Materials with Tunable Optical Properties for Wearable Epidermal Sensing in Health Monitoring

Photo-responsive color/fluorescence efficient dual-switching properties of diarylethene modified Eu-doped ZnO@Silane quantum dots

Photo-responsive switches are very promising materials due to their potential applications in many fields. However, photo-controlled color/fluorescence dual-switching with higher modulation efficiency has rarely been reported. Herein, two new optical responsive switches were constructed by Eu-doped ZnO@Silane quantum dots (Eu:ZnO@Silane QDs) and two diarylethenes. The two switches exhibited strong fluorescence intensity and excellent photochromic properties due to the doping of Eu and the diarylethene modification on the surface of the QDs. The switches showed outstanding photo-controlled color/fluorescence dual-switching performance with UV/Vis lights irradiation. Although the structure of the two diarylethenes affected the fatigue resistance of the two switches, their fluorescence modulation efficiency (FME) could reach 100%. Furthermore, the two switches were successfully applied to bioimaging, and provide an alternative way for the design and construction of novel efficient color/fluorescence dual-switching.

The novel nanozyme-based electrochemical-driven electrochromic visual biosensor based on PEDOT:PSS/RGO conductive film for rapid detection of nitrite in food samples

An efficient and facile nitrite (NO2-) detection system was developed using Fe3O4@Au-Cu/MOF, which was manufactured through self-assembly as the nanozyme, and a PEDOT:PSS/RGO thin film produced by chemical synthesis as the counter electrode, in conjunction with smartphone-based colorimetry. The Fe3O4@Au-Cu/MOF nanozyme exhibits remarkable catalytic efficiency and can significantly enhance the conversion of NO2-. PEDOT:PSS/RGO films exhibit outstanding electron transport and electrochromic properties. The color of PEDOT:PSS/RGO films can be modified by applying voltage and the electronic current generated by NO2- within the reaction system. The colorimetric assessment of film color alteration using a smartphone, supplemented by electrochemical validation. Under ideal conditions, the sensor detected NO2- within a linear range of 0.01 to 100 mmol/L and exhibited a detection limit of 3.37 μmol/L. This method demonstrated no significant difference compared to the results obtained using the electrochemical method and was effectively employed for the detection of NO2- in real samples.

Self-Assembled Hydroxypropyl Celluloses With Structural Colors for Biomedical Applications

Hydroxypropyl cellulose (HPC), a cellulose derivative with biocompatibility, edibility, and exceptional solubility in many polar solvents, holds significant potential for biomedical applications. Within a specific concentration range, HPC undergoes self-assembly to form cholesteric liquid crystals, which display distinct structural colors. These colors result from the interaction between incident light and the periodic nano-architecture of HPC, providing long-lasting visual effects that can be dynamically adjusted by factors such as concentration, temperature, and functional additives. This review includes the mechanisms underlying the genesis of structural colors and the regulation of HPCs while summarizing advanced techniques for fabricating HPC-based materials with diverse configurations. Furthermore, through representative examples, we highlight the multifaceted applications of these materials in sensors, bionic skins, drug delivery, and anti-counterfeiting labels. We also propose strategies to address current research and application challenges with the goal of exploring the potential of structural color HPCs for scientific breakthroughs and societal well-being. We hope this review catalyzes HPC-based structural color materials' advancement and future biomedical applications.

Wireless Organic Light-Emitting Diode Contact Lenses for On-Eye Wearable Light Sources and Their Application to Personalized Health Monitoring

On-eye optoelectronic systems can address unmet needs across various healthcare applications, including monitoring of physiological signals related to vision or other diseases. In this context, this work introduces wearable light sources that combine ultrathin organic light-emitting diodes (OLEDs) with contact lenses. As an illustration, we demonstrate their efficacy as a robust lighting solution for electroretinography (ERG). Through an in vivo experiment with a rabbit, we show that our system can generate an ERG signal comparable to that of conventional full-field light stimulation. Furthermore, we design a configuration and process flow that integrates an ultrathin OLED as well as an antenna and a controller chip for wireless power reception. It is shown that this wireless OLED contact lens system can also be used for ERG measurement with little signal interference. Considering the need for close proximity between the power source and receiver, we further envision a scenario in which patients wear a sleep mask equipped with an embedded microcontroller. Unlike conventional ERG, which requires a darkened space, our work allows patients to rest comfortably, even with their eyes closed during measurements, illustrating its potential to significantly benefit both patients and ophthalmologists and thus contributing to further advancements in on-eye digital healthcare.

A lignin-based multifunctional elastomer via facile fabrication for wearable strain sensors

Elastomer-based electronics have the potential to maintain both electrochemical conductivity and mechanical properties simultaneously, paving the way for the development of stretchable sensors. However, creating sensors that are extremely stretchable, wearable, and highly sensitive remains a significant challenge. In this work, we present a novel fully bio-based multifunctional polymer (FPL/PTA) designed using thioctic acid (TA) and formaldehyde-protected lignin (FPL). The FPL serves as both a cross-linking agent and a free radical quenching agent. The FPL/PTA features an adaptive polymer network cross-linked by dynamic covalent disulfide bonds and enhanced by multiple non-covalent interactions. This unique structure imparts the material with stretchability, self-healing properties, hydrophobicity, swelling resistance, self-adhesion, and fully recyclable, degradable characteristics. In addition, the simple preparation route, multiple functions, and green characteristics give the elastomer and choline chloride combination as a strain sensor. The sensor exhibits exceptional sensitivity to strain, repeatability, and durability, enabling it to monitor a wide range of human movements, including joint articulation, speech, and swallowing. We believe that the overall performance and feasibility of manufacturing the developed FPL/PTA marks it as a promising multifunctional strain sensor for applications in flexible wearable electronic devices and humanoid robots.

Future of magnetic sensors applications in early prediction of cardiac health status

The evolution of health monitoring technologies has highlighted the need for accurate and reliable sensors, particularly in the context of cardiac health. This review examines the potential of magnetic sensors as a superior alternative to optical sensors for the early prediction of cardiac health status. Optical sensors face significant challenges, especially for individuals with darker skin tones, where increased light absorption adversely affects measurement accuracy. Additionally, issues such as sensor-skin coupling and motion artifacts further compromise the performance of optical devices. In contrast, magnetic sensors offer a compelling solution by providing consistent readings irrespective of skin tone, thereby enhancing inclusivity in health monitoring. These sensors leverage magnetic fields, which do not rely on light penetration, allowing for improved coupling with the skin's surface and maintaining accuracy during motion. This paper discusses recent advancements in magnetic sensor technology and their implications for cardiac health applications, emphasizing the potential for increased accuracy and reliability in predicting cardiac outcomes. As healthcare shifts toward more personalized and precise monitoring solutions, magnetic sensors emerge as a promising frontier, addressing critical challenges in current health status prediction methods. By focusing on these innovative technologies, we aim to contribute to the ongoing discourse on enhancing cardiac health monitoring and fostering more equitable healthcare solutions.

Highly sensitive and multifunctional Fe3+ enhanced PVA/gelatin multi-network hydrogels with wide temperature range environmental stability for wearable sensors

Hydrogels are one of the ideal materials for preparing new flexible sensor devices. In this work, based on the freeze-thaw cycle and Hoffmeister effect to improve the mechanical properties, a low-temperature resistant multifunctional conductive hydrogel with excellent mechanical properties, high transparency, high adhesion, high conductivity, high sensitivity, excellent sensing ability and self-healing ability was prepared by diffusing Fe3+ and ethylene glycol (EG) in the hydrogel network via immersion method. Moreover, wearable sensors assembled from it could accurately monitor the movement of various parts of the human body, and it could accurately identify tiny strain and pressure sensing. In addition, the hydrogel had long-term stability (after 7 days at room temperature, it still maintained 88.03 % of its original weight), strong adhesion (after repeated adhesion for 3 times, the adhesion to 6 materials was more than 72 % of the original value), excellent frost resistance (not freeze under -50 °C), and good self-repairing ability (the cut hydrogel would be self-repaired after 5 min and could be stretched to more than 2 times the original length). The hydrogel's multifunctionality and high sensitivity to small strains will make it show a high application prospect in the field of precision sensing.

