Leaf-inspired homeostatic cellulose biosensors

Electronic Skin for Health Monitoring Systems: Properties, Functions, and Applications

Electronic skin (e-skin), a skin-like wearable electronic device, holds great promise in the fields of telemedicine and personalized healthcare because of its good flexibility, biocompatibility, skin conformability, and sensing performance. E-skin can monitor various health indicators of the human body in real time and over the long term, including physical indicators (exercise, respiration, blood pressure, etc.) and chemical indicators (saliva, sweat, urine, etc.). In recent years, the development of various materials, analysis, and manufacturing technologies has promoted significant development of e-skin, laying the foundation for the application of next-generation wearable medical technologies and devices. Herein, the properties required for e-skin health monitoring devices to achieve long-term and precise monitoring and summarize several detectable indicators in the health monitoring field are discussed. Subsequently, the applications of integrated e-skin health monitoring systems are reviewed. Finally, current challenges and future development directions in this field are discussed. This review is expected to generate great interest and inspiration for the development and improvement of e-skin and health monitoring systems.

What Exactly Can Bionic Strategies Achieve for Flexible Sensors?

Flexible sensors have attracted great attention in the field of wearable electronic devices due to their deformability, lightness, and versatility. However, property improvement remains a key challenge. Fortunately, natural organisms exhibit many unique response mechanisms to various stimuli, and the corresponding structures and compositions provide advanced design ideas for the development of flexible sensors. Therefore, this Review highlights recent advances in sensing performance and functional characteristics of flexible sensors from the perspective of bionics for the first time. First, the "twins" of bionics and flexible sensors are introduced. Second, the enhancements in electrical and mechanical performance through bionic strategies are summarized according to the prototypes of humans, plants, and animals. Third, the functional characteristics of bionic strategies for flexible sensors are discussed in detail, including self-healing, color-changing, tangential force, strain redistribution, and interfacial resistance. Finally, we summarize the challenges and development trends of bioinspired flexible sensors. This Review aims to deepen the understanding of bionic strategies and provide innovative ideas and references for the design and manufacture of next-generation flexible sensors.

Flexible Electrodes for Brain-Computer Interface System

Brain-computer interface (BCI) has been the subject of extensive research recently. Governments and companies have substantially invested in relevant research and applications. The restoration of communication and motor function, the treatment of psychological disorders, gaming, and other daily and therapeutic applications all benefit from BCI. The electrodes hold the key to the essential, fundamental BCI precondition of electrical brain activity detection and delivery. However, the traditional rigid electrodes are limited due to their mismatch in Young's modulus, potential damages to the human body, and a decline in signal quality with time. These factors make the development of flexible electrodes vital and urgent. Flexible electrodes made of soft materials have grown in popularity in recent years as an alternative to conventional rigid electrodes because they offer greater conformance, the potential for higher signal-to-noise ratio (SNR) signals, and a wider range of applications. Therefore, the latest classifications and future developmental directions of fabricating these flexible electrodes are explored in this paper to further encourage the speedy advent of flexible electrodes for BCI. In summary, the perspectives and future outlook for this developing discipline are provided.

Cellulose-Based Conductive Materials for Energy and Sensing Applications

Cellulose-based conductive materials (CCMs) have emerged as a promising class of materials with various applications in energy and sensing. This review provides a comprehensive overview of the synthesis methods and properties of CCMs and their applications in batteries, supercapacitors, chemical sensors, biosensors, and mechanical sensors. Derived from renewable resources, cellulose serves as a scaffold for integrating conductive additives such as carbon nanotubes (CNTs), graphene, metal particles, metal-organic frameworks (MOFs), carbides and nitrides of transition metals (MXene), and conductive polymers. This combination results in materials with excellent electrical conductivity while retaining the eco-friendliness and biocompatibility of cellulose. In the field of energy storage, CCMs show great potential for batteries and supercapacitors due to their high surface area, excellent mechanical strength, tunable chemistry, and high porosity. Their flexibility makes them ideal for wearable and flexible electronics, contributing to advances in portable energy storage and electronic integration into various substrates. In addition, CCMs play a key role in sensing applications. Their biocompatibility allows for the development of implantable biosensors and biodegradable environmental sensors to meet the growing demand for health and environmental monitoring. Looking to the future, this review emphasizes the need for scalable synthetic methods, improved mechanical and thermal properties, and exploration of novel cellulose sources and modifications. Continued innovation in CCMs promises to revolutionize sustainable energy storage and sensing technologies, providing environmentally friendly solutions to pressing global challenges.

