Processing Natural Wood into a High-Performance Flexible Pressure Sensor

Utilizing Natural Materials in Electronic Devices: Inching Toward "Herbal Electronics"

Sustainable development is the primary key to address global energy challenges. Though the scientific community is engaged in developing efficient ways to not only maximize energy production from natural resources like sun, wind, water, etc. but also to make all the electronic gadgets power efficient, despite all this, the materials used in most of the electronic devices are largely produced using various materials processing techniques and semiconductors, polymers, dielectrics, etc. which again increases the burden on energy and in turn affects the environment. While addressing these challenges, it is very important to explore the possibility to directly, or with minimum processing, utilize the potential of natural resources in the development of electronic devices. Recent articles are focused on the development of herbal electronic devices that essentially implement natural resources, like plants, leaves, etc., either in their raw or extracted form in the device assembly. This review encompasses the recent research developments around herbal electronic devices. Furthermore, herbal electronics has been discussed for several functional applications including electrochromism, energy storage, memresistor, LED, solar cell, water purification, pressure sensor, etc. Moreover, advantages, disadvantages, and challenges encountered in the realization of "herbal electronics" have been discussed at length.

Calligraphic interdigitated capacitive sensors for green electronics

This study presents a novel approach to fabricating interdigitated capacitive (IDC) touch sensors using graphite-based pencils on a wood substrate. The sensors were designed to detect touches and pressure variations, offering a cost-effective and environmentally friendly solution for sensor fabrication. The fabrication process involved abrasion of graphite pencils on a wooden substrate to create conductive traces, followed by the integration of interdigitated electrode structures. Capacitance variations resulting from touch interactions were investigated to calibrate sensor responses for tailored tasks. The sensitivity of the sensor was found to be 1.2 pF/kPa, highlighting its responsiveness to pressure variations. Additionally, the sensors were interfaced with an Arduino Uno microcontroller board to demonstrate practical applications, such as replicating arrow key functionality. Additionally, the sensors exhibit sensitivity to environmental factors, with the relative change in capacitance increasing from 0.1 to 0.65 as relative humidity ranges from 30 to 90%. Furthermore, variations in temperature from 30 to 60ºC result in a relative change in capacitance increasing to approximately 0.5. The results indicate the feasibility and versatility of using wood-based substrates and graphite-based pencils for fabricating IDC touch sensors, offering promising prospects for sustainable and accessible sensor technology.

Microneedle Patches-Integrated Transdermal Bioelectronics for Minimally Invasive Disease Theranostics

Wearable epidermal electronics with non- or minimally-invasive characteristics can collect, transduce, communicate, and interact with accessible physicochemical health indicators on the skin. However, due to the stratum corneum layer, rich information about body health is buried under the skin stratum corneum layer, for example, in the skin interstitial fluid. Microneedle patches are typically designed with arrays of special microsized needles of length within 1000 µm. Such characteristics potentially enable the access and sample of biomolecules under the skin or give therapeutical treatment painlessly and transdermally. Integrating microneedle patches with various electronics allows highly efficient transdermal bioelectronics, showing their great promise for biomedical and healthcare applications. This comprehensive review summarizes and highlights the recent progress on integrated transdermal bioelectronics based on microneedle patches. The design criteria and state-of-the-art fabrication techniques for such devices are initially discussed. Next, devices with different functions, including but not limited to health monitoring, drug delivery, and therapeutical treatment, are highlighted in detail. Finally, key issues associated with current technologies and future opportunities are elaborated to sort out the state of recent research, point out potential bottlenecks, and provide future research directions.

