Flexible and Stretchable Smart Display: Materials, Fabrication, Device Design, and System Integration

Wearable sensors have gained much attention due to their potential in personal health monitoring in a timely, cost-effective, easy-operating, and noninvasive way. In recent studies, nanomaterials have been employed in wearable sensors to improve the sensing performance in view of their excellent properties. Here, focus is mainly on the nanomaterial-enabled wearable sensors and their latest advances in personal health monitoring. Different kinds of nanomaterials used in wearable sensors, such as metal nanoparticles, carbon nanomaterials, metallic nanomaterials, hybrid nanocomposites, and bio-nanomaterials, are reviewed. Then, the progress of nanomaterial-based wearable sensors in personal health monitoring, including the detection of ions and molecules in body fluids and exhaled breath, physiological signals, and emotion parameters, is discussed. Furthermore, the future challenges and opportunities of nanomaterial-enabled wearable sensors are discussed.

The continued growth of mobile and interactive computing requires devices manufactured with low-cost processes, compatible with large-area and flexible form factors, and with additional functionality. We review recent advances in the design of electronic and optoelectronic devices that use colloidal semiconductor quantum dots (QDs). The properties of materials assembled of QDs may be tailored not only by the atomic composition but also by the size, shape, and surface functionalization of the individual QDs and by the communication among these QDs. The chemical and physical properties of QD surfaces and the interfaces in QD devices are of particular importance, and these enable the solution-based fabrication of low-cost, large-area, flexible, and functional devices. We discuss challenges that must be addressed in the move to solution-processed functional optoelectronic nanomaterials.