Solvent-Free and Skin-Like Supramolecular Ion-Conductive Elastomers with Versatile Processability for Multifunctional Ionic Tattoos and On-Skin Bioelectronics

The development of stable and biocompatible soft ionic conductors, alternatives to hydrogels and ionogels, will open up new avenues for the construction of stretchable electronics. Here, a brand-new design, encapsulating a naturally occurring ionizable compound by a biocompatible polymer via high-density hydrogen bonds, resulting in a solvent-free supramolecular ion-conductive elastomer (SF-supra-ICE) that eliminates the dehydration problem of hydrogels and possesses excellent biocompatibility, is reported. The SF-supra-ICE with high ionic conductivity (>3.3 × 10-2  S m-1 ) exhibits skin-like softness and strain-stiffening behaviors, excellent elasticity, breathability, and self-adhesiveness. Importantly, the SF-supra-ICE can be obtained by a simple water evaporation step to solidify the aqueous precursor into a solvent-free nature. Therefore, the aqueous precursor can act as inks to be painted and printed into customized ionic tattoos (I-tattoos) for the construction of multifunctional on-skin bioelectronics. The painted I-tattoos exhibit ultraconformal and seamless contact with human skin, enabling long-term and high-fidelity recording of various electrophysiological signals with extraordinary immunity to motion artifacts. Human-machine interactions are achieved by exploiting the painted I-tattoos to transmit the electrophysiological signals of human beings. Stretchable I-tattoo electrode arrays, manufactured by the printing method, are demonstrated for multichannel digital diagnosis of the health condition of human back muscles and spine.

Cellulose-Based Ionic Conductor: An Emerging Material toward Sustainable Devices

Ionic conductors (ICs) find widespread applications across different fields, such as smart electronic, ionotronic, sensor, biomedical, and energy harvesting/storage devices, and largely determine the function and performance of these devices. In the pursuit of developing ICs required for better performing and sustainable devices, cellulose appears as an attractive and promising building block due to its high abundance, renewability, striking mechanical strength, and other functional features. In this review, we provide a comprehensive summary regarding ICs fabricated from cellulose and cellulose-derived materials in terms of fundamental structural features of cellulose, the materials design and fabrication techniques for engineering, main properties and characterization, and diverse applications. Next, the potential of cellulose-based ICs to relieve the increasing concern about electronic waste within the frame of circularity and environmental sustainability and the future directions to be explored for advancing this field are discussed. Overall, we hope this review can provide a comprehensive summary and unique perspectives on the design and application of advanced cellulose-based ICs and thereby encourage the utilization of cellulosic materials toward sustainable devices.

Plant Leaf-Inspired Separators with Hierarchical Structure and Exquisite Fluidic Channels for Dendrite-Free Lithium Metal Batteries

Lithium (Li) metal batteries are among the most promising devices for high energy storage applications but suffer from severe and irregular Li dendrite growth. Here, it is demonstrated that the issue can be well tackled by precisely designing the leaf-like membrane with hierarchical structure and exquisite fluidic channels. As a proof of concept, plant leaf-inspired membrane (PLIM) separators are prepared using natural attapulgite nanorods. The PLIM separators feature super-electrolyte-philicity, high thermal stability and high ion-selectivity. Thus, the separators can guide uniform and directed Li growth on the Li anode. The Li//PLIM//Li cell with limited Li anode shows high Coulombic efficiency and cycling stability over 1500 h with small overpotential and interface impedance. The Li//PLIM//S battery exhibits high initial capacity (1352 mAh g-1 ), cycling stability (0.019% capacity decay per cycle at 1 C over 500 cycles), rate performance (673 mAh g-1 at 4 C), and high operating temperature (65 °C). The separators can also effectively improve reversibility and cycling stability of the Li/Li cell and Li//LFP battery with carbonate-based electrolyte. As such, this work provides fresh insights into the design of bioinspired separators for dendrite-free metal batteries.

