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Stimuli-Responsive Flexible Sensors
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Stimuli-Responsive Flexible Sensors

A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Electronic Sensors".

Deadline for manuscript submissions: closed (20 November 2022) | Viewed by 19013

Special Issue Editor

Dr. Sungryul Yun

E-Mail Website
Guest Editor
Human Enhancement & Assistive Technology Research Section, Electronics and Telecommunications Research Institute (ETRI), 218 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
Interests: electroactive polymers; active nanocomposite; stretchable electronics; biocompatible polymers; wearable electronic systems; haptic interfaces; soft robotics; smart optics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Over the last decade, in compliance with the rapid increase in worldwide demands for wearable technologies that improve the quality of human life, the thin-film sensors with flexible form factor have been receiving great attention as one of the key elements for human-friendly electronic systems. Recently, due to revolutionary advances in sensing material, fabrication technique, and structural design, state-of-the-art flexible sensors that enable highly sensitive and real-time detection of various stimuli, including strain, pressure, biological molecules, gas, ions, pH, light, and temperature, have been rapidly developed. Their stable performance on curvilinear or even free-formed interfaces increases the feasibility of the electronic systems to be used for human health monitoring, motion detection, medical diagnosis, sensory capability enhancement, human–robot interaction, environmental detection, food freshness detection, etc. However, for practical use, there are still challenges related to biocompatibility, highly selective detection, mechanical robustness, long-term integration, and versatility with qualified performance in miniaturized design, as well as environmental effects.

This Special Issue of Sensors (MDPI) is focused on all aspects of research relevant to flexible sensors for the detection of physical/chemical/biological stimuli. This issue invites research and review papers that report novel contributions with respect to sensing materials, fabrication methodologies, structural designs, sensing performance, advanced wearable sensing systems, and innovative applications for flexible sensors.

Dr. Sungryul Yun
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Sensors is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Flexible physical sensors 
  • Flexible biosensors 
  • Flexible electrochemical sensors 
  • Inorganic/organic sensing materials 
  • Stretchable skin electronics 
  • Biocompatible functional materials 
  • Surface engineering 
  • Human health monitoring 
  • Human–robot interface 
  • Motion detection 
  • Food freshness detection 
  • Environmental detection 
  • Medical diagnosis 
  • Sensory capability enhancement 
  • Wearable multifunctional sensing systems

Published Papers (7 papers)

