Linear Actuators Based on Polyvinyl Alcohol/Lithium Chloride Hydrogels Activated by Low AC Voltage

Abstract

The development of fast-responding electrically conductive polymers as actuators activated by low electrical current is now regarded as an urgent matter. Due to their limited electrical conductivity, actuators based on polymeric hydrogels must be activated using a high voltage (up to 200 V) and frequency (up to 500 Hz) when employing AC power. In this work, to improve the electrical conductivity of the hydrogel and decrease the required activation voltage of the hydrogel actuators, lithium chloride (LiCL) was added as a conductive filler to the polymer matrix of polyvinyl alcohol (PVA). In order to ascertain the deformation of actuators, activation and relaxation times, actuator efficiencies, and generated force under the conditions of activation, linear actuators that can be activated by extension/contraction (swelling/shrinking) cycles were prepared and investigated depending on the LiCl content, applied voltage, and frequency. Under a load of approximately 20 kPa and using a 90 V AC power at a 50 Hz frequency with a 30 wt.% LiCl content, it was found that the actuators’ total contraction, reinforced by a woven mesh braided material, was about 20% with a ~2.2 s activation time, while the actuators’ total extension, reinforced by a spiral weave material, was about 52% with a ~2.5 s activation time, after applying a 110 V AC at a 50 Hz frequency with a 10 wt.% LiCl content. Furthermore, as the lowest voltage, a 20 V AC power can operate these actuators by increasing the LiCl weight content to the same PVA mass content. Moreover, the PVA/LiCl hydrogels’ activation force can be greater than 0.5 MPa. The actuators that have been developed have broad applications in soft robotics, artificial muscles, medicine, and aerospace fields.

1. Introduction

Recently, soft robotics and biomedicine are becoming more interested in polymer actuators because of their unique features, which include their light weight, simplicity of use, quietness, biodegradability, fast response, and acceptable mechanical properties [1]. Electroactive polymers (EAPs) are a significant and fascinating class of actuators that exhibit deformation in response to an external electric field. They are appealing for use as actuators, sensors, and artificial muscles [2,3,4,5]. EAPs are an attractive choice for electromechanical actuator applications because of their ability to precisely control the electrical to mechanical energy conversion and vice versa when exposed to an electrical current [6,7]. The well-known mechanism for ionic EAP activation under DC power is ion diffusion, which causes an alteration in the size of the actuator and its shape (bending) due to ion diffusion and mobility in the electrolyte [8]. Ionic EAPs activated by a DC current have several drawbacks, including poor strength, slow activation, and difficulty preserving humidity. They also have trouble maintaining continuous deformation when DC power is used [9,10]. Hydrogels prepared based on polyvinyl alcohol (PVA) are a famous example of ionic EAPs. PVA is a synthetic polymer that is hydrophilic, biodegradable, and biocompatible, and which possesses excellent adhesive properties [11]. It is widely utilized in various industries, including applications in biomedicine, wastewater treatment, food packaging, textiles, and paper [12,13].

In our previous work [14], a novel method for activating hydrogels of polyvinyl alcohol as actuators by using AC power was suggested. Within this approach, the actuation mechanism of the actuators are the extension/contraction cycles, due to the ion’s local vibrations, without any motion toward the electrodes, that induce the hydrogel to heat up, which in turn causes the actuator to enlarge and the water molecules to change into vapor. In further detail, hydrogel exhibits a small swelling ratio at room conditions because of the hydrogen bond-compressed network structure, and when it heats up after applying the AC power, the hydrogen bonds become weak, and the twisted polymer molecular chains are stretched. Moreover, when the hydrogel temperature exceeds 95 °C, the hydrogen bond-compressed network structure will be broken, and the water molecules will change into a vaporous state, which will lead to the actuator swelling. However, the actuator will return to its original form once the power is switched off, because the state of the water will alter from a gas to a liquid and will be adsorbed by the hydrogel, as in Figure 1.

