Tough Photocrosslinked Silk Fibroin/Graphene Oxide Nanocomposite Hydrogels

Advances in Membrane Separation for Biomaterial Dewatering

Biomaterials often contain large quantities of water (50-98%), and with the current transition to a more biobased economy, drying these materials will become increasingly important. Contrary to the standard, thermodynamically inefficient chemical and thermal drying methods, dewatering by membrane separation will provide a sustainable and efficient alternative. However, biomaterials can easily foul membrane surfaces, which is detrimental to the performance of current membrane separations. Improving the antifouling properties of such membranes is a key challenge. Other recent research has been dedicated to enhancing the permeate flux and selectivity. In this review, we present a comprehensive overview of the design requirements for and recent advances in dewatering of biomaterials using membranes. These recent developments offer a viable solution to the challenges of fouling and suboptimal performances. We focus on two emerging development strategies, which are the use of electric-field-assisted dewatering and surface functionalizations, in particular with hydrogels. Our overview concludes with a critical mention of the remaining challenges and possible research directions within these subfields.

3D-Printable Sustainable Bioplastics from Gluten and Keratin

Bioplastic films comprising both plant- and animal-derived proteins have the potential to integrate the optimal characteristics inherent to the specific domain, which offers enormous potential to develop polymer alternatives to petroleum-based plastic. Herein, we present a facile strategy to develop hybrid films comprised of both wheat gluten and wool keratin proteins for the first time, employing a ruthenium-based photocrosslinking strategy. This approach addresses the demand for sustainable materials, reducing the environmental impact by using proteins from renewable and biodegradable sources. Gluten film was fabricated from an alcohol-water mixture soluble fraction, largely comprised of gliadin proteins. Co-crosslinking hydrolyzed low-molecular-weight keratin with gluten enhanced its hydrophilic properties and enabled the tuning of its physicochemical properties. Furthermore, the hierarchical structure of the fabricated films was studied using neutron scattering techniques, which revealed the presence of both hydrophobic and hydrophilic nanodomains, gliadin nanoclusters, and interconnected micropores in the matrix. The films exhibited a largely (>40%) β-sheet secondary structure, with diminishing gliadin aggregate intensity and increasing micropore size (from 1.2 to 2.2 µm) with an increase in keratin content. The hybrid films displayed improved molecular chain mobility, as evidenced by the decrease in the glass-transition temperature from ~179.7 °C to ~173.5 °C. Amongst the fabricated films, the G14K6 hybrid sample showed superior water uptake (6.80% after 30 days) compared to the pristine G20 sample (1.04%). The suitability of the developed system for multilayer 3D printing has also been demonstrated, with the 10-layer 3D-printed film exhibiting >92% accuracy, which has the potential for use in packaging, agricultural, and biomedical applications.

Electrogelated drug-embedded silk/gelatin/rGO degradable electrode for anti-inflammatory applications in brain-implant systems

Implantable electrodes have raised great interest over the last years with the increasing incidence of neurodegenerative disorders. For brain implant devices, some key factors resulting in the formation of glial scars, such as mechanical mismatch and acute injury-induced inflammation, should be considered for material design. Therefore, in this study, a new biocompatible flexible electrode (e-SgG) with arbitrary shapes on a positive electrode was developed via electrogelation by applying a direct electrical voltage on a silk fibroin/gelatin/reduced graphene oxide composite hydrogel. The implantable flexible e-SgG-2 film with 1.23% rGO content showed high Young's modulus (11-150 MPa), which was sufficient for penetration under dried conditions but subsequently became a biomimetic brain tissue with low Young's modulus (50-3200 kPa) after insertion in the brain. At the same time, an anti-inflammatory drug (DEX) incorporated into the e-SgG-2 film can be electrically stimulated to exhibit two-stage release to overcome tissue inflammation during cyclic voltammetry via degradation by applying an AC field. The results of cell response to the SF/gelatin/rGO/DEX composite film showed that the released DEX could interrupt astrocyte growth to reduce the inflammatory response but showed non-toxicity toward neurons, which demonstrated a great potential for the application of the biocompatible and degradable e-SgG-D electrodes in the improvement of nerve tissue repair.

