Professor Zhang Shiwu’s research team from the Institute of Robotics and Intelligent Equipment, School of Engineering Sciences, University of Science and Technology of China, and the joint research group of British and Australian collaborators have successfully developed a new composite material that can respond to changes in external mechanical loads and electrical signals to autonomously adjust mechanical stiffness, conductivity and sensitivity. The results were published in Science Advances on January 25, 2023, under the title “Electro-mechano responsive elastomers with self-tunable conductivity and stiffness” (DOI: 10.1126/sciadv.adf1141).
Physical properties such as conductivity and mechanical stiffness of traditional materials are often fixed. But today’s growing number of applications, such as soft robotics, medical surgical equipment, and reconfigurable electronics, require a smart material that actively adjusts physical properties in response to environmental changes. Composites consisting of low-melting alloys and elastomeric polymers are a widely adopted solution that enables switching between soft/hard and conductive/insulating by melting/solidifying alloy fillers near room temperature. However, existing materials of this type often require external controls to regulate temperature and cannot autonomously respond to environmental changes such as pressure or deformation. These materials can only switch between insulators and conductors, and cannot achieve continuous adjustment of resistance.
Fig. 1 (A) microstructure and (B) electrical properties of FMHE
To fill this research gap, the joint research group developed a composite conductive elastomer (FMHE) composed of nickel micron particles, low-melting field alloys (FM) and polymer matrices. Thanks to a multi-packing conductive network formed by irregular nickel particles and FM particles (Figure 1A), the conductivity of this material can be exponentially enhanced by more than ten million times under mechanical loads including compression, tension, torsion, bending. When the material is heated above 60°C, the FM particles in it melt. Melted FM droplets cannot touch each other like solid FM particles to form a conductive path, but instead deform with the polymer matrix under load. This significantly reduces the material’s elastic modulus, conductivity, and strain sensitivity (Figure 1B). Since the resistance of the material is significantly reduced during deformation, composites powered by a 3V voltage can be heated at a specific pressure to melt FM particles, enabling self-triggering coordinated adjustment of stiffness and resistance.
Figure 2 (A) variable stiffness compensator and (B) reusable fuse developed based on FMHE
By combining the adjustable resistance/stiffness properties of this material, the research group developed a variable stiffness multiaxial flexible compensator that can be used in robotic arm joints (Figure 2A). This compensator can compensate the robotic arm for position and angle errors through deformation, thus avoiding damage to motors and equipment due to bumps in complex operating environments. In addition, when the joint deformation reaches a preset amplitude, the compensator can be triggered to reduce the stiffness to further increase the amount of compensation. Compared to the state-of-the-art commercial compensation unit (bendable 1°), the device developed by the research group provides much larger bending compensation (bendable 16.5°). In addition, the research group developed a reusable current-limiting cryogenic fuse based on this material (Figure 2B). When the preset fusing current is reached, the resistance of the fuse can be increased 1000 times in 0.1 seconds to cut off the circuit and return to a usable state within 10 seconds. Compared to commercial reusable fuses, it has a more compact structure (<1mm), lower fusing temperature (65°C), and faster fusing and recovery speed. This smart material that responds to environmental changes enables the synergistic use of adjustable electrical and mechanical properties, demonstrating its potential to revolutionize the next generation of soft robots and electronic devices.
Guolin Xian, a postdoctoral fellow at the School of Engineering Sciences, University of Science and Technology of China, is the first author of the paper. Professor Shiwu Zhang of the University of Science and Technology of China, Associate Professor Shiyang Tang of the University of Birmingham, and Professor Weihua Li of the University of Wollongong in Australia are co-corresponding authors of the paper, and the collaborators include Professor Tawfique Hasan of the University of Cambridge, UK. This research was supported by the National Natural Science Foundation of China. (Source: University of Science and Technology of China)
Related paper information:https://doi.org/10.1126/sciadv.adf1141
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