Important Progress in Ultrastable Stretchable Electrodes In Ningbo Materials Institute

In the field of smart wearable electronics, stable and durable flexible stretchable conductors are still a great challenge. Especially in the process of collecting physiological signals of human epidermis, stable and stretchable electrodes can realize long-term and accurate signal collection. At present, it is difficult to achieve stable electrical properties under dynamic deformation whether it is a surface structure design type, a conductive material composite type or an authentic stretchable electrode. Therefore, it is still a great challenge to prepare electrodes with high and stable electrical properties.

Recently, under the leadership of researcher Li Runwei, the flexible magnetoelectric functional materials and devices team of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, inspired by artificial fishing nets, imitated the “water film-fishing net” structure to design an ultra-stable ultra-stable with flexible adaptive conductive interface. For stretchable electrodes, it is proposed to construct a two-dimensional “water-like film-fishnet” structure film of liquid metal polyurethane (TPU) by electrospinning method, which achieves a very low initial square resistance (52mΩ sq-1) and solves the problem of electrical conductivity in elastic electrodes. The problems of incompatibility of rate and elongation rate, and unstable electrical properties under cyclic deformation. Under strain, the liquid metal is bound by the mesh to expand outward and the liquid metal flows adaptively in the mesh to achieve low resistance and high stability. The dynamic adaptive conductive network of the electrode makes it have strong dynamic cycle stability. After 330,000 100% tensile strain cycles, the resistance only changes by 5%. At the same time, the electrode faces changes in the service environment such as cold and heat, acid and alkali, and water immersion. , still showing stable electrical properties. The electrode can be used in all-weather human epidermal physiological signal monitoring, intelligent human-machine interface and human body hyperthermia, etc., and is expected to help the sustainable development of wearable health monitoring systems and electronic skin human-machine interface based on the Internet of Everything. The work is published in InfoMat in a paper titled “Ultra-robust stretchable electrode for e-skin: In situ assembly using a nanofiber scaffold and liquid metal to mimic water-to-net interaction” (DOI: 10.1002/inf2.12302) , and was selected as the cover article (Figure 1).

Liquid metal-based ultra-stable stretchable electrodes and applications InfoMat cover

Liquid metal-based ultra-stable stretchable electrodes and applications InfoMat cover

Figure 1

The team prepared a stretchable electrode imitating “water film-fishing net” by in-situ composite of TPU electrospinning and electrostatic spraying of liquid metal micro-nano particles, and subsequent mechanical activation. The ultra-stable electrical properties of the electrode are mainly due to its imitative “water film-fishing net” structure, which can also be called a liquid metal dynamic adaptive network. Under high strain (<100% strain), it was observed in situ by SEM that the liquid metal could achieve adaptive flow, relieve local stress, and keep the conductive film continuous; under large strain (300%-500% strain), although the liquid metal film would break, but the polyurethane spun mesh prevented it from breaking and allowed it to wrap around the filaments, maintaining the stability of the overall conductive network (Fig. 2a). The authors also thoroughly analyze how the liquid metal micro-nanospheres can be combined with the nanofibrous filament network through size effects and microscopic bundling structures.

At the same time, through local activation and laser cutting, the polyurethane liquid metal composite material can be prepared into a multi-layer multi-functional human-computer interaction system. The upper-layer capacitive sensing array is connected to the integrated circuit and the Bluetooth module, which can realize wireless signal transmission, and can input wireless commands to the computer in the stretching and bending state, which can be applied in smart wearable game control and other aspects. The lower layer serpentine heater exhibits good electrothermal stability, can achieve stable heating at 45°C-90°C, and exhibits excellent heating cycle performance, which can be used for human body heating therapy. The locally activated circuit exhibits good resistance to mechanical damage, and the electrode can achieve immediate conductive path reconstruction, allowing the electrode to function normally even in the broken and stretched state (Fig. 2b). In the 100% strain tensile cycle test, the electrode exhibited a slight increase in the first tensile resistance, and in the subsequent 330,000 cycles, its resistance increased by only 5%, which is far superior to other reported stretchable electrodes (Fig. 2c).

Ultrastable Electrode Mechanism and Application

Ultrastable Electrode Mechanism and Application

Fig. 2

The electrode can realize all-weather ECG signal detection on human epidermis. First, in vitro cell experiments proved that the electrode has good biocompatibility and extremely low toxicity, and can be used in human epidermis for ECG monitoring, which exhibits similar impedance performance to commercial gel electrodes. Secondly, according to the human activity scene, this work designs three working scenarios for the electrode: static, motion, and water flushing. The ultra-stable electrode exhibits excellent ECG signal collection ability, and the signal-to-noise ratio reaches 0.43, especially in the water flushing environment. The electrode can still collect stable and clear ECG signals, which can be used for all-weather ECG diagnosis (Figure 3).

Biocompatibility study of ultra-stable electrodes and their application in all-weather ECG monitoring

Biocompatibility study of ultra-stable electrodes and their application in all-weather ECG monitoring

Figure 3

In summary, this work designs and realizes ultra-durable stretchable electrodes based on an adaptive conductive network composed of liquid metal and polyurethane spinning networks, which can achieve high performance in mechanical deformation, prolonged oxidation, cyclic immersion, heating, acid-base Stable electrical properties under various environmental stimuli such as immersion, in particular, extremely small resistance changes under 330,000 stretching cycles. The electrode can be used in all-weather ECG monitoring, intelligent human-computer interaction systems, etc., and shows great potential in long-term surface electronic skin, in vivo biocompatible devices, etc. Important Progress in Ultrastable Stretchable Electrodes

The work was completed by Cao Jinwei, Liang Fei, Li Huayang, etc. under the joint guidance of Researcher Li Runwei and Professor Zhu Guang of the University of Nottingham Ningbo, and was awarded the National Natural Science Foundation of China (51525103, 51701231, 51931011), Ningbo 3315 Talent Program, Ningbo Science and Technology Innovation 2025 Project (2018B10057), Zhejiang Provincial Natural Science Foundation (LR19F010001), Zhejiang Provincial Science Fund for Distinguished Young Scholars (2016YFA0202703) and Wang Kuancheng Education Fund of Chinese Academy of Sciences (GJTD-2020-11). Important Progress in Ultrastable Stretchable Electrodes

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