Sericin Promotes Fibroin Silk I Stabilization Across a Phase-Separation

Recent advances of silk fibroin materials: From molecular modification and matrix enhancement to possible encapsulation-related functional food applications

Silk fibroin materials are emergingly explored for food applications due to their inherent properties including safe oral consumption, biocompatibility, gelatinization, antioxidant performance, and mechanical properties. However, silk fibroin possesses drawbacks like brittleness owing to its inherent specific composition and structure, which limit their applications in this field. This review discusses current progress about molecular modification methods on silk fibroin such as extraction, blending, self-assembly, enzymatic catalysis, etc., to address these limitations and improve their physical/chemical properties. It also summarizes matrix enhancement strategies including freeze drying, spray drying, electrospinning/electrospraying, microfluidic spinning/wheel spinning, desolvation and supercritical fluid, to generate nano-, submicron-, micron-, or bulk-scale materials. It finally highlights the food applications of silk fibroin materials, including nutraceutical improvement, emulsions, enzyme immobilization and 3D/4D printing. This review also provides insights on potential opportunities (like safe modification, toxicity risk evaluation, and digestion conditions) and possibilities (like digital additive manufacturing) in functional food industry.

Decoding silkworm spinning programmed by pH and metal ions

Silk is one of the toughest fibrous materials known despite spun at ambient temperature and pressure with water as a solvent. It is a great challenge to reproduce high-performance artificial fibers comparable to natural silk by bionic for the incomplete understanding of silkworm spinning in vivo. Here, we found that amphipol and digitonin stabilized the structure of natural silk fibroin (NSF) by a large-scale screening in vitro, and then studied the close-to-native ultrastructure and hierarchical assembly of NSF in the silk gland lumen. Our study showed that NSF formed reversible flexible nanofibrils mainly composed of random coils with a sedimentation coefficient of 5.8 S and a diameter of about 4 nm, rather than a micellar or rod-like structure assembled by the aggregation of globular NSF molecules. Metal ions were required for NSF nanofibril formation. The successive pH decrease from posterior silk gland (PSG) to anterior silk gland (ASG) resulted in a gradual increase in NSF hydrophobicity, thus inducing the sol-gelation transition of NSF nanofibrils. NSF nanofibrils were randomly dispersed from PSG to ASG-1, and self-assembled into anisotropic herringbone patterns at ASG-2 near the spinneret ready for silkworm spinning. Our findings reveal the controlled self-assembly mechanism of the multi-scale hierarchical architecture of NSF from nanofibrils to herringbone patterns programmed by metal ions and pH gradient, which provides novel insights into the spinning mechanism of silk-secreting animals and bioinspired design of high-performance fibers.

Silk Fibroin Materials: Biomedical Applications and Perspectives

The golden rule in tissue engineering is the creation of a synthetic device that simulates the native tissue, thus leading to the proper restoration of its anatomical and functional integrity, avoiding the limitations related to approaches based on autografts and allografts. The emergence of synthetic biocompatible materials has led to the production of innovative scaffolds that, if combined with cells and/or bioactive molecules, can improve tissue regeneration. In the last decade, silk fibroin (SF) has gained attention as a promising biomaterial in regenerative medicine due to its enhanced bio/cytocompatibility, chemical stability, and mechanical properties. Moreover, the possibility to produce advanced medical tools such as films, fibers, hydrogels, 3D porous scaffolds, non-woven scaffolds, particles or composite materials from a raw aqueous solution emphasizes the versatility of SF. Such devices are capable of meeting the most diverse tissue needs; hence, they represent an innovative clinical solution for the treatment of bone/cartilage, the cardiovascular system, neural, skin, and pancreatic tissue regeneration, as well as for many other biomedical applications. The present narrative review encompasses topics such as (i) the most interesting features of SF-based biomaterials, bare SF's biological nature and structural features, and comprehending the related chemo-physical properties and techniques used to produce the desired formulations of SF; (ii) the different applications of SF-based biomaterials and their related composite structures, discussing their biocompatibility and effectiveness in the medical field. Particularly, applications in regenerative medicine are also analyzed herein to highlight the different therapeutic strategies applied to various body sectors.

