Electrospinning of reconstituted silk fiber from aqueous silk fibroin solution

Microstructured silk fiber scaffolds with enhanced stretchability

Despite extensive research, current methods for creating three-dimensional (3D) silk fibroin (SF) scaffolds lack control over molecular rearrangement, particularly in the formation of β-sheet nanocrystals that severely embrittle SF, as well as hierarchical fiber organization at both micro- and macroscale. Here, we introduce a fabrication process based on electrowriting of aqueous SF solutions followed by post-processing using an aqueous solution of sodium dihydrogen phosphate (NaH2PO4). This approach enables gelation of SF chains via controlled β-sheet formation and partial conservation of compliant random coil structures. Moreover, this process allows for precise architecture control in microfiber scaffolds, enabling the creation of 3D flat and tubular macro-geometries with square-based and crosshatch microarchitectures, featuring inter-fiber distances of 400 μm and ∼97% open porosity. Remarkably, the crosslinked printed structures demonstrated a balanced coexistence of β-sheet and random coil conformations, which is uncommon for organic solvent-based crosslinking methods. This synergy of printing and post-processing yielded stable scaffolds with high compliance (modulus = 0.5-15 MPa) and the ability to support elastic cyclic loading up to 20% deformation. Furthermore, the printed constructs supported in vitro adherence and growth of human renal epithelial and endothelial cells with viability above 95%. These cells formed homogeneous monolayers that aligned with the fiber direction and deposited type-IV collagen as a specific marker of healthy extracellular matrix, indicating that both cell types attach, proliferate, and organize their own microenvironment within the SF scaffolds. These findings represent a significant development in fabricating organized stable SF scaffolds with unique microfiber structures and mechanical and biological properties that make them highly promising for tissue engineering applications.

A systematic review on green and natural polymeric nanofibers for biomedical applications

Electrospinning is the simplest technique to produce ultrathin nanofibers, which enables the use of nanotechnology in various applications. Nanofibrous materials produced through electrospinning have garnered significant attention in biomedical applications due to their unique properties and versatile potential. In recent years, there has been a growing emphasis on incorporating sustainability principles into material design and production. However, electrospun nanofibers, owing to their reliance on solvents associated with significant drawbacks like toxicity, flammability, and disposal challenges, frequently fall short of meeting environmentally friendly standards. Due to the limited solvent choices and heightened concerns for safety and hygiene in modern living, it becomes imperative to carefully assess the implications of employing electrospun nanofibers in diverse applications and consumer products. This systematic review aims to comprehensively assess the current state of research and development in the field of "green and natural" electrospun polymer nanofibers as well as more fascinating and eco-friendly commercial techniques, solvent preferences, and other green routes that respect social and legal restrictions tailored for biomedical applications. We explore the utilization of biocompatible and biodegradable polymers sourced from renewable feedstocks, eco-friendly processing techniques, and the evaluation of environmental impacts. Our review highlights the potential of green and natural electrospun nanofibers to address sustainability concerns while meeting the demanding requirements of various biomedical applications, including tissue engineering, drug delivery, wound healing, and diagnostic platforms. We analyze the advantages, challenges, and future prospects of these materials, offering insights into the evolving landscape of environmentally responsible nanofiber technology in the biomedical field.

Silk fibroin-derived electrospun materials for biomedical applications: A review

Electrospinning is a versatile technique for fabricating polymeric fibers with diameters ranging from micro- to nanoscale, exhibiting multiple morphologies and arrangements. By combining silk fibroin (SF) with synthetic and/or natural polymers, electrospun materials with outstanding biological, chemical, electrical, physical, mechanical, and optical properties can be achieved, fulfilling the evolving biomedical demands. This review highlights the remarkable versatility of SF-derived electrospun materials, specifically focusing on their application in tissue regeneration (including cartilage, cornea, nerves, blood vessels, bones, and skin), disease treatment (such as cancer and diabetes), and the development of controlled drug delivery systems. Additionally, we explore the potential future trends in utilizing these nanofibrous materials for creating intelligent biomaterials, incorporating biosensors and wearable sensors for monitoring human health, and also discuss the bottlenecks for its widespread use. This comprehensive overview illuminates the significant impact and exciting prospects of SF-derived electrospun materials in advancing biomedical research and applications.

