Template-guided capillary force-driven self-assembly technology is increasingly being considered as an alternative for fabricating a range of micro/nanostructures. Because of the advantages of rapid preparation of large-area and controllable complex hierarchical microstructures, the combination of femtosecond laser and capillary force-driven assembly (LPCS) technology has become an attractive method. In the LPCS method, the stability of the microstructure depends on the competition between the contact force, e.g., the van der Waals force, and elastic force of the microstructure after self-assembly. The contact force between microstructures is positively correlated with the contact area of the microstructure. A larger contact area corresponds to the larger van der Waals force and more stable assembly. When the contact area is greater than the critical value, the microstructure assembly behavior is irreversible. In our previous research, we found that microstructures with linear contact, such as microwalls, or even microstructures with surface contact could not be restored to the upright state after immersion in a liquid because of the large contact area after assembly, which greatly limits the application range of microstructures. In this study, the reversible self-assembly of linear contact microstructures prepared by LPCS is realized by taking advantage of the significant deformation ability of temperature-responsive hydrogels.

In this study, the temperature-responsive hydrogel is mainly composed of N-isopropylacrylamide, acrylamide, and phosphine oxide, where N-isopropylacrylamide is the monomer, acrylamide is the crosslinking agent, and phosphine oxide is the photoinitiator. We successively put the 400 mg N-isopropylacrylamide, 30 mg acrylamide, 30 mg phosphine oxide, and 450 μL glycol into a bottle and then place the bottle in a water bath at 50 ℃ for ultrasound. Finally, the 50 mg polyethylene pyrrolidone is added as a viscosifier. The femtosecond laser source used in the experiment has a pulse width of 75 fs, repetition frequency of 80 MHz, average output power of 2.5 W, and wavelength of 800 nm. The microscope system consists of a lens with a numerical aperture of 1.35. A scanning galvanometer is used to control the movement of the laser in the xy plane, and a nanostage is used to control the precise movement of the sample in the z direction to realize 3D printing of any shape. During processing, the measured average laser power is 24 mW, and the hatching distances used for anisotropic microstructures are 150 nm and 350 nm. After processing, the samples are placed in a developer (ethanol) for 10 min to remove any unpolymerized hydrogel.

We investigate the effects of different hatching distances (HDs) on the degree of hydrogel polymerization. With an increase in HD, the shrinkage rate of the hydrogel increases. To achieve significant deformation and good surface appearance, HDs of 150 nm and 350 nm and their area width ratio of 3∶7 are selected, as shown in Figs.1(c) and (d). Anisotropic arm microstructures with a length of 30 μm are designed as shown in Fig.2. When the solution temperature is lower or higher than the critical temperature, arm bending deformation occurs in the opposite direction and exhibits good reversibility. In addition, the arm shows good transportation ability. As shown in Fig.3, we apply LPCS technology to temperature-responsive hydrogels to prepare a variety of assembly patterns. Compared with the traditional LPCS preparation, the requirements for spatial distribution are reduced. Finally, Figure 5 shows that based on the remarkable shrinkage property of temperature-responsive hydrogels at high temperatures, linear contact microgrippers with reversible assembly are prepared, and their applications in the field of microsensing are explored. Microgrippers with good airtightness and fatigue resistance can be repeatedly used.

In this study, anisotropic hydrogel microstructures are printed using a femtosecond laser, and the direction of their movement is controlled by temperature regulation to realize the directional transport of micro-objects. Second, laser-printed hydrogel microstructures are combined with capillary force-driven self-assembly to obtain rich micropatterns. This further demonstrates the flexibility of femtosecond laser two-photon processing and the convenience of preparing hierarchical microstructures combined with capillary force-driven self-assembly. More importantly, microgrippers with line contacts can be reversibly prepared using the deformation characteristics of temperature-responsive hydrogels. The closed microgrippers can be opened through the deformation force generated by the significant shrinkage of the hydrogel at high temperatures, which resolves the irreversibility of the assembly behavior of line contact microstructures prepared by the previous LPCS method. By sensing the surrounding environment, the microstructures can be closed and opened, greatly enriching the application of LPCS technology in the field of sensors.