Flexing the Future: Strategic Insights into Wearable Sensor Development

Flexible wearable sensors show great potential for applications in wearable devices, remote health monitoring, artificial intelligence, soft robotics, and artificial skin due to their stretchability, bendability, thinness and portability, and excellent electrical properties. Hydrogels have tunable mechanical properties, excellent biocompatibility, and flexibility, making them attractive candidates for wearable flexible sensors. Among them, tremendous efforts have focused on the advancement of chitosan-based hydrogels (CS-Gels) to realize multifunctional wearable sensing by modifying hydrogel networks with additives/nanofillers/functional groups. Recently, remarkable progress has been made in flexible wearable sensors. Herein, this review summarizes recent advances in CS-Gels wearable sensors for applications such as human motion monitoring, health monitoring, human-machine interface and soft robotics. Representative synthesis methods and strategies for CS-Gels are briefly described, the problems and deficiencies of CS-Gels for wearable sensors are discussed. Finally, the possible opportunities and challenges for the future development of CS-Gels flexible wearable devices are proposed.

Chapter 12 - Wearable and Flexible Sensors Based on 2D and Nanomaterials

Changes in the biomechanical functions of artery serve as important indicators for prediction of cardiovascular disease. One of the challenges of the ultrasonic arterial measurement is the compression of the artery due to the weight of a conventional ultrasound probe employed and/or undesired probe motion, resulting in artifacts on the measurement results. An approach to address this issue with a low-cost consideration is to use a flexible, lightweight wearable ultrasound sensor which could reduce or avoid deformation of the artery of interest beneath the sensor during the measurement. A flexible lightweight sensor was constructed from a polyvinylidene difluoride (PVDF) piezoelectric film having an active sensing area of 20 mm by 20 mm. The total thickness and weight of the sensor was 0.75 mm and 2g, respectively. The flexible sensor was applied to perform measurements of the brachial artery of a healthy subject. It was successfully demonstrated that the flexible sensor was able to measure the arterial diameter changes corresponding to the cardiac cycle with less motion artifacts.

mwymwAbstract: In recent years, flexible wearable sensors have received widespread attention for their potential application value in motion monitoring, healthcare and humancomputer interaction. Graphene, as a two-dimensional zero-bandgap semi-metallic material, is an ideal material for the preparation of wearable flexible sensors due to its high carrier mobility, mechanical flexibility, and biocompatibility. Based on target species and performance requirements, researchers utilize graphene and its redox products to form carbon nanocomposites by combining them with polymers, metals and other substrates through specific preparation methods. This helps to increase the number of active sites and functional groups of modified graphene, achieving dispersion and functionalization of graphene. Such materials can effectively solve the current problems of low sensitivity and poor sustainability faced by pure graphene flexible sensors in the wearable field and promote the maturity of next-generation flexible electronic products in diversified directions. This review starts from the types and classic preparation methods of graphene composites. Then it elaborates the principles and applications of existing pressure/strain, biological, and humidity graphene composites based wearable flexible sensors. Finally, the paper comprehensively summaries the problems and challenges at the present stage and proposes the future development trends.

Wearable biosensors have received tremendous attention over the past decade owing to their great potential in predictive analytics and treatment toward personalized medicine. Flexible electronics could serve as an ideal platform for personalized wearable devices because of their unique properties such as light weight, low cost, high flexibility and great conformability. Unlike most reported flexible sensors that mainly track physical activities and vital signs, the new generation of wearable and flexible chemical sensors enables real-time, continuous and fast detection of accessible biomarkers from the human body, and allows for the collection of large-scale information about the individual's dynamic health status at the molecular level. In this article, we review and highlight recent advances in wearable and flexible sensors toward continuous and non-invasive molecular analysis in sweat, tears, saliva, interstitial fluid, blood, wound exudate as well as exhaled breath. The flexible platforms, sensing mechanisms, and device and system configurations employed for continuous monitoring are summarized. We also discuss the key challenges and opportunities of the wearable and flexible chemical sensors that lie ahead.

Flexible and wearable electrochemical sensors for health and safety monitoring.

