Hydrogel: Diversity of Structures and Applications in Food Science

Figures & data

Table 1. Classification of hydrogels

Classification basis	Hydrogel types	Classification basis	Hydrogel types	Classification basis	Hydrogel types
Polymer source	Natural hydrogels Synthetic hydrogels Hybrid hydrogel systems	Configuration	Amorphous (non-crystalline) hydrogels Semicrystalline hydrogels Crystalline hydrogels	Anisotropy	Homogeneous/random hydrogels Oriented structural hydrogels
Polymeric composition	Homopolymeric hydrogels Copolymeric hydrogels Multipolymer hydrogels: Interpenetrating network (IPN) hydrogels Double network (DN) hydrogels	Physical appearance and size	Macroscopic hydrogels (matrix, film, etc., typically on the order of millimeters to centimeters) Microgels (microspheres, 100 nm–10 μm) Nanogels (nanoparticles, 10–100 nm)	Electrical charge	Nonionic (neutral) Ionic (anionic or cationic) Amphoteric electrolyte (ampholytic) species containing both acidic and basic groups Zwitterionic (polybetaines) containing both anionic and cationic groups in each structural repeating unit
Crosslinking mechanisms	Physical crosslinking (noncovalent interactions): electrostatic interactions hydrogen bonds crystallization metal-ligand coordination stereocomplex crystallization hydrophobic interactions conformation transformation host-guest interactions molecular-specific binding π-π stacking Chemical crosslinking (covalent junctions) Via monomers Via polymers Enzymatic crosslinking (covalent junctions) Multi-crosslinking			Functions	Smart hydrogels Self-healing/self-recovery hydrogels Injectable hydrogels Strong adhesive hydrogels High strength hydrogels Superabsorbent hydrogels Bionic hydrogels
				Stimuli response	Conventional hydrogels (non-stimuli responsive) Responsive (smart) hydrogels: Physical stimuli: temperature, magnetic field, light, electric field, pressure, and sound Chemical stimuli: pH, solvent composition, ionic strength, and molecular species

Figure 1. ‘Egg-box’ structure of hydrogel formed between G residues of alginate and Ca²⁺. Dimensions are not drawn to scale

Figure 2. Schematic of the micellar copolymerization of HA-gels. Dimensions are not drawn to scale

Figure 3. Schematic diagram of κ-carrageenan hydrogel formation via aggregation of helices. Dimensions are not drawn to scale

Figure 4. Structures of polyrotaxane and pseudo-polyrotaxane formed via CD-based host-guest interactions

Figure 5. Formation of polyrotaxane hydrogels from α-CD and high molecular weight PEG

Figure 6. Schematic diagram of the four common free-radical polymerization methods. Dimensions are not drawn to scale

Figure 7. Schematic diagram of the hydrogels prepared via UV-induced free-radical polymerization. (a) Solution polymerization pathway, and (b) emulsion/micellar polymerization pathway. Dimensions are not drawn to scale

Figure 8. Illustrations of core-shell-structured hydrogels

Figure 9. Different types of multi-layered hydrogels

Figure 10. Three different hydrogel-nanoparticle structural designs exist: a) micro- or nano-sized hydrogel particles stabilizing inorganic or polymer nanoparticles, b) nanoparticles non-covalently immobilized in a hydrogel matrix, and c) nanoparticles covalently immobilized in a hydrogel matrix. Reproduced from ref. ^[430] with permission. Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 11. Five main approaches used to obtain hydrogel-nanoparticle conjugates with uniform distributions: 1) hydrogel formation in a nanoparticle suspension, 2) physically embedding the nanoparticles into a hydrogel matrix after gelation, 3) reactive nanoparticle formation within a preformed gel, 4) crosslinking using nanoparticles to form hydrogels, 5) gel formation using nanoparticles, polymers, and distinct gelator molecules. Reproduced from ref. ^[430] with permission. Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 12. Schematic illustrations of an MMC hydrogel structure. The MC hydrogel structure is similar to that of the MMC hydrogel structure as long as the macromolecular microsphere is replaced by a microgel

Figure 13. Main possible interactions contributing to the formation and toughening of MMC hydrogels. Reproduced from ref. ^[452] with permission. Copyright 2013 American Chemical Society

Figure 14. Schematic illustrations of an SIPN (A) and a FIPN (B)

Figure 15. Schematic illustrations of an AB-type-crosslinked polymer

Figure 16. Schematic of the in situ and sequential pathways used to prepare IPNs and semi-IPNs. Reproduced from ref. ^[464] with permission. Copyright 2011 Elsevier

Figure 17. Topological hydrogel structure of typical figure-eight crosslinked networks

Figure 18. Schematic illustration of tetra-PEG hydrogels

Figure 19. Schematic representation of emulsion-filled gels and emulsion particulate gels. Reproduced from ref. ^[508] with permission. Copyright 2019 Elsevier

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