Deformable devices with integrated functional nanomaterials for wearable electronics

Customized surface adhesive and wettability properties of conformal electronic devices

Conformal and body-adaptive electronics have revolutionized the way we interact with technology, ushering in a new era of wearable devices that can seamlessly integrate with our daily lives. However, the inherent mismatch between artificially synthesized materials and biological tissues (caused by irregular skin fold, skin hair, sweat, and skin grease) needs to be addressed, which can be realized using body-adaptive electronics by rational design of their surface adhesive and wettability properties. Over the past few decades, various approaches have been developed to enhance the conformability and adaptability of bioelectronics by (i) increasing flexibility and reducing device thickness, (ii) improving the adhesion and wettability between bioelectronics and biological interfaces, and (iii) refining the integration process with biological systems. Successful development of a conformal and body-adaptive electronic device requires comprehensive consideration of all three aspects. This review starts with the design strategies of conformal electronics with different surface adhesive and wettability properties. A series of conformal and body-adaptive electronics used in the human body under both dry and wet conditions are systematically discussed. Finally, the current challenges and critical perspectives are summarized, focusing on promising directions such as telemedicine, mobile health, point-of-care diagnostics, and human-machine interface applications.

Degradation phenomena of quantum dot light-emitting diodes induced by high electric field

Quantum dots possess exceptional optoelectronic properties, such as narrow bandwidth, controllable wavelength, and compatibility with solution-based processing. However, for efficient and stable operation in electroluminescence mode, several issues require resolution. Particularly, as device dimensions decrease, a higher electric field may be applied through next-generation quantum dot light-emitting diode (QLED) devices, which could further degrade the device. In this study, we conduct a systematic analysis of the degradation phenomena of a QLED device induced by a high electric field, using scanning probe microscopy (SPM) and transmission electron microscopy (TEM). We apply a local high electric field to the surface of a QLED device using an atomic force microscopy (AFM) tip, and we investigate changes in morphology and work function in the Kelvin probe force microscopy mode. After the SPM experiments, we perform TEM measurements on the same degraded sample area affected by the electric field of the AFM tip. The results indicate that a QLED device could be mechanically degraded by a high electric field, and work function changes significantly in degraded areas. In addition, the TEM measurements reveal that In ions migrate from the indium tin oxide (ITO) bottom electrode to the top of the QLED device. The ITO bottom electrode also deforms significantly, which could induce work function variation. The systematic approach adopted in this study can provide a suitable methodology for investigating the degradation phenomena of various optoelectronic devices.

Liquid Metal Patterning and Unique Properties for Next-Generation Soft Electronics

Room-temperature liquid metal (LM)-based electronics is expected to bring advancements in future soft electronics owing to its conductivity, conformability, stretchability, and biocompatibility. However, various difficulties arise when patterning LM because of its rheological features such as fluidity and surface tension. Numerous attempts are made to overcome these difficulties, resulting in various LM-patterning methods. An appropriate choice of patterning method based on comprehensive understanding is necessary to fully utilize the unique properties. Therefore, the authors aim to provide thorough knowledge about patterning methods and unique properties for LM-based future soft electronics. First, essential considerations for LM-patterning are investigated. Then, LM-patterning methods-serial-patterning, parallel-patterning, intermetallic bond-assisted patterning, and molding/microfluidic injection-are categorized and investigated. Finally, perspectives on LM-based soft electronics with unique properties are provided. They include outstanding features of LM such as conformability, biocompatibility, permeability, restorability, and recyclability. Also, they include perspectives on future LM-based soft electronics in various areas such as radio frequency electronics, soft robots, and heterogeneous catalyst. LM-based soft devices are expected to permeate the daily lives if patterning methods and the aforementioned features are analyzed and utilized.

Smart Mask as Wearable for Post-Pandemic Personal Healthcare

A mask serves as a simple external barrier that protects humans from infectious particles from poor air conditions in the surrounding environment. As an important personal protective equipment (PPE) to protect our respiratory system, masks are able not only to filter pathogens and dust particles but also to sense, reflect or even respond to environmental conditions. This smartness is of particular interest among academia and industries due to its potential in disease detection, health monitoring and caring aspects. In this review, we provide an overlook of the current air filtration strategies used in masks, from structural designs to integrated functional modules that empower the mask's ability to sense and transfer physiological or environmental information to become smart. Specifically, we discussed recent developments in masks designed to detect macroscopic physiological signals from the wearer and mask-based disease diagnoses, such as COVID-19. Further, we propose the concept of next-generation smart masks and the requirements from material selection and function design perspectives that enable masks to interact and play crucial roles in health-caring wearables.

