Textile-Based Sensors for Inspection of Composite Materials

The monitoring of mechanical deformation and damage of composite materials is normally performed by established analytical methods,such as strain gauges and opt...     

Fabrication and Characterization of Single-Layer Textile-Based Flexible Pressure Sensors for Smart Wearable Electronics Applications

Textile-based sensors have been widely studied for wearable monitoring. The sensor systems demand a large sensing area, flexibility, and scalable fabrication method. Herein, single-layer piezoresistive sensors are developed by a machine stitching technique using metallic and graphene nanoplatelets-coated conductive threads and fabrics. The pressure-sensing mechanism is based on measuring the electrical resistance due to the change in the contact area between the conductive thread and fabric as pressure on the sensor varies. The single-layer sensor design provides flexibility and overcomes the physical drift of the sensor during human activities, which enhances wearability and performance. The coated textiles are characterized by scanning electron microscopy and Fourier-transform infrared spectroscopy. Physical and electromechanical tests are performed on the sensors to evaluate their wearability and sensing performance. The sensors exhibit a wide working range of up to 100 kPa and good sensitivity with excellent durability against repeated mechanical deformations. The application potential of the sensors in real-time monitoring is demonstrated by embedding them into clothing as a wearable device. Moreover, the effectiveness of the sensors is tested for posture correction. This article suggests a novel technique to fabricate durable, flexible, and highly efficient pressure sensors for smart wearable applications.     

Analysis of a Direct Interface Circuit for Capacitive Sensors

We present the theoretical analysis and performance results of a direct microcontroller unit (MCU) interface circuit for capacitive sensors based on the charge-transfer method, when stray capacitances are considered. The interface circuit can implement two alternative two-point calibration techniques that reduce the effects of stray capacitance, temperature, and MCU parameters that depend on the power supply voltage. The best measurement deviation achieved from 0 $^{circ}hbox{C}$ to 50 $^{circ}hbox{C}$ and for power supply voltage from 4.0 to 5 V is below 0.01 full-scale range (FSR) for the two subranges from 10 to 100 pF and from 100 pF to 1 nF and 0.08 FSR for the subrange from 2 to 10 pF.      

Capacitive Transduction for Liquid Crystal-Based Sensors, Part I: Ordered System

In this paper, the theory for tracking the average molecular orientation of a liquid crystal (LC) material via capacitive sensing of anisotropic media is presented. The candidate LC sensor structure utilizes interdigital electrodes. Two capacitive measurements in orthogonal direction can track the director axis of nematic LC material in a homogenous ordered LC film. The sensitivity for the sensor at different alignments is studied. The candidate sensors have been fabricated and experimentally verified. Both experimental and calculated values for capacitances of selected interdigital fingers sensor structures are presented.     

Self-Powered Sensors for Monitoring of Highway Bridges

The task of structural health monitoring (SHM) of aging highway bridges and overpasses is important not only from the point of preventing economic losses from traffic delays and detours but also is a matter of preventing catastrophic failures and loss of human life. In recent years, wireless sensor technologies have been used extensively to develop SHM platforms for bridges. A limitation of wireless sensors is the finite life span of batteries and high cost of battery replacements, which make such systems prohibitively expensive in many cases. Energy harvesting is a solution capable to alleviate this problem. A novel wireless sensor system is presented that harvests vibrations of the bridge created by passing traffic, which is converted into usable electrical energy by means of a linear electromagnetic generator. Utilization of an electromagnetic generator allows harvesting of up to 12.5 mW of power in the resonant mode with the frequency of excitation at 3.1 Hz, in this particular design. The novelty of the system also includes tight integration of the power generator and a smart algorithm for energy conversion that switches between the low-power mode and the impedance matching mode. Finally, results of field experiments are presented in which the wireless system is operated exclusively by the harvested energy of vibration on a rural highway bridge with low traffic volume.     

