Silk fibroin produced by transgenic silkworms overexpressing the Arg‐Gly‐Asp motif accelerates cutaneous wound healing in mice

Advancements in silk fibroin and silk sericin-based biomaterial applications for cancer therapy and wound dressing formulation: A comprehensive review

Silks are a class of proteins generated naturally by different arthropods, including silkworms, spiders, scorpions, mites, wasps, and bees. This review discusses the silk fibroin and silk sericin fabricated by Bombyx mori silkworm as versatile fibers. This silk fiber is predominantly composed of hydrophobic silk fibroin and hydrophilic silk sericin. Fibroin is defined as a structural protein that bestows silk with strength, while sericin is characterized as a gum-like protein, tying the two fibrous proteins together and endowing silk proteins with elasticity. Due to their versatile structures, biocompatibility, and biodegradability, they could be tailored into intricate structures to warrant particular demands. The intrinsic functional groups of both proteins enable their functionalization and cross-linking with various biomaterials to endow the matrix with favorable antioxidant and antibacterial properties. Depending on the target applications, they can be integrated with other materials to formulate nanofibrous, hydrogels, films, and micro-nanoparticles. Given the outstanding biological and controllable physicochemical features of fibroin and sericin, they could be exploited in pharmaceutical applications involving tissue engineering, wound repair, drug delivery, and cancer therapy. This review comprehensively discusses the advancements in the implementation of different formulations of silk fibroin and sericin in wound healing and drug delivery systems, particularly for cancer treatment.

Modeling Wound Chronicity In Vivo: The Translational Challenge to Capture the Complexity of Chronic Wounds

In an aging society with common lifestyle-associated health issues such as obesity and diabetes, chronic wounds pose a frequent challenge that physicians face in everyday clinical practice. Therefore, nonhealing wounds have attracted much scientific attention. Several in vitro and in vivo models have been introduced to deepen our understanding of chronic wound pathogenesis and amplify therapeutic strategies. Understanding how wounds become chronic will provide insights to reverse or avoid chronicity. Although choosing a suitable model is of utmost importance to receive valuable outcomes, an ideal in vivo model capturing the complexity of chronic wounds is still missing and remains a translational challenge. This review discusses the most relevant mammalian models for wound healing studies and provides guidance on how to implement the hallmarks of chronic wounds. It highlights the benefits and pitfalls of established models and maps out future avenues for research.

Silk fibroin-based scaffolds for tissue engineering

Silk fibroin is an important natural fibrous protein with excellent prospects for tissue engineering applications. With profound studies in recent years, its potential in tissue repair has been developed. A growing body of literature has investigated various fabricating methods of silk fibroin and their application in tissue repair. The purpose of this paper is to trace the latest developments of SF-based scaffolds for tissue engineering. In this review, we first presented the primary and secondary structures of silk fibroin. The processing methods of SF scaffolds were then summarized. Lastly, we examined the contribution of new studies applying SF as scaffolds in tissue regeneration applications. Overall, this review showed the latest progress in the fabrication and utilization of silk fibroin-based scaffolds.

Bombyx mori Silk Fibroin and Model Peptides Incorporating Arg-Gly-Asp Motifs and Their Application in Wound Dressings

Skin plays an important role in protecting the human body from the environment, dehydration, and infection. Burns, wounds, and disease cause the skin to lose its role, but tissue-engineered skin substitutes offer the opportunity to restore skin loss. Silk fibroin from Bombyx mori (SF) has proven to be an excellent wound dressing material. In this study, we aim to develop an excellent wound dressing material by introducing three-residue sequence Arg-Gly-Asp (RGD), which is the most well-known adhesion site of fibronectin, in the films of SF and the model peptide. Its usefulness as a wound dressing material was evaluated both in vitro and in vivo. First, we showed that the flexible structures of the RGD sequence are still maintained in SF with a rigid antiparallel β-sheet structure using NMR in association with excellent wound dressings of SF containing RGD. Then, in in vitro experiments, two types of normal cells derived from human skin, normal human neonatal epidermal keratinocytes and normal human neonatal dermal fibroblasts, were used to evaluate the cell adhesion. On the other hand, in in vivo experiments, the study was conducted using a rat model of a whole skin layer defect wound. The results showed that the high-functionalized SF developed here has the potential to play a significant role in the field of wound dressings.

