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Zhao, Z.; Hu, Y.; Liu, K.; Yu, W.; Li, G.; Meng, C.; Guo, S. Ionic Hydrogels. Encyclopedia. Available online: https://encyclopedia.pub/entry/42760 (accessed on 21 June 2024).
Zhao Z, Hu Y, Liu K, Yu W, Li G, Meng C, et al. Ionic Hydrogels. Encyclopedia. Available at: https://encyclopedia.pub/entry/42760. Accessed June 21, 2024.
Zhao, Zhen, Yong-Peng Hu, Kai-Yang Liu, Wei Yu, Guo-Xian Li, Chui-Zhou Meng, Shi-Jie Guo. "Ionic Hydrogels" Encyclopedia, https://encyclopedia.pub/entry/42760 (accessed June 21, 2024).
Zhao, Z., Hu, Y., Liu, K., Yu, W., Li, G., Meng, C., & Guo, S. (2023, April 04). Ionic Hydrogels. In Encyclopedia. https://encyclopedia.pub/entry/42760
Zhao, Zhen, et al. "Ionic Hydrogels." Encyclopedia. Web. 04 April, 2023.
Ionic Hydrogels
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Hydrogels are three-dimensional polymer networks with excellent flexibility. Ionic hydrogels have attracted extensive attention in the development of tactile sensors owing to their unique properties, such as ionic conductivity and mechanical properties. These features enable ionic hydrogel-based tactile sensors with exceptional performance in detecting human body movement and identifying external stimuli. Currently, there is a pressing demand for the development of self-powered tactile sensors that integrate ionic conductors and portable power sources into a single device for practical applications.

ionic hydrogels self-powered sensors piezoelectric

1. Ionic Conductivity

Due to their 3D framework structure and continuous water phase, hydrogels offer a large number of pathways for ion movement. This opens up the possibility of synthesizing good ionic conductive hydrogel materials. Ionic hydrogels have so far been created for flexible electronic devices using a variety of ionic conductive materials, such as electrolytes [1], polyelectrolytes that constitute networks of hydrogels [2], and ionic liquids [3][4] that dissolve in the solvent phase. They increase the hydrogels’ resistance to freezing in addition to providing conductivity. In the meantime, ions typically improve the mechanical characteristics of hydrogels by interacting with polymer chains through processes such as the salting-out effect or coordination [5].
One easy and quick method to make an ionic conductive hydrogel is by directly doping the hydrogel with soluble inorganic salts such as LiCl [6] and NaCl [7]. Because hydrogels have a high water content and a microporous structure, resolvable salts can disperse easily, allowing the hydrogels to conduct ions. Polyelectrolytes are polymers containing ionic moieties in their repeating units [8], and they can be crosslinked chemically or physically to produce polyelectrolyte hydrogels. Polysaccharide-based polymers (alginate [9], chitosan [10], carrageenan [11]), polypeptide/protein-based polymers (gelatin [12], collagen [13]), and synthetic polymers (polyacrylic acid [14], poly(sulfobetaine) [15], carboxymethyl cellulose [16]) can be used to create polyelectrolyte hydrogels. Ionic liquids (ILs) are molten salts with melting temperatures under 100 °C that are made up of organic cations and organic or inorganic anions. In terms of conductivity and stability, ILs are a promising class of small-molecule liquids for providing free ions. By adjusting the loading amount and creating an appropriate network structure for hydrogels, ionic conductive materials can impart proper conductivity to hydrogels.
Guo et al. [17] obtained poly(amidoxime)/polyethyleneimine (PAO/PEI) hydrogel through hydrogen bond interactions. PAO with amidoxime groups has outstanding ionic adsorption properties, which results in an ultrahigh ionic conductivity of 19.1 S m−1 in 6 M LiCl for the PAO/PEI hydrogels. This hydrogel with high ionic conductivity is expected to have a broad application prospect in wearable sensing devices.
Yang et al. [18] synthesized a zwitterionic hydrogel through the random copolymerization of a zwitterionic monomer (SBMA) and 2-hydroxyethyl acrylate (HEA) with the addition of LiCl salt. The anionic and cationic counterions present on the zwitterionic chains promote the dissociation of LiCl, leading to a high room-temperature ionic conductivity of 14.6 S m−1.
Yao et al. [19] developed an ionic conductive hydrogel platform through a simple one-step method by incorporating cellulose nanofibrils (CNFs) into a crosslinked network of phenylboronic acid-ionic liquid (PBA-IL) and acrylamide. By increasing the PBA-IL content, the ionic conductivity of the hydrogels was enhanced from 0.92 ± 0.10 to 6.94 ± 0.21 mS cm−1, rendering them attractive soft sensing materials.
Ionic hydrogels generally exhibit relatively low conductivity due to the delicate balance between ion concentration and water content in the hydrogels. To fulfill the need for improved performance in sensing devices, continuous attempts have been made to produce hydrogels with enhanced ionic conductivity. The distinct attributes of the ionic hydrogels will serve as a crucial reference guide for the development of the next generation of wearable electronics and electronic skin.
It should be emphasized that ionic hydrogels conduct electrical charges through the migration of mobile ions and the “ion–electron” conversion on the electrode surfaces. As a result, AC mode is typically used to test the electrical resistance or impedance of ionic conductors. When an ionic hydrogel is in contact with a metal surface, mobile ions and electrons come together to create an electric double layer (EDL) at the interface. The EDL performs the role of a capacitor by coupling the electronic current in the metal and the ionic current in the hydrogel [20].

