Soft Wearable Healthcare Materials and Devices

Wireless breathable face mask sensor for spatiotemporal 2D respiration profiling and respiratory diagnosis

Owing to air pollution and the pandemic outbreak, the need for quantitative pulmonary monitoring has greatly increased. The COVID-19 outbreak has aroused attention for comfortable wireless monitoring of respiratory profiles and more real-time diagnosis of respiratory diseases. Although respiration sensors have been investigated extensively with single-pixel sensors, 2D respiration profiling with a pixelated array sensor has not been demonstrated for both exhaling and inhaling. Since the pixelated array sensor allowed for simultaneous profiling of the nasal breathing and oral breathing, it provides essential respiratory information such as breathing patterns, respiration habit, breathing disorders. In this study, we introduced an air-permeable, stretchable, and a pixelated pressure sensor that can be integrated into a commercial face mask. The mask sensor showed a strain-independent pressure-sensing performance, providing 2D pressure profiles for exhalation and inhalation. Real-time 2D respiration profiles could monitor various respiratory behaviors, such as oral/nasal breathing, clogged nose, out-of-breath, and coughing. Furthermore, they could detect respiratory diseases, such as rhinitis, sleep apnea, and pneumonia. The 2D respiratory profiling mask sensor is expected to be employed for remote respiration monitoring and timely patient treatment.

Evolution of Musculoskeletal Electronics

The musculoskeletal system, constituting the largest human physiological system, plays a critical role in providing structural support to the body, facilitating intricate movements, and safeguarding internal organs. By virtue of advancements in revolutionized materials and devices, particularly in the realms of motion capture, health monitoring, and postoperative rehabilitation, "musculoskeletal electronics" has actually emerged as an infancy area, but has not yet been explicitly proposed. In this review, the concept of musculoskeletal electronics is elucidated, and the evolution history, representative progress, and key strategies of the involved materials and state-of-the-art devices are summarized. Therefore, the fundamentals of musculoskeletal electronics and key functionality categories are introduced. Subsequently, recent advances in musculoskeletal electronics are presented from the perspectives of "in vitro" to "in vivo" signal detection, interactive modulation, and therapeutic interventions for healing and recovery. Additionally, nine strategy avenues for the development of advanced musculoskeletal electronic materials and devices are proposed. Finally, concise summaries and perspectives are proposed to highlight the directions that deserve focused attention in this booming field.

Low-impedance tissue-device interface using homogeneously conductive hydrogels chemically bonded to stretchable bioelectronics

Stretchable bioelectronics has notably contributed to the advancement of continuous health monitoring and point-of-care type health care. However, microscale nonconformal contact and locally dehydrated interface limit performance, especially in dynamic environments. Therefore, hydrogels can be a promising interfacial material for the stretchable bioelectronics due to their unique advantages including tissue-like softness, water-rich property, and biocompatibility. However, there are still practical challenges in terms of their electrical performance, material homogeneity, and monolithic integration with stretchable devices. Here, we report the synthesis of a homogeneously conductive polyacrylamide hydrogel with an exceptionally low impedance (~21 ohms) and a reasonably high conductivity (~24 S/cm) by incorporating polyaniline-decorated poly(3,4-ethylenedioxythiophene:polystyrene). We also establish robust adhesion (interfacial toughness: ~296.7 J/m2) and reliable integration between the conductive hydrogel and the stretchable device through on-device polymerization as well as covalent and hydrogen bonding. These strategies enable the fabrication of a stretchable multichannel sensor array for the high-quality on-skin impedance and pH measurements under in vitro and in vivo circumstances.

