Preparation of flexible pressure sensor based on MXene/PEDOT:PSS and its application in lip language recognition
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摘要: 唇语是声带损伤、喉舌损伤及听障患者的一种有效的语言沟通方式。唇语信号由嘴唇和面部肌肉运动而产生，其包含了大量的语音信息。通过柔性压力传感器来捕获肌肉运动可实现唇语信号的提取和识别，为听、说功能障碍患者提供了更加自然、便捷的无障碍交流方式。本研究采用二维材料MXene和高导电聚合物聚(3,4-乙基二氧噻吩):聚(苯乙烯磺酸盐)(PEDOT:PSS)作为复合材料，以可拉伸的且具有微结构的Ecoflex作为柔性基底，制备了一种压阻式柔性压力传感器。该传感器在0-2.5 kPa压力范围内具有42.31 kPa−1的高灵敏度及快速响应(<150 ms)，并且在10000次压缩-释放循环测试中显示出高稳定性。将该柔性压力传感器贴附于嘴角上并捕获唇语的肌肉运动，结合卷积神经网络算法对十二生肖英语单词信号进行训练和测试，平均准确率高达90.18%。该工作增加了唇语识别系统的多样性，为唇语运动信号直接转化为语音或文本奠定了重要基础。Abstract: Lip language is an effective form of verbal communication for patients with vocal cord injuries, laryngeal and tongue injuries, and hearing loss. Lip-speaking signals are generated by lip and facial muscle movements, which contain a large amount of speech information. The extraction and recognition of lip-speaking signals can be achieved by capturing the muscle movements through flexible pressure sensors, providing a more natural and convenient way of accessible communication for patients with listening and speaking dysfunction. In this study, a piezoresistive flexible pressure sensor was prepared using a two-dimensional material, MXene, and a highly conductive polymer, poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), as composites, and a stretchable and microstructured Ecoflex as a flexible substrate. The piezoresistive sensor demonstrated a high sensitivity of 42.31 kPa−1 and fast response (<150 ms) in the pressure range of 0-2.5 kPa, and shows high stability in 10,000 compression-release cycles. The flexible piezoresistive sensor was attached to the corners of the mouth and captured the muscle movements of the lips, which was combined with a convolutional neural network algorithm to train and test the signals of English words in the Chinese zodiac, with an average accuracy of up to 90.18%. This work increases the versatility of lip recognition systems and lays an important foundation for the direct conversion of lip movement signals into speech or text.
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图 1 传感器分别在(a)无负载和(b)有负载状态下的示意图；(c)传感器工作机制的等效电路图
Figure 1. Schematic of the sensor in the unloaded states (a) as well as under loading (b), respectively; (c) Equivalent circuit diagram of the sensor mechanism
图 2 基于MXene/PEDOT:PSS压阻式柔性压力传感器的制备流程图
Figure 2. Flow chart for the preparation of a piezoresistive flexible pressure sensor based on MXene/PEDOT:PSS
PEDOT:PSS—Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
图 3 (a) Ecoflex微结构基底的SEM图；(b) Ecoflex微结构基底上涂覆MXene/PEDOT:PSS导电材料的SEM图及局部放大图
Figure 3. (a) SEM image of Ecoflex microstructured substrate; (b) SEM image of MXene/PEDOT:PSS conductive material coated on Ecoflex microstructured substrates and local enlargement
图 4 (a) 传感器在不同压力下的应力分布; (b) Ecoflex微结构的接触面积随外部压力变化的模拟图
Figure 4. (a) Stress distribution in the sensor when subjected to different pressures; (b) Simulated contact areas of the Ecoflex microstructure variation versus the external pressure
图 5 (a) 不同旋涂转速下基底薄膜的电阻率; (b) 不同旋涂转速的传感器相对电阻随压力的变化; (c) 1500 r/min制备的传感器灵敏度; (d) 250 μm,180 μm和120 μm砂纸模板制备的压力传感器的相对电阻随压力的变化
