It is discovered that poly(vinyl alcohol) (PVA) hydrogel prepared using the freezing/thawing method can selfrepair at room temperature without the need for any stimulus or healing agent. The autonomous self-healing process can be fast for mechanically strong PVA hydrogel yielding a high fracture stress. Investigation on the effect of the hydrogel preparation conditions points out that hydrogen bonding between PVA chains across the interface of the cut surfaces is at the origin of the phenomenon. The key for an effective selfhealing is to have an appropriate balance between high concentration of free hydroxyl groups on PVA chains on the cut surfaces prior to contact and sufficient PVA chain mobility in the hydrogel. S elf-healing materials possess the capability of repairing themselves after damages, which is a striking property that can prolong the lifetime of these materials and, thus, lower the cost. In recent years there is fast growing interest on a variety of self-healing polymers. 1−11 Of them, self-healing hydrogels have attracted much attention due to their great potential in biomedical applications. 12−24 It is no surprise to see that the main strategies for making self-healable hydrogels are all built up with the use of dynamic covalent bonds 16 or supramolecular interactions such as hydrogen bonding, electrostatic interaction, host−guest recognition, metal−ligand coordination, hydrophobic association, and π−π stacking. 17−24 Despite the exciting progress made on self-healing hydrogels, important challenges still remain. On the one hand, hydrogels for biomedical applications must have good biocompatibility and nontoxicity, whereas the designed self-healable hydrogels generally put the emphasis on their self-healing property without taking the biocompatibility and toxicity issues into great account. On the other hand, the current generation of self-healable supramolecular hydrogels generally suffers from low mechanical strength, which may be problematic for some biomedical applications such as tissue engineering scaffolds. Basically, the self-healing ability of a hydrogel is antagonist of its mechanical strength, because good polymer chain mobility, which favors chain diffusion across an interface of cut or fractured surfaces and predominantly influences the efficiency of self-healing, often means low mechanical strength of the hydrogel. It can also be noticed that many self-healing hydrogels are stimulihealable hydrogels because their repairing process requires the input of a stimulus to be activated. 16,18,22−24 Here we report the discovery that poly(vinyl alcohol) (PVA) hydrogel prepared under appropriate conditions can autonomously self-heal, exhibiting both good self-healing capability and mechanical strength. This finding is important because PVA hydrogel has been extensively studied and considered as one of the hydrogels the most suitable for biomedical applications due to its biocompatibility and nontoxicity. 25 The unveiled self-healing nature of PVA hydrogel adds a new appealing property to t...

A new strategy for enhancing the photoinduced mechanical force is demonstrated using a reprocessable azobenzene-containing liquid crystalline network (LCN). The basic idea is to store mechanical strain energy in the polymer beforehand so that UV light can then be used to generate a mechanical force not only from the direct light to mechanical energy conversion upon the trans-cis photoisomerization of azobenzene mesogens but also from the light-triggered release of the prestored strain energy. It is shown that the two mechanisms can add up to result in unprecedented photoindued mechanical force. Together with the malleability of the polymer stemming from the use of dynamic covalent bonds for chain crosslinking, large-size polymer photoactuators in the form of wheels or spring-like "motors" can be constructed, and, by adjusting the amount of prestored strain energy in the polymer, a variety of robust, light-driven motions with tunable rolling or moving direction and speed can be achieved. The approach of prestoring a controllable amount of strain energy to obtain a strong and tunable photoinduced mechanical force in azobenzene LCN can be further explored for applications of light-driven polymer actuators.

A near-infrared-light (NIR)- and UV-light-responsive polymer nanocomposite is synthesized by doping polymer-grafted gold nanorods into azobenzene liquid-crystalline dynamic networks (AuNR-ALCNs). The effects of the two different photoresponsive mechanisms, i.e., the photochemical reaction of azobenzene and the photothermal effect from the surface plasmon resonance of the AuNRs, are investigated by monitoring both the NIR- and UV-light-induced contraction forces of the oriented AuNR-ALCNs. By taking advantage of the material's easy processability, bilayer-structured actuators can be fabricated to display photocontrollable bending/unbending directions, as well as localized actuations through programmed alignment of azobenzene mesogens in selected regions. Versatile and complex motions enabled by the enhanced photocontrol of actuation are demonstrated, including plastic "athletes" that can execute light-controlled push-ups or sit-ups, and a light-driven caterpillar-inspired walker that can crawl forward on a ratcheted substrate at a speed of about 13 mm min . Moreover, the photomechanical effects arising from the two types of light-triggered molecular motion, i.e., the trans-cis photoisomerization and a liquid-crystalline-isotropic phase transition of the azobenzene mesogens, are added up to design a polymer "crane" that is capable of performing light-controlled, robot-like, concerted macroscopic motions including grasping, lifting up, lowering down, and releasing an object.

Liquid crystal elastomers (LCEs), a class of soft materials capable of a large and reversible change in the shape under the trigger of external stimuli, can be fabricated into diverse architectures with complicated deformation modes through four-dimensional (4D) printing. However, the printable LCE ink is only in the form of monomeric precursors and the deformation mode is limited to contraction/extension deformation. Herein, we report a novel approach to break through these limitations. We achieved 4D printing of a single-component liquid crystal polymer ink in its isotropy state through direct ink writing (DIW) technology. The drawing force imposed by the movement of nozzle in the extruded printing process was able to align the mesogen units along the specific printing path. An orientation gradient perpendicular to the printing direction was obtained due to the existence of a temperature gradient between the two sides of printed samples and could be further fixed by post-photo-cross-linking treatment through the dimerizable groups in the LCE, realizing a new actuation mode in the field of extrusion-based printing of LCE actuators. The printed film was able to change reversibly from a strip to a tightly hollow cylinder and could reversibly lift up an object with roughly 600 times its own weight. The orientation gradient can be patterned through liquid-assistant printing or programmed structure design to integrate both bending and contraction actuation modes on the same printed sample, leading to complex deformation and two-dimensional (2D) planar porous structure to three-dimensional (3D) porous cylinder transition. This study opens up a new prospect to directly print a wide variety of LCE actuators with versatile actuation modes.

A new optically triggered shape memory composite material was prepared and investigated. Poly(3-caprolactone) (PCL)-surface functionalized AuNPs were loaded in a thermosensitive shapememory polymer (SMP) matrix of biodegradable, branched oligo(3-caprolactone) (bOCL) cross-linked with hexamethylene diisocyanate (HMDI), referred to as XbOCL. By making use of a localized photothermal effect arising from the SPR absorption of AuNPs, we are able to demonstrate an optically triggered and spatially selective shape recovery process, with a stretched AuNP-loaded XbOCL film undergoing stepwise contraction and lifting of a load. Since the shape recovery process can be halted at any time by turning off the light exposure, multiple intermediate shapes can readily be obtained. These are appealing features that cannot be obtained from thermally activated SMPs based on a bulk thermal effect. Moreover, the magnitude of the photoinduced temperature increase of the material can be controlled by adjusting the laser power, it is also possible to use the same AuNP-loaded composite material for applications with different environmental temperatures below T transition , since the thermal transition at T > T transition can be optically induced by a laser from different environmental temperatures.