Generally, in any electrochemical cell, the electric field can be partially blocked as a result of the movement of electrolyte ions toward the electrodes and their accumulation at their surfaces [15,16]. Under DC power, this effect will be obvious, but under AC power, the ion motion related to the current frequency will make the effect more complicated to observe, since the change in the transferred voltage value across the electrolyte, when the current frequency alters, can be noticeable [17,18]. However, in AC power systems, ohmic current leads to the heating of the electrolyte because of Joule heating that happens regardless of blocked electrodes, and it will be produced continually due to the ions’ periodical motion in the electrolyte. Moreover, Joule heating will cause a density variation in the electrolyte, formation of an AC electrothermal flow, and also boiling of the electrolyte [19,20]. Furthermore, Joule heating can also create AC electro-osmosis flows [21], which have an important effect on the activation process of the hydrogel actuator. In other words, water transports across the hydrogel, functioning as a semipermeable barrier starting from the hydrogel side that has a high water’s chemical potential to the one that has a lower chemical potential [22]. In the ionic hydrogel network scenario, the charges’ location within the chains of polymer leads to much larger adsorbing (swelling) forces when ionized water molecules (H₃O⁺, OH⁻) are dissolved. At the swelling equilibrium, the total variation in free energy is at its lowest; it is comparable with the state where each mobile species’ coexisting phase of chemical potential equalizes. This suggests that the way hydrogels expand and contract in response to temperature variations under AC voltage is likewise related to the forward osmosis hydration and dehydration processes [23,24].

Nevertheless, the main drawback of our previous work [14] was that to obtain the actuators’ maximal deformation, a high voltage of 200 V and a high frequency of 500 Hz were required. To reduce the required AC voltage needed to activate these proposed actuators, fillers, such as salts, can be added to polymer hydrogels to increase their electrical conductivity. Both water and salt, in this hydrogel type, play essential roles: water acts as a solvent and salt provides conducting ions. A hydrogel with a content of salt can conduct electrical current so long as it contains water, and the lower the percentage of the contained water, the stiffer the hydrogel will be. Therefore, this fundamental property of the hydrogel, the capacity to retain water, affects its other properties [25]. MgCl₂ (magnesium chloride), LiCl (lithium chloride), NaCl (sodium chloride), and CH₃COOK (potassium acetate) are considered to be the most commonly added salts for polymer hydrogels to improve their electrical conductivity. Lithium chloride is regarded as a hygroscopic salt for polymer hydrogels leading, consequently, to an increase in the capacity of water retention and to an increase in the heat and moisture exchangers’ efficiency. Moreover, due to its higher hydration energy, LiCl can absorb moisture from the surrounding medium and maintain the moisture content of the hydrogel. In the reference [25], polyacrylamide hydrogels with LiCl were prepared as highly stretchable transparent electrodes in flexible electronics to enhance the polyacrylamide hydrogel capacity to retain water by incorporating an easily hydratable salt in the hydrogel. The authors showed that the prepared hydrogels showed an increased water retention capacity at different levels. In particular, a polyacrylamide hydrogel with a high lithium chloride content can retain more than 70% of the original water even in an environment with only 10% relative humidity. In the reference [26], a hydrogel was prepared to be used as an electrode in a transparent loudspeaker, and NaCl was added as a conductive filler, since the sodium cation (Na⁺) and chloride anion (Cl⁻) act as conducting ions. In the reference [27], an acrylic acid was polymerized in an ionic liquid to prepare a polyacrylic acid ionogel that is transparent. The prepared ionogel used in transparent actuators exhibited a 0.22 S/m conductivity, ~3 kPa of elasticity, and ~4.6 of tensile strength.

