Flexible Silk–Inorganic Nanocomposites: From Transparent to Highly Reflective

A novel type of all‐natural, biocompatible, and very robust nanoscale free‐standing biohybrids are reported. They are obtained by integrating a silk fibroin matrix with functional inorganic nanoplatelets using a spin‐assisted layer‐by‐layer assembly. The organized assembly of the silk fibroin with clay (montmorillonite) nanosheets results in highly transparent nanoscale films with significantly enhanced mechanical properties, including strength, toughness, and elastic modulus, as compared to those for the pristine silk nanomaterials. Moreover, replacing clay nanoplatelets with a highly reflective Langmuir monolayer of densely packed silver nanoplates causes a similar enhancement of the mechanical properties, but in contrast to the materials above, highly reflective, mirror‐like, nanoscale flexible films are created. This strategy offers a new perspective for the fabrication of robust all‐natural flexible nanocomposites with exceptional mechanical properties important for biomedical applications, such as reinforced tissue engineering. On the other hand, the ability to convert silk‐based nanoscale films into mirror‐like biocompatible flexible films can be intriguing for prospective photonics and optical exploitation of these nanobiohybrids.     

Toward Biodegradable Mg–Air Bioelectric Batteries Composed of Silk Fibroin–Polypyrrole Film

Biodegradable active implantable devices can be used to diagnose and/or treat disease and eventually disappear without surgical removal. If an “external” energy source is required for effective operation then a biocompatible and biodegradable battery would be ideal. In this study, a partially biodegradable Mg‐air bioelectric battery (biobattery) is demonstrated using a silk fibroin‐polypyrrole (SF‐PPy) film cathode coupled with bioresorbable Mg alloy anode in phosphate buffered saline (PBS) electrolyte. PPy is chemically coated onto one side of the silk substrate. SF‐PPy film shows a conductivity of ≈1.1 S cm^?1 and a mild catalytic activity toward oxygen reduction. It degrades in a concentrated buffered protease XIV solution, with a weight loss of 82% after 15 d. The assembled Mg‐air biobattery exhibits a discharge capacity up to 3.79 mA h cm^?2 at a current of 10 μA cm^?2 at room temperature, offering a specific energy density of ≈4.70 mW h cm^?2. This novel partially biodegradable battery provides another step along the route to biodegradable batteries.     

Biofunctional Silk/Neuron Interfaces

Silk fibroin (SF) is a biocompatible and slowly biodegradable material with excellent mechanical properties and huge potential for use as biofunctional interface in electronic devices that aim to stimulate and control neural network activity and peripheral nerve repair. It is shown that SF films act as material interfaces that support the adherence and neurite outgrowth of dorsal root ganglion (DRG) neurons and preserve neuronal functions. Silk films preserve the capability of neuronal cells to fire and DRG neurons on silk films retain the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) response to capsaicin, a typical noxious stimulus for this neuronal culture model. It is also demonstrated that nerve growth factor (NGF)‐functionalized silk films promote neurite outgrowth and modulate functional properties of DRG neurons. The results show that silk preserves DRG neuronal physiology and is a promising biomaterial platform for the future development of devices with goals including functional recovery of injured neurons, neurite functional outgrowth in vitro, or functional electrostimulation in vivo.     

A Facile In Situ Approach to Polypyrrole Functionalization Through Bioinspired Catechols