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Research

12 pages, 1463 KiB  
Article
Inflammatory Stimuli Responsive Non-Faradaic, Ultrasensitive Combinatorial Electrochemical Urine Biosensor
by Antra Ganguly, Varun Gunda, Kevin Thai and Shalini Prasad
Sensors 2022, 22(20), 7757; https://doi.org/10.3390/s22207757 - 13 Oct 2022
Cited by 2 | Viewed by 1650
Abstract
In this work, we propose a novel diagnostic biosensor that can enable stratification of disease states based on severity and hence allow for clear and actionable diagnoses. The scheme can potentially boost current Point-Of-Care (POC) biosensors for diseases that require time-critical stratification. Here, [...] Read more.
In this work, we propose a novel diagnostic biosensor that can enable stratification of disease states based on severity and hence allow for clear and actionable diagnoses. The scheme can potentially boost current Point-Of-Care (POC) biosensors for diseases that require time-critical stratification. Here, two key inflammatory biomarkers—Interleukin-8 and Interleukin-6—have been explored as proof of concept, and a four-class stratification of inflammatory disease severity is discussed. Our method is superior to traditional lab techniques as it is faster (<4 minutes turn-around time) and can work with any combination of disease biomarkers to categorize diseases by subtypes and severity. At its core, the biosensor relies on electrochemical impedance spectroscopy to transduce subtle inflammatory stimuli at the input for IL-8 and IL-6 for a limit of detection (LOD) of 1 pg/mL each. The biosensing scheme utilizes a two-stage random forest machine learning model for 4-state output disease classification with a 98.437% accuracy. This scheme can potentially boost the diagnostic power of current electrochemical biosensors for better precision therapy and improved patient outcomes. Full article
(This article belongs to the Special Issue Stimuli-Responsive Flexible Sensors)
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12 pages, 4863 KiB  
Article
Cellulose Nanofiber Films and Their Vibration Energy Harvesting
by Seok-Hyun Lee and Jaehwan Kim
Sensors 2022, 22(16), 6280; https://doi.org/10.3390/s22166280 - 21 Aug 2022
Cited by 1 | Viewed by 1843
Abstract
Cellulose, the most abundant sustainable material on Earth, has excellent mechanical and physical properties, high optical transparency, biocompatibility, and piezoelectricity. So, it has many possibilities for future materials, and many researchers are interested in its application. In this paper, cellulose nanofiber (CNF) and [...] Read more.
Cellulose, the most abundant sustainable material on Earth, has excellent mechanical and physical properties, high optical transparency, biocompatibility, and piezoelectricity. So, it has many possibilities for future materials, and many researchers are interested in its application. In this paper, cellulose nanofiber (CNF) and CNF/polyvinyl alcohol (PVA) films are made, and their vibration energy harvesting is studied. CNF was isolated by chemical and physical methods, and the CNF suspension was cast on a flat substrate to make a film. A cast CNF wet film stayed in a 5 Tesla superconductor magnet for 7 days, which resulted in CNF alignment perpendicular to the magnetic field. To further improve the mechanical properties of the CNF film, mechanical stretching was applied. The CNF suspension was mixed with PVA, giving the film toughness. The cast CNF/PVA wet film was mechanically stretched and dried, which improved the CNF alignment. The fabricated CNF and CNF/PVA films were characterized using scanning electron microscopy and X-ray diffraction to verify the alignment. By stretching, the aligned CNF/PVA film exhibits the largest mechanical properties along the aligned direction. The maximum Young’s modulus and tensile strength of the 50% stretched CNF/PVA film are 14.9 GPa and 170.6 MPa, respectively. Finally, a vibration energy harvesting experiment was performed by invoking the piezoelectric behavior of the pure CNF, and 50% stretched CNF/PVA films. The harvester structure was innovated by adopting a cymbal structure, which was beneficial to producing large in-plane strain on the films. The designed cymbal structure was analyzed using ANSYS, and its natural frequency was experimentally verified. The CNF/PVA film performs better vibration energy harvesting than the pure CNF film. The CNF/PVA film is applicable for biocompatible and flexible vibration energy harvesting. Full article
(This article belongs to the Special Issue Stimuli-Responsive Flexible Sensors)
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16 pages, 7852 KiB  
Article
Compact Four-Port Circularly Polarized MIMO X-Band DRA
by Ahmed A. Ibrahim, Hijab Zahra, Syed Muzahir Abbas, Mohamed I. Ahmed, Gaurav Varshney, Subhas Mukhopadhyay and Abdelhady Mahmoud
