Fundamental Concepts of Hydrogels: Synthesis, Properties, and Their Applications

Abstract

In the present review, we focused on the fundamental concepts of hydrogels-classification, the polymers involved, synthesis methods, types of hydrogels, properties, and applications of the hydrogel. Hydrogels can be synthesized from natural polymers, synthetic polymers, polymerizable synthetic monomers, and a combination of natural and synthetic polymers. Synthesis of hydrogels involves physical, chemical, and hybrid bonding. The bonding is formed via different routes, such as solution casting, solution mixing, bulk polymerization, free radical mechanism, radiation method, and interpenetrating network formation. The synthesized hydrogels have significant properties, such as mechanical strength, biocompatibility, biodegradability, swellability, and stimuli sensitivity. These properties are substantial for electrochemical and biomedical applications. Furthermore, this review emphasizes flexible and self-healable hydrogels as electrolytes for energy storage and energy conversion applications. Insufficient adhesiveness (less interfacial interaction) between electrodes and electrolytes and mechanical strength pose serious challenges, such as delamination of the supercapacitors, batteries, and solar cells. Owing to smart and aqueous hydrogels, robust mechanical strength, adhesiveness, stretchability, strain sensitivity, and self-healability are the critical factors that can identify the reliability and robustness of the energy storage and conversion devices. These devices are highly efficient and convenient for smart, light-weight, foldable electronics and modern pollution-free transportation in the current decade.

Keywords: applications; hydrogel electrolytes; hydrogels; natural and synthetic polymers; properties; synthesis of hydrogels.

Figure 1

Classification of hydrogels.

Figure 2

The reaction mechanism of the poly (acrylic acid) hydrogel [38].

Figure 3

Preparation of vinyl hybrid silica nanoparticles (VSNPs) from vinyl-triethoxysilane nanoparticles followed by the synthesis of VSNP-PAM from VSNPs (crosslinking agent), ammonium persulfate (APS, initiator), acrylamide (AM, main monomer), and phosphoric acid (proton source) [11].

Figure 4

Schematic of the synthesis route to form solid-state electrolytes by grafting PAM on cellulose nanofibers (CNFs) via a facile free radical polymerization approach [40].

Figure 5

(a) Synthesis mechanism of poly (N-isopropylacrylamide)/mesoporous silica nanoparticle (PNIPAM/MSN) composite hydrogels; (b) digital photographs showing the PNIPAM/MSN-0, PNIPAM/MSN-1, and PNIPAM/MSN-5 hydrogels at 20 °C and 40 °C [48].

Figure 6

Dual physically crosslinked pectin-Fe³⁺ ion/hydrophobically modified acrylamide hydrogels under ultraviolet light in three steps [74].

Figure 7

Synthetic routes for chitosan hydrogel preparation and schematic illustrations of the expected network structures of RP_L (radical polymerization at low UV intensity) and RP_H (left) radical polymerization at high UV intensity) or RP_CT (chain-transfer radical polymerization) and SP (step polymerization) (right). Open circles represent the point at which the chain moves out of the plane. Red and green linkages indicate inter- and intra-chain crosslinking, respectively [88].

Figure 8

The plausible mechanism of carboxymethyl cellulose-g-poly (acrylamide-co-acrylic acid-co-2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS)/montmorillonite (MMT)) hydrogel synthesis [95].

Figure 9

Proposed reaction mechanism of interpenetrating network (IPN) formation: (a) simultaneous strategy; (b) sequential strategy; (c) selective crosslinking of a linear polymer entrapped in semi-IPN [96].

Figure 10

The proposed reaction mechanism of a semi-IPN hydrogel of chitosan/acrylamide-g-hydroxyethyl cellulose [100].

Figure 11

(a) Schematic structure of the ACC/PAA/alginate mineral hydrogel. (b) SEM image of the freeze-dried ACC/PAA/alginate hydrogel. (c) Frequency dependencies of the storage (G’) and loss (G″) moduli of the ACC/PAA/alginate and ACC/PAA hydrogels. (d) The ACC/PAA/alginate hydrogel can be manipulated into various shapes. (e) When a hydrogel film is attached to a prosthetic finger, it dynamically adapts to the highly nonlinear surface and accommodates the finger movements [139].

Figure 12

(a) Mechanical strength of the hydrogels, (b) strain amplitude test, and (c) frequency sweep study [140].

Figure 13

(a) Illustration of the fabrication of a crosslinked conducting polymer hydrogel (CPH) following the addition of tannic acid (TA), polypyrrole (Py), and Fe³⁺ ion. TA acts as a crosslinking agent and dopant and Fe³⁺ ion acts as an oxidant and ionic crosslinking agent. (b) Graphical representation of a “C”-shape, semi-tubular CPH that was implanted as a bridge to cover the spinal cord hemi-section gap. (c) Locomotor recovery of the animals was measured using the standard BBB scale in an open field; ** p < 0.01, **** p < 0.0001; n = 5 animals in each group. Error bars represent the standard deviation [150].