Hydrogel electrodes with conductive and substrate-adhesive layers for noninvasive long-term EEG acquisition

Noninvasive brain-computer interfaces (BCIs) show great potential in applications including sleep monitoring, fatigue alerts, neurofeedback training, etc. While noninvasive BCIs do not impose any procedural risk to users (as opposed to invasive BCIs), the acquisition of high-quality electroencephalograms (EEGs) in the long term has been challenging due to the limitations of current electrodes. Herein, we developed a semidry double-layer hydrogel electrode that not only records EEG signals at a resolution comparable to that of wet electrodes but is also able to withstand up to 12 h of continuous EEG acquisition. The electrode comprises dual hydrogel layers: a conductive layer that features high conductivity, low skin-contact impedance, and high robustness; and an adhesive layer that can bond to glass or plastic substrates to reduce motion artifacts in wearing conditions. Water retention in the hydrogel is stable, and the measured skin-contact impedance of the hydrogel electrode is comparable to that of wet electrodes (conductive paste) and drastically lower than that of dry electrodes (metal pin). Cytotoxicity and skin irritation tests show that the hydrogel electrode has excellent biocompatibility. Finally, the developed hydrogel electrode was evaluated in both N170 and P300 event-related potential (ERP) tests on human volunteers. The hydrogel electrode captured the expected ERP waveforms in both the N170 and P300 tests, showing similarities in the waveforms generated by wet electrodes. In contrast, dry electrodes fail to detect the triggered potential due to low signal quality. In addition, our hydrogel electrode can acquire EEG for up to 12 h and is ready for recycled use (7-day tests). Altogether, the results suggest that our semidry double-layer hydrogel electrodes are able to detect ERPs in the long term in an easy-to-use fashion, potentially opening up numerous applications in real-life scenarios for noninvasive BCI.

Hairy-Skin-Adaptive Viscoelastic Dry Electrodes for Long-Term Electrophysiological Monitoring

Long-term epidermal electrophysiological (EP) monitoring is crucial for disease diagnosis and human-machine synergy. The human skin is covered with hair that grows at an average rate of 0.3 mm per day. This impedes a stable contact between the skin and dry epidermal electrodes, resulting in motion artifacts during ultralong-term EP monitoring. Therefore, accurate and high-quality EP signal detection remains challenging. To address this issue, a new solution-the hairy-skin-adaptive viscoelastic dry electrode (VDE) is reported. This innovative technology is capable of bypassing hair and filling into the skin wrinkles, leading to long-lasting and stable interface impedance. The VDE maintains a stable interface impedance for a remarkable period of 48 days and 100 cycles. The VDE is highly effective in shielding against hair disturbances in electrocardiography (ECG) monitoring, even during intense chest expansion, and in electromyography (EMG) monitoring during large strain. Furthermore, the VDE is easily attachable to the skull without requiring any electroencephalogram (EEG) cap or bandage, making it an ideal solution for EEG monitoring. This work represents a substantial breakthrough in the field of EP monitoring, providing a solution for the previously challenging issue of monitoring human EP signals on hairy skin.

Recent Progress of Biomaterials-Based Epidermal Electronics for Healthcare Monitoring and Human-Machine Interaction

Epidermal electronics offer an important platform for various on-skin applications including electrophysiological signals monitoring and human-machine interactions (HMI), due to their unique advantages of intrinsic softness and conformal interfaces with skin. The widely used nondegradable synthetic materials may produce massive electronic waste to the ecosystem and bring safety issues to human skin. However, biomaterials extracted from nature are promising to act as a substitute material for the construction of epidermal electronics, owing to their diverse characteristics of biocompatibility, biodegradability, sustainability, low cost and natural abundance. Therefore, the development of natural biomaterials holds great prospects for advancement of high-performance sustainable epidermal electronics. Here, we review the recent development on different types of biomaterials including proteins and polysaccharides for multifunctional epidermal electronics. Subsequently, the applications of biomaterials-based epidermal electronics in electrophysiological monitoring and HMI are discussed, respectively. Finally, the development situation and future prospects of biomaterials-based epidermal electronics are summarized. We expect that this review can provide some inspirations for the development of future, sustainable, biomaterials-based epidermal electronics.

UV and IR dual light triggered cellulose-based invisible actuators with high sensitivity

Actuators are widely used in bionic devices and soft robots, among which invisible actuators have some unique applications, including performing secret missions. In this paper, highly visible transparent cellulose-based UV-absorbing films were prepared by dissolving cellulose raw materials using N-methylmorpholine-N-oxide (NMMO) and using ZnO nanoparticles as UV absorbers. Furthermore, transparent actuator was fabricated by growing highly transparent and hydrophobic polytetrafluoroethylene (PTFE) film on regenerated cellulose (RC)-ZnO composite film. In addition to its sensitive response to Infrared (IR) light, the as-prepared actuator also shows a highly sensitive response to UV light, which is attributed to the strong absorption of UV light by ZnO NPs. Thanks to the drastic differences in adsorption capacity between the RC-ZnO and PTFE materials for water molecules, the asymmetrically- assembled actuator demonstrates extremely high sensitivity and excellent actuation performance, with a force density of 60.5, a maximum bending curvature of 3.0 cm^-1, and a response time of below 8 s. Bionic bug, smart door and the arm of excavator made from the actuator all exhibit sensitive responses to UV and IR lights.