Assessment of skin sensitization properties of few-layer graphene and graphene oxide through the Local Lymph Node Assay (OECD TG 442B)

Skin contact is one of the most common exposure routes to graphene-based materials (GBMs) during their small-scale and industrial production or their use in technological applications. Nevertheless, toxic effects in humans by cutaneous exposure to GBMs remain largely unexplored, despite skin contact to other related materials has been associated with adverse effects. Hence, this in vivo study was carried out to evaluate the cutaneous effects of two GBMs, focusing on skin sensitization as a possible adverse outcome. Skin sensitization by few-layer graphene (FLG) and graphene oxide (GO) was evaluated following the Organization for Economic Cooperation and Development (OECD) guideline 442B (Local Lymph Node Assay; LLNA) measuring the proliferation of auricular lymph node cells during the induction phase of skin sensitization. Groups of four female CBA/JN mice (8-12 weeks) were daily exposed to FLG or GO through the dorsal skin of each ear (0.4-40 mg/mL, equal to 0.01-1.00 mg/ear) for 3 consecutive days, and proliferation of auricular lymph node cells was evaluated 3 days after the last treatment. During this period, no clinical signs of toxicity and no alterations in body weight and food or water consumptions were observed. In addition, no ear erythema or edema were recorded as signs of irritation or inflammation. Bromo-deoxyuridine (BrdU) incorporation in proliferating lymphocytes from ear lymph nodes (stimulation indexes <1.6) and the histological analysis of ear tissues excluded sensitizing or irritant properties of these materials, while myeloperoxidase activity in ear biopsies confirmed no inflammatory cells infiltrate. On the whole, this study indicates the absence of sensitization and irritant potential of FLG and GO.

The Use of Electrochemical Impedance Spectroscopy as a Tool for the In-Situ Monitoring and Characterization of Carbon Nanotube Aqueous Dispersions

So far, there is no validated technology for characterizing the dispersion and morphology state of carbon nanotubes (CNTs) aqueous dispersions during sonication. Taking advantage of the conductive nature of CNTs, the main hypothesis of the current study is that Electrochemical Impedance Spectroscopy (EIS) is an appropriate technique for the in-situ monitoring and qualification of the dispersion state of CNTs in aqueous media. To confirm our hypothesis, we monitored the Impedance |Z| during the sonication process as a function of type CNTs/admixtures used for the preparation of the aqueous solutions and of crucial process parameters, such as the applied sonication power and duration (i.e., sonication energy). For dispersions above the percolation threshold, a drop of |Z| by approximately seven orders of magnitude was observed, followed by a linear reduction. The dramatic change in |Z| is regarded as an indication of the formation of a conductive path or destruction of an existing one during sonication and can be used to characterize the dispersion and morphology state of CNTs. The results of the EIS provide, straightforwardly and reliably, the required information to create an optimum dispersion protocol for conductive CNT suspensions. The produced dispersions are part of research focusing on the manufacturing of cement-based composite materials with advanced thermoelectric functionalities for energy harvesting. Such dispersions are not only limited to energy harvesting applications but also to applications where functionalities are introduced through the use of conductive-based suspensions.

Slippery Epidural ECoG Electrode for High-Performance Neural Recording and Interface

Chronic implantation of an epidural Electrocorticography (ECoG) electrode produces thickening of the dura mater and proliferation of the fibrosis around the interface sites, which is a significant concern for chronic neural ECoG recording applications used to monitor various neurodegenerative diseases. This study describes a new approach to developing a slippery liquid-infused porous surface (SLIPS) on the flexible ECoG electrode for a chronic neural interface with the advantage of increased cell adhesion. In the demonstration, the electrode was fabricated on the polyimide (PI) substrate, and platinum (Pt)-gray was used for creating the porous nanocone structure for infusing the silicone oil. The combination of nanocone and the infused slippery oil layer created the SLIPS coating, which has a low impedance (4.68 kΩ) level favourable for neural recording applications. The electrochemical impedance spectroscopy and equivalent circuit modelling also showed the effect of the coating on the recording site. The cytotoxicity study demonstrated that the coating does not have any cytotoxic potentiality; hence, it is biocompatible for human implantation. The in vivo (acute recording) neural recording on the rat model also confirmed that the noise level could be reduced significantly (nearly 50%) and is helpful for chronic ECoG recording for more extended neural signal recording applications.