Spidroins under the Influence of Alcohol: Effect of Ethanol on Secondary Structure and Molecular Level Solvation of Silk-Like Proteins

Future sustainable materials based on designer biomolecules require control of the solution assembly, but also interfacial interactions. Alcohol treatments of protein materials are an accessible means to this, making understanding of the process at the molecular level of seminal importance. We focus here on the influence of ethanol on spidroins, the main proteins of silk. By large-scale atomistically detailed molecular dynamics (MD) simulations and interconnected experiments, we characterize the protein aggregation, secondary structure changes, molecular level origins of them, and solvation environment changes for the proteins, as induced by ethanol as a solvation additive. The MD and circular dichoroism (CD) findings jointly show that ethanol promotes ordered structure in the protein molecules, leading to an increase of helix content and turns but also increased aggregation, as revealed by dynamic light scattering (DLS) and light microscopy. The structural changes correlate at the molecular level with increased intramolecular hydrogen bonding. The simulations reveal that polar amino acids, such as glutamine and serine, are most influenced by ethanol, whereas glycine residues are most prone to be involved in the ethanol-induced secondary structure changes. Furthermore, ethanol engages in interactions with the hydrophobic alanine-rich regions of the spidroin, significantly decreasing the hydrophobic interactions of the protein with itself and its surroundings. The protein solutes also change the microstructure of water/ethanol mixtures, essentially decreasing the level of larger local clustering. Overall, the work presents a systematic characterization of ethanol effects on a widely used, common protein type, spidroins, and generalizes the findings to other intrinsically disordered proteins by pinpointing the general features of the response. The results can aid in designing effective alcohol treatments for proteins, but also enable design and tuning of protein material properties by a relatively controllable solvation handle, the addition of ethanol.

Facile construction of calcium titanate-loaded silk fibroin scaffolds hybrid frameworks for accelerating neuronal cell growth in peripheral nerve regeneration

Different concentrations of calcium titanate (CaTiO3) nanoparticles were loaded into the Silk fibroin (SF) solution to construct porous SF@CaTiO3 hybrid scaffolds, which were shown to have enhanced properties for stimulating peripheral nerve regeneration. Surface charges, crystallization intensity, wettability, porosity, and morphology were measured and analyzed. We analyzed the hybrid porous SF@CaTiO₃ scaffolds that affected the expansion of Schwann cells. The results demonstrated a concentration-dependent influence on the dispersion of nanoparticles in the CaTiO₃ hybridized SF scaffolds. Incorporating CaTiO₃-NPs into the porous SF@CaTiO₃ hybrid scaffolds can boost hydrophobicity while decreasing surface charge density and porosity. The hybridized scaffolds mostly had an orthorhombic calcium titanate crystal structure with amorphous Silk fibroin mixed. Schwann cell cultures revealed that SF@CaTiO₃ hybrid scaffolds containing an optimal CaTiO₃-NPs concentration could stimulate the proliferation, attachment, and protection of Schwann cell biological functions, suggesting the scaffolds' potential for use in peripheral nerve regeneration.

Electrochemical and Electroconductive Behavior of Silk Fibroin Electrospun Membrane Coated with Gold or Silver Nanoparticles

The surface modification of materials obtained from natural polymers, such as silk fibroin with metal nanoparticles that exhibit intrinsic electrical characteristics, allows the obtaining of biocomposite materials capable of favoring the propagation and conduction of electrical impulses, acting as communicating structures in electrically isolated areas. On that basis, this investigation determined the electrochemical and electroconductive behavior through electrochemical impedance spectroscopy of a silk fibroin electrospun membrane from silk fibrous waste functionalized with gold or silver nanoparticles synthetized by green chemical reduction methodologies. Based on the results obtained, we found that silk fibroin from silk fibrous waste (SF_w) favored the formation of gold (AuNPs-SF_w) and silver (AgNPs-SF_w) nanoparticles, acting as a reducing agent and surfactant, forming a micellar structure around the individual nanoparticle. Moreover, different electrospinning conditions influenced the morphological properties of the fibers, in the presence or absence of beads and the amount of sample collected. Furthermore, treated SF_w electrospun membranes, functionalized with AuNPs-SF_w or AgNPS-SF_w, allowed the conduction of electrical stimuli, acting as stimulators and modulators of electric current.

Development of New Bio-Composite of PEO/Silk Fibroin Blends Loaded with Piezoelectric Material