A template‐free benign approach to modify polypyrrole (PPy) with bioinspired catechol derivatives dopamine (DA), 1,2‐dihydroxybenzene or catechol (CA), and l ‐3,4‐dihydroxyphenylalanine (DOPA) is reported. It is found that PPy functionalized with these catechol derivatives (DA, CA, and DOPA) exhibited fibrous structure, smaller particle size, good water dispersibility, and enhanced film adhesion. Surprisingly, it is found that adding a small amount of catechols can also improve PPy's electrical conductivity. This rapid, one‐step, in situ, template‐free method provided an alternative strategy to the facile production of PPy fibers. Among these three catechols, functionalized PPy and DA‐PPy exhibits the smallest particle size and best performance in both adhesion and electrical conductivity. In contrast, the control phenylethlamine (PA) modification had almost negligible influence on the PPy properties, which provides strong evidence that instead of amine functional group or coexistence of both catechol and amine moieties, catechol itself is responsible for the successful functionalization of PPy and overall performance improvement. Furthermore, catechol‐PPy nanofibers are blended into polyvinyl alcohol (PVA) aqueous solution and casted to form thin films; as‐synthesized conductive films are found able to bond strongly onto the surface and may find broad applications in manufacturing biosensors and electronic devices.     

Mechanical Properties of Robust Ultrathin Silk Fibroin Films

Robust ultrathin multilayer films of silk fibroin were fabricated by spin coating and spin‐assisted layer‐by‐layer assembly and their mechanical properties were studied both in tensile and compression modes for the first time. The ultrathin films were characterized by a high elastic modulus of 6–8 GPa (after treatment with methanol) with the ultimate tensile strength reaching 100 MPa. The superior toughness is also many times higher than that usually observed for conventional polymer composites (328 kJ m^–3 for the silk material studied here versus typical values of < 100 kJ m^–3). These outstanding properties are suggested to be caused by the gradual development of the self‐reinforcing microstructure of highly crystalline β‐sheets, serving as reinforcing fillers and physical crosslinks, a process that is well known for bulk silk materials but it is demonstrated here to occur in ultrathin films as well, despite their limited dimensions. However, the confined state within films thinner than the lengths of the extended domains causes a significantly reduced elasticity which should be considered in the design of nanosized films from silk materials. Such regenerated silk fibroin films with outstanding mechanical strength have potential applications in microscale biodevices, biocompatible implants, and synthetic coatings for artificial skin.     

Fabrication of a Highly Transparent Conductive Thin Film from Polypyrrole/Poly(methyl methacrylate) Core/Shell Nanospheres

Polypyrrole (PPy)/poly(methyl methacrylate) (PMMA) core/shell nanospheres with diameters of several tens of nanometers have been synthesized by two‐step microemulsion polymerization, and highly transparent conductive thin films have been fabricated using the nanospheres as a filler in a PMMA matrix. The PPy/PMMA core/shell nanoparticles and their composite films have been extensively characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy (EDX), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier‐transform infrared (FT‐IR) and UV‐vis spectroscopies, and electrical‐conductivity measurements. The fabricated polymer films containing the PPy/PMMA core/shell nanofillers show a much better transparent conductive performance than that of uncoated PPy nanoparticles with similar dimensions or bulk PPy particles with diameters of several hundreds of nanometers. The PMMA shell promotes compatibility of the conductive fillers with the PMMA matrix and enhances dispersion of the PPy/PMMA core/shell nanofillers. In addition, the nanometer‐thick PMMA shell has a lower glass‐transition temperature (T_g), and can be effectively annealed to form a conductive‐filler network with a high electrical conductivity at a relatively low filler content.     

Nanoengineered Polypyrrole‐Coated Fe2O3@C Multifunctional Composites with an Improved Cycle Stability as Lithium‐Ion Anodes

Novel multifunctional composites composed of highly dispersed nanosized Fe₂O₃ particles, a tubular mesoporous carbon host, and a conductive polypyrrole (PPy) sealing layer are hierarchically assembled via two facile processes, including bottom‐up introduction of Fe₂O₃ nanoparticles in tubular mesoporous carbons, followed by in situ surface sealing with the PPy coating. Fe₂O₃ particles are well‐dispersed within the carbon matrix and PPy is spatially and selectively coated onto the external surface and the pore entrances of the Fe₂O₃@C composite, thereby bridging the composite particles together into a larger unit. As an anode material for Li‐ion batteries (LIBs), the PPy‐coated Fe₂O₃@C composite exhibits stable cycle performance. Additionally, the PPy‐coated Fe₂O₃@C composite also possesses fast electrode reaction kinetics, high Fe₂O₃ use efficiency, and large volumetric capacity. The excellent electrochemical performance is associated with a synergistic effect of the highly porous carbon matrix and the conducting PPy sealing layer. Such multifunctional configuration prevents the aggregation of NPs and maintains the structural integrity of active materials, in addition to effectively enhancing the electronic conductivity and warranting the stability of as‐formed solid electrolyte interface (SEI) films. This nanoengineering strategy might open new avenues for the design of other multifunctional composite architectures as electrode materials in order to achieve high‐performance LIBs.     

