Fatigue fracture of tough hydrogels

Tough hydrogels of many chemical compositions have been developed in recent years, but their fatigue fracture has not been studied. The lack of study hinders further development of hydrogels for applications that require long lifetimes under cyclic loads. Examples include tissue engineering, soft robots, and stretchable electronics. Here we study the fatigue fracture of a polyacrylamide-alginate tough hydrogel. We find that the stress-stretch curve changes cycle by cycle, and reaches a steady state after thousands of cycles. The threshold for fatigue fracture is about 53 J/m2, much below the fracture energy (~10,000 J/m2) measured under monotonic load. Nonetheless, the extension of crack per cycle in the polyacrylamide-alginate tough hydrogel is much smaller than that in a single-network polyacrylamide hydrogel.Engineering and Applied Science

Self-sensing magnetic actuators of bilayer hydrogels

ABSTRACTHard magnetic soft robots have been widely used in biomedical engineering. In these applications, it is crucial to sense the movement of soft robots and their interaction with target objects. Here, we propose a strategy to fabricate a self-sensing bilayer actuator by combining magnetic and ionic conductive hydrogels. The magnetic hydrogel containing NdFeB particles exhibits rapid response to magnetic field and achieve bending deformation. Meanwhile, the polyacrylamide (PAAm) hydrogel with lithium chloride (LiCl) allows for the sensing of deformation. The bending behavior of the bilayer under magnetic field is well captured by theoretical and simulated models. Additionally, the bilayer strain sensor shows good sensitivity, stability and can endure a wide-range cyclic stretching (0–300%). These merits qualify the self-sensing actuator to monitor the motion signals, such as bending of fingers and grasping process of an intelligent gripper. When subject to an external magnetic field, the gripper can grab a cube and sense the resistance change simultaneously to detect the object size. This work may provide a versatile strategy to integrate actuating and self-sensing ability in soft robots

Fatigue of double-network hydrogels

The discovery of tough hydrogels of many chemical compositions, and their emerging applications in medicine, clothing, and engineering, has raised a fundamental question: How do hydrogels behave under many cycles of stretch? This paper initiates the study of the fatigue behavior of the classic PAMPS/PAAM double network hydrogels discovered by Gong and her co-workers (Advanced Materials 15, 1155, 2003). We reproduce the hydrogels, and prepare samples of two types, with or without a crack cut before the test. When an uncut sample is subject to cyclic stretches, internal damage accumulates over thousands of cycles until a steady state is reached. When a cut sample is subject to cyclic stretches, the crack extends cycle by cycle if the amplitude of stretch is above a certain value. A threshold of energy release rate exists, below which the crack remains stationary as the sample is cycled. We find a threshold around 400 J/m2 for hydrogels containing PAAM networks of a low density of crosslinkers, and around 200 J/m2 for hydrogels containing PAAM networks of a high density of crosslinkers. The experimental findings are compared to the Lake-Thomas model adapted to the double-network hydrogels.Engineering and Applied Science

Stretchable Seal

Many stretchable electronic devices require stretchable hermetic seals. However, stretchability and permeability are inextricably linked at the molecular level: stretchable, low-permeability materials do not exist. We collect data for the permeation of water and oxygen in many materials and describe the scaling relations for both flat and wrinkled seals. Whereas flat seals struggle to fulfill the simultaneous requirements of stretchability, low stiffness, and low transmissibility, wrinkled seals can fulfill them readily. We further explore the behavior of wrinkled seals under cyclic stretch using aluminum, polyethylene, and silica films on elastomer substrates. The wrinkled aluminum develops fatigue cracks after a small number of cycles, but the wrinkled polyethylene and silica maintain low transmissibility after 10 000 cycles of tensile strain

Theoretical Analysis on the Nonlinear Free Vibration of a Tri-Cross String

Here we present a theoretical analysis on the nonlinear free vibration of a tri-cross string system, which is an element of space net-antennas. We derived the governing equations from Hamilton’s principle and obtained a linearized solution by the standard perturbation method. The semi-analytical solutions of the governing equations have not been provided referring to the solution of plate vibrating problem. This analysis revealed that natural frequencies of the tri-cross string depend on the vibration amplitude due to the geometrical nonlinearity in the constitutive equation. The geometric parameters, such as the diameters and the lengths of the constituent strings, also affect the frequency through the nonlinearity of the tri-cross string. The nonlinear natural frequency shows coupled characteristic; that is, the natural frequency of the tri-cross string varies with that of the constituent strings, but the contribution of each constituent string to the natural frequency is in different proportions