Nacre-inspired, strong, tough silk fibroin hydrogels based on biomineralization and the layer-by-layer assembly of ordered silk fabric

Hydrogels are attractive materials with structures and functional properties similar to biological tissues and widely used in biomedical engineering. However, traditional synthetic hydrogels possess poor mechanical strength, and their applications are limited. Herein, a multidimensional material design method is developed; it includes the in situ gelation of silk fabric and nacre-inspired layer-by-layer assembly, which is used to prepare silk fibroin (SF) hydrogels. The in situ gelation method of silk fabric introduces a directionally ordered fabric network in a silk substrate, considerably enhancing the strength of hydrogels. Based on the nacre structure, the layer-by-layer assembly method enables silk hydrogels to break through the size limit and increase the thickness, realizing the longitudinal extension of the hydrogels. The application of the combined biomineralization and hot pressing method can effectively reduce interface defects and improve the interaction between organic and inorganic interfaces. The multidimensional material design method helps increase the strength (287.78 MPa), toughness (18.43 MJ m-3), and fracture energy (50.58 kJ m-2) of SF hydrogels; these hydrogels can weigh 2000 times their own weight. Therefore, SF hydrogels designed using the aforementioned combined method can realize the combination of strength and toughness and be used in biological tissue engineering and structural materials.

Superelastic and multifunctional fibroin aerogels from multiscale silk micro-nanofibrils exfoliated via deep eutectic solvent

Superelastic silk fibroin (SF)-based aerogels can be used as multifunctional substrates, exhibiting a promising prospect in air filtration, thermal insulation, and biomedical materials. However, fabrication of the superelastic pure SF aerogels without adding synthetic polymers remains challenging. Here, the SF micro-nano fibrils (SMNFs) that preserved mesostructures are extracted from SF fibers as building blocks of aerogels by a controllable deep eutectic solvent liquid exfoliation technique. SMNFs can assemble into multiscale fibril networks during the freeze-inducing process, resulting in all-natural SMNF aerogels (SMNFAs) with hierarchical cellular architectures after lyophilization. Benefiting from these structural features, the SMNFAs demonstrate desirable properties including ultra-low density (as low as 4.71 mg/cm³) and superelasticity (over 85 % stress retention after 100 compression cycles at 60 % strain). Furthermore, the potential applications of superelastic SMNFAs in air purification and thermal insulation are investigated to exhibit their functionality, mechanical elasticity, and structural stability. This work provides a reliable approach for the fabrication of highly elastic SF aerogels and endows application prospects in air purification and thermal insulation opportunities.

Silk fibroin scaffolds: A promising candidate for bone regeneration

It remains a big challenge in clinical practice to repair large-sized bone defects and many factors limit the application of autografts and allografts, The application of exogenous scaffolds is an alternate strategy for bone regeneration, among which the silk fibroin (SF) scaffold is a promising candidate. Due to the advantages of excellent biocompatibility, satisfying mechanical property, controllable biodegradability and structural adjustability, SF scaffolds exhibit great potential in bone regeneration with the help of well-designed structures, bioactive components and functional surface modification. This review will summarize the cell and tissue interaction with SF scaffolds, techniques to fabricate SF-based scaffolds and modifications of SF scaffolds to enhance osteogenesis, which will provide a deep and comprehensive insight into SF scaffolds and inspire the design and fabrication of novel SF scaffolds for superior osteogenic performance. However, there still needs more comprehensive efforts to promote better clinical translation of SF scaffolds, including more experiments in big animal models and clinical trials. Furthermore, deeper investigations are also in demand to reveal the degradation and clearing mechanisms of SF scaffolds and evaluate the influence of degradation products.

4D Printing of Hydrogels: Innovation in Material Design and Emerging Smart Systems for Drug Delivery

Advancements in the material design of smart hydrogels have transformed the way therapeutic agents are encapsulated and released in biological environments. On the other hand, the expeditious development of 3D printing technologies has revolutionized the fabrication of hydrogel systems for biomedical applications. By combining these two aspects, 4D printing (i.e., 3D printing of smart hydrogels) has emerged as a new promising platform for the development of novel controlled drug delivery systems that can adapt and mimic natural physio-mechanical changes over time. This allows printed objects to transform from static to dynamic in response to various physiological and chemical interactions, meeting the needs of the healthcare industry. In this review, we provide an overview of innovation in material design for smart hydrogel systems, current technical approaches toward 4D printing, and emerging 4D printed novel structures for drug delivery applications. Finally, we discuss the existing challenges in 4D printing hydrogels for drug delivery and their prospects.

