Design, fabrication and sensing application of hierarchical microstructures based on micro/nano fibers
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摘要: 微结构化是提高柔性压力传感器性能的重要技术手段之一，本文提出一种基于微纳米纤维的多层次微结构设计与快速制备方法。首先采用近场直写与熔融沉积成型一体化成形工艺制备出具有多尺度纤维的牺牲支架，在聚二甲基硅氧烷(Polydimethylsiloxane，PDMS)中掺杂碳纳米管(Carbon nanotubes，CNTs)作为柔性传感器的介电层材料，通过牺牲模板法制备出具有多层次微结构的CNTs/PDMS柔性介电层；进一步研究了多层次微结构的设计参数对其传感性能的影响。实验结果表明：设计的微结构能显著增强柔性传感器的输出电性能。其中高度为1.3 mm、间距为1.5 mm的多层次微结构在频率为3 Hz、压力载荷为14 N下输出电性能最为优异；此外，制备的传感器经过20000次循环测试，表现出良好的稳定性与耐久性。最后，设计了一款用于观察足底压力分布及步态检测的柔性压力传感鞋垫，结果表现出良好灵敏度和稳定性。本文为低成本快速制备多层次微结构提供新的思路，为制备高性能柔性压力传感器提供参考与借鉴。Abstract: Microstructuring is one of the important techniques to improve the performance of flexible pressure sensors. In this paper, a method for designing and fabricating hierarchical microstructures based on micro/nano fibers was proposed. First, a sacrificial mold with hierarchical microstructures was prepared by integrating near-field direct writing and fused deposition modeling. Carbon nanotubes (CNTs) were doped into polydimethylsiloxane (PDMS) as the dielectric layer material for the flexible sensor. The CNTs/PDMS flexible dielectric layer with hierarchical microstructures was then prepared by sacrificial template method. Furthermore, the effect of design parameters of hierarchical microstructures on the sensing performance was studied. The experimental results show that the designed microstructures can significantly enhance the output electrical performance of the flexible sensor. Among them, the hierarchical microstructure with a height of 1.3 mm and a spacing of 1.5 mm exhibits the best output electrical performance under a pressure load of 14 N at a frequency of 3 Hz. In addition, the fabricated sensor exhibits good stability and durability after 20000 cycles of testing. Finally, a flexible pressure sensing insole was designed for observing the distribution of foot pressure and gait detection, which demonstrates good sensitivity and stability. This study provides a new approach for low-cost and rapid fabrication of hierarchical microstructures and serves as a reference for the development of high-performance flexible pressure sensors.
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图 1 (a) 垂直接触分离式摩擦纳米发电机(TENG)压力传感器结构图；(b) 传感原理示意图；(c) 微结构层压缩变形应力云图
Figure 1. (a) Composition of vertical contact separation-type triboelectric nanogenerator (TENG) structure; (b) Diagram of sensing principle; (c) Stress cloud of dielectric layer with microstructures
PET—Polyethylene terephthalate; CNTs—Carbon nanotubes; PDMS—Polydimethylsiloxane
图 2 聚己内酯(PCL)纤维支架实物图：(a) 细纤维；(b) 粗纤维；(c) 多尺度纤维
Figure 2. Photograph of polycaprolactone (PCL) fiber film: (a) Fine fiber; (b) Coarse fiber; (c) Multi-scale fiber
图 3 (a) CNTs/PDMS薄膜制备流程图；(b) 微结构CNTs/PDMS薄膜的光学图像
Figure 3. (a) Schematic of CNTs/PDMS film preparation; (b) Optical image of microstructured CNTs/PDMS film
DCM—Dichloromethane
