Strong, Stretchable, Dual-Responsive PNIPAM Nanogel Cross-Linked UCST-type Macrogels for Biomedical Applications

Dual-responsive nano-structured poly(N-acryloyl glycinamide) (NSG PNAGA) hydrogels were prepared in a cross-linking polymerization reaction of activated poly(N-isopropylacrylamide) (PNIPAM) nanogels with N-acryloyl glycinamide (NAGA). Reactive double bonds on the nanogel were accessed by prematurely stopping the precipitation polymerization of PNIPAM nanogels. The nano-structured hydrogels retained a high mechanical strength (storage modulus G′ ≥ 10,000 Pa and elasticity modulus Emod ∼ 100 kPa), elasticity (L ≥ 600%), and lower (LCST) and upper critical solution temperature (UCST)-type swelling properties. Turbidity measurements showed LCST-type behavior from 0 to 34 °C and UCST-type behavior from 34 to 50 °C. The rheological behavior of NSG PNAGA hydrogels follows a dual-responsive UCST- and LCST-type behavior. At the LCST, a leap of storage modulus G′ of up to 3700 Pa was observed. Scanning electron microscopy showed a distinct morphology compared to the neat PNAGA hydrogel due to the incorporation of PNIPAM into the network. The self-healing properties of NSG PNAGA hydrogels were successfully demonstrated. The nano-structured PNAGA hydrogels could be used as temperature sensors or biological scaffolds in future applications.