The Dynamic Mortise-and-Tenon Interlock Assists Hydrated Soft Robots Toward Off-Road Locomotion

Abstract

Natural locomotion such as walking, crawling, and swimming relies on spatially controlled deformation of soft tissues, which could allow efficient interaction with the external environment. As one of the ideal candidates for biomimetic materials, hydrogels can exhibit versatile bionic morphings. However, it remains an enormous challenge to transfer these in situ deformations to locomotion, particularly above complex terrains. Herein, inspired by the crawling mode of inchworms, an isotropic hydrogel with thermoresponsiveness could evolve to an anisotropic hydrogel actuator via interfacial diffusion polymerization, further evolving to multisection structure and exhibiting adaptive deformation with diverse degrees of freedom. Therefore, a dynamic mortise-and-tenon interlock could be generated through the interaction between the self-deformation of the hydrogel actuator and rough terrains, inducing continual multidimensional locomotion on various artificial rough substrates and natural sandy terrain. Interestingly, benefiting from the powerful mechanical energy transfer capability, the crawlable hydrogel actuators could also be utilized as hydrogel motors to activate static cargos to overstep complex terrains, which exhibit the potential application of a biomimetic mechanical discoloration device. Therefore, we believe that this design principle and control strategy may be of potential interest to the field of deformable materials, soft robots, and biomimetic devices.

Fig. 1.

The adaptive evolution and all-terrain locomotion of the PNIPAm sponge. (A) An isotropic PNIPAm sponge could be endowed with an anisotropic structure by growing a passive hydrogel layer via IDP after preparation. (B) The bilayer hydrogel was cut into 3 blocks and reassembled via IDP to form the new anisotropic structure. Based on the anisotropic structure, the linear hydrogel actuator was capable of moving on a rugged substrate via adaptive deformation like the inchworm crawling above branches. (C) The linear hydrogel actuator would evolve into the hexapod actuator and pass through the complex and narrow space.

Fig. 2.

The fabrication and performance of the thermoresponsive hydrogel actuator. (A) The IDP-induced hydrogel growing process. The thermoresponsive hydrogel actuator was prepared by pouring the hydrogel precursor onto the surface of the PNIPAm sponge containing the initiator. (B) The actuating velocity of the bilayer hydrogel actuator with different thicknesses of the ordinary PNIPAm layer and the PNIPAm sponge layer. (C) Comparison of bending velocity of the existing hydrogel actuator [,–,–53].

Fig. 3.

Evolution-based multifunctional hydrogel actuator. (A) Illustration schematic showing the preparation of the hydrogel oscillator. The photothermal hydrogel oscillator was fabricated via a growing photothermal hydrogel layer containing Fe₃O₄ in the middle area and a passive hydrogel layer in the rest area, respectively. (B) Images show the phototaxis of the hydrogel actuator. (C) Illustration schematic showing the oscillating mechanism of the hydrogel oscillator. The hydrogel oscillator would bend toward the direction of the NIR source. (D) Tip trajectory of the hydrogel oscillator during the oscillating process. (E) Illustration schematic showing the fabricating process of the hydrogel actuator with a multisection structure. A novel hydrogel actuator with a multisection structure was fabricated by cutting a bilayer hydrogel actuator into 4 sections and rearranging them via IDP. (F) Images show the multidimensional deformation of the hydrogel actuator with a multisection structure with the assistance of programmable NIR. Scale bars: 1 cm.

Fig. 4.

Inchworm-inspired locomotion of the hydrogel actuator. (A) The inchworm-inspired locomotion of the bilayer hydrogel actuator. The hydrogel actuator is anchored to the substrate via deformation, inducing mortise-and-tenon interlock and crawling forward. Then, the body would deform to an “S” shape, removing the mortise-and-tenon interlock and beginning the next cycle. (B) The feature trajectories of the head and tail of the hydrogel actuator within one crawling cycle. (C) The crawling velocity of the bilayer hydrogel actuator with different ratios of head and tail. (D and E) The locomotion velocity and process of the hydrogel actuator in different terrains. (F) Comparison of moving velocity of the existing soft robots[19,20,22,27,48,54,55]. Scale bars: 1 cm.

Fig. 5.

The multimode locomotion of the bilayer hydrogel actuator. (A) Schematic showing the dual crawling mode where both the head and tail of the hydrogel actuator could export forward force. (B) The feature trajectories of the head and tail of the hydrogel actuator in dual crawling mode. (C) The bilayer hydrogel actuator tended to crawl to the side with stronger NIR when a weak NIR and stronger NIR are irradiated successively. (D) The 2D locomotion of the bilayer hydrogel actuator within the included angle between the left and right arm.

Fig. 6.

Two-dimensional off-road locomotion and applications of bilayer hydrogel motors. (A) Schematic showing the assembly process of the hydrogel motor. The model component was loaded into the PNIPAm sponge and assembled with 3 bilayer hydrogel motors via IDP. (B) The composite soft robot could crawl and pass through a 2D maze. (C) The fabrication process of the off-road hydrogel robot. A 6-claw-shaped bilayer hydrogel was cut into 7 parts and reconnected via IDP. (D) Off-road process of the hydrogel robot. The reconfigured 6-claw-shaped hydrogel actuator could pass through the narrow mountain pass and off-road the complex 2D terrain via the coupling deformation of 6 claws. Scale bars: 2 cm.