Table 1.

Physical crosslinking methods for algal polysaccharides-based hydrogel preparation.

Crosslinking	Materials	Important reagents	Gelation	Properties	Ref.
Ionic interaction	Alginate/alginate sodium (brown algae such as Laminaria hyperborea and lessonia)	Divalent cations-containing solutions, such as calcium chloride, zinc chloride, etc.	Divalent cations interacted with their carboxyl groups to form intermolecular crosslinking “egg-box” structures	Biocompatible, biodegradable, divalent cations-affected mechanical properties	[124,125,126,128,129,130]
	Alginate/Sodium alginate (brown algae) Chitosan derivatives	5% acetic acid or other dilute organic/inorganic acids	Anionic sodium alginate interacted with cationic chitosan derivatives	pH-sensitive, biocompatible, biodegradable, high capacity to bind heavy metal ions, acidic gas, and basic gas	[130,131,132]
	Fucoidan (brown algae Fucus vesiculosus) Collagen Chitosan	-	Ionic electrostatic interactions between the positively charged groups of collagens and/or chitosan and the negatively charged groups of fucoidan	Degradable, biocompatible	[54]
	Lambda-carrageenan (red algae Sarcothalia lanceata)	Aluminium(III) chloride/iron(III) chloride/iron(III) chloride hexahydrate	Ionic interactions in the presence of specific trivalent ions	Thermostable, biocompatible, biodegradable	[26,139]
	Kappa-carrageenan (red algae) Sodium alginate (brown algae)	Calcium chloride and potassium chloride	Combined algal polysaccharides to interact with divalent calcium ions and monovalent potassium ions, forming alginate-calcium cation and kappa-carrageenan-potassium cation crosslinked networks	Thermostable, biocompatible, biodegradable	[140]
	Ulvan (green algae Ulva spp.)	Borate, calcium chloride	Boric acid and divalent cations such as calcium cations initiated ionic crosslinking. It also involved the chelation of calcium with hydroxyl groups of borates	Thermoreversible, biocompatible, biodegradable	[142]
Freezing-thawing method	Sodium alginate (brown algae) PVA	Calcium chloride, and/or diclofenac sodium (changed from transparent to white and opaque solution)	Repeated freezing-thawing cycles on PVA-containing ionically crosslinked sodium alginate hydrogels for two times	Biocompatible, pH-sensitive, improved swelling behaviors and encapsulation efficiency	[152]
Freezing-thawing method	Agarose (red algae Rhodophyta) PVA	-	Thermal-induced aggregation (above 85 °C) of agarose followed by fabricating PVA hydrogels via the repeating freezing-thawing cycles. They entangled through van der Waals force and hydrogen bonding	Robust mechanical property, biocompatible, self-healing	[154]
	Sodium alginate (brown algae) Poly(vinyl alcohol) (PVA)	Sodium chloride	Repeated freezing-thawing cycles on the homogeneous PVA/sodium alginate solution and then immersed the virgin hydrogel in the saturated sodium chloride solution	Biocompatible, high toughness and electric conductivity	[155]
	Kappa-carrageenan (red algae) PVA	Iron salts (iron(II) sulfate heptahydrate and iron(III) chloride hexahydrate), ammonia solution, potassium chloride	Mixed iron salts, PVA and kappa-carrageenan, followed by adding ammonia solution to adjust pH at 10 until magnetic nanoparticles with the dark color were formed. The polymer networks were crosslinked by the repeating freezing-thawing cycles and then ionic interaction with potassium cations	Magnetic	[156]
Secondary structure	Native agarose (red algae Gelidium and Gracilaria) Kappa-carrageenan (red algae)	(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO), sodium bromide, sodium hypochlorite, sodium hydroxide, ethanol, sodium chloride, and sodium borohydride	Blended α-helix-rich agarose/kappa-carrageenan with the β-sheet-rich carboxylated derivatives in the hot deionized water, achieving converting an α-helix to a β-sheet. Followed by aggregation of polymer chains through β-sheet motifs and elongation of these aggregates into high-aspect-ratio structure	Injectable, tunable mechanical and structural properties, biocompatible, biodegradable, formation in vivo	[28,161]
