Flexible capacitive pressure sensor based on interdigital electrodes with porous microneedle arrays for physiological signal monitoring

Flexible pressure sensors have many potential applications in the monitoring of physiological signals because of their good biocompatibil-ity and wearability.However,their relatively low sensitivity,linearity,and stability have hindered their large-scale commercial application.Herein,aflexible capacitive pressure sensor based on an interdigital electrode structure with two porous microneedle arrays(MNAs)is pro-posed.The porous substrate that constitutes the MNA is a mixed product of polydimethylsiloxane and NaHCO3.Due to its porous and interdigital structure,the maximum sensitivity(0.07 kPa-1)of a porous MNA-based pressure sensor was found to be seven times higher than that of an imporous MNA pressure sensor,and it was much greater than that of aflat pressure sensor without a porous MNA structure.Finite-element analysis showed that the interdigital MNA structure can greatly increase the strain and improve the sensitivity of the sen-sor.In addition,the porous MNA-based pressure sensor was found to have good stability over 1500 loading cycles as a result of its bilayer parylene-enhanced conductive electrode structure.Most importantly,it was found that the sensor could accurately monitor the motion of afinger,wrist joint,arm,face,abdomen,eye,and Adam’s apple.Furthermore,preliminary semantic recognition was achieved by monitoring the movement of the Adam’s apple.Finally,multiple pressure sensors were integrated into a 33 array to detect a spatial pressure distribu-×tion.Compared to the sensors reported in previous works,the interdigital electrode structure presented in this work improves sensitivity and stability by modifying the electrode layer rather than the dielectric layer.

Recent progress in flexible capacitive sensors:Structures and properties

The future intelligent era that will be brought about by 5G technology can be well predicted.For example,the connection between humans and smart wearable devices will become increasingly more intimate.Flexible wearable pressure sensors have received much attention as a part of this process.Nevertheless,there is a lack of complete and detailed discussion on the recent research status of capacitive pressure sensors composed of polymer composites.Therefore,this article will mainly discuss the key concepts,preparation methods and main performance of flexible wearable capacitive sensors.The concept of a processing“toolbox”is used to review the developmental status of the dielectric layer as revealed in highly cited literature from the past five years.The preparation methods are categorized into types of processing:primary and secondary.Using these categories,the preparation methods and structure of the dielectric layer are discussed.Their influence on the final capacitive sensing behavior is also addressed.Recent developments in the electrode layer are also systematically reviewed.Finally,the results of the above discussion are summarized and future development trends are discussed.

A Novel Uniplanar Multi-Electrode Capacitive Sensor for In-Situ Weathering Damage Detection of Nonmetallic Materials

A uniplanar capacitive sensor with 5-electrodes on one plane substrate and a large reflector electrode,was designed to get the corresponding capacitance information for weathering damage detection of non-metallic materials exposed to a service environment.A 2-D finite-element method was employed to simulate the electric potential distribution and capacitance measurements for the sensor.2 marble slabs,one was healthy and the other was notched,were experimentally detected.Both the simulation and the preliminary experimental results show that the measured capacitances decrease after weathering damage occurs in nonmetallic material.The reflector can enlarge the sensitive depth.The weathering assessment of nonmetallic materials can be done by processing the measured capacitances.The proposed approach can effectively detect the weathering damage of nonmetallic material and can be practically used for in-situ weathering damage evaluation.

Progress of Proximity Sensors for Potential Applications in Electronic Skins

Recently,electronic skins and fl exible wearable devices have been developed for widespread applications in medical monitoring,artifi cial intelligence,human–machine interaction,and artifi cial prosthetics.Flexible proximity sensors can accurately perceive external objects without contact,introducing a new way to achieve an ultrasensitive perception of objects.This article reviews the progress of fl exible capacitive proximity sensors,fl exible triboelectric proximity sensors,and fl exible gate-enhanced proximity sensors,focusing on their applications in the electronic skin fi eld.Herein,their working mechanism,materials,preparation methods,and research progress are discussed in detail.Finally,we summarize the future challenges in developing fl exible proximity sensors.

Dielectric Properties of Capacitive-Type Humidity Sensor Made under Different Pressures

Capacitive humidity sensors were made of nanometer barium titanate.The pellets were prepared under different pressures between 3920N to 7850N force.The capacitance changes in three orders of magnitude in the relative humidity range of 10%～98%,indicating high humidity sensitivity of the sensors.At a certain measuring frequency,the capacitance of the sensors increases as increasing of the preparation pressure,while the sensitivity of the sensors basically remains the same.The frequencies corresponding to the peaks of the dielectric loss of the sensors move to the higher frequency direction as increasing of the relative humidity.At a certain humidity,the frequencies corresponding to the peaks of the dielectric loss move to the higher frequency direction as increasing of the preparation pressure.

