Self-regulated reversal deformation and locomotion of structurally homogenous hydrogels subjected to constant light illumination

Abstract

Environmentally adaptive hydrogels that are capable of reconfiguration in response to external stimuli have shown great potential toward bioinspired actuation and soft robotics. Previous efforts have focused mainly on either the sophisticated design of heterogeneously structured hydrogels or the complex manipulation of external stimuli, and achieving self-regulated reversal shape deformation in homogenous hydrogels under a constant stimulus has been challenging. Here, we report the molecular design of structurally homogenous hydrogels containing simultaneously two spiropyrans that exhibit self-regulated transient deformation reversal when subjected to constant illumination. The deformation reversal mechanism originates from the molecular sequential descending-ascending charge variation of two coexisting spiropyrans upon irradiation, resulting in a macroscale volumetric contraction-expansion of the hydrogels. Hydrogel film actuators were developed to display complex temporary bidirectional shape transformations and self-regulated reversal rolling under constant illumination. Our work represents an innovative strategy for programming complex shape transformations of homogeneous hydrogels using a single constant stimulus.

Fig. 1. Design of light-induced bidirectional contraction-expansion of MCH(1 + 2) hydrogels.

a Chemical structures and net charge states of the protonated ring-opening merocyanine form (MCH1, MCH2) in the dark and ring-closed spiropyran form (SP1, SP2) before and after irradiation with visible light. The photoisomerization rate k₁ is designed to be much greater than that of k₂. b Plot of the characteristic absorbance of MCH1 at 422.5 nm (red) and MCH2 at 450.3 nm (black) versus irradiation time (450 nm, 154.6 mW/cm²), followed by fitting to the ExpDec1 function to obtain the photoisomerization rates. c Schematic representation of the transient sequential contraction-expansion of MCH(1 + 2) hydrogels due to the stepwise descending-ascending variation in the total net charge. d Plot of hydrogel size (%, defined as the percentage change in diameter of the dish-shaped hydrogel) as a function of two-sided irradiation (8.99 mW/cm²) time. The green dashed curve represents the calculated hydrogel size, obtained by averaging the changes in size of MCH1 (red, expansion) and MCH2 (black, contraction). e Rheological measurements of MCH(1 + 2) hydrogels before (blue) and after irradiation (8.99 mW/cm²) for 21 min (orange) and 100 min (green). The insets are corresponding SEM images showing the changes in microporosity. Error bars represent standard deviations of data collected from three separate samples.

Fig. 2. Light-induced reversal bending deformation of MCH(1 + 2) hydrogel thin films.

a Schematic representation of transient bidirectional positive and negative bending deformation of a ribbon-shaped MCH(1 + 2) hydrogel upon irradiation from above. b Plot of the bending angles of the MCH1 hydrogel (red), MCH2 hydrogel (black), and MCH(1 + 2) hydrogel (blue, molar ratio is 1:1) as a function of irradiation duration (450 nm, 15.24 mW/cm²). On the right are photographs of the bending geometry of the MCH(1 + 2) hydrogel ribbon at a specific irradiation time. c Plot of the positive and negative bending angles of MCH(1 + 2) hydrogel ribbons upon irradiation at different pH values. d Plot of the net charge changes in mixed MCH1 and MCH2 with variable ratios as a function of irradiation time, showing positively and negatively charged regions with tunable charge transition time points. e Plot of the bending angles of MCH(1 + 2) hydrogel ribbons with variable mixing ratios under constant irradiation (450 nm, 15.24 mW/cm²). f Plot of the bending angles of MCH(1 + 2) hydrogel ribbons with variable total grafting densities (MCH1:MCH2 = 1:1). g Plot of the bending angles of MCH(1 + 2) hydrogel ribbons containing a fixed mixing ratio of 1:1 and a fixed total grafting density of 2.0% upon irradiation with variable light intensities. The labeling time represents the illumination time when the hydrogel reaches its maximum bending angle. Error bars in (e–g) represent standard deviations of data collected from three separate samples.