Rapid Self-Healing Hydrogel with Ultralow Electrical Hysteresis for Wearable Sensing

Self-healing hydrogels are in high demand for wearable sensing applications due to their remarkable deformability, high ionic and electrical conductivity, self-adhesiveness to human skin, as well as resilience to both mechanical and electrical damage. However, these hydrogels face challenges such as delayed healing times and unavoidable electrical hysteresis, which limit their practical effectiveness. Here, we introduce a self-healing hydrogel that exhibits exceptionally rapid healing with a recovery time of less than 0.12 s and an ultralow electrical hysteresis of less than 0.64% under cyclic strains of up to 500%. This hydrogel strikes an ideal balance, without notable trade-offs, between properties such as softness, deformability, ionic and electrical conductivity, self-adhesiveness, response and recovery times, durability, overshoot behavior, and resistance to nonaxial deformations such as twisting, bending, and pressing. Owing to this unique combination of features, the hydrogel is highly suitable for long-term, durable use in wearable sensing applications, including monitoring body movements and electrophysiological activities on the skin.

Advancing spinal cord injury care through non-invasive autonomic dysreflexia detection with AI

This paper presents an AI-powered solution for detecting and monitoring Autonomic Dysreflexia (AD) in individuals with spinal cord injuries. Current AD detection methods are limited, lacking non-invasive monitoring systems. We propose a model that combines skin nerve activity (SKNA) signals with a deep neural network (DNN) architecture to overcome this limitation. The DNN is trained on a meticulously curated dataset obtained through controlled colorectal distension, inducing AD events in rats with spinal cord surgery above the T6 level. The proposed system achieves an impressive average classification accuracy of 93.9% ± 2.5%, ensuring accurate AD identification with high precision (95.2% ± 2.1%). It demonstrates a balanced performance with an average F1 score of 94.4% ± 1.8%, indicating a harmonious balance between precision and recall. Additionally, the system exhibits a low average false-negative rate of 4.8% ± 1.6%, minimizing the misclassification of non-AD cases. The robustness and generalizability of the system are validated on unseen data, maintaining high accuracy, F1 score, and a low false-negative rate. This AI-powered solution represents a significant advancement in non-invasive, real-time AD monitoring, with the potential to improve patient outcomes and enhance AD management in individuals with spinal cord injuries. This research contributes a promising solution to the critical healthcare challenge of AD detection and monitoring.

Materials-Driven Soft Wearable Bioelectronics for Connected Healthcare

In the era of Internet-of-things, many things can stay connected; however, biological systems, including those necessary for human health, remain unable to stay connected to the global Internet due to the lack of soft conformal biosensors. The fundamental challenge lies in the fact that electronics and biology are distinct and incompatible, as they are based on different materials via different functioning principles. In particular, the human body is soft and curvilinear, yet electronics are typically rigid and planar. Recent advances in materials and materials design have generated tremendous opportunities to design soft wearable bioelectronics, which may bridge the gap, enabling the ultimate dream of connected healthcare for anyone, anytime, and anywhere. We begin with a review of the historical development of healthcare, indicating the significant trend of connected healthcare. This is followed by the focal point of discussion about new materials and materials design, particularly low-dimensional nanomaterials. We summarize material types and their attributes for designing soft bioelectronic sensors; we also cover their synthesis and fabrication methods, including top-down, bottom-up, and their combined approaches. Next, we discuss the wearable energy challenges and progress made to date. In addition to front-end wearable devices, we also describe back-end machine learning algorithms, artificial intelligence, telecommunication, and software. Afterward, we describe the integration of soft wearable bioelectronic systems which have been applied in various testbeds in real-world settings, including laboratories that are preclinical and clinical environments. Finally, we narrate the remaining challenges and opportunities in conjunction with our perspectives.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

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.

