Flexible Sensor Research Articles

Intrinsically conductive polymer reinforced hydrogel with synergistic strength, toughness, and sensitivity for flexible motion-monitoring sensors

Intrinsically conductive polymer reinforced hydrogel with synergistic strength, toughness, and sensitivity for flexible motion-monitoring sensors

Application of Flexible Wearable Sensors for Revolution of Personalized Medicine in the Age of Digital Health

Application of Flexible Wearable Sensors for Revolution of Personalized Medicine in the Age of Digital Health

Thin, flexible hybrid-structured piezoelectric sensor array with enhanced resolution and sensitivity

Wearable piezoelectric electronics have garnered significant interest in healthcare and human-machine interfaces due to their unique ability to convert mechanical stimuli into electrical signals. However, current wearable piezoelectric sensors struggle with low spatial resolution, limited sensitivity, and insufficient mechanical flexibility. Here, we report materials and designs of a thin, flexible piezoelectric sensor array with a balanced performance that could be used for spatiotemporal pressure mapping on human skin. The device with a soft-rigid hybrid structure integrates small-pixel, highly sensitive piezoceramic thick films into a conformally adaptable format that achieves satisfactory spatial resolution (6 mm), high sensitivity (15.08 mV/kPa) and optimal flexibility (bending radius of 4.23 mm). Theoretical analysis indicates that the thickness of substrate presents a trade-off between output performance and mechanical flexibility. To demonstrate the capabilities of the device, it has been employed to monitor joint bending, human gait, and throat sounds. Furthermore, it incorporates a convolutional neural network (CNN-2D) for voice recognition, achieving an impressive accuracy of 98.18 %. These capabilities underscore the potential of the device in advanced wearable applications, offering a robust platform for real-time, high-resolution physiological monitoring.

Electrocatalysis in MOF Films for Flexible Electrochemical Sensing: A Comprehensive Review.

Flexible electrochemical sensors can adhere to any bendable surface with conformal contact, enabling continuous data monitoring without compromising the surface's dynamics. Among various materials that have been explored for flexible electronics, metal-organic frameworks (MOFs) exhibit dynamic responses to physical and chemical signals, offering new opportunities for flexible electrochemical sensing technologies. This review aims to explore the role of electrocatalysis in MOF films specifically designed for flexible electrochemical sensing applications, with a focus on their design, fabrication techniques, and applications. We systematically categorize the design and fabrication techniques used in preparing MOF films, including in situ growth, layer-by-layer assembly, and polymer-assisted strategies. The implications of MOF-based flexible electrochemical sensors are examined in the context of wearable devices, environmental monitoring, and healthcare diagnostics. Future research is anticipated to shift from traditional microcrystalline powder synthesis to MOF thin-film deposition, which is expected to not only enhance the performance of MOFs in flexible electronics but also improve sensing efficiency and reliability, paving the way for more robust and versatile sensor technologies.

The phase transition and enhanced dielectric properties in La0.7Ba0.3MnO3 /P(VDF-HFP) composited films

Polymer-based ferroelectric composites, particularly those containing Poly (vinylidene fluoride) (PVDF), have emerged as preferred materials for flexible electronics and sensors owing to their excellent electro-mechanical attributes and ease of fabrication. The efficacy of PVDF composites is significantly influenced by the fillers and the proportion of the β-phase present. This research details the synthesis of La0.7Ba0.3MnO3 (LBMO) particles via solid-state reaction, surface-functionalized with acetic acid, and subsequently introduced as nucleating agents into Poly (vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) to fabricate LBMO/P(VDF-HFP) composite membranes. The LBMO fillers before modification significantly increased the β-phase content from 32.7 % to 87.8 % in 5 wt% unmodified LBMO/P(VDF-HFP) membranes. After modification, the hydroxyl groups attached to acetic acid-modified LBMO form hydrogen bonds with -CF2 groups in P(VDF-HFP), inducing the formation of the β phase while enhancing macroscopic polarization and piezoelectricity. At a 7.5 wt% acetate-modified LBMO doping level, the composite membrane exhibits a β-phase content of 64.5 %, but concurrently enhancing the film's dielectric properties. At 103 Hz, the dielectric constant of 5 wt% modified LBMO/P(VDF-HFP) membrane increases to 24.8, with a minimal rise in dielectric loss from 0.0293 to 0.0314. Moreover, under a 10 N compressive load at 1 Hz, the 5 wt% modified LBMO/P(VDF-HFP) composite achieves an open-circuit voltage of 4.3 V. These improved dielectric and piezoelectric properties indicate the composite's suitability for energy harvesting applications.

