Facile fabrication of thermo/redox responsive hydrogels based on a dual crosslinked matrix for a smart on–off switch

Abstract

Stimuli-responsive or "smart" soft materials have raised considerable attention due to their ability to spontaneously respond to external environmental variations and have a great potential for wide applications. Herein, a thermo/redox responsive hydrogel is facilely constructed based on a dual crosslinked matrix: the primary chemical crosslinked copolymer is composed of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) and poly(ionic liquid), and the secondary physical crosslinking component is generated by the ionic coordination between iron ions and carboxyl groups in the poly(ionic liquid). The non-covalent ion coordination crosslinking is introduced into a covalently crosslinked network, which further strengthens the soft PNIPAM matrix and enhances the mechanical performances of the hydrogels. The excellent thermosensitivity of PNIPAM and the good conductive property of poly(ionic liquid) provide the hydrogel with an attractive performance as a thermo-responsive switch. Moreover, the trapped iron ions in the network endow the hydrogels with redox-responsiveness, which could be reversibly chemically oxidized and reduced. The mechanical strength of hydrogels could also be tuned by the crosslinked capacity of iron ions within the gel matrix between the strong binding of the oxidized state (Fe3+) and poor coordination of the reduced state (Fe2+). These stimuli-responsive hydrogels have the potential to be used as smart materials for stimuli-responsive devices.

Thermo-Sensitive Nanomaterials: Recent Advance in Synthesis and Biomedical Applications

“…Several natural polymers that demonstrate thermally sensitive properties have been used for the synthesis of hydrogels such as polysaccharides (cellulose, chitosan and xiloglucan) [ 123 , 124 , 125 ] and proteins (gelatin) [ 126 ]. Different polymers such as poly(ethylene glycol)-poly(3-caprolactone)-poly(ethylene glycol) [ 127 ], poly n -isopropylacrylamide/polyacrylic acid (PNIPAm/PAA) [ 128 ], poly( d , l -lactide)-block poly(ethylene glycol)-block-poly( d , l -lactide) (PDLLA-PEG-PDLLA) [ 129 ], poly(ethylene oxide)- b -poly(propylene oxide)- b -poly(ethylene oxide) (PEO-PPO-PEO) [ 130 ], and PNIPAm-based hydrogels have also been reported in drug delivery [ 131 , 132 , 133 ] (see Table 1 ).…”

Smart hydrogels with high tunability of stiffness as a biomimetic cell carrier

“…Sun et al () introduced ionic coordination into chemically crosslinked PNIPAM network, obtaining porous hydrogels which demonstrate high thermosensitivity, as well as perfect mechanical strength. The hydrogels displayed increased stiffness with increased content of polymer chain density and ionic coordination.…”

Temperature and Redox‐Responsive Hydrogels Based on Nitroxide Radicals and Oligoethyleneglycol Methacrylate

“…Among the few works performed in this direction, the results obtained by Zheng and co‐workers [ 8 ] can be cited here, where thermo‐ and redox‐responsive hydrogels based on copolymers of N ‐isopropylacrylamide (NIPAM) and an ionic liquid‐containing monomer were synthetized. [ 8 ] Those hydrogels presented a high thermoresponsive sensitivity and good redox‐responsiveness originating from the transition of iron ions between two oxidation states in the gel matrix and making them very interesting candidates for stimuli‐responsive electronic devices or other self‐adaptive electronics. [ 8 ] In another example, a dual redox‐ and thermoresponsive hydrogel was synthesized based on NIPAM and ferrocenyl silane‐vinyl imidazolium (FS‐Vim) ionic liquid monomers.…”

“…[ 8 ] Those hydrogels presented a high thermoresponsive sensitivity and good redox‐responsiveness originating from the transition of iron ions between two oxidation states in the gel matrix and making them very interesting candidates for stimuli‐responsive electronic devices or other self‐adaptive electronics. [ 8 ] In another example, a dual redox‐ and thermoresponsive hydrogel was synthesized based on NIPAM and ferrocenyl silane‐vinyl imidazolium (FS‐Vim) ionic liquid monomers. [ 9 ] In this system, the FS‐Vim poly(ionic liquid) was used as macrocrosslinker and provided redox‐responsive properties to the system while the lower critical transition temperature (LCST) of poly(NIPAM) was influenced by the incorporation of the poly(ionic liquid).…”