Andreev and spin transport in carbon nanotube quantum dot hybrid devices

Advances in Soft Bioelectronics for Brain Research and Clinical Neuroengineering

Open AccessCCS ChemistryMINI REVIEWS7 Jul 2022Autonomous Chemistry Enabling Environment-Adaptive Electrochemical Energy Storage Devices Zhisheng Lv, Wenlong Li, Jiaqi Wei, Fanny Ho, Jie Cao and Xiaodong Chen Zhisheng Lv Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 138634 Singapore , Wenlong Li Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 138634 Singapore , Jiaqi Wei Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore , Fanny Ho Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore , Jie Cao Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore and Xiaodong Chen *Corresponding author: E-mail Address: [email protected] Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 138634 Singapore Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore https://doi.org/10.31635/ccschem.022.202202153 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Next-generation electronics that are fused into the human body can play a key role in future intelligent communication, smart healthcare, and human enhancement applications. As a promising energy supply component for smart biointegrated electronics, environment-adaptive electrochemical energy storage (EES) devices with complementary adaptability and functions have garnered huge interest in the past decade. Owing to the advancements in autonomous chemistry, which regulate the constitutional dynamic networks in materials, EES devices have witnessed higher freedom of autonomous adaptability in terms of mechano-adaptable, biocompatibility, and stimuli-response properties for biointegrated and smart applications. In this mini-review, we summarize the recent progress in emerging environment-adaptive EES devices enabled by the constitutional dynamic network of mechanical adaptable materials, biocompatible materials, and stimuli-responsive supramolecular polymer materials. Finally, the challenges and perspectives of autonomous chemistry on the environment-adaptive EES devices are discussed. Download figure Download PowerPoint Introduction Advancements made in the merging of human consciousness and machine capabilities have driven the emergence of smart electronics.1–6 The fusion of electronics into human beings to form biointegrated systems has motivated the development of electronics with a higher degree of adaptability in ever-changing environments. To intimately integrate electronics with the human body, electronics are becoming more flexible and stretchable to accommodate the arbitrary shapes of the human body and mechanical deformation during human motion. Alongside mechanical adaptability, biointegrated electronics are also required to be biocompatible so as to operate in physiological environments, such as near-body, on-skin, and in vivo environments. Moreover, by combining with other smart functions, such as biometric sensing and stimuli-responsive actuating, smart biointegrated electronics can allow humans to share their senses and engage in collaborative work with machines to extend the limitation of the human body and machines. Progress in such adaptability of electronics and emerging symbiosis between humans and machines has unlocked exciting opportunities in fabricating biointegrated electronics for smart healthcare and human enhancement applications. To seamlessly power the emerging environment-adaptative electronics, in the last decade, the indispensable electrochemical energy storage (EES) devices, mainly supercapacitors and batteries, have witnessed complementary changes in mechanical adaptability, biocompatibility, and smart functionalities for biointegrated applications.7–9 The environment-adaptive transformations in the EES devices would not be realized without the participation of autonomous chemistry. Autonomous chemistry is an adaptive and self-evolved chemical system that manipulates the covalent and noncovalent constitutional dynamic networks to autonomously adapt and respond to internal and external stimuli, such as light, temperature, pH, and biological enzymes (Figure 1). The molecular chemistry governing the dynamic covalent bonds (Figure 1a) and supramolecular chemistry implementing the intermolecular interactions (Figure 1b) create building blocks of constitutional dynamic chemistry networks.10–14 These dynamic chemistry networks extend and combine with molecules of various sizes to allow for variation and adaptation through autonomous component selection and response to internal and external environmental stimuli (Figure 1c). The introduction of autonomous chemistry into materials science opens up new perspectives on self-adaptive materials. These autonomously adaptive materials integrated with device technologies provide great opportunities for the development of environment-adaptive EES devices. Figure 1 | The emergence of autonomous chemistry toward adaptive and self-evolved chemical systems. (a) Schematic representation of the dynamic reversible and irreversible molecular interactions for the formation of the crystalline and amorphous covalent organic framework. Adapted with permission from ref 14. Copyright 2019 American Chemical Society. (b) Schematic representation of the noncovalent constitutional dynamic network for supramolecular interactions. (c) Autonomous chemistry with covalent and noncovalent constitutional dynamic networks to construct adaptive and self-evolved chemical systems. Download figure Download PowerPoint Autonomous chemistry allows for regulating the functions of environment-adaptive EES devices. By using autonomously adaptive materials, autonomously operating chemical systems in devices can adapt and respond to environmental changes (Figure 2a–f, h). Since the first transformation from the original rigid prototype into flexible devices in 2007, environment-adaptive EES devices have ushered in a new era.7,15 With enhanced flexibility of electrode materials via flexible and stretchable molecular and supramolecular design, intrinsically flexible and stretchable electrodes can endow the mechano-adaptable EES devices with adaptability to arbitrary shapes of the human body.16–22 Accompanying the enhanced mechanical adaptability, the molecular designs of biodegradable and bioresorbable materials enable EES devices to be autonomously biocompatible with the human body for skin-mounted, ingestible, and implantable applications.23–26 Furthermore, the incorporation of stimuli-responsive alloys and supramolecular polymers has spurred EES devices towards smart transformations (e.g., self-healing, shape memory, thermal protection, and electrochromism), providing environment-adaptive EES devices with self-responsive and self-protective functions under external mechanical, thermal, and/or electrical stimuli.27–30 The design and synthesis of autonomous molecular and supramolecular networks with autonomous operation in mechano-adaptable materials, biocompatible materials, and stimuli-responsive supramolecular polymer materials have enabled the development of EES devices with programmable environment-adaptive functions, offering new opportunities to power the next-generation smart and biointegrated electronics.31,32 Figure 2 | The transformation of traditional EES devices into environment-adaptive EES devices enabled by autonomous chemistry. Autonomous chemistry modulates the constitutional dynamic (a) rigid and elastic networks, (b) biological regulatory networks, and (c) the stimulus-response networks for environment-adaptive EES with (d–f) desired functionalities. (a) Chemical structure and illustration of stretchable and adhesive water-dispersible PUs with both soft and hard segments. Reproduced with permission from ref 22. Copyright 2021 Wiley-VCH. (b) Colorimetric detection of glucose using cascade enzyme reaction systems with glucose oxidase and horseradish peroxidase. Adaptable with permission from ref 26. Copyright 2019 Springer Nature Limited. (c) Stimuli-responsive polymers designed with different kinds of topological structures and morphologies. (d) The representative mechanical deformations. (e) The biofriendly interfaces with the human body that require biocompatible properties. (f) The stimulus-responsive smart functions. (g) Schematic device structure of conventional EES devices. (h) The schematic device structure of the environment-adaptive EES devices. Download figure Download PowerPoint Herein, we review recent important advances in environment-adaptive