Integration of stiff graphene and tough silk for the design and fabrication of versatile electronic materials

High-Performance Graphene Biocomposite Enabled by Fe3+ Coordination for Thermal Management

Emerging biocomposites with excellent heat dissipation capabilities and inherent sustainability are urgently needed to address the cooling issues of modern electronics and growing environmental concerns. However, the moisture stability, mechanical performance, thermal conductivity, and even flame retardancy of biomass-based materials are generally insufficient for practical thermal management applications. Herein, we present a high-performance graphene biocomposite consisting of carboxylated cellulose nanofibers and graphene nanosheets through an evaporation-induced self-assembly and subsequent Fe3+ cross-linking strategy. The Fe3+ coordination plays a critical role in stabilizing the material structure, thereby improving the mechanical strength and water stability of the biocomposite films, and its effect is revealed by density functional theory calculations. The hierarchical structure of the biocomposite films also leads to a high in-plane thermal conductivity of 42.5 W m-1 K-1, enabling a superior heat transfer performance. Furthermore, the resultant biocomposite films exhibit outstanding Joule heating performance with a fast thermal response and long-term stability, improved thermal stability, and flame retardancy. Therefore, such a general strategy and the desired overall properties of the biocomposite films offer wide application prospects for functional and safe thermal management.

Graphene oxide nanosheets augment silk fibroin aerogels for enhanced water stability and oil adsorption

Nanocomposite aerogels exhibit high porosity and large interfacial surface areas, enabling enhanced chemical transport and reactivity. Such mesoporous architectures can be prepared by freeze-casting naturally-derived biopolymers such as silk fibroin, but often form mechanically weak structures that degrade in water, which limits their performance under ambient conditions. Adding 2D material fillers such as graphene oxide (GO) or transition metal carbides (e.g. MXene) could potentially reinforce these aerogels via stronger intermolecular interactions with the polymeric binder. Here, we show that freeze-casting of GO nanosheets with silk fibroin results in a highly water-stable, mechanically robust aerogel, with considerably enhanced properties relative to silk-only or silk-MXene aerogels. These silk-GO aerogels exhibit high contact angles with water and are highly water stable. Moreover, aerogels can adsorb up 25-35 times their mass in oil, and can be used robustly for selective oil separation from water. This increased stability may occur due to strengthened intermolecular interactions such as hydrogen bonding, despite the random coil and α-helix conformation of silk fibroin, which is typically more soluble in water. Finally, we show these aerogels can be prepared at scale by freeze-casting on a copper mesh. Ultimately, we envision that these multicomponent aerogels could be widely utilized for molecular separations and environmental sensing, as well as for thermal insulation and electrical conductivity.

Printed sustainable elastomeric conductor for soft electronics

The widespread adoption of renewable and sustainable elastomers in stretchable electronics has been impeded by challenges in their fabrication and lacklustre performance. Here, we realize a printed sustainable stretchable conductor with superior electrical performance by synthesizing sustainable and recyclable vegetable oil polyurethane (VegPU) elastomeric binder and developing a solution sintering method for their composites with Ag flakes. The binder impedes the propagation of cracks through its porous network, while the solution sintering reaction reduces the resistance increment upon stretching, resulting in high stretchability (350%), superior conductivity (12833 S cm-1), and low hysteresis (0.333) after 100% cyclic stretching. The sustainable conductor was used to print durable and stretchable impedance sensors for non-obstructive detection of fruit maturity in food sensing technology. The combination of sustainable materials and strategies for realizing high-performance stretchable conductors provides a roadmap for the development of sustainable stretchable electronics.

A Simplified Method for the Preparation of Highly Conductive and Flexible Silk Nanofibrils/MXene Membrane

Silk nanofibers (SNF) have great applications in high-performance functional nanocomposites due to their excellent mechanical properties, biocompatibility, and degradability. However, the preparation of SNF by traditional methods often requires the use of some environmentally harmful or toxic reagents, limiting its application in green chemistry. In this paper, we successfully prepared SNF using natural silk as raw material and solvent stripping technology by adjusting the solvent concentration and solution ratio (the diameter of about 120 nm). Using the above SNFs as raw materials, SNF membranes were prepared by vacuum filtration technology. In addition, we prepared an SNF/MXene nanocomposite material with excellent humidity sensitivity by simply coating MXene nanosheets with silk fibers. The conductivity of the material can approach 1400.6 S m-1 with excellent mechanical strength (51.34 MPa). The SNF/MXene nanocomposite material with high mechanical properties, high conductivity, and green degradability can be potentially applied in the field of electromagnetic interference (EMI) shielding, providing a feasible approach for the development of functional nanocomposite materials.

Portable Electrochemical Sensor for Micromotor Speed Monitoring

Autonomous movement promotes practical applications of micromotors. Understanding the moving speeds is a crucial step in micromotor studies. The current analysis method relies on an expensive optical microscope, which is limited to laboratory settings. Herein, we have developed a lightweight (0.15 g), portable (2.0 × 3.5 cm2), and low-cost (approximately $0.26) micromotor sensor (μ-Motor sensor), composed of water-sensitive materials for micromotor speed monitoring. Moving micromotors induce fluid flow, enhancing the evaporation rate of the liquid medium. Consequently, a high correlation between motor speed and water molecule concentration above the moving medium has been established. The μ-Motor sensor enables a real-time readout of the moving speed in various settings, with high accuracy (≥95% in the lab and ≥90% in field studies at a local beach). The μ-Motor sensor opens up a new way for detecting micro/nanomachine movements, illuminating future applications of micro/nanorobotics for diverse scenarios.

Chemresistor Smart Sensors from Silk Fibroin-Graphene Composites for Touch-free Wearables

With the rapid development of wearable electronics, low-cost, multifunctional, ultrasensitive touch-free wearables for human-machine interaction and human/plant healthcare management have attracted great attention. The experience of fighting the COVID-19 epidemic has also confirmed the great significance of contactless sensation. Herein, a wearable smart-sensing platform using silk fibroin-reduced graphene oxide (SF-rGO) as bifunctional sensing active layers has been fabricated and integrated with a noncontact moisture/thermo sensor and Joule heater. As a result, the as-prepared smart sensor operated at 0.1 V exhibits good stability and sensitivity (sensor response of 60 for 97% RH) under a wide linear range of 6-97% RH, fast response/recover speed (real test: 21.51 s/85.62 s) toward touch-free humidity/temperature sensing for wearables, and thermal readings that can be accurately corrected by Joule heater. Impressively, it can achieve breath monitoring, mental state prediction, or elevator switching by identifying fingertip humidity variation. Prospectively, this all-in-one wearable smart sensor would set an example for improving sensing performance from structure-function relationship points of view and building a noncontact sensing system for daily life.