New bio-composite nanofibers composed of polyethylene oxide (PEO)/silk fibroin (SF)/barium titanate (BaTiO₃) are introduced in this study. The SF solution was added to the PEO solution to form a PEO/SF blend with different weight percentages (5, 10, 15, 20 wt.%). The PEO/15 wt.% SF blend was selected to continue the experimental plan based on the optimum nanofiber morphology. Different wt.% of BaTiO₃ particles (0.2, 0.4, 0.8, 1 wt.%) were added to the PEO/15 wt.% SF blend solution, and the suspensions obtained were introduced to an electrospinning device. The fabricated tissue was characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. The zeta potential of the solution and the piezoelectric performance of the fabricated tissue were characterized. A newly designed pizoTester was used to investigate piezoelectric properties. The results showed that a well-organized, smooth PEO/15 wt.% SF/0.2 wt.% BaTiO₃ nanofiber composite with low bead contents was obtained. Improved properties and electrical coupling were achieved in the newly introduced material. Electrospun PEO/15 wt.% SF/0.2 wt.% BaTiO₃ mats increased the output voltage (1150 mV) compared to pristine PEO and PEO/SF composite fibers (410 and 290 mV, respectively) upon applying 20 N force at 5 Hz frequency. The observed enhancement in piezoelectric properties suggests that the prepared composite could be a promising material in cardiac tissue engineering (CTE).

Bioinspired silk fibroin materials: From silk building blocks extraction and reconstruction to advanced biomedical applications

Silk fibroin has become a promising biomaterial owing to its remarkable mechanical property, biocompatibility, biodegradability, and sufficient supply. However, it is difficult to directly construct materials with other formats except for yarn, fabric and nonwoven based on natural silk. A promising bioinspired strategy is firstly extracting desired building blocks of silk, then reconstructing them into functional regenerated silk fibroin (RSF) materials with controllable formats and structures. This strategy could give it excellent processability and modifiability, thus well meet the diversified needs in biomedical applications. Recently, to engineer RSF materials with properties similar to or beyond the hierarchical structured natural silk, novel extraction and reconstruction strategies have been developed. In this review, we seek to describe varied building blocks of silk at different levels used in biomedical field and their effective extraction and reconstruction strategies. This review also present recent discoveries and research progresses on how these functional RSF biomaterials used in advanced biomedical applications, especially in the fields of cell-material interactions, soft tissue regeneration, and flexible bioelectronic devices. Finally, potential study and application for future opportunities, and current challenges for these bioinspired strategies and corresponding usage were also comprehensively discussed. In this way, it aims to provide valuable references for the design and modification of novel silk biomaterials, and further promote the high-quality-utilization of silk or other biopolymers.

Preparation and characterization of silk fibroin/polyethylene oxide nanofiber membranes with antibacterial activity

Bacterial infection is among the most common diseases that threaten human health. Antibiotics are effective in treating bacterial infections. However, the overuse of antibiotics will lead to an increase in bacterial resistance. To reduce the overuse of antibiotics and improve the effective use of antibiotics through slow release, silk fibroin (SF)/polyethylene oxide (PEO) nanofiber membranes with different SF and PEO proportions were prepared by electrospinning. The ecofriendly solvent ethanol solution was used for electrospinning for better protection of antibiotic activity. The SEM showed that the surface of SF/PEO (2:8) and SF/PEO (3:7) was smoother and more uniform. With the increase of SF content, the thermal stability and hydrophilicity of SF/PEO nanofiber membranes were improved. The SF/PEO (3:7) nanofiber membrane had the best mechanical property and its maximum stress and strain were 4.6 1 ± 0.24 MPa and 16.36 ± 0.41%, respectively. Based on these good properties, SF/PEO (3:7) nanofiber membrane was chosen for loading and releasing gentamicin sulfate (GS). The fabricated (GS)/SF/PEO (3:7) nanofiber membrane exhibited good release efficiency and showed the good antibacterial activity against Staphylococcus aureus and Escherichia coli. These investigations indicated the GS/SF/PEO (3:7) nanofiber membrane (GS/SF/PEO) has a great potential for application in antibacterial materials.

Fabrication and Characterization of Silk Fibroin-Based Nanofibrous Scaffolds Supplemented with Gelatin for Corneal Tissue Engineering