Mechano-Regulable and Healable Silk-Based Materials for Adaptive Applications

Mechanically adaptive materials responsive to environmental stimuli through changing mechanical properties are highly attractive in intelligent devices. However, it is hard to regulate the mechanical properties of most mechanically adaptive materials in a facile way. Moreover, it remains a challenge to achieve mechano-regulable materials with mechanical properties ranging from high strength to extreme toughness. Here, inspired by the reversible nanofibril network structure of skeletal muscle to achieve muscle strength regulation, we present a mechano-regulable biopolymeric silk fibroin (SF) composite through regulating dynamic metal-ligand coordination bonds by using water molecules as competitive regulators. Efficient interfacial hydrogen bonds between tannic acid-tungsten disulfide nanohybrids and the SF matrix endow the composite with high mechanical strength and self-healing ability. The resulting composite exhibits 837-fold change in Young's modulus (5.77 ± 0.61 GPa to 6.89 ± 0.64 MPa) after water vapor triggering, high mechanical properties (72.5 ± 6.3 MPa), and excellent self-healing efficiency (nearly 100%). The proof-of-concept ultraconformable iontronic skin and smart actuators are demonstrated, thereby providing a direction for future self-adaptive smart device applications.

Advancements and Applications in the Composites of Silk Fibroin and Graphene-Based Materials

Silk fibroin and three kinds of graphene-based materials (graphene, graphene oxide, and reduced graphene oxide) have been widely investigated in biomedical fields. Recently, the hybrid composites of silk fibroin and graphene-based materials have attracted much attention owing to their combined advantages, i.e., presenting outstanding biocompatibility, mechanical properties, and excellent electrical conductivity. However, maintaining bio-toxicity and biodegradability at a proper level remains a challenge for other applications. This report describes the first attempt to summarize the hybrid composites’ preparation methods, properties, and applications to the best of our knowledge. We strongly believe that this review will open new doors for coming researchers.

Conductive silk fibroin hydrogel with semi-interpenetrating network with high toughness and fast self-recovery for strain sensors

Regenerated silk fibroin (RSF) hydrogels have been extensively studied in the fields of biomedicine and wearable devices in recent years due to their outstanding biocompatibility. However, the pure RSF hydrogels usually exhibited frangibility and low ductility, limiting their application in many aspects severely. Herein, we demonstrate a tough RSF/poly (N, N-dimethylallylamine) hydrogel with semi-interpenetrating network, which possesses good mechanical properties with high stretchability (ε_b = 900%), tensile strength (σ_b = 101.7 kPa), toughness (W_f = 516.7 kJ/m³) and tearing fracture energy (T = 407.3 J/m²). Besides, the gels show low residual strain in the cyclic tests and rapid self-recovery (80% toughness recovery within 5 min with the maximum strain of 400%). Moreover, the gels also show high ionic conductivity due to the incorporation of the NaCl and the hydrogel can act as an ideal candidate for strain sensor with high sensitivity (GF = 1.84), admirable linearity, and good durability (1000 cycles with the strain of 100%). When used as a wearable strain sensor for monitoring human movements, it also can detect small and large deformations with high sensitivity. It is expected that this work can provide a new strategy for the fabrication of smart RSF-based hydrogels and expand their application in multiple scenarios.

Liquid metal-tailored gluten network for protein-based e-skin

Designing electronic skin (e-skin) with proteins is a critical way to endow e-skin with biocompatibility, but engineering protein structures to achieve controllable mechanical properties and self-healing ability remains a challenge. Here, we develop a hybrid gluten network through the incorporation of a eutectic gallium indium alloy (EGaIn) to design a self-healable e-skin with improved mechanical properties. The intrinsic reversible disulfide bond/sulfhydryl group reconfiguration of gluten networks is explored as a driving force to introduce EGaIn as a chemical cross-linker, thus inducing secondary structure rearrangement of gluten to form additional β-sheets as physical cross-linkers. Remarkably, the obtained gluten-based material is self-healing, achieves synthetic material-like stretchability (>1600%) and possesses the ability to promote skin cell proliferation. The final e-skin is biocompatible and biodegradable and can sense strain changes from human motions of different scales. The protein network microregulation method paves the way for future skin-like protein-based e-skin.

E-skins currently suffer from issues to do with the predominantly non-biological materials they are made from. Here, the authors report on a gluten network which is cross-linked with EGaIn liquid metal to make a self-healing, biocompatible, biodegradable, stretchable and conductive material which is demonstrated as a movement strain sensor.

