Multi-responsive hydrogels with UCST- and LCST-induced shrinking and controlled release behaviors of rhodamine B

•Apparent UCST and LCST volume phase transitions of P(DMAEMA-SS-SBMA) copolymeric hydrogels were observed.•Different pore size and response sensitivity of shrunken structures below UCST and above LCST were visualized by SEM images.•A synergistically sustained release and a burst release were obtained below UCST and above LCST, respectively.•pH-Tunable swelling and redox-sensitive degradation were also observed.

By using a disulfide-functionalized crosslinker, a pH- and thermo-responsive 2-(dimethylamino) ethyl methacrylate (DMAEMA) monomer and a zwitterionic sulfobetaine methacrylate (SBMA) monomer were conjugated to fabricate a multi-responsive P(DMAEMA-SS-SBMA) copolymeric hydrogel. Apparent UCST and LCST volume transitions were observed in the P(DMAEMA-SS-SBMA) hydrogels with equivalent weight fractions of monomers. Different pore size and response sensitivity of shrunken structures below UCST and above LCST were visualized by SEM images. The hydrogel exhibited a highly swollen state with a swelling ratio of 17.8 and a pore size of 106 μm at 45 °C, they deswelled unequally at 5 °C with a compact surface with pore size of 30 μm and a loose bulk with pore size of 83 μm, while they deswelled uniformly at 65 °C with dense shrunken structure with small pore size of 12 μm. The dual-thermoresponsive hydrogel was promising in controlled drug release. The initial drug release was predominantly controlled by diffusion, and the long-term release was influenced by the swelling ratio. Below UCST, the relatively hydrophilic shrunken structure and slow diffusion had a synergistic effect on the sustained release. Above LCST, the fast diffusion and the rapid “off” effect of hydrophobic skin layer resulted in a burst release. Additionally, pH-tunable swelling and redox-sensitive degradation were also observed.

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