In this article, PVA hydrogels were prepared, and LiCl was added as a conductive ion filler to decrease the required actuating voltage. External reinforcement materials (spiral weave and woven mesh braided) were used to carry out linear actuator motion. Actuator efficiency, deformation, activation and relaxation times, and generated forces during activation were examined depending on LiCl concentration, and on the applied voltage and frequency.

2. Materials and Methods

2.1. Materials

The average molecular weight of polyvinyl alcohol (PVA) purchased from “Ruskhim Ltd.” (Moscow, Russia) was 105,000 g/mol, and the hydrolysis degree was 99%. As a crosslinking agent, sodium tetraborate (borax) was used. The solvent used during the preparation of PVA hydrogels was distilled water. Anhydrous pure lithium chloride was used as a conductive filler. For obtaining hydrogel actuators, the following materials were used:

• Latex balloons as an elastomeric shell;
• Spiral weave material of 8 mm internal diameter made of 0.5 mm diameter polycaprolactam fiber as an external reinforcement material;
• Woven mesh braided material of 8 mm internal diameter made of polyethylene terephthalate which has the ability to stretch twice;
• Conductive copper wires used as electrodes;
• Heat-shrinkable tubes made of polyolefins, 10 mm long with a diameter of 4- and 2-mm. Heat-shrinkable tubes are required to properly connect and fix the external reinforcements, elastomeric shell, and electrodes in the prepared the actuators.

2.2. Preparation of PVA Hydrogels

First, we heated the PVA/LiCl solution in distilled water at 140 °C temperature for 1 h using a magnetic stirrer. Then, we added a borax solution in distilled water stirred at the same temperature for 15 min to the solution. After that, we removed the air bubbles from the PVA/LiCl/borax solutions using vacuuming (Figure 2). Finally, we kept the prepared solutions in the refrigerator at 4 °C to use them later.

Hydrogels containing different amounts of LiCl were prepared to study the influence of LiCl concentration on the actuator’s deformation and its activation and relaxation times. A total of 100 mL of distilled water was used to obtain all PVA hydrogels with a 7 wt.% (0.00067 mol/L) PVA concentration and a 2 wt.% (0.00367 mol/L) borax content of PVA mass content. LiCl was added to the PVA hydrogel in different contents, which were 10, 30, 50, and 100 wt.% of the PVA mass content.

Table 1. The contents of LiCl in the hydrogels that were prepared.

Material No.	Material Code	LiCl Concentration, wt. % of PVA Mass Content	LiCl Concentration, mol/L
1	PS1	10	0.1652
2	PS3	30	0.4956
3	PS5	50	0.8261
4	PS10	100	1.6521

demonstrates the contents of LiCl in the hydrogels that were prepared. Figure 3 presents the crosslinking process and the chemical reaction of PVA and borax.

2.3. Preparation of Hydrogel Actuators

The procedure of the preparation of the hydrogel actuators was described in detail in our previous work [14]. Briefly, 4 mL of hydrogel was inserted into a stretchable elastomeric shell with a 50 ± 5 mm length and a 7 ± 0.5 mm inner diameter using a syringe. The two ends of the shell were closed and fixed to the external reinforcement and copper electrodes using heat-shrinkable tubes. Figure 4 shows a schematic design for the hydrogel actuators and two types of external reinforcement. The two external reinforcement internal diameters were 7 ± 0.5 mm.

2.4. Procedures

2.4.1. Hydrogel Actuators’ Electrical Resistance Measurements

The electrical resistance of the actuators based on PVA hydrogels was measured using a four-point probe resistivity measurement system (RLC meter AKIP-6112/2, AKIP™, Moscow, Russia) with an electrical capacitance range of between 0.01 nF and 10 F. At least 3 measurements for each hydrogel composite were caried out under a voltage of 2 V using 2 values of frequency of 50 and 500 Hz.

