PEG-Induced Controllable Thin-Thickness Gradient and Water Retention: A Simple Way to Programme Deformation of Hydrogel Actuators

Cartilage-Inspired, High-Strength, and Heat-Tolerant Lubricating Hydrogels by Macrophase Separation

Hydrogels are considered as a potential cartilage replacement material based on their structure being similar to natural cartilage, which are of great significance in repairing cartilage defects. However, it is difficult for the existing hydrogels to combine the high load bearing and low friction properties (37 °C) of cartilage through sample methods. Herein, we report a facile and new fabrication strategy to construct the PNIPAm/EYL hydrogel by using the macrophase separation of supersaturated N-isopropylacrylamide (NIPAm) monomer solution to promote the formation of liposomes from egg yolk lecithin (EYL) and asymmetric template method. The PNIPAm/EYL hydrogels possess a relatively high compressive strength (more than 12 MPa), fracture energy (9820 J/m2), good fatigue resistance, lubricating properties, and excellent biocompatibility. Compared with the PNIPAm hydrogel, the friction coefficient (COF 0.046) of PNIPAm/EYL hydrogel is reduced by 50%. More importantly, the COF (0.056) of PNIPAm/EYL hydrogel above lower critical solution temperature (LCST) does not increase significantly, exhibiting heat-tolerant lubricity. The finite element analysis further proves that PNIPAm/EYL hydrogel can effectively disperse the applied stress and dissipate energy under load conditions. This work not only provides new insights for the design of high-strength lubricating hydrogels but also lays a foundation for the treatment of cartilage injury as a substitute material.

Amphibious Nastic Hydrogel Based on the Tropic Movement of Gelatin and Its Opposite Phase Transition to PNIPAm

Mimicking the anisotropic structure and environmental adaptation of organisms in nature remains a key objective in the field of hydrogels. However, it has been very challenging due to complex fabrication and confined application only in water. Here, we demonstrate a new strategy of spontaneous fabrication of an anisotropic hydrogel based on our finding in the tropic movement of gelatin toward the Teflon template. The obtained hydrogel exhibits fast response and recovery under temperature stimuli both in aqueous and non-aqueous environments, making use of the approximate transition temperature and opposite phase transition behavior of gelatin and poly(N-isopropylacrylamide) (PNIPAm). Its recovery performance in water is more than 50 times faster than that of the PNIPAm hydrogel. Furthermore, the PNIPAm/gelatin hydrogel can achieve 3D complex deformations, stealth deformation, erasable and reprogrammed surface patterning, and multistage encryption by simply modulating the location and shape of gelatin to achieve an anisotropic structure. The work provides a simple and versatile way to obtain an anisotropic hydrogel with a definite and predictable structure, which is demonstrated across a range of different monomers. It improves the responsive performance and broadens the hydrogel application to the non-aqueous environment. Additionally, this tropic movement of gelatin can be extended for the design of new types of anisotropic materials and thus endows the materials with diverse functionality.

Gradient hydrogel actuator with fast response and self-recovery in air

The driving principle of a thermal-responsive hydrogel that loses water at high temperature and absorbs water at low temperature limits its application in an aqueous environment. Here, a gradient hydrogel actuator was developed by introducing sodium hyaluronate into poly(N-isopropylacrylamide) hydrogel by an asymmetric mold method. The hydrogel exhibited a fast response above the LCST in air and unusual self-recovery without the need for further temperature stimuli. The actuation behavior was related to conversion from free water to bound water and water retention within the gradient matrix. The self-recovery mechanism was explored. This work provides a new insight into designing bionic hydrogels applied in a non-aqueous environment.

Programmable Multistimuli-Responsive and Multimodal Polymer Actuator Based on a Designed Energy Transduction Network

A polymer actuator typically responds to only one or two types of stimuli, where sensing and actuation are simultaneously exerted by the same responsive polymer. In cells, sensing and actuation are exerted separately by different biomolecules, which are integrated into nanoscale assemblies to construct the signaling network, making cells a multistimuli responsive and multimodal system. Inspired by the structure-function relationship of the signaling network in cells, we have developed a strategy to select and assemble proper functional polymers into assemblies, where sensing and actuation are exerted by different polymers, and the assemblies can present novel functions beyond that of each polymer component. Three polymers [polyaniline, PANi; poly(N-isopropylacrylamide), PNIPAm; and polydimethylsiloxane, PDMS] are integrated as nodes into a simple energy transduction network, which can be regulated by three molecular factors (pH, kosmotropic anions, and polyethylene glycol). PANi converts the light or electric stimulus into heat, which triggers the actuation of PNIPAm and PDMS. Relying on this energy transduction network, the polymer assembly can respond to six types of stimuli (light, electricity, temperature, water, ions, and organic solvents) and perform different actuation modes, serving as a powerful actuator. Programmable complex deformation upon multiple simultaneous or sequential stimuli has also been achieved by this actuator. An adaptive gripper to catch thin objects and a self-regulating switch to maintain environmental humidity illustrate the wide potential of this actuator for next-generation smart materials and soft robots.