2. Mechanical Properties

Excellent mechanical strength of soft conductive materials is essential for wearable tactile sensors in order to maintain excellent structural integrity and reliable signal monitoring capability under complex loads and repeated large deformations. Since ionic hydrogels often have weak but adaptable mechanical characteristics and excellent stretchability, they have shown to be an attractive alternative for the creation of flexible tactile sensors. As a result, significant efforts have been devoted to improving the mechanical properties of these hydrogels, especially for applications such as ionotronic skin (i-skin).
The most commonly employed method for producing tough and highly conductive hydrogels involves creating a double-network (DN) structure that utilizes reversible crosslinking [21]. A DN is made up of two networks: a second network that is soft and ductile and a first network that is rigid and brittle [22]. Sun et al. [23] developed a DN ionic conductive hydrogel that possesses impressive mechanical properties and conductivity. The hydrogel was created by immersing a virgin gellan gum/gelatin composite hydrogel in a mixed solution of Na2SO4 and (NH4)2SO4. This hydrogel exhibits adjustable Young’s modulus (ranging from 0.08 to 42.6 MPa), fracture stress (0.05 to 7.5 MPa), fracture stretch (1.4 to 7.1), high fracture toughness (up to 27.7 kJ m−2), and superior ionic conductivity (up to 11.4 S m−1 at f = 1 kHz). The enhancement in mechanical properties of the DN gel is attributed to the introduction of chain-entanglement crosslinking points by SO24SO42− in the gelatin network and electrostatic interaction crosslinking points by Na+ in the gellan gum network. Meanwhile, soaking in salt solutions is an important way to strengthen DN conductive hydrogels.
Polyols were employed as effective physical crosslinking agents to enhance the strength and toughness of ionic hydrogels. Peng et al. [24] described the fabrication of a PVA-NaCl-glycerol hydrogel through the incorporation of glycerol and NaCl into an aqueous solution of PVA, followed by storing at room temperature for 2.0 h. Benefiting from the hydrogen bonding and salting-out effect, the PVA-NaCl-glycerol hydrogels demonstrated exceptional toughness, exhibiting a tensile strength of 0.57 (±0.02) MPa and an elongation at break of 575 (±29)%. Moreover, these hydrogels exhibited remarkable conductivity, reaching as high as 92.5 (±0.18) mS/cm.
Ionic hydrogels show more suitable mechanical adaptability to epidermal or muscle tissues than conventional polymeric materials [25]. Hence, there is a need to dedicate further efforts towards advancing the development of ionic hydrogels in order to enhance the performance of tactile sensors.

