Hydrogels are hydrophilic, three-dimensional polymeric networks, which can absorb and retain a large amount of water, but do not dissolve in it due to the existence of cross-linking points. In the past three decades, stimuli-responsive hydrogels, which exhibit drastic volume phase transitions in response to environmental changes, have been considerably investigated. Poly(N-isopropylacrylamide) (PNIPAAm) hydrogels undergo sharp swelling/deswelling behavior at around 32-34℃, which is called lower critical solution temperature (LCST). Due to the unique property, PNIPAAm-related hydrogels have been developed to design different biodevices, such as drug release systems, biosensors, matrix for tissue engineering, etc.
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