High-Conductivity, Self-Healing, and Adhesive Ionic Hydrogels for Health Monitoring and Human-Machine Interactions Under Extreme Cold Conditions

Ionic conductive hydrogels (ICHs) are emerging as key materials for advanced human-machine interactions and health monitoring systems due to their unique combination of flexibility, biocompatibility, and electrical conductivity. However, a major challenge remains in developing ICHs that simultaneously exhibit high ionic conductivity, self-healing, and strong adhesion, particularly under extreme low-temperature conditions. In this study, a novel ICH composed of sulfobetaine methacrylate, methacrylic acid, TEMPO-oxidized cellulose nanofibers, sodium alginate, and lithium chloride is presented. The hydrogel is designed with a hydrogen-bonded and chemically crosslinked network, achieving excellent conductivity (0.49 ± 0.05 S m-1), adhesion (36.73 ± 2.28 kPa), and self-healing capacity even at -80 °C. Furthermore, the ICHs maintain functionality for over 45 days, showcasing outstanding anti-freezing properties. This material demonstrates significant potential for non-invasive, continuous health monitoring, adhering conformally to the skin without signal crosstalk, and enabling real-time, high-fidelity signal transmission in human-machine interactions under cryogenic conditions. These ICHs offer transformative potential for the next generation of multimodal sensors, broadening application possibilities in harsh environments, including extreme weather and outer space.

Smart Bacterial Cellulose-Methylacrylated Chitosan Composite Hydrogel: Multifunctional Characterization for Real-Time pH Monitoring

pH is a critical parameter that influences biochemical and environmental processes. Real-time and accurate pH detection is essential for monitoring health and the environment. Herein, a bacterial cellulose and methylacrylated chitosan (BC-MACS) composite hydrogel was prepared to achieve rapid pH detection. The integration of MACS reduced the crystallinity of pristine BC, with no adverse effects on thermal stability. SEM images validated the fibrous nature of the BC-MACS composite, indicating that MACS was successfully infiltrated into the pores of BC. By incorporating MACS into the BC matrix, the exceptional biocompatibility of BC was maintained, while simultaneously augmenting its mechanical properties. Due to the excellent swelling ability of MACS, the fabricated BC-MACS hydrogel exhibited superior swelling behavior compared to the BC hydrogel, which facilitated the absorption of the solution under test. A BC-MACS pH sensor was fabricated by introducing the pH indicator solution, and the color variation across the pH range (2-12) demonstrated a clear response to pH changes. Therefore, the BC-MACS pH sensor holds potential for use as a visual indicator in a diverse range of applications, especially for health and environmental monitoring.

Anisotropic cellulose nanofibril piezoionic organohydrogel fabricated by directional freezing for flexible strain sensors

Flexible self-powered strain sensors based on piezoionic hydrogels demonstrate great promise for wearable devices, e-skins, and health monitoring. However, existing piezoionic hydrogels typically suffer from limited piezoionic output performance, falling short of the demands of high-performance, self-powered wearable sensors. This study addresses this challenge by constructing directional ion transport channels using a cellulose nanofibril (CNF)-assisted directional freezing technique. CNF acts as nanofillers that enhance ion selectivity in the channels while improving the mechanical properties of polyvinyl alcohol (PVA)/CNF organohydrogels, enabling efficient directional ion transport with K3[Fe(CN)]6 as an ion source. The resulting organohydrogel exhibits high extensibility (479.61 %), ultimate stress (3.63 MPa), and transparency (90 %). Under a pressure of 5 N, it generates an output voltage of up to 199.45 mV within 150 ms, demonstrating a strong linear relationship between stress and output voltage in the range of 2-5 N (R2 = 0.99). After directional freezing, the output voltage and current increased by 2.49 and 1.94 times, respectively. Additionally, the selective enhancement of ion mobility by CNF has improved the piezoionic output, resulting in a 2.02-fold increase in piezoionic voltage. These high-sensitivity piezoionic organohydrogels, with their efficient electromechanical conversion capabilities, indicate a promising future for flexible strain sensors.

Microgels sense wounds' temperature, pH and glucose

Wound healing concerns almost all bed-side related diseases. With our increasing comprehension of healing nature, the physical and chemical natures behind the wound microenvironment have been decoupled. Wound care demands timely screening and prompt diagnosis of wound complications such as infection and inflammation. Biosensor by the way of exhaustive collection, delivery, and analysis of data, becomes indispensable to arrive at an ideal healing upshot and controlling complications by capturing in-situ wound status. Electrochemical based sensors carry some potential unstable performance subjected to the electrical circuitry and power access and contamination. The colorimetric sensors are free from those concerns. We report that microsensors designed from O/W/O of capillary fluids can continuously monitor wound temperature, pH and glucose concentration. We combined three different types of microgels to encapsulate liquid crystals of cholesterol, nontoxic fuel litmus and two glucose-sensitizing enzymes. A smartphone applet was then developed to convert wound healing images to RGB of digitalizing data. The microgel dressing effectively demonstrates the local temperature change, pH and glucose levels of the wound in high resolution where a microgel is a 'pixel'. They are highly responsive, reversible and accurate. Monitoring multiple physicochemical and physiological indicators provides tremendous potential with insight into healing processing.

Dynamically mechanochromic, fluorescence-responsive, and underwater sensing cellulose nanocrystal-based conductive elastomers

Utilizing cellulose nanocrystals (CNCs) to mimic biological skin capable of converting external stimuli into optical and electrical signals represents a significant advancement in the development of advanced photonic materials. However, traditional CNC photonic materials typically exhibit static and singular optical properties, with their structural color and mechanical performance being susceptible to water molecules, thereby limiting their practical applications. In this study, CNC-based conductive elastomers with dynamic mechanochromism, fluorescence responsiveness, and enhanced water resistance were developed by incorporating carbon quantum dots (C QDs) and hydrophobic deep eutectic solvents (HDES) into CNC photonic films via an in-situ swelling-photopolymerization method. Additionally, the abundant non-covalent and hydrophobic interactions within poly(HDES) endow the resulting poly(HDES)/C-CNC elastomer with excellent structural stability, ionic conductivity, and self-adhesive properties, allowing for encrypted information transmission in both air and aquatic environments. The design and application of poly(HDES)/C-CNC elastomers hold tremendous potential for various applications, including anti-counterfeiting labels, human-machine interactions, and wearable devices.

Biomaterials for reliable wearable health monitoring: Applications in skin and eye integration

Recent advancements in biomaterials have significantly impacted wearable health monitoring, creating opportunities for personalized and non-invasive health assessments. These developments address the growing demand for customized healthcare solutions. Durability is a critical factor for biomaterials in wearable applications, as they must withstand diverse wearing conditions effectively. Therefore, there is a heightened focus on developing biomaterials that maintain robust and stable functionalities, essential for advancing wearable sensing technologies. This review examines the biomaterials used in wearable sensors, specifically those interfaced with human skin and eyes, highlighting essential strategies for achieving long-lasting and stable performance. We specifically discuss three main categories of biomaterials-hydrogels, fibers, and hybrid materials-each offering distinct properties ideal for use in durable wearable health monitoring systems. Moreover, we delve into the latest advancements in biomaterial-based sensors, which hold the potential to facilitate early disease detection, preventative interventions, and tailored healthcare approaches. We also address ongoing challenges and suggest future directions for research on material-based wearable sensors to encourage continuous innovation in this dynamic field.