Synergic adhesive chemistry-based fabrication of BMP-2 immobilized silk fibroin hydrogel functionalized with hybrid nanomaterial to augment osteogenic differentiation of rBMSCs for bone defect repair

Bone defect repair and tissue engineering is specifically challenging process because of the distinctive morphological and structural behaviours of natural bone with complex healing and biochemical mechanisms. In the present investigation, we designed dopamine adhesive chemistry-based fabrication of silk fibroin hydrogel (SFD) with incorporation of nano-hydroxyapatite (nHA)-graphene oxide (GO) hybrid nanofillers with well-arranged porous morphology immobilized with bone morphogenic protein-2 (BMP-2) for the effective in vitro rabbit bone marrow derived mesenchymal stem cells loading compatibility and in vivo new bone regrowth and collagen deposition ability. We have achieved bone-specific hydrogel scaffolds with upgraded structural features, mechanical properties and particularly promoted in vitro osteogenic differentiation and compatibility of rabbit bone marrow mesenchymal stem cells (rBMSCs). Structural and microscopic analyses established greater distributions of components and well-ordered and aligned porous structure of the hydrogel network. In vivo result of new bone regrowth was promisingly higher in the Bm@nHG-SFD hydrogel (85%) group as compared to the other treatment groups of nHG-SFD (77%) and nH-SFD (64%) hydrogel. Overall, we summarized that morphologically improved hydrogel material with immobilization of BMP-2 could be have more attentions for new generation bone regeneration therapies.

Bioactive Silk Fibroin-Based Hybrid Biomaterials for Musculoskeletal Engineering: Recent Progress and Perspectives

Musculoskeletal engineering has been considered as a promising approach to customize regenerated tissue (such as bone, cartilage, tendon, and ligament) via a self-healing performance. Recent advances have demonstrated the great potential of bioactive materials for regenerative medicine. Silk fibroin (SF), a natural polymer, is regarded as a remarkable bioactive material for musculoskeletal engineering thanks to its biocompatibility, biodegradability, and tunability. To improve tissue-engineering performance, silk fibroin is hybridized with other biomaterials to form silk-fibroin-based hybrid biomaterials, which achieve superior mechanical and biological performance. Herein, we summarize the recent development of silk-based hybrid biomaterials in musculoskeletal tissue with reasonable generalization and classification, mainly including silk fibroin-based inorganic and organic hybrid biomaterials. The applied inorganics are composed of calcium phosphate, graphene oxide, titanium dioxide, silica, and bioactive glass, while the polymers include polycaprolactone, collagen (or gelatin), chitosan, cellulose, and alginate. This article mainly focuses on the physical and biological performances both in vitro and in vivo study of several common silk-based hybrid biomaterials in musculoskeletal engineering. The timely summary and highlight of silk-fibroin-based hybrid biomaterials will provide a research perspective to promote the further improvement and development of silk fibroin hybrid biomaterials for improved musculoskeletal engineering.

3D Printable Soy/Silk Hybrid Hydrogels for Tissue Engineering Applications

The development of protein-based 3D printable hydrogel systems with tunable structure and properties is a critical challenge in contemporary biomedicine. Particularly, 3D printing of modular hydrogels comprising different types of protein tertiary structure, such as globular and fibrous, has not yet been achieved. Here we report the extrusion-based 3D printing of hybrid hydrogels photochemically co-cross-linked between globular soy protein isolate (SPI) and fibrous silk fibroin (SF) for the first time. The hierarchical structure and organization of pristine SPI and SF, and 1:3 (SPI/SF) hybrid inks under various shear stress were investigated using in situ rheology combined with small-/ultra-small-angle neutron scattering (Rheo-SANS/USANS). The hybrid ink exhibited an isotropic mass fractal structure that was stable between tested shear rates of 0.1 and 100 s^-1 (near printing shear). The kinetics of sol-gel transition during the photo-cross-linking reaction and the micromechanical properties of fabricated hydrogels were investigated using photorheology and atomic force microscopy, where the hybrid hydrogels exhibited tunable storage and Young's moduli in the range of 13-29 and 214-811 kPa, respectively. The cross-link density and printing accuracy of hybrid hydrogels and inks were observed to increase with the increase in SF content. The 3D printed hybrid hydrogels exhibited a micropore size larger than that of solution casted hydrogels; where the 3D printed 1:3 (SPI/SF) hybrid hydrogel showed a pore size about 7.6 times higher than that of the casted hydrogel. Moreover, the fabricated hybrid hydrogels exhibit good mouse fibroblast cell attachment, viability, and proliferation, demonstrating their potential for tissue engineering applications.