Paracetamol sensing with a flexible graphene paper electrode modified with MnCo2O4

AbstractBACKGROUNDThe design of new‐generation flexible sensors for medical applications has gained considerable interest in the research community in recent past years. Herein, a free‐standing flexible electrode was developed for the electrochemical determination of Paracetamol (PC). This graphene‐based paper electrode was prepared using graphene and MnCo2O4 dispersion with a mold‐casting method followed by chemical reduction. The morphology and structure of our flexible paper electrode (MnCo2O4/GP) were characterized by scanning electron microscopy, energy‐dispersive, X‐ray diffraction, X‐ray photoelectron, and Raman spectroscopy.RESULTSThe as‐prepared flexible MnCo2O4/GP showed outstanding sensing performance for PC, as compared to GP, due to the excellent electrochemical properties of MnCo2O4. The proposed sensor exhibited a wide linear range of 4.64–1000 μM for electrochemical determination of PC with a low detection limit of 1.39 μM. Furthermore, MnCo2O4/GP was successfully applied for PC syrup and tablets with good accuracy results.CONCLUSIONSFor the first time, the present study demonstrated the fabrication and utilization of MnCo2O4‐modified graphene paper to develop a flexible and sensitive electrode for monitoring PC. © 2024 The Author(s). Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

High-Performance Flexible Sensor with Sensitive Strain/Magnetic Dual-Mode Sensing Characteristics Based on Sodium Alginate and Carboxymethyl Cellulose.

Flexible sensors can measure various stimuli owing to their exceptional flexibility, stretchability, and electrical properties. However, the integration of multiple stimuli into a single sensor for measurement is challenging. To address this issue, the sensor developed in this study utilizes the natural biopolymers sodium alginate and carboxymethyl cellulose to construct a dual interpenetrating network, This results in a flexible porous sponge that exhibits a dual-modal response to strain and magnetic stimulation. The dual-mode flexible sensor achieved a maximum tensile strength of 429 kPa and elongation at break of 24.7%. It also exhibited rapid response times and reliable stability under both strain and magnetic stimuli. The porous foam sensor is intended for use as a wearable electronic device for monitoring joint movements of the body. It provides a swift and stable sensing response to mechanical stimuli arising from joint activities, such as stretching, compression, and bending. Furthermore, the sensor generates opposing response signals to strain and magnetic stimulation, enabling real-time decoupling of different stimuli. This study employed a simple and environmentally friendly manufacturing method for the dual-modal flexible sensor. Because of its remarkable performance, it has significant potential for application in smart wearable electronics and artificial electroskins.

A Highly Sensitive 3D-Printed Flexible Sensor for Sensing Small Pressures in Deep-Sea High-Pressure Environment.

The origin of life on Earth is believed to be from the ocean, which offers abundant resources in its depths. However, deep-sea operations are limited due to the lack of underwater robots and rigid grippers with sensitive force sensors. Therefore, it is crucial for deep-sea pressure sensors to be integrated with mechanical hands for manipulation. Here, a flexible stress sensor is presented that can function effectively under high water pressure in the deep ocean. Inspired by biological structures found in the abyssal zone, our sensor is designed with an internal and external pressure balance structure (hollow interlocking spherical structure). The digital light processing (DLP) three-dimensional (3D) printing technology is utilized to construct this complex structure after obtaining optimized structural parameters using finite element simulation. The sensor exhibits linear sensitivity of 0.114 kPa-1 within the range of 0-15 kPa and has an extremely short response time of 32 ms, good dynamic-static load response capability, and excellent resistance cycling stability. It shows high sensitivity of 1.74 kPa-1 even under 30 MPa static water pressures and the theoretical working pressure can exceed 110 MPa, which provides a new solution for deep-sea robots.

