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Developing stimulus‐responsive materials (SRMs) with reversible, large, and reprogrammable multi‐responsiveness is crucial for soft actuators and intelligent devices, which remains challenging. In this study, regenerated silk fibroin with hierarchical structure is utilized to specially design a reprogrammable multi‐stimuli‐responsive protein film and versatile soft actuators. The freestanding fibroin films exhibit notable thermal contraction (−1383 ppm K−1) and humidity‐responsiveness that can be repeatedly regulated by easy post‐treatment with Ca2+ as desired. The actuating and regulating mechanisms involve reversible conformational change that is magnified into macroscopic deformations by hierarchical structure, and the roles of material structure and ambient conditions in determining actuating performances are analyzed based on thermodynamics. Driven by humidity gradient, the fibroin film demonstrates spontaneous flipping locomotion, self‐oscillation with tunable frequencies, bio‐butterfly wing flapping, and transformation from 2D to 3D structure. Moreover, reprogrammable deformations at specific regions are achieved in multi‐stimuli‐driven PET/fibroin film actuators owing to the straightforward Ca2+‐content‐based tunability of the responsiveness. The fibroin‐based actuators can be used as artificial muscles to drive the high‐frequency wing‐flapping of a bio‐dragonfly and soft gripper to grasp, lift, and transfer objects. The simple yet effective strategy presented herein provides valuable inspiration for designing advanced SRMs and soft actuators with reprogrammable multi‐responsiveness. [ABSTRACT FROM AUTHOR]

Owing to its excellent mechanical and tribological properties, graphene has been proposed to be a promising atomically-thin solid lubricant for engineering applications. However, as a typical two-dimensional (2D) material, graphene has an exceptionally high surface-to-volume ratio and is very susceptible to the surrounding environments. By performing nanoscale scratch tests on graphene deposited on four different substrates, we have shown that the anti-wear performance of graphene, characterized by the maximum load carrying capacity, is not an intrinsic material property. Instead, its value is significantly affected by the stiffness the substrates: Stiffer substrate typically results in a higher load carrying capacity. As revealed by finite element simulations, stiffer substrate can effectively share the normal load and reduce the in-plane stress of graphene by limiting graphene deformation, which enhances the overall load carrying capacity. In addition to the load sharing mechanism, the experimental results also suggest that the frictional shear stress during scratch tests may facilitate wear of graphene by lowering its equivalent strength. The deformation mechanism of graphene/substrate systems revealed in this work provides guidelines for optimizing the mechanical performance of 2D materials for a wide range of tribological applications. [ABSTRACT FROM AUTHOR]

The unprecedentedly-high mechanical strength and the intrinsically lubricating property make graphene an ideal candidate for atomically-thin solid lubricant. Despite its high potential, graphene was often found to get worn in micro- or macroscale friction tests in ambient conditions. To explore the detailed wear process and evaluate its impact on lubricating performance, we performed tribological experiments on monolayer graphene using a macroscale glass lens under normal loads down to 50 μN. Our ambient experiments show that there existed a critical normal load, below which wear of graphene was undetectable (with friction coefficient on the order of 10 −2 ). Beyond the load threshold, graphene started to become worn and friction slowly increased. In contrast to the abrupt change in nanoscale tests, friction increase due to graphene wear was gradual at the macroscale and it only became apparent (with friction coefficient on the order of 10 −1 ) when graphene damage was substantial. This suggests that a partially worn graphene can remain effectively lubricious at the macroscale. Our comparative experiments also demonstrate that humidity can noticeably minimize graphene wear and substantially extend lubrication life time. The study provides new insights on macroscale wear characteristics of graphene, which is crucial for mechanical applications of this atomically-thin lubricant. [ABSTRACT FROM AUTHOR]