Advances in Photonic Materials and Integrated Devices for Smart and Digital Healthcare: Bridging the Gap Between Materials and Systems

Recent advances in developing photonic technologies using various materials offer enhanced biosensing, therapeutic intervention, and non-invasive imaging in healthcare. Here, this article summarizes significant technological advancements in materials, photonic devices, and bio-interfaced systems, which demonstrate successful applications for impacting human healthcare via improved therapies, advanced diagnostics, and on-skin health monitoring. The details of required materials, necessary properties, and device configurations are described for next-generation healthcare systems, followed by an explanation of the working principles of light-based therapeutics and diagnostics. Next, this paper shares the recent examples of integrated photonic systems focusing on translation and immediate applications for clinical studies. In addition, the limitations of existing materials and devices and future directions for smart photonic systems are discussed. Collectively, this review article summarizes the recent focus and trends of technological advancements in developing new nanomaterials, light delivery methods, system designs, mechanical structures, material functionalization, and integrated photonic systems to advance human healthcare and digital healthcare.

Photonic Nanomaterials for Wearable Health Solutions

This review underscores the transformative potential of photonic nanomaterials in wearable health technologies, driven by increasing demands for personalized health monitoring. Their unique optical and physical properties enable rapid, precise, and sensitive real-time monitoring, outperforming conventional electrical-based sensors. Integrated into ultra-thin, flexible, and stretchable formats, these materials enhance compatibility with the human body, enabling prolonged wear, improved efficiency, and reduced power consumption. A comprehensive exploration is provided of the integration of photonic nanomaterials into wearable devices, addressing material selection, light-matter interaction principles, and device assembly strategies. The review highlights critical elements such as device form factors, sensing modalities, and power and data communication, with representative examples in skin patches and contact lenses. These devices enable precise monitoring and management of biomarkers of diseases or biological responses. Furthermore, advancements in materials and integration approaches have paved the way for continuum of care systems combining multifunctional sensors with therapeutic drug delivery mechanisms. To overcome existing barriers, this review outlines strategies of material design, device engineering, system integration, and machine learning to inspire innovation and accelerate the adoption of photonic nanomaterials for next-generation of wearable health, showcasing their versatility and transformative potential for digital health applications.

Advanced Design for Stimuli-Reversible Chromic Wearables With Customizable Functionalities

Smart wearable devices with dynamically reversible color displays are crucial for the next generation of smart textiles, and promising for bio-robots, adaptive camouflage, and visual health monitoring. The rapid advancement of technology brings out different categories that feature fundamentally different color-reversing mechanisms, including thermochromic, mechanochromic, electrochromic, and photochromic smart wearables. Although some reviews have showcased relevant developments from unique perspectives, reviews focusing on the advanced design of flexible chromic wearable devices within each category have not been reported. In this review, the development history and recent progress in smart chromic wearables across each category are systematically examined. The design strategies for each chromic wearable device are outlined with a focus on functional materials, synthesis processes, and advanced applications. Furthermore, integrated devices based on dual-stimuli and multi-stimuli responsive chromics with customizable functionalities are summarized. Finally, challenges and perspectives on the future development of smart chromic wearables are proposed. Such a systematic summary will serve as a valuable insight for researchers in this field.

A Streamlined Approach to Anticounterfeiting Technologies: Patterned AAO Membranes Based on Photonic Crystal Effects with Tunable Color Shifts and pH Responsiveness

Anticounterfeiting technologies have become increasingly crucial due to the growing issue of counterfeit goods, particularly in high-value industries. Traditional methods such as barcodes and holograms are prone to replication, prompting the need for advanced, cost-effective, and efficient solutions. In this work, a practical application of anodic aluminum oxide (AAO) membranes are presented for anticounterfeiting, which addresses the challenges of high production costs and complex fabrication processes. Unlike previous approaches requiring metal coatings for color generation, this method uses commercial aluminum foils to produce colorful AAO membranes without metal layers. Elemental mapping suggests that impurities on the aluminum surface contribute to enhanced reflectivity, aiding photonic crystal formation. A two-step anodization process that creates patterned AAO membranes is further introduced, with the pattern clarity controlled by anodization time. Additionally, a pH-responsive film composed of 2-anilino-6-dibutylaminofluoran (ODB-2) and thermoplastic polyurethane (TPU) is integrated, enabling dynamic color changes under varying pH conditions, further enhancing the anticounterfeiting functionality. This streamlined approach provides a scalable and cost-effective solution for developing versatile AAO membranes for industrial anticounterfeiting applications.

Fundamentals and Advances in Stimuli-Responsive Hydrogels and Their Applications: A Review

This review summarizes the fundamental concepts, recent advancements, and emerging trends in the field of stimuli-responsive hydrogels. While numerous reviews exist on this topic, the field continues to evolve dynamically, and certain research directions are often overlooked. To address this, we classify stimuli-responsive hydrogels based on their response mechanisms and provide an in-depth discussion of key properties and mechanisms, including swelling kinetics, mechanical properties, and biocompatibility/biodegradability. We then explore hydrogel design, synthesis, and structural engineering, followed by an overview of applications that are relatively well established from a scientific perspective, including biomedical uses (biosensing, drug delivery, wound healing, and tissue engineering), environmental applications (heavy metal and phosphate removal from the environment and polluted water), and soft robotics and actuation. Additionally, we highlight emerging and unconventional applications such as local micro-thermometers and cell mechanotransduction. This review concludes with a discussion of current challenges and future prospects in the field, aiming to inspire further innovations and advancements in stimuli-responsive hydrogel research and applications to bring them closer to the societal needs.

Recent Advances in Wearable Sweat Sensor Development

Wearable sweat sensors for detecting biochemical markers have emerged as a transformative research area, with the potential to revolutionize disease diagnosis and human health monitoring. Since 2016, a substantial body of pioneering and translational work on sweat biochemical sensors has been reported. This review aims to provide a comprehensive summary of the current state-of-the-art in the field, offering insights and perspectives on future developments. The focus is on wearable microfluidic platforms for sweat collection and delivery and the analytical chemistry applicable to wearable devices. Various microfluidic technologies, including those based on synthetic polymers, paper, textiles, and hydrogels, are discussed alongside diverse detection methods such as electrochemistry and colorimetry. Both the advantages and current limitations of these technologies are critically examined. The review concludes with our perspectives on the future of wearable sweat sensors, with the goal of inspiring new ideas, innovations, and technical advancements to further the development and practical application of these devices in promoting human health.

Advances in Cellulose-Based Hydrogels: Current Trends and Challenges

This paper provides a solid foundation for understanding the synthesis, properties, and applications of cellulose-based gels. It effectively showcases the potential of these gels in diverse applications, particularly in biomedicine, and highlights key synthesis methods and properties. However, to push the field forward, future research should address the gaps in understanding the environmental impact, mechanical stability, and scalability of cellulose-based gels, while also considering how to overcome barriers to their industrial use. This will ultimately allow for the realization of cellulose-based gels in large-scale, sustainable applications.

Liquid Metal-Enabled Epidermal Interfaces

Soft materials are crucial for epidermal interfaces in biomedical devices due to their capability to conform to the body compared to rigid inorganic materials. Gels, liquids, and polymers have been extensively explored, but they lack sufficient electrical and thermal conductivity required for many application settings. Gallium-based alloys are molten metals at room temperature with exceptional electrical and thermal conductivity. These liquid metals and their composites can be directly applied onto the skin as interface materials. In this Spotlight on Applications, we focus on the rapidly evolving field of liquid metal-enabled epidermal interfaces featuring unique physical properties beyond traditional gels and polymers. We delve into the role of liquid metal in electrical and thermal biointerfaces in various epidermal applications. Current challenges and future directions in this active area are also discussed.