Self-Assembled Protein Nanofilm Regulating Uniform Zn Nucleation and Deposition Enabling Long-Life Zn Anodes

Rechargeable zinc-ion batteries (RZIBs) have gained promising attention as a feasible alternative for large-scale energy storage by the virtue of their intrinsic security, environmental benignity, low cost, and high volumetric capacity (5849 mAh cm-3 ). Nevertheless, the deep-rooted issues of dendrite formation and side reactions in unstable Zn metal anode have impeded RZIBs from being dependably deployed in their proposed applications. Herein, silk fibroin (SF) and lysozyme (ly), as natural biomacromolecules with abundant polar groups arranged in polypeptide backbones, are in situ self-assembled on the Zn anode surface to construct a homogeneous and compact protein nanofilm. Such protein nanofilm protecting layer presents a negative charge surface and significantly regulates Zn2+ deposition behavior. Meanwhile, synergistic flexible and robust features of protein nanofilm function as artificial solid electrolyte interface (SEI), accommodates the dynamic volume deformation during deposition/dissolution, and blocks corrosion of side reactions. Consequently, the electrochemical stability of protein nanofilm-modified Zn anode is greatly improved, with an excellent extended lifespan of over 1100 h at a high current density of 10 mA cm-2 and a high cycling capacity of 10 mAh cm-2 , corresponding to a high depth of discharge (83% DODZn ). Furthermore, the highly reversible Zn electrode remarkably improved the overall performance of MnO2 ||Zn full-cells.

Colloidal stability of graphene in aqueous medium: a theoretical approach through molecular dynamics

CONTEXT

Graphene has been used as reinforcement of polymeric nanocomposites to increase mechanical and electrical properties. Recently, graphene suspensions have been used for the development of nanofluids in automotive applications, where improvements in convection heat transfer coefficients and pressure drops have been reported. However, dispersions of graphene sheets in a polymeric matrix as well as in a solvent medium are difficult to achieve; that is because Van der Waals, [Formula: see text] and Coulombic interactions cause agglomerations. Surface chemical modifications have been considered as viable options to improve the graphene integration. In this work, we studied the colloidal stability of aqueous solutions of graphene sheets functionalized with (i) carboxylic groups, (ii) 3-amino-propyl tri-ethoxy silane (amphiphilic behavior), (iii) graphene oxide, and (iv) pristine graphene. Results show that the lower sedimentation velocity corresponds to the graphene functionalized with carboxylic groups, which presents the higher colloidal stability. However, the amphiphilic group enhances the interaction energy between graphene and the solvent; we believe that there is a threshold percentage of functionalization that improves the colloidal stability of graphene.

METHOD

Transport properties of graphene solutions were estimated by using Non-Equilibrium Molecular Dynamics simulations to generate Poiseuille flow in an NVT ensemble. Simulations were developed in the LAMMPS code. The COMPASS Force Field was used for the graphene systems and the TIP3P for the water molecules. Bonds and angles of hydrogen atoms were kept rigid by using the shake algorithm. The molecular models were built through MedeA and visualized with the Ovito software.

In Situ Mineralizing Spinning of Strong and Tough Silk Fibers for Optical Waveguides

Biopolymer-based optical waveguides with low-loss light guiding performance and good biocompatibility are highly desired for applications in biomedical photonic devices. Herein, we report the preparation of silk optical fiber waveguides through bioinspired in situ mineralizing spinning, which possess excellent mechanical properties and low light loss. Natural silk fibroin was used as the main precursor for the wet spinning of the regenerated silk fibroin (RSF) fibers. Calcium carbonate nanocrystals (CaCO₃ NCs) were in situ grown in the RSF network and served as nucleation templates for mineralization during the spinning, leading to the formation of strong and tough fibers. CaCO₃ NCs can guide the structure transformation of silk fibroin from random coils to β-sheets, contributing to enhanced mechanical properties. The tensile strength and toughness of the obtained fibers are up to 0.83 ± 0.15 GPa and 181.98 ± 52.42 MJ·m^-3, obviously higher than those of natural silkworm silks and even comparable to spider silks. We further investigated the performance of the fibers as optical waveguides and observed a low light loss of 0.46 dB·cm^-1, which is much lower than natural silk fibers. We believed that these silk-based fibers with excellent mechanical and light propagation properties are promising for applications in biomedical light imaging and therapy.

Light-driven textile sensors with potential application of UV detection

Smart textiles based on monitoring systems of health conditions, structural behaviour, and external environmental conditions have been presented as elegant solutions for the increasing demands of health care. In this study, cotton fabrics (CFs) were modified by a common strategy with a dipping-padding procedure using reduced graphene oxide (RGO) and a photosensitive dye, spiropyran (SP), which can detect environmental UV light. The morphology of the CF is observed by scanning electron microscopy (SEM) measurements showing that the topography structure of coatings is related to the SP content. The resistance of the textile sensors decreases after UV radiation, which may be attributed to the easier electron transmission on the coatings of the CF. With the increase of SP content, the introduction of a large amount of SP within the composites could cause discontinuous distributions of RGO in the fiber surfaces, preventing electron transmission within the coatings of the RGO. The surface wettability of the coatings and the sweat sensitivity are also studied before and after UV radiation.