Sonication and heat-mediated synthesis, characterization and larvicidal activity of sericin-based silver nanoparticles against dengue vector (Aedes aegypti)

Fabrication, characterization and evaluation of the larvicidal potential of novel silk protein (sericin)-based silver nanoparticles (Se-AgNPs) were the prime motives of the designed study. Furthermore, investigation of the sericin as natural reducing or stabilizing agent was another objective behind this study. Se-AgNPs were synthesized using sonication and heat. Fabricated Se-AgNPs were characterized using particle size analyzer, UV spectrophotometry, FTIR and SEM which confirmed the fabrication of the Se-AgNPs. Size of sonication-mediated Se-AgNPs was smaller (7.49 nm) than heat-assisted Se-AgNPs (53.6 nm). Being smallest in size, sonication-assisted Se-AgNPs revealed the significantly highest (F4,10  = 39.20, p = .00) larvicidal activity against fourth instar lab and field larvae (F4,10  = 1864, p = .00) of dengue vector (Aedes aegypti) followed by heat-assisted Se-AgNPs and positive control (temephos). Non-significant larvicidal activity was showed by silver (without sericin) which made the temperature stability of silver, debatable. Furthermore, findings of biochemical assays (glutathione-S transferase, esterase, and acetylcholinesterase) showed the levels of resistance in field strain larvae. Aforementioned findings of the study suggests the sonication as the best method for synthesis of Se-AgNPs while the larvicidal activity is inversely proportional to the size of Se-AgNPs, i.e., smallest the size, highest the larvicidal activity. Conclusively, status of the sericin as a natural reducing/stabilizing agent has been endorsed by the findings of this study. RESEARCH HIGHLIGHTS: Incorporation of biocompatible and inexpensive sericin as a capping/reducing agent for synthesis of Se-AgNPs. A novel sonication method was used for the fabrication of Se-AgNPs which were thoroughly characterized by particle size analyzer, UV-visible spectrophotometry, SEM and FTIR. Analysis of enzymatic (GSTs, ESTs) levels in field and lab strains of Aedes aegypti larvae for evaluation of insecticides resistance.

Development of a More Environmentally Friendly Silk Fibroin Scaffold for Soft Tissue Applications

A push for environmentally friendly approaches to biomaterials fabrication has emerged from growing conservational concerns in recent years. Different stages in silk fibroin scaffold production, including sodium carbonate (Na₂CO₃)-based degumming and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)-based fabrication, have drawn attention for their associated environmental concerns. Environmentally friendly alternatives have been proposed for each processing stage; however, an integrated green fibroin scaffold approach has not been characterized or used for soft tissue applications. Here, we show that the combination of sodium hydroxide (NaOH) as a substitute degumming agent with the popular "aqueous-based" alternative silk fibroin gelation method yields fibroin scaffolds with comparable properties to traditional Na₂CO₃-degummed aqueous-based scaffolds. The more environmentally friendly scaffolds were found to have comparable protein structure, morphology, compressive modulus, and degradation kinetics, with increased porosity and cell seeding density relative to traditional scaffolds. Human adipose-derived stem cells showed high viability after three days of culture while seeded in each scaffold type, with uniform cell attachment to pore walls. Adipocytes from human whole adipose tissue seeded into scaffolds were found to have similar levels of lipolytic and metabolic function between conditions, in addition to a healthy unilocular morphology. Results indicate that our more environmentally friendly methodology for silk scaffold production is a viable alternative and well suited to soft tissue applications.

Mesoscale Confinement in Bagworm Silk: A Hidden Structural Organization

While silk fibers produced by silkworms and spiders are frequently described as a network of amorphous protein chains reinforced by crystalline β-sheet nanodomains, the importance of higher-order, self-assembled structures has been recognized for advanced modeling of mechanical properties. General acceptance of hierarchical structural models is, however, currently limited by lack of experimental results. Indeed, X-ray scattering studies of spider's dragline-type fibers have been particularly limited by low crystallinities. Here we are reporting on probing the local structure of exceptionally crystalline bagworm silk fibers by X-ray nanobeam scattering. Probing the comparable thickness of cross sections with an X-ray nanobeam allows removing the strong scattering background from the outer sericin layer and reveals a hidden structural organization due to a radial gradient in diameters of mesoscale nanofibrillar bundles in the fibroin phase. Our results provide direct support for lateral interactions between nanofibrils.

