SENSING MECHANISM AND APPLICATION OF MECHANICAL STRAIN SENSOR: A MINI-REVIEW

This study reviews the potential of flexible strain sensors based on nanomaterials such as carbon nanotubes (CNTs), graphene, and metal nanowires (NWs). These nanomaterials have excellent flexibility, conductivity, and mechanical properties, which enable them to be integrated into clothing or attached to the skin for the real-time monitoring of various activities. However, the main challenge is balancing high stretchability and sensitivity. This paper explains the basic concept of strain sensors that can convert mechanical deformation into electrical signals. Moreover, this paper focuses on simple, flexible, and stretchable resistive and capacitive sensors. It also discusses the important factors in choosing materials and fabrication methods, emphasizing the crucial role of suitable polymers in high-performance strain sensing. This study reviews the fabrication processes, mechanisms, performance, and applications of stretchable strain sensors in detail. It analyzes key aspects, such as sensitivity, stretchability, linearity, response time, and durability. This review provides useful insights into the current status and prospects of stretchable strain sensors in wearable technology and human–machine interfaces

Screen Printing of Surface-Modified Barium Titanate/Polyvinylidene Fluoride Nanocomposites for High-Performance Flexible Piezoelectric Nanogenerators

Piezoelectric energy harvesters are appealing for the improvement of wearable electronics, owing to their excellent mechanical and electrical properties. Herein, screen-printed piezoelectric nanogenerators (PENGs) are developed from triethoxy(octyl)silane-coated barium titanate/polyvinylidene fluoride (TOS-BTO/PVDF) nanocomposites with excellent performance based on the important link between material, structure, and performance. In order to minimize the effect of nanofiller agglomeration, TOS-coated BTO nanoparticles are anchored onto PVDF. Thus, composites with well-distributed TOS-BTO nanoparticles exhibit fewer defects, resulting in reduced charge annihilation during charge transfer from inorganic nanoparticles to the polymer. Consequently, the screen-printed TOS-BTO/PVDF PENG exhibits a significantly enhanced output voltage of 20 V, even after 7500 cycles, and a higher power density of 15.6 μW cm−2, which is 200 and 150% higher than those of pristine BTO/PVDF PENGs, respectively. The increased performance of TOS-BTO/PVDF PENGs is due to the enhanced compatibility between nanofillers and polymers and the resulting improvement in dielectric response. Furthermore, as-printed devices could actively adapt to human movements and displayed excellent detection capability. The screen-printed process offers excellent potential for developing flexible and high-performance piezoelectric devices in a cost-effective and sustainable way

PEDOT:PSS Overcoating Layer for Mechanically and Chemically Stable Ag Nanowire Flexible Transparent Electrode

We investigated the effect of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) deposition on the chemical and mechanical stability of Ag nanowire flexible electrodes. A large number of bending cycles, up to 500,000 cycles, were imposed on the Ag nanowire electrodes with and without PEDOT:PSS overcoating layer. In situ resistance measurement during bending tests revealed that the Ag nanowire electrode with PEDOT:PSS overcoating layer was mechanically reliable, showing a 21.9% increase in resistance after 500,000 cycles of bending. Scanning electron microscope images revealed that the failure of the Ag nanowire network occurred along with cracks initiated in the PEDOT:PSS layer, which resulted in the increase in resistance under bending. Furthermore, the PEDOT:PSS deposition enhanced the chemical stability of Ag nanowire electrode, which showed no significant increase in resistance after exposure in air for 50 days. Our study underscored that PEDOT:PSS is effective in protecting the Ag nanowires, while maintaining the high mechanical stability

A Screen-Printed Metal Hybrid Composite for Wireless Wind Sensing

Wind sensing has become a key component in various fields with the growing trend of assessing air conditions for energy conversion. In this study, we demonstrated a wireless screen-printable flexible strain sensor system based on Ag/MWCNT composite for wind sensing. To achieve high printability with the metal hybrid composite for the fabrication of a screen-printed flexible sensor, we systematically investigated the rheological properties, resulting in the high shear thinning and thixotropic behavior of the composite. After confirming the suitability for screen printing, we investigated the performance of the printed strain sensor, obtaining a gauge factor (G.F.) of 2.08 with 90% sensitivity and high durability after 6000 bending cycles. In addition, the sensor showed 98% temperature sensitivity during a wind sensing test due to the intrinsic properties of the metal hybrid composite. In an application based on an IoT system, we verified that the response of the wireless sensor corresponded with that of a wired sensor, indicating the expansion of low-cost, mass-produced screen-printed wind sensors

