Editorial

3 pages, 185 KiB

Open AccessEditorial

Editorial for the Special Issue on Flexible and Wearable Sensors

by Libo Gao and Zhuoqing Yang

Micromachines 2023, 14(7), 1400; https://doi.org/10.3390/mi14071400 - 09 Jul 2023

Viewed by 881

Flexible wearable sensors have garnered significant interest in the fields of human-computer interaction, materials science, and biomedicine [...] Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

Research

Jump to: Editorial, Review

20 pages, 4113 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Laser-Formed Sensors with Electrically Conductive MWCNT Networks for Gesture Recognition Applications

by Natalia A. Nikitina, Dmitry I. Ryabkin, Victoria V. Suchkova, Artem V. Kuksin, Evgeny S. Pyankov, Levan P. Ichkitidze, Aleksey V. Maksimkin, Evgeny P. Kitsyuk, Ekaterina A. Gerasimenko, Dmitry V. Telyshev, Ivan Bobrinetskiy, Sergey V. Selishchev and Alexander Yu. Gerasimenko

Micromachines 2023, 14(6), 1106; https://doi.org/10.3390/mi14061106 - 24 May 2023

Cited by 2 | Viewed by 1793

Abstract

Currently, an urgent need in the field of wearable electronics is the development of flexible sensors that can be attached to the human body to monitor various physiological indicators and movements. In this work, we propose a method for forming an electrically conductive [...] Read more.

Currently, an urgent need in the field of wearable electronics is the development of flexible sensors that can be attached to the human body to monitor various physiological indicators and movements. In this work, we propose a method for forming an electrically conductive network of multi-walled carbon nanotubes (MWCNT) in a matrix of silicone elastomer to make stretchable sensors sensitive to mechanical strain. The electrical conductivity and sensitivity characteristics of the sensor were improved by using laser exposure, through the effect of forming strong carbon nanotube (CNT) networks. The initial electrical resistance of the sensors obtained using laser technology was ~3 kOhm (in the absence of deformation) at a low concentration of nanotubes of 3 wt% in composition. For comparison, in a similar manufacturing process, but without laser exposure, the active material had significantly higher values of electrical resistance, which was ~19 kOhm in this case. The laser-fabricated sensors have a high tensile sensitivity (gauge factor ~10), linearity of >0.97, a low hysteresis of 2.4%, tensile strength of 963 kPa, and a fast strain response of 1 ms. The low Young’s modulus values of ~47 kPa and the high electrical and sensitivity characteristics of the sensors made it possible to fabricate a smart gesture recognition sensor system based on them, with a recognition accuracy of ~94%. Data reading and visualization were performed using the developed electronic unit based on the ATXMEGA8E5-AU microcontroller and software. The obtained results open great prospects for the application of flexible CNT sensors in intelligent wearable devices (IWDs) for medical and industrial applications. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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15 pages, 5498 KiB

Open AccessArticle

Parameter Study on Force Curves of Assembled Electronic Components on Foils during Injection Overmolding Using Simulation

by Martin Hubmann, Mona Bakr, Jonas Groten, Martin Pletz, Jan Vanfleteren, Frederick Bossuyt, Behnam Madadnia and Barbara Stadlober

Micromachines 2023, 14(4), 876; https://doi.org/10.3390/mi14040876 - 19 Apr 2023

Cited by 2 | Viewed by 1423

Abstract

The integration of assembled foils in injection-molded parts is a challenging step. Such assembled foils typically comprise a plastic foil on which a circuit board is printed and electronic components are mounted. Those components can detach during overmolding when high pressures and shear [...] Read more.

The integration of assembled foils in injection-molded parts is a challenging step. Such assembled foils typically comprise a plastic foil on which a circuit board is printed and electronic components are mounted. Those components can detach during overmolding when high pressures and shear stresses prevail due to the injected viscous thermoplastic melt. Hence, the molding settings significantly impact such parts’ successful, damage-free manufacturing. In this paper, a virtual parameter study was performed using injection molding software in which 1206-sized components were overmolded in a plate mold using polycarbonate (PC). In addition, experimental injection molding tests of that design and shear and peel tests were made. The simulated forces increased with decreasing mold thickness and melt temperature and increasing injection speed. The calculated tangential forces in the initial stage of overmolding ranged from 1.3 N to 7.3 N, depending on the setting used. However, the experimental at room temperature-obtained shear forces at break were at least 22 N. Yet, detached components were present in most of the experimentally overmolded foils. Hence, the shear tests performed at room temperature can only provide limited information. In addition, there might be a peel-like load case during overmolding where the flexible foil might bend during overmolding. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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12 pages, 5847 KiB

Open AccessArticle

A Pressure and Temperature Dual-Parameter Sensor Based on a Composite Material for Electronic Wearable Devices

by Zhidong Zhang, Huinan Zhang, Qingchao Zhang, Xiaolong Zhao, Bo Li, Junbin Zang, Xuefeng Zhao and Tiansheng Zhang

Micromachines 2023, 14(3), 690; https://doi.org/10.3390/mi14030690 - 21 Mar 2023

Cited by 2 | Viewed by 1272

Abstract

Wearable sensors integrating multiple functionalities are highly desirable in artificial wearable devices, which are of great significance in the field of biomedical research and for human–computer interactions. However, it is still a great challenge to simultaneously perceive multiple external stimuli such as pressure [...] Read more.