Anti-Oxidized Self-Assembly of Multilayered F-Mene/MXene/TPU Composite with Improved Environmental Stability and Pressure Sensing Performances

MXenes, as emerging 2D sensing materials for next-generation electronics, have attracted tremendous attention owing to their extraordinary electrical conductivity, mechanical strength, and flexibility. However, challenges remain due to the weak stability in the oxygen environment and nonnegligible aggregation of layered MXenes, which severely affect the durability and sensing performances of the corresponding MXene-based pressure sensors, respectively. Here, in this work, we propose an easy-to-fabricate self-assembly strategy to prepare multilayered MXene composite films, where the first layer MXene is hydrogen-bond self-assembled on the electrospun thermoplastic urethane (TPU) fibers surface and the anti-oxidized functionalized-MXene (f-MXene) is subsequently adhered on the MXene layer by spontaneous electrostatic attraction. Remarkably, the f-MXene surface is functionalized with silanization reagents to form a hydrophobic protective layer, thus preventing the oxidation of the MXene-based pressure sensor during service. Simultaneously, the electrostatic self-assembled MXene and f-MXene successfully avoid the invalid stacking of MXene, leading to an improved pressure sensitivity. Moreover, the adopted electrospinning method can facilitate cyclic self-assembly and the formation of a hierarchical micro-nano porous structure of the multilayered f-MXene/MXene/TPU (M-fM2T) composite. The gradient pores can generate changes in the conductive pathways within a wide loading range, broadening the pressure detection range of the as-proposed multilayered f-MXene/MXene/TPU piezoresistive sensor (M-fM2TPS). Experimentally, these novel features endow our M-fM2TPS with an outstanding maximum sensitivity of 40.31 kPa-1 and an extensive sensing range of up to 120 kPa. Additionally, our M-fM2TPS exhibits excellent anti-oxidized properties for environmental stability and mechanical reliability for long-term use, which shows only ~0.8% fractional resistance changes after being placed in a natural environment for over 30 days and provides a reproducible loading-unloading pressure measurement for more than 1000 cycles. As a proof of concept, the M-fM2TPS is deployed to monitor human movements and radial artery pulse. Our anti-oxidized self-assembly strategy of multilayered MXene is expected to guide the future investigation of MXene-based advanced sensors with commercial values.

Rational Design of Amorphous Carbon-Coated Laminar-Structured Wood for Integrating Repeatable Early Fire Detection and High-Temperature Affordable Flexible Pressure Sensing in One System

High-temperature affordable flexible polymer-based pressure sensors integrated with repeatable early fire warning service are strongly desired for harsh environmental applications, yet their creation remains challenging. This work proposed an approach for preparing such advanced integrated sensors based on silver nanoparticles and an ammonium polyphosphate (APP)-modified laminar-structured bulk wood sponge (APP/Ag@WS). Such integrated sensors demonstrated excellent fire warning performance, including a short response time (minimum of 0.44 s), a long-lasting alarm time (>750 s), and reliable repeatability. Moreover, it achieved high-temperature affordable flexible pressure sensing that exhibited an almost unimpaired working range of 0-7.5 kPa and a higher sensitivity (in the low-pressure range, maximum to 226.03 kPa-1) after fire. The high stability was attributed to reliable structural elasticity, and the wood-derived amorphous carbon is capable of repeatable fire warnings. Finally, a Ag@APP/WS-based wireless fire alarm system that realized reliable house fire accident detection was demonstrated, showing great promise for smart firefighting application.

Transparent and Efficient Wood-Based Triboelectric Nanogenerators for Energy Harvesting and Self-Powered Sensing

Wood possesses several advantageous qualities including innocuity, low cost, aesthetic appeal, and excellent biocompatibility, and its naturally abundant functional groups and diverse structural forms facilitate functionalization modification. As the most sustainable bio-based material, the combination of wood with triboelectric nanogenerators (TENGs) stands poised to significantly advance the cause of green sustainable production while mitigating the escalating challenges of energy consumption. However, the inherent weak polarizability of natural wood limits its development for TENGs. Herein, we present the pioneering development of a flexible transparent wood-based triboelectric nanogenerator (TW-TENG) combining excellent triboelectrical properties, optical properties, and wood aesthetics through sodium chlorite delignification and epoxy resin impregnation. Thanks to the strong electron-donating groups in the epoxy resin, the TW-TENG obtained an open-circuit voltage of up to ~127 V, marking a remarkable 530% enhancement compared to the original wood. Furthermore, durability and stability were substantiated through 10,000 working cycles. In addition, the introduction of epoxy resin and lignin removal endowed the TW-TENG with excellent optical characteristics, with optical transmittance of up to 88.8%, while preserving the unique texture and aesthetics of the wood completely. Finally, we show the application prospects of TW-TENGs in the fields of self-power supply, motion sensing, and smart home through the demonstration of a TW-TENG in the charging and discharging of capacitors and the output of electrical signals in different scenarios.