A Comprehensive Review on Silk Fibroin as a Persuasive Biomaterial for Bone Tissue Engineering

Bone tissue engineering (BTE) utilizes a special mix of scaffolds, cells, and bioactive factors to regulate the microenvironment of bone regeneration and form a three-dimensional bone simulation structure to regenerate bone tissue. Silk fibroin (SF) is perhaps the most encouraging material for BTE given its tunable mechanical properties, controllable biodegradability, and excellent biocompatibility. Numerous studies have confirmed the significance of SF for stimulating bone formation. In this review, we start by introducing the structure and characteristics of SF. After that, the immunological mechanism of SF for osteogenesis is summarized, and various forms of SF biomaterials and the latest development prospects of SF in BTE are emphatically introduced. Biomaterials based on SF have great potential in bone tissue engineering, and this review will serve as a resource for future design and research.

Type 1 diabetes and engineering enhanced islet transplantation

The development of new therapeutic approaches to treat type 1 diabetes mellitus (T1D) relies on the precise understanding and deciphering of insulin-secreting β-cell biology, as well as the mechanisms responsible for their autoimmune destruction. β-cell or islet transplantation is viewed as a potential long-term therapy for the millions of patients with diabetes. To advance the field of insulin-secreting cell transplantation, two main research areas are currently investigated by the scientific community: (1) the identification of the developmental pathways that drive the differentiation of stem cells into insulin-producing cells, providing an inexhaustible source of cells; and (2) transplantation strategies and engineered transplants to provide protection and enhance the functionality of transplanted cells. In this review, we discuss the biology of pancreatic β-cells, pathology of T1D and current state of β-cell differentiation. We give a comprehensive view and discuss the different possibilities to engineer enhanced insulin-secreting cell/islet transplantation from a translational perspective.

Structure of silk I (Bombyx mori silk fibroin before spinning) in the dry and hydrated states studied using 13C solid-state NMR spectroscopy

Nowadays, much attention has been paid to Bombyx mori silk fibroin (SF) by many researchers because of excellent physical properties and biocompatibility. These superior properties originate from the structure of SF and therefore, the structural analysis is a key to clarify the superiority. Here we concentrated on silk I structure (SF structure before spinning). We showed that silk I* (the structure of (GAGAGS)_n which is a main part of SF) is a repeated type II β-turn, neither α-helix nor random coil, from the conformation-dependent ¹³C NMR chemical shift data. This conclusion is different from that obtained using IR by many researchers. Next, the formation of silk I* structure was investigated at molecular level using ¹³C solid-state NMR spectroscopy. Three kinds of ¹³C INEPT, CP/MAS and DD/MAS NMR spectra were observed for SF, [3-¹³C] Ser- and [3-¹³C] Tyr-SF, the crystalline fraction obtained by chymotrypsin treatment of SF and their model peptide with silk I structures in the dry and hydrated states. Especially, the presence of the sequences containing Tyr, (((GX)_m1GY)_m2 where X = A or V) with random coil conformations adjacent to (GAGAGS)_n is an essence to get water-soluble SF and the formation of silk I* structure of (GAGAGS)_n.

Near-Infrared-Enpowered Nanomotor-Mediated Targeted Chemotherapy and Mitochondrial Phototherapy to Boost Systematic Antitumor Immunity

