Table 1.

Different gatekeepers that can be used to maintain the “zero release” of the drug and to trigger drug release

External stimuli				Internal stimuli
Magnetic field		Light		pH Sensitive systems		Redox sensitive systems		Enzyme sensitive systems
Magnetic particles	Magnetic nanocrystal	Gold nanoparticles	Photolabile molecules	Polyelectrolytes	Ester bond	Acetal bond	Peptides	Disulfude bond