Printability of the Screen-Printed Strain Sensor with Carbon Black/Silver Paste for Sensitive Wearable Electronics

Printing technology enables not only high-volume, multipurpose, low-impact, low-cost manufacturing, but also the introduction of flexible electronic devices, such as displays, actuators, and sensors, to a wide range of consumer markets. Consequently, in the past few decades, printed electronic products have attracted considerable interest. Although flexible printed electronic products are attracting increasing attention from the scientific and industrial communities, a systematic study on their sensing performance based on printability has not been reported so far. In this study, carbon black/Ag nanocomposites were utilized as pastes for a flexible wearable strain sensor. The effects of the rheological property of the pastes and the pattern dimensions of the printed electrodes on the sensor’s performance were investigated. Consequently, the printed sensor demonstrated a high gauge factor of 444.5 for an applied strain of 0.6% to 1.4% with a durability of 1000 cycles and a linearity of R2 = 0.9974. The sensor was also stable under tough environmental conditions

Fabrication of Enzyme-Free and Rapid Electrochemical Detection of Glucose Sensor Based on ZnO Rod and Ru Doped Carbon Nitride Modified Gold Transducer

Over 3 in 4 adults with diabetes live in low- and middle-income counties and health expenditure also increased 316% over the last 15 years. In this regard, we fabricate low cost, reusable and rapid detection of diabetes sensor based on zinc oxide rod inserted ruthenium-doped carbon nitride (ZnO–g–Ru–C3N4) modified sensor device. Developed sensor device physically and electrochemically characterized using X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), chronoamperometry (CA) and differential pulse voltammetry (DPV). Sensing device as an effective enzyme-free glucose detection with high sensitivity (346 μA/mM/cm2) over the applied lower potential of +0.26 V (vs. Ag/AgCl), fast response (3 s) and broad linear range of (2–28) mM, coupled with a lower limit of detection (3.5 nM). The biosensing device gives better anti-interference ability with justifiable reproducibility, reusability (single electrode re-use 26 times in physiological buffer and 3 times in serum) and stability. Moreover, the real-time applicability of the sensor device was evaluated in human blood, serum and urine samples

Effect of a Urethane Acrylate-Based Photosensitive Coating on the Reliability of Ag Nanowire Transparent Electrodes

Due to the susceptibility of Ag nanowires to external mechanical and chemical damage, maintaining high optical performance and ambient and mechanical stability during the fabrication process is important for the industrial use of Ag nanowire transparent electrodes (TEs). In this study, urethane acrylate-based photosensitive resin (UAPR) is used as the coating material for Ag nanowire TEs to improve their optical transmittance, ambient stability, and resistance to external wiping damage. In the proposed method, UV-curable UAPR is coated onto Ag nanowire TEs using a simple doctor blade, forming a protective coating that increases the optical transmittance of the electrodes due to the refractive index of the UAPR between the air and the substrate. The UAPR coating successfully protects the Ag nanowires from corrosion in ambient air, with no significant change in their optical or electrical properties observed after 180 h of exposure to ambient air. Mechanical wiping tests also confirm that the UAPR coating is effective in protecting the Ag nanowires from external wiping damage, with no degradation of the optical or electrical properties observed after six wiping cycles

Rapid and Label-Free Detection of 5-Hydroxymethylcytosine in Genomic DNA Using an Au/ZnO Nanorods Hybrid Nanostructure-Based Electrochemical Sensor

