Development of Thermoinks for 4D Direct Printing of Temperature-Induced Self-Rolling Hydrogel Actuators

4D printing has emerged as an important technique for fabricating 3D objects from programmable materials capable of time-dependent reshaping. In the present investigation, novel 4D thermoinks composed of laponite (LAP), an interpenetrating network of poly(N-isopropylacrylamide) (PNIPAAm), and alginate (ALG) are developed for direct printing of shape-morphing structures. This approach consists of the design and fabrication of 3D honeycomb-patterned hydrogel discs self-rolling into tubular constructs under the stimulus of temperature. The shape morphing behavior of hydrogels is due to shear-induced anisotropy generated via 3D printing. The compositionally tunable hydrogel discs can be programmed to exhibit different actuation behaviors at different temperatures. Upon immersion in 12 °C water, singly crosslinked sheets roll up into a tubular construct. When transferred to 42 °C water, the tubes first rapidly unfold and then slightly curve up in the opposite direction. Through a dual photocrosslinking of PNIPAAm, it is possible to inverse temperature-dependent shape morphing and induce self-folding at higher and unrolling at lower temperatures. The extensive self-assembling motion is essential to developing thermal actuators with broad applications in, e.g., soft robotics and active implantology, whereas controllable self-rolling of planar hydrogels is of the highest interest to biomedical engineering as it allows for effective fabrication of hollow tubes.     

Actuation of Three-Dimensional-Printed Nanocolloidal Hydrogel with Structural Anisotropy

Polymer hydrogels exhibit actuation properties that result in reversible shape transformations and have promising applications in soft robotics, drug delivery systems, sensors, and microfluidic devices. Actuation occurs due to differential hydrogel swelling and is generally achieved by modulating hydrogel composition. Here a different approach to hydrogel actuation that originates solely from its structural anisotropy is reported. For 3D-printed single-layer hydrogels formed by cellulose nanocrystals (CNCs) and gelatin methacryloyl it is shown that shear-induced orientation of CNCs results in anisotropic mechanical and swelling properties of the hydrogel. Upon swelling in water, planar hydrogels acquire multiple complex 3D shapes that are achieved by i) varying CNC orientation with respect to the shape on the hydrogel sheet and ii) patterning the hydrogel with the regions of shear-mediated and random CNC orientation. This study shows the capability to generate multiple shapes from the same hydrogel actuator based on the degree of its structural anisotropy. In addition, it introduces a biocompatible nanocolloidal ink with shear-thinning and self-healing properties for additive manufacturing of hydrogel actuators.     

Starfish-Inspired Cut-Resistant Hydrogel with Self-Growing Armor: Where Softness Meets Toughness

Many soft animals like starfish have developed armors to protect their soft bodies in order to survive in harsh environment. Inspired by this fact, special hydrogels with self-growing protective armors are developed by allowing sodium acetate to crystallize on hydrogel surface. Poly(acrylic acid) chains limit the crystalline region to the surface of the hydrogel by decreasing the pH value and limiting the movement of ions, and then creating a set of tough armor. This armor-protected hydrogel is able to withstand a high pressure (> 78 MPa) under cutting and prevent the penetration of sharp objects. Interestingly, the unique stimulated-precipitation mechanism allows the armor to repair itself after damage. Besides, the surface of hydrogel changes from “sticky” to “slippery”, which also helps to improve its protective ability. Moreover, the armor helps to retrain water in hydrogel network, and significantly improve the mechanical properties of hydrogels (maximum compressive stress >18 MPa, compressive strain > 90%). In addition, the hydrogel can keep soft and work durably at extreme temperatures (−50 °C and 80 °C) due to the high salt concentration. This study provides an innovative approach for designing armor-protected hydrogels with great potential in engineering applications such as actuators and sensors for harsh environments.     

