Aqueous Silk Fibroin Solution Research Articles

Low voltage electric field governs fibrous silk electrogels

Silk fibroin gels, formed with electric fields, have been studied and potential utility in medical materials. However, silk electrogels with controllable nanofiber network morphology have rarely been reported. In this work, low voltage electric field was developed to control the nanofiber network morphology formation and resulting electrogelation of silk fibroin aqueous solution, inducing a transition from an overall protein structure that is initially rich in random coil to one that is rich in β-sheet. Under low voltage (3–24 V), solution concentration (≤5.0 wt%), and neutral, alkaline conditions, silk fibroin fibrous electrogels were easily formed. At the same time, compression test result exhibited the compressive stress of silk fibroin fibrous electrogels was 250.17 ± 16.50 kPa at dry state, which formed from silk fibroin concentration 3.0 wt%. Moreover, the recovery rate of electrogels can reach more than 50% after 10 compression-recovery circles, exhibiting excellent flexibility. In addition, silk fibroin electrogels loaded cefixime drugs have obvious antibacterial effect against Staphylococcus aureus and Escherichia coli than commercial medical band-aids. Drug-loaded silk fibroin fibrous electrogels supported mouse embryonic fibroblasts 3 T3 cells attachment and proliferation over 7 days. This study offers an effective approach to design and synthesis of new dressings.

Formation, Structure, and Mechanical Performance of Silk Nanofibrils Produced by Heat-Induced Self-Assembly.

The heat-induced self-assembly of silk fibroin (SF) is studied by combing fluorescence assessment, infrared nanospectroscopy, wide-angle X-ray scattering, and Derjaguin-Muller-Toporov coupled with atomic force microscopy. Several fundamental issues regarding the formation, structure, and mechanical performance of silk nanofibrils (SNFs) under heat-induced self-assembly are discussed. Accordingly, SF in aqueous solution is rod-like in shape and not micellar. The formation of SNFs occurs through nucleation-dependent aggregation, but the assembly period is variable and irregular. SF shows inherent fractal growth, and this trend is critical for the short-term assembly. The long-term assembly of SF, however, mainly involves an elongation growth process. SNFs produced by different methods, such as ethanol treatment and heat incubation, have similar secondary structure and mechanical properties. These investigations improve the in-depth understanding of fundamental issues related to self-assembly of SNFs, and thus provide inspiration and guidance in designing of silk nanomaterials.

Stretchable silk fibroin hydrogels

Hydrogels derived from silk fibroin (SF) are attractive soft materials in biomedical applications such as drug delivery and tissue engineering. However, SF hydrogels reported so far are generally brittle in tension limiting their load-bearing applications. We present here a novel strategy for preparing stretchable SF hydrogels by incorporating flexible polymer chains into the brittle SF network, which strengthen the interconnections between SF globules. We included N, N-dimethylacrylamide (DMAA) monomer and ammonium persulfate initiator into an aqueous SF solution containing a diepoxide cross-linker to in situ generate flexible poly (N,N-dimethylacrylamide) (PDMAA) chains. Moreover, instead of SF, methacrylated SF was used for the gel preparation to create an interconnected SF/PDMAA network. The free-radical polymerization of DMAA leads to the formation of PDMAA chains interconnecting globular SF molecules via their pendant vinyl groups. Incorporation of 2 w/v% DMAA into the SF network turns the brittle hydrogel into a stretchable one sustaining up to 370% elongation ratio. The mechanical properties of SF hydrogels could be adjusted by the amount of PDMAA incorporated into the SF network. The stretchable and tough SF hydrogels thus developed are suitable as a scaffold in tissue engineering and offer an advantage as a biomaterial over other SF-based biomaterials.

