Rheology and Electrospinning of Regenerated Bombyx mori Silk Fibroin Aqueous Solutions

Silk Fibroin as a Bioink - A Thematic Review of Functionalization Strategies for Bioprinting Applications

Bioprinting is an emerging tissue engineering technique that has attracted the attention of researchers around the world, for its ability to create tissue constructs that recapitulate physiological function. While the technique has been receiving hype, there are still limitations to the use of bioprinting in practical applications, much of which is due to inappropriate bioink design that is unable to recapitulate complex tissue architecture. Silk fibroin (SF) is an exciting and promising bioink candidate that has been increasingly popular in bioprinting applications because of its processability, biodegradability, and biocompatibility properties. However, due to its lack of optimum gelation properties, functionalization strategies need to be employed so that SF can be effectively used in bioprinting applications. These functionalization strategies are processing methods which allow SF to be compatible with specific bioprinting techniques. Previous literature reviews of SF as a bioink mainly focus on discussing different methods to functionalize SF as a bioink, while a comprehensive review on categorizing SF functional methods according to their potential applications is missing. This paper seeks to discuss and compartmentalize the different strategies used to functionalize SF for bioprinting and categorize the strategies for each bioprinting method (namely, inkjet, extrusion, and light-based bioprinting). By compartmentalizing the various strategies for each printing method, the paper illustrates how each strategy is better suited for a target tissue application. The paper will also discuss applications of SF bioinks in regenerating various tissue types and the challenges and future trends that SF can take in its role as a bioink material.

3D Printing of Monolithic Proteinaceous Cantilevers Using Regenerated Silk Fibroin

Silk fibroin, regenerated from Bombyx mori, has shown considerable promise as a printable, aqueous-based ink using a bioinspired salt-bath system in our previous work. Here, we further developed and characterized silk fibroin inks that exhibit concentration-dependent fluorescence spectra at the molecular level. These insights supported extrusion-based 3D printing using concentrated silk fibroin solutions as printing inks. 3D monolithic proteinaceous structures with high aspect ratios were successfully printed using these approaches, including cantilevers only supported at one end. This work provides further insight and broadens the utility of 3D printing with silk fibroin inks for the microfabrication of proteinaceous structures.

Solvent Effects on the Elasticity of Electrospinnable Polymer Solutions

Ultrafine fibers manufactured through electrospinning are a frontrunner for advanced fiber applications, but transitioning from potential to commercial applications for ultrafine fibers requires a better understanding of the behavior of polymer solutions in electrospinning to enable the design of more complex spinning dopes. In complex fluids, there are viscoelastic stresses and microstructural transitions that alter free surface flows. These may not be seen in shear rheology; therefore, an in-depth analysis of the extensional rheological behavior must be performed. In this work, we use dripping-onto-substrate rheometry to characterize the extensional viscosities of electrospinning dopes from four polymer solutions commonly used in electrospinning (low- and high-molecular-weight polyvinylpyrrolidone in methanol and water as well as poly(ethylene oxide) and poly(vinyl alcohol) in water). We link the electrospinnability, characterized through fiber morphology, to the extensional rheological properties for semidilute and entangled polymer solutions and show that high-surface-tension solvents require higher extensional viscosities and relaxation times to form smooth fibers and that the Deborah and Ohnesorge numbers are a promising method of determining electrospinnability. Through this tie between solvent characteristics, viscoelasticity, and electrospinnability, we will enable the design of more complex spinning dopes amenable to applications in wearable electronics, pharmaceuticals, and more.

Toughening Wet-Spun Silk Fibers by Silk Nanofiber Templating

Regenerated silk fibers typically fall short of silkworm cocoon fibers in mechanical properties due to reduced fiber crystal structure and alignment. One approach to address this has been to employ inorganic materials as reinforcing agents. The present study avoids the need for synthetic additives, demonstrating the first use of exfoliated silk nanofibers to control silk solution crystallization, resulting in all-silk pseudocomposite fibers with remarkable mechanical properties. Incorporating only 0.06 wt. % silk nanofibers led to a ∼44% increase in tensile strength (over 600 MPa) and ∼33% increase in toughness (over 200 kJ/kg) compared with fibers without silk nanofibers. These remarkable properties can be attributed to nanofiber crystal seeding in conjunction with fiber draw. The crystallinity nearly doubled from ∼17% for fiber spun from pure silk solution to ∼30% for the silk nanofiber reinforced sample. The latter fiber also shows a high degree of crystal orientation with a Herman's orientation factor of 0.93, a value which approaches that of natural degummed B. mori silk cocoon fiber (0.96). This study provides a strong foundation to guide the development of simple, eco-friendly methods to spin regenerated silk with excellent properties and a hierarchical structure that mimics natural silk. This article is protected by copyright. All rights reserved.