Figure 14

(a) Synthesis of polyacrylamide/chitosan (PAM/CS) interpenetrating (PAM/CS IPN) hydrogels; (b) polypyrrole was absorbed into the IPN hydrogel, fixed on CS chains, and accumulated in the zone of CS entanglement; (c) polypyrrole (PPy) was polymerized in situ in the hydrogel under the controlling of CS molecular templates; PPy-conductive pathway (purple line) intertwisted along CS chains (purple line), and PPy nanorods aggregated on the chain entanglement zone of CS hydrogels for repairing full thickness defects on rats. (d) Schematics of hydrogel implantation and conductive properties of the hydrogel. (e) H&E (histomorphological evaluation) staining of wound sections after 21 days; S, sample; BV, blood vessel. (f) PPy–PAM/CS hydrogel was connected to a circuit and illuminated an LED; (g) conductivity of the PPy–PAM/CS hydrogel with different contents of PPy [151].

Figure 15

(a) Morphology of karaya gum-g-poly (acrylic acid) hydrogel, (b) swelling–deswelling–reswelling at different pH levels, and (c) swelling–deswelling–reswelling in salt solutions [161].

Figure 16

Swelling–reswelling at different pH levels, in salt solutions, morphology, and water uptake of hydrogels [162].

Figure 17

Environmental stimuli sensitive to hydrogels [163].

Figure 18

(a) Synthesis and supercapacitor assembly containing prepared covalently carboxylated chitosan hydrogel electrolytes [178]. (b) Synthesis of physically crosslinked cellulose hydrogel electrolytes and zinc ion hybrid supercapacitor assembly [180].

Figure 19

The comparison of (a) tensile strength and elongation at break and (b) toughness of HA-GPE-x. (c) Digital images of HA-GPE-1M with high flexibility (left) and stretchability (right). Cycling (d) tensile stress–strain and (e) compressive stress–strain curves of HA-GPE at a strain of 400% and 95% and corresponding recovery ratio. (f) Tensile stress–strain curves of HA-GPE-x with different LiClO₄ concentrations. (g) Cyclic voltammetry (CV) curves at normal and varied bending angles (45°, 90°). (h) Cycling performance collected for the current density of 1 Ag⁻¹ (black: capacity retention; red: Coulombic efficiency as a function of cycle number) [192].

Figure 20

(a) Stretching ability of sodium alginate/poly (acrylamide)/poly (acrylic acid)/zinc sulfate (SA-Zn) hydrogel electrolytes. (b–d) Flexibility demonstration of SA-Zn hydrogel electrolytes. (e-I) Schematic of the structure and three key parameters (θ: bending angle, R: bending radius of curvature, and L: length of the device) that are used to demonstrate the bending state of H-ZHS. (e-II) Capacity retentions of H-ZHS at different bending angles (0–180°), fixed length (2 cm), and bending radius (0.15 cm). (e-III) Capacity retentions of H-ZHS upon 100 bending cycles at fixed bending angle (90°), bending radius (0.15 cm), and length (2 cm). (f) Photographs of an electrical watch powered by H-ZHS at different bending angles (0–180°). (g) An electrical watch and (h) a green LED powered by H-ZHS compressed by a 200-g load. (h) Photographs of H-ZHS attached on a wristband to power an electrical watch [179].

Figure 21

(a) Optical images showing self-healing of the PVA-g-PAA/KCl pieces under ambient conditions. (b) Fluorescence microscopy images of the electrolytes after healing for (1) 0 min, (2) 20 min, and (3) 60 min. (c) Effect of healing time on the mechanical properties and mechanical healing efficiency of the PVA-g-PAA/KCl electrolytes; the grafting amounts of PAA and KCl contents were 3.5 wt.% and 200 mM, respectively. (d) Ionic conductivity of the electrolytes after different cutting/healing cycles; the inset shows the mechanical properties after the 1st and 15th cutting/healing cycles. (e) Effect of urea and glucose treatments on the mechanical properties of the healed electrolytes. (f) Schematic illustration of the self-healing mechanism [195].

Figure 22

(a) Schematic illustration of the internal crosslinking effect of the KCl–Fe³⁺/PAA hydrogel. (b–d) Self-healing of a dumbbell-shaped hydrogel at room temperature: (b) the hydrogel after being cut, (c) the hydrogel after healing for 12 h, and (d) the hydrogel after healing for 24 h. (e) Stretching the self-healed hydrogel up to 200%. (f) Comparison of stress–strain curves between the original and self-repaired KCl–Fe³⁺/PAA hydrogel. (g) CV curves of the KCl–Fe³⁺/PAA F-supercapacitor and KCl L-supercapacitor at a scan rate of 5 mV s⁻¹. (h) Comparison of the charge/discharge profile between the KCl–Fe³⁺/PAA F-supercapacitor and KCl L-supercapacitor at a current density of 0.5 A g⁻¹. (i) Comparison of the Nyquist impedance plot of the KCl–Fe³⁺/PAA F-supercapacitor and KCl L-supercapacitor [196].

