Microelectronic devices play a vital role in healthcare monitoring, intelligent robotics, and human-machine interfaces. In this field, how to make these devices more practical under the trend of integration and miniaturization is a recognized key challenge. MXenes (Ti3C2Tx) as a new type of accordion-like laminated two-dimensional material can be used as a piezoresistive material. Its compressible layered laminate structure is a key factor to change the internal resistance and conductivity of MXene. However, current stress sensors based on MXene quickly reach the deformation limit of MXene in a two-dimensional limited space under external stimulation, which greatly limits the performance of the sensor. In addition, most previous studies only focused on stress detection or other indicators (sensitivity or limit), and few jobs can perceive multifunctional micro-forces (such as sound, touch, and motion recognition) in a simple structure.

【Research results】

The research group of teachers Weili Deng, Yong Fang and Weiqing Yang from Southwest Jiaotong University published a research paper entitled  Microchannel-Confined MXene Based Flexible Piezoresistive Multifunctional Micro-Force Sensor  in the internationally renowned journal Advanced Functional Matterials. Through the designed channel seal and compressible laminated MXene (Ti3C2Tx), a high-sensitivity micro-channel sealed MXene-based flexible multifunctional piezoresistive stress sensor has been developed. The sensor of this design not only achieves low detection limit (<9 Pa), high sensitivity (99.5 kPa-1) and fast response (4 ms), but it can also achieve multi-functional micro-force sensing in one device at the same time, such as detection Wrist pulse, sound, fretting, and even acceleration of the host. This multi-functional sensing characteristic makes the sensor have broad application prospects in smaller flexible electronic devices such as wearable medical monitoring equipment, smart robots and efficient human-machine interfaces.

[Picture and text quick view]

Figure 1. Schematic diagram of the stress sensor. a) A beam of sound waves hit the surface of the stress sensor. b) The overall schematic diagram and detailed structure of the stress sensor; left: the structure of the stress sensor; upper right: SEM image of accordion-like MXene; lower right: schematic diagram of MXene filled in the microchannel. c) 3D white light interference image of fingerprint-like microchannel PET film. Illustration: Enlarged 3D image. The scale bars are 500 and 100 µm respectively. d) Photograph of flexible stress sensor array. e) The molecular structure of MXene.

Figure 2. The working mechanism of a MXene-based piezoresistive stress sensor with limited microchannels and the basic performance of the sensor. a) Equivalent resistance model of the prepared sensor. b) The equivalent circuit diagram of the stress sensor. c) Under external stress, the thickness of a single MXene (D1/D2) and the wider distance between MXene (DL1/DL2) decrease. d–f) SEM images of laser-engraved microstructures, the channels are 9, 39, 78μm. Illustration: FEA simulation of MXenes stress state in channels at different depths under the same stress. g) Electrical performance of stress sensors with different channel depths. h) Real-time current response of a stress sensor with a channel depth of 78 µm under a load of 0.052 to 3.56kPa. i) The sensitivity of a stress sensor with a channel depth of 78 µm.

Figure 3. Electrical performance of a stress sensor with a channel depth of 78 µm. a) A photo of a tiny object placed on the stress sensor. b) The stress sensor can detect tiny stress as low as 9Pa. c) Response time and recovery time of the stress sensor. d) Place the bottle with ink on the stress sensor connected in series with the LED. As the ink volume increases, the LED will become brighter. e) Current response under step stress. f) I-V curve of stress sensor under various stresses. g) Current response under various stresses at a bias voltage of 0.1V. h) The durability test is performed under a stress of 364 Pa. i) Place four bottles with different water content on the 9×9 sensor array and plot the corresponding values.

Figure 4. Application of stress sensor in small signal detection. a) A schematic diagram of a stress sensor used as a wearable device for physiological signal detection. b) Say  Hello  and the corresponding signal of  Sensor . Illustration: Photo of the sensor attached to the throat. c) Corresponding signal of wrist pulse. Illustration: Photo of the sensor attached to the wrist. d) The corresponding signals of the stress sensor vibrating at different frequencies on the vibration table. e) The corresponding signal of the stress sensor vibrating on the board. f) In the vibration mode, the relationship between the corresponding current value, displacement, velocity and acceleration. g) Schematic diagram of MXene density at different stages of vibration. h) Schematic diagram of sonic impact stress sensor. i) The sound pressure map of the sensor surface related to sound waves and pulses. j–k) Corresponding detection signals and source waves of the two bells.

Literature link: https://doi.org/10.1002/adfm.201909603.

Information source: flexible electronic materials

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