Flexible Temperature Sensor Research Articles

Integrated Piezo-Triboelectric Nanogenerators-Based Self-Powered Flexible Temperature and Pressure Sensor

Efficient harvesting of ubiquitous ambient mechanical energy using various types of nanogenerators (NGs) has attracted considerable interest in recent years. Herein, we present a textile triboelectric nanogenerator (T-TENG) using Polytetrafluoroethylene (PTFE) and Nylon based counter-surfaces in fabric form (both fixed to conductive fabric electrodes). T-TENG performance is enhanced by exposing PTFE film with Argon plasma. The output voltage of the plasma treated devices increased by a factor of ~10, and their short circuit current density increased by a factor of ~9 (compared to pristine non-plasma treated devices). Additionally, we demonstrate that the fabricated T-TENG can be used as a self-powered temperature sensor, tested in the 25-90 °C range. The T-TENG output voltage decreased linearly with increasing temperature exhibiting a sensitivity of ~ 1%/(°C). Further, we integrated a flexible piezoelectric nanogenerator (PENG) on top of the T-TENG to detect the contact forces. The PENG output is also used to compensate the pressure dependent output of TENG underneath, and allows distinct temperature and pressure measurements. The excellent results observed here show the potential of T-TENGs for use as self-powered sensors in applications such as health monitoring, wearables and interactive systems.

Fully Printed Sensors for In Situ Temperature, Heat Flow, and Thermal Conductivity Measurements in Flexible Devices.

Flexible temperature sensors allow temperature monitoring in wearable healthcare devices. A temperature sensor, which can be printed on flexible substrates, is designed and fabricated using a low-cost silver particle ink and a fast and scalable screen-printing process. A high temperature resolution of 10 m°C is reached. The versatility of this temperature sensor design is demonstrated for various applications, including in situ heat flux measurements, where a 2 mW cm-2 resolution is reached, and thermal conductivity measurements on polymer films as thin as 25 μm, with a wide range of accessible values from ∼0.1 to 0.8 W K-1 m-1.

Fabrication of Temperature Sensors with High-Performance Uniformity through Thermal Annealing.

It is considered to be of great significance to monitor human health and track the effect of drugs by measuring human temperature mapping through flexible temperature sensors. In this work, we found that the thermal annealing of flexible temperature sensors based on graphite-acrylate copolymer composites can not only improve the temperature coefficient of resistance (TCR) values of the devices, but also greatly improve the uniformity of the performance of the devices prepared in parallel. The best results were obtained when the devices were annealed at 100 °C, which is believed to be due to the rearrangement of graphite particles to generate more uniform and numerous conductive channels within the conductive composite. We believe this finding might promote the practical development of flexible temperature sensors in body temperature sensing for health maintenance and medical applications.

A flexible humidity and temperature sensor based on calligraphy itself for calligraphy conservation application

A flexible humidity and temperature sensor based on calligraphy itself for calligraphy conservation application

Development of a Highly Sensitive and Stable Flexible Temperature Sensor Based on All‐Inorganic Mn–Co–Ni–O Thin Films

AbstractDespite the fact that flexible temperature sensors, which can be mechanically deformed, are used for the fabrication of next‐generation smart electronics over the past few years, the improvement in their sensitivity and stability remains a major challenge. Along these lines, in this work, an all‐inorganic flexible temperature sensor consisting of MnCoNiO (MCNO) spinel structural ceramics film and a flexible 40 µm‐thick mica substrate is fabricated. By precisely controlling the production process, the pure spinel phase MCNO/mica films with low defect density can be obtained. In addition, the activation energy of the proposed flexible ceramic temperature sensor film remains stable in the natural bending state and at radii of curvature of 10, 15, and 20 mm, respectively, accordingly, B25/50 is relatively stable around the range of 3450–3550 K. Meanwhile, its sensitivity temperature coefficient of resistance value reaches −3.9% °C−1. More specifically, ΔB25/50 is less than 1.83%, 1.57%, and 1.32% after thermal shock, application of the bending cycle, and vibration, respectively. The superior performance of this all‐inorganic ceramic film is due to the excellent bonding between the film and the substrate. This work provides an alternative approach to exploit the development of the next‐generation high‐sensitivity flexible electronic devices.