The Manufacture of Unbreakable Bionics via Multifunctional and Self-Healing Silk-Graphene Hydrogels

Biomaterials capable of transmitting signals over longer distances than those in rigid electronics can open new opportunities for humanity by mimicking the way tissues propagate information. For seamless mirroring of the human body, they also have to display conformability to its curvilinear architecture, as well as, reproducing native-like mechanical and electrical properties combined with the ability to self-heal on demand like native organs and tissues. Along these lines, a multifunctional composite is developed by mixing silk fibroin and reduced graphene oxide. The material is coined "CareGum" and capitalizes on a phenolic glue to facilitate sacrificial and hierarchical hydrogen bonds. The hierarchal bonding scheme gives rise to high mechanical toughness, record-breaking elongation capacity of ≈25 000%, excellent conformability to arbitrary and complex surfaces, 3D printability, a tenfold increase in electrical conductivity, and a fourfold increase in Young's modulus compared to its pristine counterpart. By taking advantage of these unique properties, a durable and self-healing bionic glove is developed for hand gesture sensing and sign translation. Indeed, CareGum is a new advanced material with promising applications in fields like cyborganics, bionics, soft robotics, human-machine interfaces, 3D-printed electronics, and flexible bioelectronics.

Novel fabrication of antibiotic containing multifunctional silk fibroin injectable hydrogel dressing to enhance bactericidal action and wound healing efficiency on burn wound: In vitro and in vivo evaluations

The development of biologically active multifunctional hydrogel wound dressings can assist effectively to wound regeneration and also has influenced multiple functions on wound injury. Herein, we designed a carbon-based composited injectable silk fibroin hydrogel as multifunctional wound dressing to provide effective anti-bacterial, cell compatibility and in vivo wound closure actions. Importantly, the fabricated injectable hydrogel exhibit sustained drug delivery properties, anti-oxidant and self-healing abilities, which confirm that composition of hydrogel is highly beneficial to tissue adhesions and burn wound regeneration ability. Frequently, designed injectable hydrogel can be injected into deep and irregular burn wound sites and would provide rapid self-healing and protection from infection environment with thoroughly filled wound area. Meanwhile, incorporated carbon nanofillers improve injectable hydrogel strength and also offer high fluid uptake to hydrogel when applied on the wound sites. In vitro MTT cytotoxicity assay on human fibroblast cell lines establish outstanding cytocompatibility of the injectable hydrogel and also have capability to support cell growth and proliferations. In vivo burn wound animal model results demonstrate that the hydrogel dressings predominantly influenced enhanced wound contraction and also promoted greater collagen deposition, granulation tissue thickness and vascularization. This investigation's outcome could open a new pathway to fabricate multifunctional biopolymeric hydrogel for quicker burn wound therapy and effectively prevents microenvironment bacterial infections.

Processing, mechanical properties and bio-applications of silk fibroin-based high-strength hydrogels

Hydrogels are an attractive class of materials that possess similar structural and functional characteristics to wet biological tissues and demonstrate a diversity of applications in biomedical engineering. Silk fibroin (SF) is a unique natural polymer due to its fibrous protein nature, versatile formats, biocompatibility, tunable biodegradation and is thus a good hydrogel candidate for bio-applications. Compared to synthetic polymer hydrogels, poor mechanical performance is still a fatal drawback that hinders the application of SF hydrogels as structural materials. Researchers have attempted to develop strategies to construct silk fibroin-based high-strength hydrogels (SF-HSHs). Herein, we firstly provide an overview of the approaches of processing SF-HSHs with a focus on the physical/non-covalent crosslinking mechanisms. The examples of SF-HSHs are discussed in detail under four categories, including physical-crosslinked, dual-crosslinked, double network and composite hydrogels respectively. A brief section follows to elucidate on the gelation mechanisms of SF-HSHs before a description of the utility of SF-HSHs in biomedicine and devices is presented. Finally, the potential challenges and future development of SF-HSHs are briefly discussed. This review aims to enhance our understanding of the structure-mechanical property-function relationships of soft materials made from natural polymers and guide future research of silk fibroin-based hydrogels for biomedical applications. STATEMENT OF SIGNIFICANCE: Silk fibroin (SF) extracted from silk fibres is increasingly applied in the biomedical field, and SF hydrogel has been an emerging area for frontier bio-research. Since SF biopolymer has an intrinsic tendency to form regular β-sheet stacks, it can be processed into purely physically crosslinked hydrogels, thus avoiding the use of chemical crosslinkers. Nevertheless, akin to other natural polymers, lab-produced SF is variable (i.e. the molecular weight and distribution), and the gelation of SF hydrogel is challenging to control. In addition, hydrogels made from SF are usually weak and brittle, which hinders the wide use of this biofriendly and biodegradable hydrogel. Recently, there is a pressing need for high strength hydrogels from natural polymers for biomedical applications, and SF is proposed as a strong candidate. Therefore, we have studied the literature in the past 10 years and would like to focus on the gelation mechanism and mechanical strength of SF hydrogels for the review.