Wearable sensors integrating multiple functionalities are highly desirable in artificial wearable devices, which are of great significance in the field of biomedical research and for human–computer interactions. However, it is still a great challenge to simultaneously perceive multiple external stimuli such as pressure and temperature with one single sensor. Combining the piezoresistive effect with the negative temperature coefficient of resistance, in this paper, we report on a pressure–temperature dual-parameter sensor composed of a polydimethylsiloxane film, carbon nanotube sponge, and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate). The proposed multifunctional sensor can stably monitor pressure signals with a high sensitivity of 16 kPa⁻¹, has a range of up to 2.5 kPa, and also has a fast response time. Meanwhile, the sensor can also respond to temperature changes with an ultrahigh sensitivity rate of 0.84% °C⁻¹ in the range of 20 °C to 80 °C. To validate the applicability of our sensor in practical environments, we conducted real-scene tests, which revealed its capability for monitoring = human motion signals while simultaneously sensing external temperature stimuli, reflecting its great application prospects for electronic wearable devices. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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14 pages, 3955 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Triboelectric-Electromagnetic Hybrid Wind-Energy Harvester with a Low Startup Wind Speed in Urban Self-Powered Sensing

by Gang Li, Juan Cui, Tingshan Liu, Yongqiu Zheng, Congcong Hao, Xiaojian Hao and Chenyang Xue

Micromachines 2023, 14(2), 298; https://doi.org/10.3390/mi14020298 - 23 Jan 2023

Cited by 6 | Viewed by 1697

Abstract

Wind energy as a renewable energy source is easily available and widely distributed in cities. However, current wind-energy harvesters are inadequate at capturing energy from low-speed winds in urban areas, thereby limiting their application in distributed self-powered sensor networks. A triboelectric–electromagnetic hybrid harvester [...] Read more.

Wind energy as a renewable energy source is easily available and widely distributed in cities. However, current wind-energy harvesters are inadequate at capturing energy from low-speed winds in urban areas, thereby limiting their application in distributed self-powered sensor networks. A triboelectric–electromagnetic hybrid harvester with a low startup wind speed (LSWS-TEH) is proposed that also provides output power within a wide range of wind speeds. An engineering-implementable propeller design method is developed to reduce the startup wind speed of the harvester. A mechanical analysis of the aerodynamics of the rotating propeller is performed, and optimal propeller parameter settings are found that greatly improved its aerodynamic torque. By combining the high-voltage output of the triboelectric nanogenerator under low-speed winds with the high-power output of the electromagnetic generator under high-speed winds, the harvester can maintain direct current output over a wide wind-speed range after rectification. Experiments show that the harvester activates at wind speeds as low as 1.2 m/s, powers a sensor with multiple integrated components in 1.7 m/s wind speeds, and drives a Bluetooth temperature and humidity sensor in 2.7 m/s wind speeds. The proposed small, effective, inexpensive hybrid energy harvester provides a promising way for self-powered requirements in smart city settings. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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13 pages, 6308 KiB

Open AccessArticle

Flexible Capacitive Pressure Sensor Based on a Double-Sided Microstructure Porous Dielectric Layer

by Qingyang Yu and Jian Zhang

Micromachines 2023, 14(1), 111; https://doi.org/10.3390/mi14010111 - 30 Dec 2022

Cited by 8 | Viewed by 2077

Abstract

In the era of intelligent sensing, there is a huge demand for flexible pressure sensors. High sensitivity is the primary requirement for flexible pressure sensors, whereas pressure response range and resolution, which are also key parameters of sensors, are often ignored, resulting in [...] Read more.

In the era of intelligent sensing, there is a huge demand for flexible pressure sensors. High sensitivity is the primary requirement for flexible pressure sensors, whereas pressure response range and resolution, which are also key parameters of sensors, are often ignored, resulting in limited applications of flexible pressure sensors. This paper reports a flexible capacitive pressure sensor based on a double-sided microstructure porous dielectric layer. First, a porous structure was developed in the polymer dielectric layer consisting of silicon rubber (SR)/NaCl/carbon black (CB) using the dissolution method, and then hemisphere microstructures were developed on both sides of the layer by adopting the template method. The synergistic effect of the hemispheric surface microstructure and porous internal structure improves the deformability of the dielectric layer, thus achieving high sensitivity (3.15 kPa⁻¹), wide response range (0–200 kPa), and high resolution (i.e., the minimum pressure detected was 27 Pa). The proposed sensing unit and its array have been demonstrated to be effective in large-area pressure sensing and object recognition. The flexible capacitive pressure sensor developed in this paper is highly promising in applications of robot skin and intelligent prosthetic hands. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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14 pages, 3055 KiB

Open AccessArticle

A Framework to Predict Gastric Cancer Based on Tongue Features and Deep Learning

by Xiaolong Zhu, Yuhang Ma, Dong Guo, Jiuzhang Men, Chenyang Xue, Xiyuan Cao and Zhidong Zhang

Micromachines 2023, 14(1), 53; https://doi.org/10.3390/mi14010053 - 25 Dec 2022

Cited by 6 | Viewed by 2264

Abstract

Gastric cancer has become a global health issue, severely disrupting daily life. Early detection in gastric cancer patients and immediate treatment contribute significantly to the protection of human health. However, routine gastric cancer examinations carry the risk of complications and are time-consuming. We [...] Read more.