图 4 (a) 粗纤维微结构(高度0.15 mm)；(b) 粗细纤维结合微结构(高度0.15 mm)；(c) 粗纤维微结构(高度1.30 mm)；(d) 粗细纤维结合微结构(高度1.30 mm，间距2.50 mm)；(e) 粗细纤维结合微结构(高度1.30 mm，间距2.00 mm)；(f) 粗细纤维结合微结构(高度1.30 mm，间距1.50 mm)；(g) CNTs/PDMS薄膜截面微观形貌SEM图像；(h) 拉伸弯曲测试示意图
Figure 4. (a) Coarse fiber pattern with height of 0.15 mm; (b) Pattern of combined fiber with height of 0.15 mm; (c) Coarse fiber pattern with height of 1.30 mm; (d) Pattern of combined fiber with height of 1.30 mm and spacing of 2.50 mm; (e) Pattern of combined fiber with height of 1.30 mm and spacing of 2.00 mm; (f) Pattern of combined fiber with height of 1.30 mm and spacing of 1.50 mm; (g) SEM images of microstructure morphology in the cross-section of CNTs/PDMS thin film; (h) Tensile and bending test
图 5 单轴拉伸实验与多层次微结构CNTs/PDMS介电层应力-应变曲线
Figure 5. Uniaxial tensile test and stress-strain curves of hierarchical microstructure CNTs/PDMS dielectric layer
图 6 CNTs/PDMS薄膜表面微结构对传感器输出电性能影响：(a) 传感性能测试装置示意图；(b) 开路电压；(c) 短路电流
Figure 6. Influence of surface microstructure of CNTs/PDMS films on sensor electrical performance: (a) Schematic diagram of the test setup; (b) Open-circuit voltage; (c) Short-circuit current
图 7 CNTs/PDMS薄膜表面微结构尺寸对传感器输出电性能影响：(a) 开路电压；(b) 短路电流
Figure 7. Influence of surface microstructure size of CNTs/PDMS films on sensor electrical performance: (a) Open-circuit voltage; (b) Short-circuit current
图 8 外部载荷对传感器输出电性能影响：(a) 开路电压；(b) 短路电流；(c) 灵敏度
Figure 8. Influence of external load on sensor electric performance: (a) Open-circuit voltage; (b) Short-circuit current; (c) Sensitivity
图 9 传感器循环耐久实验：(a) 载荷频率对传感器输出电性能的影响；(b) 20000次循环下传感器的电压稳定性测试
Figure 9. Sensor cycle durability experiment: (a) Influence of load frequency on sensor output electrical performance; (b) Voltage stability test of the sensor under 20000 cycles
图 10 微结构传感单元的足底压力分布与步态检测：(a) 传感单元分布示意图；(b) 负摩擦面实物图；(c) 触地期；(d) 支撑期；(e) 离地期
Figure 10. Plantar pressure distribution and gait detection of microstructured sensor unit: (a) Schematic diagram of sensor unit distribution; (b) Photo of negative friction surface; (c) Contact phase; (d) Support phase; (e) Swing phase
图 11 不同运动状态下微结构传感单元的压力分布与输出电压：(a) 行走时输出电压信号；(b) 奔跑时输出电压信号；(c) 跳跃时输出电压信号
Figure 11. Pressure distribution and output voltage of microstructured sensor unit under different motion states: (a) Output voltage signal during walking; (b) Output voltage signal during running; (c) Output voltage signal during jumping
表 1 多层次微结构CNTs/PDMS传感器与其他同类压力传感器的性能比较
Table 1. Performance comparison between hierarchical microstructured CNTs/PDMS sensors and other similar pressure sensors
Key material Sensitivity Open-circuit voltage/V Short-circuit current/μA Cycle Ref. CNT/PDMS 0.5 V/kPa 4.0 3.0 10000 [28] CNT/PDMS 0.122 V/kPa 31 — 10000 [35] CNT/PDMS — 42 1.6 — [36] Polyacrylamide (PAAm)-LiCl 0.013 V/kPa 4.0 1.5 5000 [37] Fluorinated ethylene propylene (FEP) 0.04 V/kPa 15 — 10000 [38] PDMS — 16.2 0.512 — [30] CNTs/PDMS 0.437 V/kPa, 0.015 μA/kPa 50.8 1.85 20000 This work 
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