupling temperatures were shifted more pronouncedly into lower temperature by silver rather than polypyrrole. This effect might be assigned to polypyrrole effect on hydrogel conductivity which is mainly electronic conductivity rather than ionic one. Polypyrrole clearly diminishes the jumping at the definite temperature of each charging–discharging process, in other words diminish the effect of water transformations (Fig. 4).

Fig. 4
figure 4

Activation plots for the onset frequencies of each charging–discharging process appeared in G1, G2, G3 and G4 samples. The solid lines are the linear fits

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Table 3 shows the decrease in loss activation energies of the electrode-blocking steps by polypyrrole addition. So higher amount of energy could be stored, due to lower energy required for the charging process. The stored energy, \(U\), is a linear function of the capacitance and could be calculated approximately for half of the sinusoidal cycle from \(U = 0.5 CV^{2}\). Its values are found to be in order of 5 mJ for G4 sample (Fig. 5).

Table 3 Activation energies of each Arrhenius step of the electrode polarization onset frequencies for the investigated samples
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Fig. 5
figure 5

Capacitance of G4 sample at different temperatures labeled in the graph

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The comparison of complex conductivity of G4 with ionic liquid of 1-butyl methyl imidazolium (BMIM-BF4) as representative example measured at 294 K (Fig. 6) shows generally similar trends. Although BMIM-BF4 has the advantage of lower conductivity that means lower leakage current, G4 hydrogel has nearly the same charge density for blocking of electrodes. On the other hand, G4 overcomes the liquid discharge probability of ionic liquid and with actually lower preparation cost. Details of ac-conductivity of ionic liquids could be found elsewhere [37,38,39,40].

Fig. 6
figure 6

Compassion of the complex electrical conductivity (σ′ and σ″) for polypyyrole-hydrogel (G4) and ionic liquid (BMIM-BF4)

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3.3 Surface Area and Pore Size Distribution

N2 adsorption–desorption isotherms at 77 K for tested samples are illustrated in Fig. 7. The isotherm types of tested samples have appeared that the sample G1 belongs to type H3 while samples G2, G3 and G4 belong to type H2 (a) following the IUPAC classification according to Thommes et al. [41]. Type H3 loops are: the adsorption branch resembles a Type II isotherm, and the lower limit of the desorption branch is normally located at the cavitation-induced p/p0. Loops of this type are given by non-rigid aggregates of plate-like particles (e.g., certain clays) but also if the pore network consists of microporous, which are not completely filled with the pore condensate. H2 (a) loops are for instance given by many silica gels, some porous glasses as well as some ordered mesoporous materials (e.g., SBA-16 and KIT-5 silica). The Type H2 (b) loop is also associated with pore blocking, but the size distribution of neck widths is now much larger. Examples of this type of hysteresis loops have been observed with mesocellular silica foams and certain mesoporous ordered silica after hydrothermal treatment.

Fig. 7
figure 7

Complete isotherm adsorption (A) and desorption (D) of tested hydrogels

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Figure 8 illustrates the pore size distribution of the prepared hydrogels using the Barrett–Joyner–Halenda (BJH) model from the desorption branch of the nitrogen isotherm. These plots illustrated that the pore size distribution for these hydrogels is centered at 58.5, 64, 70 and 62 nm for G1, G2, G3, and G4, respectively.

Fig. 8
figure 8

BJH pore size distribution adsorption and desorption results

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3.4 Biological Activity Studies

3.4.1 Antimicrobial Activity

The antimicrobial activity was carried out using agar diffusion method [4, 5, 21, 22, 42,43,44]. The four tested samples showed antimicrobial activity which varies according to the number of samples components as cleared in Fig. 9 as well as the clear zone measurements are tabulated in Table 4. G1 sample showed the lowest efficiency as antimicrobial agent. In addition, the antimicrobial activity of G2 was improved due to the addition of silver nanoparticles. Hence G3 and G4 hydrogels showed the highest antimicrobial activity according to the synergic effect of silver nanoparticles and pyrrole [22, 44]. On the other hand, G4 exhibited excellent antimicrobial activity against all tested microorganisms. Many suggested antimicrobial mechanisms may interpret the antimicrobial activity of the prepared hydrogels. The antimicrobial activity of polypyrrole can be attributed to lone pair of electrons of pyrrole nitrogen atom which bind with the microbial cell protein and deactivate. Also, silver nanoparticles play an important role in antimicrobial activity. AgNPs work by inhibiting the enzymatic systems that are involved in the respiratory chain altering the DNA configuration [45]. In context, due to their small size, nanoparticles have a large contact area compared to other salts and even the silver particulate, thus providing a better contact with the microorganisms by binding to the cell membrane and also penetrating inside. Studies in the medicine field have shown that silver is effective against more than 650 pathogens, having a broad spectrum of activity. Its use in the form of nanoparticles enhances this property, allowing its use in a wide range of applications [46, 47]. Therefore, AgNPs are now considered one of the most viable alternatives to antibiotics because of their high potency to overcome the problem of multidrug resistance, which is often developed by several bacterial strains [47].

Fig. 9
figure 9

Antimicrobial activity of prepared hydrogels by agar plat diffusion method: E. coli (a), P. aeruginosa (b), B. subtilis (c), C. albicans (d), and A. niger (e)

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Table 4 The inhibition zone in mm for tested samples (100 µg/mL) against tested organisms
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3.4.2 Cytocompatibility Test

The cytocompatibility test is a useful tool used in evaluation of the biocompatibility value of materials. Hydrogels were evaluated for their cytotoxic effect against Vero cell line (Fig. 10). Up to 125 µg/mL hydrogels concentration, there is no significant toxic effect on the cells; the cell viability was found to be 91% after 24 h. The IC50 values of sample were recorded an acceptable value 250, 195, 169 and 151 µg/mL for G1, G2, G3 and G4, respectively. Despite the presence of silver nanoparticles, the cells grew normally without any deformation. These results are in agreement with other previous work [19, 28]. Moreover, these results are confirming the previous conclusion [28], where the surface of prepared hydrogels revealed the formation of a dense layer of silver nanoparticles/polypyrrole on the surface of the hydrogel. Hence the well silver nanoparticle arranged appearing in the biological activity.

Fig. 10
figure 10

Morphological characteristics of Vero cells treated with and untraded sample observed after 24 h treatment under an inverted microscope. a Control Vero cells, b Vero cells treated with (G1 125 μg/mL), c Vero cells treated with (G2 125 μg/mL), d Vero cells treated with (G2 125 μg/mL) and e Vero cells treated with (G3 125 μg/mL). Magnification: × 40

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4 Conclusion

Polypyrrole addition to the hydrogel motivated us to use hydrogel in medical applications. Due to this, it diminishes the water transformation effect in contrary to silver nano-particles, increases permittivity and lowers the energy loss during the charging–discharging cycles. Moreover, the cytocompatibility cleared that the prepared hydrogels have not any toxic effect at high concentration towered the normal Vero cells. In addition, the antimicrobial activity of the native hydrogel was improved via the synergetic effect of polypyrrole and silver nanoparticles as well as the activity increased due to the increasing of silver nanoparticles concentration. The main objective of this work is to correlate between biological activity and the conductivity contribution in hydrogel. From dielectric view point, the prepared hydrogel showed similar behavior of ionic liquid [BMIM-BF4], but the hydrogel has structural advantage of the mechanical properties over the liquid nature of the ionic liquid.