Water‐Stable Silk Films with Reduced β‐Sheet Content

Silk fibers have outstanding mechanical properties. These fibers are insoluble in organic solvents and water, are biocompatible, and exhibit slow biodegradation in vitro and in vivo due to the hydrophobic nature of the protein and the presence of a high content of β‐sheet structure. Regenerated silk fibroin can be processed into a variety of materials normally stabilized by the induction of β‐sheet formation through the use of solvents or by physical stretching. To extend the biomaterial utility of silk proteins, options to form water‐stable silk‐based materials with reduced β‐sheet formation would be desirable. To address this need for more rapidly degradable silk biomaterials, we report the preparation of water‐stable films from regenerated silk fibroin solutions, with reduced β‐sheet content. The keys to this process are the preparation of concentrated (8 % by weight) aqueous solutions of fibroin and a subsequent water‐based annealing procedure. These new materials degrade more rapidly due to the reduced β‐sheet content, as determined in vitro via enzymatic hydrolysis, yet support human adult stem‐cell expansion in vitro in a similar or improved fashion to the crystallized proteins in film form. These new silk‐based materials extend the range of biomaterial properties that can be generated from this unique family of proteins.     

Silk Flexible Electronics: From Bombyx mori Silk Ag Nanoclusters Hybrid Materials to Mesoscopic Memristors and Synaptic Emulators

Functionalization of flexible materials based on mesoscopic reconstruction is a key strategy in fabricating biocompatible flexible electronics. This work is to acquire new mesoscopic bioelectronic hybrid materials of silk fibroin (SF)‐Ag nanoclusters (AgNCs@BSA; BSA: bovine serum albumin), which enhance significantly the performance of silk memristors. It is to build AgNCs@BSA into SF mesoscopic networks by templated β‐crystallization. Atomic force microscopy potential probing indicates that AgNCs@BSA serve as electronic potential wells that change completely the transport behavior of charge particles within the SF films. This leads to significant enhancement in the switching speed (≈10 ns), very good switching stability, extremely low set/reset voltages (0.3/?0.18 V) of SF meso‐hybrid memristors, compared with the original and other organic memristors, and displays unique synapse characteristics and the capability of synapse learning. Classical density functional theory Poisson–Nernst–Planck simulations indicate that the enhanced performance is subject to the low potential paths interconnecting the AgNCs@BSA, which guide charges' transport (Ag⁺) and deposition in SF films.     

Construction of White‐Light‐Emitting Silk Protein Hybrid Films by Molecular Recognized Assembly among Hierarchical Structures

The fabrication of bio‐hybrid functional films is demonstrated by applying a materials assembly technique. Based on the hierarchical structures of silk fibroin materials, functional molecular/materials, i.e., quantum dots (QDs), can be fixed to amino acid groups in silk fibroin films. It follows that white‐light‐emitting QD silk hybrid films are obtained by hydrogen bond molecular recognition to the –COO^– groups functionalized to blue luminescent ZnSe (5.2 nm) and yellow luminescent CdTe (4.1 nm) QDs in a molar ratio of 30:1 of ZnSe to CdTe QDs. Simultaneously, a systematic blue shift in the emission peak is observed from the QD solution to QDs silk fibroin films. The significant blue shift hints the appearance of the strong interaction between QDs and silk fibroins, which causes strong white‐light‐emitting uniform silk films. The molecular recognized interactions are confirmed by high resolution transmission electron microscopy, field scanning electron microscope, and attenuated total internal reflectance Fourier transform infrared spectroscopy. The QD silk films show unique advantages, including simple preparation, tunable white‐light emission, easy manipulation, and low fabrication costs, which make it a promising candidate for multicomponent optodevices.     