Tissue engineering is a promising approach to overcome the severe worldwide shortage of healthy donor corneas. In this work, we have developed a silk-gelatin composite scaffold using electrospinning and permeation techniques to achieve the properties comparable to cornea analog. In particular, we present the fabrication and comparative evaluation of the novel gelatin sheets consisting of silk fibroin nanofibers, which are prepared using silk fibroin (SF) (in formic acid) and SF (in aqueous) electrospun scaffolds, for its suitability as corneal stromal analogs. All the fabricated samples were treated with ethanol vapor (T) to physically crosslink the silk nanofibers. Micro/nano-scale features of the fabricated scaffolds were analyzed using scanning electron microscopy micrographs. Fourier transform infrared spectroscopy revealed characteristic peaks of polymeric functional groups and modifications upon ethanol vapor treatment. Transparency of the scaffolds was determined using UV-visible spectra. Among all the fabricated samples, the gelatin-permeated SF (in formic acid; T) scaffold showed the highest level of transparency, i.e., 77.75 ± 2.3%, which is similar to that of the native cornea (∼70%-90% [variable with age group]) with healthy acute vision. Contact angle of the samples was studied to estimate the hydrophilicity of the scaffolds. All the scaffolds except non-treated SF (in aqueous; NT) were found to be significantly stable up to 14 days when incubated in phosphate buffered saline at 37°C. Treated samples showed significantly better stability, both physically and microscopically, in comparison to nontreated samples. Proliferation and viability assays of rabbit corneal fibroblast cells (SIRC) and mouse fibroblast cells (L929 RFP) when cultured on fabricated scaffolds revealed remarkable cellular compatibility with gelatin-permeated SF (in formic acid; T) scaffolds compared to SF (in aqueous; T). Unlike other reports in the existing literature, this work presents the design and development of a nanofibrous silk-gelatin composite that exhibits acceptable transparency, cellular biocompatibility, as well as improved mechanical stability comparable to that of native cornea. Therefore, we anticipate that the fabricated novel scaffold is likely to be a good candidate for corneal tissue construct. Moreover, among the fabricated scaffolds, the outcomes depict gelatin-permeated SF (in formic acid; T) composite scaffold to be a better candidate as a corneal stromal analog that carries properties of both the silk and gelatin, i.e., optimal transparency, better stability, and enhanced cytocompatibility.

Silk-based hybrid microfibrous mats as guided bone regeneration membranes

LAPONITE® (LAP) nanoplatelets were incorporated within a regenerated silk fibroin (RSF) microfibrous mat via electrospinning, which exhibited better cell adhesion and proliferation of bone marrow mesenchymal stem cells (BMSCs) than the pristine RSF ones.

Usnic-Acid-Functionalized Silk Fibroin Composite Scaffolds for Cutaneous Wounds Healing

Despite the progress in chronic wound treatment, antibacterial cutaneous scaffold with high efficiency in wound healing is still the hot spot in the field. In present study, a functionalized silk fibroin (SF) cutaneous scaffold incorporated with natural medicine usnic acid (UA) is investigated, in which UA is used as an antibacterial and wound-healing reagent. Via electrospinning, UA-SF mixture is fabricated into UA-SF composite scaffold (USCS), which is composed of uniform nanofibers with average diameters of around 360 ± 10 nm. The interwoven nanofibers form mesh structure providing sufficient moisture permeability for scaffold. With methanol treatment, USCS presents improved mechanical properties and stability to protease XIV. In the presence of USCS, the growth rate of both Gram-positive and Gram-negative bacteria, including Staphylococcus aureus, Streptococci pyogenes, Escherichia coli, and Pseudomonas aeruginosa, is significantly inhibited in plate culture and suspension assays. In a cutaneous excisional mouse wound model, USCS presents a significant increase of wound closure rate, compared with pure SF scaffold and commercial dressing, Tegaderm Hydrocolloid ^3M . The histological assessments further prove that USCS can enhance re-epithelialization, vascularization, and collagen deposition in wound site to promote the wound-healing process, which indicates the potential application of USCS in chronic wound treatment.

Evaluation of silk-based bioink during pre and post 3D bioprinting: A review

During past few decades, the demand for the replacement of damaged organs is increasing consistently. This is due to the advancement in tissue engineering, which opens the possibility of regeneration of damaged organs or tissues into functional parts with the help of 3D bioprinting. Bioprinting technology presents an excellent potential to develop complex structures with precise control over cell suspension and structure. A brief description of different types of 3D bioprinting techniques, including inkjet-based, laser-based, and extrusion-based bioprinting is presented here. Due to innate advantageous features like tunable biodegradability, biocompatibility, elasticity and mechanical robustness, silk has carved a niche in the realm of tissue engineering. In this review article, the focus is to highlight the possible approach of exploring silk as bioink for fabrication of bioprinted implants using 3D bioprinting. This review discusses different type of degumming, dissolution techniques for extraction of proteins from different sources of silk. Different recently reported 3D bioprinting techniques suitable for silk-based bioink are further elaborated. Postprinting characterization of resultant scaffolds are also describe here. However, there is an astounding progress in 3D bioprinting technology, still there is a need to develop further the current bioprinting technology to make it suitable for generation of heterogeneous tissue construct. The possibility of utilizing the adhesive property of sericin to consider it as bioink is elaborated.