EES devices that are enabled by autonomous chemistry. Specifically, electrodes, electrolytes, and devices associated with constitutional dynamic networks of mechano-adaptable materials, biocompatible materials, and stimuli-responsive supramolecular polymers will be discussed. The emerging mechano-adaptable, biocompatible, and smart EES devices for environment-adaptive applications will also be emphasized. Finally, this review concludes with an outlook on the challenges and opportunities in this fast-developing field. Constitutional Dynamic Networks for Mechano-Adaptable EES Devices The basic mechanical deformations of EES devices involve bending and stretching to cover the curved surface of human skin and soft tissues. Conventional EES devices with packaging consist of two electrodes separated by an electrically insulating separator immersed in a liquid electrolyte (Figure 2g). However, the maximum elastic strains these components in conventional EES devices, including electrodes, separators, and packaging, are typically less than 1%, which cannot match the stretchability (>50%) required for biointegrated applications.33,34 Clearly, improving the tensile strains of these components is essential to designing mechano-adaptable EES devices. To make the components in EES devices stretchable, two strategies have been exploited, including directly replacing inelastic materials with elastic alternatives and constructing structural nanomaterial networks for stretchable electrodes. The inextensible components—electrolytes, separators, and packaging—can be directly replaced by their stretchable alternatives. For example, silicone rubbers such as polydimethylsiloxane and Ecoflex are frequently adopted as stretchable packaging materials, and gel electrolytes with stretchable molecular design can serve as both electrolytes and stretchable separators to eliminate the necessity of poorly elastic separators and possible leakage of liquid electrolyte. As for electrodes, the available inelastic materials can be reinforced with conductive nanomaterials (e.g., graphene, carbon nanotubes (CNTs), and conductive polymers) that have higher tensile strains. However, the tensile strain of composite electrodes is still less than 10%.35,36 In this regard, directly replacing electrode materials with elastic alternatives is not enough. As a complementary method, a stretchable structural design has been developed to improve the stretchability of the electrode. Based on thin and porous nanomaterials, more bendable electrodes cooperate with elastic polymer substrates and proper electrode layouts, such as wave-like wrinkled structures, coiled and braided structures, and kirigami structures, which are highly stretchable and can accommodate tensile strain greater than 100%.16,20,37–42 Instead of using structural designs, the molecular design of intrinsically stretchable polymers is an effective way to realize flexible and stretchable electrodes and electrolytes for mechano-adaptable EES devices. Common conjugated polymers, including polypyrrole (PPy), polyaniline (PANI), and poly(3,4-ethylenedioxythiophene) (PEDOT), when doped into a conducting state, are promising pseudocapacitive materials for flexible supercapacitors.21,43,44 However, the doped conductive polymers usually possess a close-packed and laminated structure that is not capable of releasing mechanical stresses induced by ion exchange in the charging/discharging process and mechanical deformation. This leads to the deterioration of both electrochemical and mechanical performances of as-fabricated devices during mechanical deformation.21 To address this issue, partially doping anion into a conductive polymer matrix has been applied to prevent the overstacking of polymer chains. Taking the electropolymerization of PPy as an example, a porous PPy/black phosphorus oxide (BPO) composite electrode was prepared through a two-step electropolymerization method by utilizing partially oxidized black phosphorus (BP) as dopant.21 The BPO surface was partially doped into the PPy matrix, while the unoxidized BP surface helped to form micropores during the deposition of PPy/BPO composites on the CNT films. As a comparison, the doped PPy composite electrode showed densely packed structures when the anion dopants (SO42− and graphene oxides (GO)) with fully covered oxygen functional groups were employed for the electropolymerization process. Owing to the porous structures to buffer the strain under deformation, the as-prepared stretchable supercapacitors with the porous PPy/BPO-CNT electrode exhibited 97% capacitance retention when being stretched to 2400%, which is superior to that of dense stacking PPy/SO42− and PPy/GO electrode-enabled stretchable supercapacitors (78% and 87% respectively). Besides tuning the dopant in conductive polymers, the additional template and solvent precursors also affect the polymerization process, thus affecting the resultant morphology and stretchability of the conductive polymer-based electrodes. With the template of ice crystals, anisotropic polyvinyl alcohol (PVA)/PANI hybrid hydrogels were synthesized through a cryopolymerization strategy (Figure 3a).43 During the freezing process, the 3D-ordered honeycomb structure of PVA was formed along the growing direction of vertically aligned ice crystals. Followed by cryopolymerization, the polymerization of PANI nanofiber was confined within the boundaries between PVA cell walls and ice crystals. The honeycomb structures with the PVA and PANI interpenetrating networks enabled isotropic PVA/PANI hybrid hydrogels with maximum elongation up to 416%. The as-fabricated supercapacitors delivered 85% capacitance under 200% tensile strain, demonstrating an exciting avenue to synthesize conductive polymer hybrid hydrogels as intrinsically stretchable electrodes for mechano-adaptable EES devices. Additionally, adding dimethyl sulfoxide (DMSO) into an aqueous PEDOT:polystyrene sulfonate (PSS) solution for dry annealing and rehydration processes can lead to a well-controlled phase separation to form interconnected networks of PEDOT:PSS nanofibrils in the as-prepared pure PEDOT:PSS hydrogels. In contrast, fragmented PEDOT:PSS microgel was obtained from the PEDOT:PSS aqueous solution without DMSO due to the separation between the soft PSS-rich domain and the rigid PEDOT-rich domain under the same drying and swelling process.44 The pure PEDOT:PSS hydrogels (20 vol % DMSO in the preparation) can be used as intrinsically stretchable electrodes for supercapacitors and reach a maximum stretchability up to 35% in phosphate-buffered saline (PBS) solution, which closely matches the stretchability of biological tissues (∼20% for neural tissues and ∼50% for skin).35 Figure 3 | Molecular design of flexible and stretchable polymers for mechano-adaptable EES devices. (a) The fabrication of the anisotropic hybrid porous PVA/PANI hydrogels for stretchable supercapacitors with diverse shapes under stretching and compression. Reproduced with permission from ref 43. Copyright 2019 Springer Nature Limited. (b) The synthesis process of VSNPs-PAM electrolytes for stretchable and compressible supercapacitors by cross-linking the VSNPs from vinyltriethoxysilane with acrylamide monomers at the presence of the ammonium persulfate initiator and phosphoric acid. Reproduced with permission from ref 45. Copyright 2017 Wiley-VCH. (c) Illustration of the VSNPs cross-linking PAM network for superstretchability and high compressibility. Reproduced with permission from ref 45. Copyright 2017 Wiley-VCH. (d) The chemical structure of the lithium-ion conductor (LIC) and diagram showing the LIC polymer electrolyte upon stretching. The orange squares represent hydrogen-bonding UPy moieties, black wires are poly(propylene glycol)-pol(ethylene glycol)-poly(propylene glycol) (PPG-PEG-PPG) chains, and the blue circles are lithium ions. Reproduced with permission from ref 46. Copyright 2019 Springer Nature Limited. (e) The application of the supramolecular LIC for stretchable LIBs. Reproduced with permission from ref 46. Copyright 2019 Springer Nature Limited. Download figure Download PowerPoint Apart from the stretchable electrodes, the molecular design of supramolecular dynamic polymers with both mechanically robust and ionically conductive segments is also used to fabricate intrinsically stretchable electrolytes for mechano-adaptable EES devices.45 Pure polyacrylamide (PAM) hydrogels with weak hydrogen bonds are hard to make ultrastretchable, which limits their applications as