Flexible high electrochemical active hydrogel for wearable sensors and supercapacitor electrolytes

With the rapid advancement of flexible, portable devices, hydrogel electrolytes have gained considerable attention as potential replacements for conventional liquid electrolytes. A hydrogel electrolyte was synthesised by cross-linking acrylic acid (AA), acrylamide (AM), carboxymethyl cellulose (CMC), and zinc sulphate (ZnSO4). The formation of hydrogen bonds and chelate interactions between the P(AA-co-AM) polymer, CMC, and ZnSO4 created a robust network, enhancing the mechanical properties of the hydrogel electrolytes. Notably, the hydrogel electrolyte containing 0.6 % CMC demonstrated superior mechanical strength (compression strength of 1.22 MPa, tensile stress of 230 kPa, tensile strain of 424 %, adhesion strength of 1.98 MPa on wood). Additionally, the CMC/P(AA-co-AM) hydrogels exhibited commendable electrical performance (38 mS/cm) and a high gauge factor (2.9), enabling the precise detection of physiological activity signals through resistance measurements. The unique network structure of the hydrogel electrolyte also ensured a stable bonding interface between the electrode and the electrolyte. After 2000 charge-discharge cycles, the supercapacitor maintained good capacitance characteristics, with a capacitance retention rate of 71.21 % and a stable Coulombic efficiency of 98.85 %, demonstrating excellent cyclic stability. This study introduces a novel methodology for fabricating multifunctional all-solid-state supercapacitors and suggests that the hydrogel can significantly advance the development of wearable energy storage devices.

Fabrication of Flexible Wearable Mechanosensors Utilizing Piezoelectric Hydrogels Mechanically Enhanced by Dipole-Dipole Interactions

Conductive hydrogels have been increasingly employed to construct wearable mechanosensors due to their excellent mechanical flexibility close to that of soft tissues. In this work, piezoelectric hydrogels are prepared through free radical copolymerization of acrylamide (AM) and acrylonitrile (AN) and further utilized in assembling flexible wearable mechanosensors. Introduction of the polyacrylonitrile (PAN) component in the copolymers endows the hydrogels with excellent piezoelectric properties. Meanwhile, significant enhancement of mechanical properties has been accessed by forming dipole-dipole interactions, which results in a tensile strength of 0.51 MPa. Flexible wearable mechanosensors are fabricated by utilizing piezoelectric hydrogels as key signal converting materials. Self-powered piezoelectric pressure sensors are assembled with a sensitivity (S) of 0.2 V kPa-1. Additionally, resistive strain sensors (gauge factor (GF): 0.84, strain range: 0-250%) and capacitive pressure sensors (S: 0.23 kPa-1, pressure range: 0-8 kPa) are fabricated by utilizing such hydrogels. These flexible wearable mechanosensors can monitor diverse body movements such as joint bending, walking, running, and stair climbing. This work is anticipated to offer promising soft materials for efficient mechanical-to-electrical signal conversion and provides new insights into the development of various wearable mechanosensors.

Preparation and Stability Study of an Injectable Hydrogel for Artificial Intraocular Lenses

Currently available intraocular lenses (IOLs) on the market often differ significantly in elastic modulus compared to the natural human lens, which impairs their ability to respond effectively to the tension of the ciliary muscles for focal adjustment after implantation. In this study, we synthesized a polyacrylamide-sodium acrylate hydrogel (PAH) through the cross-linking polymerization of acrylamide and sodium acrylate. This hydrogel possesses excellent biocompatibility and exhibits several favorable properties. Notably, the hydrogel demonstrates high transparency (94%) and a refractive index (1.41 ± 0.07) that closely matches that of the human lens (1.42). Additionally, it shows strong compressive strength (14.00 kPa), good extensibility (1400%), and an appropriate swelling ratio (50 ± 2.5%). Crucially, the tensile modulus of the hydrogel is 2.07 kPa, which closely aligns with the elastic modulus of the human lens (1.70-2.10 kPa), enabling continuous focal adjustment under the tension exerted by the ciliary muscles.

A Cost-Effective and Easy-to-Fabricate Conductive Velcro Dry Electrode for Durable and High-Performance Biopotential Acquisition

Compared with the traditional gel electrode, the dry electrode is being taken more seriously in bioelectrical recording because of its easy preparation, long-lasting ability, and reusability. However, the commonly used dry AgCl electrodes and silver cloth electrodes are generally hard to record through hair due to their flat contact surface. Claw electrodes can contact skin through hair on the head and body, but the internal claw structure is relatively hard and causes discomfort after being worn for a few hours. Here, we report a conductive Velcro electrode (CVE) with an elastic hook hair structure, which can collect biopotential through body hair. The elastic hooks greatly reduce discomfort after long-time wearing and can even be worn all day. The CVE electrode is fabricated by one-step immersion in conductive silver paste based on the cost-effective commercial Velcro, forming a uniform and durable conductive coating on a cluster of hook microstructures. The electrode shows excellent properties, including low impedance (15.88 kΩ @ 10 Hz), high signal-to-noise ratio (16.0 dB), strong water resistance, and mechanical resistance. After washing in laundry detergent, the impedance of CVE is still 16% lower than the commercial AgCl electrodes. To verify the mechanical strength and recovery capability, we conducted cyclic compression experiments. The results show that the displacement change of the electrode hook hair after 50 compression cycles was still less than 1%. This electrode provides a universal acquisition scheme, including effective acquisition of different parts of the body with or without hair. Finally, the gesture recognition from electromyography (EMG) by the CVE electrode was applied with accuracy above 90%. The CVE proposed in this study has great potential and promise in various human-machine interface (HMI) applications that employ surface biopotential signals on the body or head with hair.

Eye of the future: Unlocking the potential utilization of hydrogels in intraocular lenses

Hydrogels are distinguished by their exceptional ability to absorb and retain large volumes of water within their complex three-dimensional polymer networks, which is advantageous for the development of intraocular lenses (IOLs). Their innate hydrophilicity offers an optimal substrate for the fabrication of IOLs that simulate the natural lens' accommodation, thereby reducing irritation and facilitating healing after surgery. The swelling and water retention characteristics of hydrogels contribute to their notable biocompatibility and versatile mechanical properties. However, the clinical application of hydrogels faces challenges, including managing potential adverse postimplantation effects. Rigorous research is essential to ascertain the safety and effectiveness of hydrogels. This review systematically examines the prospects and constraints of hydrogels as innovative materials for IOLs. Our comprehensive analysis examines their inherent properties, various classification strategies, cross-linking processes, and sensitivity to external stimuli. Additionally, we thoroughly evaluate their interactions with ocular tissues, underscoring the potential for hydrogels to be refined into seamless and biologically integrated visual aids. We also discuss the anticipated technological progress and clinical uses of hydrogels in IOL manufacturing. With ongoing technological advancements, the promise of hydrogels is poised to evolve from concept to clinical reality, marking a significant leap forward in ophthalmology characterized by improved patient comfort, enhanced functionality, and reliable safety.

Wearable smart contact lenses: A critical comparison of three physiological signals outputs for health monitoring

Smart contact lenses (SCLs) have been considered as novel wearable devices for out-of-hospital and self-monitoring applications. They are capable of non-invasively and continuously monitoring physiological signals in the eyes, including vital biophysical (e.g., intraocular of pressure, temperature, and electrophysiological signal) and biochemical signals (e.g., pH, glucose, protein, nitrite, lactic acid, and ions). Recent progress mainly focuses on the rational design of wearable SCLs for physiological signal monitoring, while also facilitating the treatment of various ocular diseases. It covers contact lens materials, fabrication technologies, and integration methods. We also highlight and discuss a critical comparison of SCLs with electrical, microfluidic, and optical signal outputs in health monitoring. Their advantages and disadvantages could help researchers to make decisions when developing SCLs with desired properties for physiological signal monitoring. These unique capabilities make SCLs promising diagnostic and therapeutic tools. Despite the extensive research in SCLs, new technologies are still in their early stages of development and there are a few challenges to be addressed before these SCLs technologies can be successfully commercialized particularly in the form of rigorous clinical trials.