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

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

Artificial ligament made from silk protein/Laponite hybrid fibers

With developments in tissue engineering, artificial ligaments are expected to be future materials for anterior cruciate ligament (ACL) reconstruction. However, poor healing of the intraosseous part after ACL reconstruction significantly hinders their applications in this field. In this study, a bioactive clay Laponite (LAP) was introduced into the regenerated silk fibroin (RSF) spinning dope to produce functional RSF/LAP hybrid fibers by wet-spinning. These RSF/LAP hybrid fibers were then woven into artificial ligament for ACL reconstruction. The structure and mechanical properties of RSF/LAP hybrid fibers were extensively studied by different means. Results confirmed the presence of LAP in RSF fibers and revealed that the addition of LAP slightly deteriorated the comprehensive mechanical properties of RSF fibers. However, they were still much tougher (with higher breaking energy) than those of degummed natural silkworm silk that was earlier used for making artificial ligament. The artificial ligament woven from RSF/LAP hybrid fibers showed better cytocompatibility and osteogenic differentiation with mouse pre-osteoblasts in vitro than those made from degummed natural silkworm silks and pure RSF fibers. Furthermore, in vivo study in a rat ACL reconstruction model demonstrated that the presence of LAP in the artificial ligament could significantly enhance the graft osseointegration process and also improve the corresponding biomechanical properties of the artificial ligament. Based upon these results, the RSF/LAP hybrid fibers, which can be mass produced by wet-spinning process, are believed to have a great potential for use as artificial ligament materials for ACL reconstruction. STATEMENT OF SIGNIFICANCE: In this study, we successfully introduced Laponite (LAP), a kind of clay that has the function of osteogenic induction, into regenerated silk fibroin (RSF) fibers, which was prepared by a mature wet-spinning method developed in our lab. We believe that through artificial spinning, additional functional components can be added into RSF fibers, which one can hardly achieve with natural silks. We showed that the artificial ligament woven from RSF/LAP hybrid fibers had better cytocompatibility and osteogenic differentiation for mouse pre-osteoblasts in vitro, and significantly enhanced the graft osseointegration process and improved the corresponding biomechanical properties in a rat ACL reconstruction model in vivo, compared to those artificial ligaments made from degummed natural silkworm silks and pure RSF fibers.

Bio-inspired human in vitro outer retinal models: Bruch's membrane and its cellular interactions

Retinal degenerative disorders, such as age-related macular degeneration (AMD), are one of the leading causes of blindness worldwide, however, treatments to completely stop the progression of these debilitating conditions are non-existent. Researchers require sophisticated models that can accurately represent the native structure of human retinal tissue to study these disorders. Current in vitro models used to study the retina are limited in their ability to fully recapitulate the structure and function of the retina, Bruch's membrane and the underlying choroid. Recent developments in the field of induced pluripotent stem cell technology has demonstrated the capability of retinal pigment epithelial cells to recapitulate AMD-like pathology. However, such studies utilise unsophisticated, bio-inert membranes to act as Bruch's membrane and support iPSC-derived retinal cells. This review presents a concise summary of the properties and function of the Bruch's membrane-retinal pigment epithelium complex, the initial pathogenic site of AMD as well as the current status for materials and fabrication approaches used to generate in vitro models of this complex tissue. Finally, this review explores required advances in the field of in vitro retinal modelling. STATEMENT OF SIGNIFICANCE: Retinal degenerative disorders such as age-related macular degeneration are worldwide leading causes of blindness. Previous attempts to model the Bruch's membrane-retinal pigment epithelial complex, the initial pathogenic site of age-related macular degeneration, have lacked the sophistication to elucidate valuable insights into disease mechanisms. Here we provide a detailed account of the morphological, physical and chemical properties of Bruch's membrane which may aid the fabrication of more sophisticated and physiologically accurate in vitro models of the retina, as well as various fabrication techniques to recreate this structure. This review also further highlights some recent advances in some additional challenging aspects of retinal tissue modelling including integrated fluid flow and photoreceptor alignment.

Evaluation of the potential of chimeric spidroins/poly(L-lactic-co-ε-caprolactone) (PLCL) nanofibrous scaffolds for tissue engineering

In this study, a novel type of chimeric spider silk proteins (spidroins) NTW_1-4CT was blended with poly(L-lactic-co-ε-caprolactone) (PLCL) to obtain nanofibrous scaffolds via electrospinning. Spidroins are composed of a N-terminal module (NT) from major ampullate spidroins, a C-terminal module (CT) from minor ampullate spidroins and 1-4 repeat modules (W) from aciniform spidroins. Physical characteristics and structures of NTW_1-4CT/PLCL (25/75, w/w) blend scaffolds were carried out by scanning electron microscope (SEM), water contact angles measurements, Fourier transform infrared (FTIR) spectroscopy and tensile mechanical tests. Results showed that blending with spidroins decreased diameters of nanofibers and increased porosity and wettability of scaffolds. Additionally, chimeric spidroins undergone a similar structural transition in electrospinning process as with the formation process of native and artificial spider silks from other spidroins. With amounts of W modules increasing, the tensile strength and elongation of blend scaffolds were also increased. Particularly, NTW₄CT/PLCL (25/75) scaffolds revealed much higher breaking stress than pure PLCL scaffolds. In vitro experiments, human umbilical vein endothelial cells (HUVEC) cultured on NTW₄CT/PLCL (25/75) scaffolds displayed significantly higher activity of proliferation and adhesion than on pure PLCL scaffolds. All results suggested that chimeric spidroins/PLCL, especially NTW₄CT/PLCL (25/75) blend nanofibrous scaffolds had promising potential for vascular tissue engineering.