Flexible Te/PEDOT:PSS thin films with high thermoelectric power factor and their application as flexible temperature sensors.

Flexible, lightweight, and low-cost thermoelectric thin films are promising for self-powered wearable electronics and sensors. In this work, we report on flexible Te nanostructures/PEDOT:PSS composite thin films with high power factor and their application as flexible temperature sensors. Te nanostructures with high crystallinity and high aspect ratios were synthesized through an environmentally friendly method without using highly toxic chemicals. Individual Te nanostructures achieve a thermoelectric figure of merit (ZT) of 0.13 at 300 K, indicating good potential as inorganic fillers for nanostructures/polymer hybrid materials. Based on the synthesized Te nanostructures, flexible p-type Te/PEDOT:PSS thin films were fabricated through a simple dilution and vacuum filtration method. The power factor of the as-prepared composite thin film outperforms that of either a Te or DMSO-treated PEDOT:PSS thin film, and importantly, it can be further enhanced to 149 μW m-1 K-2 by hot pressing, which is nearly threefold enhancement compared to the values reported for the vacuum-filtered flexible Te/PEDOT:PSS thin films in the literature. The hot-pressed composite thin film shows high flexibility with the electrical conductivity remaining almost unchanged after 1000 bending cycles under a bending radius of 5 mm. Flexible temperature sensors were fabricated based on the hot-pressed Te/PEDOT:PSS thin film, which exhibited high sensitivity in detecting temperature stimuli. The developed temperature sensors were applied onto a two-finger flexible mechanical claw for identifying hot/cold objects in robotic grasping. This work demonstrates an effective approach to enhance the thermoelectric power factor of flexible Te nanostructures/polymer composites and their promising application in flexible thermal sensing.

An ultrasensitive flexible pressure, temperature, and humidity sensor based on structurally adjustable nano-through-hole array films

A pressure, temperature and humidity flexible multiresponsive sensor was fabricated, which has independent responses to triple stimulus signals without crosstalk.

Remote Monitoring of Skin Temperature Through a Wristband Employing a Printed VO Sensor

The need for highly sensitive, environmentally stable, mechanically flexible, and low-cost temperature sensors for on-body measurements has been increasing with the wide adoption of personal healthcare-Internet-of-Things (H-IoT) devices. Printed electronics (PE) is a good platform for such sensors because it enables the realization of flexible devices through simple and rapid methods at a relatively low cost. However, previously reported printed temperature sensors suffer from poor sensitivity and/or environmental instability. In this article, we report a custom tungsten (W)-doped vanadium dioxide (VO2) ink-based screen-printed temperature sensor having the highest temperature coefficient of resistance (TCR) of 2.78% C-1 with a resolution of 0.1 °C between 30 °C and 40 °C. To protect it from environmental effects, a fluoropolymer-basedpassivation layer is added for accurate temperature readings even in 90% relative humidity. The sensor is printed on a flexible substrate and shows minimal deterioration in performance over 1000 bending cycles. For wearability and remote monitoring, the sensor is integrated with a custom Bluetooth low energy (BLE) wireless readout in the form of wristband. The BLE readout comprises an ultrathin and flexible patch antenna optimized for both BLE bandwidth (BW) and human wearability. It demonstrates a minimal specific absorption rate (SAR) value of only 0.068 W/kg, making it safe to wear. Despite the antenna’s thin structure (0.004λ), it has a gain of 1.65 dBi, enabling an excellent communication range. The proposed wristband is tested on ten volunteers and under daily activities, which shows promising results with a maximum error of 0.16 °C with reference to those of a commercial thermometer.