Ice‐Templated Protein Nanoridges Induce Bone Tissue Formation

Little is known about the role of biocompatible protein nanoridges in directing stem cell fate and tissue regeneration due to the difficulty in forming protein nanoridges. Here an ice‐templating approach is proposed to produce semi‐parallel pure silk protein nanoridges. The key to this approach is that water droplets formed in the protein films are frozen into ice crystals (removed later by sublimation), pushing the surrounding protein molecules to be assembled into nanoridges. Unlike the flat protein films, the unique protein nanoridges can induce the differentiation of human mesenchymal stem cells (MSCs) into osteoblasts without any additional inducers, as well as the formation of bone tissue in a subcutaneous rat model even when not seeded with MSCs. Moreover, the nanoridged films induce less inflammatory infiltration than the flat films in vivo. This work indicates that decorating biomaterials surfaces with protein nanoridges can enhance bone tissue formation in bone repair.     

Moisture Sensitive Smart Yarns and Textiles from Self‐Balanced Silk Fiber Muscles

Smart textiles that sense, interact, and adapt to environmental stimuli have provided exciting new opportunities for a variety of applications. However, current advances have largely remained at the research stage due to the high cost, complexity of manufacturing, and uncomfortableness of environment‐sensitive materials. In contrast, natural textile materials are more attractive for smart textiles due to their merits in terms of low cost and comfortability. Here, water fog and humidity‐driven torsional and tensile actuation of thermally set twisted, coiled, plied silk fibers, and weave textiles from these silk fibers are reported. When exposed to water fog, the torsional silk fiber provides a fully reversible torsional stroke of 547° mm^?1. Coiled‐and‐thermoset silk yarns provide a 70% contraction when the relative humidity is changed from 20% to 80%. Such an excellent actuation behavior originates from water absorption‐induced loss of hydrogen bonds within the silk proteins and the associated structural transformation, which are corroborated by atomistic and macroscopic characterization of silk and molecular dynamics simulations. With its large abundance, cost‐effectiveness, and comfortability for wearing, the silk muscles will open up additional possibilities in industrial applications, such as smart textiles and soft robotics.     

Calcium‐Modified Silk as a Biocompatible and Strong Adhesive for Epidermal Electronics

With the increasing interest and demand for epidermal electronics, a strong interface between a sensor and a biological surface is essential, yet achieving such interface is still a challenge. Here, a calcium (Ca)‐modified biocompatible silk fibroin as a strong adhesive for epidermal electronics is proposed and the physical principles behind its interfacial and adhesive properties are reported. A strong adhesive characteristic (>800 N m^?1) is observed because of the increase in both viscoelastic property and mechanical interlocking through the incorporation of Ca ions. Furthermore, additional key characteristics of the Ca‐modified silk: reusability, stretchability, biocompatibility, and conductivity, are reported. These characteristics enable a wide range of applications as demonstrated in four epidermal electronic systems: capacitive touch sensor, resistive strain sensor, hydrogel‐based drug delivery, and electrocardiogram monitoring sensor. As a reusable, biocompatible, conductive, and strong adhesive with water‐degradability, the Ca‐modified silk adhesive is a promising candidate for the next‐generation adhesive for epidermal biomedical sensors.     