Gastric cancer has become a global health issue, severely disrupting daily life. Early detection in gastric cancer patients and immediate treatment contribute significantly to the protection of human health. However, routine gastric cancer examinations carry the risk of complications and are time-consuming. We proposed a framework to predict gastric cancer non-invasively and conveniently. A total of 703 tongue images were acquired using a bespoke tongue image capture instrument, then a dataset containing subjects with and without gastric cancer was created. As the images acquired by this instrument contain non-tongue areas, the Deeplabv3+ network was applied for tongue segmentation to reduce the interference in feature extraction. Nine tongue features were extracted, relationships between tongue features and gastric cancer were explored by using statistical methods and deep learning, finally a prediction framework for gastric cancer was designed. The experimental results showed that the proposed framework had a strong detection ability, with an accuracy of 93.6%. The gastric cancer prediction framework created by combining statistical methods and deep learning proposes a scheme for exploring the relationships between gastric cancer and tongue features. This framework contributes to the effective early diagnosis of patients with gastric cancer. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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18 pages, 8336 KiB

Open AccessArticle

Textile One-Component Organic Electrochemical Sensor for Near-Body Applications

by Rike Brendgen, Carsten Graßmann, Sandra Gellner and Anne Schwarz-Pfeiffer

Micromachines 2022, 13(11), 1980; https://doi.org/10.3390/mi13111980 - 15 Nov 2022

Cited by 2 | Viewed by 1770

Abstract

The need for more efficient health services and the trend of a healthy lifestyle pushes the development of smart textiles. Since textiles have always been an object of everyday life, smart textiles promise an extensive user acceptance. Thereby, the manufacture of electrical components [...] Read more.

The need for more efficient health services and the trend of a healthy lifestyle pushes the development of smart textiles. Since textiles have always been an object of everyday life, smart textiles promise an extensive user acceptance. Thereby, the manufacture of electrical components based on textile materials is of great interest for applications as biosensors. Organic electrochemical transistors (OECTs) are often used as biosensors for the detection of saline content, adrenaline, glucose, etc., in diverse body fluids. Textile-based OECTs are mostly prepared by combining a liquid electrolyte solution with two separate electro-active yarns that must be precisely arranged in a textile structure. Herein, on the other hand, a biosensor based on a textile single-component organic electrochemical transistor with a hardened electrolyte was developed by common textile technologies such as impregnation and laminating. Its working principle was demonstrated by showing that the herein-produced transistor functions similarly to a switch or an amplifier and that it is able to detect ionic analytes of a saline solution. These findings support the idea of using this new device layout of textile-based OECTs as biosensors in near-body applications, though future work must be carried out to ensure reproducibility and selectivity, and to achieve an increased level of textile integration. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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14 pages, 5146 KiB

Open AccessArticle

A Flexible Thermocouple Film Sensor for Respiratory Monitoring

by Xiaodan Miao, Xiang Gao, Kaiming Su, Yahui Li and Zhuoqing Yang

Micromachines 2022, 13(11), 1873; https://doi.org/10.3390/mi13111873 - 31 Oct 2022

Cited by 4 | Viewed by 1803

Abstract

A novel flexible thermocouple film sensor on a polyimide substrate is proposed that is simple and flexible for monitoring the respiratory signal. Several thermocouples were connected in series and patterned on the polyimide substrate, and each one is formed by copper and a [...] Read more.

A novel flexible thermocouple film sensor on a polyimide substrate is proposed that is simple and flexible for monitoring the respiratory signal. Several thermocouples were connected in series and patterned on the polyimide substrate, and each one is formed by copper and a constant line connected to each other at two nodes. The respiratory signal was measured by the output voltage, which resulted from the temperature difference between the hot and cold junctions. The sensors were fabricated with surface-microfabrication technology with three sputtering steps. The measurement results showed that the peak voltage decreased by 90% in the case of apnea compared with normal breathing. The sensor has potential application for wearable detection of sleep apnea hypopnea syndrome (OSAHS). Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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27 pages, 17589 KiB

Open AccessArticle

A Study on Over-Molded Copper-Based Flexible Electronic Circuits

by Mona Bakr, Martin Hubmann, Frederick Bossuyt and Jan Vanfleteren

Micromachines 2022, 13(10), 1751; https://doi.org/10.3390/mi13101751 - 16 Oct 2022

Cited by 2 | Viewed by 1788

Abstract

Over-molding has been proposed in recent years as an integrated functional flexible circuit board in a plastic part. This method uses the conventional process for film insert technology. Over-molding has attracted significant attention across many industries due to its potential to deliver different [...] Read more.

Over-molding has been proposed in recent years as an integrated functional flexible circuit board in a plastic part. This method uses the conventional process for film insert technology. Over-molding has attracted significant attention across many industries due to its potential to deliver different electrical functions in a variety of different part geometries, especially in automotive interiors and home appliances. While it has great application potential, manufacturing challenges continue throughout foil fabrication and injection molding. This raises challenges for designers and researchers responsible for maintaining the reliability of such electronic flexible circuits. Therefore, the purpose of this research paper is to improve some of the over-molding process parameters. On 0805 and 1206 over-molded zero-ohm resistors, electrical, mechanical, and failure characterization was performed. Those components were mounted in parallel, perpendicular, and 45° angled arrangements on two different polymer substrates, polyimide (PI) and polyethylene terephthalate (PET) using lead-free solder, low-melt solder, and conductive adhesive paste. Moreover, as an over-molding material, polycarbonate (PC) with medium viscosity was used. The effect of using different mold shapes (corner mold, 2 mm flat mold, and 3 mm flat mold) and injection molding process parameters (injection speeds and melt temperature) was studied. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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18 pages, 3687 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Electromechanical Properties of 3D-Printed Stretchable Carbon Fiber Composites

by Teemu Salo, Donato Di Vito, Aki Halme and Jukka Vanhala

Micromachines 2022, 13(10), 1732; https://doi.org/10.3390/mi13101732 - 13 Oct 2022

Cited by 8 | Viewed by 2017

Abstract

The addition of fillers has been implemented in fused filament fabrication (FFF), and robust carbon fillers have been found to improve the mechanical, electrical, and thermal properties of 3D-printed matrices. However, in stretchable matrices, the use of fillers imposes significant challenges related to [...] Read more.