Rheological behavior of regenerated silk fibroin/polyvinyl alcohol blended solutions in steady and dynamic state and the effect of temperature

Since the silk fibroin is a precious material widely used in various fields, the discarded silk textile is a profound waste in resource-constrained settings. In this paper, with recyclability in mind, we selected waste silk from discarded silk quilt to make the regenerated silk fibroin (RSF) aqueous solutions. Mixing with water-soluble polyvinyl alcohol (PVA), the RSF/PVA blended solutions were made as a spinning dope for preparing the regenerated fibers. To optimize the parameters during the spinning process, we used the rheological measurement firstly to investigate the fundamental characters of solutions with various blending ratios and the effect of temperature on solutions. The results showed the RSF/PVA blended aqueous solutions were non-Newtonian fluids, and the apparent viscosity rose with the increasing PVA contents in co-solutions and decreased with the growth of the shear rate. The blending ratio had a certain influence on the viscoelasticity of co-solutions, the more the percentage of PVA, the higher the value of the viscoelasticity of co-solutions would be, and the co-solution with blending ratio of 6:4 (RSF:PVA) showed the greatest influence on storage modulus, while the solutions with more RSF exhibited more effect on loss modulus. Too much PVA content or too high RSF proportion in solutions was unfavorable to spinning. What’s more, the co-solutions with RSF content less than 40% had the most similar morphological structural systems, and the co-solutions containing more than 20% PVA had narrow distribution, linear and high molecular weight. For preventing protein denaturation, the temperature should not be higher than around 30 °C.

Preparation and Drug Release Behavior of Tussah Silk Fibroin Composite Membrane

Silk fibroin (SF) aqueous solution was obtained by the degumming and dissolution of tussah silk, and then the obtained solution was chemically cross-linked with sodium alginate (SA) to form composite membrane, which could be used as a carrier for sustained drug release. The mechanical properties and dissolution rate of such composite membrane together with the drug release behavior under different conditions were investigated. A series of structure-tunable composite membranes were prepared through altering the ingredients proportion, the pH value during fabrication, the concentration of Ca2+, and cross-linking time. The drug release behavior of the corresponding drug-loaded composite membrane was elaborate studied using artemisinin as the model drug. It was found that the drug release rate exhibited a decreasing trend with increasing the SF content, the Ca2+ concentration and the cross-linking time of the corresponding composite membrane. Moreover, the drug release behavior exhibited pH sensitivity, and the drug release rate could be promoted under weak alkaline conditions.

Silk succumbs to heat processing

The silk fibroin protein that silkworms spin into cocoons is prized for its mechanical properties. But the complex network of hydrogen-bonded β-sheets that give silk its strength and flexibility also make it difficult to process with heat. Silk will burn rather than melt. To make materials from silk protein, scientists have used solution-processing methods, but these require large amounts of solvent and the silk protein tends to recrystallize in solution over time. Now, researchers led by Tufts University’s Chunmei Li and David L. Kaplan have figured out a way to turn silk protein into solid pellets that melt like plastic (Nat. Mater. 2019, DOI: 10.1038/s41563-019-0560-8). The researchers make the tiny pellets by freeze-drying an aqueous solution of silk fibroin. They then melt the pellets by heating them under pressure so that they fuse into larger pieces of material. Bulk solids made from these pellets can be machined into medical devices,

Preparation, Characterization and Antifungal Activity Studies of AgNPs Loaded Silk Fibroin Hydrogels

Silk fibroin hydrogels are promising materials for controlled drug delivery device due to their aqueous process ability, biocompatibility, and biodegradability. The research work is aimed to prepare silk fibroin (SF) hydrogels loaded with Ag nanoparticles and to evaluate its antifungal activities. The Silk fibroin hydrogels were formulated at 37°C using 2% (w/v) silk fibroin aqueous solution either by treating 50% (v/v) of ethanol, or 50% (v/v) of propanol, or 50% (v/v) of glycerol, respectively. Above these, the rate of gelation was sufficiently accelerated by addition of glycerol. The silk fibroin hydrogels and prepared silver nanoparticles (AgNPs) were characterized by using Scanning Electron Microscopy (SEM), Thermo Gravimetrical Analysis (TGA). The encapsulation efficiency and release profile of AgNPs were studied by UV-vis spectrometry. The particle size of AgNPs was measured by Malvern Zetasizer Nano and found 93±5 nm. The encapsulation efficiency and morphology of the hydrogels was affected by the formulation conditions. The in vitro release profile showed an initial burst release of AgNPs followed by controlled release for next 20 hours. The antifungal activity of AgNPs loaded SF hydrogels showed a positive response to Aspergillus Niger pathogen. Therefore, silk fibroin hydrogels might be an effective biopolymeric matrix for antifungal applications.