2.4.2. Actuation Tests

To study the actuation deformation, activation and relaxation times, and activation force of the prepared PVA hydrogels, actuators with different LiCl contents were subjected to different AC voltage values in the range of 20–110 V and two values of frequency of 50 and 500 Hz under a load of 50 g (see Video S1). For generating AC voltage, AC power (MATRIX APS-7100 AC Power Source) with an operating voltage range of 0–310 V and a frequency range of 45–500 Hz was used.

2.4.3. Actuation Deformation Measurement

The values of contraction/extension deformation of the hydrogel actuators were measured using a high-performance laser distance sensor, namely the Wenglor YP11MGVL80 (Wenglor sensoric GmbH, Tettnang, Germany), with a linearity of 0.5%, a measuring range of 50 mm, and a resolution of 20 um, as in Figure 5a. Using a USB interface (LA-2USB-14) (LLC Rudnev-Shilyaev, Moscow, Russia) and PowerGraph software 3.3.11 (DISoft, Moscow, Russia), the data of deformation values, activation and relaxation times, and activation force were collected and processed. The activation time was calculated as the time between the moment of applying voltage and the maximum height of the deformation peak, whereas the relaxation time was calculated as the time between the moment of switching the voltage off and the initial value of displacement.

The calculation of the actuator efficiency was carried out as follows:

ƞ = P₂/P₁ × 100

(1)

where ƞ is the efficiency (%), P₁ is the supplied electrical power (watt), and P₂ is the useful mechanical power generated by the actuator (watt). Equation (2) is as follows:

P₁ = V·I

(2)

where V is the voltage (volts) and I is the current (Amps). Equation (3) is as follows:

P₂ = m·a·L/t

(3)

where m is the applied load (kg), a is the acceleration (9.81 m/s²), L is the displacement (m), and t is the activation time (sec).

Here, the useful mechanical power generated by the hydrogel actuator was calculated for 1 m of actuator length.

2.4.4. PVA/LiCl Hydrogels’ Generated Force Measurement

An SUP 1 load cell (ShengWanJiang Transducer Technology (Shenzhen) Co., Ltd., Shenzhen, China) with a maximum measurement limit of 20 kg and a complex error of 0.05% was used to measure the generated force of the hydrogels under AC voltage (Figure 5b). Here, 4 mL of PVA/LiCl hydrogel was put in a syringe with a volume of 5 mm (Figure 5b).

3. Results

Table 2. Electrical resistance of PVA hydrogels as determined by the four-probe method (2 V).

Material	PVA		SP1		SP3		SP5		SP10
Frequency, Hz	50	500	50	500	50	500	50	500	50	500
Electrical resistance, Ω	1352 ± 11	1102 ± 8	890 ± 12	813 ± 6	414 ± 10	393 ± 2	151 ± 7	137 ± 6	111 ± 2	101 ± 1

shows the electrical resistance of the PVA/LiCl hydrogels under different frequencies. As can be seen, the addition LiCl and the increasing frequency value led to a decrease in the values of the electrical resistance of the PVA/LiCl hydrogels. This is related to the fact that by increasing the number of ions in the water (solvent), these ions are considered charged particles, which can carry an electric current.

Understanding the structure of liquid water, hydration processes, and ion–solvent interactions is crucial for understanding material science and solution chemistry in general. To explain the features of aqueous solutions, it is essential to comprehend the molecular structure of liquid water, specifically the intermolecular hydrogen bonds [28]. As was mentioned in the introduction, hydrogen bonds in the hydrogel network play the key role in the process on the activation of the actuator under AC voltage. In other words, when the hydrogel temperature increases as a result of Joule heating effect, the water molecules will be transformed into a gaseous state after the destruction of the hydrogen bonds, which in turn leads to swelling of the volume of the elastomeric shell that covers the hydrogel, activating the actuator. In the AC systems, ohmic current will activate the Joule heating effect, which in its turn will form an AC electrothermal flow and result in the boiling of the water molecules [19,20]. In the aqueous systems, under the effect AC voltage with low frequences (0−1 GHz) and low electric field strength (0−100 MV/m), the change in the dielectric constant of the aqueous solution can be negligible [29,30], whereas for highly conductive solutions (water with salts), the temperature changes due to the Joule heating effect will be dependent on the conductivity and effective voltage drop across the electrolyte [31]. Here, the temperature changes in the electrolyte are related to the changes in its chemical potential and permittivity [31]. Under an AC electric field, the materials that have high permittivity can be more polarized in comparison with those that have low permittivity, which means that they can store more energy [32]. This means that by adding LiCl to the hydrogel, its conductivity will be increased (see Table 2), which in turn will lead to a decrease in the permittivity of the hydrogel. As such, the effect of the Joule heating will be reached at a lower AC voltage, which in turn means that the hydrogel actuators based on PVA/LiCl can be activated at a lower voltage in comparison with the ones based on PVA.