Sensors are core elements to directly obtain information from surrounding objects for further detecting, judging and controlling purposes. With the rapid development of soft electronics, flexible sensors have made considerable progress, and can better fit the objects to detect and, thus respond to changes more sensitively. Recently, as a newly emerging electronic ink, liquid metal is being increasingly investigated to realize various electronic elements, especially soft ones. Compared to conventional soft sensors, the introduction of liquid metal shows rather unique advantages. Due to excellent flexibility and conductivity, liquid-metal soft sensors present high enhancement in sensitivity and precision, thus producing many profound applications. So far, a series of flexible and wearable sensors based on liquid metal have been designed and tested. Their applications have also witnessed a growing exploration in biomedical areas, including health-monitoring, electronic skin, wearable devices and intelligent robots etc. This article presents a systematic review of the typical progress of liquid metal-enabled soft sensors, including material innovations, fabrication strategies, fundamental principles, representative application examples, and so on. The perspectives of liquid-metal soft sensors is finally interpreted to conclude the future challenges and opportunities.

Flexible wearable sensors are emerging as next-generation tools to collect information from the human body and surroundings in a smart, friendly, and real-time manner. A new class of such sensors with various functionality and amenability for the human body is essential for this goal. Unfortunately, the majority of the wearable sensors reported so far in the literature were of a single function (mostly strain sensors) and just a prototype without thinking of continuous mass production. In this paper, we report a series of multifunctional conductive hydrogel/ thermochromic elastomer hybrid fibers with core-shell segmental configuration and their application as flexible wearable strain and temperature sensors to monitor human motion and body/surrounding temperatures. Specifically, a conductive reduced-graphene-oxide-doped poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamide (rGO-poly(AMPS-co-AAm)) hydrogel and a thermochromic elastomer containing silicon rubber and thermochromic microcapsules are chosen as strain-sensitive and thermosensitive materials, respectively. A core-shell segmental structure is realized by programming the extrusion of either conductive hydrogel precursor solution or a thermochromic elastomer prepolymer as a core layer via dual-core coaxial wet spinning. Depending on the assembly order and length of the conductive hydrogel and the thermochromic elastomer, the as-prepared hybrid fibers can be used for different purposes, i.e., human-motion monitoring, body or room temperature detection, and color decoration. The strategy described above, i.e., fabrication of core-shell segmental fibers via the wet-spinning method, is especially suitable for mass production in industry and can be further extended to fabricate flexible wearable devices with more components and more functions such as transistors, sensors, displays, and batteries.

Flexible wearable sensors, emerging as pivotal toolsfor healthdiagnosis and treatment, have garnered significant attention in thepresent. They facilitate real-time monitoring of human physiological,biochemical, and environmental parameters, demonstrating considerablepotential for personalized health monitoring. However, existing literaturereviews on wearable sensors predominantly focus on single-signal monitoring.In this paper, the progress of flexible wearable sensors in healthmonitoring is systematically reviewed, encompassing physiological,biochemical, and environmental signals relevant to human health. Simultaneously,this review provides a comprehensive overview of the design and sensingprinciples of wearable sensors as well as the strategies for improvingtheir performance. Finally, the development prospects are also discussed,aiming to offer references for the advancement of wearable sensorsin health monitoring.

Conductive hydrogels are promising multifunctional materials for wearable sensors, but their practical applications require combined properties that are difficult to achieve. Herein, we developed a flexible wearable sensor with double-layer structure based on conductive composite hydrogel, which included the outer layer of silicone elastomer (Ecoflex)/silica microparticle composite film and the inner layer of P(AAm-co-HEMA)-MXene-AgNPs hydrogel. Through covalently cross-linking silicone elastomer on the surface of the hydrogel polymer, we bonded a thin Ecoflex film (100 μm) on the P(AAm-co-HEMA)-MXene-AgNPs hydrogel with robust interface, which can easily adhere to the Ecoflex/SiO2 microparticle composite film by silicone glue. The Ecoflex/SiO2 microparticle composite film endows the strain wearable sensor with superhydrophobic function that could maintain the stability under stretching or bending. Moreover, it can effectively resist the interference of water droplets and water flow. The P(AAm-co-HEMA)-MXene-AgNPs hydrogel exhibits outstanding antibacterial activity to inhibit Staphylococcus aureus, Escherichia coli, and even drug-resistant Escherichia coli. In addition, the flexible wearable sensor exhibited good self-adhesive performance by changing the reaction temperature of hydrogel and can adhere strongly onto various materials. The conductive composite hydrogel reported in this work contributes an innovative strategy for the preparation of multifunctional flexible wearable sensor.