Spinning Regenerated Silk Fibers with Improved Toughness by Plasticizing with Low Molecular Weight Silk

Low-molecular weight (LMW) silk was utilized as a LMW silk plasticizer for regenerated silk, generating weak physical crosslinks between high-molecular weight (HMW) silk chains in the amorphous regions of a mixed solution of HMW/LMW silk. The plasticization effect of LMW silk was investigated using mechanical testing, Raman spectroscopy, and wide-angle X-ray scattering (WAXS). Small amounts (10%) of LMW silk resulted in a 19.4% enhancement in fiber extensibility and 37.8% increase in toughness. The addition of the LMW silk facilitated the movement of HMW silk chains during drawing, resulting in an increase in molecular chain orientation when compared with silk spun from 100% HMW silk solution. The best regenerated silk fibers produced in this work had an orientation factor of 0.94 and crystallinity of 47.82%, close to the values of natural degummedBombyx mori silk fiber. The approach and mechanism elucidated here can facilitate artificial silk systems with enhanced properties.

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.

Tailoring the Diameters of Electro-Mechanically Spun Fibers by Controlling Their Deborah Numbers

Polymer solutions with different concentrations of SU-8 2002/poly(ethylene) glycol/tetrabutyl ammonium tetrafluoroborate (SU-8/PEO/TBATFB) were electrospun in a low-voltage near-field electrospinning platform (LVNFES) at different velocities. Their diameters were related to the concentration contents as well as to their Deborah (De) numbers, which describes the elasticity of the polymer solution under determined operating conditions. We found a direct correlation between the concentration of PEO/TBATFB, the De and the diameter of the fibers. Fibers with diameters as thin as 465 nm can be achieved for De ≈ 1.

Effect of process parameters on additive-free electrospinning of regenerated silk fibroin nonwovens

Silk fibroin is a biomaterial with multiple beneficial properties for use in regenerative medicine and tissue engineering. When dissolving and processing the reconstituted silk fibroin solution by electrospinning, the arrangement and size of fibers can be manifold varied and according fiber diameters reduced to the nanometer range. Such nonwovens show high porosity as well as potential biocompatibility. Usually, electrospinning of most biomaterials demands for the application of additives, which enable stable electrospinning by adjusting viscosity, and are intended to evaporate during processing or to be washed out afterwards. However, the use of such additives increases costs and has to be taken into account in terms of biological risks when used for biomedical applications.

In this study, we explored the possibilities of additive-free electrospinning of pure fibroin nonwovens and tried to optimize process parameters to enable stable processing. We used natural silk derived from the mulberry silkworm Bombyx mori. After degumming, the silk fibroin was dissolved and the viscosity of the spinning solution was controlled by partial evaporation of the initial solving agent. This way, we were able to completely avoid the use of additives and manufacture nonwovens, which potentially offer higher biocompatibility and reduced immunogenicity. Temperature and relative humidity during electrospinning were systematically varied (25–35 °C, 25–30% RH). In a second step, the nonwovens optionally underwent methanol treatment to initiate beta-sheet formation in order to increase structural integrity and strength. Comprehensive surface analysis on the different nonwovens was performed using scanning electron microscopy and supplemented by additional mechanical testing. Cytotoxicity was evaluated using BrdU-assay, XTT-assay, LDH-assay and live-dead staining. Our findings were, that an increase of temperature and relative humidity led to unequal fiber diameters and defective nonwovens. Resistance to penetration decreased accordingly. The most uniform fiber diameters of 998 ± 63 nm were obtained at 30 °C and 25% relative humidity, also showing the highest value for resistance to penetration (0.20 N). The according pure fibroin nonwoven also showed no signs of cytotoxicity. However, while the biological response showed statistical evidence, the material characteristics showed no statistically significant correlation to changes of the ambient conditions within the investigated ranges. We suggest that further experiments should explore additional ranges for temperature and humidity and further focus on the repeatability of material properties in dependency of suitable process windows.

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Usually, electrospinning of most biomaterials demands for the application of additives. However, the use of such additives increases costs and has to be taken into account in terms of biological risks.

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After degumming, fibroin was dissolved and the viscosity of the spinning solution was controlled by partial evaporation of the initial solving agent. In this way, we were able to completely avoid the use of additives.

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Using a pure fibroin solution contributes to higher biocompatibility and reduces immunogenicity of the products.

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Increase of temperature and humidity led to unequal fiber diameters and defective nonwovens. The most uniform fiber diameters of 998 ± 63 nm were obtained at 30 °C and 25% RH.