Materials and design strategies for stretchable electroluminescent devices

Stretchable displays have recently received increasing attention as input and/or output interfaces for next-generation human-friendly electronic systems. Stretchable electroluminescent (EL) devices are a core component of stretchable displays, and they can be classified into two types, structurally stretchable EL devices and intrinsically stretchable EL devices, according to the mechanism for achieving their stretchability. We herein present recent advances in materials and design strategies for stretchable EL devices. First, stretchable devices based on ultrathin EL devices are introduced. Ultrathin EL devices are mechanically flexible like thin paper, and they can become stretchable through various structural engineering methods, such as inducing a buckled structure, employing interconnects with stretchable geometries, and applying origami/kirigami techniques. Secondly, intrinsically stretchable EL devices can be fabricated by using inherently stretchable electronic materials. For example, light-emitting electrochemical cells and EL devices with a simpler structure using alternating current have been developed. Furthermore, novel stretchable semiconductor materials have been presented for the development of intrinsically stretchable light-emitting diodes. After discussing these two types of stretchable EL devices, we briefly discuss applications of deformable EL devices and conclude the review.

Soft Bioelectronics Based on Nanomaterials

Recent advances in nanostructured materials and unconventional device designs have transformed the bioelectronics from a rigid and bulky form into a soft and ultrathin form and brought enormous advantages to the bioelectronics. For example, mechanical deformability of the soft bioelectronics and thus its conformal contact onto soft curved organs such as brain, heart, and skin have allowed researchers to measure high-quality biosignals, deliver real-time feedback treatments, and lower long-term side-effects in vivo. Here, we review various materials, fabrication methods, and device strategies for flexible and stretchable electronics, especially focusing on soft biointegrated electronics using nanomaterials and their composites. First, we summarize top-down material processing and bottom-up synthesis methods of various nanomaterials. Next, we discuss state-of-the-art technologies for intrinsically stretchable nanocomposites composed of nanostructured materials incorporated in elastomers or hydrogels. We also briefly discuss unconventional device design strategies for soft bioelectronics. Then individual device components for soft bioelectronics, such as biosensing, data storage, display, therapeutic stimulation, and power supply devices, are introduced. Afterward, representative application examples of the soft bioelectronics are described. A brief summary with a discussion on remaining challenges concludes the review.

Additively manufactured nano-mechanical energy harvesting systems: advancements, potential applications, challenges and future perspectives

Additively manufactured nano-MEH systems are widely used to harvest energy from renewable and sustainable energy sources such as wind, ocean, sunlight, raindrops, and ambient vibrations. A comprehensive study focusing on in-depth technology evolution, applications, problems, and future trends of specifically 3D printed nano-MEH systems with an energy point of view is rarely conducted. Therefore, this paper looks into the state-of-the-art technologies, energy harvesting sources/methods, performance, implementations, emerging applications, potential challenges, and future perspectives of additively manufactured nano-mechanical energy harvesting (3DP-NMEH) systems. The prevailing challenges concerning renewable energy harvesting capacities, optimal energy scavenging, power management, material functionalization, sustainable prototyping strategies, new materials, commercialization, and hybridization are discussed. A novel solution is proposed for renewable energy generation and medicinal purposes based on the sustainable utilization of recyclable municipal and medical waste generated during the COVID-19 pandemic. Finally, recommendations for future research are presented concerning the cutting-edge issues hurdling the optimal exploitation of renewable energy resources through NMEHs. China and the USA are the most significant leading forces in enhancing 3DP-NMEH technology, with more than 75% contributions collectively. The reported output energy capacities of additively manufactured nano-MEH systems were 0.5-32 mW, 0.0002-45.6 mW, and 0.3-4.67 mW for electromagnetic, piezoelectric, and triboelectric nanogenerators, respectively. The optimal strategies and techniques to enhance these energy capacities are compiled in this paper.

