Autonomous lifting: Swelling-activated 3D-printed actuators

Stimuli-responsive actuators respond to external cues such as water uptake, thermal excitation, and electric fields to perform mechanical work. Typically, such systems comprise active (or responsive) components and require an external power source to initiate actuation. These two factors may pose limitations on performance and implementation. In this paper, we present a two-component autonomous system that addresses these potential drawbacks. Specifically, the proposed actuator is made of an active (responsive) component and a non-responsive structural element that performs the mechanical work. The design features non-responsive 3D-printed elastic rods positioned between superabsorbent polymer gel beads placed in cylindrical hinges. As the gels swell, a compressive force is exerted on the elastic rods, and their buckling is used to obtain mechanical energy. We first investigated a single-rod system, where submersion in water caused the gels to swell, applying axial forces on the rod. Once a critical threshold force was reached, the rod experienced Euler buckling and a significant vertical displacement. Expanding the design to configurations with four and eight rods that are cross-braced, we observed that the amplitude followed the swelling kinetics of the gels, as expected. To assess load-bearing capabilities, we placed weights at the intersection of the rods. The four- and the eight-rod systems are capable of lifting up to (Formula presented) and (Formula presented), respectively. In the case of the eight-rod design, the actuator can lift more than 40% of its own weight. To better understand the experimental data, an elastic stability analysis was carried out and revealed that the critical buckling force is independent of the applied load. By decoupling actuation from structural support, our approach provides a versatile design strategy for autonomous load-adaptive devices that can operate in partially aqueous environments.