	Kappa-carrageenan (red algae) Gelatin	-	Host (kappa-carrageenan)-guest (geletin) interaction generating electrostatic interaction, hydrophobic interaction, and hydrogen bonding, a coil to helix transition of gelatin, followed by aggregation of helices	Biocompatible, biodegradable	[179]
Hydrogen bonding	Sodium alginate (brown algae) Carboxymethyl chitosan	Citric acid	Blended those two biopolymers in the citric acid solution, resulting in hydrogen bonding between the polymers and citric acid under an acidic environment	pH-sensitive, thermally stable, biocompatible, biodegradable	[25]
	Sodium carboxymethyl cellulose (not specified)	Hydrochloric acid/citric acid	Mixed sodium carboxymethyl cellulose with acid, replacing sodium in carboxymethyl group with hydrogen. Carboxymethyl cellulose molecules aggregated because of reduction of the polymer solubility in water	Stable, biocompatible, durable	[180]
Hydrogen bonding	Fucoidan (brown algae) Kappa-carrageenan (red algae)	-	Non-gelling polysaccharide fucoidan interacted with kappa-carrageenan under high temperature (approximately 95 °C), forming hydrogel bonds	Biocompatible, biodegradable, improved water retention and frost resistance, thermal stable, enhanced rheological properties	[181]
Freezing-thawing method	Agarose (red algae Rhodophyta) PVA	-	Thermal-induced aggregation (above 85 °C) of agarose followed by fabricating PVA hydrogels via the repeating freezing-thawing cycles. They entangled through van der Waals force and hydrogen bonding	Robust mechanical property, biocompatible, self-healing	[154]
	Sodium alginate (brown algae) Poly(vinyl alcohol) (PVA)	Sodium chloride	Repeated freezing-thawing cycles on the homogeneous PVA/sodium alginate solution and then immersed the virgin hydrogel in the saturated sodium chloride solution	Biocompatible, high toughness and electric conductivity	[155]
	Kappa-carrageenan (red algae) PVA	Iron salts (iron(II) sulfate heptahydrate and iron(III) chloride hexahydrate), ammonia solution, potassium chloride	Mixed iron salts, PVA and kappa-carrageenan, followed by adding ammonia solution to adjust pH at 10 until magnetic nanoparticles with the dark color were formed. The polymer networks were crosslinked by the repeating freezing-thawing cycles and then ionic interaction with potassium cations	Magnetic	[156]
Secondary structure	Native agarose (red algae Gelidium and Gracilaria) Kappa-carrageenan (red algae)	(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO), sodium bromide, sodium hypochlorite, sodium hydroxide, ethanol, sodium chloride, and sodium borohydride	Blended α-helix-rich agarose/kappa-carrageenan with the β-sheet-rich carboxylated derivatives in the hot deionized water, achieving converting an α-helix to a β-sheet. Followed by aggregation of polymer chains through β-sheet motifs and elongation of these aggregates into high-aspect-ratio structure	Injectable, tunable mechanical and structural properties, biocompatible, biodegradable, formation in vivo	[28,161]
	Kappa-carrageenan (red algae) Gelatin	-	Host (kappa-carrageenan)-guest (geletin) interaction generating electrostatic interaction, hydrophobic interaction, and hydrogen bonding, a coil to helix transition of gelatin, followed by aggregation of helices	Biocompatible, biodegradable	[179]
Hydrogen bonding	Sodium alginate (brown algae) Carboxymethyl chitosan	Citric acid	Blended those two biopolymers in the citric acid solution, resulting in hydrogen bonding between the polymers and citric acid under an acidic environment	pH-sensitive, thermally stable, biocompatible, biodegradable	[25]
	Sodium carboxymethyl cellulose (not specified)	Hydrochloric acid/citric acid	Mixed sodium carboxymethyl cellulose with acid, replacing sodium in carboxymethyl group with hydrogen. Carboxymethyl cellulose molecules aggregated because of reduction of the polymer solubility in water	Stable, biocompatible, durable	[180]
Hydrogen bonding	Fucoidan (brown algae) Kappa-carrageenan (red algae)	-	Non-gelling polysaccharide fucoidan interacted with kappa-carrageenan under high temperature (approximately 95 °C), forming hydrogel bonds	Biocompatible, biodegradable, improved water retention and frost resistance, thermal stable, enhanced rheological properties	[181]