Advanced Wearable Microfluidic Sensors for Healthcare Monitoring

Wearable flexible sensors based on integrated microfluidic networks with multiplex analysis capability are emerging as a new paradigm to assess human health status and show great potential in application fields such as clinical medicine and athletic monitoring. Well‐designed microfluidic sensors can be attached to the skin surface to acquire various pieces of physiological information with high precision, such as sweat loss, information regarding metabolites, and electrolyte balance. Herein, the recent progress of wearable microfluidic sensors for applications in healthcare monitoring is summarized, including analysis principles and microfabrication methods. Finally, the challenges and opportunities for wearable microfluidic sensors in practical applications are discussed.     

Intraocular Pressure Monitoring Sensors

Continuous measurement of intraocular pressure is important in the detection and treatment of glaucoma. While a point check of intraocular pressure in a doctor's office using indirect measurements such as the tonometer is helpful, it is inadequate to track circadian variation. Circadian variation is an independent risk factor in addition to elevated pressure levels. This paper is aimed at providing an up-to-date review of various intraocular pressure sensing techniques and in vivo sensor design approaches. The basic operating principles of various implantable sensors are reviewed and categorized into groups to delineate their differences. A discussion is presented identifying the drawbacks of existing designs and key design questions are proposed for future progress.     

Transparent,Flexible, Conformal Capacitive Pressure Sensors with Nanoparticles

The fundamental challenge in designing transparent pressure sensors is the ideal combination of high optical transparency and high pressure sensitivity. Satisfying these competing demands is commonly achieved by a compromise between the transparency and usage of a patterned dielectric surface, which increases pressure sensitivity, but decreases transparency. Herein, a design strategy for fabricating high‐transparency and high‐sensitivity capacitive pressure sensors is proposed, which relies on the multiple states of nanoparticle dispersity resulting in enhanced surface roughness and light transmittance. We utilize two nanoparticle dispersion states on a surface: (i) homogeneous dispersion, where each nanoparticle (≈500 nm) with a size comparable to the visible light wavelength has low light scattering; and (ii) heterogeneous dispersion, where aggregated nanoparticles form a micrometer‐sized feature, increasing pressure sensitivity. This approach is experimentally verified using a nanoparticle‐dispersed polymer composite, which has high pressure sensitivity (1.0 kPa^–1), and demonstrates excellent transparency (>95%). We demonstrate that the integration of nanoparticle‐dispersed capacitor elements into an array readily yields a real‐time pressure monitoring application and a fully functional touch device capable of acting as a pressure sensor‐based input device, thereby opening up new avenues to establish processing techniques that are effective on the nanoscale yet applicable to macroscopic processing.     

Smooth Contact Capacitive Pressure Sensors in Touch- and Peeling-Mode Operation

Capacitive (C) pressure sensors typically sense quadratic changes in $C$ as a pressure difference $(P)$ deflects a flexible conducting diaphragm near a rigid ground plane. Touch-mode capacitive pressure (C-P) sensors, where the conducting diaphragm touches a dielectric coated ground plane, often show a more linear response, but with less sensitivity, particularly at low-$P$ . Initial contact of the diaphragm often occurs at a critical $P$. Until $P_{rm crit}$ is reached, the sensitivity is typically too low for accurate measurements. In this work, two different types of electrodes with “parabolic” and “donut” cavity-shapes have been designed, fabricated, and tested to achieve high-sensitivity at low-pressures. A flexible conducting diaphragm touches the bottom electrode smoothly, and both cavity shapes permit initial contact at a zero-pressure differential. This type of C-P sensors can have touch-mode and peeling-mode operations. The sensitivities of these sensors in two operation modes were measured, and their resolutions were smaller than 0.1 Pa at a mean pressure of ${10} ^{5}~{rm Pa}$. Both sensors in two modes have the resolution over total-pressure less than ${10} ^{-6}$, which is difficult to achieve at atmospheric pressure.      