High-Sensitivity Flexible Capacitive Pressure Sensors Based on Biomimetic Hibiscus Flower Microstructures

The integration of low-dimensional nanomaterials with microscale architectures in flexible pressure sensors has garnered significant interest due to their outstanding performance in healthcare monitoring. However, achieving high sensitivity across different magnitudes of external pressure remains a critical challenge. Herein, we present a high-performance flexible pressure sensor crafted from biomimetic hibiscus flower microstructures coated with silver nanowires. When compared with a flat electrode, these microstructures as electrodes display significantly enhanced sensitivity and an extended stimulus-response range. Furthermore, we utilized an ionic gel film as the dielectric layer, resulting in an enhancement of the overall performance of the flexible pressure sensor through an increase in interfacial capacitance. Consequently, the capacitive pressure sensor exhibits an extraordinary ultrahigh sensitivity of 48.57 [Kpa]-1 within the pressure range of 0-1 Kpa, 15.24 [Kpa]-1 within the pressure range of 1-30 Kpa, and 3.74 [Kpa]-1 within the pressure range of 30-120 Kpa, accompanied by a rapid response time (<58 ms). The exceptional performance of our flexible pressure sensor serves as a foundation for its numerous applications in healthcare monitoring. Notably, the flexible pressure sensor excels not only in detecting subtle physiological signals such as finger and wrist pulse signals, vocal cord vibrations, and breathing intensity but also demonstrates excellent performance in monitoring higher pressures, such as plantar pressure. We foresee that this flexible pressure sensor possesses significant potential in the field of wearable electronics.

Design of Double Conductive Layer and Grid-Assistant Face-to-Face Structure for Wide Linear Range, High Sensitivity Flexible Pressure Sensors

Recently, flexible pressure sensors have drawn great attention because of their potential application in human-machine interfaces, healthcare monitoring, electronic skin, etc. Although many sensors with good performance have been reported, researchers mostly focused on surface morphology regulation, and the effect of the resistance characteristics on the performance of the sensor was still rarely systematically investigated. In this paper, a strategy for modulating electron transport is proposed to adjust the linear range and sensitivity of the sensor. In the modulating process, we constructed a double conductive layer (DCL) and grid-assistant face-to-face structure and obtained the sensor with a wide linear range of 0-700 kPa and a high sensitivity of 57.5 kPa-1, which is one of the best results for piezoresistive sensors. In contrast, the sensor with a single conductive layer (SCL) and simple face-to-face structure exhibited a moderate linear range (7 kPa) and sensitivity (2.8 kPa-1). Benefiting from the great performance, the modulated sensor allows for clear pulse wave detection and good recognition of gait signals, which indicates the great application potential in human daily life.

High-Strength, High-Swelling-Resistant, High-Sensitivity Hydrogel Sensor Prepared with Wood That Retains Lignin

Wood-derived hydrogels possess satisfactory longitudinal strength but lack excellent swelling resistance and dry shrinkage resistance when achieving high anisotropy. In this study, we displayed the preparation of highly dimensional stable wood/polyacrylamide hydrogels (wood/PAM-Al3+). The alkali-treated wood retains lignin as the skeleton of the hydrogel. Second, Al ions were added to the metal coordination with lignin. Finally, by employing free radical polymerization, we construct a conductive electronic network using polyaniline within the wood/PAM-Al3+ matrix to create the flexible sensor. This approach leverages lignin's integrated structure within the middle lamella to provide enhanced swelling resistance and stronger binding strength in the transverse direction. Furthermore, coordination between lignin and Al ions improves the mechanical strength of the wood hydrogel. Polyaniline provides stable linear pressure and temperature responses. The wood/PAM-Al3+ exhibits a transverse swelling ratio of 3.90% while achieving a longitudinal tensile strength of 20.5 MPa. This high-strength and high-stability sensor is capable of monitoring macroscale human behavior. Therefore, this study presents a simple yet innovative strategy for constructing tough hydrogels while also establishing an alternative pathway for exploring lignin networks in new functional materials development.