stretchable electrolytes for stretchable supercapacitors. To enhance the toughness and stretchability of the PAM hydrogel, the hydrogel skeleton was reinforced by the strong covalent bonding between the PAM chains and vinyl hybrid silica nanoparticles (VSNPs) (Figure 3b).45 The VSNPs-PAM hydrogels with ionically conductive PAM polyelectrolyte matrix and stress-buffering of VSNP cross-linkers enabled stretchable supercapacitors to possess intrinsic superstretchability (up to 1000% strain) and compressibility (up to 50% strain) without degradation of their initial capacitance (Figure 3c).45 Similarly, the crosslinking of hairy nanoparticles helped strengthen the stretchable hydrogel electrolytes. The stretchable supramolecular lithium-ion conductor with mechanically reinforced hydrogen bonds was designed to serve as the polymer electrolyte for stretchable lithium-ion batteries (LIBs) (Figure 3d).46 To decouple mechanical robustness from ionic conductivity in low-Tg (the glass transition temperature) polymer electrolytes, a copolymer was created by introducing a dynamic bonded ureido-pyrimidinone (UPy) backbone into low-Tg polyether backbone, wherein the low-Tg polyether backbone provided the polymer electrolyte with high ionic conductivity (1.2 ± 0.21 × 10−4 S cm−1), and the Upy group enhanced the mechanical toughness of the polymer electrolyte (29.3 ± 0.21 × 10−4 MJ m−3, three times higher than reported polymer electrolytes) by dynamic hydrogen bonds. The as-prepared polymer electrolyte-based stretchable LIBs, with a capacity density of 1.1 mAh cm−2 function well to power light-emitting diodes even when stretched up to 70%, suggesting the promising application of tough ion-conducting polymers for conformable EES devices (Figure 3e). Constitutional Dynamic Networks for Biocompatible EES Devices The boom in environment-adaptive electronics for real-time in vivo health monitoring and diagnosis has stimulated the development of EES devices with not only mechano-adaptability but also biocompatibility, so as to adapt to biological environments and intimately integrate onto/into essential organs of the human body.47 However, developing biocompatible EES devices that avoid the likelihood of infection remains a great challenge, especially in the exploration of nontoxic and biodegradable devices. To safely use biocompatible EES devices for in vivo applications, the original corrosive and toxic materials in EES devices should be replaced with nontoxic and biodegradable alternatives that can be autonomously dissolved and resorbed or disposed of by the body through biochemical processes like metabolization and bioabsorption. Such biodegradable materials include inorganic materials and organic polymers. Inorganic metals (e.g., Mg, Mo, and Li) that can react and dissolve in aqueous solutions are suitable materials to fabricate biodegradable electrodes for EES devices. Likewise, biodegradable polymers that can undergo chemical or enzymatical hydrolysis and/or oxidation are suitable as well.48–52 A representative biodegradable battery system is shown in Figure 4a, wherein the primary Mg–Mo battery is packaged with polyanhydride materials to provide a constant current density of 0.1 mA cm−2 at a voltage of 1.6 V for around 6 h.49 Because all the constituent materials in the battery are water-soluble, the Mg–Mo battery is fully degradable after 11 days in PBS at 37 °C followed by another 8 days in PBS at 85 °C. The biodegradation of polyanhydride in the Mg–Mo battery stems from the fact that its ester bonds are susceptible to hydrolysis. Other hydrolytically degradable moieties as shown in Figure 4b, like the amide, thioester, and imine, can serve as the synthetic polymer backbone for eco-friendly degradation.48 Complementary to hydrolysis, oxidation is another way to biologically degrade polymers. Polymers designed with oxidizable moieties, such as ethers, alcohols, and phenols, are susceptible to oxidative cleavage (Figure 4b).53 Currently, previous reports about the degradation of biocompatible EES devices are mainly limited to hydrolysis of electrode materials in physiological conditions, and the mechanism for the oxidation of polymers in biodegradable EES devices has yet to be thoroughly explored. Figure 4 | Biodegradable and biocompatible materials for biocompatible EES devices. (a) The dissolution of the biodegradable Mg–Mo battery with degradable inorganic metallic electrodes and an organic polyanhydride spacer. Reproduced with permission from ref 49. Copyright 2014 Wiley-VCH. (b) The chemical structures of moieties tend to be hydrolyzed and oxidized, and the red marks indicate the hydrolyzation and oxidation sites. Images adapted with permission from ref 48. Copyright 2018 American Chemical Society. (c) The biophilized graphene oxide (bGO)-Mb electrode for biocompatible supercapacitors. Upper: the synthesis of bGO with the negative charge by absorbing cationized bovine serum albumin (cBSA) ion of cBSA onto GO sheet. Down: dose-dependent toxicity in (COS-7) and MEF cells coincubated with GO and bGO/Mb as measured by the intracellular dehydrogenases activity. Reproduced with permission from ref 56. Copyright 2017 Wiley-VCH. (d) An implantable NAD/BQ/CNT yarn supercapacitor. Top: Fabrication scheme of NAD/BQ/CNT yarn electrode and the reversible redox reaction of NAD with the assistance of BQs (oxidized and reduced forms of NAD abbreviated as NAD+ and reduced nicotinamide adenine dinucleotide [NADH], respectively). Down: Implantation of the yarn supercapacitors into the abdominal cavity of a mouse and the capacitance retention of the implantable supercapacitors on the day of surgery, 3 and 14 days after implantation. Reproduced with permission from ref 58. Copyright 2021 Wiley-VCH. Download figure Download PowerPoint As the EES devices become incorporated into the digestive system, biocompatible EES devices that can be ingested by individuals also emerge, with emphasis on biodegradable, bioresorbable, and noncytotoxic characteristics of the devices. Conventional active materials, such as MnO2, which is a constituent material of supercapacitors, pose a threat to the human body (causing abdominal pain and nausea) when ingested. Moreover, the toxicity of many nanomaterials is still unknown.24,54 In this case, naturally derived materials (e.g., biochar, cellulose, silk, and collagen) provide alternatives to fabricate biocompatible and edible devices due to their intrinsic nontoxicity and enzymatic degradability.48,55 Edible supercapacitors with naturally-derived food materials, including active charcoal as the electrode material, egg whites as the edible binder, and high-purity gold leaf as the current collector, can be connected in to power a red and dissolved in the This that naturally derived materials are promising nontoxic materials to biocompatible EES As for the implantable EES devices, of the to degradable devices has only degradation in or stimulated such as PBS that the of in vivo degradation is different from that of in degradation due to the between electrode materials and biological components (e.g., and To improve the properties and the for implantable applications, the used active materials for supercapacitors, such as CNT and have been with functional and conductive A biocompatible with aligned by oxygen was to operate in biological such as and The has that the synthesized with oxygen including and and for cell and than biophilized reduced graphene oxide as the electrode for nontoxic supercapacitors (Figure toxicity of materials (up to high of to mouse and cell was in the biocompatible The maximum nontoxic was times higher than that of GO electrodes. Besides surface and the conductive composites also for implantation. A of CNT electrodes, was into the abdominal cavity of a infection was during the implantation. Moreover, the implantable capacitance retention after 8 days of implantation. In energy the redox like the nicotinamide adenine dinucleotide to the energy storage by this redox system, an implantable CNT yarn was by CNT electrodes with NAD and Figure The biocompatible NAD/BQ/CNT yarn electrodes into the abdominal cavity of a exhibited the in vivo electrical of a even after 14 days (Figure Constitutional Dynamic Networks for EES Devices With the of electronics into soft and smart EES devices with and to respond to external stimuli have as a new of biointegrated devices. In kinds of smart EES devices have been including self-healing, thermal and EES devices. These emerging smart EES devices cannot be realized