Figure 5. (a) Resistivity of the substrate film at different spin-coating speeds; (b) Variation of relative resistance versus pressure for sensors with different spin-coating speeds; (c) Sensor sensitivity prepared from 1500 r/min; (d) Variation of relative resistance versus pressure for pressure sensors prepared using 250 μm, 180 μm and 120 μm sandpaper templates
图 6 (a) 不同MXene/PEDOT:PSS 掺杂比例条件下传感器的相对电阻随压力的变化; (b) MXene/PEDOT:PSS 质量比为2∶1时传感器的灵敏度; (c) 平面结构和微结构传感器的相对电阻随外加压力的变化; (d) MXene, PEDOT:PSS和MXene/PEDOT:PSS 复合材料的拉曼光谱
Figure 6. (a) Variation of relative resistance versus pressure of sensors under different ratios of MXene/PEDOT:PSS doping conditions; (b) Sensitivity of the sensor at a MXene/PEDOT:PSS mass ratio of 2∶1; (c) Variation of relative resistance of planar structured and microstructured sensors against applied pressure; (d) Raman spectra of MXene, PEDOT:PSS, and MXene/PEDOT:PSS composites
图 7 (a) 压力传感器在0.1 kPa和0.2 kPa压力下的响应和恢复时间; (b) 不同压力下传感器相对电阻的变化
Figure 7. (a) Response and relaxation time of the pressure sensor under pressures of 0.1 kPa and 0.2 kPa; (b) Variation of relative resistance versus different pressure for sensors
图 8 (a) 传感器在不同频率压力下的相对电阻随压力的变化; (b)在1.0 kPa的压力下, 加载和卸载重复10000次，相对电阻变化
Figure 8. (a) Variation of relative resistance of sensors with pressure at different frequency pressures; (b) Variation of relative resistance after 10,000 repetitions of loading and unloading at a pressure of 1.0 kPa.
图 9 (a) 以两种不同的速度读出"ai", "pin", "cai", "hui", "ying"的唇语信号; (b) 五位志愿者(Zhong, Xu, Jia, Zhang和Wang)读"ai", "pin","cai", "hui"和"ying"时的唇语信号
Figure 9. (a) Lip language signals of "ai", "pin", "cai", "hui", "ying" are read out at two different speeds; (b) Lip language signals of five volunteers (Zhong, Xu, Jia, Zhang and Wang) when pronouncing "ai", "pin", "cai", "hui" and "ying"
图 10 十二生肖英语单词的唇语信号
Figure 10. Lip language signals for English words in the Chinese zodiac
图 11 (a) 传感器捕捉唇语信号的照片; (b) 卷积神经网络模型结构示意图
Figure 11. (a) Photograph of the sensor capturing the lip language signal; (b) Schematic of the structure of the convolutional neural network model
图 12 (a) 12个英语单词(十二生肖)生成的唇语信号三维图; (b) 12个英语单词(十二生肖)唇语信号的混淆矩阵; 跟踪训练集和测试集唇语识别的(c)准确率和(d)损失率的变化
Figure 12. (a) 3 D plot of lip language signals generated for 12 English words (Chinese zodiac) in the dataset; (b) Confusion matrix representing the lip language signals for 12 English words (Zodiac); Tracking variations of training set and test set in lip language recognition’s accuracy (c) and (d) loss rate
表 1 实验试剂及实验设备
Table 1. Laboratory reagents and experimental equipment
Name of reagents and instruments Norm Manufacturer Ecoflex 00-30 Beijing Angelcrete Art Landscaping Co.,Ltd PEDOT:PSS PH 1000 Xi'an Qiyue Biotechnology Co.,Ltd. MXene 5 mg/mL Jilin 11 Technology Co.,Ltd Sandpaper 120/180/250 μm Shinkong Technology Co.,Ltd. Drying oven GZX-9070 Shanghai Boxun Co.,Ltd. Oxygen plasma PLUTO-T Shanghai Pei Yuan Instrument Co.,Ltd. Heating stage HP550-S Dragon laboratory instruments.Co.,Ltd Digital source meter Keithley2614 Tektronix USA, Inc. Spin coater KW-4 A Shanghai Chemat Advanced Ceramics Technology.Co.,Ltd. Universal testing machine CMT6502 Shenzhen suns technology stock.Co.,Ltd Scanning electron microscopy(SEM) S-4800 Hitachi.,Ltd Raman spectrometer Qontor Renishaw,UK 
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