PDMS Film-Based Flexible Pressure Sensor Array with Surface Protruding Structure for Human Motion Detection and Wrist Posture Recognition

Flexible pressure sensors have been widely concerned because of their great application potential in the fields of electronic skin, human-computer interaction, health detection, and so on. In this paper, a flexible pressure sensor is designed, with polydimethylsiloxane (PDMS) films with protruding structure as elastic substrate and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS)/cellulose nanocrystals (CNC) as conductive-sensitive material. The flexible pressure sensor has a wide linear detection range (0-100 kPa), outstanding sensitivity (2.32 kPa-1), and stability of more than 2000 cycles. The sensor has been proven to be able to detect a wide range of human movements (finger bending, elbow bending, etc.) and small movements (breathing, pulse, etc.). In addition, the pressure sensor array can detect the pressure distribution and judge the shape of the object. A smart wristband equipped with four flexible pressure sensors is designed. Among them, the k-nearest neighbor (KNN) algorithm is used to classify sensor data to achieve high accuracy (99.52%) recognition of seven kinds of wrist posture. This work provides a new opportunity to fabricate simple, flexible pressure sensors with potential applications in the next-generation electronic skin, health detection, and intelligent robotics.

Artificial Neuron Devices

Efforts to design devices emulating complex cognitive abilities and response processes of biological systems have long been a coveted goal. Recent advancements in flexible electronics, mirroring human tissue's mechanical properties, hold significant promise. Artificial neuron devices, hinging on flexible artificial synapses, bioinspired sensors, and actuators, are meticulously engineered to mimic the biological systems. However, this field is in its infancy, requiring substantial groundwork to achieve autonomous systems with intelligent feedback, adaptability, and tangible problem-solving capabilities. This review provides a comprehensive overview of recent advancements in artificial neuron devices. It starts with fundamental principles of artificial synaptic devices and explores artificial sensory systems, integrating artificial synapses and bioinspired sensors to replicate all five human senses. A systematic presentation of artificial nervous systems follows, designed to emulate fundamental human nervous system functions. The review also discusses potential applications and outlines existing challenges, offering insights into future prospects. We aim for this review to illuminate the burgeoning field of artificial neuron devices, inspiring further innovation in this captivating area of research.

Polyvinyl alcohol/propylene glycol facilitates reversible thermochromism of passive energy-saving flexible wood films at low (brightness) to high (depth) temperatures

Reversible thermochromism presents depth at low temperatures and brightness at high temperatures, which is not conducive to the application of the passive energy-saving and shading effect. Here, we propose an innovative strategy for unconventional reversible thermochromic energy-storage flexible wood films (FT-PCMs-2) by the contribution of polyvinyl alcohol/propylene glycol (PVA/PG). Upon heating, the FT-PCMs-2 changes from light red/green to dark red/green, in contrast to the color change of the non-flexible composites without PVA/PG. The FT-PCMs-2 has good flexibility, mechanical strength and thermal stability. Among them, MSR-DEW-6PVA and MSG-DEW-6PVA have better comprehensive properties, including suitable phase change temperatures (38.6 °C and 37.2 °C), high latent heat values (59.79 J g-1 and 73.02 J g-1), and low supercooling degrees (2 °C and 0.6 °C). FTIR, XRD and SEM analysis show that the binary fatty acid eutectics were successfully loaded into the eucalyptus fibre skeleton with physical interaction with PVA/PG. In addition, MSR-DEW-6PVA and MSG-DEW-6PVA have photothermal conversion rates of 48.3% and 36%, good cycling stability and anti-leakage performance after 50 cycles, and have promising applications in the fields of building energy saving, intelligent windows and decoration.