Fig. 3. Design of light-induced bidirectional contraction-expansion of MCH(3 + 2) hydrogels.

a Chemical structures and net charge states of the protonated ring-opening merocyanine form (MCH3, MCH2) in the dark and ring-closed spiropyran form (SP3, SP2) before and after irradiation with visible light. The photoisomerization rate k₃ is designed to be as slow as that of k₂. b Plot of the characteristic absorbance of MCH3 at 456 nm (red) and MCH2 at 450 nm (black) versus irradiation time (450 nm, 154.6 mW/cm²), followed by fitting to the ExpDec1 function to obtain the photoisomerization rates. c Schematic representation of the transient sequential contraction-expansion of MCH(3 + 2) hydrogels due to the stepwise descending-ascending variation in the total net charge. d Plot of hydrogel size (%, defined as the percentage change in diameter of the dish-shaped hydrogel) as a function of two-sided irradiation (8.99 mW/cm²) time. The green dashed curve represents the calculated hydrogel size obtained by averaging the changes in size of MCH3 (red, expanded) and MCH2 (black, contracted). e Rheological measurements of MCH(3 + 2) hydrogels before (blue) and after irradiation (8.99 mW/cm²) for 17 min (orange) and 140 min (green). The insets are corresponding SEM images showing the changes in microporosity. Error bars represent standard deviations of data collected from three separate samples.

Fig. 4. Light-induced reversal bending deformation of MCH(3 + 2) hydrogel thin films.

a Schematic representation of transient bidirectional positive and negative bending deformation of a ribbon-shaped MCH(3 + 2) hydrogel upon irradiation from above. b Plot of the bending angles of the MCH3 hydrogel (red), MCH2 hydrogel (black), and MCH(3 + 2) hydrogel (blue, molar ratio is 1:1) as a function of irradiation duration (450 nm, 15.24 mW/cm²). On the right are photographs of the bending geometry of the MCH(3 + 2) hydrogel ribbon at a specific irradiation time. c Plot of the positive and negative bending angles of the MCH(3 + 2) hydrogel ribbons upon irradiation at different pH values. d Plot of the net charge changes in mixed MCH3 and MCH2 with variable ratios as a function of irradiation time, showing positively and negatively charged regions with tunable charge transition time points. e Plot of the bending angles of MCH(3 + 2) hydrogel ribbons with variable mixing ratios under constant irradiation (450 nm, 15.24 mW/cm²). f Plot of the bending angles of MCH(3 + 2) hydrogel ribbons containing variable total grafting densities (MCH3:MCH2 = 1:1). g Plot of the bending angles of MCH(3 + 2) hydrogel ribbons containing a fixed mixing ratio of 1:1 and a fixed total grafting density of 2.0% upon irradiation with variable light intensities. The labeling time represents the illumination time when the hydrogel reaches its maximum bending angle. Error bars in (e–g) represent standard deviations of data collected from three separate samples.

Fig. 5. Self-regulated bidirectional shape transformation.

a Photographs of transient bidirectional bending deformation of MCH(1 + 2) hydrogels with multiple inverted shapes, including fish tails, crosses, stars, and brushes. b Plot of transient positive (red) and negative (blue) bending angles of a star-shaped MCH(1 + 2) hydrogel for eight repeated cycles by alternatively switching light (450 nm, 15.24 mW/cm²) on and off. For each cycle, the hydrogel was incubated in acidic water (5 mM) for 8 h to fully relax to its original state. c Photographs of a single dual-star-shaped MCH(1 + 2) hydrogel that displays programmable transient deformation configurations by manipulation of different irradiation pathways.

Fig. 6. Self-regulated reversal rolling motion.

a Schematic representation of the self-regulated bidirectional rolling motion of an O-ring-shaped MCH(1 + 2) hydrogel under constant irradiation from the left. b Photographs of an O-ring MCH2 hydrogel showing monotonous unidirectional rolling toward a light source giving a positive rolling distance of d₂. c Photographs of an O-ring-shaped MCH(1 + 2) hydrogel showing self-regulated nonmonotonous bidirectional rolling motion. The white arrow indicates the instant rolling direction; d₊ and d_- represent the positive and negative rolling distance, respectively. d Photographs of an O-ring-shaped MCH1 hydrogel showing monotonous unidirectional rolling away from the light source giving a negative rolling distance of d₁. e Plot of the rolling distances of MCH1 (black), MCH2 (red), and MCH(1 + 2) hydrogels (blue) as a function of irradiation time under constant illumination (450 nm, 10.67 mW/cm²) from the left.