Field effect transistor based wearable biosensors for healthcare monitoring

The rapid advancement of wearable biosensors has revolutionized healthcare monitoring by screening in a non-invasive and continuous manner. Among various sensing techniques, field-effect transistor (FET)-based wearable biosensors attract increasing attention due to their advantages such as label-free detection, fast response, easy operation, and capability of integration. This review explores the innovative developments and applications of FET-based wearable biosensors for healthcare monitoring. Beginning with an introduction to the significance of wearable biosensors, the paper gives an overview of structural and operational principles of FETs, providing insights into their diverse classifications. Next, the paper discusses the fabrication methods, semiconductor surface modification techniques and gate surface functionalization strategies. This background lays the foundation for exploring specific FET-based biosensor designs, including enzyme, antibody and nanobody, aptamer, as well as ion-sensitive membrane sensors. Subsequently, the paper investigates the incorporation of FET-based biosensors in monitoring biomarkers present in physiological fluids such as sweat, tears, saliva, and skin interstitial fluid (ISF). Finally, we address challenges, technical issues, and opportunities related to FET-based biosensor applications. This comprehensive review underscores the transformative potential of FET-based wearable biosensors in healthcare monitoring. By offering a multidimensional perspective on device design, fabrication, functionalization and applications, this paper aims to serve as a valuable resource for researchers in the field of biosensing technology and personalized healthcare.

Anisotropic Piezoresistive Sensors Made with Magnetically Induced Vertically Aligned Carbon Nanotubes/Polydimethylsiloxane

A piezoresistive material consisting of internal vertically aligned carbon nanotubes acting in concert with an external microdome structure is prepared to obtain a flexible piezoresistive sensor with high anisotropy. Here, we first obtained flexible piezoresistive composites (VCP) with anisotropic properties by inducing the vertical alignment of multiwalled carbon nanotubes in the pressure direction under a weak magnetic field of 0.6 T. Then, the composite with a microdome structure on the surface (m-VCP) was fabricated by a mold with a microstructure to further increase the anisotropy of the composite. The m-VCP microstructure was docked with VCP and placed between two layers of copper foil. With the synergistic effect of vertically aligned carbon nanotubes and the microdome structure, the sensitivity of the flexible sensor in the pressure direction was dramatically increased. In the low-strain range (0-6%), the sensitivity of m-VCP (GF = 9.208) is improved by 49% compared to m-CP and by 86% compared to VCP. The sensor has high anisotropy in the piezoresistive direction and retains good fatigue resistance under fatigue testing for 2000 cycles. This means that the sensor can be used in emerging fields such as human health monitoring, wearable electronics, and intelligent human-computer interaction.

Ultrathin Self-Powered Heavy-Metal-Free Cu-In-Se Quantum Dot Photodetectors for Wearable Health Monitoring

Mechanically deformable photodetectors (PDs) are key device components for wearable health monitoring systems based on photoplethysmography (PPG). Achieving high detectivity, fast response time, and an ultrathin form factor in the PD is highly needed for next-generation wearable PPG systems. Self-powered operation without a bulky power-supply unit is also beneficial for point-of-care application. Here, we propose ultrathin self-powered PDs using heavy-metal-free Cu-In-Se quantum dots (QDs), which enable high-performance wearable PPG systems. Although the light-absorbing QD layer is extremely thin (∼40 nm), the developed PD exhibits excellent performance (specific detectivity: 2.10 × 1012 Jones, linear dynamic range: 102 dB, and spectral range: 250-1050 nm at zero bias), which is comparable to that of conventional rigid QD-PDs employing thick Pb-chalcogenide QD layers. This is attributed to material and device strategies─materials that include Cu-In-Se QDs, a MoS2-nanosheet-blended poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer, a ZnO nanoparticle electron transport layer, Ag and ITO electrodes, and an ultrathin form factor (∼120 nm except the electrodes) that enable excellent mechanical deformability. These allow the successful application of QD-PDs to a wearable system for real-time PPG monitoring, expanding their potential in the field of mobile bioelectronics.