Porous, Aligned, and Biomimetic Fibers of Regenerated Silk Fibroin Produced by Solution Blow Spinning

Solution blow spinning (SBS) has emerged as a rapid and scalable technique for the production of polymeric and ceramic materials into micro-/nanofibers. Here, SBS was employed to produce submicrometer fibers of regenerated silk fibroin (RSF) from Bombyx mori (silkworm) cocoons based on formic acid or aqueous systems. Spinning in the presence of vapor permitted the production of fibers from aqueous solutions, and high alignment could be obtained by modifying the SBS setup to give a concentrated channeled airflow. The combination of SBS and a thermally induced phase separation technique (TIPS) resulted in the production of macro-/microporous fibers with 3D interconnected pores. Furthermore, a coaxial SBS system enabled a pH gradient and kosmotropic salts to be applied at the point of fiber formation, mimicking some of the aspects of the natural spinning process, fostering fiber formation by self-assembly of the spinning dope. This scalable and fast production of various types of silk-based fibrous scaffolds could be suitable for a myriad of biomedical applications.

Silk nanofibril self-assembly versus electrospinning

Natural silk fibers represent one of the most advanced blueprints for (bio)polymer scientists, displaying highly optimized mechanical properties due to their hierarchical structures. Biotechnological production of silk proteins and implementation of advanced processing methods enabled harnessing the potential of these biopolymer not just based on the mechanical properties. In addition to fibers, diverse morphologies can be produced, such as nonwoven meshes, films, hydrogels, foams, capsules and particles. Among them, nanoscale fibrils and fibers are particularly interesting concerning medical and technical applications due to their biocompatibility, environmental and mechanical robustness as well as high surface-to-volume ratio. Therefore, we introduce here self-assembly of silk proteins into hierarchically organized structures such as supramolecular nanofibrils and fabricated materials based thereon. As an alternative to self-assembly, we also present electrospinning a technique to produce nanofibers and nanofibrous mats. Accordingly, we introduce a broad range of silk-based dopes, used in self-assembly and electrospinning: natural silk proteins originating from natural spinning glands, natural silk protein solutions reconstituted from fibers, engineered recombinant silk proteins designed from natural blueprints, genetic fusions of recombinant silk proteins with other structural or functional peptides and moieties, as well as hybrids of recombinant silk proteins chemically conjugated with nonproteinaceous biotic or abiotic molecules. We highlight the advantages but also point out drawbacks of each particular production route. The scope includes studies of the natural self-assembly mechanism during natural silk spinning, production of silk fibrils as new nanostructured non-native scaffolds allowing dynamic morphological switches, as well as studying potential applications. This article is categorized under:  Biology-Inspired Nanomaterials > Peptide-Based Structures  Nanotechnology Approaches to Biology > Nanoscale Systems in Biology  Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Precise Protein Photolithography (P3): High Performance Biopatterning Using Silk Fibroin Light Chain as the Resist

Precise patterning of biomaterials has widespread applications, including drug release, degradable implants, tissue engineering, and regenerative medicine. Patterning of protein-based microstructures using UV-photolithography has been demonstrated using protein as the resist material. The Achilles heel of existing protein-based biophotoresists is the inevitable wide molecular weight distribution during the protein extraction/regeneration process, hindering their practical uses in the semiconductor industry where reliability and repeatability are paramount. A wafer-scale high resolution patterning of bio-microstructures using well-defined silk fibroin light chain as the resist material is presented showing unprecedent performances. The lithographic and etching performance of silk fibroin light chain resists are evaluated systematically and the underlying mechanisms are thoroughly discussed. The micropatterned silk structures are tested as cellular substrates for the successful spatial guidance of fetal neural stems cells seeded on the patterned substrates. The enhanced patterning resolution, the improved etch resistance, and the inherent biocompatibility of such protein-based photoresist provide new opportunities in fabricating large scale biocompatible functional microstructures.