Ten-eleven-translocation (TET) proteins modify DNA methylation by oxidizing 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). Loss of 5hmC, a widely accepted epigenetic hallmark of cancers, is proposed as a biomarker for early cancer diagnosis and prognosis. Thus, precise quantification of 5hmC holds great potential for diverse clinical applications. DNAs containing 5mC or 5hmC display different adsorption affinity toward the gold surface, thus producing different electrochemical responses. Here a novel, label-free electrochemical sensor based on gold nanoparticles (Au NPs)/zinc oxide nanorods (ZnO NRs) nanostructure for the facile and real-time detection of 5hmC-enriched DNAs is reported. The hybrid structure is fabricated by the vertical hydrothermal growth of ZnO NRs onto indium tin oxide glass substrate, followed by the decoration of ZnO NRs with Au NPs via sputtering. Successful fabrication is confirmed by analyzing the morphology and chemical composition of the sensor. By coupling the fabricated sensor with cyclic voltammetry, its functionality in distinguishing genomic DNAs containing different levels of 5hmC is validated. Notably, the sensor device successfully and consistently detects 5hmC loss in primary hepatocellular carcinoma, compared to the normal tissues. Thus, the novel sensing strategy to assess DNA hydroxymethylation will likely find broad applications in early cancer diagnosis and prognosis evaluation

Highly selective and real-time detection of 5-hydroxymethylcytosine in genomic DNA using a carbon nitride-modified gold transducer-based electrochemical sensor

Cancer biomarkers are crucial indicators of cancer status and progression that aid in early detection and more effective treatment of the disease. The loss of 5-hydroxymethylcytosine (5hmC), an oxidation product of 5-methylcytosine (5mC), is a recurrent epigenetic biomarker across various types of cancers. Therefore, accurately quantifying 5hmC holds great potential for various clinical applications. However, distinguishing 5hmC from 5mC using conventional methods is challenging. In this study, we developed a rapid and highly selective electrochemical sensor for label-free detection of 5hmC-enriched DNAs using a graphitic carbon nitride (g-C3N4)-modified gold transducer. Two-dimensional g-C3N4 sheets were synthesized via direct pyrolysis of urea under ambient or nitrogen atmospheres and drop-cast onto the gold electrode. Subsequently, 5hmC-containing DNAs were immobilized onto g-C3N4 via hydrogen bonding between the ???OH of 5hmC and the -NH2 of g-C3N4. The developed sensor demonstrated high sensitivity, selectivity, remarkable reproducibility, and stability, with a low oxidation potential (0.23 V) and an extremely low limit of detection (0.316 pM) for 5hmC. The sensor was also tested for its applicability to real samples using primary liver samples from mouse models, in which 5hmC levels were diminished due to either Tet gene knockout or hepatocellular carcinogenesis. The sensor effectively detected reduced genomic 5hmCs in TET-deficient livers and hepatocellular carcinomas compared to controls. Thus, this novel sensing strategy has the potential to develop clinically applicable sensors for early cancer diagnosis and prognosis evaluation by rapidly quantifying genomic 5hmC

A fast and label-free detection of hydroxymethylated DNA using a nozzle-jet printed AuNPs@Ti3C2 MXene-based electrochemical sensor

5-hydroxymethylcytosine (5hmC) is a key epigenetic mark in the mammalian genome that has been proposed as a promising cancer biomarker with diagnostic and prognostic potentials. A new type of two-dimensional (2D) material called MXene includes transition metal carbides and nitrides and possesses unique physico-chemical properties suitable for diverse applications, including electrochemical sensors. Here, we report a new nozzle-jet printed electrochemical sensor using gold nanoparticles (AuNPs)@Ti3C2 MXene nanocomposite for the real-time and label-free detection of 5hmC in the genome. We utilized Ti3C2 MXene as a platform to immobilize AuNPs, which have been shown to exhibit different affinity interactions toward 5-methylcytosine (5 mC) and 5hmC, and thus produce distinct electrochemical responses. To fabricate the electrode, a highly conductive and adhesive silver ink was prepared to generate a silver line onto polyethylene terephthalate (PET) substrate using nozzle-jet printing, followed by deposition of AuNPs@Ti3C2 MXene ink at one end via dropcasting. Analyses of morphology and chemical composition showed that all steps of the sensor fabrication were successful. The fabricated sensor coupled with cyclic voltammetry showed excellent performance in distinguishing 5 mC- or 5hmC-enriched cellular genomic DNAs. As a proof-of-concept investigation, we confirmed that our sensor readily and consistently detected 5hmC diminution in multiple tumors, compared to the paired normal tissues. Thus, our simple and cost-effective sensing strategy using printable AuNPs@Ti3C2 MXene ink holds promise for a wide range of practical applications in epigenetic studies as well as clinical settings