Wireless Power Transfer and Telemetry for Implantable Bioelectronics

Implantable bioelectronic devices are becoming useful and prospective solutions for various diseases owing to their ability to monitor or manipulate body functions. However, conventional implantable devices (e.g., pacemaker and neurostimulator) are still bulky and rigid, which is mostly due to the energy storage component. In addition to mechanical mismatch between the bulky and rigid implantable device and the soft human tissue, another significant drawback is that the entire device should be surgically replaced once the initially stored energy is exhausted. Besides, retrieving physiological information across a closed epidermis is a tricky procedure. However, wireless interfaces for power and data transfer utilizing radio frequency (RF) microwave offer a promising solution for resolving such issues. While the RF interfacing devices for power and data transfer are extensively investigated and developed using conventional electronics, their application to implantable bioelectronics is still a challenge owing to the constraints and requirements of in vivo environments, such as mechanical softness, small module size, tissue attenuation, and biocompatibility. This work elucidates the recent advances in RF-based power transfer and telemetry for implantable bioelectronics to tackle such challenges.

Recent advances in liquid-metal-based wearable electronics and materials

Soft wearable electronics are rapidly developing through exploration of new materials, fabrication approaches, and design concepts. Although there have been many efforts for decades, a resurgence of interest in liquid metals (LMs) for sensing and wiring functional properties of materials in soft wearable electronics has brought great advances in wearable electronics and materials. Various forms of LMs enable many routes to fabricate flexible and stretchable sensors, circuits, and functional wearables with many desirable properties. This review article presents a systematic overview of recent progresses in LM-enabled wearable electronics that have been achieved through material innovations and the discovery of new fabrication approaches and design architectures. We also present applications of wearable LM technologies for physiological sensing, activity tracking, and energy harvesting. Finally, we discuss a perspective on future opportunities and challenges for wearable LM electronics as this field continues to grow.

Lateral dimension and amino-functionalization on the balance to assess the single-cell toxicity of graphene on fifteen immune cell types

Given the wide variety of potential applications of graphene oxide (GO), its consequent release into the environment poses serious concerns on its safety. The future production and exploitation of graphene in the years to come should be guided by its specific chemical-physical characteristics. The unparalleled potential of single-cell mass cytometry (CyTOF) to dissect by high-dimensionality the specific immunological effects of nanomaterials, represents a turning point in nanotoxicology. It helps us to identify the safe graphene in terms of physical-chemical properties and therefore to direct its future safe production. Here we present a high-dimensional study to evaluate two historically indicated as key parameters for the safe exploitation: functionalization and dimension. The role of lateral dimension and the amino-functionalization of GO on their immune impact were here evaluated as synergistic players. To this end, we dissected the effects of GO, characterized by a large or small lateral size (GO 1.32 μm and GO 0.13 μm, respectively), and its amino-functionalized counterpart (GONH₂ 1.32 μm and GONH₂ 0.13 μm, respectively) on fifteen cell types of human primary peripheral blood mononuclear cells (PBMCs). We describe how the smallest later size not only evokes pronounced toxicity on the pool of PBMCs compared to larger GOs but also towards the distinct immune cell subpopulations, in particular on non-classical monocytes, plasmacytoid dendritic cells (pDCs), natural killer cells (NKs) and B cells. The amino-functionalization was able to improve the biocompatibility of classical and non-classical monocytes, pDCs, NKs, and B cells. Detailed single-cell analysis further revealed a complex interaction of all GOs with the immune cells, and in particular monocyte subpopulations, with different potency depending on their physicochemical properties. Overall, by high-dimensional profiling, our study demonstrates that the lateral dimension is an important factor modulating immune cells and specifically monocyte activation, but a proper surface functionalization is the dominant characteristic in its immune effects. In particular, the amino-functionalization can critically modify graphene impact dampening the immune cell activation. Our study can serve as a guide for the future broad production and use of graphene in our everyday life.