Bioengineered and functionalized silk proteins accelerate wound healing in rat and human dermal fibroblasts

Wound healing is an intrinsic process directed towards the restoration of damaged or lost tissue. The development of a dressing material having the ability to control the multiple aspects of the wound environment would be an ideal strategy to improve wound healing. Though natural silk proteins, fibroin, and sericin have demonstrated tissue regenerative properties, the efficacy of bioengineered silk proteins on wound healing is seldom assessed. Furthermore, silk proteins sans contaminants, having low molecular masses, and combining with other bioactive factors can hasten the wound healing process. Herein, recombinant silk proteins, fibroin and sericin, and their fusions with cecropin B were evaluated for their wound-healing effects using in vivo rat model. The recombinant silk proteins demonstrated accelerated wound closure in comparison to untreated wounds and treatment with Povidone. Among all groups, the treatment with recombinant sericin-cecropin B (RSC) showed significantly faster healing, greater than 90% wound closure by Day 12 followed by recombinant fibroin-cecropin B (RFC) (88.86%). Furthermore, histological analysis and estimation of hydroxyproline showed complete epithelialization, neovascularization, and collagenisation in groups treated with recombinant silk proteins. The wound healing activity was further verified by in vitro scratch assay using HADF cells, where the recombinant silk proteins induced cell proliferation and cell migration to the wound area. Additionally, wound healing-related gene expression showed recombinant silk proteins stimulated the upregulation of EGF and VEGF and regulated the expression of TGF-β1 and TGF-β3. Our results demonstrated the enhanced healing effects of the recombinant silk fusion proteins in facilitating complete tissue regeneration with scar-free healing. Therefore, the recombinant silks and their fusion proteins have great potential to be developed as smart bandages for wound healing.

In vitro biological activities of the flexible and virus nanoparticle-decorated silk fibroin-based films

Flexible films were prepared from silk fibroin (SF) and gelatin (GA) with a presence of glycerol (Gly), followed by water vapor annealing to achieve water-insoluble matrices. The blended SF/GA/Gly films were chemically conjugated with tobacco mosaic virus (TMV), either native (TMV-wt) or genetically modified with Arg-Gly-Asp (RGD) sequences (TMV-rgd), to improve cellular responses. The attachment and proliferation of L929 cells on TMV-decorated films were improved, possibly due to enhanced surface roughness. The cellular responses were pronounced with TMV-rgd, due to the proper decoration of RGD, which is an integrin recognition motif supporting cell binding. However, the biological results were inconclusive for human primary cells because of an innate slow growth kinetic of the cells. Additionally, the cells on SF/GA/Gly films were greater populated in S and G2/M phase, and the cell cycle arrest was notably increased in the TMV-conjugated group. Our findings revealed that the films modified with TMV were cytocompatible and the cellular responses were significantly enhanced when conjugated with its RGD mutants. The biological analysis on the cellular mechanisms in response to TMV is further required to ensure the safety concern of the biomaterials toward clinical translation.

One-Step FRESH Bioprinting of Low-Viscosity Silk Fibroin Inks

Silk fibroin (SF) is an attractive material for composing bioinks suitable for three-dimensional (3D) bioprinting. However, the low viscosity of SF solutions obtained through common dissolution methods limits 3D-bioprinting applications without the addition of thickeners or partial gelation beforehand. Here, we report a method of 3D bioprinting low-viscosity SF solutions without additives. We combined a method of freeform reversible embedding of suspended hydrogels, known as the FRESH method, with horseradish peroxidase-catalyzed cross-linking. Using this method, we successfully fabricated 3D SF hydrogel constructs from low-viscosity SF ink (10% w/w, 50 mPa s at 1 s^-1 shear rate), which does not yield 3D constructs when printed onto a plate in air. Studies using mouse fibroblasts confirmed that the printing process was cell-friendly. Additionally, cells enclosed in printed SF hydrogel constructs maintained > 90% viability for 11 days of culture. These results demonstrate that the 3D bioprinting technique developed in this study enables new 3D bioprinting applications using SF inks and thus has a great potential to contribute to tissue engineering and regenerative medicine.