Flexible wearable sweat sensors allow continuous, real-time, noninvasive detection of sweat analytes, provide insight into human physiology at the molecular level, and have received significant attention for their promising applications in personalized health monitoring. Electrochemical sensors are the best choice for wearable sweat sensors due to their high performance, low cost, miniaturization, and wide applicability. Recent developments in soft microfluidics, multiplexed biosensing, energy harvesting devices, and materials have advanced the compatibility of wearable electrochemical sweat-sensing platforms. In this review, we summarize the potential of sweat for medical detection and methods for sweat stimulation and collection. This paper provides an overview of the components of wearable sweat sensors and recent developments in materials and power supply technologies and highlights some typical sensing platforms for different types of analytes. Finally, the paper ends with a discussion of the challenges and a view of the prospective development of this exciting field.

Wearable electronic sensors have recently attracted tremendous attention in applications such as personal health monitoring, human movement detection, and sensory skins as they offer a promising alternative to counterparts made from traditional metallic conductors and bulky metallic conductors. However, the real-world use of most wearable sensors is often hindered by their limited stretchability and sensitivity, and ultimately, their difficulty to integrate into textiles. To overcome these limitations, wearable sensors can incorporate flexible conductive fibers as electrically active components. In this study, we adopt a scalable wet-spinning approach to directly produce flexible and conductive fibers from aqueous mixtures of Ti3C2Tx MXene and natural rubber (NR). The electrical conductivity and stretchability of these fibers were tuned by varying their MXene loading, enabling knittability into textiles for wearable sensors. As individual filaments, these MXene/NR fibers exhibit suitable conductivity dependence on strain variations, making them ideal for motivating sensors. Meanwhile, textiles from knitted MXene/NR fibers demonstrate great stability as capacitive touch sensors. Collectively, we believe that these elastic and conductive MXene/NR-based fibers and textiles are promising candidates for wearable sensors and smart textiles.