Antibiotic‐Releasing Silk Biomaterials for Infection Prevention and Treatment

Effective treatment of infections in avascular and necrotic tissues can be challenging due to limited penetration into the target tissue and systemic toxicities. Controlled‐release polymer implants have the potential to achieve the high local concentrations needed while also minimizing systemic exposure. Silk biomaterials possess unique characteristics for antibiotic delivery, including biocompatibility, tunable biodegradation, stabilizing effects, water‐based processing, and diverse material formats. The functional release of antibiotics spanning a range of chemical properties from different material formats of silk (films, microspheres, hydrogels, coatings) is reported. The release of penicillin and ampicillin from bulk‐loaded silk films, drug‐loaded silk microspheres suspended in silk hydrogels and bulk‐loaded silk hydrogels is investigated and the in vivo efficacy of the ampicillin‐releasing silk hydrogels is demonstrated in a murine infected‐wound model. Silk sponges with nanofilm coatings are loaded with gentamicin and cefazolin, and release is sustained for 5 and 3 days, respectively. The capability of silk antibiotic carriers to sequester, stabilize, and then release bioactive antibiotics represents a major advantage over implants and pumps based on liquid drug reservoirs, where instability at room or body temperature is limiting. The present studies demonstrate that silk biomaterials represent a novel, customizable antibiotic platform for focal delivery of antibiotics using a range of material formats (injectable to implantable).     

聚吡咯/氧化锌纳米复合材料的电化学制备与性能研究

以草酸和十二烷基苯磺酸为电解质,利用电化学方法,基于镍片制备了ZnO质量分数不同(10%～30%)的聚吡咯/氧化锌(PPy/ZnO)纳米复合材料,并对其进行了表征和性能分析。结果表明基于镍片的复合材料中,聚吡咯呈典型的菜花状结构,白色的Zn0颗粒夹杂在PPy中,填充在PPy颗粒间的缝隙中,防止基底与溶液的直接接触,这种结构对基底有着很好的防腐蚀保护性能。而且随着质量比的增加,PPy/Zn0复合材料中的白色ZnO颗粒更为密集。研究发现PPy/ZnO复合材料比纯PPy具有更好的电化学性能,而且随着质量比的增加,PPy/ZnO复合材料的氧化还原可逆性、电化学交换反应与容纳电荷的能力等电化学性能有增强的趋势。     

Flexible and Biocompatible Physical Unclonable Function Anti-Counterfeiting Label

Optical physical unclonable functions (PUFs) have been proven to be one of the most effective anti-counterfeiting strategies. However, optical PUFs endowed with flexibility and biocompatibility have not been developed, limiting their application scenarios. Herein, biocompatible and flexible optical PUF labels are developed by randomly embedding microdiamonds in silk fibroin films. The PUF labels can be conformally attached onto the surface of complex shaped objects, providing the desired protection against fake and interior products. In this system, silk fibroin films serve as a flexible and biocompatible substrate, while the Raman signal of the microdiamonds serves as response of the excitation. The extremely high stability and random distribution of the microdiamonds ensure the performance of PUFs, and the maximum encoding capability of 2¹⁰⁰⁰⁰ is finally realized. The cytotoxicity analysis results also verify the biosafety of the PUF system. In addition, the as-prepared PUF labels are attached onto the surface of polyethylene material, and human skin, and even have been implanted under chicken skin tissue, promising their practical applications.     

Biomimetic Chitin–Silk Hybrids: An Optically Transparent Structural Platform for Wearable Devices and Advanced Electronics

The cuticles of insects and marine crustaceans are fascinating models for man‐made advanced functional composites. The excellent mechanical properties of these biological structures rest on the exquisite self‐assembly of natural ingredients, such as biominerals, polysaccharides, and proteins. Among them, the two commonly found building blocks in the model biocomposites are chitin nanofibers and silk‐like proteins with β‐sheet structure. Despite being wholly organic, the chitinous protein complex plays a key role for the biocomposites by contributing to the overall mechanical robustness and structural integrity. Moreover, the chitinous protein complex alone without biominerals is optically transparent (e.g., dragonfly wings), thereby making it a brilliant model material system for engineering applications where optical transparency is essentially required. Here, inspired by the chitinous protein complex of arthropods cuticles, an optically transparent biomimetic composite that hybridizes chitin nanofibers and silk fibroin (β‐sheet) is introduced, and its potential as a biocompatible structural platform for emerging wearable devices (e.g., smart contact lenses) and advanced displays (e.g., transparent plastic cover window) is demonstrated.     

