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As a flexible sensor network, electronic skin (e-skin) is becoming an important interactive medium for human-machine interfaces, bio-integrated devices, and personalized medicine. MXenes are a new type of two-dimensional nanomaterials with excellent electrical conductivity and hydrophilicity, showing great potential in next-generation electronic skin sensors. However, current researches on MXene-based electronic skins mainly focus on a single electrical sensing mode, and studies on key functions such as visual recognition are rarely reported. Therefore, it is very necessary to explore the construction of efficient, multimodal fusion MXene-based electronic skin devices.


Bioinspired MXene-Based User-Interactive Electronic Skin for Digital and Visual Dual-Channel Sensing

Wentao Cao, Zheng Wang, Xiaohao Liu, Zhi Zhou, Yue Zhang, Shisheng He*, Daxiang Cui*, and Feng Chen*

Nano-Micro Letters (2022) 14:119

https://doi.org/10.1007/s40820-022-00838-0

Highlights of this article

1. Using a convenient design concept, a biomimetic MXene-based electronic skin is constructed for digital and visual dual-signal sensing.

2. The MXene-based electronic skin has excellent electromechanical sensing performance, enabling real-time monitoring of human activities such as writing, drinking, walking, and speaking.
3. Utilizing the excellent Joule heating properties of MXene films, MXene-based electronic skins can achieve a wide range of dynamic coloring for passive display and visual recognition of various human actions.
brief introduction

The team of researcher Feng Chen and Professor He Shisheng from the Tenth Peoples Hospital Affiliated to Tongji University and the team of Professor Cui Daxiang of Shanghai Jiaotong University have proposed a bionic flexible electronic device with a simple structure that can simultaneously realize digital electrical signal response and optical visualization under external mechanical stimuli. skin. The electronic skin is composed of a carbon nanotube/cellulose nanofiber/MXene conductive layer and a silica gel/thermochromic pigment elastic layer, two-dimensional MXene nanosheets and cellulose nanofibers (CNFs) dispersed one-dimensional carbon nanotubes (CNTs) phase Combined, the electronic skin has excellent electromechanical response behavior and can achieve accurate monitoring of all-round human activities. Furthermore, the Joule heating properties of MXene/CNTs can transfer local thermal energy to thermochromic pigments in silicon-based elastomers for passive color-changing displays, camouflage, and visual surveillance. Therefore, compared with the traditional MXene-based electronic skin, the electronic skin can simultaneously realize digital/electrical signal response and optical visualization to external mechanical stimuli. This study not only demonstrates a biomimetic user-interactive electronic skin with significant digital-visual synergy, but also provides a good platform for the development of next-generation smart and flexible electronics.

Graphical guide

I Preparation of MXene-based smart electronic skin (CCM e-skin)
By transferring the conductive CCM layer inside the silica gel, we fabricated a flexible, strain-sensitive, and human-computer-interactive CCM electronic skin (Fig. 1). After the CCM electronic skin is attached to the human skin, it can monitor human activities through two artificial sensory channels, digital and visual. The electromechanical/digital channel consists of strain sensors that detect human motion by analyzing changes in resistive signals, while the visual channel is mainly based on Joule heating and thermomechanical color-changing mechanisms. This multimodal fusion strategy enables the CCM e-skin to be more directly and precisely applied to the study of human activities.


Figure 1. Schematic diagram of the fabrication of MXene-based electronic skins. First, the mixed solution of CNTs, CNFs and MXene nanosheets was suction filtered to obtain a composite membrane; then, the silica gel was pre-cured to prepare a sticky substrate, which was colored with the aid of a polytetrafluoroethylene mold; finally, the composite membrane was Transfer to silica gel. Among them, the composite film serves as the strain sensing layer and Joule heating layer, while the silica gel/chromic pigment layer serves as the thermochromic component and coating layer. Therefore, CCM electronic skin can realize intelligent monitoring of human movement by means of digital-visual fusion.

Characterization and Molecular Dynamics Simulation of II CCM Thin Films

The physicochemical properties of the material were systematically studied, and the formation mechanism of the CNTs/CNFs hybrid was further explored through molecular dynamics simulations (Fig. 2). As shown in Figure 2h, the CNFs were initially dispersed around the CNTs (0 ns), and then gradually attached to the CNT surface around 5 ns. As the simulation time increases, it can be seen that the region of high estimated density of glucose units (red region) gradually concentrates to around 0.4 nm at around 5 ns, and then remains relatively stable (Fig. 2i). Different ratios of MXene nanosheets were dispersed into the CNFs/CNTs mixture, and the membrane structure was prepared by vacuum-assisted filtration and hot-press drying with a microporous filter membrane; the CNTs on the filter membrane showed brittleness and poor uniformity due to their poor dispersibility in water. , and the obtained CCM films showed good uniformity and integrity (Fig. 2j). Figure 2k shows that 1D carbon nanotubes weave loose MXene nanosheets into a bridged structure. Therefore, the well-integrated structure of CNTs, CNFs, and MXene nanosheets can provide continuous electronic channels, making CCM films highly elastic and conductive.