Flexible room temperature sensor with modulation of polyaniline interfacial polymerization by CTAB for ppb-level ammonia detection

A highly selective and sensitive flexible room temperature ammonia (NH3) sensor was produced by in situ polymerizations of polyaniline (PANI) with modulation of Hexadecyl trimethyl ammonium Bromide (CTAB) on a porous Polyvinylidene fluoride film (PVDF). CTAB allows the PANI nanoparticles to grow uniformly on the surface and in the internal pores of the PVDF film, which increases the sensor’s response to the NH3. As a result, the sensor’s response values to 5 ppm NH3 and 0.01 ppm NH3 are 81.3 % and 4.6 % at 20 °C, respectively. The sensor also shows excellent selectivity for NH3 and maintains superior stability. Such a highly responsive and sensitive sensor is expected to be applied to detect chronic kidney disease.

Flexible temperature sensor with high sensitivity ranging from liquid nitrogen temperature to 1200 °C

Flexible temperature sensors have been extensively investigated due to their prospect of wide application in various flexible electronic products. However, most of the current flexible temperature sensors only work well in a narrow temperature range, with their application at high or low temperatures still being a big challenge. This work proposes a flexible thermocouple temperature sensor based on aerogel blanket substrate, the temperature-sensitive layer of which uses the screen-printing technology to prepare indium oxide and indium tin oxide. It has good temperature sensitivity, with the test sensitivity reaching 226.7 μV °C−1 . Most importantly, it can work in a wide temperature range, from extremely low temperatures down to liquid nitrogen temperature to high temperatures up to 1200 °C, which is difficult to be achieved by other existing flexible temperature sensors. This temperature sensor has huge application potential in biomedicine, aerospace and other fields.

High‐performance flexible temperature sensor from hybrid nanocomposite for continuous human body temperature monitoring

AbstractCombining high sensitivity with fast response and high resolution remains a critical challenge for flexible temperature sensors. The present study leverages the intrinsically high surface‐to‐volume ratio of nanocomposite fibers as well as the high mechanical properties of nanomaterials for achieving conformable temperature sensors with accurate and fast detection of temperature. To achieve this, nanocomposite films of electrospun polyvinylidene fluoride (PVDF) with embedded silver (Ag) nanoparticles were layered with multiwall carbon nanotubes (MWCNT). The sensor showed a negative temperature coefficient (NTC) with excellent sensitivity of −0.18%/°C and a quick response rate of 11 s. The sensor also exhibited low self‐heating errors for an activation current of 1 mA and excellent anti‐interference ability when tested for bending forces and wet environments. The nanocomposite fiber‐based sensor can be used for real‐time monitoring of human body temperature as confirmed by successful experiments. The present work lays the foundation for integrating the sensor further with a user interface to create a wearable temperature monitoring system for mobile healthcare.

Advanced Functional Composite Materials toward E‐Skin for Health Monitoring and Artificial Intelligence

AbstractElectronic skin (E‐skin), especially the wearable sensors efficiently detect various stimuli attracted huge research interest owing to their potential applications in health monitoring and artificial intelligence. On the other hand, functional polymer composites possessing excellent properties such as light weight, good flexibility, and superior electrical performances, are promising candidates as building blocks for flexible electronics. Accordingly, tremendous efforts are devoted to the development of polymer composites with functional properties for E‐skin applications. Here, recent advances on the controlled fabrication of various fillers based functional polymer composites and their diverse applications in E‐skin are reviewed. In addition, contemporary studies on fabrication strategies, working mechanisms, and device performance for E‐skin based on the functional polymer composites are reviewed, including flexible pressure sensors, strain sensors, temperature sensors, energy harvesters, and transistors. Furthermore, the applications of functional composite based flexible electronics in healthcare and artificial intelligence are discussed. Finally, the existing challenges and opportunities for the functional composite materials based E‐skin are summarized.