Flexible Piezoresistive Sensors from Polydimethylsiloxane Films with Ridge-like Surface Structures

Developing flexible sensors and actuators is of paramount importance for wearable devices and systems. In this research, we developed a simple and facile technique to construct flexible piezoresistive sensors from polydimethylsiloxane films with ridge-like surface structures and laser-induced porous graphene. Using a replication strategy, we prepared the ridge-like surface structures from sandpapers. The piezoresistive sensors exhibit excellent sensitivity with a response time of less than 50 ms and long-term cyclic stability under mechanical loading. The smallest weight they can sense is ~96 mg. We demonstrated applications of the piezoresistive sensors in the sensing of bio-related activities, including muscle contraction, finger flexion, wrist flexion, elbow bending, knee bending, swallowing, respiration, sounds, and pulses.

Carbon Nanotube Network Topology-Enhanced Iontronic Capacitive Pressure Sensor with High Linearity and Ultrahigh Sensitivity

Flexible capacitive pressure sensors with high sensitivity over a wide pressure range are highly anticipated in the fields of tactile perception and physiological signal monitoring. However, despite the introduction of microstructures on the electrolyte layer, the deformability is still limited due to the size limitation of the microstructures, making it difficult to significantly improve the sensitivity of iontronic capacitive pressure sensors (ICPSs). Here, we propose an innovative strategy of combining carbon nanotubes (CNTs) topological networks and ionic hydrogel micropyramid array microstructures that can significantly enhance the sensitivity of flexible ICPSs for ultrasensitive pressure detection. Compared with other previously reported ICPSs, the sensor developed in this work exhibits an unprecedented sensitivity (Smin > 1050 kPa-1) and a high linear response (R2 > 0.99) in a wide pressure range (0.03-28 kPa) enabled by CNT percolation networks inside the microstructred electrolyte layer. This ultrasensitive and flexible ICPS also can effectively detect pressure from a variety of sources, including sound waves, lightweight objects, and tiny physiological signals, such as pulse rate and heartbeat. This work provides a general strategy to achieve an ICPS with both broader pressure-response range and higher sensitivity, which provides a stable and efficient way for a low-cost, scalable sensor for sensitive tactile sensing in human-computer interaction applications.

A broad range and piezoresistive flexible pressure sensor based on carbon nanotube network dip-coated porous elastomer sponge

Flexible pressure sensors have provided an attractive option for potential applications in wearable fields like human motion monitoring or human-machine interfaces. For the development of flexible pressure sensors, achieving high performance or multifunctions are popular research tendencies in recent years, such as improving their sensitivity, working range, or stability. Sponge materials with porous structures have been demonstrated that they are one of the potential substrates for developing novel and excellent flexible pressure sensors. However, for sponge-based pressure sensors, it is still a great challenge to realize a wide range of pressures from Pa level to hundreds kPa level. And how to achieve mechanical robustness remains unsolved. Here, we develop a flexible pressure sensor based on multicarbon nanotubes (MWCNTs) network-coated porous elastomer sponge with a broad range and robust features for use in wearable applications. Specifically, polyurethane (PU) sponge is used as the substrate matrix while dip-coated PU/MWCNTs composites as a conductive layer, achieving a highly bonding effect between the substrate and the conductive material, hence a great mechanical robust advantage is obtained and the working range also is improved. The pressure sensor show range of up to 350 kPa, while the minimum detection threshold is as low as 150 Pa. And before and after rolling by a bicycle or electric motorcycle, the sensor has the almost same responses, exhibiting great robustness.

Piezoresistive Sensor Containing Lamellar MXene-Plant Fiber Sponge Obtained with Aqueous MXene Ink

Sustainable biomass materials are promising for low-cost wearable piezoresistive pressure sensors, but these devices are still produced with time-consuming manufacturing processes and normally display low sensitivity and poor mechanical stability at low-pressure regimes. Here, an aqueous MXene ink obtained by simply ball-milling is developed as a conductive modifier to fabricate the multiresponsive bidirectional bending actuator and compressible MXene-plant fiber sponge (MX-PFS) for durable and wearable pressure sensors. The MX-PFS is fabricated by physically foaming MXene ink and plant fibers. It possesses a lamellar porous structure composed of one-dimensional (1D) MXene-coated plant fibers and two-dimensional (2D) MXene nanosheets, which significantly improves the compression capacity and elasticity. Consequently, the encapsulated piezoresistive sensor (PRS) exhibits large compressible strain (60%), excellent mechanical durability (10 000 cycles), low detection limit (20 Pa), high sensitivity (435.06 kPa^-1), and rapid response time (40 ms) for practical wearable applications.