Fabrication and characterization of electrospun laminin-functionalized silk fibroin/poly(ethylene oxide) nanofibrous scaffolds for peripheral nerve regeneration

The peripheral nerve regeneration is still one of the major clinical problems, which has received a great deal of attention. In this study, the electrospun silk fibroin (SF)/poly(ethylene oxide) (PEO) nanofibrous scaffolds were fabricated and functionalized their surfaces with laminin (LN) without chemical linkers for potential use in the peripheral nerve tissue engineering. The morphology, surface chemistry, thermal behavior and wettability of the scaffolds were examined to evaluate their performance by means of scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and water contact angle (WCA) measurements, respectively. The proliferation and viability of Schwann cells onto the surfaces of SF/PEO nanofibrous scaffolds were investigated using SEM and thiazolyl blue (MTT) assay. The results showed an improvement of SF conformation and surface hydrophilicity of SF/PEO nanofibers after methanol and O₂ plasma treatments. The immunostaining observation indicated a continuous coating of LN on the scaffolds. Improving the surface hydrophilicity and LN functionalization significantly increased the cell proliferation and this was more prominent after 5 days of culture time. In conclusion, the obtained results revealed that the electrospun LN-functionalized SF/PEO nanofibrous scaffold could be a promising candidate for peripheral nerve tissue regeneration. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1595-1604, 2018.

A compliant and biomimetic three-layered vascular graft for small blood vessels

Engineering a small diameter vascular graft with mechanical and biological properties comparable to living tissues remains challenging. Often, current devices lead to thrombosis and unsatisfactory long-term patency as a result of poor blood compatibility and a mismatch between the mechanical properties of the living tissue and the implanted biomaterial. Addressing all these requirements is essential to produce scaffolds able to survive throughout the life of the patient. For this purpose, we fabricated a novel three-layered vascular graft by combining electrospinning and braiding. Mirroring the structure of human blood vessels, the proposed device is composed of three layers: the intima, the media, and the adventitia. The intima and media layers were obtained by sequentially electrospinning silk fibroin (SF) and poly(L-lactide-co-ε-caprolactone), with ratios selected to match the mechanical properties of the native tissue. For the outer layer, the adventitia, SF yarns were braided on top of the electrospun tubes to create a structure able to withstand high pressures. Compliance, Young's modulus and deformability of the obtained scaffold were similar to that of human blood vessels. Additionally, cytocompatibility of the two layers, media and intima, was assessed in vitro by analysing cell metabolic activity and proliferation of endothelial cells and smooth muscle cells, respectively. Furthermore, heparin functionalization of the scaffolds led to improved anticoagulant properties upon incubation in whole blood. The obtained results indicate a potential application of the herewith designed three-layered construct as a vascular graft for small diameter blood vessel engineering.

Electrospun protein nanofibers in healthcare: A review

Electrospun nanofibers are being utilized for a wide range of healthcare applications. A plethora of natural and synthetic polymers are exploited for their ability to be electrospun and replace the complex habitat provided by the extracellular matrix for the cells. The fabrication of nanofibers can be tuned to act as a multicarrier system to deliver drugs, growth factors and health supplements etc. in a sustained manner. Owing to its pliability, nanofibers reached its heights in tissue engineering and drug delivery applications. This review mainly focuses on various standardized parameters and optimized blending ratios for animal and plant proteins to yield fine, continuous nanofibers for effective utilization in various healthcare applications.

Investigate the Effect of Thawing Process on the Self-Assembly of Silk Protein for Tissue Applications

Biological self-assembly is a process in which building blocks autonomously organize to form stable supermolecules of higher order and complexity through domination of weak, noncovalent interactions. For silk protein, the effect of high incubating temperature on the induction of secondary structure and self-assembly was well investigated. However, the effect of freezing and thawing on silk solution has not been studied. The present work aimed to investigate a new all-aqueous process to form 3D porous silk fibroin matrices using a freezing-assisted self-assembly method. This study proposes an experimental investigation and optimization of environmental parameters for the self-assembly process such as freezing temperature, thawing process, and concentration of silk solution. The optical images demonstrated the possibility and potential of -80ST48 treatment to initialize the self-assembly of silk fibroin as well as controllably fabricate a porous scaffold. Moreover, the micrograph images illustrate the assembly of silk protein chain in 7 days under the treatment of -80ST48 process. The surface morphology characterization proved that this method could control the pore size of porous scaffolds by control of the concentration of silk solution. The animal test showed the support of silk scaffold for cell adhesion and proliferation, as well as the cell migration process in the 3D implantable scaffold.