Effects of replacing carbamate with alkyl side chains on the properties and temperature sensing performance of hemi-isoindigo-based polymers

Previously, we developed several carbamate side chain-substituted hemi-isoindigo (HID)-based π-conjugated polymers, which demonstrated excellent sensitivity and stability as the sensing layers in chemiresistive temperature sensors. This work investigated the effects of the side chains on the HID units by changing the carbamate to alkyl side chains. Specifically, a series of 2-ethylhexyl-substituted HID polymers, poly(3-((3'',4'-bis(dodecyloxy)-[2,2':5',2''-terthiophen]-5-yl)methylene)-1-(2-ethylhexyl)indolin-2-one-6,5”-diyl) (PTAB), poly(3-((3'',4'-bis(dodecyloxy)-3,4-dimethoxy-[2,2':5',2''-terthiophen]-5-yl) methylene)-1-(2-ethylhexyl)indolin-2-one-6,5”-diyl) (PMAB), and poly(3-((7-(3,3'-bis(dodecyloxy)-[2,2'-bithiophen]-5-yl)-2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)methylene)-1-(2-ethylhexyl)indolin-2-one-6,5”-diyl) (PEAB) were synthesized, and their properties and temperature sensing performance were compared with their counterpart carbamate-substituted HID polymers, poly(2-ethylhexyl-3-((3'',4'-bis(dodecyloxy)-[2,2':5',2''-terthiophen]-5-yl)methylene)-2-oxoindoline-1-carboxylate-6,5”-diyl) (PTEB), poly(2-ethylhexyl-3-((3'',4'-bis(dodecyloxy)-3,4-dimethoxy-[2,2':5',2''-terthiophen]-5-yl)methylene)-2-oxoindoline-1-carboxylate-6,5”-diyl) (PMEB), and poly(2-ethylhexyl-3-((7-(3,3'-bis(dodecyloxy)-[2,2'-bithiophen]-5-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)methylene)-2-oxoindoline-1-carboxylate-6,5”-diyl) (PEEB), and their thermally annealed products, poly(3-((3'',4'-bis(dodecyloxy)-[2,2':5',2''-terthiophen]-5-yl)methylene)indolin-2-one-6,5”-diyl) (PTNB), poly(3-((3'',4'-bis(dodecyloxy)-3,4-dimethoxy-[2,2':5',2''-terthiophen]-5-yl)methylene)indolin-2-one-6,5”-diyl) (PMNB), and poly(3-((7-(3,3'-bis(dodecyloxy)-[2,2'-bithiophen]-5-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)methylene)indolin-2-one-6,5”-diyl) (PENB). The highest occupied molecular orbital energy (E HOMO) level and crystallinity of PEAB are very similar compared to PEEB. Chemiresistor devices with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) (PEAB:F4TCNQ) fabricated on flexible plastic substrates exhibited a high temperature coefficient of resistance (TCR) of −1.09% °C−1, although the value is lower than that (−1.92% °C−1) of the device based on PENB:F4TCNQ. The device based on PEAB:F4TCNQ also showed excellent stability with no performance degradation over 1 month, which is similar to the device based on PENB:F4TCNQ. On the other hand, PTAB and PMAB showed significantly higher E HOMO levels and crystallinity compared to their counterpart polymers. Sensors based on PTAB:F4TCNQ and PMAB:F4TCNQ showed TCR values of −1.02% °C−1 and −1.15% °C−1, respectively, which are lower than their corresponding annealed carbamate-substituted HID polymers. PTAB has a much lower E HOMO level (−4.95 eV) than that of PTNB (−4.69 eV) and is more crystalline than the latter, which should lead to poorer stability of the doped complex PTAB:F4TCNQ. Surprisingly, PTAB:F4TCNQ showed much better long-term stability than PTNB:F4TCNQ. It was considered that the hydrophobic alkyl side chains in PTAB can help prevent the interaction of water in the air with the PTAB:F4TCNQ complex, thereby stabilizing the complex. This study provided new insights into the design principles of conjugated polymers for printed and flexible temperature sensors.