Phototherapy is an important strategy to inhibit tumor growth and activate antitumor immunity. However, the effect of photothermal/photodynamic therapy (PTT/PDT) is restricted by limited tumor penetration depth and unsatisfactory potentiation of antitumor immunity. Here, a near-infrared (NIR)-driven nanomotor is constructed with a mesoporous silicon nanoparticle (MSN) as the core, end-capped with Antheraea pernyi silk fibroin (ApSF) comprising arginine-glycine-aspartate (RGD) tripeptides. Upon NIR irradiation, the resulting ApSF-coated MSNs (DIMs) loading with photosensitizers (ICG derivatives, IDs) and chemotherapeutic drugs (doxorubicin, Dox) can efficiently penetrate into the internal tumor tissues and achieve effective phototherapy. Combined with chemotherapy, a triple-modal treatment (PTT, PDT, and chemotherapy) approach is developed to induce the immunogenic cell death of tumor cells and to accelerate the release of damage-associated molecular patterns. In vivo results suggest that DIMs can promote the maturation of dendritic cells and surge the number of infiltrated immune cells. Meanwhile, DIMs can polarize macrophages from M2 to M1 phenotypes and reduce the percentages of immunosuppressive Tregs, which reverse the immunosuppressive tumor microenvironment and activate systemic antitumor immunity. By achieving synergistic effects on the tumor inhibition and the antitumor immunity activation, DIMs show great promise as new nanoplatforms to treat metastatic breast cancer.

Silk fibroin and sericin polymer blends for sustainable battery separators

Natural polymers are a promising alternative for reducing the environmental impact of batteries. For this reason, it is still necessary to study their behavior and implement its use in these devices, especially in separator membranes. This work reports on new separator membranes based on silk fibroin (SF) and silk sericin (SS) prepared by salt leaching method. The effect of the different SS relative content on the physiochemical properties of the membranes and on the electrochemical performance of the corresponding batteries with lithium iron phosphate (LFP) as cathodes has been reported. It is observed that the increasing of SS content leads to a decrease of the overall crystallinity of the membranes. All SF/SS membranes presented a well-defined porosity above 75% with a uniform distribution of interconnected micropores. The electrolyte uptake and the ionic conductivity are dependent on the relative SS content. The addition of 10 wt% of SS into SF membranes, induce a high ionic conductivity of 4.09 mS.cm^-1 and high lithium transference number (0.52), due to the improvement of the Li⁺ ions conduction paths within the blended structure. Charge/discharge tests performed in Lithium/C-LFP half-cells reveal a discharge capacity of 85 mAh.g^-1 at 2C after 100 cycles for batteries with a SF/SS separator, containing a 10 wt% of SS, which suggests a stabilizing effect of Sericin on discharge capacity. Further, a 50% and 35% of capacity of retention and capacity fade, respectively, is observed. The presented SF/SS membrane show high electrochemical stability, being suitable for implementation in a next generation of sustainable battery systems. This could allow the SS valorization considering that 150,000 tons of SS are abandoned each year, reducing the contamination of environmental effluents.

Macroscopic Assembly of Sericin toward Self-Healable Silk

Silk contains an adhesive glycoprotein, silk sericin, in which silk fibroins can be enfolded and chemically stabilized. Silk sericin is gaining importance as the material for the creation of functional bioscaffolds. However, the assembly of silk sericin is generally limited to the blend of polymers or proteins due to its inherent poor mechanical strength. Here, we report a simple macroscopic controlled assembly of silk sericin fibers based on their secondary structure via wet-spinning. In addition, plasticization of silk sericin using glycerol immobilized with glutaraldehyde was found to induce dimensional stability, affording stable linear fibers with self-adhesion. Furthermore, cyclo-phenylalanine nanowires were incorporated into the silk sericin dope for a practical demonstration of their potential in artificial silk production with superstructure formation. The physicochemical characteristics of the spun fibers have also been elucidated using Fourier-transform infrared spectroscopy, electron microscopy, tensile test, differential scanning calorimetry, and 2D X-ray diffraction.

Silk Fibroin as a Green Material

Silk fibroin has been explored as a suitable biomaterial due to its biocompatibility, tunable degradability, low toxicity, and mechanical properties. To harness silk fibroin's innate properties, it is purified from native silkworm cocoons by removing proteins and debris that have the potential to cause inflammatory responses. Typically, within the purification and fabrication steps, chemical solvents, energy-intensive equipment, and large quantities of water are used to reverse engineer silk fibroin into an aqueous solution and then process into the final material format. Gentler, green methods for extraction and fabrication have been developed that reduce or remove the need for harmful chemical additives and energy-inefficient equipment while still producing mechanically robust biomaterials. This review will focus on the alternative green processing and fabrication methods that have proven useful in creating silk fibroin materials for a range of applications including consumer and medical materials.

