Recent developments in stimuli-responsive polymer nanogels for drug delivery and diagnostics: A review

Polymer nanogels (NGs) are water-swellable, cross-linked 3D network structures with size typically range from 1 to 1000 nm. Especially, biocompatible and "smart" NGs engineered from stimuli-responsive polymers are attractive because of its capability to respond the endogenous biological triggers of pH, bioreduction, biomolecule recognition, as well as the exogenous stimuli-triggers like temperature and light. Importantly, on exposing to these physical or biochemical signals, the responsive NGs can be utilized for therapeutic delivery and diagonostic applications. In the past decade, substantial developments were achieved in the development of "smart" NGs for theranostic and diagnostic applications such as intracellular delivery of drug and nucleic acids, photodynamic/photothermal therapy, bioimaging and sensing. Herein, we exclusively review the recent exciting developments in synthetic methods as well as biomedical applications of successfully employed "smart" NGs which can respond to a single, dual or multiple stimulus- responsive triggers. The prospects in the application of the stimuli-responsive and multifunctional NGs also will be addressed in this review.