Silk Fibroin-Based Biomaterials for Hemostatic Applications

Hemostasis plays an essential role in all surgical procedures. Uncontrolled hemorrhage is the primary cause of death during surgeries, and effective blood loss control can significantly reduce mortality. For modern surgeons to select the right agent at the right time, they must understand the mechanisms of action, the effectiveness, and the possible adverse effects of each agent. Over the past decade, various hemostatic agents have grown intensely. These agents vary from absorbable topical hemostats, including collagen, gelatins, microfibrillar, and regenerated oxidized cellulose, to biologically active topical hemostats such as thrombin, biological adhesives, and other combined agents. Commercially available products have since expanded to include topical hemostats, surgical sealants, and adhesives. Silk is a natural protein consisting of fibroin and sericin. Silk fibroin (SF), derived from silkworm Bombyx mori, is a fibrous protein that has been used mostly in fashion textiles and surgical sutures. Additionally, SF has been widely applied as a potential biomaterial in several biomedical and biotechnological fields. Furthermore, SF has been employed as a hemostatic agent in several studies. In this review, we summarize the several morphologic forms of SF and the latest technological advances on the use of SF-based hemostatic agents.

Engineering Gelation Kinetics in Living Silk Hydrogels by Differential Dynamic Microscopy Microrheology and Machine Learning

Microbes embedded in hydrogels comprise one form of living material. Discovering formulations that balance potentially competing for mechanical and biological properties in living hydrogels-for example, gel time of the hydrogel formulation and viability of the embedded organisms-can be challenging. In this study, a pipeline is developed to automate the characterization of the gel time of hydrogel formulations. Using this pipeline, living materials comprised of enzymatically crosslinked silk and embedded E. coli-formulated from within a 4D parameter space-are engineered to gel within a pre-selected timeframe. Gelation time is estimated using a novel adaptation of microrheology analysis using differential dynamic microscopy (DDM). In order to expedite the discovery of gelation regime boundaries, Bayesian machine learning models are deployed with optimal decision-making under uncertainty. The rate of learning is observed to vary between artificial intelligence (AI)-assisted planning and human planning, with the fastest rate occurring during AI-assisted planning following a round of human planning. For a subset of formulations gelling within a targeted timeframe of 5-15 min, fluorophore production within the embedded cells is substantially similar across treatments, evidencing that gel time can be tuned independent of other material properties-at least over a finite range-while maintaining biological activity.

Nonmulberry silk proteins: multipurpose ingredient in bio-functional assembly

The emerging field of tissue engineering and regenerative medicines utilising artificial polymers is facing many problems. Despite having mechanical stability, non-toxicity and biodegradability, most of them lack cytocompatibility and biocompatibility. Natural polymers (such as collagen, hyaluronic acid, fibrin, fibroin, and others), including blends, are introduced to the field to solve some of the relevant issues. Another natural biopolymer: silkworm silk gained special attention primarily due to its specific biophysical, biochemical, and material properties, worldwide availability, and cost-effectiveness. Silk proteins, namely fibroin and sericin extracted from domesticated mulberry silkwormBombyx mori, are studied extensively in the last few decades for tissue engineering. Wild nonmulberry silkworm species, originated from India and other parts of the world, also produce silk proteins with variations in their nature and properties. Among the nonmulberry silkworm species,Antheraea mylitta(Indian Tropical Tasar),A. assamensis/A. assama(Indian Muga), andSamia ricini/Philosamia ricini(Indian Eri), along withA. pernyi(Chinese temperate Oak Tasar/Tussah) andA. yamamai(Japanese Oak Tasar) exhibit inherent tripeptide motifs of arginyl glycyl aspartic acid in their fibroin amino acid sequences, which support their candidacy as the potential biomaterials. Similarly, sericin isolated from such wild species delivers unique properties and is used as anti-apoptotic and growth-inducing factors in regenerative medicines. Other characteristics such as biodegradability, biocompatibility, and non-inflammatory nature make it suitable for tissue engineering and regenerative medicine based applications. A diverse range of matrices, including but not limited to nano-micro scale structures, nanofibres, thin films, hydrogels, and porous scaffolds, are prepared from the silk proteins (fibroins and sericins) for biomedical and tissue engineering research. This review aims to represent the progress made in medical and non-medical applications in the last couple of years and depict the present status of the investigations on Indian nonmulberry silk-based matrices as a particular reference due to its remarkable potentiality of regeneration of different types of tissues. It also discusses the future perspective in tissue engineering and regenerative medicines in the context of developing cutting-edge techniques such as 3D printing/bioprinting, microfluidics, organ-on-a-chip, and other electronics, optical and thermal property-based applications.