Flexible and Shape‐Morphing Plant Sensors Designed for Microenvironment Temperature Monitoring of Irregular Surfaces

AbstractFlexible plant sensors play a critical role in smart agriculture due to their advantages in real‐time monitoring physiological signals of plants, and are experiencing growth in recent years. Such devices are expected to be directly placed on surfaces of plant organs for better detection. However, most existing sensors based on the planar substrate are not able to adapt to nondevelopable surfaces of plants, and are unsatisfactory in biocompatibility. Herein, considering the complexity of the plant surface, flexible temperature sensors for leaves and fruits are developed. The leaf temperature sensor is based on the porous substrate, which is designed to minimize its effect on plant respiration, and is demonstrated to measure temperature changes accurately after long‐time integration with a leaf. By mechanical design, the fruit temperature sensor realizes the transformation from a planar shape to a tridimensional shape, and is demonstrated to work on a variety of complex curved surfaces without loss of performance. The proposed shape‐morphing structure expands the capabilities of current planar electronics, and links thin‐film technology to spatial deformable devices. Results of the in vitro experiments show that these two proposed sensors hold promise to monitor microenvironment temperature in plant biology.

Wearable Temperature Sensor with High Resolution for Skin Temperature Monitoring.

Flexible temperature sensors with high resolution and good reliability under deformation are a major research focus for wearable electronic devices for skin temperature monitoring. In this study, a fiber-like temperature sensor is fabricated by in situ growing poly(3,4-ethylenedioxythiophene) (PEDOT) on the surface of thermoplastic polyurethane (TPU) fiber. The temperature sensor achieves a high sensitivity of 0.95%·°C-1 with a high linearity between 20 and 40 °C. Most importantly, the sensor achieves a high temperature resolution of 0.2 °C. Due to its structure, the temperature-sensitive fiber is easily embedded into textiles. By sewing the fiber into normal textiles in an S-shape, the interference of strain can be nearly avoided, even when the textile is stretched to 140%. Also, the obtained sensors can monitor skin temperature during exercise, which demonstrates the potential of the sensor's application in healthcare and disease diagnosis.

A biodegradable and flexible temperature sensor supported on avocado peel and its enhancement of detection by sensitizing with the La0.5Sr0.5CoO3 perovskite

We report the performance of flexible and biodegradable temperature sensors fabricated with avocado peel (biodegradable waste used as support), graphene (G), carbon nanotubes (CNTs) and La0.5Sr0.5CoO3 (LSCO) perovskite. The LSCO powder is formed by agglomerates of microparticles with average size of 1.63 μm and they are decorated by smaller nanoplates with sizes of 50–150 nm. According to the X-ray diffraction (XRD) analysis, the LSCO presents a rhombohedral crystalline phase. The performance of the temperature sensor made without LSCO (it contained only G + CNTs as active materials) was firstly evaluated to detect a temperature of 60 °C and response/recovery times of 35.3/155.5 s were obtained. Later, the sensor was sensitized with LSCO microparticles deposited on its surface and the response/recovery times were lowered to 8.5/27.6 s for the same detection temperature of 60 °C, thus, the response time was decreased by ≈ 76%. The sensor made with LSCO was also subjected to 300 bending cycles. Interestingly, it produced a low response/recovery times of 9.7/7.1 s for the detection temperature of 90 °C. Moreover, the temperature sensor made with LSCO microparticles could detect temperatures in the range of 60–140 °C and the response/recovery times increased from 8.5/27.6 s to 15.3/122.3 s after increasing the detection temperature from 60 °C to 140 °C. Furthermore, the XPS, Raman and FTIR analysis revealed the formation of amorphous carbon and structural defects on the sensor's surface after its use for the temperature detection, which contributed to the enhancement of the sensor's sensitivity by ≈ 139%. Overall, the new class of temperature sensor presented in this work can pave the way for the development of eco-friendly sensors, which can find applications in wearable/industrial devices.

