Fig. 4

a Schematic of the synthesis of REO NPs combined with rod shaped hydrogels for bending actuation. b Images of the as-prepared rod shaped hydrogels with 808 nm (Nd₂O₃ NPs selective absorbance) and 980 nm (Yb₂O₃ NPs selective absorbance) irradiation demonstrating the bending actuation. This NIR light irradiation exploited at 3 W/cm² for 60 s at 28 °C. (a, b were reproduced with permission from Ref. [39], © 2018, Inc. The Authors; published by Springer Nature). c Schematic of the synthesis of PNIPAAm/PDA-NPs hydrogel by the oxidative self-polymerization of PDA-NPs and free radical polymerization of PNIPAAm. d Bending motion of the bilayered hydrogel actuator, composed of PNIPAAm/PDA-NPs and PNIPAAm hydrogel layers, upon NIR laser irradiation. (c, d were reproduced with permission from Ref. [40] © 2016, American Chemical Society)