A Comprehensive Review of Self-Healing Polymer, Metal, and Ceramic Matrix Composites and Their Modeling Aspects for Aerospace Applications

Composites can be divided into three groups based on their matrix materials, namely polymer, metal and ceramic. Composite materials fail due to micro cracks. Repairing is complex and almost impossible if cracks appear on the surface and interior, which minimizes reliability and material life. In order to save the material from failure and prolong its lifetime without compromising mechanical properties, self-healing is one of the emerging and best techniques. The studies to address the advantages and challenges of self-healing properties of different matrix materials are very limited; however, this review addresses all three different groups of composites. Self-healing composites are fabricated to heal cracks, prevent any obstructed failure, and improve the lifetime of structures. They can self-diagnose their structure after being affected by external forces and repair damages and cracks to a certain degree. This review aims to provide information on the recent developments and prospects of self-healing composites and their applications in various fields such as aerospace, automobiles etc. Fabrication and characterization techniques as well as intrinsic and extrinsic self-healing techniques are discussed based on the latest achievements, including microcapsule embedment, fibers embedment, and vascular networks self-healing.

Molecular Design and Preparation of Protein-Based Soft Ionic Conductors with Tunable Properties

Protein-based soft ionic conductors have attracted considerable research interest in recent years with great potential in applications at the human-machine interfaces. However, a fundamental mechanistic understanding of the ionic conductivity of silk-based ionic conductors is still unclear. Here, we first developed an environmental-friendly and scalable method to fabricate silk-based soft ionic conductors using silk proteins and calcium chloride. The mechanistic understanding of the ion transport and molecular interactions between calcium ions and silk proteins at variable water contents was investigated in-depth by combining experimental and simulation approaches. The results show that calcium ions primarily interact with amide groups in proteins at a low water content. The ionic conductivity is low since the calcium ions are confined around silk proteins within 2.0-2.6 Å. As water content increases, the calcium ions are hydrated with the formation of water shells, leading to the increased distance between calcium ions and silk proteins (3.3-6.0 Å). As a result, the motion of the calcium ions increased to achieve a higher ionic conductivity. By optimizing the ratio of the silk proteins, calcium ions, and water, silk-based soft ionic conductors with good stretchability and self-healing properties can be obtained. Such protein-based soft ionic conductors can be further used to fabricate smart devices such as electrochromic devices.

Thrombus-targeted nano-agents for NIR-II diagnostic fluorescence imaging-guided flap thromboembolism multi-model therapy

In oral and maxillofacial surgery, flap repair is essential to the quality of postoperative life. Still, thrombosis is fatal for the survival of the flaps. Besides, some postoperative thrombotic diseases, such as pulmonary embolism, also intimidate patients’ life. The traditional diagnostic methods are still limited by a large amount of hardware and suffer from inconvenience, delay, and subjectivity. Moreover, the treatments mainly rely upon thrombolytics, such as urokinase (UK) plasminogen activator, which may cause bleeding risk, especially intracerebral hemorrhage. Herein, a kind of poly (lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) containing a first near-infrared window (NIR-I) phototheranostic agent Y8 and urokinase plasminogen activator (UK) as the core, and modified with the fibrin-targeting peptide Gly–Pro–Arg–Pro–Pro (GPRPP) were developed for the flap and postoperative thromboembolism treatment (named GPRPP-Y8U@P). The conjugated molecule Y8 endows GPRPP-Y8U@P with the capacity of NIR-II imaging and excellent photothermal/photodynamic therapeutic effects. In vivo experiments demonstrated that GPRPP-Y8U@P could quickly locate thrombus by NIR-II fluorescence imaging, and semi-quantitative analysis of the embolized blood vessels' paraffin section verified its thrombolytic efficiency. Additionally, the urokinase trapped in the NPs would not result in nonspecific bleeding, tremendously improving physical security and curative effects with minimizing side effects. Overall, the advantages of GPRPP-Y8U@P, such as precise localization of the thrombus, thrombus ablation in the site, and mild side effects, demonstrated the attractiveness of this approach for effective clinical monitoring of thrombus therapy.

Preparation of a novel regenerated silk fibroin-based hydrogel for extrusion bioprinting

Three-dimensional (3D) bioprinting technology, allowing rapid prototyping and personalized customization, has received much attention in recent years, while regenerated silk fibroin (RSF) has also been widely investigated for its excellent biocompatibility, processibility, and comprehensive mechanical properties. However, due to the difficulty in curing RSF aqueous solution and the tendency of conformational transition of RSF chains under shearing, it is rather complicated to fabricate RSF-based materials with high mechanical strength through extrusion bioprinting. To solve this problem, a printable hydrogel with thixotropy was prepared from regenerated silk fibroin with high-molecular-weight (HMWRSF) combined with a small amount of hydroxypropyl methylcellulose (HPMC) in urea containing aqueous solution. It was found that the introduction of urea could not only vary the solid content of the hydrogel to benefit the mechanical properties of the 3D-bioprinted pre-cured hydrogels or 3D-bioprinted sponges, but also expand the "printable window" of this system. Indeed, the printability and rheological properties could be modulated by varying the solid content, the heating time, the urea/HMWRSF weight ratio, etc. Moreover, the microstructure of nanospheres stacked in these lyophilized 3D-bioprinted sponges was interesting to observe, which indicated the existence of microhydrogels and both "the reversible network" and "the irreversible network" in this HMWRSF-based pre-cured hydrogel. Like other HMWRSF materials fabricated in other ways, these 3D-bioprinted HMWRSF-based sponges exhibited good cytocompatibility for dental pulp mesenchymal stem cells. This work may inspire the design of functional HMWRSF-based materials by regulating the relationship between structure and properties.

Supertough and Highly Stretchable Silk Protein-based Films with Controlled Biodegradability

Naturally derived protein-based biopolymers are considered potential biomaterials in biomedical applications and eco-friendly materials for replacing current petroleum-based polymers due to their good biocompatibility, low environmental impact, and tunable degradability. However, current strategies for fabricating protein-based materials with superior properties and tailored functionality in a scalable manner are still lacking. Here, we demonstrate an aqueous-based scalable approach for fabricating silk protein-based films through controlled molecular self-assembly (CMS) of silk proteins with plasticizers and salt ions. The films fabricated using this method can achieve a toughness of up to 64±5 MJ/m³ with a stretchability of up to 574±31%. We also demonstrate the tunable enzymatic degradability, low in vitro cytotoxicity, and good in vivo biocompatibility of the films. Furthermore, the films can be patterned with predesigned complex structures through laser cutting and functionalized with bioactive components. The functional silk protein-based films show great potential in various applications, including flexible electronics, bioelectronics, tissue engineering, and bioplastic packaging. STATEMENT OF SIGNIFICANCE: Inspired by the naturally optimized multi-scale self-assembly of silk proteins in natural silks, we develop an aqueous-based approach for scalable production of superior protein-based films through controlled molecular self-assembly (CMS) of silk proteins with glycerol and calcium ions. The prepared silk films present outstanding mechanical properties, controlled enzymatic biodegradability, low in vitro cytotoxicity, and good in vivo biocompatibility. Notably, the films fabricated using this method can achieve a high toughness of 64±5 MJ/m³ with a stretchability of 594±31%. The approach introduced in this work provides a facile route toward making silk-based materials with superior properties. It also paves new avenues for developing functional protein-based materials with precisely controlled structures and properties for various applications.

