This paper investigates the entire process of 4D printing of shape memory alloys (SMAs), characterized by pre-programming of the future responsive shape of a deformable structure during its 3D forming. Taking an inchworm-inspired crawling robot as an application case, we propose a system prototype composed of two deformable SMA structures, i.e. a curved sheet and a spring coil. From the behavioral synergism of these two antagonistically configured structures, the initial and responsive shapes of each binary structure were collaboratively designed. The feature shapes of different structures were then pairwise combined to generate the robot’s gait, which was realized through the alternating activations of the two components. We also investigated the processing strategy and methods of the presented structures with small feature sizes and unique configurations, based on the optimal 3D printing process of selective laser melting of nickel–titanium alloys. The experimental results verified the design expectations and demonstrated the performance stability of the SMA structures under multiple testing cycles. This research confirmed the possible use of hard materials to create customized smart structures with high levels of flexibility and deformation control, providing new ideas for further development of intelligent, flexible robots.

This article presents a soft crawling robot prototype with a simple architecture inspired by inchworms. The robot functionally integrates the torso (body) and feet in a monolithic curved structure that only needs a single shape memory alloy coil and differential friction to actuate it. A novel foot configuration is proposed, which makes the two feet, with an anti-symmetrical friction layout, can be alternately anchored, to match the contraction–recovery sequence of the body adaptively. Based on the antagonistic configuration between the shape memory alloy actuator and the elastic body, a vertically auxiliary spring was adopted to enhance the interaction mechanism. Force and kinematic analysis was undertaken, focusing on the parametric design of the special foot configuration. A miniature robot prototype was then 3D-printed (54 mm in length and 9.77 g in weight), using tailored thermoplastic polyurethane elastomer as the body material. A series of experimental tests and evaluations were carried out to assess its performance under different conditions. The results demonstrated that under appropriate actuation conditions, the compact robot prototype could accomplish a relative speed of 0.024 BL/s (with a stride length equivalent to 27% of its body length) and bear a load over five times to its own weight.

Two-dimensional SMA structures have great potential in intelligent and highly flexible robots. However, they have challenges in direct electric heating. This paper proposes a solution to heat SMA sheets using carbon fibers as surface heat sources and indirectly transmits Joule heat generated by the carbon fibers to the SMA sheets. The preparation process of the carbon fiber-based thermoelectric composite film is illustrated in this study. The effect of heat source (carbon fiber) distribution on heat transfer characteristics and the temperature field of the SMA surface is simulated. Then, a functional prototype integrated thermoelectric film and flexible SMA is fabricated, and its effectiveness is verified by local and global activation, respectively. The results show that the composite structure can recover deformation rapidly in 2 s with only 0.7 A running current (applied to each carbon fiber). In addition, the local temperature control method can also realize distributed (spatial dimension) and sequential (temporal dimension) control of the structure. The proposed solution is expected to expand further the design and control of complex, intelligent structures with the development of 4D printing technology.

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