Medical devices are designed, tested, and placed on the market in a highly regulated environment. Wearable sensors are crucial components of various medical devices: design and validation of wearable sensors, if managed according to international standards, can foster innovation while respecting regulatory requirements. The purpose of this paper is to review the upcoming European Union (EU) Medical Device Regulations 2017/745 and 2017/746, the current and future International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO) standards that set methods for design and validation of medical devices, with a focus on wearable sensors. Risk classification according to the regulation is described. The international standards IEC 62304, IEC 60601, ISO 14971, and ISO 13485 are reviewed to define regulatory restrictions during design, pre-clinical validation and clinical validation of devices that include wearable sensors as crucial components. This paper is not about any specific innovation but it is a toolbox for interpreting current and future regulatory restrictions; an integrated method for design planning, validation and clinical testing is proposed. Application of this method to design wearable sensors should be evaluated in the future in order to assess its potentially positive impact to fostering innovation and to ensure timely development.

All-inorganic perovskite quantum dots (QDs) have drawn much attention due to their prominent quantum-size effects and highly tunable optical properties. Tuning the size of perovskite QDs is attractive for many potential applications. For instance, smaller QDs exhibit more evident quantum properties than larger QDs, but present a blue-shifted spectrum, which limits their applications. Here, we conduct a systematically theoretical analysis about the optical response and plasmon resonance of comparatively small barium titanate quantum dots (BTO–QDs) coupled with silver (Ag) nanowires based on time-dependent density functional theory (TDDFT). Our results show that the silver nanowires can induce an intense optical response respectively in the infrared and visible region to eliminate the spectrum-shift. Furthermore, the absorption spectrum and plasmon resonance can be effectively modified by either altering the position of the silver nanowires or changing the thickness of the BTO–QDs. More importantly, these two methods can act simultaneously, this maybe provide a new approach to implementing the quantum control.

Recent advances in materials science, microelectronics, and data analytics have propelled the flexible and wearable sensors field forward. This comprehensive review summarizes the current state of research in flexible and wearable sensors, shedding light on their evolving technology, applications, and potential impact on various industries. The key developments in sensor materials, fabrication techniques, and integration methods, highlighting their implications for human–machine interaction, health monitoring, sports and fitness, and beyond are explored. Additionally, the difficulties and possibilities in this quickly developing field are addressed and perspectives on the future paths and possible uses of wearable and flexible sensors, highlighting their influence on environmental monitoring, personalized technology, and the current healthcare landscape are provided. This review serves as a resource for researchers, engineers, and innovators interested in cutting‐edge developments and emerging opportunities in the realm of flexible and wearable sensors.