Conductive In Situ Reduced Graphene Oxide-Silk Fibroin Bionanocomposites

This research paper describes the fabrication of bionanocomposites (BNCs) based on silk fibroin (SF) and reduced graphene oxide (rGO). The recorded UV-visible (UV-vis) spectra of the sample confirm the reduction of GO to rGO in SF by showing a plasmon resonance band within the wavelength range of 261-268 nm. The X-ray diffraction (XRD) peak at 11.6° corresponding to the GO intensity decreases with increasing reaction time, resulting in rGO in the SF host matrix. The morphological behavior of the SF-rGO BNCs is scrutinized using scanning electron microscopy (SEM), and the images clearly indicate the existence of rGO within the matrix. The increasing amount of GO in the SF shows broken graphene sheets, which can increase the surface roughness and establish a strong physical contact between the SF and rGO nanosheets. The high-resolution transmission electron microscope (HR-TEM) image of the bionanocomposite showed that the formed rGO encompassments of fewer layers are stacked, each with fewer wrinkles and folding. The Raman spectroscopy confirmed the formation of rGO by showing the increased intensity ratio of D to G band (I D/I G) in the bionanocomposite samples. The rGO effect on the electrical conductivity is measured, and the results show that DC conductivity increases from 1.28 × 10-9 to 82.4 × 10-9 S/cm with an increase in the GO content in the SF biopolymer. The investigations demonstrate loss of the insulation property and improved conducting behavior of the SF biopolymer.

3D Silk Fiber Construct Embedded Dual-Layer PEG Hydrogel for Articular Cartilage Repair - In vitro Assessment

Since articular cartilage does not regenerate itself, researches are underway to heal damaged articular cartilage by applying biomaterials such as a hydrogel. In this study, we have constructed a dual-layer composite hydrogel mimicking the layered structure of articular cartilage. The top layer consists of a high-density PEG hydrogel prepared with 8-arm PEG and PEG diacrylate using thiol-norbornene photo-click chemistry. The compressive modulus of the top layer was 700.1 kPa. The bottom layer consists of a low-density PEG hydrogel reinforced with a 3D silk fiber construct. The low-density PEG hydrogel was prepared with 4-arm PEG using the same cross-linking chemistry, and the compressive modulus was 13.2 kPa. Silk fiber was chosen based on the strong interfacial bonding with the low-density PEG hydrogel. The 3D silk fiber construct was fabricated by moving the silk fiber around the piles using a pile frame, and the compressive modulus of the 3D silk fiber construct was 567 kPa. The two layers were joined through a covalent bond which endowed sufficient stability against repeated torsions. The final 3D silk fiber construct embedded dual-layer PEG hydrogel had a compressive modulus of 744 kPa. Chondrogenic markers confirmed the chondrogenic differentiation of human mesenchymal stem cells encapsulated in the bottom layer.

Engineering silk sericin decorated zeolitic imidazolate framework-8 nanoplatform to enhance chemotherapy

The low therapeutic effect and strong side-effect are the major barriers for clinical chemotherapy. Herein, a pH-responsive nanoplatform based-silk sericin-zeolitic imidazolate framework-8 was designed for the delivery of chemotherapeutic doxorubicin (denoted as ZIF-8@DOX@SS, ZDS), which can overcome the limitation of poor circulation stability and unexpected drug leakage in blood circulation, producing a satisfactory chemotherapy. Concretely, ZIF-8 structure shows better stability and biocompatibility owing to the protection of a nature and non-toxic sericin protein. When it comes to low pH environment (e.g. in tumor cell microenvironment), the coordination effect in ZIF-8 will be broken and release DOX drugs. The nano-sized morphology endow ZDS an efficient drug uptake and significant tumor permeability efficiency. Furthermore, the tumor-specific biodegradability makes ZDS possible to realize targeted and enhanced chemotherapy. Considering all the advantages in the study, this silk sericin-based nanosystem represent a promising strategy for the design of stimuli-responsive by using natural polymer to improve the treatment effect of chemotherapy.

