TABLE 1.

The preparation mechanisms and relevant drug release behaviors of NG-DDS.

Network structure	Cross-linking mechanism	The nanogel size	The loaded molecule	Stimuli-responsive drug release	References
Chemical cross-linking	Traditional free radical polymerization	90–230 nm	Doxorubicin	pH/thermo	Wang et al. (2015)
	Controllable/living free radical polymerization	82–153 nm	Coumarin 102	UV light	Xin et al. (2020)
	Disulfide, imine cross-linking	250 nm	Doxorubicin	pH/redox	Pei et al. (2018)
	Click reaction	236 nm	Labeled insulin	Glucose/H2O2	Li et al. (2019a)
Physical cross-linking	Hydrophilic and hydrophobic interactions	55.7–259.2 nm	Nile Red	pH	Wang et al. (2018)
	Supramolecular host-guest assembly	108.1–121.4 nm	Doxorubicin and indocyanine green	NIR light	Zan et al. (2015)
	Hydrophobic/electrostatic interactions	140–230 nm	Curcumin	pH/oxidation	Dong et al. (2020)
	Hydrogen bonds	230 nm	DAPI, fluorescein, Nile red	Thermo	Zavgorodnya et al. (2017)
Both	Disulfide cross-linking/hydrophobic interactions	66.95 ± 1.93 nm	Purpurin 18 and 10-hydroxycamplothecin	Redox	Ma et al. (2021)
	Free radical polymerization/hydrogen bonds	18 nm	Curcumin	Thermo/NIR light	Wang et al. (2014)
	Free radical polymerization/disulfide cross-linking/ electrostatic interactions	70 nm	Doxorubicin	pH/thermo/photo	Chen et al. (2017)