Shape-Engineerable Silk Fibroin Papers for Ideal Substrate Alternatives of Plastic Electronics

Plastic-based electronics fill the gaps in conventional rigid silicon-based devices toward the applications in soft interfaces. However, people in the future should also consider their potential environmental impact if tons of non-degradable plastics are applied. Silk fibroin is a superior substrate alternative for the development of “green” electronics; whereas, the brittleness of silk films is still a major limitation impeding their practical use. Different from the widely reported polyphasic composite approaches, here a trace-ion-assisted plasticization strategy is developed, and shape-engineerable pure silk fibroin paper (PSFP) is prepared for the first time, which can be engraved and crumpled like a sheet of paper in the dry state. The PSFPs exhibit higher tensile fracture energy (14.4 ± 4 kJ m⁻²) than any typical plastic-electronic-substrates as far as it is known. The intrinsic brittleness of pure silk films is overcome, and the PSFP can be easily engineered to form periodic meshes, electronic prototypes, and kirigami-based devices, which are beyond the reported regenerated silk films or silk composite films. Moreover, the scrape coating method employed here is simple, highly repeatable, and suitable for scaled production of low-cost PSFP continuously. Collectively, the PSFP is generalizable to various shapes and devices, represents an ideal substrate alternative to plastic electronics.     

High Electromechanical Response of Ionic Polymer Actuators with Controlled‐Morphology Aligned Carbon Nanotube/Nafion Nanocomposite Electrodes

Recent advances in fabricating controlled‐morphology vertically aligned carbon nanotubes (VA‐CNTs) with ultrahigh volume fraction create unique opportunities for markedly improving the electromechanical performance of ionic polymer conductor network composite (IPCNC) actuators. Continuous paths through inter‐VA‐CNT channels allow fast ion transport, and high electrical conduction of the aligned CNTs in the composite electrodes lead to fast device actuation speed (>10% strain/second). One critical issue in developing advanced actuator materials is how to suppress the strain that does not contribute to the actuation (unwanted strain) thereby reducing actuation efficiency. Here, experiments demonstrate that the VA‐CNTs give an anisotropic elastic response in the composite electrodes, which suppresses the unwanted strain and markedly enhances the actuation strain (>8% strain under 4 V). The results reported here suggest pathways for optimizing the electrode morphology in IPCNCs using ultrahigh volume fraction VA‐CNTs to further enhanced performance.     

Carbon Nanotubes: High Electromechanical Response of Ionic Polymer Actuators with Controlled‐Morphology Aligned Carbon Nanotube/Nafion Nanocomposite Electrodes (Adv. Funct. Mater. 19/2010)

Recent advances in fabricating controlled‐morphology vertically aligned carbon nanotubes (VA‐CNTs) with ultrahigh volume fraction create unique opportunities for markedly improving the electromechanical performance of ionic polymer conductor network composite (IPCNC) actuators. Continuous paths through inter‐VA‐CNT channels allow fast ion transport, and high electrical conduction of the aligned CNTs in the composite electrodes lead to fast device actuation speed (>10% strain/second). One critical issue in developing advanced actuator materials is how to suppress the strain that does not contribute to the actuation (unwanted strain) thereby reducing actuation efficiency. Here, experiments demonstrate that the VA‐CNTs give an anisotropic elastic response in the composite electrodes, which suppresses the unwanted strain and markedly enhances the actuation strain (>8% strain under 4 V). The results reported here suggest pathways for optimizing the electrode morphology in IPCNCs using ultrahigh volume fraction VA‐CNTs to further enhanced performance.