Progress in recycling and valorization of waste silk

Due to the improvements in living standards and the "throw away" culture of mankind, large amount of waste textiles is constantly generated. In particular, silk is an abundant high-grade textile material with characteristics of wear comfort, high profit, and low supply with high demand, but it transforms into waste when discarded. This paper reviews the current progress of recycling and reuse of waste silk from the aspects of energy, yarn and fabric, reinforcement of composites, silk fibroin, biological tissue engineering, filtration of air and water, and electrode. The modification, optimization and application of regenerated silk fibroin extracted from waste silk are promising to industrialization and sustainable development. Making waste silk functional and intelligently wearable are two ways of recycling waste silk with low cost and high return value in the near future. The recovery and utilization of waste silk provide a paradigm for valorization of other fiber-based waste such as wool, cotton, bast and synthetic fibers.

Global Trends in Natural Biopolymers in the 21st Century: A Scientometric Review

Since the 21st century, natural biopolymers have played an indispensable role in long-term global development strategies, and their research has shown a positive growth trend. However, these substantive scientific results are not conducive to our quick grasp of hotspots and insight into future directions and to understanding which local changes have occurred and which trend areas deserve more attention. Therefore, this study provides a new data-driven bibliometric analysis strategy and framework for mining the core content of massive bibliographic data, based on mathematical models VOS Viewer and CiteSpace software, aiming to understand the research prospects and opportunities of natural biopolymers. The United States is reported to be the most important contributor to research in this field, with numerous publications and active institutions; polymer science is the most popular subject category, but the further emphasis should be placed on interdisciplinary teamwork; mainstream research in this field is divided into five clusters of knowledge structures; since the explosion in the number of articles in 2018, researchers are mainly engaged in three fields: “medical field,” “biochemistry field,” and “food science fields.” Through an in-depth analysis of natural biopolymer research, this article provides a better understanding of trends emerging in the field over the past 22 years and can also serve as a reference for future research.

Realizing Spontaneously Regular Stacking of Pristine Graphene Oxide by a Chemical-Structure-Engineering Strategy for Mechanically Strong Macroscopic Films

Mechanical-electrical properties of macroscopic graphene films derived from graphene oxide (GO) sheets are substantially restricted by their surface wrinkles and structural misalignment. Herein, we propose a chemical-structure-engineering strategy to realize the spontaneously regular stacking of modified GO (GO-m) with trace carboxyl. The highly aligned GO-m film delivers a fracture strength and modulus of nearly 3- and 5-fold higher than a wrinkled film with conventional Hummer's method derived GO (GO-c). The favorable assembly pattern of GO-m sheets is attributed to their decreased interfacial friction on the atomic scale, which weakens their local gelation capability for freer configuration adjustment during the assembly process. The chemical structure of GO-m can be further engineered by an epoxide-to-hydroxyl reaction, achieving a record high tensile strength of up to 631 MPa for the pristine GO film. By exploring the relationship between the surface terminations of GO and its stacking mode, this work proves the feasibility to realize high-performance macroscopic materials with optimized microstructure through the chemical modulation of nanosheet assembly.

Silk Fiber Multiwalled Carbon Nanotube-Based Micro-/Nanofiber Composite as a Conductive Fiber and a Force Sensor

Silk cocoon fibers (SFs) are natural polymers that are made up of fibroin protein. These natural fibers have higher mechanical stability and good elasticity properties. In this work, we coated multiwalled carbon nanotubes (MWCNTs) on the surface of SFs using a simple stirring technique with vinegar as the medium. This SF-MWCNT micro-/nanofiber composite was prepared without any adhesives. The characterization results revealed that the SF-MWCNT micro-/nanofiber composite exhibited excellent electrical conductivity (995 Ω cm^-1), tensile strength (up to 200% greater elongation), and durability characteristics. In addition, this micro-/nanofiber composite shows a change in resistance from 1450 to 960 Ω cm^-1 for an applied mechanical force of 0.3-1 N kg^-1. Based on our findings, SF-MWCNT micro-/nanofiber composite-based conductive fibers (CFs) and force sensors (FSs) were developed.

Natural Polymer in Soft Electronics: Opportunities, Challenges, and Future Prospects

Pollution caused by nondegradable plastics has been a serious threat to environmental sustainability. Natural polymers, which can degrade in nature, provide opportunities to replace petroleum-based polymers, meanwhile driving technological advances and sustainable practices. In the research field of soft electronics, regenerated natural polymers are promising building blocks for passive dielectric substrates, active dielectric layers, and matrices in soft conductors. Here, the natural-polymer polymorphs and their compatibilization with a variety of inorganic/organic conductors through interfacial bonding/intermixing and surface functionalization for applications in various device modalities are delineated. Challenges that impede the broad utilization of natural polymers in soft electronics, including limited durability, compromises between conductivity and deformability, and limited exploration in controllable degradation, etc. are explicitly inspected, while the potential solutions along with future prospects are also proposed. Finally, integrative considerations on material properties, device functionalities, and environmental impact are addressed to warrant natural polymers as credible alternatives to synthetic ones, and provide viable options for sustainable soft electronics.