Intelligent Polymers, Fibers and Applications

Intelligent materials, also known as smart materials, are capable of reacting to various external stimuli or environmental changes by rearranging their structure at a molecular level and adapting functionality accordingly. The initial concept of the intelligence of a material originated from the natural biological system, following the sensing–reacting–learning mechanism. The dynamic and adaptive nature, along with the immediate responsiveness, of the polymer- and fiber-based smart materials have increased their global demand in both academia and industry. In this manuscript, the most recent progress in smart materials with various features is reviewed with a focus on their applications in diverse fields. Moreover, their performance and working mechanisms, based on different physical, chemical and biological stimuli, such as temperature, electric and magnetic field, deformation, pH and enzymes, are summarized. Finally, the study is concluded by highlighting the existing challenges and future opportunities in the field of intelligent materials.

Advances in Materials for Soft Stretchable Conductors and Their Behavior under Mechanical Deformation

Soft stretchable sensors rely on polymers that not only withstand large deformations while retaining functionality but also allow for ease of application to couple with the body to capture subtle physiological signals. They have been applied towards motion detection and healthcare monitoring and can be integrated into multifunctional sensing platforms for enhanced human machine interface. Most advances in sensor development, however, have been aimed towards active materials where nearly all approaches rely on a silicone-based substrate for mechanical stability and stretchability. While silicone use has been advantageous in academic settings, conventional silicones cannot offer self-healing capability and can suffer from manufacturing limitations. This review aims to cover recent advances made in polymer materials for soft stretchable conductors. New developments in substrate materials that are compliant and stretchable but also contain self-healing properties and self-adhesive capabilities are desirable for the mechanical improvement of stretchable electronics. We focus on materials for stretchable conductors and explore how mechanical deformation impacts their performance, summarizing active and substrate materials, sensor performance criteria, and applications.

Nanomaterial-Enabled Flexible and Stretchable Sensing Systems: Processing, Integration, and Applications

Nanomaterial-enabled flexible and stretchable electronics have seen tremendous progress in recent years, evolving from single sensors to integrated sensing systems. Compared with nanomaterial-enabled sensors with a single function, integration of multiple sensors is conducive to comprehensive monitoring of personal health and environment, intelligent human-machine interfaces, and realistic imitation of human skin in robotics and prosthetics. Integration of sensors with other functional components promotes real-world applications of the sensing systems. Here, an overview of the design and integration strategies and manufacturing techniques for such sensing systems is given. Then, representative nanomaterial-enabled flexible and stretchable sensing systems are presented. Following that, representative applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human-machine interactions are provided. To conclude, perspectives on the challenges and opportunities in this burgeoning field are considered.

Wearable Strain Sensors Using Light Transmittance Change of Carbon Nanotube-Embedded Elastomers with Microcracks

A number of flexible and stretchable strain sensors based on piezoresistive and capacitive principles have been recently developed. However, piezoresistive sensors suffer from poor long-term stability and considerable hysteresis of signals. On the other hand, capacitive sensors exhibit limited sensitivity and strong electromagnetic interference from the neighboring environment. In order to resolve these problems, a novel stretchable strain sensor based on the modulation of optical transmittance of carbon nanotube (CNT)-embedded Ecoflex is introduced in this paper. Within the film of multiwalled CNTs embedded in the Ecoflex substrate, the microcracks are propagated under tensile strain, changing the optical transmittance of the film. The proposed sensor exhibits good stretchability (ε ≈ 400%), high linearity (R² > 0.98) in the strain range of ε = 0-100%, excellent stability, high sensitivity (gauge factor ≈ 30), and small hysteresis (∼1.8%). The sensor was utilized to detect the bending of the finger and wrist for the control of a robot arm. Furthermore, the applications of this sensor to the real-time posture monitoring of the neck and to the detection of subtle human motions were demonstrated.