Bioactive polymer-enabled conformal neural interface and its application strategies

Neural interface is a powerful tool to control the varying neuron activities in the brain, where the performance can directly affect the quality of recording neural signals and the reliability of in vivo connection between the brain and external equipment. Recent advances in bioelectronic innovation have provided promising pathways to fabricate flexible electrodes by integrating electrodes on bioactive polymer substrates. These bioactive polymer-based electrodes can enable the conformal contact with irregular tissue and result in low inflammation when compared to conventional rigid inorganic electrodes. In this review, we focus on the use of silk fibroin and cellulose biopolymers as well as certain synthetic polymers to offer the desired flexibility for constructing electrode substrates for a conformal neural interface. First, the development of a neural interface is reviewed, and the signal recording methods and tissue response features of the implanted electrodes are discussed in terms of biocompatibility and flexibility of corresponding neural interfaces. Following this, the material selection, structure design and integration of conformal neural interfaces accompanied by their effective applications are described. Finally, we offer our perspectives on the evolution of desired bioactive polymer-enabled neural interfaces, regarding the biocompatibility, electrical properties and mechanical softness.

Advances in machine learning for high value-added applications of lignocellulosic biomass

Lignocellulose can be converted into biofuel or functional materials to achieve high value-added utilization. Biomass utilization process is complex and multi-dimensional. This paper focuses on the biomass conversion reaction conditions, the preparation of biomass-based functional materials, the combination of biomass conversion and traditional wet chemistry, molecular simulation and process simulation. This paper analyzes the mechanism, advantages and disadvantages of important machine learning (ML) methods. The application examples of ML in different aspects of high value utilization of lignocellulose are summarized in detail. The challenges and future prospects of ML in this field are analyzed.

Nanoengineering and green chemistry-oriented strategies toward nanocelluloses for protein sensing

As one of the most important functional organic macromolecules of life, proteins not only participate in the cell metabolism and gene regulation, they also earnestly protect the body's immunity system, leading to a powerful biological shield and homeostasis. Advances in nanomaterials are boosting the significant progress in various applications, including the sensing and examination of proteins in trace amount. Nanocellulose-oriented protein sensing is at the forefront of this revolution. The inherent feature of high biocompatibility, low cytotoxicity, high specific area, good durability and marketability endow nanocellulose with great superiority in protein sensing. Here, we highlight the recent progress of protein sensing using nanocellulose as the biosensor in trace amount. Besides, various kinds of construction strategies for nanocelluloses-based biosensors are discussed in detail, to enhance the agility and accuracy of clinical/medical diagnostics. Finally, several challenges in the approbatory identification of new approaches for the marketization of biomedical sensing that need further expedition in the future are highlighted.

Electrochemical Detection of a Few Copies of Unamplified SARS-CoV-2 Nucleic Acids by a Self-Actuated Molecular System

The existing electrochemical biosensors lack controllable and intelligent merit to modulate the sensing process upon external stimulus, leading to challenges in analyzing a few copies of biomarkers in unamplified samples. Here, we present a self-actuated molecular-electrochemical system that consists of a tentacle and a trunk modification on a graphene microelectrode. The tentacle that contains a probe and an electrochemical label keeps an upright orientation, which increases recognition efficiency while decreasing the pseudosignal. Once the nucleic acids are recognized, the tentacles nearby along with the labels are spontaneously actuated downward, generating electrochemical responses under square wave voltammetry. Thus, it detects unamplified SARS-CoV-2 RNAs within 1 min down to 4 copies in 80 μL, 2-6 orders of magnitude lower than those of other electrochemical assays. Double-blind testing and 10-in-1 pooled testing of nasopharyngeal samples yield high overall agreement with reverse transcription-polymerase chain reaction results. We fabricate a portable prototype based on this system, showing great potential for future applications.

2020 International brain-computer interface competition: A review

The brain-computer interface (BCI) has been investigated as a form of communication tool between the brain and external devices. BCIs have been extended beyond communication and control over the years. The 2020 international BCI competition aimed to provide high-quality neuroscientific data for open access that could be used to evaluate the current degree of technical advances in BCI. Although there are a variety of remaining challenges for future BCI advances, we discuss some of more recent application directions: (i) few-shot EEG learning, (ii) micro-sleep detection (iii) imagined speech decoding, (iv) cross-session classification, and (v) EEG(+ear-EEG) detection in an ambulatory environment. Not only did scientists from the BCI field compete, but scholars with a broad variety of backgrounds and nationalities participated in the competition to address these challenges. Each dataset was prepared and separated into three data that were released to the competitors in the form of training and validation sets followed by a test set. Remarkable BCI advances were identified through the 2020 competition and indicated some trends of interest to BCI researchers.