Wearable Pressure Sensors for Pulse Wave Monitoring

Cardiovascular diseases remain the leading cause of death worldwide. The rapid development of flexible sensing technologies and wearable pressure sensors have attracted keen research interest and have been widely used for long-term and real-time cardiovascular status monitoring. Owing to compelling characteristics, including light weight, wearing comfort, and high sensitivity to pulse pressures, physiological pulse waveforms can be precisely and continuously monitored by flexible pressure sensors for wearable health monitoring. Herein, an overview of wearable pressure sensors for human pulse wave monitoring is presented, with a focus on the transduction mechanism, microengineering structures, and related applications in pulse wave monitoring and cardiovascular condition assessment. The conceptualizations and methods for the acquisition of physiological and pathological information related to the cardiovascular system are outlined. The biomechanics of arterial pulse waves and the working mechanism of various wearable pressure sensors, including triboelectric, piezoelectric, magnetoelastic, piezoresistive, capacitive, and optical sensors, are also subject to systematic debate. Exemple applications of pulse wave measurement based on microengineering structured devices are then summarized. Finally, a discussion of the opportunities and challenges that wearable pressure sensors face, as well as their potential as a wearable intelligent system for personalized healthcare is given in conclusion.

Preparation of a Vertical Graphene-Based Pressure Sensor Using PECVD at a Low Temperature

Flexible pressure sensors have received much attention due to their widespread potential applications in electronic skins, health monitoring, and human-machine interfaces. Graphene and its derivatives hold great promise for two-dimensional sensing materials, owing to their superior properties, such as atomically thin, transparent, and flexible structure. The high performance of most graphene-based pressure piezoresistive sensors relies excessively on the preparation of complex, post-growth transfer processes. However, the majority of dielectric substrates cannot hold in high temperatures, which can induce contamination and structural defects. Herein, a credibility strategy is reported for directly growing high-quality vertical graphene (VG) on a flexible and stretchable mica paper dielectric substrate with individual interdigital electrodes in plasma-enhanced chemical vapor deposition (PECVD), which assists in inducing electric field, resulting in a flexible, touchable pressure sensor with low power consumption and portability. Benefitting from its vertically directed graphene microstructure, the graphene-based sensor shows superior properties of high sensitivity (4.84 KPa^-1) and a maximum pressure range of 120 KPa, as well as strong stability (5000 cycles), which makes it possible to detect small pulse pressure and provide options for preparation of pressure sensors in the future.

Force-Sensitive Interface Engineering in Flexible Pressure Sensors: A Review

Flexible pressure sensors have received extensive attention in recent years due to their great importance in intelligent electronic devices. In order to improve the sensing performance of flexible pressure sensors, researchers are committed to making improvements in device materials, force-sensitive interfaces, and device structures. This paper focuses on the force-sensitive interface engineering of the device, which listing the main preparation methods of various force-sensitive interface microstructures and describing their respective advantages and disadvantages from the working mechanisms and practical applications of the flexible pressure sensor. What is more, the device structures of the flexible pressure sensor are investigated with the regular and irregular force-sensitive interface and accordingly the influences of different device structures on the performance are discussed. Finally, we not only summarize diverse practical applications of the existing flexible pressure sensors controlled by the force-sensitive interface but also briefly discuss some existing problems and future prospects of how to improve the device performance through the adjustment of the force-sensitive interface.