Recent Research on Hybrid Hydrogels for Infection Treatment and Bone Repair

The repair of infected bone defects (IBDs) is still a great challenge in clinic. A successful treatment for IBDs should simultaneously resolve both infection control and bone defect repair. Hydrogels are water-swollen hydrophilic materials that maintain a distinct three-dimensional structure, helping load various antibacterial drugs and biomolecules. Hybrid hydrogels may potentially possess antibacterial ability and osteogenic activity. This review summarizes the recent progress of different kinds of antibacterial agents (including inorganic, organic, and natural) encapsulated in hydrogels. Several representative hydrogels of each category and their antibacterial mechanism and effect on bone repair are presented. Moreover, the advantages and disadvantages of antibacterial agent hybrid hydrogels are discussed. The challenge and future research directions are further prospected.

Graphene-enabled wearable sensors for healthcare monitoring

Wearable sensors in healthcare monitoring have recently found widespread applications in biomedical fields for their non- or minimal-invasive, user-friendly and easy-accessible features. Sensing materials is one of the major challenges to achieve these superiorities of wearable sensors for healthcare monitoring, while graphene-based materials with many favorable properties have shown great efficiency in sensing various biochemical and biophysical signals. In this paper, we review state-of-the-art advances in the development and modification of graphene-based materials (i.e., graphene, graphene oxide and reduced graphene oxide) for fabricating advanced wearable sensors with 1D (fibers), 2D (films) and 3D (foams/aerogels/hydrogels) macroscopic structures. We summarize the structural design guidelines, sensing mechanisms, applications and evolution of the graphene-based materials as wearable sensors for healthcare monitoring of biophysical signals (e.g., mechanical, thermal and electrophysiological signals) and biochemical signals from various body fluids and exhaled gases. Finally, existing challenges and future prospects are presented in this area.

Structure of Animal Silks

As an abundant fibrous protein, animal silks have received a variety of interests in both traditional and high-tech industries, such as textiles, decoration, and biomedicine, due to their unique advantages in mechanical performance, sustainability, biocompatibility, and biodegradability. While developing applications of animal silks, the structure of animal silks has also received more and more attention in these decades. Briefly, most animal silks can be considered as semicrystalline fibers, which are composed of β-sheet nanocrystals and amorphous regions. However, different animal silks have similarities and also have obvious differences at different structural levels. In this chapter, we will introduce the structures of the three most representative animal silks, that is, spider dragline silk, tussah silk, and mulberry silk. The similarities and differences in their structures will be highlighted, so as to provide fundamental guidance for the research and use of these animal silks.

Activation of extracellular electron network in non-electroactive bacteria by Bombyx mori silk

Extracellular electron transfer material (EETM) has increasingly attracted attentions for the enhancing effect on multiple microbial reactions. Especially, EETM is known to be essential to activate the energy network in non-electroactive bacteria. It is motivated to find out an EETM which is natural-based, environmentally friendly, and easily produced at large-scale. In this study, Bombyx mori silk is found, for the first time, to function as an EETM by using an EETM-dependent pentachlorophenol (PCP) dechlorinating anaerobic microbial culture. Subsequently, by dividing fibroin fiber into different soluble/insoluble fractions and correlating their EET functions with their structural properties based on various spectroscopic analyses, the β-sheet configuration is suggested as an essential structure supporting the EET function of silk materials. The analyses also suggested the involvement of sulfur-containing amino acids in this function. The EET function is not degraded by boiling or acid/alkaline treatments and the material can be utilized multiple times, although it is susceptible to UV irradiation. Bombyx mori silk also enhance other microbial reactions, including Fe(III)OOH reduction, CO₂ reduction to acetate, and nitrogen fixation. This discovery provides a basis for developing biotechnology for environmental remediation, global warming reduction, and biofertilizer production using Bombyx mori silk and its wastes.

New Silk Road: From Mesoscopic Reconstruction/Functionalization to Flexible Meso-Electronics/Photonics Based on Cocoon Silk Materials

Two of the key questions to be addressed are whether and how one can turn cocoon silk into fascinating materials with different electronic and optical functions so as to fabricate the flexible devices. In this review, a comprehensive overview of the unique strategy of mesoscopic functionalization starting from silk fibroin (SF) materials to the fabrication of various meso flexible SF devices is presented. Notably, SF materials with novel and enhanced properties can be achieved by mesoscopically reconstructing the hierarchical structures of SF materials. This is based on rerouting the refolding process of SF molecules by meso-nucleation templating. As-acquired functionalized SF materials can be applied to fabricate bio-compatible/degradable flexible/implantable meso-optical/electronic devices of various types. Consequently, functionalized SF can be fabricated into optical elements, that is, nonlinear photonic and fluorescent components, and make it possible to construct silk meso-electronics with high-performance. These advances enable the applications of SF-material based devices in the areas of physical and biochemical sensing, meso-memristors, transistors, brain electrodes, and energy generation/storage, applicable to on-skin long-term monitoring of human physiological conditions, and in-body sensing, information processing, and storage.

Biospired Janus Silk E-Textiles with Wet-Thermal Comfort for Highly Efficient Biofluid Monitoring

Functionalized textiles capable of biofluid administration are favorable for enhancing the wet-thermal comfort of the wearer and healthcare performance. Herein, inspired by the Janus wettability of lotus leaf, we propose a skin-comfortable Janus electronic textile (e-textile) based on natural silk materials for managing and analysis of biofluid. Silk materials are chosen and modified as both a textile substrate and a sensing electrode due to its natural biocompatibility. The unidirectional biofluid behavior of such Janus silk substrate facilitates a comfortable skin microenvironment, including weakening the undesired wet adhesion (∼0 mN cm^-2) and avoiding excessive heat or cold on the epidermis. We noninvasively analyze multiple targets of human sweat with less required liquid volume (∼5 μL) and a faster (2-3 min) response time based on the silk-based yarn electrode woven into the hydrophilic side of Janus silk. This work bridges the gap between physiological comfort and sensing technology using biomass-derived elements, presenting a new type of smart textiles for wet-thermal management and health monitoring.

