Hydrogels, exhibiting wide applications in soft robotics, tissue engineering, implantable electronics, etc., often require sophisticately tailoring of the hydrogel mechanical properties to meet specific demands. For examples, soft robotics necessitates tough hydrogels; stem cell culturing demands various tissue‐matching modulus; and neuron probes desire dynamically tunable modulus. Herein, a strategy to broadly alter the mechanical properties of hydrogels reversibly via tuning the aggregation states of the polymer chains by ions based on the Hofmeister effect is reported. An ultratough poly(vinyl alcohol) (PVA) hydrogel as an exemplary material (toughness 150 ± 20 MJ m−3), which surpasses synthetic polymers like poly(dimethylsiloxane), synthetic rubber, and natural spider silk is fabricated. With various ions, the hydrogel's various mechanical properties are continuously and reversibly in situ modulated over a large window: tensile strength from 50 ± 9 kPa to 15 ± 1 MPa, toughness from 0.0167 ± 0.003 to 150 ± 20 MJ m−3, elongation from 300 ± 100% to 2100 ± 300%, and modulus from 24 ± 2 to 2500 ± 140 kPa. Importantly, the ions serve as gelation triggers and property modulators only, not necessarily required to remain in the gel, maintaining the high biocompatibility of PVA without excess ions. This strategy, enabling high mechanical performance and broad dynamic tunability, presents a universal platform for broad applications from biomedicine to wearable electronics.

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.

No abstract

Smart interactive electronic devices can dynamically respond to and visualize environmental stimuli. Inspired by the rapid color changes of natural creatures, an interactive electronic fiber sensor with high stretchability and tunable coloration is presented. It is based on an ingenious multi-sheath design on a piezoresistive electronic fiber coupled with a mechanochromic photonic crystal microtubule. It has the unique capabilities of sensing and visualizing its deformation simultaneously, by reconstructing conductive paths and regulating the lattice spacing of the photonic sheath. In particular, it exhibits dynamic color switching spanning the full visible region (from red to blue), fast optical/electrical response (≈80 ms), and a large working range (0-200%), allowing its application as a user-interactive sensor for dynamically monitoring large joint movements and muscle microvibrations of the human body in real time. This investigation provides a general platform for emerging interactive devices, which are promising for applications in wearable electronics, human-machine interactions, and intelligent robots.

Structural colors of 2D gratings are iridescent, color‐tunable, and never fade, which renders them appealing for anti‐counterfeiting applications. However, for advanced security, it still remains a challenge to completely hide the encrypted color patterns and reveal them on demand. In this work, a water‐responsive photonic grating consisting of a micropillar array and a hydrogel overcoat with a similar refractive index, termed “hydrocipher”, is presented. The joint effect of stimuli‐reversible refractive‐index (mis)match and reconfigurable grating‐based diffraction coloration enables a complete encryption of the structural color and rapid decryption. The photonic structure shows a strong iridescence due to the angle‐dependent diffraction when the hydrogel overcoat is swollen from water. Upon drying, the micropillars bend and the refractive index contrast disappears, which dramatically lessens the diffraction intensity and renders the surface highly transparent. The dehydrated‐to‐hydrated state transition can occur within 1 s, enabling fast decryption. The color switching is highly reversible over a prolonged hydration/dehydration cycle, and the dehydrated hydrogel layer protects the delicate micropillar array from external mechanical stress. The creative combination of hydrogel material and the 2D grating structure offers a new and simple strategy for realizing reversible, durable, and fast‐response cryptography with potentially broad impact on the anti‐counterfeiting technology market.