The addition of fillers has been implemented in fused filament fabrication (FFF), and robust carbon fillers have been found to improve the mechanical, electrical, and thermal properties of 3D-printed matrices. However, in stretchable matrices, the use of fillers imposes significant challenges related to quality and durability. In this work, we show that long carbon staple fibers in the form of permeable carbon fiber cloth (CFC) can be placed into a stretchable thermoplastic polyurethane (TPU) matrix to improve the system. Four CFC sample series (nominally 53–159-µm-thick CFC layers) were prepared with a permeable and compliant thin CFC layer and a highly conductive and stiff thick CFC layer. The sample series was tested with single pull-up tests and cyclic tensile tests with 10,000 cycles and was further studied with digital image correlation (DIC) analyses. The results showed that embedded CFC layers in a TPU matrix can be used for stretchable 3D-printed electronics structures. Samples with a thin 53 µm CFC layer retained electrical properties at 50% cyclic tensile deformations, whereas the samples with a thick >150-µm CFC layer exhibited the lowest resistance (5 Ω/10 mm). Between those structures, the 106-µm-thick CFC layer exhibited balanced electromechanical properties, with resistance changes of 0.5% in the cyclic tests after the orientation of the samples. Furthermore, the suitability of the structure as a sensor was estimated. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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8 pages, 3098 KiB

Open AccessArticle

Refractive Index Sensor Based on the Fano Resonance in Metal–Insulator–Metal Waveguides Coupled with a Whistle-Shaped Cavity

by Bo Li, Huarong Sun, Huinan Zhang, Yuetang Li, Junbin Zang, Xiyuan Cao, Xupeng Zhu, Xiaolong Zhao and Zhidong Zhang

Micromachines 2022, 13(10), 1592; https://doi.org/10.3390/mi13101592 - 25 Sep 2022

Cited by 9 | Viewed by 1407

Abstract

A plasmonic refractive index sensor based on surface plasmon polaritons (SPPs) that consist of metal–insulator–metal (MIM) waveguides and a whistle-shaped cavity is proposed. The transmission properties were simulated numerically by using the finite element method. The Fano resonance phenomenon can be observed in [...] Read more.

A plasmonic refractive index sensor based on surface plasmon polaritons (SPPs) that consist of metal–insulator–metal (MIM) waveguides and a whistle-shaped cavity is proposed. The transmission properties were simulated numerically by using the finite element method. The Fano resonance phenomenon can be observed in their transmission spectra, which is due to the coupling of SPPs between the transmission along the clockwise and anticlockwise directions. The refractive index-sensing properties based on the Fano resonance were investigated by changing the refractive index of the insulator of the MIM waveguide. Modulation of the structural parameters on the Fano resonance and the optics transmission properties of the coupled structure of two MIM waveguides with a whistle-shaped cavity were designed and evaluated. The results of this study will help in the design of new photonic devices and micro-sensors with high sensitivity, and can serve as a guide for future application of this structure. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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11 pages, 33982 KiB

Open AccessArticle

Additive-Manufactured Platinum Thin-Film Strain Gauges for Structural Microstrain Testing at Elevated Temperatures

by Xiaochuan Pan, Fan Lin, Chao Wu, Yingjun Zeng, Guochun Chen, Qinnan Chen, Daoheng Sun and Zhenyin Hai

Micromachines 2022, 13(9), 1472; https://doi.org/10.3390/mi13091472 - 05 Sep 2022

Cited by 9 | Viewed by 2339

Abstract

This paper investigates the feasibility and performance of the fabrication of platinum high-temperature thin-film strain sensors on nickel-based alloy substrates by additive manufacturing. The insulating layer was made of a dielectric paste by screen printing process. A 1.8-micron-thick platinum film was deposited directly [...] Read more.

This paper investigates the feasibility and performance of the fabrication of platinum high-temperature thin-film strain sensors on nickel-based alloy substrates by additive manufacturing. The insulating layer was made of a dielectric paste by screen printing process. A 1.8-micron-thick platinum film was deposited directly on the insulating layer. The four-wire resistance measurement method was used to eliminate the contact resistance of the solder joints. Comprehensive morphological and electrical characterization of the platinum thin-film strain gauge was carried out, and good static and dynamic strain responses were obtained, which confirmed that the strain gauge was suitable for in situ strain monitoring of high-temperature complex components. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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10 pages, 3963 KiB

Open AccessArticle

ZrB₂/SiCN Thin-Film Strain Gauges for In-Situ Strain Detection of Hot Components

by Fan Lin, Xiaochuan Pan, Chao Wu, Yingjun Zeng, Guochun Chen, Qinnan Chen, Daoheng Sun and Zhenyin Hai

Micromachines 2022, 13(9), 1467; https://doi.org/10.3390/mi13091467 - 04 Sep 2022

Cited by 6 | Viewed by 2302

Abstract

The in-situ strain/stress detection of hot components in harsh environments remains a challenging task. In this study, ZrB₂/SiCN thin-film strain gauges were fabricated on alumina substrates by direct writing. The effects of ZrB₂ content on the electrical conductivity and strain [...] Read more.