Protein composites from silkworm cocoons as versatile biomaterials

Silk is a naturally occurring biopolymer formed into fibers composed primarily of fibroin and sericin proteins. The outstanding mechanical properties of silk fibroin (SF) provides numerous applications for silk-based biomaterials. However, the canonical approaches for fabricating silk-based biomaterials typically involve degumming to remove the silk sericin (SS) to avoid adverse biological effects. Meanwhile, sericin has multiple biological functions including outstanding hydrophilicity, promoting cell attachment that are useful to exploit in new materials, inspiring the use of sericin-based biomaterials for biomedical applications. However, compared to fibroin, sericin is not a structural protein, thus sericin-based materials do not provide robust mechanical properties. To address this problem, we report an effective method for fabricating silk fibroin-sericin protein (SS-SF) composites directly from whole cocoons, negating the traditional extraction step to remove the sericin. This approach combines the material features from both fibroin as a structural unit and sericin as a biological functional unit, to achieve advantages regarding processing and materials properties, not only simplifying processing and maintaining the mechanical properties of the fibroin by avoiding degumming, but also endowing these SS-SF composite materials with enhanced hydrophilicity and cell adhesion performance to promote cell growth and proliferation. In addition, these protein composites could be fabricated into a variety of materials formats (e.g. films, sponges, monoliths) to fit different biomedical applications.

New insight into the mechanism of in vivo fibroin self-assembly and secretion in the silkworm, Bombyx mori

Fibroin of the silkworm consists of fibroin heavy chain (Fib-H) with hydrophobic intermediate repeats flanked by hydrophilic N and C terminal domains (NTD and CTD, respectively), fibroin light chain (Fib-L), and P25. However, the respective roles of each polypeptide in silk processing remain largely unknown. Here, a series of transgenic silkworms with different fusion gene expression cassettes were created in order to selectively express different fluorescent fusion proteins in silk glands. The roles of different components in silk processing were investigated via observing and analyzing the movement and distribution of these proteins in the silk gland and in cocoon silk. The data showed that hydrophilic NTDs were distributed on the surface of micelles, providing sufficient electrostatic repulsion to prevent premature crystallization of silk proteins. Hydrophilic CTD==Ls ("==" represents the disulfide bond) were located on the inner layer of micelles to control the solubility of large micelles. The results presented here elucidated the underlying mechanisms of silkworm silk processing in vivo. This is significant for the development of artificial spinning technology, novel silk biomaterials, and silk gland expression systems.

Multi-responsive nanococktails with programmable targeting capacity for imaging-guided mitochondrial phototherapy combined with chemotherapy

The integration of multimodal functions into one nanoplatform holds great promise for enhancing anticancer drug action and mitigating adverse effects. Herein, we prepared hyaluronic acid-functionalized regenerated silk fibroin-based nanoparticles (NPs) loading with photosensitizer (NIR770) and doxorubicin (DOX). The resultant HNDNPs had a desirable diameter of 161.0 nm and a negative zeta-potential of -30.5 mV. Interestingly, they showed excellent responses when triggered with various stimuli (acidity, reactive oxygen species, glutathione, hyaluronidase, or hyperthermia). Cell experiments revealed that HNDNPs could be specifically internalized by A549 cells, and efficiently released the payloads into the cytoplasm. Moreover, NIR770 was preferentially retained in mitochondria due to its lipophilic and cationic properties, which exhibited highly efficient photothermal therapy and photodynamic therapy upon near infrared (NIR) irradiation. Meanwhile, DOX molecules were mainly accumulated in the nucleus. Intravenous injection of HNDNPs into mice followed by NIR irradiation provided excellent multimodal imaging (NIR, photothermal, and photoacoustic imaging), almost eliminated the entire tumor, and greatly prolonged mice survival time with no side effects. Our study demonstrates that this HNDNP, which integrates the functions of tumor targeting, on-demand drug release, multimodal imaging, mitochondrial phototherapy, and chemotherapy, can be exploited as a promising nanococktail for imaging-guided synergistic treatment of cancer.

