Table 1.

Stimuli-responsive hydrogels with their key features, properties, and applications.

Type of Hydrogels	Examples	Key Features	Properties	Applications	References
pH-responsive	Chitosan, guar gum succinate, kappa-carrageenan, PEI, PAM, PAA, PDEAEMA, PDMAEMA, PEAAc, pHEMA, PMAA, PPAA, and PVA	pH variation results in swelling/deswelling behavior due to the changes in hydrophobicity of the polymeric chains and increase in electrostatic repulsion between chains	Biocompatibility, sustained release of incorporated drugs, increased hydrophilicity, and swelling, strong electrostatic interactions, and stability	Drug delivery, Sensing, inflammation responsive hydrogels, wound and skin healing.	[19,20]
Temperature responsive	Poloxamer, Pluronic, PAA, PNIPA, PNVCL grafted with PEO, TMC crosslinked with PEG, glycerophosphate, and methoxy poly(ethylene glycol)-poly(pyrrolidone-co-lactide)	Temperature variation disturbs the equilibrium exists between hydrophobic and hydrophilic segments of the polymeric chain and increase the sol-gel transformation rate	Unique physical properties similar to the extracellular matrix, easy functionalization with drug molecules, controlled degradation	Drug delivery, intraocular lenses, tissue engineering.	[21,22,23]
Electric field responsive	PPy nanoparticles loaded in PLGA, PEG hydrogels, Agarose, calcium alginate, carbomer, chondroitin sulphate, hyaluronic acid, partially hydrolyzed PAM, PDMA, and xanthan gum	Upon the application of an electric field, deswelling or bending takes place, based on the shape and position of the gel relative to the electrodes.	Biocompatibility, minimal invasiveness, controlled release of the cargo depending on the strength or the duration of applied electric field	Drug delivery, creams and suspensions as emulsion stabilizer, in cosmetics as thickener and stabilizer, buccal delivery.	[24,25,26]
Magnetic field responsive	Alginate-xanthan cross-linked with Ca2+ magnetic nanoparticles, Hemicellulose crosslinked with GGM, hemicellulose hydrogels with magnetic iron oxide (Fe3O4), methacrylate chondroitin sulfate with magnetic nanoparticles, PNIPA, and xanthan-bovine serum albumin-magnetic nanoparticles	Application of heating, mechanical deformation, or external magnetic field to magnetic nanoparticles, such as nanoparticles of magnetite, maghemite, and ferrite	Swelling behavior responsive to temperature too, some of them dispose of anisotropic properties, successful absorption and controlled release of drugs	Drug delivery, sensing, microfluidics, tissue engineering.	[27,28,29]
Light responsive	Poly [2-((4,5-dimethoxy-2-nitrobenzyl) oxy)-N-(2-(methacryloyloxy)ethyl)-N,N-dimethyl-2-oxoethan-1-aminium, HPMC, Carbopol hydrogels containing diclofenac-sodium chitosan microspheres, Azo benzene-pHEMA, azo benzene-bovine albumin, triphenylmethane leuco derivatives, and trisodium salt of copper chlorophyllin-PNIPAM23	External stimulus of either visible or UV light initiates sol-gel transformation	Control release, reversible and irreversible, spatiotemporal control over functional groups, reasonable strengthens according to application.	Drug delivery, optical delivery, microfluidics, self-sterilization and self-cleaning.	[30,31,32,33]
Biomolecules responsive	Insulin, phenylborate derivative 4-(1,6-dioxo-2,5-diaza-7-oxamyl) phenylboronic acid in combination with PNIPA, and poly(2-hydroxyethyl methacrylate-co-N,N-dimethylaminoethyl methacrylate) in combination with glucose oxidase	Changes in biomolecule concentration and pH in hydrogel as a self-regulated, can expand the polyelectrolytes resulting in swelling/deswelling behavior.	Enzyme responsive, achieves molecular recognition, high affinity, and specificity, controlled release, biocompatibility.	Drug delivery, insulin-delivery system, cell culture, sensing, tissue engineering.	[34,35,36]

Abbreviations (pH-responsive hydrogels): Poly(ethyleneimine) (PEI); Poly(acrylamide) (PAM); Poly(acrylicacid) (PAA); Poly(diethylaminoethyl methacrylate) (PDEAEMA); Poly(dimethylaminoethyl methacrylate) (PDMAEMA); Poly(ethylacrylic acid) (PEAAc); Poly(hydroxyethyl methacrylate) (pHEMA); Poly(methacrylic acid) (PMAA); Poly(propylacrylic acid) (PPAA); Poly(vinyl alcohol) (PVA). Abbreviations (temperature-responsive hydrogels): Poly(N-isopropylacrylamide) (PNIPA); Poly(N-vinyl caprolactam) (PNVCL); Poly(ethylene oxide) (PEO); N-trimethyl chitosan chloride (TMC); Poly(ethylene glycol) (PEG). Abbreviations (electric field-responsive hydrogels): Polypyrrole (PPy); Poly lactic-co-glycolic acid (PLGA); Polydimethylaminopropyl acrylamide (PDMA). Abbreviations (magnetic field-responsive hydrogels): O-acetyl-galactoglucomannan (GGM); Poly(N-isopropylacrylamide) (PNIPA). Abbreviations (light field-responsive hydrogels): Hydroxypropyl methylcellulose (HPMC); Poly(N-isopropylacrylamide) (PNIPAM).