Sensors 2022, 22(12), 4461; https://doi.org/10.3390/s22124461 - 13 Jun 2022
Cited by 9 | Viewed by 1658
Abstract
A circularly polarized (CP) multi-input multioutput (MIMO) dielectric resonator (DR) antenna (DRA) with compact size and four ports is implemented. CP radiation was achieved using the deformed DR geometry excited with aperture coupled feeding. A CPDRA with a single and two ports is [...] Read more.
A circularly polarized (CP) multi-input multioutput (MIMO) dielectric resonator (DR) antenna (DRA) with compact size and four ports is implemented. CP radiation was achieved using the deformed DR geometry excited with aperture coupled feeding. A CPDRA with a single and two ports is investigated. The defected ground structure (DGS) was incorporated into the antenna for improving the isolation between the ports. The DGS was incorporated in such a way that the required phase difference between the generated orthogonal degenerate modes is preserved. This concept could be utilized in implementing a compact four-port CP antenna. The MIMO antenna provides a 10 dB impedance bandwidth of 38% (8.5–12.5 GHz) and a 3 dB AR bandwidth of 9.32% (9.2–10.1 GHz). The gain of the implemented antenna was around 6 dBi in the band where CP radiation was achieved. The MIMO performance parameters were calculated, and their values remained within the acceptable limits. The implemented antenna could suitably be used in X-band applications. Full article
(This article belongs to the Special Issue Stimuli-Responsive Flexible Sensors)
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11 pages, 2689 KiB  
Article
Whey Protein Isolate Film and Laser-Ablated Textured PDMS-Based Single-Electrode Triboelectric Nanogenerator for Pressure-Sensor Application
by Minwoo Lee, Jonghwan Shin, Sunkook Kim and Srinivas Gandla
Sensors 2022, 22(6), 2154; https://doi.org/10.3390/s22062154 - 10 Mar 2022
Cited by 4 | Viewed by 2335
Abstract
The use of biopolymers for realizing economical and eco-friendly triboelectric nanogenerators (TENGs) widens the application prospects of TENGs. Herein, an animal-sourced whey protein isolate (WPI) film, processed and prepared by a simple aqueous solution preparation and drop-casting technique, is applied to demonstrate its [...] Read more.
The use of biopolymers for realizing economical and eco-friendly triboelectric nanogenerators (TENGs) widens the application prospects of TENGs. Herein, an animal-sourced whey protein isolate (WPI) film, processed and prepared by a simple aqueous solution preparation and drop-casting technique, is applied to demonstrate its potential use in bio-TENGs. With the addition of formaldehyde in WPI, the films result in a free-standing and flexible film, whereas the pure WPI films are difficult to handle and lack flexibility. A TENG device based on the WPI and the laser-ablated textured polydimethylsiloxane (PDMS) for pressure-sensor application were developed. The output voltage of the TENG comprising WPI increased nearly two-fold compared to the TENG without WPI. A simple single-electrode TENG device configuration was adopted so that it could be easily integrated into a wearable electronic device. Moreover, WPI film exhibited tribo-negative-like material characteristics. This study provides new insights into the development of biocompatible and eco-friendly biopolymers for various electronic devices and sensors. Full article
(This article belongs to the Special Issue Stimuli-Responsive Flexible Sensors)
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12 pages, 4083 KiB  
Article
A Dielectric Elastomer-Based Multimodal Capacitive Sensor
by Yuting Zhu, Tim Giffney and Kean Aw
Sensors 2022, 22(2), 622; https://doi.org/10.3390/s22020622 - 14 Jan 2022
Cited by 11 | Viewed by 3075
Abstract
Dielectric elastomer (DE) sensors have been widely used in a wide variety of applications, such as in robotic hands, wearable sensors, rehabilitation devices, etc. A unique dielectric elastomer-based multimodal capacitive sensor has been developed to quantify the pressure and the location of any [...] Read more.
Dielectric elastomer (DE) sensors have been widely used in a wide variety of applications, such as in robotic hands, wearable sensors, rehabilitation devices, etc. A unique dielectric elastomer-based multimodal capacitive sensor has been developed to quantify the pressure and the location of any touch simultaneously. This multimodal sensor is a soft, flexible, and stretchable dielectric elastomer (DE) capacitive pressure mat that is composed of a multi-layer soft and stretchy DE sensor. The top layer measures the applied pressure, while the underlying sensor array enables location identification. The sensor is placed on a passive elastomeric substrate in order to increase deformation and optimize the sensor’s sensitivity. This DE multimodal capacitive sensor, with pressure and localization capability, paves the way for further development with potential applications in bio-mechatronics technology and other humanoid devices. The sensor design could be useful for robotic and other applications, such as fruit picking or as a bio-instrument for the diabetic insole. Full article