Influence of pH on the surface and foaming properties of aqueous silk fibroin solutions.

Silk fibroin (SF) adsorbs at the air/water interface, reduces the surface tension, and forms interfacial layers suppressing bubble coalescence and stabilizing foam. Variation of pH alters the inter-molecular interactions of SF in the interfacial layers and thus interfacial network formation, dilatational visco-elasticity and foaming properties. At pH 4, around the isoelectric point, the reduced electrostatic repulsion between the SF molecules results in thicker adsorbed layers, but adsorption rate, foaming rate and foam stability are lower than at pH 3 and pH 7. At the highest pH investigated (pH 7), the small aggregate size and high protein flexibility lead to the formation of more ordered and stable viscoelastic interfacial networks, which are resistant to deformation breakage and generate homogeneous, denser and more stable foams.

Preparation of bacterial cellulose/silk fibroin double-network hydrogel with high mechanical strength and biocompatibility for artificial cartilage

Bacterial cellulose (BC) and silk fibroin (SF) double-network hydrogel with high mechanical strength and biocompatibility was synthesized by using BC as the novel polymer substrate to absorb aqueous SF solution as the modifier. Fundamental physical characterizations were carried out by scanning electron microscopy, energy dispersive spectrometry, fourier transform infrared spectrometry, X-ray diffraction and X-ray photoelectron spectroscopy. Thermogravimetric and differential thermal analysis was used to investigate the stability of the hydrogel. Strength and biocompatibility of the BC/SF hydrogel for cartilage tissue engineering were evaluated by tensile strength tester and cell culture experiments. Results reveal that the BC/SF double-network hydrogel was prepared successfully, and the hydrogel could be used as a cartilage repair material in clinical application.

Silk Fibroin-Sodium Dodecyl Sulfate Gelation: Molecular, Structural, and Rheological Insights.

A gelling agent is necessary to accelerate sol to gel transition in an aqueous solution of silk fibroin (SF), which otherwise takes several days to complete. In this paper, we investigate the mechanism of gelation of Bombyx mori SF by a model anionic surfactant, sodium dodecyl sulfate (SDS). Even though interactions between SDS and proteins have been extensively investigated, most of these studies have focused on globular proteins, which undergo denaturation. The interaction with a fibrous protein such as SF is different and results in an altered secondary structure leading to gelation. In this work, the concentration-dependent gelation process of the SF-SDS system is examined using rheology, SANS, FTIR, and NMR. We observed preferential binding of SDS to specific regions on the SF chain, which aids structural changes favoring β-sheet formation. We propose a mechanism for the accelerated sol-gel transition in the SF-SDS system.

Evaluation of silk fibroin electrogel coating for zirconia material surface.

Zirconia is commonly used in dental applications. It has been reported that surface-modified zirconia implants showed better performance in vivo than machined zirconia implants. Silk fibroin electrogel is a good candidate for controlled drug delivery; however, the use of silk fibroin electrogel on zirconia implants has not previously been reported. The aim of this study was to investigate a method to coat zirconia implants with silk fibroin electrogel and evaluate the mechanical and biological properties of the coating. The results show that the wettability of the coating was close to that of sand-blasted and acid-etched (SLA)-treated zirconia, and the bond strength was larger than that of the coating prepared from silk fibroin aqueous solution. ATR-FTIR spectra provided evidence that the secondary structure changed during the electrogelation process. Culturing cells on the coating revealed its nontoxicity to osteoblast-like cells. Thus, it can be suggested that a silk fibroin electrogel coating is a promising biocompatible and degradable drugdelivery material for zirconia implants.