The in-situ strain/stress detection of hot components in harsh environments remains a challenging task. In this study, ZrB₂/SiCN thin-film strain gauges were fabricated on alumina substrates by direct writing. The effects of ZrB₂ content on the electrical conductivity and strain sensitivity of ZrB₂/SiCN composites were investigated, and based on these, thin film strain gauges with high electrical conductivity (1.71 S/cm) and a gauge factor of 4.8 were prepared. ZrB₂/SiCN thin-film strain gauges exhibit excellent static, cyclic strain responses and resistance stability at room temperature. In order to verify the high temperature performance of the ZrB₂/SiCN thin-film strain gauges, the temperature-resistance characteristic curves test, high temperature resistance stability test and cyclic strain test were conducted from 25 °C to 600 °C. ZrB₂/SiCN thin-film strain gauges exhibit good resistance repeatability and stability, and highly sensitive strain response, from 25 °C to 600 °C. Therefore, ZrB₂/SiCN thin-film strain gauges provide an effective approach for the measurement of in-situ strain of hot components in harsh environments. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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9 pages, 3554 KiB

Open AccessArticle

A SiCN Thin Film Thermistor Based on DVB Modified Polymer-Derived Ceramics

by Chao Wu, Fan Lin, Xiaochuan Pan, Yingjun Zeng, Guochun Chen, Lida Xu, Yingping He, Daoheng Sun and Zhenyin Hai

Micromachines 2022, 13(9), 1463; https://doi.org/10.3390/mi13091463 - 03 Sep 2022

Cited by 2 | Viewed by 1693

Abstract

Carbon-rich SiCN ceramics were prepared by divinylbenzene (DVB)-modified polysilazane (PSN2), and a high-conductivity SiCN thin film sensor suitable for medium-low temperature sensing was fabricated. The modified liquid precursors were patterned by direct ink writing to produce SiCN resistive grids with line widths of [...] Read more.

Carbon-rich SiCN ceramics were prepared by divinylbenzene (DVB)-modified polysilazane (PSN2), and a high-conductivity SiCN thin film sensor suitable for medium-low temperature sensing was fabricated. The modified liquid precursors were patterned by direct ink writing to produce SiCN resistive grids with line widths of several hundreds of micrometers and thicknesses of several micrometers. The introduction of DVB not only increases the critical thickness of SiCN ceramics several times, but also significantly improves the conductivity of SiCN, making it meet the conductivity requirements of sensing applications in the mid-low temperature range. The electrical conductivity and microstructure of DVB-modified SiCN ceramics were studied in detail. In the temperature range of 30~400 °C, the temperature resistance performance of DVB modified SiCN resistance grid was measured. The SiCN ceramics with low DVB content not only have excellent electrical conductivity, but also have good oxidation resistance. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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11 pages, 5536 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

On-Skin Flexible Pressure Sensor with High Sensitivity for Portable Pulse Monitoring

by Weihao Zheng, Hongcheng Xu, Meng Wang, Qikai Duan, Yangbo Yuan, Weidong Wang and Libo Gao

Micromachines 2022, 13(9), 1390; https://doi.org/10.3390/mi13091390 - 25 Aug 2022

Cited by 11 | Viewed by 2479

Abstract

Radial artery pulse pressure contains abundant cardiovascular physiological and pathological information, which plays an important role in clinical diagnosis of traditional Chinese medical science. However, many photoelectric sensors and pressure sensors will lose a large number of waveform features in monitoring pulse, which [...] Read more.

Radial artery pulse pressure contains abundant cardiovascular physiological and pathological information, which plays an important role in clinical diagnosis of traditional Chinese medical science. However, many photoelectric sensors and pressure sensors will lose a large number of waveform features in monitoring pulse, which will make it difficult for doctors to precisely evaluate the patients’ health. In this letter, we proposed an on-skin flexible pressure sensor for monitoring radial artery pulse. The sensor consists of the MXene (Ti₃C₂T_x)-coated nonwoven fabrics (n-WFs) sensitive layer and laser-engraved interdigital copper electrodes. Benefiting from substantially increased conductive paths between fibers and electrodes during normal compression, the sensor obtains high sensitivity (3.187 kPa⁻¹), fast response time (15 ms), low detection limit (11.1 Pa), and long-term durability (20,000 cycles). Furthermore, a flexible processing circuit was connected with the sensor mounted on wrist radial artery, achieving wirelessly precise monitoring of the pulse on smart phones in real time. Compared with the commercial flexible pressure sensor, our sensor successfully captures weak systolic peak precisely, showing its great clinical potential and commercial value. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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14 pages, 4098 KiB

Open AccessArticle

Ramie Fabric Treated with Carboxymethylcellulose and Laser Engraved for Strain and Humidity Sensing

by Shangxuan Shi, Jiao Liang, Chenkai Qu, Shangbi Chen and Bin Sheng

Micromachines 2022, 13(8), 1309; https://doi.org/10.3390/mi13081309 - 13 Aug 2022

Cited by 10 | Viewed by 2093

Abstract

Wearable fabric sensors have attracted enormous attention due to their huge potential in human health and activity monitoring, human–machine interaction and the Internet of Things (IoT). Among natural fabrics, bast fabric has the advantage of high strength, good resilience and excellent permeability. Laser [...] Read more.