Graphene Oxide-Doped Gellan Gum-PEGDA Bilayered Hydrogel Mimicking the Mechanical and Lubrication Properties of Articular Cartilage

Articular cartilage (AC) is a specialized connective tissue able to provide a low-friction gliding surface supporting shock-absorption, reducing stresses, and guaranteeing wear-resistance thanks to its structure and mechanical and lubrication properties. Being an avascular tissue, AC has a limited ability to heal defects. Nowadays, conventional strategies show several limitations, which results in ineffective restoration of chondral defects. Several tissue engineering approaches have been proposed to restore the AC's native properties without reproducing its mechanical and lubrication properties yet. This work reports the fabrication of a bilayered structure made of gellan gum (GG) and poly (ethylene glycol) diacrylate (PEGDA), able to mimic the mechanical and lubrication features of both AC superficial and deep zones. Through appropriate combinations of GG and PEGDA, cartilage Young's modulus is effectively mimicked for both zones. Graphene oxide is used as a dopant agent for the superficial hydrogel layer, demonstrating a lower friction than the nondoped counterpart. The bilayered hydrogel's antiwear properties are confirmed by using a knee simulator, following ISO 14243. Finally, in vitro tests with human chondrocytes confirm the absence of cytotoxicity effects. The results shown in this paper open the way to a multilayered synthetic injectable or surgically implantable filler for restoring AC defects.

2D Nanomaterials for Tissue Engineering and Regenerative Nanomedicines: Recent Advances and Future Challenges

Regenerative medicine has become one of the hottest research topics in medical science that provides a promising way for repairing tissue defects in the human body. Due to their excellent physicochemical properties, the application of 2D nanomaterials in regenerative medicine has gradually developed and has been attracting a wide range of research interests in recent years. In particular, graphene and its derivatives, black phosphorus, and transition metal dichalcogenides are applied in all the aspects of tissue engineering to replace or restore tissues. This review focuses on the latest advances in the application of 2D-nanomaterial-based hydrogels, nanosheets, or scaffolds that are engineered to repair skin, bone, and cartilage tissues. Reviews on other applications, including cardiac muscle regeneration, skeletal muscle repair, nerve regeneration, brain disease treatment, and spinal cord healing are also provided. The challenges and prospects of applications of 2D nanomaterials in regenerative medicine are discussed.

Crosslinking strategies for silk fibroin hydrogels: promising biomedical materials

Due to their strong biomimetic potential, silk fibroin (SF) hydrogels are impressive candidates for tissue engineering, due to their tunable mechanical properties, biocompatibility, low immunotoxicity, controllable biodegradability, and a remarkable capacity for biomaterial modification and the realization of a specific molecular structure. The fundamental chemical and physical structure of SF allows its structure to be altered using various crosslinking strategies. The established crosslinking methods enable the formation of three-dimensional (3D) networks under physiological conditions. There are different chemical and physical crosslinking mechanisms available for the generation of SF hydrogels (SFHs). These methods, either chemical or physical, change the structure of SF and improve its mechanical stability, although each method has its advantages and disadvantages. While chemical crosslinking agents guarantee the mechanical strength of SFH through the generation of covalent bonds, they could cause some toxicity, and their usage is not compatible with a cell-friendly technology. On the other hand, physical crosslinking approaches have been implemented in the absence of chemical solvents by the induction of β-sheet conformation in the SF structure. Unfortunately, it is not easy to control the shape and properties of SFHs when using this method. The current review discusses the different crosslinking mechanisms of SFH in detail, in order to support the development of engineered SFHs for biomedical applications.

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.

