Silk Fibroin Films Research Articles

Preparation of Highly Crystalline Silk Nanofibrils and Their Use in the Improvement of the Mechanical Properties of Silk Films.

Due to their commendable biocompatibility, regenerated silk fibroin (RSF) films have attracted considerable research interest. However, the poor mechanical properties of RSF films have limited their use in various biomedical applications. In this study, a novel, highly crystalline silk fibril was successfully extracted from silk by combining degumming with ultrasonication. Ultrasonication accelerated the development of silk nanofibrils measuring 130–200 nm on the surface of the over-degummed silk fibers, which was confirmed via scanning electron microscopy. Additionally, the crystallinity index of silk fibril was found to be significantly higher (~68%) than that of conventionally degummed silk (~54%), as confirmed by the Fourier-transform infrared (FTIR) spectroscopy results. Furthermore, the breaking strength and elongation of the RSF film were increased 1.6 fold and 3.4 fold, respectively, following the addition of 15% silk nanofibrils. Thus, the mechanical properties of the RSF film were remarkably improved by the addition of the silk nanofibrils, implying that it can be used as an excellent reinforcing material for RSF films.

Wearable five-finger keyboardless input system based on silk fibroin electronic skin

To get rid of the limitations of traditional keyboards, we report a new generation wearable keyboardless input system (WKIS). Based on the coupling of triboelectrification and electrostatic induction, the single-electrode mode triboelectric nanogenerator (TENG) worn on the five fingers can convert finger tapping into electrical signals. Meanwhile, by developing a number pair coding table in vowel mode, we integrated the 26 English letters and necessary instructions into WKIS with five-finger tapping. The main body of the device is an ultra-thin silk fibroin film (SF), which is biocompatible, water-permeable, breathable and skin conformal, enabling it suitable for long-term wear as a ring on the finger. Data coding and transmission process are completed through a printed circuit board (PCB) and Wi-Fi module to realize keyboard communication. Feature engineering and machine learning are employed to identify WKIS registered users with an accuracy of 92%. In addition, the WKIS provides an effective solution in smart home control, which has potential applications in future human-computer interaction, Internet of Things and VR scenarios.

Fiber Humidity Sensor Based on SF-LiBr Composite Film

A novel sensor based on a silk fibroin (SF) and lithium bromide (LiBr) composite film is proposed to sense relative humidity (RH). We fabricate this sensor by coating a tapered single-mode optical fiber (TSOF) with a SF-LiBr composite film. The evanescent field generated in the taper region responds to the surrounding refractive index (RI). The SF-LiBr composite film immobilized on the surface of the taper region is sensitive to RH. As the humidity increases, more water molecules interact with the hydrogen bonds in the SF film. The LiBr enhances the water absorption properties of SF film. Therefore, the RI of SF-LiBr composite film changes according to the humidity of the surrounding ambient, resulting in transmission spectra finally shifts. We can monitor the RH by observing the wavelength shift of the transmission spectrum. The results show that the sensor has a nonlinear response in the range from 29%RH to 99%RH, and the sensitivity can reach −2.3nm/%RH in the range from 29%RH to 39%RH. Furthermore, the sensor possesses excellent stability and remarkable repeatability.

Optically Clear Silk Fibroin Films with Tunable Properties for Potential Corneal Tissue Engineering Applications: A Process-Property-Function Relationship Study.

Owing to the shortage of donor corneas and issues associated with conventional corneal transplantation, corneal tissue engineering has emerged as a promising therapeutic alternative. Biocompatibility and other attractive features make silk fibroin a biomaterial of choice for corneal tissue engineering applications. The current study presents three modes of silk fibroin film fabrication by solvent casting with popular solvents, viz. aqueous (aq), formic acid (FA), and hexafluoroisopropanol (HFIP), followed by three standard modes of postfabrication annealing with water vapor, methanol vapor, and steam, and systematic characterization studies including corneal cell culture in vitro. The results indicated that silk fibroin films made from aq, FA, and HFIP solvents had surface roughness (Rq) of 1.39, 0.32, and 0.13, contact angles of 73°, 85°, and 89°, water uptake% of 58, 29, and 27%, swelling ratios of 1.58, 1.3, and 1.28, and water vapor transmission% of 39, 26, and 22%, respectively. The degradation rate was in the order of aq > HF > FA, whereas the tensile strength was in the order of aq < HF < FA. Further, the results of the annealing process indicated notable changes in morpho-topographical, physical, degradation, and tensile properties. However, the films showed no detectable changes in chemical composition and remained optically clear with >90% transmission in the visible range, irrespective of fabrication and postfabrication processing conditions. The films were noncytotoxic against L929 cells and were cytocompatible with rabbit cornea-derived SIRC cells in vitro. The study demonstrated the potential of fine-tuning various properties of silk fibroin films by varying the fabrication and postfabrication processing conditions.