Ti3C2Tx MXene-Based Flexible Piezoresistive Physical Sensors

MXenes have received increasing attention due to their two-dimensional layered structure, high conductivity, hydrophilicity, and large specific surface area. Because of these distinctive advantages, MXenes are considered as very competitive pressure-sensitive materials in applications of flexible piezoresistive sensors. This work reviews the preparation methods, basic properties, and assembly methods of MXenes and their recent developments in piezoresistive sensor applications. The recent developments of MXene-based flexible piezoresistive sensors can be categorized into one-dimensional fibrous, two-dimensional planar, and three-dimensional sensors according to their various structures. The trends of multifunctional integration of MXene-based pressure sensors are also summarized. Finally, we end this review by describing the opportunities and challenges for MXene-based pressure sensors and the great prospects of MXenes in the field of pressure sensor applications.

Reversible Photo-, Thermal-, and pH-Responsive Functionalized Wood with Fluorescence Emission

A reversible photo-, thermal-, and pH-responsive high-performance functional wood with fluorescence has been prepared. The properties, structure, multi-response, fluorescence, water resistance, and corrosion resistance of original wood (ORW) and functional wood (FUW) were investigated with an X-ray photoelectron spectroscopy (XPS) spectrometer, a Fourier-transform infrared (FTIR) spectrometer, a N₂ adsorption–desorption analyzer, an atomic force microscope (AFM), tensile tests, a scanning electron microscope (SEM), an ultraviolet–visible (UV–Vis) spectrophotometer, a fluorescence spectrometer, the equilibrium swelling ratio (ESR), and corrosion tests. The results of XPS, FTIR, N₂ adsorption–desorption, and AFM exhibited that FUW was successfully prepared. Additionally, the results of the tensile test and SEM revealed that FUW had better mechanical properties than ORW, due to the filling of epoxy resin in the pores of the wood. Moreover, the UV–Vis and fluorescence spectra demonstrated that the introduction of epoxy resin induced multi-response and fluorescence functions to FUW. Furthermore, the data of ESR and corrosion test showed that the introduction of epoxy resin greatly improved the water and corrosion resistance of wood. This study provides ideas and methods for preparing novel high-performance multi-response FUW.

Plant-Based Structures as an Opportunity to Engineer Optical Functions in Next-Generation Light Management

This review addresses the reconstruction of structural plant components (cellulose, lignin, and hemicelluloses) into materials displaying advanced optical properties. The strategies to isolate the main building blocks are discussed, and the effects of fibrillation, fibril alignment, densification, self-assembly, surface-patterning, and compositing are presented considering their role in engineering optical performance. Then, key elements that enable lignocellulosic to be translated into materials that present optical functionality, such as transparency, haze, reflectance, UV-blocking, luminescence, and structural colors, are described. Mapping the optical landscape that is accessible from lignocellulosics is shown as an essential step toward their utilization in smart devices. Advanced materials built from sustainable resources, including those obtained from industrial or agricultural side streams, demonstrate enormous promise in optoelectronics due to their potentially lower cost, while meeting or even exceeding current demands in performance. The requirements are summarized for the production and application of plant-based optically functional materials in different smart material applications and the review is concluded with a perspective about this active field of knowledge.

Fingerprint-Inspired Strain Sensor with Balanced Sensitivity and Strain Range Using Laser-Induced Graphene

Sensitivity and strain range are two mutually exclusive features of strain sensors, where a significant improvement in flexibility is usually accompanied by a reduction in sensitivity. The skin of a human fingertip, due to its undulating fingerprint pattern, can easily detect environmental signals and enhances sensitivity without losing elasticity. Inspired by this characteristic, laser-induced graphene (LIG) with a fingerprint structure is prepared in one step on a polyimide (PI) film and transferred into an Ecoflex substrate to assemble resistive strain sensors. Experimentally, the fingerprint-inspired strain sensor exhibits a superfast response time (∼70 ms), balanced sensitivity and strain range (a gauge factor of 191.55 in the 42-50% strain range), and good reliability (>1500 cycles). Self-organized microcracks, initiated in weak mechanical areas, cause prominent resistance changes during reconnection/disconnection but irreversibly fail after excessive stretching. The robust function of fingerprint-inspired sensors is further demonstrated by real-time monitoring of tiny pulses, large body movements, gestures, and voice recognition.