Flexible room temperature gas sensor based on α-Fe2O3/Ti3C2Tx MXene composites for ppb-level H2S detection

Ti3C2Tx MXene have attracted enormous attention in portable, wearable electronics areas due to its excellent flexible, electrical conductivity and outstanding physical and chemical properties. Here, 2D α-Fe2O3/Ti3C2Tx MXene composites were synthesized by in-suit growth of α-Fe2O3 on the surface of Ti3C2Tx MXene and between the layers of Ti3C2Tx MXene by solvothermal and annealing methods. Compared with pure α-Fe2O3, the optimal α-Fe2O3/Ti3C2Tx MXene composites exhibited excellent gas sensitive performance (100 ppm can reach 20.27) and a theoretical ultra-low limit-of-detection of 10.19 ppb to H2S gas at room temperature (25 ○C). The combination of α-Fe2O3 and Ti3C2Tx lead to the layer spacing of Ti3C2Tx MXene are further enlarged which is more favorable for H2S gas adsorption. And the excellent performance is maintained even under bending conditions. This work provides a new idea for MOS/Ti3C2Tx MXene in the field of flexible room temperature sensors.

A Strong, Tough, and Self-Healing Strengthening Thioctic Acid-based Elastomer for Highly Reliable Flexible Strain Sensor

A Strong, Tough, and Self-Healing Strengthening Thioctic Acid-based Elastomer for Highly Reliable Flexible Strain Sensor

Fabrication of anti-freezing and self-healing hydrogel sensors based on carboxymethyl guar gum and poly(ionic liquid)

As classical soft materials, conductive hydrogels have attracted wide attention in the field of strain sensors due to their unique flexibility and conductivity. However, there are still challenges in developing conductive hydrogels with comprehensive mechanical strength, self-healing ability and sensitive sensing properties. In this paper, a novel PAV/CMGG hydrogel was prepared by a simple one-pot method through the introduction of 1-vinyl-3-butylimidazolium bromide (VBIMBr), acrylic acid (AA), carboxymethyl guar gum (CMGG) and AlCl3. The coordination bond between Al3+ and –COO− groups on PAA and CMGG, the hydrogen bond between PAA and CMGG, and the electrostatic interaction between [VBIM]+ and –COO− endow the hydrogel with good mechanical properties, self-recovery ability, fatigue resistance and great self-healing properties. PAV/CMGG hydrogel had good conductivity of 2.31 S/m which could successfully light up the bulb. The hydrogel as the strain sensor had not only a wide strain sensing capability (strain ranging from 0 to 800 %), but also a high strain sensitivity (gauge factor (GF) = 28.50 for the strain ranging from 600 to 800 %). This study can provide inspiration for the construction of new high-performance flexible sensors.

Highly conductive and stable double network carrageenan organohydrogels for advanced strain sensing and signal recognition arrays

Conductive hydrogels with excellent mechanical properties, a broad detection range, and stability in complex environments have remained a significant challenge for the development of flexible sensors. In this study, a straightforward freeze-thaw cycles strategy was developed to fabricate a polyvinyl alcohol (PVA)/carrageenan (CA)/calcium chloride (CaCl2)/MXene-based double network organohydrogel (PCCME) for highly flexible and responsive strain detection across a broad temperature spectrum. The PCCME organohydrogel features multiple interactive forces including hydrogen bonding, ionic interactions, and microphase crystallization, which contribute to the organohydrogel's exceptional mechanical and electrical performance. The PCCME organohydrogel exhibited excellent performance in a load-unload test repeated 100 times after being maintained at room temperature for 7 days, with a minimal mechanical decay of only 2.6%. Furthermore, the repaired PCCME organohydrogel retained its robust stability after storage at low temperatures followed by placement at room temperature. The organohydrogel sensor not only detects various movement amplitudes of the human body but also recognizes arrays of pressure signals and converts these into digital images, highlighting its significant potential for applications in rehabilitation monitoring, pressure sensing, and human-computer interaction.

Recent Advances in Flexible Self-Powered Sensors in Piezoelectric, Triboelectric, and Pyroelectric Fields

The rise of the Internet of things has catalyzed extensive research in the realm of flexible wearable sensors. In comparison with conventional sensor power supply methods that are reliant on external sources, self-powered sensors offer notable advantages in wearable comfort, device structure, and functional expansion. The energy-harvesting modes dominated by piezoelectric nanogenerators (PENGs), triboelectric nanogenerators (TENGs), and pyroelectric nanogenerators (PyENGs) create more possibilities for flexible self-powered sensors. This paper meticulously examines the progress in flexible self-powered devices harnessing TENG, PENG, and PyENG technologies and highlights the evolution of these sensors concerning the material selection, pioneering manufacturing techniques, and device architecture. It also focuses on the research progress of sensors with composite power generation modes. By amalgamating pivotal discoveries and emerging trends, this review not only furnishes a comprehensive portrayal of the present landscape but also accentuates avenues for future research and the application of flexible self-powered sensor technology.