Bionic Silk Fibroin Film Induces Morphological Changes and Differentiation of Tendon Stem/Progenitor Cells

Purpose

Tendon injuries are common musculoskeletal system disorders, but the ability for tendon regeneration is limited. Silk fibroin (SF) film may be suitable for tendon regeneration due to its excellent biocompatibility and physical properties. This study is aimed at evaluating the application value of bionic SF film in tendon regeneration.

Methods

Tendon stem/progenitor cells (TSPCs) were isolated from rat Achilles tendon and characterized based on their surface marker expression and multilineage differentiation potential. SF films with smooth or bionic microstructure surfaces (5, 10, 15, 20 μm) were prepared. The morphology and mechanical properties of natural tendons and SF films were characterized. TSPCs were used as the seed cells, and the cell viability and cell adhesion morphology were analyzed. The tendongenesis-related gene expression of TSPCs was also evaluated using quantitative polymerase chain reaction.

Results

Compared to the native tendon, only the 10, 15, and 20 μm SF film groups had comparable maximum loading and ultimate stress, with the exception of the breaking elongation rate. The 10 μm SF film group had the highest percentage of oriented cells and the most significant changes in cell morphology. The most significant upregulations in the expression of COL1A1, TNC, TNMD, and SCX were also observed in the 10 μm SF film group.

Conclusion

SF film with a bionic microstructure can serve as a tissue engineering scaffold and provide biophysical cues for the use of TSPCs to achieve proper cellular adherence arrangement and morphology as well as promote the tenogenic differentiation of TSPCs, making it a valuable customizable biomaterial for future applications in tendon repair.

Effects of transgenic silk materials that incorporate FGF-7 protein microcrystals on the proliferation and differentiation of human keratinocytes

The silk glands of silkworms produce large quantities of fibroin, which is a protein that can be physically processed and used as a biodegradable carrier for cell growth factors in tissue engineering applications. Meanwhile, protein microcrystals known as polyhedra, which are derived from cypovirus 1, have been used as a vehicle to protect and release encapsulated cell growth factors. We report the generation of transgenic silkworms that express recombinant fibroblast growth factor-7 (FGF-7) fused with the polyhedron-encapsulating signal in polyhedra produced in the middle (MSG) and posterior (PSG) silk glands. Immunofluorescence showed that polyhedra from silk glands are associated with FGF-7. The MSG and PSG from transgenic silkworms were processed into fine powdery materials, from which FGF-7 activity was released to stimulate the proliferation of human keratinocyte epidermal cells. Powders from PSGs exhibited higher FGF-7 activity than those from MSGs. Moreover, PSG powder showed a gradual release of FGF-7 activity over a long period and induced keratinocyte proliferation and differentiation in 3D culture to promote the formation of stratified epidermis expressing positive differentiation marker proteins. Our results indicate that powdery materials incorporating the FGF-7-polyhedra microcrystals from silk glands are valuable for developing cell/tissue engineering applications in vivo and in vitro.

Bionic Silk Fibroin Film Promotes Tenogenic Differentiation of Tendon Stem/Progenitor Cells by Activating Focal Adhesion Kinase

Background

Tendon injuries are common musculoskeletal disorders in clinic. Due to the limited regeneration ability of tendons, tissue engineering technology is often used as an effective approach to treat tendon injuries. Silk fibroin (SF) films have excellent biological activities and physical properties, which is suitable for tendon regeneration. The present study is aimed at preparing a SF film with a bionic microstructure and investigating its biological effects.

Methods

A SF film with a smooth surface or bionic microstructure was prepared. After seeding tendon stem/progenitor cells (TSPCs) on the surface, the cell morphology, the expression level of tenogenic genes and proteins, and the focal adhesion kinase (FAK) activation were measured to evaluate the biological effect of SF films.

Results

The TSPCs on SF films with a bionic microstructure exhibited a slender cell morphology, promoted the expression of tenogenic genes and proteins, such as SCX, TNC, TNMD, and COLIA1, and activated FAK. FAK inhibitors blocked the enhanced expression of tenogenic genes and proteins.

Conclusion

SF films with a bionic microstructure may serve as a scaffold, provide biophysical cues to alter the cellular adherence arrangement and cell morphology, and enhance the tenogenic gene and protein expression in TSPCs. FAK activation plays a key role during this biological response process.