Health Monitoring via Heart, Breath, and Korotkoff Sounds by Wearable Piezoelectret Patches

Real-time monitoring of vital sounds from cardiovascular and respiratory systems via wearable devices together with modern data analysis schemes have the potential to reveal a variety of health conditions. Here, a flexible piezoelectret sensing system is developed to examine audio physiological signals in an unobtrusive manner, including heart, Korotkoff, and breath sounds. A customized electromagnetic shielding structure is designed for precision and high-fidelity measurements and several unique physiological sound patterns related to clinical applications are collected and analyzed. At the left chest location for the heart sounds, the S1 and S2 segments related to cardiac systole and diastole conditions, respectively, are successfully extracted and analyzed with good consistency from those of a commercial medical device. At the upper arm location, recorded Korotkoff sounds are used to characterize the systolic and diastolic blood pressure without a doctor or prior calibration. An Omron blood pressure monitor is used to validate these results. The breath sound detections from the lung/ trachea region are achieved a signal-to-noise ration comparable to those of a medical recorder, BIOPAC, with pattern classification capabilities for the diagnosis of viable respiratory diseases. Finally, a 6×6 sensor array is used to record heart sounds at different locations of the chest area simultaneously, including the Aortic, Pulmonic, Erb's point, Tricuspid, and Mitral regions in the form of mixed data resulting from the physiological activities of four heart valves. These signals are then separated by the independent component analysis algorithm and individual heart sound components from specific heart valves can reveal their instantaneous behaviors for the accurate diagnosis of heart diseases. The combination of these demonstrations illustrate a new class of wearable healthcare detection system for potentially advanced diagnostic schemes.

Gout therapeutics and drug delivery

Gout is a common inflammatory arthritis caused by persistently elevated uric acid levels. With the improvement of people's living standards, the consumption of processed food and the widespread use of drugs that induce elevated uric acid, gout rates are increasing, seriously affecting the human quality of life, and becoming a burden to health systems worldwide. Since the pathological mechanism of gout has been elucidated, there are relatively effective drug treatments in clinical practice. However, due to (bio)pharmaceutical shortcomings of these drugs, such as poor chemical stability and limited ability to target the pathophysiological pathways, traditional drug treatment strategies show low efficacy and safety. In this scenario, drug delivery systems (DDS) design that overcome these drawbacks is urgently called for. In this review, we initially describe the pathological features, the therapeutic targets, and the drugs currently in clinical use and under investigation to treat gout. We also comprehensively summarize recent research efforts utilizing lipid, polymeric and inorganic carriers to develop advanced DDS for improved gout management and therapy.

Soft Bioelectronics for Therapeutics

Soft bioelectronics play an increasingly crucial role in high-precision therapeutics due to their softness, biocompatibility, clinical accuracy, long-term stability, and patient-friendliness. In this review, we provide a comprehensive overview of the latest representative therapeutic applications of advanced soft bioelectronics, ranging from wearable therapeutics for skin wounds, diabetes, ophthalmic diseases, muscle disorders, and other diseases to implantable therapeutics against complex diseases, such as cardiac arrhythmias, cancer, neurological diseases, and others. We also highlight key challenges and opportunities for future clinical translation and commercialization of soft therapeutic bioelectronics toward personalized medicine.

Wearable sensor platforms for real-time monitoring and early warning of metabolic disorders in humans

Nowadays, the prevalence of metabolic syndromes (MSs) has attracted increasing concerns as it is closely related to overweight and obesity, physical inactivity and overconsumption of energy, making the diagnosis and real-time monitoring of the physiological range essential and necessary for avoiding illness due to defects in the human body such as higher risk of cardiovascular disease, diabetes, stroke and diseases related to artery walls. However, the current sensing techniques are inconvenient and do not continuously monitor the health status of humans. Alternatively, the use of recent wearable device technology is a preferable method for the prevention of these diseases. This can enable the monitoring of the health status of humans in different health domains, including environment and structure. The use wearable devices with the purpose of facilitating rapid treatment and real-time monitoring can decrease the prevalence of MS and long-time monitor the health status of patients. This review highlights the recent advances in wearable sensors toward continuous monitoring of blood pressure and blood glucose, and further details the monitoring of abnormal obesity, triglycerides and HDL. We also discuss the challenges and future prospective of monitoring MS in humans.

