Table 1. Categories of pH-responsive hydrogel with example polymers and applications for oral drug delivery.

	Polymers	Polymer type	Delivery site	Model drug and ref.
Anionic	P(MAA-g-EG)	Synthetic	Small intestine	Insulin (Bell & Peppas, 1996; Lowman et al., 1999; Ichikawa & Peppas, 2003), calcitonin (Torres-Lugo et al., 2002; Kamei et al., 2009), IFN-α (Kamei et al., 2009)
	P(IA-co-NVP)	Synthetic	Small intestine	Salmon calcitonin, urokinase, rituximab (Koetting et al., 2016)
	P(MAA-co-NVP)	Synthetic	Small intestine	siRNA (Knipe et al., 2016)
	Alginate-based	Natural	Small intestine and colon	Heparin (Huang et al., 2000), hemoglobin (George & Abraham, 2006), melatonin (Chen et al., 2004a), vaccines (Chen et al., 2004b; Kulkarni et al., 2001), peptides (Edelman et al., 2000), probiotic yeast (Rasmussen et al., 2003), cedroxil (Peppas & Huang, 2004)
	Hyaluronic acid-based	Natural	Small intestine	Insulin (Hurteaux et al., 2005), thrombin (Kim et al., 2002), α-chymotrypsin (Fiorica et al., 2013)
Cationic	Chitosan-based	Natural	Small intestine	Insulin (Li et al., 2016), BSA (Patel & Amiji, 1996; Kamei et al., 2009)
Amphiphilic	P(MAA-g-EG) with PMMA nanoparticles	Synthetic	Colon	Doxorubicin (Schoener et al., 2013)
Degrading  polymers	Dextran-based	Natural	Colon	Hydrocortisone (Lee et al., 2008b), salmon calcitonin (Zhou et al., 2013)
	Gelatin-based	Natural	Small intestine and colon	5-fluorouracil (Anirudhan & Mohan, 2014)
	Carboxymethyl cellulose/poly(acrylic acid) hybrid hydrogels	Synthetic	Small intestine	Insulin (Gao et al., 2014)
	Maleic acid cross linked poly (vinyl alcohol)	Synthetic	Colon	Vitamin B12, salicylic acid (Basak & Adhikari, 2009)
	Azoaromatic crosslinks	Synthetic	Colon	siRNA, DNA (Chang Kang & Bae, 2011; Thambi et al., 2011) camptothecin
	BC-g-P(AA)	Combination of synthetic and natural	Small intestine	Insulin (Ahmad et al., 2016)
	Guar gum-poly(acrylic acid)-(-cyclodextrin) (GG-PAA-CD)	Combination of synthetic and natural	Small intestine and colon	Dexamethasone (Das & Subuddhi, 2015)