Improvement of High Dynamic Range Capacitive Displacement Sensor by a Globalm Planarization

This study presents an improvement of high dynamic range contact-type capacitive displacement sensor by applying planarization. The sensor is called the contact-type linear encoder-like capacitive displacement sensor (CLECDiS), is a nano-meter-resolution sensor with a wide dynamic range. However, height differences due to patterned electrodes may cause a variety of problems or performance degradation. In devices of two glass wafer surfaces with patterned structures assembled face-to-face and in sliding contact, the heights of the patterns crucially affect their performance and practicality, so it should be planarized for reducing the problem. A number of techniques for planarizing glass wafer surfaces with patterned chrome electrodes were evaluated and the following three were selected as adequate: lift-off, etch-back, and chemical mechanical polishing (CMP). The fabricated samples showed that CMP provided the best planarization. CMP was successfully employed to produce CLECDiS with improved signal reliability due to reduced collisions between electrodes.

Nondestructive testing method of wood moisture content based on a planar capacitance sensor model

For our research, a new hybrid experimental-computational method is presented. We applied a least squares fitting method （LSFM） to reconstruct the wood moisture content （WMC） from the data measured with a planar capacitance sensor. A boundary element method （BEM） was used to compute the relationship between capacitance and the dielectric constant. A functional relationship between MC and the dielectric constant was identified by LSFM. The agreement of this final computation result with the experimental data indicates that this method can be used to estimate the WMC quickly and effectively with engineering analysis. Compared with popular statistical methods, a large number of experiments are avoided, some costs of testing are reduced and the efficiency of testing is enhanced.

Development of MEMS-based micro capacitive tactile probe

In this paper, a micro capacitive sensor with nanometer resolution is presented for ultra-precision measurement of micro components, which is fabricated by the MEMS （micro electromechanical systems） non-silicon technique. Based on the sensor, a micro capacitive tactile probe is constructed by stylus assembly and packaging design for dimension metrology on micro/nano scale, in which a data acquiring system is developed with AD7747. Some measurements of the micro capacitive tactile probe are performed on a nano positioning and measuring machine （NMM）. The measurement results show good linearity and hysteresis with a range of 11.6 μm and resolution of better than 5 nm. Hence, the micro capacitive tactile probe can be integrated on NMM to realize measurement of micro structures with nanometer accuracy.

A New Humidity-Sensitive Material Based on PPBT Prepared with Palladium Complex Catalyst

Soluble Poly(propargyl benzoate) (PPBT) with pi -conjugated structure was synthesized using a novel bis(triphenylphosphine)-bisacetylide palladium complex catalyst [Pd(PPh3)(2)(C equivalent to CCH2OOCPh)(2)] (PPB). An interdigital gold electrode was covered by screen printing films of doped PPBT (DPPBT) to prepare a resistance-type humidity sensor, which exhibits electrical response towards relative humidity (RH%) variations in the range 11%-96%. PPBT shows promise as a new humidity-sensitive material.

Textile-Based Capacitive Pressure Distribution Measurement System for Human Sitting Posture Monitoring

Flexible pressure monitoring device can help correct the sitting posture and prevent health problems(e.g.,deformity of spinal column and musculoskeletal disease).Currently,most measurement systems hinder their wide applications owing to the high cost or low accuracy.In this study,a flexible sitting pressure measurement system was proposed based on a textile-based capacitive pressure sensor array in order to measure sitting pressure distribution simply and conveniently.The capacitive pressure sensor array is sandwich structure composed of a high-density sponge layer and two electrode array fabrics,which possesses high resolution(2.26 sensors/cm2),high sensitivity(0.701 kPa-1)and fast response(≤35 ms).It is worth noting that the raw materials of the sensing fabric include commercialized copper sheets and polyester yarns.The as-prepared pressure measurement system can accurately measure the pressure distribution nephogram for sitting posture analysis.The sitting pressure of 10 volunteers was measured and six types of posture were distinguished clearly.

Design of capacitance sensor system for void fraction measurement

Simulation and optimization were applied to a capacitive sensor system based on electrical tomography technology. Sensors, consisting of Morgantown Energy Technology Center (METC) axial synchro driving guard electrodes and two sets of detecting electrodes, make it possible to obtain simultaneously two groups of signals of the void fraction in oil-gas two-phase flow. The computational and experimental results showed that available sensors, charactered by high resolution and fast real-time response can be used for real-time liquid-gas two-phase flow pattern determination.

Sensor monitoring and online measurement correction in electric capacitance tomography system

The recalibration of electrical capacitance tomography (ECT) system is one of the key problems in keeping the system running steadily.However, for engineering application in solids/gas transport,online calibration can not be implemented and the data from this sensor may be unreliable due to the sensor pipe interior wall abrasion during pneumatic transport,so the solids concentration calculated from the reconstructed image based on these data will be highly inaccurate.The simulations show that, the inter-electrode relative capacitance variation of electrode pair spacing 1 is the most sensitive to the abrasion of sensor pipe interior wall, so this relative capacitance variation when the sensor is filled with air can be used as an indicator demanding offline system recalibration when the wall abrasion goes significant.Furthermore, while the pipe interior wall abrasion is not very serious, online correcting measured inter-electrode capacitance with wall capacitance variation can improve the accuracy of concentration calculation.