Substrate Optimization of Flexible Temperature Sensor for Wearable Applications

In flexible electronic devices and sensor fabrication, substrate selection is essential as it determines different parameters and techniques. This paper aims to find an efficient substrate selection approach for flexible and wearable electronic applications. A hybridized Multi-Criteria Decision-Making (MCDM) technique, namely the Analytic Hierarchy Tradeoff Ranking Technique (AH-TOR), is used to select the most suitable flexible substrate out of the several widely used options for the temperature sensor. The selection of the best substrate for the said application is made based on various properties. In this case, AHP is used to evaluate the criteria weights, while the TOR method determines the weights of alternatives. Further, different PEDOT: PSS-based temperature sensors have been fabricated to validate the optimization technique. Based on the performance of the sensors, it is found that the sensitivity of Polyethylene Terephthalate (PET) is superior, and the sensitivity of PET is approximately 53.3% better than Polydimethylsiloxane (PDMS) and 47.4% superior to Polyvinyl chloride (PVC). The result is in sync with the optimization, where PET is at the top among all other alternatives. This method will be helpful in large-area sensor fabrication and manufacturing of flexible electronic devices.

Simulation Study of High Sensitivity Fiber SPR Temperature Sensor with Liquid Filling.

In this paper, a high sensitivity fiber temperature sensor based on surface plasmon resonance is designed and studied. In the simulation, the single mode fiber is polished to remove most of the cladding, and then gold and silver films are added. Finally, it is embedded in the heat shrinkable tube filled with a thermo-optic coefficient liquid for curing. The numerical simulation results show that the sensing characteristics are sensitive to the remaining cladding thickness of the fiber, the thickness of the gold film and the thickness of the silver film. When the thermo-optic coefficient of the filling liquid is −2.8 × 10−4/°C, the thickness of the gold film, the thickness of the silver film and the thickness of the remaining cladding of the fiber are 30 nm, 20 nm and 1 μm, respectively. The sensitivity of the sensor designed in this paper can reach −6 nm/°C; this result is slightly higher than that of similar research in recent years. It will have a promising application prospect in flexible wearable temperature sensors, smart cities and other fields.

Flexible Temperature Sensor Based on Binary Polymer Composite Thin Film

A high performance flexible temperature sensor for environmental and health monitoring has been fabricated using various combinations of composite blend of poly vinylidene fluoride / poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PVDF/PEDOT:PSS). The response curves and working principle were investigated and sensors were then fabricated to achieve highly linear and stable response for a wide range of temperature sensing (25°C to 120°C). The film was fabricated on flexible polyethylene terephthalate (PET) substrate using spin coating. The copper electrodes were fabricated using copper tape. The sensors showed stable and close to linear response of impedance change by varying temperature in the range 25°C to 120°C. The resistance of the sensors changed from ~70MΩ to ~52MΩ for the temperature change in the range 25°C to 120°C. The sensors are aimed to replace low performance, complex and expensive sensors in the market for environmental and health monitoring applications.

Fast-Response Flexible Temperature Sensors with Atomically Thin Molybdenum Disulfide.

Real-time thermal sensing on flexible substrates could enable a plethora of new applications. However, achieving fast, sub-millisecond response times even in a single sensor is difficult, due to the thermal mass of the sensor and encapsulation. Here, we fabricate flexible monolayer molybdenum disulfide (MoS2) temperature sensors and arrays, which can detect temperature changes within a few microseconds, over 100× faster than flexible thin-film metal sensors. Thermal simulations indicate the sensors' response time is only limited by the MoS2 interfaces and encapsulation. The sensors also have high temperature coefficient of resistance, ∼1-2%/K and stable operation upon cycling and long-term measurement when they are encapsulated with alumina. These results, together with their biocompatibility, make these devices excellent candidates for biomedical sensor arrays and many other Internet of Things applications.