Bio-inspired graphene-based nano-systems for biomedical applications

The increasing demands of environmentally sustainable, affordable, and scalable materials have inspired researchers to explore greener nanosystems of unique properties which can enhance the performance of existing systems. Such nanosystems, extracted from nature, are state-of-art high-performance nanostructures due to intrinsic hierarchical micro/nanoscale architecture and generous interfacial interactions in natural resources. Among several, bio-inspired nanosystems graphene nanosystems have emerged as an essential nano-platform wherein a highly electroactive, scalable, functional, flexible, and adaptable to a living being is a key factor. Preliminary investigation project bio-inspired graphene nanosystems as a multi-functional nano-platform suitable for electronic devices, energy storage, sensors, and medical sciences application. However, a broad understanding of bio-inspired graphene nanosystems and their projection towards applied application is not well-explored yet. Considering this as a motivation, this mini-review highlights the following; the emergence of bio-inspired graphene nanosystems, over time development to make them more efficient, state-of-art technology, and potential applications, mainly biomedical including biosensors, drug delivery, imaging, and biomedical systems. The outcomes of this review will certainly serve as a guideline to motivate scholars to design and develop novel bio-inspired graphene nanosystems to develop greener, affordable, and scalable next-generation biomedical systems.

A Review on the Production Methods and Applications of Graphene-Based Materials

Graphene-based materials in the form of fibres, fabrics, films, and composite materials are the most widely investigated research domains because of their remarkable physicochemical and thermomechanical properties. In this era of scientific advancement, graphene has built the foundation of a new horizon of possibilities and received tremendous research focus in several application areas such as aerospace, energy, transportation, healthcare, agriculture, wastewater management, and wearable technology. Although graphene has been found to provide exceptional results in every application field, a massive proportion of research is still underway to configure required parameters to ensure the best possible outcomes from graphene-based materials. Until now, several review articles have been published to summarise the excellence of graphene and its derivatives, which focused mainly on a single application area of graphene. However, no single review is found to comprehensively study most used fabrication processes of graphene-based materials including their diversified and potential application areas. To address this genuine gap and ensure wider support for the upcoming research and investigations of this excellent material, this review aims to provide a snapshot of most used fabrication methods of graphene-based materials in the form of pure and composite fibres, graphene-based composite materials conjugated with polymers, and fibres. This study also provides a clear perspective of large-scale production feasibility and application areas of graphene-based materials in all forms.

Isolation of Nanofibrils from Animal Silks

The presence of well-organized nanofibrils in animal silks is considered to provide them excellent mechanical and biochemical properties. To direct utilize these unique natural nanomaterials, a variety of physical and/or chemical processes have been developed for directly isolating silk nanofibrils from animal silks. The yield and processability of these techniques as well as the morphologies of resultant silk nanofibrils have apparent differences but also have their own merits. In this chapter, I presented the protocols for isolation silk nanofibrils, including a physical approach of sonication, a chemical approach of salt-formic acid dissolution, as well as three combination approaches, hexafluoroisopropanol liquid exfoliation, urea-guanidine hydrochloride dissolution, and sodium hypochlorite partial dissolution.

Self-adhesive and contractile silk fibroin/graphene nano-ionotronic skin for strain sensing of irregular surfaces

Nanofiber-based artificial skin has shown promise for application in flexible wearable electronics due to its favorable breathability and comfortable wearability. However, the electrospinning method commonly used for nanofiber preparation suffers from poor spinning performance when used for ionotronic solutions. Moreover, the resulting membrane usually lacks self-adhesive and self-adapting properties when it is attached to an irregular subject, which greatly hinders its practical usage. Herein, a self-adhesive and contractile silk fibroin/graphene nano-ionotronic skin was successfully prepared using a high-yield electro-blowing technique. The electro-blowing technique was able to effectively overcome the instability of the spinning jet and raise the feed rate to at least 5 ml h^-1. The high Ca²⁺content provided the fabricated nano-ionotronic skin with humidity-induced stretchability and robusticity. More importantly, the ionotronic skin also possessed a self-adhesive property and was able to contract to adapt to irregular surfaces. Additionally, an analytical piezoresistive model was successfully built to predict the response of the sensors to stress. Furthermore, due to its stable conductivity, sensitivity, and self-adapting property, the obtained nano-ionotronic skin can be used for body monitoring, for example, for bending of the arm and hand gestures. The design and manufacture concept proposed in this work might inspire the development of high-yield ionotronic nanofibers and the design of self-adapting artificial skin.

Electro-Blown Spun Silk/Graphene Nanoionotronic Skin for Multifunctional Fire Protection and Alarm

Artificial protective skins are widely used in artificial intelligence robots, such as humanoid robots, mobile manipulation robots, and automatic probe robots, but their safety in use, especially flame retardancy, is rarely considered. As many artificial skins are designed for use in flammable or even explosive environments, flammability is a significant concern. Herein, a flame-retardant silk/graphene nanoionotronic (SGNI) skin is developed by using a rationally designed high-throughput electro-blown spinning technique, with a more efficient production efficiency than electrospinning. These flame retardant SGNI skins combine the advantages of nanofibrous and ionotronic materials, and they are sustainable, conductive, highly porous, mechanically robust, highly stretchable, self-adhesive, and humidity- and temperature-sensitive. These merits support the assembly of SGNI skins into a fire alarm system, with real-time alarm (response in 2 s) to mobile phones, clouds, and a central control system. The concept that combines a flame retardant and fire alarm material into an intelligent skin may provide potential solutions toward the design of protective skins for robotics and human-machine interactions.

Mechanical Training-Driven Structural Remodeling: A Rational Route for Outstanding Highly Hydrated Silk Materials

Highly hydrated silk materials (HHSMs) have been the focus of extensive research due to their usefulness in tissue engineering, regenerative medicine, and soft devices, among other fields. However, HHSMs have weak mechanical properties that limit their practical applications. Inspired by the mechanical training-driven structural remodeling strategy (MTDSRS) in biological tissues, herein, engineered MTDSRS is developed for self-reinforcement of HHSMs to improve their inherent mechanical properties and broaden potential utility. The MTDSRS consists of repetitive mechanical training and solvent-induced conformation transitions. Solvent-induced conformation transition enables the formation of β-sheet physical crosslinks among the proteins, while the repetitive mechanical loading allows the rearrangement of physically crosslinked proteins along the loading direction. Such synergistic effects produce strong and stiff mechanically trained-HHSMs (MT-HHSMs). The fracture strength and Young's modulus of the resultant MT-HHSMs (water content of 43 ± 4%) reach 4.7 ± 0.9 and 21.3 ± 2.1 MPa, respectively, which are 8-fold stronger and 13-fold stiffer than those of the as-prepared HHSMs, as well as superior to most previously reported HHSMs with comparable water content. In addition, the animal silk-like highly oriented molecular crosslinking network structure also provides MT-HHSMs with fascinating physical and functional features, such as stress-birefringence responsibility, humidity-induced actuation, and repeatable self-folding deformation.