Modification and release of calcitonin gene-related peptide on silk fibroin film

As a breakthrough in traditional textiles, silk fibroin (SF) has been widely studied for biomedical material applications. In order to enhance the electronegativity of SF for electrostatic adsorption of positively charged biological factors, a kind of blend film was designed by introducing adipic acid (AA) with two carboxyl groups using ethanol treatment or chemical crosslinking. Following this, a positively charged calcitonin gene-related peptide (CGRP, pI = 9.5) was introduced onto the SF/AA film surfaces through electrostatic interaction. The loading capacity and release kinetics of the CGRP were explored. Results showed that the zeta potential on the SF/AA film surfaces decreased with the increasing AA content. When the mass ratio of SF to AA changed from 100:0 to 100:2.5, the zeta potential of crosslinked blends decreased from –13.6 to –32.6 mV. The maximum loading dose of CGRP on the crosslinked blends (100:2.5) was 500 ng/cm2. In phosphate buffered saline (pH = 7.4), the CGRP on the film surfaces was released rapidly and in high quantity from 12 to 24 h. The release rate was relatively slow under weakly acidic conditions at pH = 5, and it also decreased when the pH value was changed from 7.4 to 9.0. This study provides a new strategy for improving the biological activity of SF used as specific tissue repair materials.

Nanogenerators integrated self-powered multi-functional wings for biomimetic micro flying robots

Under rigorous weight and power constraints, improving the energy systems and the onboard sensing approaches for flapping-wing micro air vehicles (FWMAVs) are monumental challenges. Mechanical energy transformation and integrated self-powered sensing hold great promising features to improve the flying performance of FWMAVs. Herein, we introduce novel triboelectric nanogenerators (TENGs) integrated multi-functional wings to address these challenges. Synthetically, The TENGs integrated wings can realize the mechanical energy transformation, the motion monitoring of flapping wings, and the pneumatic functions for FWMAVs. Firstly, a lightweight and ultra-thin sandwich structure integration technique with silk-fibroin film as the coated friction layer is realized for the TENGs integrated wings. Secondly, the wind tunnel test verifies the feasibility of the mechanical transformation and reveals the effects of the operating conditions of the FWMAVs on the triboelectric energy performance. The triboelectric energy performance of the TENGs integrated wings, whose entire thickness and the weight increases are less than 20 µm and 0.2 g respectively is verified to achieve 468.7 V voltage, 100.6 μA current, and 379.8 μW power. Likewise, as an attractive potential application, it is demonstrated to provide an accurate self-powered measurement of the flapping mechanism frequency based on the triboelectric principle. The aerodynamic measurements verify that the TENGs integrated wings maintain the pneumatic functions, which guarantee flight stability, and they positively enhance 29.89% of the onboard load of the FWMAV.

Preparation of SF/SF-nHA double-layer scaffolds for periodental tissue regeneration

Silk fibroin (SF) film/porous SF-nanohydroxyapatite (nHA) double-layer composite scaffolds were fabricated by a combination of electrospinning, casting and freeze-drying. The results showed that the tensile strength and elongation of the SF film layer were significantly improved through the bridging effect of SF/(ε-caprolactone) fibers in the SF matrix. The pore size in SF-nHA porous scaffold layer was approximately 213 μm, and nHA enhanced the compressive strength of the scaffold. CCK-8 assay showed that the SF/SF-nHA scaffold not only had no obvious cytotoxicity but also significantly promoted cell proliferation. The scaffold has the potential to be used for periodontal tissue regeneration.

Hemocompatibility Evaluation of Thai Bombyx mori Silk Fibroin and Its Improvement with Low Molecular Weight Heparin Immobilization.