Salt-induced silk gel-derived N and trace Fe co-doped 3D porous carbon as an oxygen reduction catalyst in microbial fuel cells

Inexpensive and high-efficiency oxygen reduction reaction (ORR) catalysts play a significant role in achieving practical applications of microbial fuel cells (MFCs). Hence, herein, novel nitrogen (N) and trace iron (Fe) co-doped three-dimensional (3D) porous carbon (NFe_x-C) was synthesized as an excellent ORR catalyst from an interesting salt-induced silk gel, which was beneficial to the spontaneously formation of porosity and boosted the ORR activity. Among the series of NFe_x-C, NFe_0.5-C (1.20% N-ORR/C, 0.07 at% Fe) possessed a higher specific surface area (538.94 m² g^-1) and pore volume (2.158 cm³ g^-1). Note that NFe_0.5-C exhibited a significantly higher positive initial potential (0.274 V vs. Ag/AgCl) and half-wave potential (0.095 V vs. Ag/AgCl) than other catalysts and commercial Pt/C (20 wt%); this implied that it possessed prominent ORR catalytic activity. In the MFC tests, the output-voltage and maximum power density of NFe_0.5-C were enhanced to 517.37 ± 7.87 mV and 605.35 ± 15.39 mW m^-2, respectively. Moreover, NFe_0.5-C (0.15 $ g^-1) exhibits excellent anti-poisoning ability and is thousands of times cheaper than commercial Pt/C (20 wt%, 220.04 $ g^-1); therefore, NFe_0.5-C should be a prospective catalyst to substitute precious commercial Pt/C in MFCs and even for application in other types of fuel cells.

High-toughness natural polymer nonwoven preforms inspired by silkworm cocoon structure

As the interest in environmentally friendly materials and concerns regarding depletion of petroleum resources has increased, the research on natural polymers is being actively pursued. Among the various materials based on natural polymeric resources, the interest in using natural fibers in bio-composites has grown due to their lightweight, non-toxicity, low cost, and abundance. However, the lack of interfacial adhesion between filaments and poor water resistance make the use of natural fiber-based polymer composites less attractive. To overcome these drawbacks, formaldehyde-based synthetic binders have been used. However, this requires an additional synthesis of the binder, and potential toxicity problems exist. In this work, robust and rigid natural polymer nonwoven preforms were prepared by mixing jute fibers with silk sericin (SS). SS was employed as a natural facile binder and the strong binding between jute fibers and SS resulted in remarkable enhancements in tensile strength, elongation, and toughness, which increased up to 539.1, 385.7, and 1943.8%, respectively, compared with the pristine jute nonwoven. In addition, the dense and rigid structure obtained through SS coating ensured the structural stability of the nonwoven preforms in moisture environments. Silkworm cocoon-structured natural polymer nonwoven preforms with excellent mechanical strength and higher physical stability may have more potential utilization in the composite material fields.

Reverse-engineered silk hydrogels for cell and drug delivery

Silk is an important biopolymer for (bio)medical applications because of its unique and highly versatile structure and its robust clinical track record in human medicine. Silk can be processed into many material formats, including physically and chemically cross-linked hydrogels that have almost limitless applications ranging from tissue engineering to biomedical imaging and sensing. This concise review provides a detailed background of silk hydrogels, including silk structure-function relationships, biocompatibility and biodegradation, and it explores recent developments in silk hydrogel utilization, with specific reference to drug and cell delivery. We address common pitfalls and misconceptions while identifying emerging opportunities, including 3D printing.

The Biomedical Use of Silk: Past, Present, Future

Humans have long appreciated silk for its lustrous appeal and remarkable physical properties, yet as the mysteries of silk are unraveled, it becomes clear that this outstanding biopolymer is more than a high-tech fiber. This progress report provides a critical but detailed insight into the biomedical use of silk. This journey begins with a historical perspective of silk and its uses, including the long-standing desire to reverse engineer silk. Selected silk structure-function relationships are then examined to appreciate past and current silk challenges. From this, biocompatibility and biodegradation are reviewed with a specific focus of silk performance in humans. The current clinical uses of silk (e.g., sutures, surgical meshes, and fabrics) are discussed, as well as clinical trials (e.g., wound healing, tissue engineering) and emerging biomedical applications of silk across selected formats, such as silk solution, films, scaffolds, electrospun materials, hydrogels, and particles. The journey finishes with a look at the roadmap of next-generation recombinant silks, especially the development pipeline of this new industry for clinical use.

