Table 1.

Categories of hydrogel systems with example polymers and applications for oral drug delivery.

	Formulation	Polymer type	Delivery site	Therapeutic and ref.
Intestinal delivery systems
Anionic	P(MAA-g-EG)	Synthetic	Small intestine	Insulin [63,64], calcitonin [28,65], IFN-α [65]
	Alginate-based	Natural	Small intestine and colon	Heparin [72], hemoglobin [73], melatonin [74], vaccines [75,76], peptides [77], probiotic yeast [78], cedroxil [71]
	Hyaluronic acid-based	Natural	Small intestine	Insulin [82], thrombin [80], α-chymotrypsin [81]
Cationic	Chitosan-based	Natural	Small intestine	Salmon calcitonin [91], peptides [92], BSA [69,93]
Amphiphilic	P(MAA-g-EG) with PMMA nanoparticles	Synthetic	Colon	Doxorubicin [44]
Degrading formulations	Dextran-based	Natural	Colon	Hydrocortisone [105], salmon calcitonin [106]
	Azoaromatic crosslinks	Synthetic	Colon	[107–109]
Intracellular delivery systems
Cationic	Dimethylaminoethyl methacrylate-based	Synthetic	Endosome	[95]
	Chitosan-based	Natural	Endosome	Methotrexate disodium [96]
	Chitosan-based	Natural	Cytosol	[101]
	Poly-L-lysine containing	Synthetic	Cytosol	[102]
	Poly(γ-benzyl-L-glutamate) containing	Synthetic	Cytosol	[103]
Acid-sensitive bonds	Containing oxime bonds	Synthetic	Endosome	Doxorubicin [97]
	Containing acetal bonds	Synthetic	Endosome	Doxorubicin [98]
Acid-degradable crosslinker	Crosslinked with 2,2-dimethacroyloxy-1-ethozypropane	Synthetic	Endosome	[99]
Enzyme-degradable crosslinker	Peptide crosslinker	Synthetic	Lysosome	DNA, antibodies [110]

BSA: Bovine serum albumin; PMMA: Poly(methyl methacrylate); P(MAA-g-EG): Poly(methacrylic acid) grafted with poly(ethylene glycol).