Wearable fabric sensors have attracted enormous attention due to their huge potential in human health and activity monitoring, human–machine interaction and the Internet of Things (IoT). Among natural fabrics, bast fabric has the advantage of high strength, good resilience and excellent permeability. Laser engraving, as a high throughput, patternable and mask-free method, was demonstrated to fabricate fabric sensors. In this work, we developed a simplified, cost-effective and environmentally friendly method for engraving ramie fabric (a kind of bast fabric) directly by laser under an ambient atmosphere to prepare strain and humidity sensors. We used carboxymethylcellulose (CMC) to pretreat ramie fabric before laser engraving and gained laser-carbonized ramie fabrics (LCRF) with high conductivity (65 Ω sq⁻¹) and good permeability. The strain and humidity sensors had high sensitivity and good flexibility, which can be used for human health and activity monitoring. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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14 pages, 3754 KiB

Open AccessArticle

A Direct-Reading MEMS Conductivity Sensor with a Parallel-Symmetric Four-Electrode Configuration

by Zhiwei Liao, Junmin Jing, Rui Gao, Yuzhen Guo, Bin Yao, Huiyu Zhang, Zhou Zhao, Wenjun Zhang, Yonghua Wang, Zengxing Zhang and Chenyang Xue

Micromachines 2022, 13(7), 1153; https://doi.org/10.3390/mi13071153 - 21 Jul 2022

Cited by 6 | Viewed by 2010

Abstract

This work proposes a design for a direct-reading conductivity sensor with a parallel symmetrical four-electrode structure, which integrates a silicon-based platinum thin-film strip electrode and a serpentine temperature compensation electrode. The optimal structural parameters of the electrode were determined by finite element simulations [...] Read more.

This work proposes a design for a direct-reading conductivity sensor with a parallel symmetrical four-electrode structure, which integrates a silicon-based platinum thin-film strip electrode and a serpentine temperature compensation electrode. The optimal structural parameters of the electrode were determined by finite element simulations performed via COMSOL Multiphysics. Next, the designed conductivity sensor chip was fabricated using MEMS technology, and subsequently, the conductivity measurement circuit was designed to test the fabricated sensor’s performance. In laboratory tests, the optimal AC excitation frequency was observed to be 1.067 kHz, while the maximum measurement range was 0–107.41 mS/cm and the measurement precision in low concentration range (0–76.422 mS/cm) was ±0.1 mS/cm. Furthermore, the maximum measurement error of the sensor evaluated using the National Center of Ocean Standards and Metrology was ±0.073 mS/cm. The designed sensor possesses the characteristics of high accuracy, high range, and miniaturization, and enables real-time reading of conductivity value and temperature compensation, which is of great significance for the on-site observation of the physical parameters of marine environment. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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12 pages, 3481 KiB

Open AccessArticle

High Sensitivity and Wide Range Biomimetic Tactile-Pressure Sensor Based on 2D Graphene Film and 3D Graphene Foam

by Baolin Sha, Xiaozhou Lü and La Jiang

Micromachines 2022, 13(7), 1150; https://doi.org/10.3390/mi13071150 - 21 Jul 2022

Cited by 4 | Viewed by 1785

Abstract

Bionic electronic skin is a system that simulates human skin and has multiple perceptions. For pressure sensors, high measurement accuracy and wide measurement range restrict each other, and it is difficult to achieve high measurement accuracy and wide measurement range simultaneously. Therefore, the [...] Read more.

Bionic electronic skin is a system that simulates human skin and has multiple perceptions. For pressure sensors, high measurement accuracy and wide measurement range restrict each other, and it is difficult to achieve high measurement accuracy and wide measurement range simultaneously. Therefore, the research and application of bionic tactile-pressure sensors are limited due to the mutual constraints of measurement accuracy and range. In this work, a flexible graphene piezoresistive tactile sensor based on a biomimetic structure that utilizes the piezoresistive properties of graphene was reported. The novel tactile-pressure sensor consists of a 2D graphene film tactile sensor and a 3D graphene foam pressure sensor that could achieve high accuracy and a wide-range measurement simultaneously. The testing results show that the measurement range of this sensor was in two intervals of 0–2 N and 2–40 N. For the 0–2 N measurement range, the sensitivity was 472.2 Ω/kPa, the force resolution was 0.01 N, and the response time was less than 40 ms. For the 2–40 N measurement range, the sensitivity was 5.05 kΩ/kPa, the force resolution was 1 N, and the response time was less than 20 ms. The new sensor can realize high-precision and large-scale force measurements and shows great application value in the field of medical instruments and artificial limbs. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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15 pages, 6849 KiB

Open AccessArticle

Design, Fabrication, and Dynamic Environmental Test of a Piezoresistive Pressure Sensor

by Rui Gao, Wenjun Zhang, Junmin Jing, Zhiwei Liao, Zhou Zhao, Bin Yao, Huiyu Zhang, Yuzhen Guo, Yanbo Xu, Yonghua Wang, Zengxing Zhang, Zhidong Zhang and Chenyang Xue

Micromachines 2022, 13(7), 1142; https://doi.org/10.3390/mi13071142 - 19 Jul 2022

Cited by 6 | Viewed by 2391

Abstract

Microelectromechanical system (MEMS) pressure sensors have a wide range of applications based on the advantages of mature technology and easy integration. Among them, piezoresistive sensors have attracted great attention with the advantage of simple back-end processing circuits. However, less research has been reported [...] Read more.

Microelectromechanical system (MEMS) pressure sensors have a wide range of applications based on the advantages of mature technology and easy integration. Among them, piezoresistive sensors have attracted great attention with the advantage of simple back-end processing circuits. However, less research has been reported on the performance of piezoresistive pressure sensors in dynamic environments, especially considering the vibrations and shocks frequently encountered during the application of the sensors. To address these issues, this paper proposes a design method for a MEMS piezoresistive pressure sensor, and the fabricated sensor is evaluated in a series of systematic dynamic environmental adaptability tests. After testing, the output sensitivity of the sensor chip was 9.21 mV∙bar⁻¹, while the nonlinearity was 0.069% FSS. The sensor overreacts to rapidly changing pressure environments and can withstand acceleration shocks of up to 20× g. In addition, the sensor is capable of providing normal output over the vibration frequency range of 0–5000 Hz with a temperature coefficient sensitivity of −0.30% FSS °C⁻¹ over the temperature range of 0–80 °C. Our proposed sensor can play a key role in applications with wide pressure ranges, high-frequency vibrations, and high acceleration shocks, as well as guide MEMS-based pressure sensors in high pressure ranges and complex environmental adaptability in their design. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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10 pages, 5618 KiB