Skin irritation potential of graphene-based materials using a non-animal test

Besides inhalation, skin contact may be considered one of the most relevant exposure routes to graphene-based materials (GBMs). However, very few data on the cutaneous toxicity of these materials are available, so far. This study is focused on skin irritation potential of a panel of GBMs: few-layer graphene (FLG), exfoliated by ball milling of graphite, FLG exfoliated by ultrasonication using sodium dodecyl sulfate (FLG-SDS) or sodium dodecylbenzenesulfonate (FLG-SDBS), CVD-graphene, obtained by chemical vapor deposition, graphene oxide (GO) and reduced GO (rGO). Skin irritation was assessed using the SkinEthic™ Reconstructed human Epidermis (RhE), following the Organisation for Economic Co-operation and Development (OECD) Test Guideline (TG) 439. Even though not validated for nanomaterials, the OCED TG 439 turned out to be applicable also for GBM testing, since no interference with the methylthiazolyldiphenyl-tetrazolium bromide (MTT) reduction, used as a final readout, was found. Furthermore, direct epidermal exposure to powdered GBMs mimics the actual human exposure, avoiding interference by the cell culture medium (protein corona formation). Only GBMs prepared with irritant surfactants (FLG-SDS and FLG-SDBS), but not the others, reduced RhE viability at levels lower than those predicting skin irritation (≤50%), suggesting irritant properties. This result was further confirmed by measuring cytokine (IL-1α, IL-6 and IL-8) release by GBM-treated RhE and by histological analysis as additional readouts to implement the guideline. On the whole, these results demonstrate that GBMs prepared with non-irritant exfoliation agents do not induce skin irritation after a single acute exposure.

Flexible Ultrasonic Transducer Array with Bulk PZT for Adjuvant Treatment of Bone Injury

Flexible electronic devices are developing rapidly, especially in medical applications. This paper reports an arrayed flexible piezoelectric micromachined ultrasonic transducer (FPMUT) with a sandwich structure for adjuvant treatment of bone injury. To make the device conformable and stretchable for attaching to the skin surface, the flexible substrate of polydimethylsiloxane (PDMS) was combined with the flexible metal line interconnection between the bulk lead zirconate titanate (PZT) arrays. Simulations and experiments were carried out to verify the resonant frequency and tensile property of the reported FPMUT device. The device had a resonant frequency of 321.15 KHz and a maximum sound pressure level (SPL) of 180.19 dB at the distance of 5 cm in water. In addition, detailed experiments were carried out to test its acoustic performance with different pork tissues, and the results indicated good ultrasound penetration. These findings confirm that the FPMUT shows unique advantages for adjuvant treatment of bone injury.

Photodynamic Therapy Based on Graphene and MXene in Cancer Theranostics

Cancer is one of the leading causes of death in the world. Therefore, the development of new advanced and targeted strategies in cancer research for early diagnosis and treatment has become essential to improve diagnosis outcomes and reduce therapy side effects. Graphene and more recently, MXene, are the main representatives of the family of two-dimensional (2D) materials and are widely studied as multimodal nanoplatforms for cancer diagnostics and treatment, in particular leveraging their potentialities as photodynamic therapeutic agents. Indeed, due to their irreplaceable physicochemical properties, they are virtuous allies for photodynamic therapy (PDT) in combination with bioimaging, photothermal therapy, as well as drug and gene delivery. In this review, the rapidly progressing literature related to the use of these promising 2D materials for cancer theranostics is described in detail, highlighting all their possible future advances in PDT.

Graphene and graphene oxide induce ROS production in human HaCaT skin keratinocytes: the role of xanthine oxidase and NADH dehydrogenase

The extraordinary physicochemical properties of graphene-based nanomaterials (GBNs) make them promising tools in nanotechnology and biomedicine. Considering the skin contact as one of the most feasible exposure routes to GBNs, the mechanism of toxicity of two GBNs (few-layer-graphene, FLG, and graphene oxide, GO) towards human HaCaT skin keratinocytes was investigated. Both materials induced a significant mitochondrial membrane depolarization: 72 h cell exposure to 100 μg mL-1 FLG or GO increased mitochondrial depolarization by 44% and 56%, respectively, while the positive control valinomycin (0.1 μg mL-1) increased mitochondrial depolarization by 48%. Since the effect was not prevented by cyclosporine-A, it appears to be unrelated to mitochondrial transition pore opening. By contrast, it seems to be mediated by reactive oxygen species (ROS) production: FLG and GO induced time- and concentration-dependent cellular ROS production, significant already at the concentration of 0.4 μg mL-1 after 24 h exposure. Among a panel of specific inhibitors of the major ROS-producing enzymes, diphenyliodonium, rotenone and allopurinol significantly reverted or even abolished FLG- or GO-induced ROS production. Intriguingly, the same inhibitors also significantly reduced FLG- or GO-induced mitochondrial depolarization and cytotoxicity. This study shows that FLG and GO induce a cytotoxic effect due to a sustained mitochondrial depolarization. This seems to be mediated by a significant cellular ROS production, caused by the activation of flavoprotein-based oxidative enzymes, such as NADH dehydrogenase and xanthine oxidase.

