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 healthcare and smart robots. Their development trend is flexibility and even flexibility. The core and key is the development of 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 make magnetoelectric functional materials flexible or elastic, or functionalize flexible or elastic materials is a major challenge in this field. To this end, the magnetoelectronic materials and device team of the Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences has conducted research work on the flexibility and elasticity of conductive and magnetic functional materials. , Conductors and sensors have made a series of progress.
(1) Large-strain elastic conductive materials and elastic heating devices
Stretchable conductive materials usually incorporate conductive fillers (graphene, carbon nanotubes, metal nanowires / nanoparticles, etc.) on the order of nanometers or micrometers into the elastic polymer, and are processed by dispersion compounding or layered compounding After that, a multiphase composite system with conductive function is obtained. Because the elastic modulus of the solid conductive filler and the elastic matrix are very different (about 1 million times), the gap between the filler particles that form the conductive path will change significantly when the strain is large, resulting in unstable conductive properties; Will improve the conductivity of conductive materials, but also deteriorate its elasticity, resulting in a limited amount of doping, so its conductivity is generally poor. How to obtain an elastic conductor compatible with high conductivity, tensile stability and large strain is still a challenge.
To solve the above problems, doctoral students Yu Zhe, researchers Shang Jie and 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 strain and further improve its strain stability. The results show that the conductivity of the stretchable conductive material can reach the conductor category (greater than 1000S / cm), and can achieve more than 1000% stretching, more importantly, the resistance fluctuation when stretching 100% is less than 4% , Compared with traditional stretchable conductive materials, the resistance change rate is reduced by 2-3 orders of magnitude, and the stability of stretchable conductors under large strain is achieved. 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-mentioned stretchable conductive material as ink, a direct-write printer is built, which realizes the direct printing and pattern design of this material on an elastic substrate. As shown in FIG. 1b, the printed elastic heating device is designed to have Good thermal stability. This work provides new materials and technologies for the preparation 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 circuit
A flexible circuit is a special circuit created on a flexible substrate. At present, there are still two major challenges in its application: one is poor fatigue characteristics, easy to fail under repeated cycling strains; the other is that it cannot be recycled, and traditional incineration, pickling and other recovery methods pollute the environment. In response to the above challenges, Ph.D. student Li Fali, 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 Recycling (Figure 2 is a circuit prepared with liquid metal before and after recycling), which realizes the greening of the entire life cycle of paper-based circuits in manufacturing, use, and recycling. The circuit line width is adjustable between 10μm-200μm, and through up to 10,000 half-fold tests, the maximum change rate of the circuit resistance is only 4%, with good strain stability. In addition, the paper-based circuit has a good heat dissipation function. Experiments show that the temperature of LED lamps working on liquid metal-based paper-based circuits is significantly lower than that of LED lamps on the surface of pure paper. This work provides a new method for the development of green and recyclable flexible circuits. The results were published in Advanced Materials Technologies (Adv. Mater. Technol. 2018, 1800131).
(3) Digital flexible tactile sensor
It is the dream of many people with disabilities to make the prosthesis tactile, and electronic skin is such a system that can make people's prostheses feel tactile. However, most electronic skins can only convert external force stimuli into analog signals, and cannot convert external force stimuli into physiological pulses like human skin, and accurately transmit them to the nervous system to the brain. In response to this problem, the doctoral student Yuanyuan Wu, associate researcher Liu Yiwei and researcher Li Runwei cleverly used the inductance-capacitance (LC) oscillation mechanism to design the circuit (Figure 3a). When the external stress / strain causes the inductance to change, the LC circuit The frequency of will change, so as to obtain the corresponding relationship between the applied stress / strain and the frequency, and by further optimizing the LC resonance circuit, it can be made to work within the physiological pulse frequency range of the human body. In addition, an "Air gap" structure (as shown in Figure 3a) was designed, using amorphous wire 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 (see Figure 3b), and by optimizing the modulus and structure of the sensor, it can be compatible with a wide detection range, not only can sense weak mosquitoes and pulse, but also can sense the pressure when lifting heavy objects. This work provides a new method for the development of digital bionic electronic skin. The results were published in Science Robotics (Sci. Robot.2018, 3, eaat0429), a sub-science of Science.
The above work was supported by the National Outstanding Youth Science Fund (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).
Figure 1 (a) Liquid metal stretchable conductor with gourd string conductive network structure; (b) Elastic heating sheet based on liquid metal stretchable conductor; (c) Paper cover
Figure 2 Comparison of performance of circuits prepared with liquid metal before and after recycling
Figure 3 (a) The principle diagram of the digital flexible tactile sensor; (b) The sensor distinguishes the micro-pressure of 0.3Pa; (c) The impulse response of the device with the pressure change; (d) The schematic diagram of the device
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