Skin-friendly corrugated multilayer microspherical sensor fabricated with silk fibroin, poly (lactic-co-glycolic acid), polyaniline, and kappa-carrageenan for wide range pressure detection

To enlarge the linear detection range without sacrificing the sensitivity is one of the urgent problems in the development of high-performance piezoresistive flexible sensors. Inspired by a multilayer corrugated board, this study develops a new multilayer microspherical sensor in which conductive core-shell SiO₂/Polyaniline (PANI) (PS) microspheres serve as active particles, while insulated silk fibroin (SF)/poly lactic-co-glycolic acid (PLGA) (SP) fibers are used as the support. The size of conductive microspheres attached to the insulated layer is controllable. The multiple layers of assembly endow the flexible sensor with a high sensitivity (0.071 kPa^-1) and a wide linear detection (from 10 Pa to 380 kPa) simultaneously. This corrugated sensor also have a fast response time (145 ms) and an excellent durability (over 2000 cycles), and it can be used to detect human joint pressure signals and transmit encrypted information. Moreover, flexible keyboard, safety protection of machinery, as well as object position tracking can be achieved based on this sensor. Most importantly, the sensor encapsulated by biological polysaccharide kappa-carrageenan (KC) is skin-friendly and breathable, and it can be decomposed in 90 °C hot water. In conclusion, this multilayer microspherical sensor presents great potential for flexible wearable devices.

Graded Microstructured Flexible Pressure Sensors with High Sensitivity and an Ultrabroad Pressure Range for Epidermal Pulse Monitoring

Precisely detecting epidermal pulse waves with pressure sensors is crucial for pulse-based personalized health-monitoring technologies. However, developing a pressure sensor that simultaneously demonstrates high sensitivity and an ultrabroad pressure range and a convenient fabrication process for large-scale production is a considerable challenge. Herein, by utilizing a commercial conductive fabric (CF) and a silica gel film, we develop a high-performance pressure sensor (HPPS) for the monitoring of human physiological signals. Based on convenient turnover formwork technology, the silica gel film was fabricated by replicating the microstructure of the sandpaper surface. This microstructure and the plain weave structure on the CF surface together provide a sharp increase in the contact-separation area and structural compressibility, which are beneficial for the enhancement of output performance. Made of these two materials, the graded microstructured HPPS holds high sensitivity (4.5 mV/Pa), an ultrabroad pressure range (0-30 kPa), a wide working frequency bandwidth (up to 35 Hz), decent stability (>50,000 cycles), and a simple fabrication process that is suitable for large-scale production. Given these noticeable features, the developed HPPS not only succeeds in precisely detecting subtle pulse waves on various positions of different people but can also objectively capture changes in cardiovascular parameters caused by exercise training at different intensities in real time. These findings exhibit the enormous potential application of HPPS in tracking an individual's health status and comprehensively evaluating exercise intensity.

Large-Scale, Cuttable, Full Tissue-Based Capacitive Pressure Sensor for the Detection of Human Physiological Signals and Pressure Distribution

The increasing demand for flexible and wearable electronics has promoted the rapid development of the pressure sensors capable of monitoring diverse human movements and physiological signals. However, more and more research requires the pressure sensor to possess high sensing performance and desires the fabrication to exhibit the characteristics of low cost, large-scale production, high reproduction, even disposability. Here, we propose a full tissue-based capacitive pressure sensor with a sandwiched structure consisting of two MXene-coated tissue electrodes and a blank tissue dielectric layer. The tight contact and adequate adsorption of the MXene sheets with the cellulose fibers endow the electrode with uniform conductivity and high stability over a large area. In addition, the flexible sensor could be conveniently cut into any shape and size to meet the diverse application requirements. Thereby, the pressure sensor exhibits a sensitivity of 0.051 kPa^-1 (<7 kPa), a wide detection range of 0.02-160 kPa, a fast response (∼100 ms), and good repeatability. The flexible device has been demonstrated to monitor a variety of human activities and physical stimuli. The assembled sensor array can accurately and reliably detect the pressure distribution.