Potential of Electrospun Nanofibers for Biomedical and Dental Applications

Electrospinning is a versatile technique that has gained popularity for various biomedical applications in recent years. Electrospinning is being used for fabricating nanofibers for various biomedical and dental applications such as tooth regeneration, wound healing and prevention of dental caries. Electrospun materials have the benefits of unique properties for instance, high surface area to volume ratio, enhanced cellular interactions, protein absorption to facilitate binding sites for cell receptors. Extensive research has been conducted to explore the potential of electrospun nanofibers for repair and regeneration of various dental and oral tissues including dental pulp, dentin, periodontal tissues, oral mucosa and skeletal tissues. However, there are a few limitations of electrospinning hindering the progress of these materials to practical or clinical applications. In terms of biomaterials aspects, the better understanding of controlled fabrication, properties and functioning of electrospun materials is required to overcome the limitations. More in vivo studies are definitely required to evaluate the biocompatibility of electrospun scaffolds. Furthermore, mechanical properties of such scaffolds should be enhanced so that they resist mechanical stresses during tissue regeneration applications. The objective of this article is to review the current progress of electrospun nanofibers for biomedical and dental applications. In addition, various aspects of electrospun materials in relation to potential dental applications have been discussed.

Wet electrospun silk fibroin/gold nanoparticle 3D matrices for wound healing applications

The effectiveness of a silk fibroin/gold nanoparticle 3D nanofibrous matrix on a rat model of full-thickness dermal wound healing was investigated.

Tough and VEGF-releasing scaffolds composed of artificial silk fibroin mats and a natural acellular matrix

The blend and coaxially electrospun RSF/BAMG composite scaffolds loaded VEGF exhibited good cell compatibility with improved mechanical properties.

A comparative mechanical and biocompatibility study of poly(ε-caprolactone), hybrid poly(ε-caprolactone)–silk, and silk nanofibers by colloidal electrospinning technique for tissue engineering

Poly(ε-caprolactone) is an established polymer used in the fabrication of scaffolds for tissue engineering applications. Poly(ε-caprolactone)’s intrinsic hydrophobicity and toxicity, however, is greater than other natural polymers which limits its applicability. In this study, these problems were addressed by the modification of poly(ε-caprolactone) nanofibers with nanoparticles made from natural polymers, such as silk fibroin. Silk fibroin nanoparticles were prepared by desolvation and blended with poly(ε-caprolactone) to form a colloidal solution capable of forming nanofibers by electrospinning. Fabricated silk fibroin nanoparticles and three different nanofibers were characterized by transmission electron microscopy, variable pressure field emission scanning electron microscope, contact angle, Fourier transform infrared spectroscopy, thermogravimetric analysis, as well as an evaluation of their mechanical properties. The hybrid nanofibers incorporated with silk nanoparticles improved water absorbability compared to pure poly(ε-caprolactone) nanofibers and had much better mechanical properties than the silk fibroin nanofibers. The cytotoxicity and cell attachment tests were carried by culturing NIH 3T3 fibroblasts with the nanofibers. The hybrid nanofibers exhibited better cell viability and cell attachment than the pure poly(ε-caprolactone) nanofibers. Furthermore, the silk fibroin nanoparticles improved the water contact angle and enhanced cell interaction compared to the unmodified poly(ε-caprolactone). Based on these results, the modification of poly(ε-caprolactone) nanofibers with silk nanoparticles is a promising strategy for the improvement of poly(ε-caprolactone)-based nanofibers for future tissue engineering applications.

Isolation and processing of silk proteins for biomedical applications

Silk proteins of silkworms are chiefly composed of core fibroin protein and glycoprotein sericin that glues fibroin. Unique mechanical properties, cyto-compatibility and controllable biodegradability facilitate the use of fibroin in biomedical applications. Sericin serves as additive in cosmetic and food industries, as mitotic factor in cell culture media, anti-cancerous drug, anticoagulant and as biocompatible coating. For all these uses; aqueous solutions of silk proteins are preferred. Therefore, an accurate understanding of extraction procedure of silk proteins from their sources is critical. A number of protocols exist, amongst which it is required to settle a precise and easy one with desired yield and least down-stream processing. Here, we report extraction of proteins employing methods mentioned in literature using cocoons of mulberry and nonmulberry silks. This study reveals sodium carbonate salt-boiling system is the most efficient sericin extraction procedure for all silk variants. Lithium bromide is observed as the effective fibroin dissolution system for mulberry silk cocoons; whereas heterogeneous species-dependent result is obtained in case of nonmulberry species. We further show the effect of common post processing on nanoscale morphology of mulberry silk fibroin films. This knowledge eases the adoption and fabrication of silk biomaterials in devices and therapeutic delivery systems.