Capacitance sensor for automatic soil retreat measurements

To continuously monitor the soil retreat due to erosion in field,provide valuable information about the erosion processes and overcome the disadvantages of inefficiency,high time-consumption and labor-intensity of existing methods,this paper describes a novel capacitance sensor for measuring the soil retreat.A capacitance sensor based probe is proposed,which can measure the depth of the soil around it automatically and the data can be recorded by a data logger.Experimental results in the lab verify its usefulness.

A Novel Humidity-Sensitive Material Based on PDEB with Nickel Complex Catalyst

Poly(p-diethynylbenzene) (PDEB) with improved solubility was synthesized using a novel catalyst bis(triphenylphosphine)bis(p-diethynylbenzene) nickel complex (NI-C). A surface acoustic wave (SAW) delay line oscillator was covered by Langmuir-Blodgett (LB) films of resulting PDEB to prepare a humidity sensor, which was found to have high sensitivity and good repeatability. PDEB shows promise as a good humidity-sensitive material.

基于MEMS技术的集成压力-湿度传感器

设计了一种高灵敏度的集成压力和湿度传感单元的芯片。压力传感单元基于SOI和蛇形电阻结构。室温下,传感器在载压范围为3~129 kPa内灵敏度为0.026 mV/kPa,与有限元仿真基本吻合。传感器在25~120℃范围内的热灵敏度漂移为0.004‰FS/℃,热零点漂移为0.25%FS/℃。湿度传感单元采用的叉指电极和电容式结构,引入含氟PI作湿度敏感膜。设计Mo电阻加热结构加快传感器降湿过程,缩短降湿时间近32%。在10%~90%RH的湿度范围,含氟PI湿度传感器的灵敏度为0.121 pF/%RH,略低于无氟PI器件。含氟基团的引入,使得传感器的湿滞较无氟PI降低16%。“电容-湿度”曲线呈指数分布,相关系数R^(2)=0.996。测试结果发现,湿度传感单元和压力传感单元拥有良好的独立工作性能。

基于电容传感器的CO_(2)气液两相流流型识别

为了实现碳捕捉与封存(CCS)管道内CO_(2)流量的精确测量,需要对CO_(2)的流型进行辨识。基于递归定量分析和模糊C均值(FCM)聚类算法,建立了CO_(2)气液两相流的流型识别模型。提出利用小波变换在多尺度下分析CO_(2)气液两相流电容信号递归图结构特性的方法,对各个尺度下的递归图分别进行定量分析,提取出不同流型下CO_(2)流动的电容传感器递归特征参数。采用FCM算法,对特征参数进行流型聚类,完成了对CO_(2)气液两相流的分层流、气泡流和雾状流3种流型的识别。

基于Modbus/TCP的电容式扭矩传感器数据采集系统设计

扭矩传感器通常安装于机械设备的旋转关节处,用于扭矩的测量。电容式扭矩传感器需要配备专门的数据采集系统才能使用,且数据采集系统的性能直接影响传感器的精确测量。针对实验室自主研制的电容式扭矩传感器的应用需求,设计了一种基于Modbus/TCP协议的数据采集系统。系统以STM32为主控芯片,通过控制电容数字转换芯片AD7147实现电容信号采集与转换,然后由W5500以太网芯片将数据上传到自主开发的上位机软件中进行处理、显示与存储。实验测试表明:系统最大测量误差为1.47%,数据输出频率为1000 Hz。

低轴间耦合的三维电场传感器

使用正交排列的电容式传感单元的三维电场传感器(Three-dimensional electric field sensor,3D EFS)测量空间电场时,轴间耦合效应会严重影响EFS的测量精度。为此,提出了一种电场屏蔽电极,以降低3D EFS的轴间耦合效应,提高测量精度。首先,利用多物理场仿真软件构建电场分布模型。其次,根据仿真结果建立带屏蔽电极的3D EFS电容式传感单元的屏蔽电极模型。最后,建立任意角度的测试平台,将带有屏蔽电极的3D EFS和无屏蔽电极的3D EFS进行实验对比。结果显示,有屏蔽电极的3D EFS的测量偏差在3.2%以内,比无屏蔽电极的3D EFS的测量偏差减少12%。因此,所设计的基于电场屏蔽结构的3D EFS可以使解耦矩阵更加可靠,有效降低空间电场测量偏差。

电容式含水传感器极板间油液更替时间研究

液压系统中的水污染严重影响系统工作可靠性,因此利用含水传感器实时准确地响应系统油液含水率的变化对液压系统的健康维护至关重要。电容式含水传感器通过测量介电常数的变化来间接测量水分含量,而电容极板间油液更替的时间直接反应传感器的动态响应特性。选择了两种仿真方法对极板间油液的更替情况进行了分析,并搭建试验系统进行了验证,结果表明,物质组分输运模型更适合仿真分析电容型含水传感器的响应特性。提出的仿真方法为电容式含水传感器动态响应特性的分析提供一种新的思路,分析结果可为电容式含水传感器的采样时间选择提供参考,以获得准确可靠的含水率检测结果。