Flexible and Stretchable Electrochemical Sensors for Biological Monitoring

The rise of flexible and stretchable electronics has revolutionized biosensor techniques for probing biological systems. Particularly, flexible and stretchable electrochemical sensors (FSECSs) enable the in situ quantification of numerous biochemical molecules in different biological entities owing to their exceptional sensitivity, fast response, and easy miniaturization. Over the past decade, the fabrication and application of FSECSs have significantly progressed. This review highlights key developments in electrode fabrication and FSECSs functionalization. It delves into the electrochemical sensing of various biomarkers, including metabolites, electrolytes, signaling molecules, and neurotransmitters from biological systems, encompassing the outer epidermis, tissues/organs in vitro and in vivo, and living cells. Finally, considering electrode preparation and biological applications, current challenges and future opportunities for FSECSs are discussed.

Simultaneous detection of ultraviolet irradiation and vitamin C using an all-carbon-based integrated wearable system powering by a micro-supercapacitor

Ultraviolet (UV) radiation is a harmful exogenous factor for human skin. Wearable UV photodetectors can monitor UV exposure in the surroundings, and wearable vitamin C (V_C) sensors tracking the levels in the human body present the potential ability to defend the UV radiation. Herein, we reported on the fabrication of an all-in-one wearable system with a UV photodetector and V_C sensor powered by a micro-supercapacitor. Based on direct laser writing carbonization of polyimide sheets, the patterned electrodes and interconnects of the circuit were fabricated by a facile one-step operation, obtaining an all-carbon-based integrated system. Such a system exhibited outstanding energy storage ability (56.2 μWh cm^-2 at 4.17 mW cm^-2), high areal capacitance (1.06 mF cm^-2 at 5 mV s^-1), satisfying capacitive stability, and good mechanical flexibility. The UV photodetector and the V_C sensor were powered to obtain a linear range of UV intensity from 11 to 44 μW cm^-2 (equivalent to Ultraviolet Index 4.4 to 17.6), and V_C levels of 1.0-200 μM with a low limit of detection of 0.83 μM. Furthermore, the integrated system was successfully applied to the determination of V_C in commercial beverage and human sweat samples. This work provided a simple and promising method to fabricate integrated wearable systems for on-site providing information on the UV intensity of the external environment and the V_C level of the human body simultaneously.

Wearable Smart Bandage-Based Bio-Sensors

Bandage is a well-established industry, whereas wearable electronics is an emerging industry. This review presents the bandage as the base of wearable bioelectronics. It begins with introducing a detailed background to bandages and the development of bandage-based smart sensors, which is followed by a sequential discussion of the technical characteristics of the existing bandages, a more practical methodology for future applications, and manufacturing processes of bandage-based wearable biosensors. The review then elaborates on the advantages of basing the next generation of wearables, such as acceptance by the customers and system approvals, and disposal.

Toward a new generation of permeable skin electronics

Skin-mountable electronics are considered to be the future of the next generation of portable electronics, due to their softness and seamless integration with human skin. However, impermeable materials limit device comfort and reliability for long-term, continuous usage. The recent emergence of permeable skin-mountable electronics has attracted tremendous attention in the soft electronics field. Herein, we provide a comprehensive and systematic review of permeable skin-mountable electronics. Typical porous materials and structures are first highlighted, followed by discussion of important device properties. Then, we review the latest representative applications of breathable skin-mountable electronics, such as bioelectrical sensors, temperature sensors, humidity and hydration sensors, strain and pressure sensors, and energy harvesting and storage devices. Finally, a conclusion and future directions for permeable skin electronics are provided.