Figure 6 and Figure 7 and Tables S1–S8 demonstrate the outcomes of the total extension and contraction of PVA/LiCl hydrogels, and their activation and relaxation times, by applying different AC voltages and frequencies. For the SP1 and SP3 hydrogels, the minimum AC voltage required to activate the actuators was 40 V, whereas the actuators of SP5 and SP10 can be activated by applying an AC voltage of 20 V. Moreover, the maximum values of the contraction/extension deformation were obtained at lower AC voltage when the LiCl content was increased. Furthermore, for the linear actuators strengthened using spiral reinforcement, the extension values of the actuators were higher under a low frequency of 50 Hz, whereas for the linear actuators strengthened using a woven mesh braided material, the contraction values of the actuators were higher under a high frequency of 500 Hz. Moreover, by increasing the LiCl content of the actuators, the activation time was reduced, whereas the relaxation time was dependent on the deformation value of the actuators, and it increased when the deformation increased. The maximum extension value was obtained for the SP1 actuator, and it was about 53% under 110 V AC at 50 Hz; the maximum contraction value was obtained for the SP3 actuator, and it was about 19% under 90 V AC at 50 Hz.

Figure 8 shows a comparison between the SP1 and SP10 actuators. Here, it can be seen that the increase in the LiCl content led to obtain higher deformation (extension and contraction) at a low AC voltage, and also that the required activation time for the actuator was lower. Moreover, the values of the relaxation time for the SP10 samples were significantly lower than the ones of the SP1 samples. This effect can be related to the decrease in the permittivity of the hydrogels with a high LiCl content, which means the PVA hydrogel with high LiCl content can store less energy compared to the ones with a low LiCl content. The relationship between the conductivity and permittivity is as follows [33]:

σ = ε·κ

(4)

where σ is the conductivity, ε is the permittivity, and κ is the dielectric constant.

For a better understanding of the frequency function, Figure 9 presents the extension/contraction values, activation time, and relaxation time of actuators based on hydrogels of PS10 at two frequency values. As can be seen in Figure 9, for the SP10 actuator, it can be activated by applying an AC voltage of 20 V, and the deformation values were 10% and 7% for extension and contraction, respectively, under a frequency of 500 Hz. When increasing the applied voltage under a frequency of 50 Hz, the deformation values were generally higher in comparison with the deformation values that obtained under a frequency of 500 Hz. Furthermore, the activation time was significantly decreased by increasing the applied voltage. As can be seen, by increasing the frequency up to 500 Hz, the highest value of deformation can be obtained at a lower AC voltage in comparison with those deformation values obtained under a low frequency of 50 Hz. This result is related to the Joule heating effect and the presence of ions of Li⁺ and Cl⁻. In AC systems, the Joule heating effect is strongly related to the voltage drop across the material. The voltage drop is the decrease in electric potential along the path of a current flowing in a circuit. For a simple RC circuit, the relationship between the voltage drop across the bulk (ψ_eff) and the diffuse layer (ψ_D) for a supplied voltage ψ(t) can be expressed as follows [31]:

$${\displaystyle \frac{\mathrm{d}}{\mathrm{d}\left(\mathrm{t}\right)}}{\mathsf{\psi }}_{\mathrm{D}}\left(\mathrm{t}\right)={\displaystyle \frac{{\mathsf{\psi }}_{\mathrm{eff}}}{{\mathrm{C}}_{\mathrm{D}}^{\mathrm{v}}{\mathrm{R}}_{\mathrm{b}}}}={\displaystyle \frac{\mathsf{\psi }\left(\mathrm{t}\right)-{\mathsf{\psi }}_{\mathrm{D}}\left(\mathrm{t}\right)}{{\mathrm{C}}_{\mathrm{D}}^{\mathrm{v}}\frac{\mathrm{d}}{{\mathsf{\sigma }}_{\mathrm{b}}}}}$$

(5)

where C_D^v is the differential capacitance, R_b is the bulk electrolyte resistance, σ_b is the bulk conductivity of the electrolyte, and d is the distance between the electrodes.

In the reference [34], the researchers suggested that because of steric repulsion and the increased viscosity of the condensed layer surrounding the electrodes, the double-layer capacitance decreases, and the electro-osmotic mobility saturates at large voltages even for dilute bulk solutions. This means that the condensed ionic layer near the electrode surface starts to grow as the voltage drop (ψ) across the electrodes increases. Here, it should be noticed that this happens even at low frequencies. Changing the applied field frequency and magnitude, however, results in a change in the potential drop across the bulk medium, which raises the effective field strength across the electrolyte [31]. At lower frequencies, the voltage drop across the diffuse layer is greater than the bulk, whereas at higher frequencies, the voltage drop across the diffuse layer is less than the bulk, which can explain the decrease in the generated heat as a result of the Joule heating effect in the hydrogel volume when applying high frequencies of 500 Hz. In comparison to the findings in our previous work [14], hydrogel actuators based on PVA without salt addition showed the best activation results in terms of deformation (contraction/extension) and activation time when a 200 V AC (high voltage) and a 500 Hz (high frequency) were applied. This means that the absence of salt ions would not lead to an increase in the volume of the condensed ionic layer near the electrode surface under a high frequency of 500 Hz, which in turn means that the voltage drop across the diffuse layer is higher and that the generated heat as a result of Joule heating will be higher. As can be seen in Figure 9, under a frequency of 500 Hz, the maximum deformation was obtained at an AC voltage of 60 V, whereas by increasing the voltage, the deformation values decreased. Here, another effect was observed related to the occurrence of plasma (electrified gases) in hydrogel (see Video S2). At a high LiCl content and at a high applied voltage (>90 V) and frequency, the actuators had a periodic type of linear movement without obtaining high deformation, which could be related to the thickness of the condensed ionic layer near the electrode surface. However, this effect needs more investigation in order to be understood.

As can be seen in

Table 3. Electrical properties of PVA/LiCl hydrogels.

Hydrogel	PS1				PS10
Voltage, V	90		110		90		110
Frequency, Hz	50	500	50	500	50	500	50	500
Current, A	0.042 ± 0.002	0.039 ± 0.006	0.008 ± 0.001	0.009 ± 0.001	0.007 ± 0.001	0.006 ± 0.001	-	-
P1, Watt	3.78	3.51	0.88	0.99	0.63	0.54	-	-
P2 (extension), Watt	0.0056	0.0054	0.0148	0.0042	0.0294	0.0176	-	-
Efficiency (extension), %	1.48	1.45	1.68	0.42	4.67	3.27	-	-
P2 (contraction), Watt	0.0024	0.0042	0.0031	0.0049	0.0122	0.0116	-	-
Efficiency (contraction), %	0.06	0.12	0.35	0.50	1.94	2.15	-	-

, the values of the electrical efficiency of the hydrogel actuators were generally low, but in comparison with those values for PVA hydrogel actuators without the LiCl addition [14], the electrical efficiency of the PVA/LiCl hydrogel actuators was several times higher. The highest value of the electrical efficiency of the hydrogel actuators reinforced by the spiral weave was 4.67%, and it was obtained for PS10 under a voltage of 90 and a frequency of 50 Hz, whereas the highest value of the electrical efficiency of the hydrogel actuators reinforced by woven mesh braided was 2.15%, and it was obtained for PS10 under a voltage of 90 and a frequency of 500 Hz.