Synthesis and fabrication of novel quinone-based chromenopyrazole antioxidant-laden silk fibroin nanofibers scaffold for tissue engineering applications

Oxidative stress is critically attributed for impeding tissue repair and regeneration process. Elimination of over-accumulated, deleterious reactive oxygen species (ROS) could be elicited to accelerate healing in tissue engineering applications. Antioxidant biomolecules play a pivotal role in attenuating oxidative stress by neutralizing the free radical effects. Herein, we describe the synthesis and fabrication of novel quinone-based chromenopyrazole (QCP) antioxidant-laden silk fibroin (SF) electrospun nanofiber scaffold (QCP-SF) for tissue engineering applications. The spectral characterization of the synthesized compounds (6a-6h) were analysed by using NMR, FTIR and mass spectra and cell viability study of all the synthesized compounds were evaluated by MTT assay in primary rat bone marrow stem cells (rBMSCs). Among the prepared molecules, compound 6h showed an excellent cell viability, and antioxidant efficacy of compound 6h (QCP) was investigated through 1,1‑diphenyl‑2‑picrylhydiazyl (DPPH) scavenging assay. QCP expressed high antioxidant activity with IC50% of DPPH scavenging was observed about 5.506 ± 0.2786 μg. Novel QCP laden SF fiber scaffolds (QCP-SF) were characterized and incorporation of QCP did not affect the nanofiber architecture of QCP-SF scaffold. QCP-SF scaffold exhibited an enhanced thermal and mechanical stability when compared to native SF fiber mat. In vitro biocompatibility studies were evaluated using NIH 3T3 fibroblasts and rBMSCs. The QCP-SF scaffold displayed an increased cell attachment and proliferation in both cell types. In vitro wound healing study (scratch assay) of QCP-SF scaffold showed an excellent cell migration with NIH 3T3 cells into scratch area and complete cell migration occurred within 24 h. Based on results, we propose that QCP-loaded SF (QCP-SF) nanofibrous scaffolds can serve as a promising potential antioxidant fibrous scaffold for skin tissue engineering applications.

3D Printing of Silk Fibroin for Biomedical Applications

Three-dimensional (3D) printing is regarded as a critical technological-evolution in material engineering, especially for customized biomedicine. However, a big challenge that hinders the 3D printing technique applied in biomedical field is applicable bioink. Silk fibroin (SF) is used as a biomaterial for decades due to its remarkable high machinability and good biocompatibility and biodegradability, which provides a possible alternate of bioink for 3D printing. In this review, we summarize the requirements, characteristics and processabilities of SF bioink, in particular, focusing on the printing possibilities and capabilities of bioink. Further, the current achievements of cell-loading SF based bioinks were comprehensively viewed from their physical properties, chemical components, and bioactivities as well. Finally, the emerging issues and prospects of SF based bioink for 3D printing are given. This review provides a reference for the programmable and multiple processes and the further improvement of silk-based biomaterials fabrication by 3D printing.

Incorporation of Tannic Acid in Food-Grade Guar Gum Fibrous Mats by Electrospinning Technique

The use of polysaccharides to produce functional micro- or nanoscale fibrous mats has attracted growing interest for their food-grade applications. In this study, the characterization and electro-spinnability of guar gum (GG) solutions loaded with tannic acid (TA) was demonstrated. Food-grade antioxidant materials were successfully produced by electrospinning while incorporating different loads of TA into GG fibers. Bead-free GG-TA fibers could be fabricated from GG solution (2 wt %) with 10 wt % TA. Increasing the amount of TA led to fibers with defects and larger diameter sizes. Fourier Transformed Infrared Spectroscopy and X-ray Diffraction of neat GG and TA loaded GG fibrous mats suggested that inclusion of TA interrupted the hydrogen bonding and that a higher density of the ordered junction zones formed with the increased TA. The high TA incorporation efficiency and retained antioxidant activity of the fibrous mats afford a potential application in active edible film or drug delivery system.

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.

Effect of molecular weight on electro-spinning performance of regenerated silk

Owing to the excellent biocompatibility of silk fibroins (SFs) and ease of fabrication of nano-fibrous webs by the electro-spinning technique, electro-spun SF webs have attracted the attention of researchers for various biomedical applications, including their use as tissue engineering scaffolds and membranes for guided bone regeneration. In this work, the effect of the molecular weight (MW) and concentration of SFs on the structure and properties of the electro-spun SF webs was examined. The fiber morphology and porosity of these SF webs were strongly affected by the viscosity of the SF dope solution. It was found that the electro-spinning rate of the SF solution could be increased significantly (7.5 fold) by controlling the MW and concentration of the SF. Interestingly, as the SF MW and concentration (i.e., s​olution viscosity) increased, the extent of β-sheet crystallization of the SF decreased, leading to a decrease in the overall crystallinity. The strength and elongation of the electro-spun SF web decreased with an increase in the web porosity (i.e., increasing SF concentration) and a decrease in the MW of the SF.