Microstructured Actuation of Liquid Crystal Polymer Networks

Smart microstructured materials enable functions such as actuation, detection, transportation, and sensing with potential applications ranging from robotics and photonics to biomedical devices. Of the many materials systems, liquid crystal polymer networks (LCN) are fascinating owing to their ability to exhibit reversible macroscopic deformation driven by a molecular order–disorder phase transition. LCN have been increasingly explored for their utility in the design and fabrication of smart actuating devices capable of complex shape changes or motions upon external stimulation of humidity, heat, light, and other stimuli, and recent studies in this field show that their actuation complexity can be enriched and actuation performance enhanced by having some sort of microstructures. Herein, the recent progress in microstructured actuation of LCN materials with substructures in scale ranging from micrometer to millimeter is reported, placing the emphasis on the main approaches to generating a microstructure in LCN, which include patterned LC director fields, patterned chain crosslinking in LCN with uniaxial orientation of mesogens, 3D/4D printing, and replica molding. The potential applications in microstructured 3D actuators and devices as well as functional LCN surfaces are also highlighted, with an outlook on important issues and future trends in smart microstructured LCN materials and actuators.     

Reversible Electrochemically Triggered Delamination Blistering of Hydrogel Films on Micropatterned Electrodes

Stimuli responsive elastic instabilities provide opportunities for controlling the structures and properties of polymer surfaces, offering a range of potential applications. Here, a surface actuator based on a temperature and electrically responsive poly(N‐isopropyl acrylamide‐co‐sodium acrylate) hydrogel that undergoes a two‐step delamination and buckling instability triggered using micropatterned electrodes is described. The electrically actuated structures entail large out‐of‐plane displacements that take place on time‐scales of less than 1 s, in response to modest triggering voltages (?3–6 V). Alongside these experimental observations, finite element simulations are conducted to better understand the two‐step nature of the instability. In the first step, hydrogel films undergo delamination and formation of blisters, facilitated by electrochemical reduction of the thiol groups anchoring the film to the electrodes. Subsequently, at larger reducing potentials, the electrolytic current is sufficient to nucleate a gas bubble between the electrode and the gel, causing the delaminated region to adopt a straight‐sided blister shape. Finally, thermally induced deswelling of the gel allows the film to be returned to its flat state and readhered to the electrode, thereby allowing for repeated actuation.     

Wet‐Responsive,Reconfigurable, and Biocompatible Hydrogel Adhesive Films for Transfer Printing of Nanomembranes

This study presents a wet‐responsive and biocompatible smart hydrogel adhesive that exhibits switchable and controllable adhesions on demand for the simple and efficient transfer printing of nanomembranes. The prepared hydrogel adhesives show adhesion strength as high as ≈191 kPa with the aid of nano‐ or microstructure arrays on the surface in the dry state. When in contact with water, the nano/microscopic and macroscopic shape reconfigurations of the hydrogel adhesive occur, which turns off the adhesion (≈0.30 kPa) with an extremely high adhesion switching ratio (>640). The superior adhesion behaviors of the hydrogels are maintained over repeating cycles of hydration and dehydration, indicating their ability to be used repeatedly. The adhesives are made of a biocompatible hydrogel and their adhesion on/off can be controlled with water, making the adhesives compatible with various materials and surfaces, including biological substrates. Based on these smart adhesion capabilities, diverse metallic and semiconducting nanomembranes can be transferred from donor substrates to either rigid or flexible surfaces including biological tissues in a reproducible and robust fashion. Transfer printing of a nanoscale crack sensor onto a bovine eye further demonstrates the potential of the reconfigurable hydrogel adhesive for use as a stimuli‐responsive, smart, and versatile functional adhesive for nanotransfer printing.     

Hole‐Programmed Superfast Multistep Folding of Hydrogel Bilayers

Two important aspects of actuation behavior of stimuli‐responsive hydrogels are the complexity of the shape change and its speed. Here, it is shown that varying the shape of simple polymer bilayers can result in very complex and very fast spontaneous folding. The complexity and high folding rate arise from the choice of the shape and from the presence of inhomogeneous swelling within the thermoresponsive layer entrapped between the top hydrophobic layer and the substrate. In contrast to homogeneous swelling of a freestanding bilayer, which leads to a gradual increase of curvature throughout the whole bilayer, inhomogeneous swelling first results in complete rolling of the periphery of the film, which changes its mechanical properties and affects the subsequent morphing process. Further swelling of the thermoresponsive layer generates more stress that builds up until a buckling threshold is overcome, allowing very fast switching from the flat edge‐rolled configuration into a folded one. The research demonstrates how the introduction of holes into actuating bilayers gives rise not only to a novel geometric control over the folding fate of the films but also adds the ability to tune the rate of folding, through the careful selection of hole size, location, and shape.     