Bombyx mori silk fibroin (SF), from Nangnoi Srisaket 1 Thai strain, has shown potential for various biomedical applications such as wound dressing, a vascular patch, bone substitutes, and controlled release systems. The hemocompatibility of this SF is one of the important characteristics that have impacts on such applications. In this study, the hemocompatibility of Thai SF was investigated and its improvement by low molecular weight heparin (LMWH) immobilization was demonstrated. Endothelial cell proliferation on the SF and LMWH immobilized SF (Hep/SF) samples with or without fibroblast growth factor-2 (FGF-2) was also evaluated. According to hemocompatibility evaluation, Thai SF did not accelerate clotting time, excess stimulate complement and leukocyte activation, and was considered a non-hemolysis material compared to the negative control PTFE sheet. Platelet adhesion of SF film was comparable to that of the PTFE sheet. For hemocompatibility enhancement, LMWH was immobilized successfully and could improve the surface hydrophilicity of SF films. The Hep/SF films demonstrated prolonged clotting time and slightly lower complement and leukocyte activation. However, the Hep/SF films could not suppress platelet adhesion. The Hep/SF films demonstrated endothelial cell proliferation enhancement, particularly with FGF-2 addition. This study provides fundamental information for the further development of Thai SF as a hemocompatible biomaterial.

Peptide-Functionalized Silk Fibers as a Platform to Stabilize Gelatin for Use in Ingestible Devices.

The combination of pharmacologic and endoscopic therapies is the gold standard for treating intestinal failures. The possibility of chemical solubility in water is mandatory for intelligent capsules. Functionalised silk fibroin with peptides and covalently linking different molecular entities to its structure make this protein a platform for preparing gels dissolving in the small and large intestine for drug delivery. In the present study, we linked a peptide containing the cell-adhesive motif Arginine–Glycine–Aspartic acid (RGD) to degummed silk fibres (DSF). Regenerated silk fibroin (RS) films obtained by dissolving functionalised DSF in formic acid were used to prepare composite gelatin. We show that such composite gelatin remains stable and elastic in the simulated gastric fluid (SGF) but can dissolve in the small and large intestines’ neutral-pH simulated intestine fluid (SIF). These findings open up the possibility of designing microfabricated and physically programmable scaffolds that locally promote tissue regeneration, thanks to bio-enabled materials based on functionalised regenerated silk.

Enhanced Silk Fibroin/Sericin Composite Film: Preparation, Mechanical Properties and Mineralization Activity

The periosteum plays an important role in bone formation and reconstruction. One of the reasons for the high failure rate of bone transplantation is the absence of the periosteum. Silk fibroin (SF) and silk sericin (SS) have excellent biocompatibility and physicochemical properties, which have amazing application prospects in bone tissue engineering, but lacked mechanical properties. We developed a series of SF/SS composite films with improved mechanical properties using boiling water degumming, which caused little damage to SF molecular chains to retain larger molecules. The Fourier transform infrared spectroscopy and X-ray diffraction results showed that there were more β-sheets in SF/SS films than in Na2CO3 degummed SF film, resulting in significantly improved breaking strength and toughness of the composite films, which were increased by approximately 1.3 and 1.7 times, respectively. The mineralization results showed that the hydroxyapatite (HAp) deposition rate on SF/SS composite films was faster than that on SF film. The SF/SS composite films effectively regulated the nucleation, growth and aggregation of HAp-like minerals, and the presence of SS accelerated the early mineralization of SF-based materials. These composite films may be promising biomaterials in the repair and regeneration of periosteum.

Amplified spontaneous emission from all-inorganic perovskite on a flexible substrate with silk fibroin

Stretchable microcavity lasers reveal potential application in flexible displays, biomedicine, and wearable devices in the near future. In this work, we investigated the characteristic of amplified spontaneous emission (ASE) from all inorganic CsPbBr3 QDs on a flexible PET substrate with the assistance of biocompatible silk fibroin (SF) film. In comparison with the sample on PET directly, the ASE of all-inorganic perovskite film revealed a lower threshold of 32.7 μJ/cm2, higher slope efficiency, and a larger gain coefficient of around 100.0 cm−1 owing to the better stack and good arrangement of the CsPbBr3 QDs on top of the SF film. For the temperature-dependent ASE measurement, the larger characteristic temperature of around 277 K is obtained from CsPbBr3 QD/SF film, and the emission peak reveals a slight shift with temperature variation, which indicates its temperature-insensitive property. As the curvature of flexible substrate increases under the mechanical bending, the lasing threshold of CsPbBr3 QD/SF film was reduced along with the increase in slope efficiency owing to the enhancement in the index guiding effect.

