No abstract

Mimicking biological neuromuscular systems’ sensory motion requires the unification of sensing and actuation in a singular artificial muscle material, which must not only actuate but also sense their own motions. These functionalities would be of great value for soft robotics that seek to achieve multifunctionality and local sensing capabilities approaching natural organisms. Here, we report a soft somatosensitive actuating material using an electrically conductive and photothermally responsive hydrogel, which combines the functions of piezoresistive strain/pressure sensing and photo/thermal actuation into a single material. Synthesized through an unconventional ice-templated ultraviolet–cryo-polymerization technique, the homogenous tough conductive hydrogel exhibited a densified conducting network and highly porous microstructure, achieving a unique combination of ultrahigh conductivity (36.8 milisiemens per centimeter, 103-fold enhancement) and mechanical robustness, featuring high stretchability (170%), large volume shrinkage (49%), and 30-fold faster response than conventional hydrogels. With the unique compositional homogeneity of the monolithic material, our hydrogels overcame a limitation of conventional physically integrated sensory actuator systems with interface constraints and predefined functions. The two-in-one functional hydrogel demonstrated both exteroception to perceive the environment and proprioception to kinesthetically sense its deformations in real time, while actuating with near-infinite degrees of freedom. We have demonstrated a variety of light-driven locomotion including contraction, bending, shape recognition, object grasping, and transporting with simultaneous self-monitoring. When connected to a control circuit, the muscle-like material achieved closed-loop feedback controlled, reversible step motion. This material design can also be applied to liquid crystal elastomers.

A strategy of creating stretchable conducting hydrogels for emerging soft electronics is reported. With ice-templated low-temperature polymerization (ITLP), the conducting gel exhibited a hierarchical dendritic microstructure with mitigated nanoaggregation and significantly enhanced electrical conductivity and toughness. Using such gels, strain sensors presented a broad sensing range and high sensitivity for health monitoring. Stretchable solid-state supercapacitors demonstrated remarkable capacitance and flexibility as wearable energy-storage devices. Such a general ITLP method may create diverse soft-electronic materials for energy, healthcare, and robotic applications.

Heterogeneous growth in a myriad of biological systems can lead to the formation of distinct morphologies during the maturation processes of different species. We demonstrate that the distinct circumferential buckling observed in pumpkins can be reproduced by a core− shell barrel structure using four-dimensional (4D) printing, taking advantage of digital light processing (DLP)-based three-dimensional (3D) printing and stimulus-responsive hydrogels. The mechanical mismatch between the stiff core and compliant shell results in buckling instability on the surface. The initiation and development of the buckling are governed by the ratio of core/shell radius, the ratio of core/ shell swelling ratios, and the mismatch between the core and shell in stiffness. Furthermore, the rigid core not only acts as a source of circumferential confinement but also sets a boundary at the poles of the entire structure. The heterogeneous structures with controllable buckling geometrically and structurally behave much like plants' fruits. This replicates the biological morphologic change and elucidates the general mechanism and dynamics of the complex instability formation of heterogeneous 3D objects.

Recent progress in the adhesion mechanism of mussels has led to great excitement in the field of adhesive materials. Although great progress has been made in the mussel adhesion mechanism and underwater adhesives, there are still many unknowns and challenges in this area. Thus, it is highly important to review the recent progress in mechanisms of mussel adhesion and mussel-inspired adhesives and predict trends for the future. In this review, we (1) summarize the research progress in fundamental interaction mechanisms in natural mussels; (2) discuss the application of the mussel interaction mechanism in the biomimetic mussel adhesive materials, from permanent/high-strength adhesives to temporary/smart adhesives; (3) briefly state the potential applications of the mussel-inspired adhesives in multiple fields, such as engineering applications, smart robotics and biomedicine; (4) summarize the future perspectives and unsolved challenges of mussel adhesion mechanisms and mussel-inspired adhesive materials. We envision that this review will provide an insightful perspective in understanding the mussel adhesion mechanism and directions to further explore, and promote the development of novel biomimetic mussel adhesive materials.