Objective Intelligent hydrogels usually exhibit environmental stimulus responsiveness when the external environment changes based on a unique three-dimensional network formed by cross-linking with chemical bonds or physical interactions, which makes it play an essential role in biomedicine, tissue engineering, soft robotics, and other fields. The femtosecond laser direct-writing (FLDW) technology based on the principle of two-photon nonlinear absorption has the advantages of nanoscale resolution. It also has three-dimensional modeling capabilities which are difficult to realize by UV lithography, electron beam etching, or nanoimprint. However, FLDW technology is still facing challenges in manufacturing smart micro-nanostructure devices, such as a single smart material system that meets the femtosecond laser manufacturing requirements. These include the lack of systematic research on the influence of FLDW process parameters on the manufacturing accuracy of intelligent materials and properties of manufactured materials, and lack of theoretical guidance in smart microstructures' design. In this study, we developed a composite intelligent hydrogel and applied the two-photon polymerization (TPP) technique to achieve four-dimensional microscale printing. We investigated the effects of femtosecond laser power and scanning speed on the line width, line height, swelling ratio, and hydrogels' mechanical modulus. We realized the controllable transformation of the three-dimensional micro-nanostructure under external environmental stimuli using the finite element simulation. Theoretical calculation and experimental results show that the controllable modulation of the three-dimensional shaping and structural performance of the smart hydrogel material can be realized using the laser parameters. However, the double-layer hydrogel microstructure can achieve the autonomous programmable transformation. This work laid a foundation for the development of soft-robots and tissue engineering.

Methods First, we manufacture smart photoresist materials composed of smart monomers, crosslinkers, and photoinitiators using the FLDW platform based on the principle of two-photon nonlinear absorption that can achieve three-dimensional manufacture with nanoresolution. A single suspension wire with a pitch of 10μm is manufactured to measure the line width and height using a scanning electron microscope. The volume swelling degree is tested under an optical microscope using a football model with a diameter of 40μm. The law between the volume swelling degree and the laser manufacturing parameters is studied. In the next step, we use the micromechanics testing system (Femto Tools AG, FT-MTA 02) to test the stress and strain of the 100-μm × 100-μm × 30-μm cuboid and obtain the law of stiffness with the FLDW parameters. Combined with the finite element simulation calculations and experiments, the designed double-layer network structure has excellent self-driving performance in the presence or absence of a water environment. The reversible deformation is repeatedly measured 50 times under an optical microscope, and the average error is counted to ensure the universality of the results.

Results and Discussions The configured intelligent photoresist material with a certain proportion of intelligent monomers, cross-linkers, and initiators could meet the femtosecond laser TPP manufacturing requirements. Both the minimum line width and minimum line-height can reach 400nm within the forming range. In general, with the increase of laser power and decrease of direct-writing speed, line width and line-height increase accordingly (Fig. 2). The unit model's swelling degree study shows that the swelling degree changes obviously under the premise of satisfying the laser power forming and direct writing speed. In this study, the maximum volume swelling degree can achieve 84%, and minimum swelling degree is only one-tenth of the maximum, which makes it possible for the structure to be self-driving (Fig. 3). Since different laser powers and direct writing speeds will make the degree of crosslinking of organic materials in the polymerization process different, the structure's mechanical modulus after manufacturing will also be different. In the mechanical modulus test, it can be seen that with the change of laser power and direct writing speed, stiffness has the same trend as line width, line height, and swelling degree. The stiffness can reach a minimum of 676N/m when the laser power is 15mW, and the direct writing speed is 1000μm/s. When the laser power is increased to 35mW and scanning speed is reduced to 250μm/s, the modulus of 1923N/m can be achieved (Fig. 4). Since the laser direct writing parameters have such significant influence on material manufacturing, the mesh structure manufactured by tuning the parameters has an excellent self-driving function (Fig. 5).

Conclusions In this study, we developed a composite hydrogel material sensitive to the water environment and suitable for femtosecond laser direct-writing. The study obtained the change law of the line width, wall height, swelling degree, and material mechanical modulus at different laser powers and direct writing speeds. Within the manufacturing parameters of laser direct-writing, the material could be shaped to achieve the minimum line width of 400nm, 400-nm minimum high wall, and micro-nano structure with a maximum swelling degree of 84%. The double-layer mesh microstructure with different laser powers, direct writing speeds, and scanning paths could quickly respond to water environment stimuli, proving that the reversible deformation of self-bending and self-curling is feasible. Furthermore, we established the double-layer structure's mathematical model and applied the finite element calculation simulation to verify the scientific method from topology design to target function realization. We realized the reversible 3D shape conversion function of programmable control of the six-leaf petal structure and mimosa structure. These studies have laid the foundation for developing and applying micro-nano soft robots, micro-nanosensing, and functional braking devices in the future.