The advancement of smart apparels capable of tracking human physiological signals and body locomotion have a great potential to revolutionize human performance sensing and personalized health monitoring through transforming daily life clothing into sensors. Quantitative evaluation of kinetic parameters of individual gait along with physiological signals can be employed in games and sports, as well as in diagnosis of many diseases such as Parkinson’s, Multiple Sclerosis, and sleep disorders. Among different methodologies in developing wearable sensors such as inertial measurement units, textile-based electromechanical sensors encompass the majority of widely adopted applications. Electromechanical sensors fall into three major categories based on their active mechanisms: piezoelectric sensors, triboelectric sensors, and piezoresistive sensors. Recently, different combination of materials and designs have been reported to develop wearable sensors, each of which provides a unique window into a slightly different range of motion sensitivity. For example, some human signals and motions lie in the small-scale range of pressures such as those of a subtle touch, arterial pulses, and sound vibrations, while the other body movements lie in medium to large range of pressures, such as joint movements, and locomotion during sleep and intense activities. The ability to design an unobtrusive wearable sensor being highly responsive in the required range of signals calls for getting an insight into the difference between the three mechanisms of electromechanical sensing and their corresponding responses under certain conditions.Here, we introduced a set of materials selection principles which gives researchers an in-depth insight into how to design a wearable electromechanical sensor when it comes to acquire data from a specific source of motion. In order to achieve this goal, we performed a set of purposefully designed experiments on three types of wash-stable fabric-based electromechanical sensors that had already been introduced by our lab, i.e., triboelectric sensor, piezoelectric sensor, and piezoionic sensor as a subset of piezoresistive ones. These experiments explored the effect of impact pressure, bending angle and speed, frequency, presence of a base pressure, response time, breathability, and having a multi-layer structure on the performance and sensitivity of each type of sensors. For an instance, it turned out that the triboelectric and piezoelectric sensors are a more reliable sensing element for dynamic pressures, such as joint movements, with the former being failed in the presence of a base pressure. Piezoresistive sensors are the one with the ability to sense both static and dynamic pressures, as well as being responsive under a base pressure. However, piezoresistive one would not be a choice when it comes to bending applications. Upon this comprehensive comparison, we demonstrated a conclusive map which can provide the researchers with distinguishing features of these three types of sensors to be used in corresponding niche applications.For example, some human signals and motions lie in the small-scale range of pressures such as those of a subtle touch, arterial pulses, and sound vibrations, while the other body movements lie in medium to large range of pressures, such as joint movements, and locomotion during sleep and intense activities. The ability to design an unobtrusive wearable sensor being highly responsive in the required range of signals calls for getting an insight into the difference between the three mechanisms of electromechanical sensing and their corresponding responses under certain conditions. Figure 1

Weaved piezoresistive triboelectric nanogenerator for human motion monitoring and gesture recognition