Plasmonic/magnetic nanoarchitectures: From controllable design to biosensing and bioelectronic interfaces

Controllable design of the nanocrystal-assembled plasmonic/magnetic nanoarchitectures (P/MNAs) inspires abundant methodologies to enhance light-matter interactions and control magnetic-induced effects by means of fine-tuning the morphology and ordered packing of noble metallic or magnetic building blocks. The burgeoning development of multifunctional nanoarchitectures has opened up broad range of interdisciplinary applications including biosensing, in vitro diagnostic devices, point-of-care (POC) platforms, and soft bioelectronics. By taking advantage of their customizability and efficient conjugation with capping biomolecules, various nanoarchitectures have been integrated into high-performance biosensors with remarkable sensitivity and versatility, enabling key features that combined multiplexed detection, ease-of-use and miniaturization. In this review, we provide an overview of the representative developments of nanoarchitectures that being built by plasmonic and magnetic nanoparticles over recent decades. The design principles and key mechanisms for signal amplification and quantitative sensitivity have been explored. We highlight the structure-function programmability and prospects of addressing the main limitations for conventional biosensing strategies in terms of accurate selectivity, sensitivity, throughput, and optoelectronic integration. State-of-the-art strategies to achieve affordable and field-deployable POC devices for early multiplexed detection of infectious diseases such as COVID-19 has been covered in this review. Finally, we discuss the urgent yet challenging issues in nanoarchitectures design and related biosensing application, such as large-scale fabrication and integration with portable devices, and provide perspectives and suggestions on developing smart biosensors that connecting the materials science and biomedical engineering for personal health monitoring.

Artificial intelligence-driven wearable technologies for neonatal cardiorespiratory monitoring. Part 2: artificial intelligence

BACKGROUND

With the development of Artificial Intelligence (AI) techniques, smart health monitoring, particularly neonatal cardiorespiratory monitoring with wearable devices, is becoming more popular. To this end, it is crucial to investigate the trend of AI and wearable sensors being developed in this domain.

METHODS

We performed a review of papers published in IEEE Xplore, Scopus, and PubMed from the year 2000 onwards, to understand the use of AI for neonatal cardiorespiratory monitoring with wearable technologies. We reviewed the advances in AI development for this application and potential future directions. For this review, we assimilated machine learning (ML) algorithms developed for neonatal cardiorespiratory monitoring, designed a taxonomy, and categorised the methods based on their learning capabilities and performance.

RESULTS

For AI related to wearable technologies for neonatal cardio-respiratory monitoring, 63% of studies utilised traditional ML techniques and 35% utilised deep learning techniques, including 6% that applied transfer learning on pre-trained models.

CONCLUSIONS

A detailed review of AI methods for neonatal cardiorespiratory wearable sensors is presented along with their advantages and disadvantages. Hierarchical models and suggestions for future developments are highlighted to translate these AI technologies into patient benefit.

IMPACT

State-of-the-art review in artificial intelligence used for wearable neonatal cardiorespiratory monitoring. Taxonomy design for artificial intelligence methods. Comparative study of AI methods based on their advantages and disadvantages.

Multiple pathways to stretchable electronics

Stretchable conductors expand the interfaces with biological structures.

Wearable smart devices in cancer diagnosis and remote clinical trial monitoring: Transforming the healthcare applications

During the past two decades, the era of digitalization in pharmaceutical device manufacturing has gained significant momentum for maintaining human health. From various available technologies, internet of things (IoT) sensors are being increasingly used as wearable devices (e.g., smart watches, wrist bands, mobile phones, tablets, implantable pumps, etc.) that enable real-time monitoring of data. Such devices are integrated with smart materials that typically monitor the real-time data (blood pressure, blood sugar, heart and pulse rate, cytokine levels, etc.) to advise patients and physicians. Hence, there has been a great demand for wearable devices as potential tools for remote clinical trial monitoring in cancers and other diseases and they are proving to be very cost-effective.

Surface Wettability for Skin-Interfaced Sensors and Devices

The practical applications of skin-interfaced sensors and devices in daily life hinge on the rational design of surface wettability to maintain device integrity and achieve improved sensing performance under complex hydrated conditions. Various bio-inspired strategies have been implemented to engineer desired surface wettability for varying hydrated conditions. Although the bodily fluids can negatively affect the device performance, they also provide a rich reservoir of health-relevant information and sustained energy for next-generation stretchable self-powered devices. As a result, the design and manipulation of the surface wettability are critical to effectively control the liquid behavior on the device surface for enhanced performance. The sensors and devices with engineered surface wettability can collect and analyze health biomarkers while being minimally affected by bodily fluids or ambient humid environments. The energy harvesters also benefit from surface wettability design to achieve enhanced performance for powering on-body electronics. In this review, we first summarize the commonly used approaches to tune the surface wettability for target applications toward stretchable self-powered devices. By considering the existing challenges, we also discuss the opportunities as a small fraction of potential future developments, which can lead to a new class of skin-interfaced devices for use in digital health and personalized medicine.