Figure 2. Characterization of MXene nanosheets, CNTs, CNFs, and CCM films. (a) XRD patterns of MAX precursor and as-prepared MXene nanosheets; (b) TEM image of as-prepared 2D MXene nanosheets; (c) AFM image and height profile of MXene nanosheets; (d) MXene nanosheets Lateral size distribution; (e) TEM image of CNTs; (f) TEM image of CNFs, inset is the photo of CNFs dispersion; (g) CNTs/CNFs ink and rheological behavior after 1 week of storage; (h) molecular dynamics (i) change in estimated density of the distance between glucose molecules and the surface of carbon nanotubes; (j) photographs of CNTs, CCM-0.2, CCM-0.5 and CCM-1 films deposited on cellulose films; (k) Top view and SEM cross-sectional image of the CCM membrane.

III Electromechanical properties and mechanism of CCM electronic skin

After transferring the CCM film onto the silicone rubber substrate, a flexible CCM electronic skin can be obtained; the successful integration of 2D MXene nanosheets and 1D CNFs/CNTs endows the flexible CCM electronic skin with great potential in electromechanical response. Before stretching, MXene nanosheets with typical 2D nanostructures were interconnected with 1D carbon nanotubes to form a continuous electronic conduction pathway (Fig. 3b). When the stretching starts, the MXene nanosheets tend to slide against each other due to their weak van der Waals interactions, while the CNTs can act as a "bridge" connecting the MXene nanosheets. As the stretching continued, the CNTs were pulled out, resulting in a change in the electrical resistance of the CCM e-skin.


Figure 3. Electromechanical properties and mechanism of CCM electronic skin. (a) Conductivity of CCM electronic skin with different MXene contents; (b) schematic diagram of electromechanical response mechanism of CCM electronic skin; (c) CNFs/CNTs, CCM-0.2, CCM-0.5, CCM-1 and pristine MXene e-skin Changes in relative resistance values under different strains; (d) relative resistance changes of CCM-0.5 electronic skin at various maximum tensile strains (5, 10, 20, 50, 100, 150, 200 and 250%); ( e) Relative resistance changes of CCM-0.5 e-skin at different frequencies under 100% strain; (f) time retention curves of resistance and strain as a function of time; (g) constant frequency 1hz, CCM-0.5 e-skin at 0 Relative resistance change during 1000 cycles of stretching/relaxing in the range of %~100% strain; (h) Resistance change of multi-cycle test: 1st (gray), 10th (blue), 100% strain 100th (orange) and 1000th (green).

IV CCM Electronic Skin for Monitoring Human Physiological Movement

CCM e-skin has excellent comprehensive properties such as strong flexibility, high sensitivity, good stability, and wide stretching range, and can realize full-scale real-time monitoring of human body large-scale motion and subtle physiological signals. We directly attached the CCM electronic skin to each joint of the human body to detect the large movements of the human body. For example, the CCM electronic skin was installed on the finger joints and wrist joints, respectively, and the response signals during bending and relaxing movements were recorded (Fig. 4a, b) . By analyzing the relative changes in resistance, different degrees of bending can be identified accurately and quickly. In addition, the CCM electronic skin was also able to monitor the arm bending motion at different bending frequencies (Fig. 4c). In addition to regular joint bending movements, more complex human activities, including handwriting, pouring water into a cup, drinking, etc., can also be detected by adhering CCM electronic skins to wrist or arm joints (Movies S1, 2 and Figure 4d-f). In addition, the special stretching capability enables the CCM e-skin to stably detect knee flexion movements that require large stretch deformations by attaching the CCM e-skin to the knee joint by observing the CCM e-skin in a highly reproducible manner. Changes in relative resistance can easily detect leg movements such as walking and running (Figure 4 and S13).