Effect of post-treatment methods and nanoparticles on the conformation of silk fibroin and their impact on electrical properties

ABSTRACT This paper investigates the effect of different plasticizers (glycerol and dextrose), post-treatment methods, and metal oxide nanoparticles, i.e. zinc oxide and copper oxide on the structural properties of silk fibroin films and their effect on electrical properties, i.e. dielectric strength. Structural properties of the silk fibroin films have been evaluated using Fourier transform infrared spectroscopy and X-ray diffraction technique. It was found that plasticizers and post-treatment applied on the regenerated silk fibroin films have impacted the β-sheet conformation significantly. The dielectric strength of the silk fibroin films was measured using the parallel plate capacitor method and an interrelation with structural properties was observed. Nanoparticles have been introduced into the silk fibroin films as a progressive method to impart substantial effect on the dielectric strength of the silk fibroin films. However, an unexpected change in dielectric strength has been obtained. The mechanism behind the change in dielectric strength has been explored in this work.

Silk-fibroin film as enzyme stabilizing material and optical signal transducer for developing alcohol oxidase-based μPAD methanol biosensor

The commercial success of paper-based microfluidic biosensor devices with enzyme as recognition elements may be achieved greatly by improving the enzyme stability, averting the coffee-ring effect on the detection zone, and making it a stand-alone system. We have addressed these issues here by introducing silk-fibroin (SF) film for immobilizing enzymes and chromogenic reagent (ABTS), and wave-design microfluidic channels in the chromatographic paper for developing a hybrid microfluidic paper-based analytical device (μPAD), for methanol detection through peroxidase reaction. The activity of alcohol oxidase (AOx), used as a biorecognition element, could be wholly retained in a non-dissolvable SF film (pore size <3.5 μm) until 40 days of storage at room temperature (RT) by reducing oxygen permeability to the film. Similarly, ABTS in a dissolvable SF film was protected from air-oxidation even up to two months of storage at RT. The detection approach exploited purple color as a high contrast response signal generated from ABTS di-cation formed from the reaction of SF protein with the ABTS radicals generated from the substrate-dependent peroxidase reaction. The wave-designed microfluidic channels could significantly reduce the coffee ring effect in the detection film. The bi-enzyme SF films prepared by co-immobilizing AOx with peroxidase (HRP) offered high sensitivity to the device with a detection limit (LoD) of 1 ± 0.05 mM and a dynamic range of 1 mM–2 M for methanol. The stand-alone hybrid μPAD biosensor for methanol has a high potential for commercial applications.

Bidirectional Actuation of Silk Fibroin Films: Role of Water and Alcohol Vapors

Three-dimensional (3D) shape morphism observed in nature inspires the development of stimuli-responsive soft actuators. Vapor-responsive actuators are promising among the different stimuli-responsive materials due to their capability to produce macroscale movements in response to a minuscule amount of specific chemical vapor. Here, we report unusual multiple vapor-responsive bidirectional macroscale actuation behaviors of single-layer regenerated silk fibroin films. The vapor-responsive silk fibroin actuator exhibits antagonistic actuation characteristics in a reversible manner to both water and ethanol vapors. For instance, it produces an upward bending in the presence of water vapor and downward bending in ethanol vapor, which demonstrates the chemical vapor-specific actuation. However, the actuation characteristics remain largely invariant upon changing the polarity of alcohol molecules. The silk fibroin actuators effectively utilize the vapor-induced minuscule expansion and contraction of the film surface to produce large-scale actuation, which is fully reversible. The intrinsic water content of the films and the vapor pressure of the stimulants are exploited to control the actuation performance. Further, we demonstrated the 3D shape morphing ability of the actuator by generating an undulating wavelike motion via preprogrammed water and ethanol vapor exposure conditions. The change in the actuation direction is instantaneous, which ensures the sensitivity and rapid response of the fabricated actuators.

Cu(II)-functionalized silk fibroin films for the catalytic generation of nitric oxide.

In situ release of nitric oxide (NO) has been suggested to be a potential functionalization strategy for blood-contacting implants. In this study, the NO generation capability catalyzed by the copper ion-incorporated silk fibroin (SF) films in the presence of S-nitroso-N-acetyl-dl-penicillamine (SNAP) is demonstrated. Cu(II) is effectively bound to the surface of the SF film based on metal-protein coordination. The x-ray photoelectron spectroscopy results indicate that copper ions may exist on the surface of the SF film in the form of Cu(II)/Cu(I) coexistence. The degradation behavior showed that the bound copper ions on the surface of the SF films can maintain a slow release in phosphate-buffered saline (PBS) or collagenase IA solution for 7 days. There was no significant difference in the release of copper ions between PBS degradation and enzyme degradation. The loading of copper ions significantly improved the release of NO from SNAP through catalysis. Based on the biological effects of copper ions and the ability to catalyze the release of NO from S-nitrosothiols, copper ion loading provides an option for the construction of bioactive SF biomaterials.