Enhanced self-driven flexible piezoelectric nanogenerator sensor based on NaNbO3/P(VDF-TrFE) films for security applications

The self-driven flexible piezoelectric sensor has wide application prospects in areas such as health monitoring, human-machine interaction, and warning detection due to its advantages of non-toxicity and high sensitivity. In this study, a composite film of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] doped with NaNbO3 (NN) was prepared by electrospinning. The highly oriented NN particles promote the formation of P(VDF-TrFE) β phase, reaching a maximum β phase content of 91%. Under mechanical motion at a frequency of approximately 3 Hz, the piezoelectric nanogenerator (PENG) fabricated from the nanofiber membrane achieved a maximum piezoelectric output of 17.3 V and 12 V in the transverse and longitudinal piezoelectric responses, respectively. With an external load of 100 MΩ, the PENG obtained a maximum power density of 0.67 μW/cm². Additionally, the PENG sensor exhibited sensitivity to various human activities and demonstrated high sensitivity and rapid response in monitoring the status of the proposed door lock opening/closing, highlighting its enormous potential in security monitoring applications.

High sensitivity flexible piezoelectric nanogenerator based on multi‐layer piezoelectric composite fiber for human motion energy harvesting

AbstractIn this study, a piezoelectric nanogenerator (PENG) based on multilayer composite fiber had good and stable piezoelectric output performance, which could realize biomechanical energy collection and human movement monitoring. The polyvinylidene fluoride‐hexafluoryl propylene (P(VDF‐HFP)) composite fiber doped with MXene was used as a promising piezoelectric functional layer (MPFP). By electrospinning, P(VDF‐HFP) composite fiber was constructed by alternating electrospinning with polyurethane (TPU) nanofibers layer by layer. The adoption of MXene as a functional filler promoted the transformation of P(VDF‐HFP) from α to piezoelectric β‐crystal, the piezoelectric β‐crystal content of P(VDF‐HFP) increased, and the dielectric properties and polarization levels were enhanced. At the same time, the multi‐layer structure design improved the piezoelectric sensitivity and mechanical properties of the composite fiber, and the composite fiber was more flexible and had longer tensile strain. With excellent dielectric and mechanical properties, 3MPFP/TPU/3MPFP/TPU multilayer composite fibers showed piezoelectric sensitivity of 10.88 V/kPa. Under the action of palm pressing, the PENG generated an open circuit voltage of 25 V, and the voltage signal of the device under different motion forms had specific characteristics such as peak value, frequency, and shape, which could be used to realize the analysis of human motion forms. This study provided an efficient, simple, and creative new idea for the application of flexible energy storage electronic devices, PENGs, and piezoelectric sensors.

Diving into Sweat: Advances, Challenges, and Future Directions in Wearable Sweat Sensing.

Sweat analysis has advanced from diagnosing cystic fibrosis and testing for illicit drugs to noninvasive monitoring of health biomarkers. This article introduces the rapid development of wearable and flexible sweat sensors, highlighting key milestones and various sensing strategies for real-time monitoring of analytes. We discuss challenges such as developing high-performance nanomaterial-based biosensors, ensuring continuous sweat production and sampling, achieving high sweat/blood correlation, and biocompatibility. The potential of machine learning to enhance these sensors for personalized healthcare is presented, enabling real-time tracking and prediction of physiological changes and disease onset. Leveraging advancements in flexible electronics, nanomaterials, biosensing, and data analytics, wearable sweat biosensors promise to revolutionize disease management, prevention, and prediction, promoting healthier lifestyles and transforming medical practices globally.