Colorimetric sensing for translational applications: from colorants to mechanisms

Colorimetric sensing offers instant reporting via visible signals. Versus labor-intensive and instrument-dependent detection methods, colorimetric sensors present advantages including short acquisition time, high throughput screening, low cost, portability, and a user-friendly approach. These advantages have driven substantial growth in colorimetric sensors, particularly in point-of-care (POC) diagnostics. Rapid progress in nanotechnology, materials science, microfluidics technology, biomarker discovery, digital technology, and signal pattern analysis has led to a variety of colorimetric reagents and detection mechanisms, which are fundamental to advance colorimetric sensing applications. This review first summarizes the basic components (e.g., color reagents, recognition interactions, and sampling procedures) in the design of a colorimetric sensing system. It then presents the rationale design and typical examples of POC devices, e.g., lateral flow devices, microfluidic paper-based analytical devices, and wearable sensing devices. Two highlighted colorimetric formats are discussed: combinational and activatable systems based on the sensor-array and lock-and-key mechanisms, respectively. Case discussions in colorimetric assays are organized by the analyte identities. Finally, the review presents challenges and perspectives for the design and development of colorimetric detection schemes as well as applications. The goal of this review is to provide a foundational resource for developing colorimetric systems and underscoring the colorants and mechanisms that facilitate the continuing evolution of POC sensors.

Full-Color "Off-On" Thermochromic Fluorescent Fibers for Customizable Smart Wearable Displays in Personal Health Monitoring

Responsive thermochromic fiber materials capable of miniaturization and integrating comfortably and compliantly onto the soft and dynamically deforming human body are promising materials for visualized personal health monitoring. However, their development is hindered by monotonous colors, low-contrast color changes, and poor reversibility. Herein, full-color "off-on" thermochromic fluorescent fibers are prepared based on self-crystallinity phase change and Förster resonance energy transfer for long-term and passive body-temperature monitoring, especially for various personalized customization purposes. The off-on switching luminescence characteristic is derived from the reversible conversion of the dispersion state and fluorescent emission by fluorophores and quencher molecules, which are embedded in the matrix of a phase-change material, during the crystallizing/melting processes. The achievement of full-color fluorescence is attributed to the large modulation range of fluorescence colors according to primary color additive theory. These thermochromic fluorescent fibers exhibit good mechanical properties, fluorescent emission contrast, and reversibility, showing their great potential in flexible smart display devices. Moreover, the response temperature of the thermochromic fibers is controllable by adjusting the phase-change material, enabling body-temperature-triggered luminescence; this property highlights their potential for human body-temperature monitoring and personalized customization. This work presents a new strategy for designing and exploring flexible sensors with higher comprehensive performances.

Multi-role conductive hydrogels for flexible transducers regulated by MOFs for monitoring human activities and electronic skin functions

Metal organic frameworks (MOFs) have garnered significant attention in the development of stretchable and wearable conductive hydrogels for flexible transducers. However, MOFs used in hydrogel networks have been hampered by low mechanical performance and poor dispersibility in aqueous solutions, which affect the performance of hydrogels, including low toughness, limited self-recovery, short working ranges, low conductivity, and prolonged response-recovery times. To address these shortcomings, a novel approach was adopted in which micelle co-polymerization was used for the ex situ synthesis of Zn-MOF-based hydrogels with exceptional stretchability, robust toughness, anti-fatigue properties, and commendable conductivity. This breakthrough involved the ex situ integration of Zn-MOFs into hydrophobically cross-linked polymer chains. Here the micelles of EHDDAB had two functions, first they uniformly dispersed the Zn-MOFs and secondly they dynamically cross-linked the polymer chains, profoundly influencing the mechanical characteristics of the hydrogels. The non-covalent synergistic interactions introduced by Zn-MOFs endowed the hydrogels with the capacity for high stretchability, high stress, rapid self-recovery, anti-fatigue properties, and conductivity, all achieved without external stimuli. Furthermore, hydrogels based on Zn-MOFs can serve as durable and highly sensitive flexible transducers, adept at detecting diverse mechanical deformations with swift response-recovery times and high gauge factor values. Consequently, these hydrogels can be tailored to function as wearable strain sensors capable of sensing significant human joint movements, such as wrist bending, and motions involving the wrist, fingers, and elbows. Similarly, they excel at monitoring subtle human motions, such as speech pronunciation, distinguishing between different words, as well as detecting swallowing and larynx vibrations during various activities. Beyond these applications, the hydrogels exhibit proficiency in distinguishing and reproducing various written words with reliability. The Zn-MOF-based hydrogels hold promising potential for development in electronic skin, medical monitoring, soft robotics, and flexible touch panels.

Smart Contact Lens for Colorimetric Visualization of Glucose Levels in the Body Fluid

Frequent blood glucose monitoring is a crucial routine for diabetic patients. Traditional invasive methods can cause discomfort and pain and even pose a risk of infection. As a result, researchers have been exploring noninvasive techniques. However, a limited number of products have been developed for the market due to their high cost. In this study, we developed a low-cost, highly accessible, and noninvasive contact lens-based glucose monitoring system. We functionalized the surface of the contact lens with boronic acid, which has a strong but reversible binding affinity to glucose. To achieve facile conjugation of boronic acid, we utilized a functional coating layer called poly(tannic acid). The functionalized contact lens binds to glucose in body fluids (e.g., tear) and releases it when soaked in an enzymatic cocktail, allowing for the glucose level to be quantified through a colorimetric assay. Importantly, the transparency and oxygen permeability of the contact lens, which are crucial for practical use, were maintained after functionalization, and the lenses showed high biocompatibility. Based on the analysis of colorimetric data generated by the smartphone application and ultraviolet-visible (UV-vis) spectra, we believe that this contact lens has a high potential to be used as a smart diagnostic tool for monitoring and managing blood glucose levels.

Hydrochromic and piezochromic dual-responsive optical film derived from poloxamer and ethyl cellulose for visual fingerprints identification

Developing new materials that could identify fingerprint using the naked eye and observe the level 3 microscopic details is challenging. Here, we designed a novel hydrochromic and piezochromic dual-responsive optical film, which achieved the visual transparency transition. The performances of hydrochromic and piezochromic responses from high transparency to opaque whiteness were attributed to the introduction of poloxamer. The hygroscopic swelling of the disordered micelles led to light scattering, causing the hydrochromic response. The piezochromic response may be ascribed to the microcracks in the fragments of poloxamer crystals, which changed the refractive index of light. The fascinating combination of hydrochromic and piezochromic response was effectively applied in fingerprint identification. Hydrochromic response accurately recognized sweat pores, and piezochromic response could gradually reveal the ridges and valleys according to the different color of imprinted fingerprints. The film could identify fake fingerprints based on the differences in sweat pores between fake fingerprints and living fingers. More importantly, the film could easily detected not only the clear ridges but also the detailed sweat pores using the naked eye, indicating that the film has profound research significance in fingerprint analysis and liveness fingerprint detection.

Membrane-anchoring selenophene viologens for antibacterial photodynamic therapy against periodontitis via restoring subgingival flora and alleviating inflammation

Antibacterial photodynamic therapy (aPDT) has emerged as a promising strategy for treating periodontitis. However, the weak binding of most photosensitizers to bacteria and the hypoxic environment of periodontal pockets severely hamper the therapeutic efficacy. Herein, two novel oxygen-independent photosensitizers are developed by introducing selenophene into viologens and modifying with hexane chains (HASeV) or quaternary ammonium chains (QASeV), which improve the adsorption to bacteria through anchoring to the negatively charged cell membrane. Notably, QASeV binds only to the bacterial surface of Porphyromonas gingivalis and Fusobacterium nucleatum due to electrostatic binding, but HASeV can insert into their membrane by strong hydrophobic interactions. Therefore, HASeV exhibits superior antimicrobial activity and more pronounced plaque biofilm disruption than QASeV when combined with light irradiation (MVL-210 photoreactor, 350-600 nm, 50 mW/cm2), and a better effect on reducing the diversity and restoring the structure of subgingival flora in periodontitis rat model was found through 16S rRNA gene sequencing analysis. The histological and Micro-CT analyses reveal that HASeV-based aPDT has a better therapeutic effect in reducing periodontal tissue inflammation and alveolar bone resorption. This work provides a new strategy for the development of viologen-based photosensitizers, which may be a favorable candidate for the aPDT against periodontitis.