Silk fibroin-based films in food packaging applications: A review.

Plastic pollution is a significant concern nowadays due to wastes generated from non-biodegradable and non-renewable synthetic materials. In particular, most plastic food packaging material ends up in landfills, creating mass wastes that clog the drainage system and pollute the ocean. Thus, studies on various biopolymers have been promoted to replace synthetic polymers in food packaging and consequently, the high number of research in biopolymers food packaging, especially in the characterization, properties and also the development of the biopolymer. For biopolymer-based food packaging, silk fibroin (SF) has been highlighted because of its biodegradability and low water vapor permeability properties. This review focuses on the different properties of SF films prepared through solution casting and electrospinning for food packaging. Discussions encompassed chemical properties, mechanical properties, permeability, and biodegradability. This review also discussed the studies that used SF as the biomaterial for food packaging.

Preparation of Water-Insoluble Silk Fibroin Films. Study of Their Structure and Properties

Preparation of Water-Insoluble Silk Fibroin Films. Study of Their Structure and Properties

Fabrication of silk fibroin film enhanced by acid hydrolyzed silk fibroin nanowhiskers to improve bacterial inhibition and biocompatibility efficacy

In this study, silk fibroin nanowhiskers (SNWs) were extracted from natural silk fiber by sulfuric acid hydrolysis with the assistance of ultrasonic wave treatment. The obtained SNWs were mixed with regenerated silk fibroin (RSF) solution to fabricate the SNWs/RSF films. The fabricating SNWs were systematically characterized by using SEM, FTIR, and the SNWs/RSF films were observed by digital camera, PM, etc. The results show that the monodisperse SNWs are evenly distributed in the RSF film. The presence of SNWs in RSF film significantly improves the performances of the film, including the swelling ability, mechanical properties, hydrophilicity, antibacterial efficacy, cytocompatibility. Meanwhile, the SNWs/RSF film can endorse the wound healing efficiency in vivo mice wound site. The proposed techniques for extracting SNWs and fabricating silk fibroin composite film may provide a valuable method for creating an ideal silk-based material for biomedical applications.

Biological SERS-active sensor platform based on flexible silk fibroin film and gold nanoislands.

In contrast to conventional surface-enhanced Raman scattering (SERS) platforms implemented on non-biological substrates, silk fibroin has the unique advantages of long-term biosafety and controllable biodegradability for in vitro and in vivo biomedical applications, as well as flexibility and process-compatibility. In this study, a silk fibroin film was developed to fabricate a flexible SERS sensor template with nanogap-rich gold nanoislands. The proposed biological SERS platform presents fairly good enhancements in detection performance such as detection limit, sensitivity, and signal-to-noise ratio. In particular, the sensitivity improvement was by more than 10 times compared to that of the counterpart sample, and an excellent spatial reproducibility of 2.8% was achieved. In addition, the near-field calculation results were consistent with the experimental results, and the effect of surface roughness of the silk substrate was investigated in a quantitative way. It is believed that biological SERS-active sensors could provide the potential for highly sensitive, cost-effective, and easily customizable nanophotonic platforms that include new capabilities for future healthcare devices.

Optical and Structural Properties of Insoluble and Flexible Biodegradable Regenerated Silk Films for Optically Transparent Hydrophilic Coating of Medical Devices

AbstractMany medical equipment, such as tubes, lenses, and disposable plastic slides, require improved wettability to perform properly with physiological fluids and tissues. The authors proposes the regeneration of silk cocoons into insoluble and flexible regenerated silk fibroin (SF) films allowed for the development of novel materials with distinct and convincing characteristics. The SF films with glycerol modifications are created using a casting/solvent evaporation technique to provide a flexible and optically transparent hydrophilic coating. Fourier‐transformed infrared spectroscopy and wide‐angle X‐ray diffraction reveal a structural shift in glycerol‐induced SF films, where the random coil and helical structure content reduce while the β‐sheet structure increases significantly. All of the proposed SF films have an outstanding transmission of approximately 90% in the visible range and a UV‐shield characteristic. Tensile tests reveal that glycerol‐plasticized SF films are soft and extremely flexible, with a high Young's modulus and moderate tensile strength. The adhesion strength of the SF films is assessed as 5B at 10–20% wt of glycerol, suggesting outstanding adhesion strength on the silanized glass surface. At 30% w/w glycerol, the SF film has the greatest swelling ratio of 95.3 ± 0.5% and the lowest water contact angle of 23.2°.