Injectable muscle-adhesive antioxidant conductive photothermal bioactive nanomatrix for efficiently promoting full-thickness skeletal muscle regeneration

The completed skeletal muscle regeneration resulted from severe injury and muscle-related disease is still a challenge. Here, we developed an injectable muscle-adhesive antioxidant conductive bioactive photothermo-responsive nanomatrix for regulating the myogenic differentiation and promoting the skeletal muscle regeneration in vivo. The multifunctional nanomatrix was composed of polypyrrole@polydopamine (PPy@PDA, 342 ± 5.6 nm) nanoparticles-crosslinked Pluronic F-127 (F127)-polycitrate matrix (FPCP). The FPCP nanomatrix demonstrated inherent multifunctional properties including excellent photothermo-responsive and shear-thinning behavior, muscle-adhesive feature, injectable ability, electronic conductivity (0.48 ± 0.03 S/m) and antioxidant activity and photothermal function. The FPCP nanomatrix displayed better photothermal performance with near-infrared irradiation, which could provide the photo-controlled release of protein (91% ± 2.6% of BSA was released after irradiated 3 times). Additionally, FPCP nanomatrix could significantly enhance the cell proliferation and myogenic differentiation of mouse myoblast cells (C2C12) by promoting the expressions of myogenic genes (MyoD and MyoG) and myosin heavy chain (MHC) protein with negligible cytotoxicity. Based on the multifunctional properties, FPCP nanomatrix efficiently promoted the full-thickness skeletal muscle repair and regeneration in vivo, through stimulating the angiogenesis and myotube formation. This study firstly indicated the vital role of multifunctional PPy@PDA nanoparticles in regulating myogenic differentiation and skeletal muscle regeneration. This work also suggests that rational design of bioactive matrix with multifunctional feature would greatly enhance the development of regenerative medicine.

•

Multifunctional muscle-adhesive nanomatrix was designed.

•

Nanomatrix showed good injectability, conductivity and antioxidant ability.

•

Nanomatrix displayed good photothermal function and biocompatibility.

•

Nanomatrix significantly enhanced cell proliferation and myogenic differentiation.

•

Nanomatrix efficiently promoted the full-thickness skeletal muscle regeneration.

Graphene oxide and electroactive reduced graphene oxide-based composite fibrous scaffolds for engineering excitable nerve tissue

This study systematically investigates the role of graphene oxide (GO) and reduced GO (rGO)/silk-based composite micro/nano-fibrous scaffolds in regulating neuronal cell behavior in vitro, given the limited comparative studies on the effects of graphene family materials on nerve regeneration. Fibrous scaffolds can mimic the architecture of the native extracellular matrix and are potential candidates for tissue engineering peripheral nerves. Silk/GO micro/nano-fibrous scaffolds were electrospun with GO loadings 1 to 10 wt.%, and optionally post-reduced in situ to explore a family of electrically conductive non-woven silk/rGO scaffolds. Conductivities up to 4 × 10^-5 S cm^-1 were recorded in the dry state, which increased up to 3 × 10^-4 S cm^-1 after hydration. Neuronoma NG108-15 cells adhered and were viable on all substrates. Enhanced metabolic activity and proliferation were observed on the GO-containing scaffolds, and these cell responses were further promoted for electroactive silk/rGO. Neurite extensions up to 100 μm were achieved by day 5, with maximum outgrowth up to ~250 μm on some of the conductive substrates. These electroactive composite fibrous scaffolds exhibit potential to enhance the neuronal cell response and could be versatile supportive substrates for neural tissue engineering applications.

Silk fibroins in multiscale dimensions for diverse applications

Silk biomaterials in different forms such as particles, coatings and their assemblies, represent unique type of materials in multiple scales and dimensions. Herein, we provide an overview of multi-scale silk fibroin materials including silk particles, silk coatings and silk assemblies, each of which represents a unique type of material with wide range of applications. They feature tunable structures and mechanical properties with excellent biocompatibility, which are essentially required for various biomedical and drug delivery applications. The review focuses on bringing a new perspective on the utilization of regenerated silk fibroins in modern biomedicine by beginning with the fabrication of silk in multiscale dimensions and their state-of-the-art applications in various biomedical and bioelectronic fields. It covers the fundamentals of processing silk fibroins in multi-dimensions (sizes and shapes) with a specific emphasis on its structural tunability at various length scales (nano-micro) by using the latest fabrication methods/mechanisms and advanced fabrication technologies, followed by their recent applications in diverse fields of biomedicine.