Rapid Fabrication of Large-Grain Opal Films and Photonic Crystal Hydrogel Sensors by a Filter Paper-Enhanced Evaporation Chip

Developing a rapid fabrication method for crack-free opal films is a significant challenge with broad applications. We developed a microfluidic platform known as the "filter paper-enhanced evaporation microfluidic chip" (FPEE-chip) for the fabrication of photonic crystal and inverse opal hydrogel (IOPH) films. The chip featured a thin channel formed by bonding double-sided adhesive poly(ethylene terephthalate) with a polymethyl methacrylate cover and a glass substrate. This channel was then filled with nanosphere colloids. The water was guided to evaporate rapidly at the surface of the filter paper, allowing the nanospheres to self-assemble and accumulate within the channel under capillary forces. Experimental results confirmed that the self-assembly method based on the FPEE-chip was a rapid platform for producing high-quality opal, with centimeter-sized opal films achievable in less than an hour. Furthermore, the filter paper altered the stress release mechanism of the opal films during drying, resulting in fewer cracks. This platform was proven capable of producing large-grain, crack-free opal films of up to 30 mm2 in size. We also fabricated crack-free IOPH pH sensors that exhibited color and size responsiveness to pH changes. The coefficient of variation of the gray color distribution for crack-free IOPH ranged from 0.03 to 0.07, which was lower than that of cracked IOPH (ranging from 0.07 to 0.14). Additionally, the grayscale peak value in 1 mm2 of the crack-free IOPH was more than twice that of the cracked IOPH at the same pH. The FPEE-chip demonstrated potential as a candidate for developing vision sensors.

Spongy Ag Foam for Soft and Stretchable Strain Gauges

Strain gauges, particularly for wearable sensing applications, require a high degree of stretchability, softness, sensitivity, selectivity, and linearity. They must also steer clear of challenges such as mechanical and electrical hysteresis, overshoot behavior, and slow response/recovery times. However, current strain gauges face challenges in satisfying all of these requirements at once due to the inevitable trade-offs between these properties. Here, we present an innovative method for creating strain gauges from spongy Ag foam through a steam-etching process. This method simplifies the traditional, more complex, and costly manufacturing techniques, presenting an eco-friendly alternative. Uniquely, the strain gauges crafted from this method achieve an unparalleled gauge factor greater than 8 × 103 at strains exceeding 100%, successfully meeting all required attributes without notable trade-offs. Our work includes systematic investigations that reveal the intricate structure-property-performance relationship of the spongy Ag foam with practical demonstrations in areas such as human motion monitoring and human-robot interaction. These breakthroughs pave the way for highly sensitive and selective strain gauges, showing immediate applicability across a wide range of wearable sensing applications.

Thermally Conductive and UV-EMI Shielding Electronic Textiles for Unrestricted and Multifaceted Health Monitoring

Skin-attachable electronics have garnered considerable research attention in health monitoring and artificial intelligence domains, whereas susceptibility to electromagnetic interference (EMI), heat accumulation issues, and ultraviolet (UV)-induced aging problems pose significant constraints on their potential applications. Here, an ultra-elastic, highly breathable, and thermal-comfortable epidermal sensor with exceptional UV-EMI shielding performance and remarkable thermal conductivity is developed for high-fidelity monitoring of multiple human electrophysiological signals. Via filling the elastomeric microfibers with thermally conductive boron nitride nanoparticles and bridging the insulating fiber interfaces by plating Ag nanoparticles (NPs), an interwoven thermal conducting fiber network (0.72 W m-1 K-1) is constructed benefiting from the seamless thermal interfaces, facilitating unimpeded heat dissipation for comfort skin wearing. More excitingly, the elastomeric fiber substrates simultaneously achieve outstanding UV protection (UPF = 143.1) and EMI shielding (SET > 65, X-band) capabilities owing to the high electrical conductivity and surface plasmon resonance of Ag NPs. Furthermore, an electronic textile prepared by printing liquid metal on the UV-EMI shielding and thermally conductive nonwoven textile is finally utilized as an advanced epidermal sensor, which succeeds in monitoring different electrophysiological signals under vigorous electromagnetic interference. This research paves the way for developing protective and environmentally adaptive epidermal electronics for next-generation health regulation.

Electro-optic tuning in composite silicon photonics based on ferroionic 2D materials

Tunable optical materials are indispensable elements in modern optoelectronics, especially in integrated photonics circuits where precise control over the effective refractive index is essential for diverse applications. Two-dimensional materials like transition metal dichalcogenides (TMDs) and graphene exhibit remarkable optical responses to external stimuli. However, achieving distinctive modulation across short-wave infrared (SWIR) regions while enabling precise phase control at low signal loss within a compact footprint remains an ongoing challenge. In this work, we unveil the robust electro-refractive response of multilayer ferroionic two-dimensional CuCrP2S6 (CCPS) in the near-infrared wavelength range. By integrating CCPS into silicon photonics (SiPh) microring resonators (MRR), we enhance light-matter interaction and measurement sensitivity to minute phase and absorption variations. Results show that electrically driven Cu ions can tune the effective refractive index on the order of 2.8 × 10-3 RIU (refractive index unit) while preserving extinction ratios and resonance linewidth. Notably, these devices exhibit low optical losses and excellent modulation efficiency of 0.25 V.cm with a consistent blue shift in the resonance wavelengths among all devices for either polarity of the applied voltage. These results outperform earlier findings on phase shifters based on TMDs. Furthermore, our study demonstrates distinct variations in electro-optic tuning sensitivity when comparing transverse electric (TE) and transverse magnetic (TM) modes, revealing a polarization-dependent response that paves the way for diverse applications in light manipulation. The combined optoelectronic and ionotronic capabilities of two-terminal CCPS devices present extensive opportunities across several domains. Their potential applications range from phased arrays and optical switching to their use in environmental sensing and metrology, optical imaging systems, and neuromorphic systems in light-sensitive artificial synapses.

Contact-separation-induced self-recoverable mechanoluminescence of CaF2:Tb3+/PDMS elastomer

Centrosymmetric-oxide/polydimethylsiloxane elastomers emit ultra-strong non-pre-irradiation mechanoluminescence under stress and are considered one of the most ideal mechanoluminescence materials. However, previous centrosymmetric-oxide/polydimethylsiloxane elastomers show severe mechanoluminescence degradation under stretching, which limits their use in applications. Here we show an elastomer based on centrosymmetric fluoride CaF2:Tb3+ and polydimethylsiloxane, with mechanoluminescence that can self-recover after each stretching. Experimentation indicates that the self-recoverable mechanoluminescence of the CaF2:Tb3+/polydimethylsiloxane elastomer occurs essentially due to contact electrification arising from contact-separation interactions between the centrosymmetric phosphors and the polydimethylsiloxane. Accordingly, a contact-separation cycle model of the phosphor-polydimethylsiloxane couple is established, and first-principles calculations are performed to model state energies in the contact-separation cycle. The results reveal that the fluoride-polydimethylsiloxane couple helps to induce contact electrification and maintain the contact-separation cycle at the interface, resulting in the self-recoverable mechanoluminescence of the CaF2:Tb3+/polydimethylsiloxane elastomer. Therefore, it would be a good strategy to develop self-recoverable mechanoluminescence elastomers based on centrosymmetric fluoride phosphors and polydimethylsiloxane.