Open AccessArticle

Sound Source Localization Based on Multi-Channel Cross-Correlation Weighted Beamforming

by Mengran Liu, Junhao Hu, Qiang Zeng, Zeming Jian and Lei Nie

Micromachines 2022, 13(7), 1010; https://doi.org/10.3390/mi13071010 - 26 Jun 2022

Cited by 5 | Viewed by 1716

Abstract

Beamforming and its applications in steered-response power (SRP) technology, such as steered-response power delay and sum (SRP-DAS) and steered-response power phase transform (SRP-PHAT), are widely used in sound source localization. However, their resolution and accuracy still need improvement. A novel beamforming method combining [...] Read more.

Beamforming and its applications in steered-response power (SRP) technology, such as steered-response power delay and sum (SRP-DAS) and steered-response power phase transform (SRP-PHAT), are widely used in sound source localization. However, their resolution and accuracy still need improvement. A novel beamforming method combining SRP and multi-channel cross-correlation coefficient (MCCC), SRP-MCCC, is proposed in this paper to improve the accuracy of direction of arrival (DOA). Directional weight (DW) is obtained by calculating the MCCC. Based on DW, suppressed the non-incoming wave direction and gained the incoming wave direction to improve the beamforming capabilities. Then, sound source localizations based on the dual linear array under different conditions were simulated. Compared with SRP-PHAT, SRP-MCCC has the advantages of high positioning accuracy, strong spatial directivity and robustness under the different signal–noise ratios (SNRs). When the SNR is −10 dB, the average positioning error of the single-frequency sound source at different coordinates decreases by 5.69%, and that of the mixed frequency sound sources at the same coordinate decreases by 5.77%. Finally, the experimental verification was carried out. The results show that the average error of SRP-MCCC has been reduced by 8.14% and the positioning accuracy has been significantly improved, which is consistent with the simulation results. This research provides a new idea for further engineering applications of sound source localization based on beamforming. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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16 pages, 3017 KiB

Open AccessArticle

Enhancing Part-to-Part Repeatability of Force-Sensing Resistors Using a Lean Six Sigma Approach

by Andrés O. Garzón-Posada, Leonel Paredes-Madrid, Angela Peña, Victor M. Fontalvo and Carlos Palacio

Micromachines 2022, 13(6), 840; https://doi.org/10.3390/mi13060840 - 27 May 2022

Cited by 2 | Viewed by 2004

Abstract

Polymer nanocomposites have found wide acceptance in research applications as pressure sensors under the designation of force-sensing resistors (FSRs). However, given the random dispersion of conductive nanoparticles in the polymer matrix, the sensitivity of FSRs notably differs from one specimen to another; this [...] Read more.

Polymer nanocomposites have found wide acceptance in research applications as pressure sensors under the designation of force-sensing resistors (FSRs). However, given the random dispersion of conductive nanoparticles in the polymer matrix, the sensitivity of FSRs notably differs from one specimen to another; this condition has precluded the use of FSRs in industrial applications that require large part-to-part repeatability. Six Sigma methodology provides a standard framework to reduce the process variability regarding a critical variable. The Six Sigma core is the DMAIC cycle (Define, Measure, Analyze, Improve, and Control). In this study, we have deployed the DMAIC cycle to reduce the process variability of sensor sensitivity, where sensitivity was defined by the rate of change in the output voltage in response to the applied force. It was found that sensor sensitivity could be trimmed by changing their input (driving) voltage. The whole process comprised: characterization of FSR sensitivity, followed by physical modeling that let us identify the underlying physics of FSR variability, and ultimately, a mechanism to reduce it; this process let us enhance the sensors’ part-to-part repeatability from an industrial standpoint. Two mechanisms were explored to reduce the variability in FSR sensitivity. (i) It was found that the output voltage at null force can be used to discard noncompliant sensors that exhibit either too high or too low sensitivity; this observation is a novel contribution from this research. (ii) An alternative method was also proposed and validated that let us trim the sensitivity of FSRs by means of changing the input voltage. This study was carried out from 64 specimens of Interlink FSR402 sensors. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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11 pages, 3810 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

High-Performance Flexible Piezoresistive Sensor Based on Ti₃C₂T_x MXene with a Honeycomb-like Structure for Human Activity Monitoring

by Yue Su, Kainan Ma, Fang Yuan, Jun Tang, Ming Liu and Xu Zhang

Micromachines 2022, 13(6), 821; https://doi.org/10.3390/mi13060821 - 25 May 2022

Cited by 9 | Viewed by 2274

Abstract

Wearable and flexible pressure sensors have sparked great interest due to their unique capacity to conformally attach to the surface of the skin and quantify human activities into recordable electric signals. As a result, more and more research efforts are being devoted to [...] Read more.