On-skin paintable biogel for long-term high-fidelity electroencephalogram recording

Long-term high-fidelity electroencephalogram (EEG) recordings are critical for clinical and brain science applications. Conductive liquid-like or solid-like wet interface materials have been conventionally used as reliable interfaces for EEG recording. However, because of their simplex liquid or solid phase, electrodes with them as interfaces confront inadequate dynamic adaptability to hairy scalp, which makes it challenging to maintain stable and efficient contact of electrodes with scalp for long-term EEG recording. Here, we develop an on-skin paintable conductive biogel that shows temperature-controlled reversible fluid-gel transition to address the abovementioned limitation. This phase transition endows the biogel with unique on-skin paintability and in situ gelatinization, establishing conformal contact and dynamic compliance of electrodes with hairy scalp. The biogel is demonstrated as an efficient interface for long-term high-quality EEG recording over several days and for the high-performance capture and classification of evoked potentials. The paintable biogel offers a biocompatible and long-term reliable interface for EEG-based systems.

Skin bioelectronics towards long-term, continuous health monitoring

Skin bioelectronics are considered as an ideal platform for personalised healthcare because of their unique characteristics, such as thinness, light weight, good biocompatibility, excellent mechanical robustness, and great skin conformability. Recent advances in skin-interfaced bioelectronics have promoted various applications in healthcare and precision medicine. Particularly, skin bioelectronics for long-term, continuous health monitoring offer powerful analysis of a broad spectrum of health statuses, providing a route to early disease diagnosis and treatment. In this review, we discuss (1) representative healthcare sensing devices, (2) material and structure selection, device properties, and wireless technologies of skin bioelectronics towards long-term, continuous health monitoring, (3) healthcare applications: acquisition and analysis of electrophysiological, biophysical, and biochemical signals, and comprehensive monitoring, and (4) rational guidelines for the design of future skin bioelectronics for long-term, continuous health monitoring. Long-term, continuous health monitoring of advanced skin bioelectronics will open unprecedented opportunities for timely disease prevention, screening, diagnosis, and treatment, demonstrating great promise to revolutionise traditional medical practices.

Recent Advances in Multiresponsive Flexible Sensors towards E-skin: A Delicate Design for Versatile Sensing

Multiresponsive flexile sensors with strain, temperature, humidity, and other sensing abilities serving as real electronic skin (e-skin) have manifested great application potential in flexible electronics, artificial intelligence (AI), and Internet of Things (IoT). Although numerous flexible sensors with sole sensing function have already been reported since the concept of e-skin, that mimics the sensing features of human skin, was proposed about a decade ago, the ones with more sensing capacities as new emergences are urgently demanded. However, highly integrated and highly sensitive flexible sensors with multiresponsive functions are becoming a big thrust for the detection of human body motions, physiological signals (e.g., skin temperature, blood pressure, electrocardiograms (ECG), electromyograms (EMG), sweat, etc.) and environmental stimuli (e.g., light, magnetic field, volatile organic compounds (VOCs)), which are vital to real-time and all-round human health monitoring and management. Herein, this review summarizes the design, manufacturing, and application of multiresponsive flexible sensors and presents the future challenges of fabricating these sensors for the next-generation e-skin and wearable electronics.

Low-Cost and Convenient Microchannel Resistance Biosensing Platform by Directly Translating Biorecognition into a Current Signal

We report a low-cost and convenient microchannel resistance (MCR) biosensing platform that uses current signal to report biorecognition. The biorecognition behavior between targets and biometric molecules (antigens, antibodies, or oligonucleotides) immobilized on magnetic beads and polystyrene (PS) microspheres induces a quantitative change in the unreacted PS microspheres. After magnetic separation, the unreacted PS microsphere solution is passed through the microchannel, leading to an obvious blocking effect, resulting in an increase in resistance, which can in turn be measured by monitoring the electric current. Thus, the biorecognition is directly converted into a detectable current signal without any bulky instruments or additional chemical reactions. The MCR biosensing platform is cost-effective and user-friendly with high accuracy. It can be an appropriate analysis technique for point-of-care testing in resource-poor settings.