Microencapsulated -octadecane with TiO2-doped silk fibroin shell for thermal energy storage and UV-shielding

The novel microencapsulated n-octadecane with TiO2-doped silk fibroin (SF) shell was synthesized using a self-assembly method in o/w emulsion. It was confirmed by Fourier transformation infrared spectroscope (FT-IR) and a three-phase contact angle that TiO2 is successfully modified by SF aqueous solution. The scanning electronic microscope (SEM) and laser scattering particle size distribution analyzer showed that the as-prepared microcapsules have regularly spherical morphology and are about 4–5 μm in average diameter. According to differential scanning calorimeter (DSC), thermogravimetric analyzer (TGA) and ultraviolet visible spectrophotometer (UV–vis) measurements, the TiO2-doped microcapsule presents high thermal storage capability, enhanced thermal reliability and stability, and have good UV-shielding property. The TiO2-doped microcapsule synthesized in the present study could prove to be path-breaking and lead to further studies in the field of thermal energy storage and UV-shielding applications.

FORMULATION, DEVELOPMENT AND CHARACTERIZATION OF DRUG DELIVERY SYSTEMS BASED TELMISARTAN ENCAPSULATED IN SILK FIBROIN NANOSPHERE’S

Objective: The aim of the present work was to formulate silk fibroin (SF) nanospheres (NS’s) for drug delivery application. The current study was designed to advance the water solubility and bio-availability of telmisartan by nanoprecipitation method.Methods: SF NS’s loaded with TS were prepared by nanoprecipitation method. The drug was dissolved in aqueous solution of SF by using acetone as a non-solvent. The prepared NS’s were then characterized by FTIR, X-ray diffraction and zeta potential, and were evaluated for its, surface morphology, %drug content, encapsulation efficiency and in vitro drug release.Results: The evaluation results of SF NS’s loaded of TS showed 74.22±0.17 % entrapment efficiency, 35.21±0.02 % of drug loading, and-4.9 mV to-13.6 mV of zeta potential due to the proper bounding of TS with the β-sheets of SF, the particle size reported was within the size range of 160-186 nm having smooth surface and were spherical in shape. The SFNS’s pattern switched from random coil to β-sheet formation on treating with acetone. FTIR and DSC studies marked no such inter-molecular interactions between SF and drug molecules. The % cumulative in vitro drug release from SF NS’s exhibited quick burst release. The in vitro cumulative drug release of SF NS’s of TS it was found that about 74% of the drug was released within 8 h and about 96% of drug released at 24 hr. The rate of drug release increased with the increase in SF ratio.Conclusion: It is believed that these SF NS’s will find potential applications in drug delivery release as drug carriers, especially poor water-soluble drugs. All these results proposed that SF NS’s are eventuality handy in various drug delivery systems.

Silk fibroin nanofibers loaded with hydroxytyrosol from hydrolysis of oleuropein in olive leaf extract

The purpose of this study was to prepare antimicrobial silk fibroin nanofibers from the aqueous formic acid solutions of silk fibroin and hydroxytyrosol with the in situ hydrolysis of oleouropein present in olive leaf extract using electrospinning method. With the use of aqueous formic acid solution of olive leaf extract and silk fibroin resulted in more uniform and beadless nanofibers. Morphological properties of electrospun nanofibers were investigated using Scanning Electron Microscope (SEM). The diameter of electrospun nanofibers ranged between 70 nm to 150 nm. The nanofiber diameter did not changed much with increasing concentration of olive leaf extract added into silk fibroin solution to be used in electrospinning process. The increase in olive leaf extract concentration resulted in beadless and uniform nanofiber structures. The average diameter of the nanofibers prepared with fibroin solution having 10 % olive leaf extract was determined as 85 ± 10 nm. Results revealing the formation of smoother and uniform nanofibers was attributed to the crosslinking effect of oleuropein and polyphenols present in olive leaf extract with certain functional groups in silk fibroin structure. Antibacterial properties of hydroxytyrosol loaded nanofibers against Staphylococcus epidermidis (Gram +) and Escherichia coli (Gram -) were confirmed with the clear inhibition zones observed in disc diffusion tests. Silk Fibroin nanofibers loaded with hydroxytyrosol may offer a new alternative biomaterial to be used in wound dressing or medical textile applications.