(This article belongs to the Special Issue Stimuli-Responsive Flexible Sensors)
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17 pages, 3718 KiB  
Article
Volitional EMG Estimation Method during Functional Electrical Stimulation by Dual-Channel Surface EMGs
by Joonyoung Jung, Dong-Woo Lee, Yong Ki Son, Bae Sun Kim and Hyung Cheol Shin
Sensors 2021, 21(23), 8015; https://doi.org/10.3390/s21238015 - 30 Nov 2021
Cited by 4 | Viewed by 2390
Abstract
We propose a novel dual-channel electromyography (EMG) spatio-temporal differential (DESTD) method that can estimate volitional electromyography (vEMG) signals during time-varying functional electrical stimulation (FES). The proposed method uses two pairs of EMG signals from the same stimulated muscle to calculate the spatio-temporal difference [...] Read more.
We propose a novel dual-channel electromyography (EMG) spatio-temporal differential (DESTD) method that can estimate volitional electromyography (vEMG) signals during time-varying functional electrical stimulation (FES). The proposed method uses two pairs of EMG signals from the same stimulated muscle to calculate the spatio-temporal difference between the signals. We performed an experimental study with five healthy participants to evaluate the vEMG signal estimation performance of the DESTD method and compare it with that of the conventional comb filter and Gram–Schmidt methods. The normalized root mean square error (NRMSE) values between the semi-simulated raw vEMG signal and vEMG signals which were estimated using the DESTD method and conventional methods, and the two-tailed t-test and analysis of variance were conducted. The results showed that under the stimulation of the gastrocnemius muscle with rapid and dynamically modulated stimulation intensity, the DESTD method had a lower NRMSE compared to the conventional methods (p < 0.01) for all stimulation intensities (maximum 5, 10, 15, and 20 mA). We demonstrated that the DESTD method could be applied to wearable EMG-controlled FES systems because it estimated vEMG signals more effectively compared to the conventional methods under dynamic FES conditions and removed unnecessary FES-induced EMG signals. Full article
(This article belongs to the Special Issue Stimuli-Responsive Flexible Sensors)
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11 pages, 1587 KiB  
Communication
Development of Flexible Ion-Selective Electrodes for Saliva Sodium Detection
by Hyo-Ryoung Lim, Soon Min Lee, Musa Mahmood, Shinjae Kwon, Yun-Soung Kim, Yongkuk Lee and Woon-Hong Yeo
Sensors 2021, 21(5), 1642; https://doi.org/10.3390/s21051642 - 26 Feb 2021
Cited by 21 | Viewed by 5099
Abstract
Saliva can be used for health monitoring with non-invasive wearable systems. Such devices, including electrochemical sensors, may provide a safe, fast, and cost-efficient way of detecting target ions. Although salivary ions are known to reflect those in blood, no available clinical device can [...] Read more.
Saliva can be used for health monitoring with non-invasive wearable systems. Such devices, including electrochemical sensors, may provide a safe, fast, and cost-efficient way of detecting target ions. Although salivary ions are known to reflect those in blood, no available clinical device can detect essential ions directly from saliva. Here, we introduce an all-solid-state, flexible film sensor that allows highly accurate detection of sodium levels in saliva, comparable to those in blood. The wireless film sensor system can successfully measure sodium ions from a small volume of infants’ saliva (<400 µL), demonstrating its potential as a continuous health monitor. This study includes the structural characterization and error analysis of a carbon/elastomer-based ion-selective electrode and a reference electrode to confirm the signal reliability. The sensor, composed of a pair of the electrodes, shows good sensitivity (58.9 mV/decade) and selectivity (log K = −2.68 for potassium), along with a broad detection range of 5 × 10−5 ≈ 1 M with a low detection limit of 4.27 × 10−5 M. The simultaneous comparison between the film sensor and a commercial electrochemical sensor demonstrates the accuracy of the flexible sensor and a positive correlation in saliva-to-blood sodium levels. Collectively, the presented study shows the potential of the wireless ion-selective sensor system for a non-invasive, early disease diagnosis with saliva. Full article
(This article belongs to the Special Issue Stimuli-Responsive Flexible Sensors)
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