Figure 4. The CCM electronic skin detects the response signals of various physiological movements. (a) Monitoring process of finger bending and (b) wrist bending at different angles; (c) Relative resistance response of CCM electronic skin when detecting arm bending at different speeds; (d) Relative resistance response of CCM electronic skin to handwritten MXene ; Detect various arm actions such as (e) pouring water into a cup and (f) drinking water; (g) CCM electronic skin detects relative resistive responses at different rotational speeds; (h) speech "MXene" and (i) swallowing Response curves were recorded at the throat.

Joule heating performance of V CCM electronic skin

For the CCM electronic skin, the applied strain produces a simultaneous change in electrical resistance, which provides an opportunity to realize dynamic Joule heating behavior. To quantitatively understand the electrothermal properties of the CCM e-skin, we installed a direct current (DC) power supply system on the CCM e-skin and wirelessly monitored temperature changes through a real-time infrared (IR) thermal imaging camera (Fig. 5a). Figure 5b shows the temperature-time curves of CCM electronic skins with different MXene contents when the external voltage is 20 V. The electrothermal performance of carbon nanotubes/carbon nanotube skins is poor, and the saturation temperature is low, about 34.5 °C. With the addition of MXene nanosheets, the Joule heating performance of CCM electronic skins has a tendency to be significantly improved.



Figure 5. Joule heating performance of CCM electronic skin under DC voltage. (a) Schematic diagram of the device for measuring Joule heat using an infrared camera; (b) temperature distribution of CCM electronic skins with different MXene contents when the input voltage is 20 V; (c) temperature curve of CCM electronic skins with different input voltages; (d) Steady-state temperature of CCM-0.5 e-skin as a function of voltage squared; (e) recorded temperature plots of CCM-0.5 e-skin at different strain levels; (f) temperature change at initial temperature of 60 °C Curves; (g) temperature distribution of CCM-0.5 e-skin when the voltage was gradually increased from 10 V to 25 V; (h) CCM-0.5 e-skin heating stability test with repeated application of 15 V voltage; (i) CCM- Long-term temperature change curve of 0.5 e-skin electric heater when the input voltage is 20v.

Thermochromic effect and application example of VI CCM electronic skin

To visualize the thermal response of the CCM e-skin to tensile strain, we employed a thermochromic pigment composite system dispersed in silicone rubber, which can reflect the strain, resistance, and temperature changes of the CCM e-skin according to color changes. Variety. The CCM film in the middle can act as a temperature-tunable heater applying mechanical strain, while the silicone rubber mixed with thermochromic pigments can act as the encapsulation and temperature display. Mixing various thermochromic pigments with liquid silicone rubber to form a uniform color-changing layer, CCM electronic skins containing thermochromic dyes maintained their original colors (such as blue, yellow, purple, red, etc.) at room temperature, but exceeded It will turn white (from black to white) at 31 or 65 ℃. Furthermore, since the thermochromic components have different response temperatures, mixing the two pigments may result in a new system that exhibits a broader and richer trichromatic state.



Figure 6. Thermochromic effect and application example of CCM electronic skin. (a) When the applied voltage is 20 V, the CCM electronic skin with different thermochromic characteristics will change color to white; (b) the graph shows the change trend of the resistance and color of the CCM electronic skin under constant stretching; (c) Photograph of CCM electronic skin being stretched while maintaining a constant applied voltage; (d) Example of thermochromic application of CCM electronic skin for military camouflage by switching the voltage.

VII Visual motion monitoring of interactive temperature/color changing electronic skins

The CCM electronic skin exhibits good temperature/color sensitivity to various tensile strains, and has important application potential in the visual monitoring of human activities. The CCM e-skin was attached to the index finger joint of the hand model, and its real-time temperature and color changes under different strains were recorded. On the basis of the above single-channel sensing, we also study a visual multi-channel sensing system, which consists of 5 CCM electronic skins and works in parallel to monitor the bending of a single finger of the manipulator with high accuracy Condition. Compared with other MXene electronic skins without visual perception ability, CCM e-skin can simultaneously realize digital electrical response and optical visualization to external mechanical stimuli. To our knowledge, MXene-based multimodal fusion strategies are lacking before we present this work. The simple design concept and reliable operation of this e-skin are expected to provide an ideal platform for next-generation flexible electronics.



Figure 7. Visual motion monitoring of interactive temperature/color changing electronic skin. (a) The temperature change of the CCM electronic skin with the bending of the finger and the corresponding recovery process; (b) the photo of the manipulator and the corresponding temperature distribution image, corresponding to the various bending states of the finger in (a); (c) The temperature follows the movement of the finger (d,c) Photographs and corresponding temperature distribution images of fingers in different motion states; (e) CCM electronic skin and other MXen-based electronic skins