Thai silk fibroin gelation process enhancing by monohydric and polyhydric alcohols

Silk fibroin hydrogel is an interesting natural material in various biomedical applications. However, the self-assembled gelation takes a long time. In this work, different alcohol types are used as gelation enhancers for aqueous silk fibroin solution. Monohydric alcohols having carbon chain length from C1 to C4 and polyhydric alcohols with the number of mono- to tri- hydroxyl groups were used as the enhancers which are effective for rapid gelation. The addition of monohydric alcohol distinctively reduced the gelation time, comparing to the polyhydric alcohol. The gelation process is directly dependent on the polarity of alcohol and hydrophobicity. The alcohol mediated gelation imparts strong viscoelastic property and enhanced compressive modulus of resulting hydrogels. This is due to the effective formation of self-assembled beta sheet network of the silk fibroin chains facilitates the gelation process.

Combinatory approach for developing silk fibroin scaffolds for cartilage regeneration

Several processing technologies and engineering strategies have been combined to create scaffolds with superior performance for efficient tissue regeneration. Cartilage tissue is a good example of that, presenting limited self-healing capacity together with a high elasticity and load-bearing properties. In this work, novel porous silk fibroin (SF) scaffolds derived from horseradish peroxidase (HRP)-mediated crosslinking of highly concentrated aqueous SF solution (16 wt%) in combination with salt-leaching and freeze-drying methodologies were developed for articular cartilage tissue engineering (TE) applications. The HRP-crosslinked SF scaffolds presented high porosity (89.3 ± 0.6%), wide pore distribution and high interconnectivity (95.9 ± 0.8%). Moreover, a large swelling capacity and favorable degradation rate were observed up to 30 days, maintaining the porous-like structure and β-sheet conformational integrity obtained with salt-leaching and freeze-drying processing. The in vitro studies supported human adipose-derived stem cells (hASCs) adhesion, proliferation, and high glycosaminoglycans (GAGs) synthesis under chondrogenic culture conditions. Furthermore, the chondrogenic differentiation of hASCs was assessed by the expression of chondrogenic-related markers (collagen type II, Sox-9 and Aggrecan) and deposition of cartilage-specific extracellular matrix for up to 28 days. The cartilage engineered constructs also presented structural integrity as their mechanical properties were improved after chondrogenic culturing. Subcutaneous implantation of the scaffolds in CD-1 mice demonstrated no necrosis or calcification, and deeply tissue ingrowth. Collectively, the structural properties and biological performance of these porous HRP-crosslinked SF scaffolds make them promising candidates for cartilage regeneration. Statement of SignificanceIn cartilage tissue engineering (TE), several processing technologies have been combined to create scaffolds for efficient tissue repair. In our study, we propose novel silk fibroin (SF) scaffolds derived from enzymatically crosslinked SF hydrogels processed by salt-leaching and freeze-drying technologies, for articular cartilage applications. Though these scaffolds, we were able to combine the elastic properties of hydrogel-based systems, with the stability, resilience and controlled porosity of scaffolds processed via salt-leaching and freeze-drying technologies. SF protein has been extensively explored for TE applications, as a result of its mechanical strength, elasticity, biocompatibility, and biodegradability. Thus, the structural, mechanical and biological performance of the proposed scaffolds potentiates their use as three-dimensional matrices for cartilage regeneration.

Silk Fibroin Acts as a Self-Emulsifier to Prepare Hierarchically Porous Silk Fibroin Scaffolds through Emulsion-Ice Dual Templates.

