Abstract
In recent years, the flexible piezoresistive pressure sensor has attracted widespread attention due to the trend of improved wearable electronics applied to the field of electronic skin, disease diagnosis, motion detection and health monitoring. Here in this paper, the latest progress of the exploitation of flexible piezoresistive pressure sensors is reviewed in terms of sensing mechanism, selection of sensing materials, structural design and their advanced application. Firstly, the sensing mechanism of piezoresistive pressure sensors is generally introduced from the band structure of semiconductor materials, seepage theory and tunneling effect of conductive polymer composites and changes in interface contact resistance. Based on these sensing mechanisms, various flexible piezoresistive pressure sensors with high sensitivity, broad sensing range and fast response time have been developed. The selection of composition materials and microstructural design in flexible piezoresistive pressure sensor to implement the optimization of sensing performance are emphatically presented in this review. The composition materials including organic polymer material and inorganic nanomaterial based on two-dimensional (2D) materials such as graphene and MXene are intensively exhibited. In addition to the above characteristics, these kinds of pressure sensors exhibit high mechanical reversibility and low detection limit, which is essential for detecting the minor motions like respiratory rate and pulse. Moreover, the well-designed structures applied to the composition analysis are also overviewed, such as the sea urchin-like structure, spongy porous structure and regular structure. Various designed structures provide further properties like stability for the flexible pressure sensor. However, comparing with traditional pressure sensor, the mass production and application of flexible pressure sensor are confronting several barriers, like the high cost of raw materials and relatively complex manufacturing processes. How to achieve the low cost and low energy consumption simultaneously on the basis of excellent performance is still a challenge to expanding the applications of flexible pressure sensor. Novel sensing mechanism, functional materials and synthetic integration are expected to be developed in the future. And also, the potential application of flexible pressure sensor will be further expanded after endowing it with more functions.
Highlights
Change based on interface contact resistance: (a) Schematic diagram of a bulk electrical interface[54]; (b) working mechanism of pressure sensor of electrode-active layer contact type[58]; (c) PEI-CNT coated non-conductive fibers under applied pressure showing the proposed sensing mechanism[59]
Interfaces 9 37921 Tee B C K, Chortos A, Berndt A, Nguyen A K, Tom A, McGuire A, Lin Z C, Tien K, Bae W G, Wang H, Mei P, Chou H H, Cui B, Deisseroth K, Ng T N, Bao Z 2015 Science 350 313
Summary
图 1 (a) 半导体硅在 [111] 和 [100] 波数方向的导带和价带 [34]; (b) 化合物 CsAuBr3 在 27 °C 下, 45 GPa 压力范围内电阻率的变 化 [35]; (c) 石墨烯片层沿指定方向均匀拉伸或压缩 [38]; (d) 通过按压导电聚合物复合材料降低电阻率的示意图 [45]; (e) 相邻纳米线 之间的不同电接触情况 [50] Fig. 1. (a) Conduction band and valence band in silicon along [111] and [100] k-directions[34]; (b) Evolution of the resistivity of CsAuBr3 at 27 °C in a pressure range up to 45 GPa[35]; (c) the graphene sheet is uniformly stretched or compressed along a prescribed direction[38]; (d) schematic illustration of decrease in resistivity by pressing a conductive polymer composite[45]; (e) different electrical interconnections between two adjacent NWs[45]. (a) Conduction band and valence band in silicon along [111] and [100] k-directions[34]; (b) Evolution of the resistivity of CsAuBr3 at 27 °C in a pressure range up to 45 GPa[35]; (c) the graphene sheet is uniformly stretched or compressed along a prescribed direction[38]; (d) schematic illustration of decrease in resistivity by pressing a conductive polymer composite[45]; (e) different electrical interconnections between two adjacent NWs[45]. 导电填料较大 的体积比及有序排布方式使得此渗透型复合材料 的逾渗阈值较低 (0.27%, 体积分数), 电导率高达 0.003 S·m–1, 光学透过率为 71.8%, 杨氏模量低至 122.8 kPa. 为 R1, R2 与隧穿电阻 Rtunnel 之和 (图 1(e)). 其中, J 为隧穿电流密度, V 为电位差, e 为单电子 电荷, m 为电子质量, h 为普朗克常数, d 为相邻的 两条纳米线的中心线之间的距离, λ为能垒高度 (PDMS λ = 1 eV), A 为横截面积 [47,51]. 该传感器在 0.03—30.2 kPa 压力范围内灵敏度为 1.5 kPa–1, 其响应机理为: AgNWs 包覆的组织纸表面多孔粗糙, 与叉指电极 接触的棉纸导电纤维数量取决于外部加载压力. 柔性基底可在 机械刺激下被拉长或压缩, 常见的柔性基底有聚二 甲基硅氧烷 (PDMS)[69−71]、聚对苯二甲酸乙二醇 酯 (PET)[72,73]、聚酰亚胺 (PI)[74]、聚乙烯醇 (PVA)[75] 一般来说, 柔性压阻式压力传感器包含了柔性 基底、电极和活性层材料三个部分. 柔性基底可在 机械刺激下被拉长或压缩, 常见的柔性基底有聚二 甲基硅氧烷 (PDMS)[69−71]、聚对苯二甲酸乙二醇 酯 (PET)[72,73]、聚酰亚胺 (PI)[74]、聚乙烯醇 (PVA)[75]
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