Transparent Conductive Films (TCFs) are used in a wide range of commercial applications, including information displays, capacitive touch sensors, solar photovoltaic modules, EMI shielding windows, transparent heaters, etc. There are many materials that are transparent and many materials that are electrically conductive, but there are very few materials that are both. Indium Tin Oxide (ITO) has dominated the TCF materials market for the past four decades, because of its excellent optoelectronic performance (high transparency, low sheet resistance, low haze, and excellent environmental stability). ITO is typically vacuum coated onto glass sheets or flexible plastic film substrates, with circuit patterns formed by laser ablation or chemical etching methods. The market for TCF materials is forecasted to exceed $5 billion by 2022, with more than half of the market being comprised of TCFs on flexible plastic films (vs. glass) by that date. It is important to note that ITO has well-known shortcomings on flexible plastic films. First, ITO is a brittle ceramic, so it can easily crack when flexed. This is a major limitation, as many product designs are moving towards thinner, flexible and 3D-shaped form factors. Second, ITO’s optoelectronic properties are much better on glass, due to the temperature limitations of plastic. This is a major limitation, as nearly all product designs work better with lower sheet resistance. Third, circuit patterning of ITO on flexible plastic films is expensive, due to handling damage (cracking of circuit features). This is a major limitation, as many product designs are moving towards thinner and larger area substrates, which are even more difficult to handle. Thus, there is an unmet market need for an ITO Alternative that addresses the shortcomings of ITO on flexible plastic films. Several TCF material categories have been established, but there has been limited commercial traction… One category is conductive polythiophene materials (PEDOT), which offer the advantages of low-cost circuit patterning (can be printed) and excellent flexibility and thermoformability. However, PEDOT is lacking in two key areas: 1) sheet resistance is higher than ITO on plastic films at the same transparency; 2) environmental stability is questionable, especially during UV and high temperature / humidity aging tests. These two shortcomings have made PEDOT a niche player. A second category is Carbon Nanotubes (CNTs), which offer all the advantages of PEDOT without compromising environmental stability. However, CNT TCFs have even higher sheet resistance than PEDOT at the same transparency. This shortcoming has made CNTs irrelevant for most TCF applications. A third category is Metal Mesh (MM), which are essentially printed (or etched) metal mesh patterns that have large enough open areas to have high transparency (like a window screen) and large enough cross-sectional area of metal to have low sheet resistance. MM TCFs are comprised of Silver &/or Copper. MM offers significantly lower sheet resistance than ITO on plastic films at the same transparency. MM also offers far superior flexibility than ITO. However, MM has is lacking in two key areas: 1) the metal lines comprising the MM are typically visible and this can be distracting – although this can be addressed with fine line printing (line widths ≤ 6µm), the MM cost is much higher; 2) creating TCF circuit patterns is expensive. A fourth category is Silver Nanowires (AgNWs), which offer all the advantages of MM without compromising aesthetics (MM visibility). However, AgNW is lacking in three key areas: 1) environmental stability is questionable, especially during UV and high temperature / high humidity aging tests; 2) robust electrical connections to AgNW are challenging due to the insulating polymer used to encapsulate AgNWs; 3) creating TCF circuit patterns is expensive. Despite their shortcomings, AgNW and MM appear to be gaining the most commercial traction, yet market penetration is still low. CHASM has recently created a new TCF category called Printed CNT Hybrids, which are made by combining CNTs with either AgNW or MM technologies to create a Hybrid solution that is superior to CNT, AgNW or MM technologies alone. CHASM is marketing its portfolio of Printed CNT Hybrids under the AgeNT™ brand. This presentation describes how CHASM combines its distinguished CNT synthesis capabilities (CoMoCAT™ technology) with proprietary ink formulation capabilities (V2V™ technology) to create AgeNT™ printed CNT Hybrid TCFs that are positioned to become a leading ITO Alternative. Figure 1

Emerging wearable cortisol sensors represent a significant innovation in personalized health monitoring, allowing for detection of cortisol, a key biomarker for stress, chronic diseases, and overall well-being. Since cortisol regulates stress and metabolic functions, monitoring its levels can aid in early disease detection and enhance health management. These sensors incorporate advanced technologies such as electrochemical sensors with molecularly imprinted polymers, immunosensors, and aptamer-based sensors, ensuring high sensitivity and accuracy. Optical sensors and FETs, using nanostructures and flexible substrates, improve detection precision and enable seamless integration into wearable devices. Recent advancements in fabrication techniques, including microfluidics, roll-to-roll printing, and nanofabrication, have made large-scale production of these sensors feasible and cost-effective, without sacrificing comfort or adaptability. Key features include reliable power sources and wireless communication for continuous data transmission. Additionally, machine learning algorithms and computational models enable in-depth analysis of cortisol fluctuations, offering valuable insights into their health implications. Applications for these sensors range from stress management and chronic disease monitoring to sports performance and personalized healthcare. Future advancements are expected to emphasize sensor miniaturization, integration with artificial intelligence, and the development of multi-modal sensing platforms, thereby enhancing the potential of wearable cortisol sensors to revolutionize healthcare through personalized, proactive, and preventive strategies.

Carbon nanotubes (CNTs) and semiconductor nanocrystals (SCNCs) are known to be interesting donor-acceptor partners due to their unique optical and electronic properties. These exciting features have led to the development of novel composites based on these two nanomaterials and to their characterization for use in various applications, such as components in sensors, transistors, solar cells and biomedical devices. Two approaches based on covalent and noncovalent methods have been suggested for coupling the SCNCs to CNTs. Most covalent conjugation methods used so far were found to disrupt the electronic structure of the CNTs or interfere with charge transfer in the CNT-SCNC interface. Moreover, it offers random and poorly organized nanoparticle coatings. Therefore, noncovalent methods are considered to be ideal for better electronic coupling. However, a key common drawback of noncovalent methods is the lack of stability which hampers their applicability. In this article, a method has been developed to couple semiconductor seeded nanorods onto CNTs through π-π interactions. The CNTs and pyrene conjugated SCNC hybrid materials were characterized by both microscopic and spectroscopic techniques. Fluorescence and photocurrent measurements suggest the proposed pi-stacking approach results in a strong electronic coupling between the CNTs and the SCNCs leading to better photocurrent efficiency than that of a covalent conjugation method reported using similar SCNC material. Overall, the CNT-SCNC films reported in the present study open the scope for the fabrication of optoelectronic devices for various applications.