Construction of 3D Shape‐Changing Hydrogels via Light‐Modulated Internal Stress Fields

Abstract

The 3D shape‐changing hydrogels are highly pursued for numerous applications. However, up to now, the construction of complex 3D shape‐changing hydrogels remains a challenge. The reported design strategies are mainly applied to fabricate 2D ones by introducing anisotropic microstructures into hydrogel sheets/membranes. Herein, we present a convenient photolithography strategy for constructing complex 3D shape‐changing hydrogels by simultaneously modulating anisotropic microstructures and internal stress fields of gel sheets. When the precursor solution containing ultraviolet (UV) absorber is irradiated by single‐side UV light, the attenuated polymerization rate can cause the generation of asymmetric internal stress field in the resulting hydrogel sheet. In the meantime, the directional diffusion of unpolymerized monomers allows for the formation of vertical gradient structure within hydrogel. Therefore, by applying different photomasks to modulate the local gradient structures and internal stress fields of the gel sheets, they can spontaneously transform into various complex 3D shape‐changing hydrogels in the air. Response to the external stimuli, these 3D shape‐changing hydrogels (e.g., fighter plane, birdie, and multi‐storey origami lattices) can deform in a novel 3D1‐to‐3D2‐to‐3D3 mode. This new design strategy contributes to the development of complex biomedical implants and soft robotics.

Biomimetic 3D Color-Changing Hydrogel Actuators Constructed Based on Soft Permeable Photonic Crystals

“…Thereinto, the PAM hydrogel networks formed early at UV-irradiated side could shrink freely, whereas the shrinkage of the PAM hydrogel networks subsequently formed at the nonirradiated side was restricted by the early formed network. Therefore, an asymmetric internal stress (i.e., compressive stress at UV-irradiated side and tensile stress at the nonirradiated side) was generated across the thickness of hydrogel sheets . When the samples were removed from the molds, the internal stress was released to drive them to roll up toward the nonirradiated side (Figure a and Movie S1).…”

“…For instance, the curved photonic hydrogel actuator prepared with 784 nm PNIPAM microspheres turned red and green in 10% and 40% (v/v) ethanol–water mixtures, respectively. In the meantime, the shape of the curved photonic hydrogel actuator was changed in the ethanol–water mixture, owing to its asymmetric microstructure-induced anisotropic strain. ,,, The photonic hydrogel actuator presents the low cross-linking density at the nonirradiation side and thus possesses higher swelling/deswelling ratios in good/poor solvents at this side, compared to that of UV-irradiation side . Therefore, the curved hydrogel actuator unrolled in the good solvent (i.e., ethanol–water mixture with a low ethanol concentration lower than 50 v/v%), while it further rolled up toward the nonirradiated side in water/ethanol mixture with a high ethanol concentration (≥50 v/v%), as shown in Figure d.…”

“…Therefore, an asymmetric internal stress (i.e., compressive stress at UV-irradiated side and tensile stress at the nonirradiated side) was generated across the thickness of hydrogel sheets. 35 When the samples were removed from the molds, the internal stress was released to drive them to roll up toward the nonirradiated side (Figure 1a and Movie S1). When the precursor solution was irradiated by UV light from both sides, the resultant hydrogel sheet exhibited the symmetrical microstructure and retained its flat configuration after being removed from the mold (Figure S4a,b).…”

“…The strong UV absorption by UV F-22 caused sharp decayed UV intensity along the thickness direction (Figure S3). Hence, the UV-irradiated side held a higher polymerization rate and a lower concentration of unpolymerized monomer than the nonirradiated side. ,, Because of the diffusion of unpolymerized monomers across the permeable photonic crystals under the concentration gradient, the UV-irradiated side of as-prepared photonic hydrogels possesses a higher polymer network density than the other side (Figure a). Moreover, the asymmetric UV photolithography could also create asymmetric internal stress across the thickness of hydrogel sheets during the polymerization.…”

“…In this regard, the hydrogel actuators can perform programmable complex deformations, realized by the construction of spatially heterogeneous structures using such as photolithography, − 3D printing, − auxiliary electric/magnetic field, , etc. The integration of photonic crystals into a hydrogel actuator may produce powerful color-changing actuators.…”