The increased prevalence of chronic disease in aging population entails health risks and imposes significant economic and social burden. It is essential to provide comfortable, cost-effective, and easy-to-use unobtrusive and wearable systems for personal well-being and healthcare. Novel flexible material-based non-invasive and wearable sensors offer an efficient and cost-effective solution, which enables the continuous and real-time monitoring of important physiological signs of the human beings, the assessment of personal health conditions and that provides feedback from remote and home monitoring. In this paper, novel flexible material-based wearable sensors, devised into body sensor networks to capture and monitor vital bio-signals, including electroencephalography (EEG), electrocardiography (ECG) and respiratory, are proposed. Silver nanowires (Ag NWs) and polydimethylsiloxane composite material, carbon foam, and graphene-based fiber are used to sense the EEG, ECG, and respiratory, respectively. With different flexible materials, the smart hat and smart jacket are designed to affix the sensors, which enable long-term health monitoring of vital signals seamlessly. Meanwhile, the corresponding acquisition circuits are developed and mounted with the proposed electrodes on the garments. More importantly, a comprehensive protocol is designed to validate the performance of the proposed system, while some standard sensors and commercial devices are used for comparison. The evaluation results demonstrate the proposed system represents a comparable performance with the existing system. In summary, the proposed sensing system offers an unobtrusive, detachable, expandable, user-friendly, and comfortable solution for physiological signal monitoring. It can be expected to use for the remote healthcare monitoring and provide personalized information of health, fitness, and diseases.

A robust, fatigue-resistant, self-healing, and recyclable κ-carrageenan-based ionic conductive hydrogel with synergistic K+/Ca2+ enhancement for multifunctional flexible sensor.

Conductive hydrogels exhibit high potential in the fields of wearable sensors, healthcare monitoring, and e-skins. However, it remains a huge challenge to integrate high elasticity, low hysteresis, and excellent stretch-ability in physical crosslinking hydrogels. This study reports the synthesis of polyacrylamide (PAM)-3-(trimethoxysilyl) propyl methacrylate-grafted super arborized silica nanoparticle (TSASN)-lithium chloride (LiCl) hydrogel sensors with high elasticity, low hysteresis, and excellent electrical conductivity. The introduction of TSASN enhances the mechanical strength and reversible resilience of the PAM-TSASN-LiCl hydrogels by chain entanglement and interfacial chemical bonding, and provides stress-transfer centers for external-force diffusion. These hydrogels show outstanding mechanical strength (a tensile stress of 80-120 kPa, elongation at break of 900-1400%, and dissipated energy of 0.8-9.6 kJ m-3 ), and can withstand multiple mechanical cycles. LiCl addition enables the PAM-TSASN-LiCl hydrogels to exhibit excellent electrical properties with an outstanding sensing performance (gauge factor = 4.5), with rapid response (210 ms) within a wide strain-sensing range (1-800%). These PAM-TSASN-LiCl hydrogel sensors can detect various human-body movements for prolonged durations of time, and generate stable and reliable output signals. The hydrogels fabricated with high stretch-ability, low hysteresis, and reversible resilience, can be used as flexible wearable sensors.

A review on fabrication, characterization and implementation of wearable strain sensors

The gradual rise of personal healthcare awareness is accelerating the deployment of wearable sensors, whose ability of acquiring physiological vital signs depends on sensing materials. MXenes have distinct chemical and physical superiorities over other 2D nanomaterials for wearable sensors. This review presents a comprehensive summary of the latest advancements in MXenes-based materials for wearable physical sensors. It begins with an introduction to special structural features of MXenes for sensing performance, followed by an in-depth exploration of versatile functionalities. A detailed description of different sensing mechanisms is also included to illustrate the contribution of MXenes to the sensing performance and its improvement. In addition, the real-world applications of MXenes-based physical sensors for monitoring different physiological signs are included as well. The remaining challenges of MXenes-based materials for wearable physical sensors and their promising opportunities are finally narrated, in conjunction with a prospective for future development.