Cephalopod-Inspired Chemical-Gated Hydrogel Actuation Systems for Information 3D-Encoding Display.

Cephalopods evolve the acetylcholine-gated actuation control function of their skin muscles, which enables their dynamic/static multimode display capacities for achieving perfectly spatial control over the colors/patterns on every inch of skin. Reproduction of artificial analogs that exhibit similar multimodal display is essential to reach advanced information three-dimensional (3D) encoding with higher security than the classic 2D-encoding strategy, but remains underdeveloped. The core difficulty is how to replicate such chemical-gated actuation control function into artificial soft actuating systems. Herein, this work proposes to develop azobenzene-functionalized poly(acrylamide) (PAAm) hydrogel systems, whose upper critical solution temperature (UCST) type actuation responsiveness can be intelligently programmed or even gated by the addition of hydrophilic α-cyclodextrin (α-CD) molecules for reversible association with pendant azobenzene moieties via supramolecular host-guest interactions. By employing such α-CD-gated hydrogel actuator as an analogue of cephalopods' skin muscle, biomimetic mechanically modulated multicolor fluorescent display systems are designed, which demonstrate a conceptually new α-CD-gated "thermal stimulation-hydrogel actuation-fluorescence output" display mechanism. Consequently, high-security 3D-encoding information carriers with an unprecedented combination of single-input multiple-output, dynamic/static dual-mode and spatially controlled display capacities are achieved. This bioinspired strategy brings functional-integrated features for artificial display systems and opens previously unidentified avenues for information security.

Multiple-Responsive and Amphibious Hydrogel Actuator Based on Asymmetric UCST-Type Volume Phase Transition.

Thermoresponsive hydrogel actuators have attracted tremendous interest due to their promising applications in artificial muscles, soft robotics, and flexible electronics. However, most of these materials are based on polymers with lower critical solution temperature (LCST), while those from upper critical solution temperature (UCST) are rare. Herein, we report a multiple-responsive UCST hydrogel actuator based on the complex of poly(acrylic acid) (PAAc) and poly(acrylamide) (PAAm). By applying a heterogeneous photopolymerization, a bilayer hydrogel was obtained, including a layer of the interpenetrating network (IPN) of PAAm/PAAc and a layer of a single network of PAAm. When cooled down below the UCST, the PAAm/PAAc layer contracted due to the hydrogen bonding of the two polymers while the PAAm layer stays in swelling state, driving the hydrogel to curl. By adjusting the composition of the two layers, the amplitude of actuation behavior could be regulated. By creating patterned IPN domains with photomasks, the hydrogel could deform into complex two-dimensional (2D) and three-dimensional (3D) shapes. An active motion was realized in both water and oil bath, thanks to the internal water exchange between the two layers. Interestingly, the hydrogel actuator is also responsive to urea and salts (Na2SO4, NaCl, NaSCN), due to that the strength of the hydrogen bonds in the IPN changes with the additives. Overall, the current study realized an anisotropic UCST transition by introducing asymmetrically distributed polymer-polymer hydrogen bonds, which would inspire new inventions of intelligent materials.