Stretchable solvent-free ionic conductor with self-wrinkling microstructures for ultrasensitive strain sensor

Stretchable solvent-free ionic conductors with outstanding physicochemical stability are attractive in emerging sensing devices. However, existing ionic conductors struggle to keep high strain sensitivity with a gauge factor (GF) smaller than 4 at a strain range of up to 100%, which seriously affects the reliability of the signal output. Herein, we prepared a novel solvent-free ionic conductor, consisting of a hybrid cross-linked polymer network and mobile ions. Exquisite reticular wrinkling microstructures with ion channels were formed spontaneously by utilizing modulus mismatch in shrinkage during polymerization. The ion channels promote the ionic transport in the polymer, and the reticular wrinkling microstructures dramatically reinforce the mechanical stability and enhance energy dissipation. After being carefully optimized, the as-prepared ionic conductor demonstrated many unique advantages including perfect physicochemical stability, wide operating temperature range, the high ionic conductivity of 1.17 mS cm^-1 at 15 °C and excellent sensitivity with GF as high as 7.03 at 100% strain. Moreover, benefiting from the abundant hydrogen bonds and ionic interactions in the polymer network, the ionic conductor also possesses self-healable and adhesion ability. The resulting devices can be used for quantitative evaluation of human joint motion and exhibit great potential in soft electronics or iontronics.

Water-Tolerant MXene Epidermal Sensors with High Sensitivity and Reliability for Healthcare Monitoring

Flexible and wearable pressure sensors have gained great popularity in consumer electronics due to their potential applications in human healthcare, E-skin, and artificial intelligence interactions. MXene is regarded as one of the most ideal candidate sensing materials due to its high electrical conductivity and controllable interlayer space. However, the easy-to-oxidize characteristic of MXene materials greatly restricts the sensitivity and reliability of sensor devices, especially in wet climates. Herein, a highly sensitive and waterproof flexible pressure sensor using a free-standing hydrophobic bacterial cellulose/Ti₃C₂T_x MXene (HBT) hybrid film as a sensing layer is fabricated by facile and effective nanocellulose intercalation and fluorine modification strategies. The obtained pressure sensor delivers high sensitivity (65.5 kPa^-1), fast response (50 ms), wide linear sensing range (0.002-30 kPa) with a low detection limit of 0.57 Pa, and excellent repeatability over 50,000 cycles. Meanwhile, owing to the highly hydrophobic surface of the HTB film, the outstanding sensing features could be well retained, although immersed in water several times. Benefiting from the excellent sensing properties and water resistance, the HBT sensor serves as a wearable force sensor to monitor the full-range human physiological motions regardless of whether the conditions are normal or wet. This work provides a new pathway to design the MXene pressure sensor with high reliability and demonstrates the promising usage of HBT sensors in portable biomedical electronics.

Wireless graphene-based thermal patch for obtaining temperature distribution and performing thermography

Thermal imaging provides information regarding the general condition of the human body and facilitates the diagnosis of various diseases. Heat therapy or thermotherapy can help in the treatment of injuries to the skin tissue. Here, we report a wearable thermal patch with dual functions of continuous skin temperature sensing and thermotherapy for effective self-care treatment. This system consists of a graphene-based capacitive sensor, a graphene thermal pad, and a flexible readout board with a wireless communication module. The wearable sensor continuously monitors the temperature variation over a large area of the skin (3 × 3cm²) with high resolution and sensitivity and performs thermotherapy via the graphene-based heater mounted at the bottom of the device. Animal studies prove that the proposed system can be used to diagnose various diseases. This technology could be useful in the development of convenient and wearable health care devices.