Selective Stiffening in Soft Actuators by Triggered Phase Transition of Hydrogel-Filled Elastomers

Nature has inspired a new generation of robots that not only imitate the behavior of natural systems but also share their adaptability to the environment and level of compliance due to the materials used to manufacture them, which are typically made of soft matter. In order to be adaptable and compliant, these robots need to be able to locally change the mechanical properties of their soft material-based bodies according to external feedback. In this work, a soft actuator that embodies a highly controllable thermo-responsive hydrogel and changes its stiffness on direct stimulation is proposed. At a critical temperature, this stimulation triggers the reversible transition of the hydrogel, which locally stiffens the elastomeric containment at the targeted location. By dividing the actuator into multiple sections, it is possible to control its macroscopic behavior as a function of the stiffened sections. These properties are evaluated by arranging three actuators into a gripper configuration used to grasp objects. The results clearly show that the approach can be used to develop soft actuators that can modify their mechanical properties on-demand in order to conform to objects or to exert the required force.     

Tunable Dry Adhesion of Soft Hollow Pillars through Sidewall Buckling under Low Pressure

Controlling adhesion on demand is essential for many manufacturing and assembly processes such as microtransfer printing. Among various strategies, pneumatics-controlled switchable adhesion is efficient and robust but currently still suffers from challenges in miniaturization and high energy cost. In this paper, a novel way to achieve tunable adhesion using low pressure by inducing sidewall buckling in soft hollow pillars (SHPs) is introduced. It is shown that the dry adhesion of these SHPs can be changed by more than two orders of magnitude (up to 151×) using low activating pressure (≈−10 or ≈20 kPa). Large enough negative pressure triggers sidewall buckling while positive pressure induces sidewall bulging, both of which can significantly change stress distribution at the bottom surface to facilitate crack initiation and reduce adhesion therein. It is shown that a single SHP can be activated by a micropump to manipulate various lightweight objects with different curvatures and surface textures. Here, it is also demonstrated that an array of SHPs can realize selective pick-and-place of an array of objects. These demonstrations illustrate the robustness, simplicity, and versatility of these SHPs with highly tunable dry adhesion.     

Humidity‐Driven Mechanical and Electrical Response of Graphene/Cloisite Hybrid Films

Humidity‐driven and electrically responsive graphene/cloisite hybrid films are obtained by casting water dispersions of graphene oxide and cloisite Na⁺. Coupling hydrophilicity and a high water vapor barrier in a homogenous film enables to realize humidity‐driven actuators which exploit the water gradient generated across the film section under asymmetric exposure to humidity. The hybrid films are self‐standing, flexible, and exhibit fast humidity‐triggered bidirectional bending up to 75°, which is tuned by varying the relative amount of the two components. Up to 60% of the bending angle can be preserved at the steady state, providing a large and reliable response to humidity. Moreover, thermal treatment results in the reduction of graphene oxide, endowing the films with humidity‐dependent electrical conductivity, which increases from 1.5 × 10^?6 S at 20% relative humidity (RH) up to 2.7 × 10^?5 S at 90% RH. The films are used to realize a humidity‐sensitive electrical switching system in which the reversible actuation is due to water desorption induced by the Joule effect. Due to their ease of preparation and tunable properties, this new class of graphene‐based materials is suitable for the realization of humidity‐driven and electrically responsive actuators and sensors.     

Engineered Elastomer Substrates for Guided Assembly of Complex 3D Mesostructures by Spatially Nonuniform Compressive Buckling

Approaches capable of creating 3D mesostructures in advanced materials (device‐grade semiconductors, electroactive polymers, etc.) are of increasing interest in modern materials research. A versatile set of approaches exploits transformation of planar precursors into 3D architectures through the action of compressive forces associated with release of prestrain in a supporting elastomer substrate. Although a diverse set of 3D structures can be realized in nearly any class of material in this way, all previously reported demonstrations lack the ability to vary the degree of compression imparted to different regions of the 2D precursor, thus constraining the diversity of 3D geometries. This paper presents a set of ideas in materials and mechanics in which elastomeric substrates with engineered distributions of thickness yield desired strain distributions for targeted control over resultant 3D mesostructures geometries. This approach is compatible with a broad range of advanced functional materials from device‐grade semiconductors to commercially available thin films, over length scales from tens of micrometers to several millimeters. A wide range of 3D structures can be produced in this way, some of which have direct relevance to applications in tunable optics and stretchable electronics.