Understanding Plant Biomass via Computational Modeling

Plant biomass, especially wood, has been used for structural materials since ancient times. It is also showing great potential for new structural materials and it is the major feedstock for the emerging biorefineries for building a sustainable society. The plant cell wall is a hierarchical matrix of mainly cellulose, hemicellulose, and lignin. Herein, the structure, properties, and reactions of cellulose, lignin, and wood cell walls, studied using density functional theory (DFT) and molecular dynamics (MD), which are the widely used computational modeling approaches, are reviewed. Computational modeling, which has played a crucial role in understanding the structure and properties of plant biomass and its nanomaterials, may serve a leading role on developing new hierarchical materials from biomass in the future.

Breathable Ti3C2Tx MXene/Protein Nanocomposites for Ultrasensitive Medical Pressure Sensor with Degradability in Solvents

Flexible, breathable, and degradable pressure sensors with excellent sensing performance are drawing tremendous attention for various practical applications in wearable artificial skins, healthcare monitoring, and artificial intelligence due to their flexibility, breathability, lightweight, decreased electronic rubbish, and environmentally friendly impact. However, traditional plastic or elastomer substrates with impermeability, uncomfortableness, mechanical mismatches, and nondegradability greatly restricted their practical applications. Therefore, the fabrication of such pressure sensors with high flexibility, facile degradability, and breathability is still a critical challenge and highly desired. Herein, we present a wearable, breathable, degradable, and highly sensitive MXene/protein nanocomposites-based pressure sensor. The fabricated MXene/protein-based pressure sensor is assembled from a breathable conductive MXene coated silk fibroin nanofiber (MXene-SF) membrane and a silk fibroin nanofiber membrane patterned with a MXene ink-printed (MXene ink-SF) interdigitated electrode, which can serve as the sensing layer and the electrode layer, respectively. The assembled pressure sensor exhibits a wide sensing range (up to 39.3 kPa), high sensitivity (298.4 kPa^-1 for 1.4-15.7 kPa; 171.9 kPa^-1 for 15.7-39.3 kPa), fast response/recovery time (7/16 ms), reliable breathability, excellent cycling stability over 10 000 cycles, good biocompatibility, and robust degradability. Furthermore, it shows great sensing performance in monitoring human psychological signals, acting as an artificial skin for the quantitative illustration of pressure distribution, and wireless biomonitoring in real time. Considering the biodegradable and breathable features, the sensor may become promising to find potential applications in smart electronic skins, human motion detection, disease diagnosis, and human-machine interaction.

A capacitive humidity sensor based on all-protein embedded with gold nanoparticles @ carbon composite for human respiration detection

Wool and silk fiber are the most extensive resources of protein fibers and have been used in the textile field for many years. The extracted biocompatible proteins are more and more widely used in flexible devices, sensors, tissue engineering, etc. Here, a fully biomaterial based flexible humidity sensor has been successfully fabricated for the first time. Interdigital electrodes of humidity sensor are printed on a transparent sensor substrate made of silk protein by inkjet printing. The humidity sensitive material is gold nanoparticles hosted nitrogen doped carbon (AuNPs@NC), which is fabricated by in situ dispersion of gold nanoparticles in a wool keratin assisted porous carbon precursor. The best treatment condition of the sensitive materials is obtained by comparing the sensitivity of humidity response. Moreover, the as-prepared biocompatible flexible sensor was successfully used to detect human respiration.

Thermochromic Silks for Temperature Management and Dynamic Textile Displays

HIGHLIGHTS

Wearable and smart textiles are constructed by integrating embroidery technology and 5G cloud communication, showing promising applications in temperature management and real-time dynamic textile displays. Thermochromism is introduced into the natural silk to produce high-performance thermochromic silks (TCSs) through a low cost, sustainable, efficient, and scalable strategy. The interfacial bonding of the continuously produced TCSs is in situ analyzed and improved through pre-solvent treatment and is confirmed using synchrotron Fourier transform infrared microspectroscopy.

ABSTRACT

Silks have various advantages compared with synthetic polymer fibers, such as sustainability, mechanical properties, luster, as well as air and humidity permeability. However, the functionalization of silks has not yet been fully developed. Functionalization techniques that retain or even improve the sustainability of silk production are required. To this end, a low-cost, effective, and scalable strategy to produce TCSs by integrating yarn-spinning and continuous dip coating technique is developed herein. TCSs with extremely long length (> 10 km), high mechanical performance (strength of 443.1 MPa, toughness of 56.0 MJ m⁻³, comparable with natural cocoon silk), and good interfacial bonding were developed. TCSs can be automatically woven into arbitrary fabrics, which feature super-hydrophobicity as well as rapid and programmable thermochromic responses with good cyclic performance: the response speed reached to one second and remained stable after hundreds of tests. Finally, applications of TCS fabrics in temperature management and dynamic textile displays are demonstrated, confirming their application potential in smart textiles, wearable devices, flexible displays, and human–machine interfaces. Moreover, combination of the fabrication and the demonstrated applications is expected to bridge the gap between lab research and industry and accelerate the commercialization of TCSs. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-021-00591-w.

Three-Dimensional Printing to Build Fibrous Protein Architectures

Fibrous proteins are promising bioinks for three-dimensional printing techniques to fabricate sophisticated structures that find applications in both biomedical engineering and materials science. The critical point of manufacturing these fibrous protein inks is to adjust the cross-linking and rheology properties of proteins that matching the requirements of various printing techniques. In recent years, 3D printing techniques such as extrusion-based printing, droplet-based printing, and light-assisted printing techniques have widely been applied to build sophisticated fibrous protein architectures. In this regard, a series of fibrous protein-based bioinks have been developed, such as bioinks prepared from silk fibroin, collagen, fibrin, gelatin, and recombinant spider silk. In this chapter, we present the protocols to make various fibrous protein inks, as well as how to use these bioinks to print 3D structures via different printing techniques.