Screen-printed, biocompatible and ultrasensitive sensor for real-time reactive oxygen species detection in human sweat

Excessive or burst generation of reactive oxygen species (ROS) can induce oxidative stress, precipitating a range of critical illnesses, including cancers, Parkinson's disease and Ischemia-reperfusion injury. Conventional biological assays for ROS, involving discrete steps of capturing, labelling, and spectrometric detection, are complex and time-intensive. Moreover, their accuracy is substantially compromised by the short lifespan (microseconds to milliseconds) of ROS. Consequently, there is a pressing need for a rapid and efficient method that enables real-time detection. In this study, we have developed a printable, flexible ROS sensor based on a robust nanoenzyme composite by direct deposition of the paste onto a flexible polyethylene terephthalate (PET) substrate. This device demonstrated the fast and real-time responses to the hydrogen peroxide (mimetic agent) in the laboratory and to total ROS in sweat of an individual, exhibiting an outstanding current response to hydrogen peroxide across a broad concentration range of 0.01–10 mM, with a limit of detection (LOD) of 1.85 μM. The device's sensitivity to hydrogen peroxide (136.59 μA mM−1 cm−2), was found to be 1.5 to 10 times higher than that of sensors previously reported. Moreover, the IFRS device successfully identified instantaneous ROS levels in the sweat of adult males in vitro, with amperometric response increased 8 times after half an hour strenuous exercise, thereby exhibiting excellent selectivity, remarkable stability, and confirmed high biosafety. Overall, the IFRS provides a viable and practical solution for simple, expedited, and real-time ROS detection in the near future.

Revolutionizing Personalized Health: The Frontier of Wearable Biomolecule Sensors Through 3D Printing Innovation

AbstractThis review article delves into the innovative intersection of 3D-printed technologies and wearable chemical sensors, highlighting a forward-thinking approach to biomarker monitoring. It emphasizes the transformative role of additive manufacturing in the development of wearable devices tailored for the precise detection of chemical biomarkers, crucial for proactive disease management and health assessment. By offering a detailed exploration of how 3D printing of nanomaterials contributes to pioneering sensor designs, this review underscores the practicality of sensor wearability, ensuring comfort and efficacy for users. We address the challenges of material resilience, sensor durability, and efficient data communication, while also charting the significant trends and future directions that promise to redefine the landscape of flexible and wearable chemical sensors. Through a comprehensive analysis, this article aims to showcase the pivotal advancements and ongoing innovations in the field, emphasizing the critical impact of 3D printing on enhancing personalized healthcare and wearable diagnostics. Graphical Abstract

Facile preparation of self-healing, adhesive and patterned gels via frontal polymerization for wearable sensing and actuating applications

Flexible intelligent gel has garnered great research attention for its potential artificial intelligence applications. Herein, a facile method is provided for fabricating patterned gel materials with self-healing, adhesive, and responsive properties via frontal polymerization within 10 min. The resulting gels are able to self-repair spontaneously after damaged, and exhibit the highest self-healing efficiency (∼99 %) benefiting from the synergistic effect of hydrogen-bond and host–guest assemblies. Moreover, the adhesive gels display satisfactory mechanical properties with great stretchability (524 %), tensile strength (200 kPa) and flexibility. With these features, the gels are served as flexible wearable sensors which can detect a series of movements such as stretch, bend and touch. Furthermore, the dual-component gels are prepared by the metal–ligand interaction (Fe3+-carboxyl group) on the one side of the gel films. The gradient structure and swelling ability endow the gel with actuating behaviors. This research explores a convenient approach to construct functional patterned gels towards significant applications in the wearable sensor and actuator fields.

Wet Spinning/UV Dual-Curing enabled 3D printable fiber for intelligent electronic devices

Conductive fibers are widely used in wearable sensors and implantable devices in real life due to their excellent properties of softness and lightweight. However, most conductive fibers obtained through the simple blending of polymer elastomers and inorganic conductive particles often leads to issues such as uneven conductor distribution and poor mechanical performance. Herein, the composite fiber consists of deep eutectic solvents (composed of choline chloride and 2-hydroxyethyl methacrylate) and thermoplastic polyurethane was prepared by wet spinning/ ultraviolet dual-curing method. The uniform distribution of deep eutectic solvents and the resilience of thermoplastic polyurethane ensure that the fiber material can maintain stable signal monitoring capability when subject to various strains over the long term. The composite fiber as motion sensor with an ultra-wide operating range (0–710 %), fast response time (186 ms), long lifetime (stable after 5 months), wide temperature range and excellent weather resistance. It is worth mention that the dual curing process combined with 3D printing technology, allows for the fabrication of flexible sensors in a variety of shapes. 3D printing sensors can be produced underwater to achieve different sensitivities, enabling customization to meet the requirements of applications. This innovative study holds tremendous potential for the next generation of flexible electronic products, such as human motion monitoring and information transmission.