Ultrastretchable Conductor Fabricated on Skin-Like Hydrogel-Elastomer Hybrid Substrates for Skin Electronics

Printing technology can be used for manufacturing stretchable electrodes, which represent essential parts of wearable devices requiring relatively high degrees of stretchability and conductivity. In this work, a strategy for fabricating printable and highly stretchable conductors are proposed by transferring printed Ag ink onto stretchable substrates comprising Ecoflex elastomer and tough hydrogel layers using a water-soluble tape. The elastic modulus of the produced hybrid film is close to that of the hydrogel layer, since the thickness of Ecoflex elastomer film coated on hydrogel is very thin (30 µm). Moreover, the fabricated conductor on hybrid film is stretched up to 1780% strain. The described transfer method is simpler than other techniques utilizing elastomer stamps or sacrificial layers and enables application of printable electronics to the substrates with low elastic moduli (such as hydrogels). The integration of printed electronics with skin-like low-modulus substrates can be applied to make wearable devices more comfortable for human skin.

High Durability and Waterproofing rGO/SWCNT-Fabric-Based Multifunctional Sensors for Human-Motion Detection

Wearable strain-pressure sensors for detecting electrical signals generated by human activities are being widely investigated because of their diverse potential applications, from observing human motion to health monitoring. In this study, we fabricated reduced graphene oxide (rGO)/single-wall carbon nanotube (SWCNT) hybrid fabric-based strain-pressure sensors using a simple solution process. The structural and chemical properties of the rGO/SWCNT fabrics were characterized using scanning electron microscopy (SEM), Raman, and X-ray photoelectron spectroscopy (XPS). Complex networks containing rGO and SWCNTs were homogeneously formed on the cotton fabric. The sensing performance of the devices was evaluated by measuring the effects of bending strain and pressure. When the CNT content was increased, the change in relative resistance decreased, while durability was significantly improved. The rGO/SWCNT (0.04 wt %) fabric sensor showed particularly high mechanical stability and flexibility during 100 000 bending tests at the extremely small bending radius of 3.5 mm (11.6% bending strain). Moreover, the rGO/SWCNT fabric device exhibited excellent water resistant properties after 10 washing tests due to its hydrophobic nature. Finally, we demonstrated a fabric-sensor-based motion glove and confirmed its practical applicability.

High Speed Epitaxial Perovskite Memory on Flexible Substrates

Single-crystal perovskite ferroelectric material is integrated at room temperature on a flexible substrate by the layer transfer technique. Two terminal memory devices fabricated with these materials exhibit faster switching speed, lower operating voltage, and superior endurance than other existing flexible counterparts. The research provides an avenue toward combining the rich functionality of charge and spin states, offered by the general class of complex oxides, onto a flexible platform.

Current achievements of nanoparticle applications in developing optical sensing and imaging techniques

Metallic nanostructures have recently been demonstrated to improve the performance of optical sensing and imaging techniques due to their remarkable localization capability of electromagnetic fields. Particularly, the zero-dimensional nanostructure, commonly called a nanoparticle, is a promising component for optical measurement systems due to its attractive features, e.g., ease of fabrication, capability of surface modification and relatively high biocompatibility. This review summarizes the work to date on metallic nanoparticles for optical sensing and imaging applications, starting with the theoretical backgrounds of plasmonic effects in nanoparticles and moving through the applications in Raman spectroscopy and fluorescence biosensors. Various efforts for enhancing the sensitivity, selectivity and biocompatibility are summarized, and the future outlooks for this field are discussed. Convergent studies in optical sensing and imaging have been emerging field for the development of medical applications, including clinical diagnosis and therapeutic applications.