Silk fibroin (SF) has shown enormous potentials in various fields; however, application of SF in emulsion technology is quite limited. Here, we use SF as a self-emulsifier to form an oil-in-water (O/W) emulsion by emulsifying 1-butanol in SF aqueous solution. This showed that SF possessed strong surface activity to stabilize the O/W emulsion without the need for any other surface-active agent until its solidification because of 1-butanol-induced conformational transition of SF to β-sheet. After freezing the preformed emulsions at −20 °C, robust three-dimensional porous SF scaffolds were prepared without the need for any further post-treatment. The evolution from the O/W emulsion to porous scaffold formation under freezing was tracked, and an emulsion–ice dual template mechanism was proposed for scaffold formation, based on which SF scaffolds with controllable hierarchically porous structures were achieved by tuning the dispersed droplet volume fraction. Furthermore, SF scaffolds with hierarchical porosity showed significantly higher bioactivity toward L929 fibroblasts than that of SF scaffolds with mono macroporosity, highlighting the great asset of this hierarchically porous SF scaffold for broad applications in tissue engineering. Therefore, the strong surface-active characteristic of SF presented here, in addition to its distinct advantages, sheds a bright light on the application of SF in the vast range of emulsion technologies, especially in cosmetic-, food-, and biomedical-related areas.

Silk Protein based Novel Matrix for Tissue Engineering Applications

Tissue engineering deals with recreating, maintaining and enhancing the damaged or lost tissue/organ of living organisms. For tissue engineering applications, biomaterials are needed, that can be derived from either natural or synthetic source. Silk is one of the best natural biomaterials because of its properties like biocompatibility, high mechanical strength and slow degradation rate in vivo. Present study describes the synthesis of silk fibroin hydrogel using photopolymerization method. Aqueous silk fibroin solution was blended with synthetic polymer. The solution was then exposed by a monochromatic light source for the polymerization process. During the polymerization process, sample was at a specific distance from the light source under controlled temperature conditions i.e; at 37⁰C. After the incubation period of around 3 hours, matrix with soft, slippery and elastic surface was obtained. The matrix obtained can further be used as a source for drug delivery carrier, for sustained delivery of drug, in the field of gene therapy for controlled release of DNA at the targeting site and also be used for bone and cartilage tissue engineering.

Transparent Silk Fibroin Microspheres from Controlled Droplet Dissolution in a Binary Solution.

Silk is a natural polymer with a broad range of potential applications in textiles, advanced materials, biomedical devices, and drug delivery. The ability to control the morphology and assembly of silk fibroin is essential for the fabrication of silk-based structured materials. Here, we report an effective and simple approach based on droplet dissolution for weaving silk fibroin into spheres of several hundred micrometers in diameter. The spheres possess regular wrinkled microstructures on the surface and switchable transparency for visible light. To produce these silk spheres, we immersed a sessile microdrop of the silk fibroin aqueous solution in a surrounding phase of ethanol in toluene at low concentration (<10%). The droplet experienced a two-phase process: the first phase of volume expansion due to the intake of organic solvents from the surrounding phase and the second phase of droplet dissolution. The dissolution rate is closely related to the dynamics of the droplet, while the resulting microstructure of the silk microsphere is simply adjusted by the composition of the surrounding solution. At high concentrations of ethanol, silk fibroin formed a thin shell around the droplet during the initial expansion of the droplet in volume. As the droplet shrank at a later stage, the shell around the droplet wrinkled and crumpled, leading to regular ridges and crevices on the microsphere surface. This work demonstrates that controlled droplet dissolution may be explored as a novel and effective way to tailor microstructures of silk assemblies. The as-prepared silk microspheres may be potentially used as optical units or microcarriers.

Intrinsic 3D Prestressing: A New Route for Increasing Strength and Improving Toughness of Hybrid Inorganic Biocements.

Cement is the most consumed resource and is the most widely used material globally. The ability to extrinsically prestress cementitious materials with tendons usually made from steel allows the creation of high-strength bridges and floors from this otherwise brittle material. Here, a dual setting cement system based on the combination of hydraulic cement powder with an aqueous silk fibroin solution that intrinsically generates a 3D prestressing during setting, dramatically toughening the cement to the point it can be cut with scissors, is reported. Changes of both ionic concentration and pH during cement setting are shown to create an interpenetrating silk fibroin inorganic composite with the combined properties of the elastic polymer and the rigid cement. These hybrid cements are self-densifying and show typical ductile fracture behavior when dry and a high elasticity under wet conditions with mechanical properties (bending and compressive strength) nearly an order of magnitude higher than the fibroin-free cement reference.