Capacitive Humidity Sensors With High Sensitivity and Subsecond Response Times

Capacitive-based humidity sensors were fabricated using coplanar interdigitated electrodes coated with nanostructured TiO₂ thin films produced by glancing angle deposition. In this letter, we show that increased sensitivity (nF/%RH) is obtained by decreasing the electrode periodicity or by increasing the planar area of the electrodes, or both. The devices were sensitive over a wide range of relative humidity levels (<1% to >92%) and exhibited extremely fast, subsecond response times. Typical adsorption and desorption response times were measured to be <220 and >400 ms, respectively     

Capacitive Transduction for Liquid Crystal Based Sensors, Part II: Partially Disordered System

This paper presents the capacitive transduction technique involved with liquid crystal (LC) based sensors in partially disordered systems. These sensors have the potential applications in chemical and biological systems. The theory for tracking the average molecular deformation (state of alignment) and degree of ordering of anisotropic and partially disordered LC film via capacitive sensing is investigated. This system is modeled using the Q-tensor approach in modeling uniaxial LC material. The proposed sensor design is an interdigitated electrodes structure. Transverse and fringing capacitances as function of the molecular deformation are calculated. It is verified that three capacitance measurements are required to track the average molecular orientation and the degree of disorder in the LC film. The sensitivity for the sensor at different alignments and ordering degree is also studied. Toward practical sensor, neuro-fuzzy system is modeled to simulate the capacitive transduction and to monitor the LC profile. Sensors are fabricated and tested. Both the experimental and calculated capacitances are presented and compared.      

Textile-Based Electrogoniometers for Wearable Posture and Gesture Capture Systems

This paper introduces a method for detecting joint angles by using piezoresistive strain sensitive materials, as carbon loaded rubbers are. Materials used can be screen-printed onto fabrics to provide garments with a sensing apparatus able to reconstruct human postures and gestures. The main differences between this approach and the previous ones, and the core of this work, is the rigorous proof that for small local curvatures of the layers the constituting electrogoniometers, the resistance depends only on the total curvature of the layers and not on the particular form that the sensor keeps in adherence with the human body. In this paper, we show that the hypothesis of small local curvature does not severely restrict the set of angles which can be detected.      

面向结构健康监测的压电传感器综述

压电传感器在结构健康监测方面的应用日益广泛。大型工程结构、混凝土结构及微电子构件的损伤监测与智能传感为其提供了广阔的应用平台。压电薄膜与涂层制备工艺不断革新,使压电传感器性能逐步优化,从而开拓了更广阔的应用领域与前景。结合国内外的研究热点和现状,简述了压电传感的动态监测原理,综合概述了外贴式压电传感、埋入式压电传感及表面涂覆式压电传感在结构智能监测中的研究进展,其中针对不同制备工艺,分别阐述了不同表面涂覆式压电传感器的应用研究,归纳并总结了3种传感器的优缺点及表面涂覆式压电传感器的制备工艺,最后展望了未来的应用前景和发展方向。     

Evanescent Field Fiber-Optic Sensors for Humidity Monitoring Based on Nanocoatings

The transmitted optical power of two different fiber optic based structures when a nanofilm is being deposited onto them is experimentally studied. The technique used to build the nanofilms is Electrostatic Self Assembly (ESA), which has been widely reported in the literature. For the shake of comprehensibility, the comparative analysis of this phenomenon is accomplished for a particular sensing measure, humidity. The two structures selected towards development of practical humidity evanescent field sensors are hollow core fibers and tapered optical fibers. Some preliminary experimental studies of depositing humidity sensitive thin films and demonstrating their feasibility are presented. Depending on the working point selected, up to 10dB of variation in the optical output power is obtained when the environmental humidity changes. Both configurations exhibit similar dynamic behavior and response times shorter than 300msec, making these evanescent field sensors good candidates to monitor human breathing