[New biomaterials and alternative stem cell sources for the reconstruction of the limbal stem cell niche]

Reconstruction of the limbal stem cell niche in patients with limbal stem cell insufficiency remains one of the most challenging tasks in the treatment of ocular surface diseases. Ex vivo expansion of limbal stem cells still has potential for optimization despite positive reports in centers worldwide. New biomaterials as well as alternative cell sources for the reconstruction of the limbal stem cell niche have been published in recent years. The aim of this review is to provide insight into new biomaterials and cell sources which may find their way into clinical routine in the upcoming decades.

Effects of electric field on the maximum electro-spinning rate of silk fibroin solutions

Owing to the excellent cyto-compatibility of silk fibroin (SF) and the simple fabrication of nano-fibrous webs, electro-spun SF webs have attracted much research attention in numerous biomedical fields. Because the production rate of electro-spun webs is strongly dependent on the electro-spinning rate used, the electro-spinning rate becomes more important. In the present study, to improve the electro-spinning rate of SF solutions, various electric fields were applied during electro-spinning of SF, and its effects on the maximum electro-spinning rate of SF solution as well as diameters and molecular conformations of the electro-spun SF fibers were examined. As the electric field was increased, the maximum electro-spinning rate of the SF solution also increased. The maximum electro-spinning rate of a 13% SF solution could be increased 12×by increasing the electric field from 0.5kV/cm (0.25mL/h) to 2.5kV/cm (3.0mL/h). The dependence of the fiber diameter on the present electric field was not significant when using less-concentrated SF solutions (7-9% SF). On the other hand, at higher SF concentrations the electric field had a greater effect on the resulting fiber diameter. The electric field had a minimal effect of the molecular conformation and crystallinity index of the electro-spun SF webs.

Aqueous-Based Coaxial Electrospinning of Genetically Engineered Silk Elastin Core-Shell Nanofibers

A nanofabrication method for the production of flexible core-shell structured silk elastin nanofibers is presented, based on an all-aqueous coaxial electrospinning process. In this process, silk fibroin (SF) and silk-elastin-like protein polymer (SELP), both in aqueous solution, with high and low viscosity, respectively, were used as the inner (core) and outer (shell) layers of the nanofibers. The electrospinnable SF core solution served as a spinning aid for the nonelectrospinnable SELP shell solution. Uniform nanofibers with average diameter from 301 ± 108 nm to 408 ± 150 nm were obtained through adjusting the processing parameters. The core-shell structures of the nanofibers were confirmed by fluorescence and electron microscopy. In order to modulate the mechanical properties and provide stability in water, the as-spun SF-SELP nanofiber mats were treated with methanol vapor to induce β-sheet physical crosslinks. FTIR confirmed the conversion of the secondary structure from a random coil to β-sheets after the methanol treatment. Tensile tests of SF-SELP core-shell structured nanofibers showed good flexibility with elongation at break of 5.20% ± 0.57%, compared with SF nanofibers with an elongation at break of 1.38% ± 0.22%. The SF-SELP core-shell structured nanofibers should provide useful options to explore in the field of biomaterials due to the improved flexibility of the fibrous mats and the presence of a dynamic SELP layer on the outer surface.

Electrospun silk fibroin fiber diameter influences in vitro dermal fibroblast behavior and promotes healing of ex vivo wound models

Replicating the nanostructured components of extracellular matrix is a target for dermal tissue engineering and regenerative medicine. Electrospinning Bombyx mori silk fibroin (BMSF) allows the production of nano- to microscale fibrous scaffolds. For BMSF electrospun scaffolds to be successful, understanding and optimizing the cellular response to material morphology is essential. Primary human dermal fibroblast response to nine variants of BMSF scaffolds composed of nano- to microscale fibers ranging from ~250 to ~1200 nm was assessed in vitro with regard to cell proliferation, viability, cellular morphology, and gene expression. BMSF support of epithelial migration was then assessed through utilization of a novel ex vivo human skin wound healing model. Scaffolds composed of the smallest diameter fibers, ~250 -300 nm, supported cell proliferation significantly more than fibers with diameters approximately 1 μm (p < 0.001). Cell morphology was observed to depart from a stellate morphology with numerous cell -fiber interactions to an elongated, fiber-aligned morphology with interaction predominately with single fibers. The expressions of extracellular matrix genes, collagen types I and III (p < 0.001), and proliferation markers, proliferating cell nuclear antigen (p < 0.001), increased with decreasing fiber diameter. The re-epithelialization of ex vivo wound models was significantly improved with the addition of BMSF electrospun scaffolds, with migratory keratinocytes incorporated into scaffolds. BMSF scaffolds with nanofibrous architectures enhanced proliferation in comparison to microfibrous scaffolds and provided an effective template for migratory keratinocytes during re-epithelialization. The results may aid in the development of effective BMSF electrospun scaffolds for wound healing applications