The various plants and animals in nature not only have various and varied forms, but also can change their own forms according to the changes of the external environment. Hydrogel has long been regarded as one of the ideal materials for intelligent bionics due to its soft and wet properties, and has been used in many fields such as soft robot, tissue engineering and drug delivery. At present, there are two main ways for hydrogel drivers to realize intelligent deformation: shape memory and drive. Shape memory hydrogel needs to be deformed under the action of external forces, and its temporary shape is fixed through reversible action under external stimulation. The hydrogels can spontaneously deform when stimulated by the outside world. The traditional hydrogel actuator can only realize simple bending deformation under external stimulation because it is prepared by template method. In recent years, researchers have realized multi-dimensional complex deformation of hydrogels by a series of methods such as photomask and patternization, but the deformation behavior (degree of deformation) of hydrogels driver cannot be adjusted again according to the needs because the anisotropic structure of hydrogels is given at the time of preparation. Therefore, the realization of programmable and multi-dimensional complex deformation of hydrogel driver is of great significance to the application of hydrogel in soft robot and other fields.

In recent years, Chen Tao and Zhang Jiawei, researchers of intelligent Polymer Materials Research Group, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, have been devoted to the research of intelligent deformed hydrogels. Recently, the team successfully combined the shape memory and driving function of hydrogel, and published a research paper entitled Actuating Supramolecular Shape Memorized Hydrogel in Small.

Researchers using ultraviolet light step by step polymerization method with double layer structure of water gel preparation, first by polymerization of light construction of sodium alginate with shape memory ability - polyacrylamide hydrogel (Alg - PAAm), then in the top down N - isopropyl acrylamide (NIPAm) performed fluid and light polymerization, again with shape memory and eventually drive function Alg - PAAm/PNIPAm double hydrogel drives.

Because the anisotropic structure of traditional double-layer hydrogels is only distributed in the direction perpendicular to the gel, they can only produce simple bending deformation under external stimulation. The researchers combined the shape memory and driving functions of the hydrogel. Through the shape memory process of the ALG-PaAM hydrogel, various temporary anisotropy that can be erased was given to the hydrogel, and then the temperature response driving process of the PNIPAm hydrogel realized the programmable multi-dimensional complex deformation. To be specific, for example, the striped gel was deformed into a crutch-like shape by external force, then the gel was locally immersed in the solution of iron ions to fix its temporary shape, and then the gel was placed in hot water at 60℃. With the contraction of PNIPAm hydrogel layer, the crutch-like double-layer gel spontaneously deformed into a note shape. When the external solution temperature drops to 15℃, the PNIPAm hydrogel layer will swell and the double-layer gel will return to the initial crutch-like shape. Finally, the gel will be soaked with EDTA to remove the metal chelation in the gel. The crutch-like gel will return to the initial strip shape and can be used for the next shape programming. In addition, when the strip gels are memorized as torsional shapes, the hydrogels can directly transform from one-dimensional shapes into three-dimensional helices, and can form a series of shapes such as left and right helices, as well as different degrees of helices, simply by changing the direction and Angle of torsion.

The combination of shape memory and driving function of hydrogels is not only suitable for strip gels, but also can realize the programable deformation of sheet gels. Researchers will two-dimensional water gel sheet bending along the Y-axis direction into a cylinder and fixed the shape in the solution of iron ion, water gel driving occurs when the temperature rises, due to the bending cylinder structure in the Y direction is blocked, the hydrogel only release of stress along the X axis direction, leading to a cylindrical water gel become flat shape along the X axis, Y axis direction at this time of the contractile force no longer constrained by water gel three-dimensional structure, so as to realize the contraction in the Y direction back into a cylinder. Similarly, if the hydrogel is bent along the X-axis for memory, the hydrogel will first expand into a flat plate along the Y-axis and then contract into a cylinder along the X-axis. The process shows good control over the direction of hydrogel drive, and the validity of the results is verified by finite element analysis. Inspired by the art of paper-cutting, the researchers used laser cutting machines to prepare gels with paper-cutting patterns. By rolling the gel with external force and fixing its two ends with iron ion, the hydrogel can maintain a good THREE-DIMENSIONAL cylindrical structure, then increase the external temperature, and the middle gel will contract and spontaneously form a three-dimensional lantern shape. In addition, the middle part of the gel can also be remembered as a THREE-DIMENSIONAL arch structure. Similarly, after increasing the external temperature, the hydrogel at the end can shrink to form a three-dimensional lantern shape. This process realizes the transformation between different three-dimensional shapes of hydrogel through reasonable shape programming. Combined with the function of supramolecular shape memory and drive of hydrogel, the work realized the programmable and multi-dimensional shape transformation of hydrogel driver, and provided a new idea for the preparation and development of new intelligent deformation materials.

The work was funded by the National Natural Science Foundation of China, the National Key Research and Development Program of China, and the Youth Innovation Promotion Association of the Chinese Academy of Sciences.

The shape memory of double-layer hydrogel is in coordination with the driving function

Source: Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences

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