Review of Graphene-Based Textile Strain Sensors, with Emphasis on Structure Activity Relationship

Graphene-based textile strain sensors were reviewed in terms of their preparation methods, performance, and applications with particular attention on its forming method, the key properties (sensitivity, stability, sensing range and response time), and comparisons. Staple fiber strain sensors, staple and filament strain sensors, nonwoven fabric strain sensors, woven fabric strain sensors and knitted fabric strain sensors were summarized, respectively. (i) In general, graphene-based textile strain sensors can be obtained in two ways. One method is to prepare conductive textiles through spinning and weaving techniques, and the graphene worked as conductive filler. The other method is to deposit graphene-based materials on the surface of textiles, the graphene served as conductive coatings and colorants. (ii) The gauge factor (GF) value of sensor refers to its mechanical and electromechanical properties, which are the key evaluation indicators. We found the absolute value of GF of graphene-based textile strain sensor could be roughly divided into two trends according to its structural changes. Firstly, in the recoverable deformation stage, GF usually decreased with the increase of strain. Secondly, in the unrecoverable deformation stage, GF usually increased with the increase of strain. (iii) The main challenge of graphene-based textile strain sensors was that their application capacity received limited studies. Most of current studies only discussed washability, seldomly involving the impact of other environmental factors, including friction, PH, etc. Based on these developments, this work was done to provide some merit to references and guidelines for the progress of future research on flexible and wearable electronics.

Photo-Crosslinked Silk Fibroin for 3D Printing

Silk fibroin in material formats provides robust mechanical properties, and thus is a promising protein for 3D printing inks for a range of applications, including tissue engineering, bioelectronics, and bio-optics. Among the various crosslinking mechanisms, photo-crosslinking is particularly useful for 3D printing with silk fibroin inks due to the rapid kinetics, tunable crosslinking dynamics, light-assisted shape control, and the option to use visible light as a biocompatible processing condition. Multiple photo-crosslinking approaches have been applied to native or chemically modified silk fibroin, including photo-oxidation and free radical methacrylate polymerization. The molecular characteristics of silk fibroin, i.e., conformational polymorphism, provide a unique method for crosslinking and microfabrication via light. The molecular design features of silk fibroin inks and the exploitation of photo-crosslinking mechanisms suggest the exciting potential for meeting many biomedical needs in the future.

Unpredictable recombination of PB transposon in Silkworm: a potential risk

The piggyBac (PB) transposon is the most widely used vector for generating transgenic silkworms. The stability of the PB transposon in the receptor is a serious concern that requires attention because of biosafety concerns. In this study, we found that the transgene silkworm developed loss of reporter gene traits. To further investigate the regularity, we traced the genes and traits of this silkworm. After successful alteration of the silkworm genome with the MASP1 gene (named red-eyed silkworm; RES), silkworm individuals with lost reporter genes were found after long-term transgenerational breeding and were designated as the white-eyed silkworm (WES). PCR amplification indicated that exogenous genes had been lost in the WES. Testing was conducted on the PB transposons, and the left arm (L arm) did not exist; however, the right arm (R arm) was preserved. Amino acid analysis showed that the amino acid content of the WES changed versus the common silkworm and RES. These results indicate that the migration of PB transposons in Bombyx mori does occur and is unpredictable. This is because the silkworm genome contains multiple PB-like sequences that might influence the genetic stability of transgenic lines. When using PB transposons as a transgene vector, it is necessary to fully evaluate and take necessary measures to prevent its re-migration in the recipient organism. Further experiments are needed if we want to clarify the regularity of the retransposition phenomenon and the direct and clear association with similar sequences of transposons.

From Silk Spinning to 3D Printing: Polymer Manufacturing using Directed Hierarchical Molecular Assembly

Silk spinning offers an evolution-based manufacturing strategy for industrial polymer manufacturing, yet remains largely inaccessible as the manufacturing mechanisms in biological and synthetic systems, especially at the molecular level, are fundamentally different. The appealing characteristics of silk spinning include the sustainable sourcing of the protein material, the all-aqueous processing into fibers, and the unique material properties of silks in various formats. Substantial progress has been made to mimic silk spinning in artificial manufacturing processes, despite the gap between natural and artificial systems. This report emphasizes the universal spinning conditions utilized by both spiders and silkworms to generate silk fibers in nature, as a scientific and technical framework for directing molecular assembly into high-performance structures. The preparation of regenerated silk feedstocks and mimicking native spinning conditions in artificial manufacturing are discussed, as is progress and challenges in fiber spinning and 3D printing of silk-composites. Silk spinning is a biomimetic model for advanced and sustainable artificial polymer manufacturing, offering benefits in biomedical applications for tissue scaffolds and implantable devices.

From Molecular Reconstruction of Mesoscopic Functional Conductive Silk Fibrous Materials to Remote Respiration Monitoring

Turning insulating silk fibroin materials into conductive ones turns out to be the essential step toward achieving active silk flexible electronics. This work aims to acquire electrically conductive biocompatible fibers of regenerated Bombyx mori silk fibroin (SF) materials based on carbon nanotubes (CNTs) templated nucleation reconstruction of silk fibroin networks. The electronical conductivity of the reconstructed mesoscopic functional fibers can be tuned by the density of the incorporated CNTs. It follows that the hybrid fibers experience an abrupt increase in conductivity when exceeding the percolation threshold of CNTs >35 wt%, which leads to the highest conductivity of 638.9 S m^-1 among organic-carbon-based hybrid fibers, and 8 times higher than the best available materials of the similar types. In addition, the silk-CNT mesoscopic hybrid materials achieve some new functionalities, i.e., humidity-responsive conductivity, which is attributed to the coupling of the humidity inducing cyclic contraction of SFs and the conductivity of CNTs. The silk-CNT materials, as a type of biocompatible electronic functional fibrous material for pressure and electric response humidity sensing, are further fabricated into a smart facial mask to implement respiration condition monitoring for remote diagnosis and medication.