New Progress in the Field of Flexible / Elastic Magnetoelectric Functional Materials and Devices

Wearable and implantable devices are the basis of human motion monitoring, health monitoring, and human-computer interaction technologies. They have huge application prospects in trillion-level industries such as smart medical care and intelligent robots. The development trend is flexibility and even elasticity. It is the core and key to develop flexible and even elastic magnetoelectric functional materials and devices. However, in general, magnetoelectric functional materials are mostly inorganic materials such as metals or oxides, which have poor flexibility; flexible or elastic materials are mostly polymer materials, which usually do not have magnetoelectric functions. How to flex or elasticize magnetoelectric functional materials, or functionalize flexible or elastic materials is a major challenge in this field. To this end, the Magnetoelectronic Materials and Devices team of the Key Laboratory of Magnetic Materials and Devices, Chinese Academy of Sciences has carried out research work on the flexibility and elasticity of functional materials such as conductive and magnetic materials, and has achieved flexible and elastic magnetoelectric functional materials, conductors and sensors. series progress.

(1) Large-strain elastic conductive materials and elastic heating devices

Stretchable conductive materials are usually nano- or micro-scale conductive fillers (graphene, carbon nanotubes, metal nanowires/nanoparticles, etc.) incorporated into elastic polymers, and processed by dispersion or layered composites. The obtained multiphase composite system with conductive function. Since the elastic modulus of the solid conductive filler and the elastic matrix is ​​very different (about 1 million times), the gap between the filler particles forming the conductive path will change significantly when the strain is large, resulting in unstable conductivity; in addition, the introduction of a large number of solid conductive fillers It will improve the conductivity of the conductive material, and it will also deteriorate its elasticity, resulting in limited doping, so its conductivity is generally poor. How to obtain elastic conductors compatible with high electrical conductivity, tensile stability and large strain remains a challenge.

In order to solve the above problems, doctoral students Yu Zhe, Researcher Shang Jie and Researcher Li Runwei used liquid metal as a conductive filler, and at the same time built a “gourd string”-shaped conductive network structure in the conductor to release the strain and further improve its strain stability. The results show that the electrical conductivity of the stretchable conductive material can reach the conductor range (greater than 1000 S/cm), and can achieve more than 1000% stretching. More importantly, the resistance fluctuation is less than 4% when stretched by 100%. , the change rate of the resistance of the traditional stretchable conductive material is reduced by 2-3 orders of magnitude, and the stability of the stretchable conductor under large strain is realized. As shown in Figure 1a. The result was published in Advanced Electronic Materials (Adv. Electron. Mater. 2018, 4, 1800137) as a bottom cover article. Further, using the above stretchable conductive material as ink, a direct-writing printer was built, and the direct printing and patterning design of this material on an elastic substrate was realized. As shown in Figure 1b, the printed elastic heating device was designed with Good thermal stability. This work provides new materials and technologies for the fabrication of flexible wearable electronic devices. The results were published in Advanced Materials Technologies (Adv. Mater. Technol. 2018, 3, 1800435).

(2) Green and environmentally friendly recyclable flexible paper-based circuits

A flexible circuit is a special circuit created on a flexible substrate. At present, there are two major challenges in its application: one is poor fatigue characteristics, and it is easy to break and fail under repeated cyclic strain; the other is that it cannot be recycled, and traditional recycling methods such as incineration and pickling pollute the environment. In response to the above challenges, Ph.D. students Li Fari, associate researcher Liu Yiwei and researcher Li Runwei prepared liquid metal-based flexible circuits on paper to replace traditional copper, aluminum, silver and other circuits, which not only solved the problem of poor bending fatigue, but also solved the problem of poor bending fatigue. Recycling (Figure 2 shows the circuit prepared with liquid metal before and after recycling), which realizes the greening of the whole life cycle of paper-based circuits in manufacturing, use, and recycling. The circuit line width can be adjusted between 10μm-200μm, and through up to 10,000 double-fold tests, it is found that the maximum change rate of the circuit resistance is only 4%, which has good strain stability. In addition, the paper-based circuit has a good heat dissipation function. Experiments show that the temperature of the LED lamp operating on the liquid metal-based paper-based circuit is significantly lower than that of the LED lamp on the surface of the pure paper. This work provides a new method for the development of green and recyclable flexible circuits, and the results were published in Advanced Materials Technologies (Adv. Mater. Technol. 2018, 1800131).

(3) Digital flexible tactile sensor

It is the dream of many disabled people to make prosthetic limbs tactile, and electronic skin is such a system that can make human prosthetic limbs tactile. However, most electronic skins can only convert external stimuli into analog signals, and cannot convert external stimuli into physiological pulses like human skin, and transmit them precisely to the nervous system to the brain. In response to this problem, doctoral students Wu Yuanzhao, associate researcher Liu Yiwei and researcher Li Runwei cleverly used the inductor-capacitor (LC) oscillation mechanism to design the circuit (as shown in Figure 3a). When the external stress/strain causes the inductance value to change, the LC circuit will The frequency will change, so as to obtain the corresponding relationship between the applied stress/strain and the frequency. By further optimizing the LC resonance circuit, it can work within the physiological pulse frequency range of the human body. In addition, an “Air gap” structure was also designed (as shown in Figure 3a), and amorphous wire was used as the magnetic core to improve its performance, and a digital flexible haptic with a sensitivity of 4.4kPa-1 and a detection limit of 10μN (equivalent to 0.3Pa) was obtained. The sensor device (as shown in Figure 3b), and by optimizing the modulus and structure of the sensor, can be compatible with a wide detection range, which can sense not only weak mosquitoes and pulses, but also the pressure when lifting heavy objects. This work provides a new approach for developing digital biomimetic electronic skin. The results were published in Science Robotics (Sci. Robot. 2018, 3, eaat0429).

The above work was supported by the National Science Foundation for Distinguished Young Scholars (51525103), the China-Japan International Cooperation Project of the Ministry of Science and Technology (2016YFE0126700), the National Natural Science Foundation of China (61704177, 11474295, 61774161) and the Ningbo Innovation Team (2015B11001).

Fig. 1 (a) Liquid metal stretchable conductor with calabash string conductive network structure; (b) elastic heating sheet based on liquid metal stretchable conductor; (c) paper cover

Figure 2 Comparison of circuit performance with liquid metal before and after recycling

Figure 3 (a) The schematic diagram of the digital flexible tactile sensor; (b) The sensor resolves the micro-pressure of 0.3Pa; (c) The impulse response of the device with the change of pressure; (d) The schematic diagram of the device

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