Wearable and flexible pressure sensors have sparked great interest due to their unique capacity to conformally attach to the surface of the skin and quantify human activities into recordable electric signals. As a result, more and more research efforts are being devoted to developing high-sensitivity and cost-effective flexible sensors for monitoring an individual’s state of activity. Herein, a high-performance flexible piezoresistive sensor was designed and fabricated by combing 2D transition metal carbides, nitrides, and carbonitrides (MXene) with a honeycomb-like structure formed by femtosecond filamentating pulses. The sensing mechanism is attributed to the change of the connecting conductive paths between the top interdigital electrodes and the bottom microstructured films coated with MXene. The obtained sensing device demonstrates high sensitivity of 0.61 kPa⁻¹, relatively short response time, and excellent reliability and stability. Benefiting from the aforementioned extraordinary sensing performance, the sensor can be used with success to monitor tiny physiological signals, detect large deformations during human movement, and distinguish finger gestures, thus demonstrating its broad prospects in physiological analysis systems, health monitoring systems, and human–machine interaction. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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11 pages, 5197 KiB

Open AccessArticle

Monolayer MoS₂-Based Flexible and Highly Sensitive Pressure Sensor with Wide Sensing Range

by Dandan Xu, Ling Duan, Suyun Yan, Yong Wang, Ke Cao, Weidong Wang, Hongcheng Xu, Yuejiao Wang, Liangwei Hu and Libo Gao

Micromachines 2022, 13(5), 660; https://doi.org/10.3390/mi13050660 - 22 Apr 2022

Cited by 9 | Viewed by 2767

Abstract

Flexible pressure sensors play an important role in flexible robotics, human-machine interaction (HMI), and human physiological information. However, most of the reported flexible pressure sensors suffer from a highly nonlinear response and a significant decrease in sensitivity at high pressures. Herein, we propose [...] Read more.

Flexible pressure sensors play an important role in flexible robotics, human-machine interaction (HMI), and human physiological information. However, most of the reported flexible pressure sensors suffer from a highly nonlinear response and a significant decrease in sensitivity at high pressures. Herein, we propose a flexible novel iontronic pressure sensor based on monolayer molybdenum disulfide (MoS₂). Based on the unique structure and the excellent mechanical properties as well as the large intercalation capacitance of MoS₂, the prepared sensor holds an ultra-high sensitivity (S_max = 89.75 kPa⁻¹) and a wide sensing range (722.2 kPa). Further, the response time and relaxation time of the flexible sensor are only 3 ms, respectively, indicating that the device can respond to external pressure rapidly. In addition, it shows long-term cycling stability (over 5000 cycles with almost no degradation) at a high pressure of 138.9 kPa. Finally, it is demonstrated that the sensor can be used in physiological information monitoring and flexible robotics. It is anticipated that our prepared sensor provide a reliable approach to advance the theory and practicality of the flexible sensor electronics. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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12 pages, 3105 KiB

Open AccessArticle

Automatic Classification Framework of Tongue Feature Based on Convolutional Neural Networks

by Jiawei Li, Zhidong Zhang, Xiaolong Zhu, Yunlong Zhao, Yuhang Ma, Junbin Zang, Bo Li, Xiyuan Cao and Chenyang Xue

Micromachines 2022, 13(4), 501; https://doi.org/10.3390/mi13040501 - 24 Mar 2022

Cited by 18 | Viewed by 2637

Abstract

Tongue diagnosis is an important part of the diagnostic process in traditional Chinese medicine (TCM). It primarily relies on the expertise and experience of TCM practitioners in identifying tongue features, which are subjective and unstable. We proposed a tongue feature classification framework based [...] Read more.

Tongue diagnosis is an important part of the diagnostic process in traditional Chinese medicine (TCM). It primarily relies on the expertise and experience of TCM practitioners in identifying tongue features, which are subjective and unstable. We proposed a tongue feature classification framework based on convolutional neural networks to reduce the differences in diagnoses among TCM practitioners. Initially, we used our self-designed instrument to capture 482 tongue photos and created 11 data sets based on different features. Then, the tongue segmentation task was completed using an upgraded facial landmark detection method and UNET. Finally, we used ResNet34 as the backbone to extract features from the tongue photos and classify them. Experimental results show that our framework has excellent results with an overall accuracy of over 86 percent and is particularly sensitive to the corresponding feature regions, and thus it could assist TCM practitioners in making more accurate diagnoses. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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Review

Jump to: Editorial, Research

26 pages, 7734 KiB

Open AccessReview

Review of Flexible Wearable Sensor Devices for Biomedical Application

by Xueli Nan, Xin Wang, Tongtong Kang, Jiale Zhang, Lanxiao Dong, Jinfeng Dong, Peng Xia and Donglai Wei

Micromachines 2022, 13(9), 1395; https://doi.org/10.3390/mi13091395 - 26 Aug 2022

Cited by 19 | Viewed by 6320

Abstract

With the development of cross-fertilisation in various disciplines, flexible wearable sensing technologies have emerged, bringing together many disciplines, such as biomedicine, materials science, control science, and communication technology. Over the past few years, the development of multiple types of flexible wearable devices that [...] Read more.

With the development of cross-fertilisation in various disciplines, flexible wearable sensing technologies have emerged, bringing together many disciplines, such as biomedicine, materials science, control science, and communication technology. Over the past few years, the development of multiple types of flexible wearable devices that are widely used for the detection of human physiological signals has proven that flexible wearable devices have strong biocompatibility and a great potential for further development. These include electronic skin patches, soft robots, bio-batteries, and personalised medical devices. In this review, we present an updated overview of emerging flexible wearable sensor devices for biomedical applications and a comprehensive summary of the research progress and potential of flexible sensors. First, we describe the selection and fabrication of flexible materials and their excellent electrochemical properties. We evaluate the mechanisms by which these sensor devices work, and then we categorise and compare the unique advantages of a variety of sensor devices from the perspective of in vitro and in vivo sensing, as well as some exciting applications in the human body. Finally, we summarise the opportunities and challenges in the field of flexible wearable devices. Full article

(This article belongs to the Special Issue Flexible and Wearable Sensors)

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