Table 4. Activation force of PVA/LiCl hydrogels.

Material	PS1				PS10
Voltage, V	90		110		90		110
Frequency, Hz	50	500	50	500	50	500	50	500
Activation force, kg	2.11 ± 0.18	2.59 ± 0.24	3.25 ± 0.16	4.56 ± 0.22	3.14 ± 0.16	2.56 ± 0.13	-	-
Activation force, kPa	263.59 ± 22.49	323.56 ± 29.98	406.01 ± 19.99	569.66 ± 27.45	392.27 ± 19.91	319.80 ± 16.18	-	-
Required time, sec	1.56 ± 0.23	1.42 ± 0.32	1.88 ± 0.26	1.36 ± 0.27	1.15 ± 0.12	0.75 ± 0.03	-	-

shows the obtained activation force of the PVA/LiCl hydrogels according to the procedure described in Section 2.4.2. As can be seen in Table 4, for the PS1 hydrogels, by increasing the values of the AC voltage and frequency, the activation force was increased up to 4.56 kg (570 kPa), and the required time for obtaining the maximum activation force was decreased to 1.36 s under an AC voltage of 110 V and a frequency of 500 Hz. For the SP10 hydrogels, it was also found that by increasing the frequency, the values of the activation forces were decreased. Furthermore, it should be noted that under an AC voltage of 110 V, the effect of the formation of plasma in the PS10 hydrogel (see Video S2) was observed.

4. Conclusions

This research project involved the preparation and investigation of fast-responding linear actuators based on PVA with the presence of LiCl as a conductive filler. By using AC power in accordance with the activation mechanism of the hydrogel’s water molecules, these molecules transformed into a gaseous state as a result of the heating process (Joule heating effect). Two reinforcement types, namely a spiral weave and woven mesh braided material, were used to transform the actuators’ swelling volumetric actuation into a linear state. The actuators’ deformations including contraction and extension modes, and the efficiencies and activation and relaxation times, including the obtained activation forces, were investigated depending on the LiCl content, applied voltage, and frequency under an approximately 20 kPa load. It was found that, by increasing the LiCl content, the required AC voltage to activate the actuators was decreased as a result of the increase in the hydrogel’s electrical conductivity. Moreover, it was found that the extension/contraction values of the hydrogel actuators were better under a low frequency of 50 Hz. In the extension mode, it was found that the best results for the actuators were obtained based on PS1 hydrogel, and that in ~2.5 s of activation time they can reach a 52% deformation value after the application of 110 V AC at a 50 Hz frequency, while in the contraction mode, the best results for the actuators were obtained based on the PS3 hydrogel; in ~2.2 s of activation time, they can reach a 20% deformation value after the application of 90 V AC at a 50 Hz frequency. In addition, the minimum required AC voltage to activate the hydrogel actuator with a LiCl content of the same PVA mass content was 20 V. Furthermore, the activation force of the prepared actuators can be more than 0.5 MPa under an AC voltage of 110 V. Among the most intriguing findings—which require further research to fully understand—is the presence of plasma, or electrified gases, in highly concentrated hydrogel containing LiCl at high AC voltages (>90 V), where the actuators exhibited a periodic kind of linear movement without experiencing significant deformation.

These prepared PVA/LiCl hydrogel-based actuators have a wide range of applications in the soft robotics, artificial muscles, medicine, and aerospace sectors (see Video S3).