Silk fibroin nanofibers: a promising ink additive for extrusion three-dimensional bioprinting

Here, we investigated the usefulness of silk fibroin nanofibers obtained via mechanical grinding of degummed silkworm silk fibers as an additive in bioinks for extrusion three-dimensional (3D) bioprinting of cell-laden constructs. The nanofibers could be sterilized by autoclaving, and addition of the nanofibers improved the shear thinning of polymeric aqueous solutions, independent of electric charge and the content of cross-linkable moieties in the polymers. The addition of nanofibers to bioinks resulted in the fabrication of hydrogel constructs with higher fidelity to blueprints. Mammalian cells in the constructs showed >85% viability independent of the presence of nanofibers. The nanofibers did not affect the morphologies of enclosed cells. These results demonstrate the great potential of silk fibroin nanofibers obtained via mechanical grinding of degummed silkworm silk fibers as an additive in bioinks for extrusion 3D bioprinting.

Enhanced wound healing via collagen-turnover-driven transfer of PDGF-BB gene in a murine wound model

Wound healing is a complex biological process that requires coordinated cell proliferation, migration, and extracellular matrix production/remodeling, all of which are inhibited/delayed in chronic wounds. In this study, a formulation was developed that marries a fibrin-based, provisional-like matrix with collagen mimetic peptide (CMP)/PDGF gene-modified collagens, leading to the formation of robust gels that supported temporally controlled PDGF expression and facile application within the wound bed. Analysis employing in vitro co-gel scaffolds confirmed sustained and temporally controlled gene release based on matrix metalloproteinase (MMP) activity, with ~30% higher PDGF expression in MMP producing fibroblasts as-compared with non-MMP-expressing cells. The integration of fibrin with the gene-modified collagens resulted in co-gels that strongly supported both fibroblast cell recruitment/invasion as well as multiple aspects of the longer-term healing process. The excisional wound healing studies in mice established faster wound closure using CMP-modified PDGF polyplex-loaded co-gels, which exhibited up to 24% more wound closure (achieved with ~2 orders of magnitude lower growth factor dosing) after 9 days as compared to PDGF-loaded co-gels, and 19% more wound closure after 9 days as compared to CMP-free polyplex loaded co-gels. Moreover, minimal scar formation as well as improved collagen production, myofibroblast activity, and collagen orientation was observed following CMP-modified PDGF polyplex-loaded co-gel application on wounds. Taken together, the combined properties of the co-gels, including their stability and capacity to control both cell recruitment and cell phenotype within the murine wound bed, strongly supports the potential of the co-gel scaffolds for improved treatment of chronic non-healing wounds.

Silk fibroin for skin injury repair: Where do things stand?

Several synthetic and natural materials are used in soft tissue engineering and regenerative medicine with varying degrees of success. Among them, silkworm silk protein fibroin, a naturally occurring protein-based biomaterial, exhibits many promising characteristics such as biocompatibility, controllable biodegradability, tunable mechanical properties, aqueous preparation, minimal inflammation in host tissue, low cost and ease of use. Silk fibroin is often used alone or in combination with other materials in various formats and is also a promising delivery system for bioactive compounds as part of such repair scenarios. These properties make silk fibroin an excellent biomaterial for skin tissue engineering and repair applications. This review focuses on the promising characteristics and recent advances in the use of silk fibroin for skin wound healing and/or soft-tissue repair applications. The benefits and limitations of silk fibroin as a scaffolding biomaterial in this context are also discussed. STATEMENT OF SIGNIFICANCE: Silk protein fibroin is a natural biomaterial with important biological and mechanical properties for soft tissue engineering applications. Silk fibroin is obtained from silkworms and can be purified using alkali or enzyme based degumming (removal of glue protein sericin) procedures. Fibroin is used alone or in combination with other materials in different scaffold forms, such as nanofibrous mats, hydrogels, sponges or films tailored for specific applications. The investigations carried out using silk fibroin or its blends in skin tissue engineering have increased dramatically in recent years due to the advantages of this unique biomaterial. This review focuses on the promising characteristics of silk fibroin for skin wound healing and/or soft-tissue repair applications.

Polydopamine modification of silk fibroin membranes significantly promotes their wound healing effect

Natural polymer-based wound dressings have gained great attention in skin tissue engineering.