Differential Scanning Calorimetry of Native Silk Feedstock

Native silk proteins, extracted directly from the silk gland prior to spinning, offer access to a naturally hydrated protein that has undergone little to no processing. Combined with differential scanning calorimetry (DSC), it is possible to probe the thermal stability and hydration status of silk and thus investigate its denaturation and solidification, echoing that of the natural spinning process. It is found that native silk is stable between -10 °C and 55 °C, and both the high-temperature enthalpy of denaturation (measured via modulated temperature DSC) and a newly reported low-temperature ice-melting transition may serve as useful quality indicators in the future for artificial silks. Finally, compared to albumin, silk's denaturation enthalpy is much lower than expected, which is interpreted within a recently proposed entropic desolvation framework which can serve to unveil the low-energy aquamelt processing pathway.

Polyethylenimine-functionalized silk sericin beads for high-performance remediation of hexavalent chromium from aqueous solution

The enhancement of the metal adsorption and remediation performance of biomass-based adsorbents is an important challenge in heavy metal removal processes. One of the most viable and practical approaches in accomplishing a high metal removal efficiency is the surface modification of natural polymer adsorbents with functional polymeric materials. In the present study, polyethylenimine (PEI)-modified silk sericin beads were fabricated. The PEI modification process was confirmed and analyzed by Fourier transform infrared spectroscopy (FTIR), field emission-scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDS)-based elemental analysis. The Cr(VI) remediation capabilities of PEI-modified sericin beads were analyzed by testing Cr(VI) removal from contaminated water. The results show that the Cr(VI) removal capacity of PEI-modified sericin beads was 365.3 mg/g, which is significantly higher than that of pristine sericin beads (34.56 mg/g). During the Cr(VI) removal process, Cr(VI) adsorption and reduction to Cr(III) occurred simultaneously. The results herein reveal that the synthesized PEI-modified sericin beads are a promising material for Cr(VI) adsorption and detoxification of aqueous solution.

Engineering the Future of Silk Materials through Advanced Manufacturing

Silk is a natural fiber renowned for its outstanding mechanical properties that have enabled the manufacturing of ultralight and ultrastrong textiles. Recent advances in silk processing and manufacturing have underpinned a re-interpretation of silk from textiles to technological materials. Here, it is argued that silk materials-optimized by selective pressure to work in the environment at the biotic-abiotic interface-can be harnessed by human micro- and nanomanufacturing technology to impart new functionalities and opportunities. A critical overview of recent progress in silk technology is presented with emphasis on high-tech applications enabled by recent innovations in multilevel modifications, multiscale manufacturing, and multimodal characterization of silk materials. These advances have enabled successful demonstrations of silk materials across several disciplines, including tissue engineering, drug delivery, implantable medical devices, and biodissolvable/degradable devices.

The rheological properties of native sericin

Unlike spider silk, spinning silkworm silk has the added intricacy of being both fibre and micron-thick glue-like coating. Whilst the natural flow properties of the fibre feedstock fibroin are now becoming more established, our understanding of the coating sericin is extremely limited and thus presents both a gap in our knowledge and a hindrance to successful exploitation of these materials. In this study we characterise sericin feedstock from the silkworm Bombyx mori in its native state and by employing both biochemical, rheological and spectroscopic tools, define a natural gold standard. Our results demonstrate that native sericin behaves as a viscoelastic shear thinning fluid, but that it does so at a considerably lower viscosity than its partner fibroin, and that its upper critical shear rate (onset of gelation) lies above that of fibroin. Together these findings provide the first evidence that in addition to acting as a binder in the construction of the cocoon, sericin is capable of lubricating the flow of fibroin within the silk gland, which has implications for future processing, modelling and biomimetic use of these materials.

STATEMENT OF SIGNIFICANCE

This study addresses one of the major gaps in our knowledge regarding natural silk spinning by providing rigorous rheological characterisation of the other major protein involved - sericin. This allows progress in silk flow modelling, biomimetic system design, and in assessing the quality of bioinspired and waste sericin materials by providing a better understanding of the native, undegraded system.