Air-Permeable Waterproofing Electrocardiogram Patch to Monitor Full-Day Activities for Multiple Days

On-skin healthcare patch-type devices have great technological challenges in monitoring full-day activities and wearing for multiple days without detachment. These challenges can be overcome when the sensor is air permeable but waterproof. This study presents a light-weight, highly stable, and stretchable Au electrode that is fabricated by sputtering on an imidized nanofiber mat. The contact surface of the electrode is hydro-wetting and the outer surface of the electrode is hydrophobic, so the porous electrode simultaneously has excellent sweat permeability and waterproofing capabilities. The electrode is applied to the electrocardiogram sensor for monitoring the cardiac signals for five consecutive days without detaching while doing various full-day activities such as relaxing, exercising, showering, and sleeping. This study suggests a modular setup of the electrodes and the cardiac signal processing unit for activating the device when cardiac monitoring is required.

A Wearable Microneedle-Based Extended Gate Transistor for Real-Time Detection of Sodium in Interstitial Fluids

Sodium is a prominent prognostic biomarker for assessing health status, such as dysnatremia. As of now, detection and monitoring of sodium levels in the human body is carried out by means of laborious and bulky laboratory equipmentand/or by offline analysis of various body fluids. Herein, an innovative stretchable, skin-conformal and fast-response microneedle extended-gate FET biosensor is reported for real-time detection of sodium in interstitial fluids for minimally invasive health monitoring along with high sensitivity, low limit of detection, excellent biocompatibility, and on-body mechanical stability. The integration of the reported device with a wireless-data transmitter and the Internet-of-Things cloud for real-time monitoring and long-term analysis is reported and discussed. This platform would eventually help bringing unlimited possibilities for effecient medical care and accurate clinical decision-making.

Recent advances in flexible and wearable sensors for monitoring chemical molecules

In recent years, real-time health management has received increasing attention, benefiting from the rapid development of flexible and wearable devices. Conventionally, flexible and wearable devices are used for collecting health data such as electrophysiological signals, blood pressure, heart rate, etc. The monitoring of chemical factors has shown growing significance, providing the basis for the screening, diagnosis, and treatment of many diseases. Nowadays, in order to understand the health status of the human body more comprehensively and accurately, researchers in the community have started putting effort into developing wearable devices for monitoring chemical factors. Progressively, more flexible chemical sensors with wearable real-time health-monitoring functionality have been developed thanks to advances relating to wireless communications and flexible electronics. In this review, we describe the variety of chemical molecules and information that can currently be monitored, including pH levels, glucose, lactate, uric acid, ion levels, cytokines, nutrients, and other biomarkers. This review analyzes the pros and cons of the most advanced wearable chemical sensors in terms of wearability. At the end of this review, we discuss the current challenges and development trends relating to flexible and wearable chemical sensors from the aspects of materials, electrode designs, and soft-hard interface connections.

Flexible Plasmonic Biosensors for Healthcare Monitoring: Progress and Prospects

The noble metal nanoparticle has been widely utilized as a plasmonic unit to enhance biosensors, by leveraging its electric and/or optical properties. Integrated with the "flexible" feature, it further enables opportunities in developing healthcare products in a conformal and adaptive fashion, such as wrist pulse tracers, body temperature trackers, blood glucose monitors, etc. In this work, we present a holistic review of the recent advance of flexible plasmonic biosensors for the healthcare sector. The technical spectrum broadly covers the design and selection of a flexible substrate, the process to integrate flexible and plasmonic units, the exploration of different types of flexible plasmonic biosensors to monitor human temperature, blood glucose, ions, gas, and motion indicators, as well as their applications for surface-enhanced Raman scattering (SERS) and colorimetric detections. Their fundamental working principles and structural innovations are scoped and summarized. The challenges and prospects are articulated regarding the critical importance for continued progress of flexible plasmonic biosensors to improve living quality.