Table 1.

Keywords co-occurrence count, centrality and year.

keyword	count	centrality	Initial year ofthe literature
Hydrogels	100	0.15	2007
Drug delivery	52	0.26	2004
Polymers	43	0.12	2006
Release	43	0.07	2010
Nanoparticles	36	0.06	2011
Light	34	0.12	2008
Behavior	30	0.17	2007
Photodegradable hydrogel	26	0.10	2010
Photoresponsive hydrogel	20	0.11	2015