Figure 23

(a) Mechanical healing efficiency and ionic conductivity of the electrolyte after multiple cut/healing cycles. (b) Effect of urea and fructose treatments on the mechanical properties of the healed pieces. (c) Illustration of the self-healing mechanism of the electrolyte. (d) Optical images of self-healed hydrogel electrolyte. (e) GCD profiles at 1.0 A g^–1 (f) CVs at 100 mV s^–1, (g) Four capacitors were connected in series to light up LED bulb through cut/healing operations. (h) Electrochemical performances of the capacitor at low temperature, CVs at 100 mV s^–1, (i) GCD profiles at 1.0 A g^–1 [197].

Figure 24

(a) Fabrication process of CNT-CNF/PVAB composite gels. (b) Multi-complexation between CNT-CNF nanohybrids and PVA dynamically cross-linked by borax. (c) G’ and G″ curves as a function of ω. (c) η* and G* versus angular frequency ranging from 0.1 to 100 rad s−1. (d) CNT-CNF/PVAB-2 hydrogel being stretched to more than 300% of the initial length. (e) The G″ and G′ versus time in continuous step strain of CNT-CNF/PVAB-2 composite gel. (f) Illustration of CNT-CNF/PVAB and CNF/PVAB hydrogels being completely merged. (g) Illustration of self-healing behavior for CNF/PVAB and the interfacial permeation during seal-healing. (h) Illustration of healing in situ for CNT-CNF/PVAB-2 gel and self-healing mechanism of composite gels [203].

Figure 25

(a) AAM3 hydrogel electrolytes before cutting, after cutting, and self-healed; (b) supercapacitor after cutting into two pieces; (c) rejoining of cut pieces; (d) graphite electrode broken ends connected using carbon tape; (e) voltage stored after charging; (f) discharging of two supercapacitors connected in series through LED. (g) Structural illustration of self-healing mechanism in poly (acrylamide) hydrogel [41].

Figure 26

Electrochemical performance of the flexible lithium ion battery under various deformation conditions. (a) Galvanostatic charge/discharge under different bending angles. (b) Demonstration of bending at different angles. (c) Powering an electronic watch when being (c-i) squeezed, (c-ii) twisted, and (c-iii,c-iv) folded. (d) Capacity retention of the battery after 500 bending cycles at a bending angle of 60° (C₀ and C correspond to the specific capacity before and after bending, respectively) [205].

Figure 27

Compressibility of the poly (acrylamide) hydrogel electrolyte (PAAm). (a) Images showing the elasticity of hydrogel under compressional and relaxed states; (b) resistance values of the poly (acrylamide) hydrogel electrolyte under different compressional strain values from 0 to 77.8%; (c,d) pictures showing the conductivity of hydrogel electrolyte under relaxed and compressional states with the ability to light a yellow light-emitting diode (LED) bulb; (e) optical images showing that two rechargeable Zn–MnO₂ batteries with poly (acrylamide) hydrogel electrolytes could be used for powering a luminescent panel under normal condition and with a 3-kg load on it; (f) comparison between the signals generated by the flexible sensor powered by the commercially available alkaline batteries and our compressible batteries without and with q load on top of it; (g) flexible smart wristband integrated from two ZIB modules and a flexible pressure sensor; (h) sensory signals of the smart wristband generated by human finger touch under different pressures on the device; (i) sensory signals of the smart wristband generated at different frequencies, from 0.3 to 4 Hz, by human finger touch [208].

Figure 28

(a) Tensile stress–strain curves of the as-synthesized PANa and PANa-cellulose hydrogel electrolytes with and without 300% 6 M KOH + 0.2 M Zn (CH₃COO)₂ intake. The insets are optical photos of the relaxed and elongated states of the 300% 6 M KOH + 0.2 M Zn (CH₃COO)₂ solution-incorporated PANa-cellulose hydrogel electrolytes showing excellent stretchability; (b) comparison of tensile properties of poly (acrylic acid) PAA, poly (acrylamide) PAM, sodium polyacrylate, PANa and PANa-cellulose hydrogel under alkaline condition. The inset is the photos of PAA, PAM, PANa and PANa-cellulose hydrogel at initial state and containing 300% 6M KOH solution for 8 h. The red and blue rectangles represent the shape of hydrogel before and after infiltrating alkaline solution, respectively; (c) schematic diagram reflecting structure of PANa-cellulose hydrogel electrolyte entrapped KOH and water via the interactions of hydrogen bonds; (d) schematic illustration of 800% stretchable flat-shape zinc–air battery; (e) polarization curves; (f) corresponding power density curves of the flat-shape highly stretchable zinc–air battery with a strain from 0 to 800%; (g) maximum power density as a function of the tensile strain. The insets are the photographs of the flat-shaped zinc–air battery at a fully released state and 800% strain; (h–k) flat-shape zinc–air battery is subjected to different mechanical deformations sequentially and (l) released; (m) galvanostatic discharge–charge cycling curves at a current density of 5 mA·cm⁻² and (n) corresponding discharging-charging voltage plateau at different stretching strains [210].

Figure 29

(a) The schematic diagram of a quasi-solid-state quantum dot-sensitized solar cell (QDSSC) device from a CdS/CdSe-sensitized TiO₂ photoanode and conducting hydrogel electrolytes [222]. (b) On–off switches and (c) photocurrent stability of the gel electrolyte-tailored QDSSCs [223].