Addition of Platelet-Rich Plasma to Silk Fibroin Hydrogel Bioprinting for Cartilage Regeneration

The recent advent of 3D bioprinting of biopolymers provides a novel method for fabrication of tissue-engineered scaffolds and also offers a potentially promising avenue in cartilage regeneration. Silk fibroin (SF) is one of the most popular biopolymers used for 3D bioprinting, but further application of SF is hindered by its limited biological activities. Incorporation of growth factors (GFs) has been identified as a solution to improve biological function. Platelet-rich plasma (PRP) is an autologous resource of GFs, which has been widely used in clinic. In this study, we have developed SF-based bioinks incorporated with different concentrations of PRP (12.5%, 25%, and 50%; vol/vol). Release kinetic studies show that SF-PRP bioinks could achieve controlled release of GFs. Subsequently, SF-PRP bioinks were successfully fabricated into scaffolds by bioprinting. Our results revealed that SF-PRP scaffolds possessed proper internal pore structure, good biomechanical properties, and a suitable degradation rate for cartilage regeneration. Live/dead staining showed that 3D, printed SF-PRP scaffolds were biocompatible. Moreover, in vitro studies revealed that tissue-engineered cartilage from the SF-PRP group exhibited improved qualities compared with the pure SF controls, according to histological and immunohistochemical findings. Biochemical evaluations confirmed that SF-PRP (50% PRP, v/v) scaffolds allowed the largest increases in collagen and glycosaminoglycan concentrations, when compared with the pure SF group. These findings suggest that 3D, printed SF-PRP scaffolds could be potential candidates for cartilage tissue engineering. Impact statement Three-dimensional bioprinting of silk fibroin (SF) hydrogel as bioinks is a promising strategy for cartilage tissue engineering, but it lacks biological activities, which favors proliferation of seeded cells and secretion of the extracellular matrix. In this study, we have successfully added platelet-rich plasma (PRP) into SF-based bioinks as an autologous source of growth factors. The 3D, printed SF-PRP scaffold showed an enhanced biological property, thus aiding in potential future development of novel cartilage tissue engineering applications.

Evolution of the Interfacial Structure of a Catalyst Ink with the Quality of the Dispersing Solvent: A Contrast Variation Small-Angle and Ultrasmall-Angle Neutron Scattering Investigation

The electrocatalyst layer (ECL) of the proton-exchange membrane fuel cell (PEMFC) is commonly fabricated from colloidal catalyst ink containing carbon-supported catalyst nanoparticles (NPs), ionomer stabilizer, and dispersion medium (DM). The structure, stability, and aggregate size distribution of fuel cell catalyst ink are critically dependent on the quality of DM. However, understanding of the influence of the quality of DM on the hierarchical structure of the ECL is lacking. This work presents a systematic investigation of the effects of reducing alcohol content in isopropyl alcohol/water (IPA/H₂O) binary mixtures as DM on the structural evolution of water-rich (green) catalyst ink using contrast-variation small-angle and ultrasmall-angle neutron scattering techniques. Both qualitative and quantitative information are extracted from the data to obtain information about the size, structure, and organization of the catalyst ink using different model functions fit to the experimental data. The catalyst ink prepared using 70% IPA (commonly employed in industry and extensively reported in the literature) is shown to consist of randomly distributed globular carbon aggregates (mean radius of gyration of ∼178.9 nm) stabilized by an ionomer mass fractal shell (thickness of ∼13.0 nm), which is dispersed in the matrix of rodlike (∼1.3 nm radius and ∼35.0 nm length) negatively surface-charged ionomer NPs. These well characterized baseline data are then compared and contrasted with DM formulations of lower IPA content. A sequential reduction in IPA content of DM shows a progressive increase in the ionomer NP radius and electrostatic repulsion, concomitantly with the decrease in the carbon aggregate size and ionomer shell thickness of the catalyst ink. Therefore, the changes in the interfacial structure via adjustments of the DM composition can be used as a controlling parameter to tailor the hierarchical structure of the colloidal fuel cell catalyst ink and to further optimize the performance of the ECL.

The assembly of silk fibroin and graphene-based nanomaterials with enhanced mechanical/conductive properties and their biomedical applications

The assembly of silk fibroin and graphene-based nanomaterials would present fantastic properties and functions via optimizing the interaction between each other, and can be processed into various formats to tailor specific biomedical applications.

Conductive regenerated silk-fibroin-based hydrogels with integrated high mechanical performances

Strong and tough RSF-based hydrogels that could be used as a strain sensor, a touch screen pen and an electronic skin were developed.