Soft microactuators, usually characterized by a small structural size and flexibility, have good application potential in biomedical fields, such as in drug delivery and tissue repair, and have therefore received significant attention from researchers. Hydrogels have good biocompatibility and are considered to be ideal materials for fabricating miniature flexible actuators. Hydrogels can not only be doped and chemically modified to impart stimulus responsiveness but also used to fabricate microscale soft actuators with certain deformation or actuation functions by optimizing their structural design. However, significant research efforts have focused on enhancing the material performance and optimizing the process, and only few studies have explored the effect of the actuator structure itself on its shape deformation capabilities. In this study, the design of a curved double-layer membrane structure is proposed based on the two-photon polymerization technology combined with the existing micro-robot bilayer membrane joint structure. A curved double-layer structure prepared from a pH-responsive hydrogel using optimized parameters exhibited 573% improvement in the deformation capacity compared with that of a straight-sided joint. This study provides a new design concept for the efficient driving and wide application of micro-robots.

First, the effects of the laser power and scanning speed on the performance of the machined structures were explored. We varied the parameters to process a set of microhand-shaped structures. As shown in Fig.1(a), the scanning speed was varied from 10000 to 25000 μm/s at 2500 μm/s intervals, and the processing power was varied from 15 to 35 mW at 5 mW intervals. Figures 1(b), (c) show the optical and SEM images of the microhand-shaped structure. By varying the processing power and scanning speed, 35 sets of data were obtained with pH response thresholds between 7 and 8 for the pH-responsive materials used in the study [see Figs.1(d), (e)]. Different laser powers or scanning speeds allow the printing of sparse and dense layers with different crosslinking densities. The dense layer printed by increasing the laser power or decreasing the scanning speed increases the cross-linking density of carboxyl groups in the hydrogel, while the sparse layer printed by decreasing the laser power or increasing the scanning speed decreases the cross-linking density of carboxyl groups in the hydrogel. This allows the dense and sparse layers to undergo differential swelling in the same pH environment. In this study, the dense layer was processed using a processing power of 25 mW and a scanning speed of 10000 μm/s, and the sparse layer was processed using 20 mW power and a scanning speed of 22500 μm/s.

First, the initial degree of bending of the arc has a significant effect on the deformation capacity of the curved double-layer joint with the same arc length (Fig.2). To further investigate the effect of the initial bending degree on the deformation performance of the curved double-layer joint, the deformation performances of different curved structures were compared (Fig.3). Second, to verify the superiority of the bending deformation performance of the curved joints, we designed straight-sided joints and characterized the bending deformation of the latter using the same characterization method as that of the curved joints (Fig.4). In addition, to demonstrate the superior deformation capability of the curved joints, the data for the curved and straight-sided joints with the greatest deformation were obtained separately and compared (Fig.5). Furthermore, the dissolution curves of the curved joints with and without the double-layer structure were compared (Fig.6). Finally, the application of the curved joints in the structural design is demonstrated (Fig.7).

In this study, various curved double-layer joint structures were prepared by varying the laser power and scanning speed of a two-photon polymerization process of the two-photon polymerization printing technology. The effect of processing parameters on the deformation of the curved double-layer structure with the same length of the curved structure was systematically analyzed, and then the design of the curved double-layer joint was optimized. The experimental results reveal that the curved double-layer membrane joint has a greater bending ability. When the length of the curved structure is the same, the deformation performance increases with increasing central angle of the corresponding curved double-layer joint structure. The curved structure studied in this investigation can be applied with a central angle range of 180°?240° and exhibits the best deformation capability (central angle is 240°). The curved joints perform better than the conventional straight-sided joints, both in terms of the deformation capacity and number of cyclic deformations. Therefore, it is inferred that the arc-shaped joint structure can be a novel alternative for designing micro-nano-robotic joint structures that respond to pH and other stimuli, and they have good prospects in biomedical applications based on micro-nano soft body robots (e.g., drug delivery and tissue repair).