Flexible Accelerated-Wound-Healing Antibacterial Hydrogel-Nanofiber Scaffold for Intelligent Wearable Health Monitoring

Flexible epidermal sensors hold significant potential in personalized healthcare and multifunctional electronic skins. Nonetheless, achieving both robust sensing performance and efficient antibacterial protection, especially in medical paradigms involving electrophysiological signals for wound healing and intelligent health monitoring, remains a substantial challenge. Herein, we introduce a novel flexible accelerated-wound-healing biomaterial based on a hydrogel-nanofiber scaffold (HNFS) via electrostatic spinning and gel cross-linking. We effectively engineer a multifunctional tissue nanoengineered skin scaffold for wound treatment and health monitoring. Key features of HNFS include high tensile strength (24.06 MPa) and elasticity (214.67%), flexibility, biodegradability, and antibacterial properties, enabling assembly into versatile sensors for monitoring human motion and electrophysiological signals. Moreover, in vitro and in vivo experiments demonstrate that HNFS significantly enhances cell proliferation and skin wound healing, provide a comprehensive therapeutic strategy for smart sensing and tissue repair, and guide the development of high-performance "wound healing-health monitoring" bioelectronic skin scaffolds. Therefore, this study provides insights into crafting flexible and repairable skin sensors, holding potential for multifunctional health diagnostics and intelligent medical applications in intelligent wearable health monitoring and next-generation artificial skin fields.

Bionic Luminescent Skin as Ultrasensitive Temperature-Acoustic Sensor for Underwater Information Perception and Transmission

Bioinspired artificial luminescent skin (L-skin) integrated with multiple sensing functions significantly promotes the development of smart devices. It is considerably challenging to realize underwater sensing technologies. Here, a sharkskin-inspired Eu@HOF-TJ-1@TA L-skin (1) is prepared for both temperature and sound sensing. 1 is an ultrathin and flexible temperature sensor, in 298.15-358.15 K, exhibiting ultrahigh maximum relative sensitivity (97.669% K-1 ) and low minimum uncertainty (0.000 952 K). The temperature response mechanism is analyzed deeply. As a waterproofing acoustic sensor, 1 can monitor sound in both air and water with the greatest sound response frequencies of 400 and 300 Hz in air and water, respectively. The maximum sensitivities of 1 in air and water are 6 593 765.2 and 1 346 124.5 cps Pa-1 , respectively. The response times of 1 in air and water are as fast as 20 and 10 ms. The sound response processes of 1 in air and water are simulated by finite element simulation. Moreover, by using sharkskin-inspired 1, the actual water temperature can be monitored, and a series of water sound information can be recognized by using an artificial neural network. This work proposes a sharkskin-inspired L-skin for temperature and acoustic sensing and promotes the development of underwater sensing technology with high performances.

A Wearable Co-Located Neural-Mechanical Signal Sensing Device for Simultaneous Bimodal Muscular Activity Detection

The co-located and concurrent measurement of both muscular neural activity and muscular deformation is considered necessary in many applications, such as medical robotics, assistive exoskeletons and muscle function evaluations. Nevertheless, conventional muscle-related signal perception systems either detect only one of these modalities, or are made with rigid and bulky components that cannot provide conformal and flexible interface. Herein, a flexible, easy-to-fabricate, bimodal muscular activity sensing device, which collects neural and mechanical signal at the same muscle location, is reported. The sensing patch includes a screen-printed sEMG sensor, and a pressure-based muscular deformation sensor (PMD sensor) based on a highly sensitive, co-planar iontronic pressure sensing unit. Both sensors are integrated on a super-thin (25 μm) substrate. The sEMG sensor shows a high signal-to-noise ratio of 37.1 dB, and the PMD sensor sensor exhibits a high sensitivity of 70.9 kPa -1. The responses of the sensor to three types of muscle activities (isotonic, isometric, and passive stretching) were analyzed and validated by ultrasound imaging. Bimodal signals during dynamic walking experiments with different level-ground walking speeds were also investigated. The application of the bimodal sensor was verified in gait phase estimation, and results show that the assembly of both modalities significantly reduce (p < 0.05) the average estimation error across all subjects and all walking speeds to 3.82%. Demonstrations show the potential of this sensing device for informative evaluation of muscular activities, and its abilities in human-robot interaction.

Metal-organic gels: recent advances in their classification, characterization, and application in the pharmaceutical field

Metal-organic gels (MOGs) are a type of functional soft substance with a three-dimensional (3D) network structure and solid-like rheological behavior, which are constructed by metal ions and bridging ligands formed under the driving force of coordination interactions or other non-covalent interactions. As the homologous substances of metal-organic frameworks (MOFs) and gels, they exhibit the potential advantages of high porosity, flexible structure, and adjustable mechanical properties, causing them to attract extensive research interest in the pharmaceutical field. For instance, MOGs are often used as excellent vehicles for intelligent drug delivery and programmable drug release to improve the clinical curative effect with reduced side effects. Also, MOGs are often applied as advanced biomedical materials for the repair and treatment of pathological tissue and sensitive detection of drugs or other molecules. However, despite the vigorous research on MOGs in recent years, there is no systematic summary of their applications in the pharmaceutical field to date. The present review systematically summarize the recent research progress on MOGs in the pharmaceutical field, including drug delivery systems, drug detection, pharmaceutical materials, and disease therapies. In addition, the formation principles and classification of MOGs are complemented and refined, and the techniques for the characterization of the structures/properties of MOGs are overviewed in this review.

A highly sensitive flexible capacitive pressure sensor with hierarchical pyramid micro-structured PDMS-based dielectric layer for health monitoring

Herein, a flexible pressure sensor with high sensitivity was created using a dielectric layer featuring a hierarchical pyramid microstructure, both in simulation and fabrication. The capacitive pressure sensor comprises a hierarchically arranged dielectric layer made of polydimethylsiloxane (PDMS) with pyramid microstructures, positioned between copper electrodes at the top and bottom. The achievement of superior sensing performance is highly contingent upon the thickness of the dielectric layer, as indicated by both empirical findings and finite-element analysis. Specifically, the capacitive pressure sensor, featuring a dielectric layer thickness of 0.5 mm, exhibits a remarkable sensitivity of 0.77 kPa-1 within the pressure range below 1 kPa. It also demonstrates an impressive response time of 55 ms and recovery time of 42 ms, along with a low detection limit of 8 Pa. Furthermore, this sensor showcases exceptional stability and reproducibility with up to 1,000 cycles. Considering its exceptional achievements, the pressure sensor has been effectively utilized for monitoring physiological signals, sign language gestures, and vertical mechanical force exerted on objects. Additionally, a 5 × 5 sensor array was fabricated to accurately and precisely map the shape and position of objects. The pressure sensor with advanced performance shows broad potential in electronic skin applications.

Smart Wearable Nanopaper Patch for Continuous Multiplexed Optical Monitoring of Sweat Parameters

Notwithstanding the substantial progress in optical wearable sensing devices, developing wearable optical sensors for simultaneous, real-time, and continuous monitoring of multiple biomarkers is still an important, yet unmet, demand. Aiming to address this need, we introduced for the first time a smart wearable optical sensor (SWOS) platform combining a multiplexed sweat sensor sticker with its IoT-enabled readout module. We employed our SWOS system for on-body continuous, real-time, and simultaneous fluorimetric monitoring of sweat volume (physical parameter) and pH (chemical marker). Herein, a variation in moisture (5-45 μL) or pH (4.0-7.0) causes a color/fluorescence change in the copper chloride/fluorescein immobilized within a transparent chitin nanopaper (ChNP) in a selective and reversible manner. Human experiments conducted on athletic volunteers during exercise confirm that our developed SWOS platform can be efficiently exploited for smart perspiration analysis toward personalized health monitoring. Moreover, our system can be further extended for the continuous and real-time multiplexed monitoring of various biomarkers (metabolites, proteins, or drugs) of sweat or other biofluids (for example, analyzing exhaled breath by integrating onto a facemask).