3. Freezing Resistance

Generally, a traditional hydrogel composed of a pure hydrophilic system inevitably freezes at temperatures below zero; the ion transmission is blocked, and the hydrogel is hardened and brittle, which seriously limits its application in the low temperature range [26][27]. Therefore, ionic hydrogel-based flexible tactile sensors with reliable antifreezing performance have received a lot of attention. In order to solve these problems, the current antifreezing strategy is to incorporate different antifreezing additives, such as ionic compounds, natural biopolymers, and organic/aqueous solvents into the polymer network, which help to change the water–ice phase equilibrium at different stages of ice nucleation and growth [28][29][30].
Inorganic salts such as CaCl2, ZnCl2, and LiCl and ionic liquids or poly(ionic liquids) are widely used as water freezing inhibitors to be introduced into hydrogels to increase their low-temperature resistance. For example, Wang et al. [31] synthesized a zwitterionic composite hydrogel with antifreezing and water retention properties, which are attributed to the incorporation of LiCl as an electrolyte and antifreeze agent. Electrostatic dissociation of LiCl with the zwitterions contributed to the high conductivity of the composite hydrogel (7.95 S m−1) and excellent antifreeze performance, reaching as low as −45.3 °C. At the same time, due to the presence of salt ions, the composite hydrogel was observed to retain 97% of its initial water content after exposure to air (25 °C, 55% RH) for one week.
Another approach is to introduce organic solutions such as ethylene glycol (EG), sorbitol, DMSO, and glycerol into the hydrogel. For example, Yu et al. [27] prepared high-ionic-conductivity hydrogels with excellent antifreeze and dehydration resistance by immersing cellulose nanofibril (CNF)-reinforced hydrogels in CaCl2/sorbitol solution for solvent replacement. The synergistic effect of sorbitol and CaCl2 makes the hydrogels exhibit excellent freezing resistance, dehydration resistance, and ionic conductivity. Strong hydrogen bonds between water and sorbitol molecules prevent the formation of ice crystals and the evaporation of water, giving CS-NC hydrogels very-low-temperature resistance to −50 °C and excellent dehydration resistance, with weight retention in excess of 90%. Wang et al. [28] introduced a water-retaining antifreeze ionic conductive hydrogel composed of silk fibroin, ionic liquid, water, and inorganic salts. Silk fibroin (SF)/1-ethyl-3-methylimidazolium acetate (EMImAc)/H2O/KCl-based hydrogel electrolytes work well at temperatures as low as −50 °C and after prolonged storage outdoors.
The inorganic salts used to create the antifreezing hydrogels have good ionic conductivity, but when added in large quantities, the inorganic salts can compromise the hydrogels’ mechanical strength. On the other hand, in the presence of organic solvents, the ionic conductivity of the hydrogel will be greatly reduced due to the reduced ion dissociation in organic solvents and limited ion mobility with enhanced crosslinking density. In addition, organic solvents are known to be environmentally hazardous and raise health and safety issues [29]. Recently, Zhang et al. [30] introduced and demonstrated a comprehensive crosslinking approach for 4,9-dioxo-5,8-dioxa-3,10-diazadodecane-1,12-diyl diacrylate (EGINA) crosslinked double-network hydrogels that exhibit inherent antifreezing properties. The antifreezing mechanism is solely derived from the formation of tightly bound water with networks via hydrogen bonds and the confinement of the water in the tightly crosslinked DN structure. In summary, antifreezing hydrogels are capable of retaining their ionic conductivity and mechanical properties even at low temperatures, making them ideal for use in extreme environments. With the continuous development of novel materials and advanced crosslinking strategies, antifreezing hydrogels are expected to achieve even higher performance and expand their range of applications.

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