Nacre-Inspired MXene Nanocomposite-based Strain Sensor with Ultrahigh Sensitivity in a Small Strain Range for Parkinson's Disease Diagnosis

Nowadays, chronic diseases are the primary threat to public health and are getting younger. By taking the advantages of continuousness, convenience, and real-time response, wearable strain sensors have been given great attention to diagnose chronic diseases via analyzing the patient's health state. However, most physiological signals, such as limb tremor of Parkinson's disease, microexpression, and slight joint movement, are tiny and difficult to be detected. Therefore, the development of strain sensors characterized with ultrahigh sensitivity in a small strain range (ε < 10%) is urgent. Inspired by nacre's hierarchical structure, we have fabricated nacre-mimetic nanocomposites with "brick-and-mortar" architecture by employing polyacrylamide (PAM) and Ti3C2Tx MXene nanosheets through a layer-by-layer (LBL) spin-coating technique. The resultant nanocomposite-based strain sensor exhibits ultrahigh sensitivity in a small strain range (GF = 296.8, ε < 10%), attributed to the bioinspired hierarchical structure and hydrogen bond-enhanced interfacial interactions. In addition, a high reliability, broad working sensing range (453%), short response time (183 ms), skin-like tensile stress (7.2 MPa), and excellent durability (2000 cycles) are also achieved. Due to the ultrahigh sensitivity within a small strain, the reported strain sensor can accurately diagnose and distinguish Parkinson's disease symptoms, including thumb pill-rolling tremor, masked face (microexpression), intermittent shaking of the head, and limb cogwheel motion. This work provides new insights to design strain sensors with high sensitivity for monitoring tiny signals and for disease diagnosis.

Remarkable sol-gel transition of PNIPAm-based nanogels via large steric hindrance of side-chains

While the block/graft/branched structures are widely studied to favor the reversible physical gelation, there are no reports regarding the steric hindrance-induced sol-gel transition of PNIPAm-based nanogels above their phase transition temperature (Tp). Generally, the introduction of hydrophobic components into poly (N-isopropylacrylamide) (PNIPAm)-based nanogels only led to collapse and lower viscosity instead of the sol-gel transition upon heating above the Tp. Herein, the results of temperature-variable 1HNMR and FTIR confirm that the introduction of hydrophobic N-tert-butylacrylamide (TBA) with the large steric hindrance of side groups of N-tert-butyl to form NIPAm/TBA copolymer nanogels can dramatically slow down the dehydration of all the hydrophobic alkyl groups, thus resulting in the formation of thermally induced sol-gel transition above the Tp. Furthermore, the N-acrylamido-L-phenylalanine (APhe) monomer composed of a strongly water absorbing carboxyl group and a phenyl group with larger steric hindrance is studied to form P(NIPAm/TBA/APhe) terpolymer nanogels which can self-assemble into colorful colloidal crystals. Surprisingly, owing to the synergistic effect between the water absorbing carboxyl group and the steric hindrance group on the same side group, these colloidal crystals can achieve sol-gel transition above Tp, forming a physically crosslinked colorful hydrogel. This work is expected to greatly advance the design, synthesis, and application of the sol-gel transition of PNIPAm-based nanogel systems.

Soft microfiber-based hollow microneedle array for stretchable microfluidic biosensing patch with negative pressure-driven sampling

Wearable point-of-care testing devices are essential for personalized and decentralized healthcare. They can collect biofluid samples from the human body and use an analyzer to detect biomolecules. However, creating an integrated system is challenging due to the difficulty of achieving conformality to the human body, regulating the collection and transport of biofluids, developing a biosensor patch capable of precise biomolecule detection, and establishing a simple operation protocol that requires minimal wearer attention. In this study, we propose using a hollow microneedle (HMN) based on soft hollow microfibers and a microneedle-integrated microfluidic biosensor patch (MIMBP) capable of integrated blood sampling and electrochemical biosensing of biomolecules. The soft MIMBP includes a stretchable microfluidic device, a flexible electrochemical biosensor, and a HMN array made from flexible hollow microfibers. The HMNs are fabricated by electroplating flexible and mechanically durable hollow microfibers made from a nanocomposite matrix of polyimide, a poly (vinylidene fluoride-co-trifluoroethylene) copolymer, and single-walled carbon nanotubes. The MIMBP uses the negative pressure generated by a single button push to collect blood and deliver it to a flexible electrochemical biosensor modified with a gold nanostructure and Pt nanoparticles. We have demonstrated that glucose can be accurately measured up to the molar range in whole human blood collected through the microneedle. The MIMBP platform with HMNs has great potential as a foundation for the future development of simple, wearable, self-testing systems for minimally invasive biomolecule detection. This platform capable of sequential blood collection and high sensitivity glucose detection, which are ideal for personalized and decentralized healthcare.

Biomaterial-Based Biomimetic Visual Sensors: Inkjet Patterning of Bacteriorhodopsin

Biomimetic visual sensors utilizing bacteriorhodopsin (bR) were fabricated by using an inkjet method. The inkjet printer facilitated the jetting of the bR suspension, allowing for the deposition of bR films. The resulting inkjet-printed bR film exhibited time-differential photocurrent response characteristics similar to those of a dip-coated bR film. By adjusting the number of printed bR film layers, the intensity of the photocurrent could be easily controlled. Moreover, the inkjet printing technique enabled unconstrained patterning, facilitating the design of various visual information processing functions, such as visual filters. In this study, we successfully fabricated two visual filters, namely, a two-dimensional Difference of Gaussian (DOG) filter and a Gabor filter. The printed DOG filter demonstrated edge detection capabilities corresponding to contour recognition in visual receptive fields. On the other hand, the printed Gabor filter proved effective in detecting objects of specific sizes as well as their motion and orientation. The integration of bR and the inkjet method holds significant potential for the widespread implementation of highly functional biomaterial-based visual sensors. These sensors have the capability to provide real-time visual information while operating in an energy-efficient manner.

Multifunctional Ionic Conductive Anisotropic Elastomers with Self-Wrinkling Microstructures by In Situ Phase Separation

Multifunctional flexible sensors are the development trend of wearable electronic devices in the future. As the core of flexible sensors, the key is to construct a stable multifunctional integrated conductive elastomer. Here, ionic conductive elastomers (ICEs) with self-wrinkling microstructures are designed and prepared by in situ phase separation induced by a one-step polymerization reaction. The ICEs are composed of ionic liquids as ionic conductors doped into liquid crystal elastomers. The doped ionic liquids cluster into small droplets and in situ induce the formation of wrinkle structures on the upper surface of the films. The prepared ICEs exhibit mechanochromism, conductivity, large tensile strain, low hysteresis, high cycle stability, and sensitivity during the tension-release process, which achieve dual-mode outputs of optical and electrical signals for information transmission and sensors.

Spray-On Colorimetric Sensors for Distinguishing the Presence of Lead Ions

Sprayable stimuli-responsive material coatings represent a new class of detection system which can be quickly implemented to transform a surface into a color-responsive sensor. In this work, we describe a dipicolylamine-terminated diacetylene-containing amphiphile formulation for spray coating on to a simple paper substrate to yield disposable test strips that can be used to detect the presence of lead ions in solution. We find the response to be very selective to only lead ions and that the sensitivity can be modulated by altering the UV curing time after spraying. Sensitive detection to at least 0.1 mM revealed a clear color change from a blue to red phase. This represents the first demonstration of a spray-on sensor system capable of detection of lead ions in solution.