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

Silk-Based 3D Porous Scaffolds for Tissue Engineering

Silk fibroin, extracted from the silk of the Bombyx mori silkworm, stands out as a biomaterial due to its nontoxic nature, excellent biocompatibility, and adjustable biodegradability. Porous scaffolds, a type of biomaterial, are crucial for creating an optimal microenvironment that supports cell adhesion and proliferation, thereby playing an essential role in tissue remodeling and repair. Therefore, this review focuses on 3D porous silk fibroin-based scaffolds, first summarizing their preparation methods and then detailing their regenerative effects on bone, cartilage, tendon, vascular, neural, skin, hepatic, and tracheal epithelial tissue engineering in recent years.

Recent advances of silk fibroin materials: From molecular modification and matrix enhancement to possible encapsulation-related functional food applications

Silk fibroin materials are emergingly explored for food applications due to their inherent properties including safe oral consumption, biocompatibility, gelatinization, antioxidant performance, and mechanical properties. However, silk fibroin possesses drawbacks like brittleness owing to its inherent specific composition and structure, which limit their applications in this field. This review discusses current progress about molecular modification methods on silk fibroin such as extraction, blending, self-assembly, enzymatic catalysis, etc., to address these limitations and improve their physical/chemical properties. It also summarizes matrix enhancement strategies including freeze drying, spray drying, electrospinning/electrospraying, microfluidic spinning/wheel spinning, desolvation and supercritical fluid, to generate nano-, submicron-, micron-, or bulk-scale materials. It finally highlights the food applications of silk fibroin materials, including nutraceutical improvement, emulsions, enzyme immobilization and 3D/4D printing. This review also provides insights on potential opportunities (like safe modification, toxicity risk evaluation, and digestion conditions) and possibilities (like digital additive manufacturing) in functional food industry.

Silk fibroin-hydroxyapatite scaffolds promote the proliferation of adipose-derived mesenchymal stem cells by activating the ERK signal

Adipose-derived mesenchymal stem cell (Ad-MSC) with capacities of releasing trophic factors and chondrogenic differentiation was a promising candidate for tracheal reconstruction. Silk fibroin (SF)- hydroxyapatite (HA) scaffolds were fabricated by the freeze-drying method. And Ad-MSCs were co-cultured on the scaffolds for 14 days in vitro. The role of the SF-HA scaffold in regulating the adhesion, growth, and proliferation of Ad-MSCs, and its potential mechanisms were investigated. The identity of Ad-MSCs was confirmed by cell morphology, surface markers, and differentiation characteristics. Cell proliferation, viability, and morphology were observed via CCK-8, live/dead assay, and scanning electron microscopy (SEM). Gene mRNA and protein levels were examined using quantitative real-time polymerase chain reaction and western blotting, respectively. SF-HA scaffolds showed excellent properties of promoting Ad-MSCs adhesion, growth, and proliferation for at least 14 days. In the CCK-8 assay, the relative OD value of Ad-MSCs cultured on SF-HA scaffolds increased (p < 0.001). Furthermore, live/dead staining showed that the fluorescent coverage increased with time (p < 0.05). SEM also showed that 3 days after inoculation, the coverage of Ad-MSCs on the SF-HA scaffolds was 78.15%, increased to 92.91% on day 7, and reached a peak of 94.38% on day 14. Extracellular signal-regulated kinase (ERK) mRNA and phosphorylated ERK (pERK) protein expression increased at day 3 (p < 0.05), followed by a significant decline at day 7 (p < 0.05). And ERK mRNA expression was positively correlated with Ad-MSCs proliferation (p < 0.05). In summary, the SF-HA scaffold co-cultured with Ad-MSCs is a promising biomaterial for tracheal repair by activating the ERK signal pathway.

Silk Fibroin-Based Shape-Memory Organohydrogels with Semicrystalline Microinclusions

Inspired by nature, we designed organohydrogels (OHGs) consisting of a silk fibroin (SF) hydrogel as the continuous phase and the hydrophobic microinclusions based on semicrystalline poly(n-octadecyl acrylate) (PC18A) as the dispersed phase. SF acts as a self-emulsifier to obtain oil-in-water emulsions, and hence, it is a versatile and green alternative to chemical emulsifiers. We first prepared a stable oil-in-water emulsion without an external emulsifier by dispersing the n-octadecyl acrylate (C18A) monomer in an aqueous SF solution. To stabilize the emulsions for longer times, gelation in the continuous SF phase was induced by the addition of ethanol, which is known to trigger the conformational transition in SF from random coil to β-sheet structures. In the second step, in situ polymerization of C18A droplets in the emulsion system was conducted under UV light in the presence of a photoinitiator to obtain high-strength OHGs with shape-memory function, and good cytocompatibility. The incorporation of hydrophilic N,N-dimethylacrylamide and noncrystallizable hydrophobic lauryl methacrylate units in the hydrogel and organogel phases of OHGs, respectively, further improved their mechanical and shape-memory properties. The shape-memory OHGs presented here exhibit switchable viscoelasticity and mechanics, a high Young's modulus (up to 4.3 ± 0.1 MPa), compressive strength (up to 2.5 ± 0.1 MPa), and toughness (up to 0.68 MPa).

Carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds for bone tissue engineering applications

In bone tissue engineering, the development of advanced biomimetic scaffolds has led to the quest for biomotifs in scaffold design that better recreate the bone matrix structure and composition and hierarchy at different length scales. In this study, an advanced hierarchical scaffold consisting of silk fibroin combined with a decellularized cell-derived extracellular matrix and reinforced with carbon nanotubes was developed. The goal of the carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds is to harvest the individual properties of their constituents to introduce hierarchical capacity in order to improve standard silk fibroin scaffolds. The scaffolds were fabricated using enzymatic cross-linking, freeze modeling, and decellularization methods. The developed scaffolds were assessed for the pore structure and mechanical properties showing satisfying results to be used in bone regeneration. The developed carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds were shown to be bioactive in vitro and expressed no hemolytic effect. Furthermore, cellular in vitro studies on human adipose-derived stem cells (hASCs) showed that scaffolds supported cell proliferation. The hASCs seeded onto these scaffolds evidenced similar metabolic activity to standard silk fibroin scaffolds but increased ALP activity. The histological staining showed cell infiltration into the scaffolds and visible collagen production. The expression of several osteogenic markers was investigated, further supporting the osteogenic potential of the developed carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds. The hemolytic assay demonstrated the hemocompatibility of the hierarchical scaffolds. Overall, the carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds presented the required architecture for bone tissue engineering applications.

Preparation and Characterization of Porous Scaffolds Based on Poly(3-hydroxybutyrate) and Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Poly(hydroxyalkanoates) (PHAs) with different material properties, namely, the homopolymer poly(3-hydroxybutyrate), P(3HB), and the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate, P(3HB-co-3HV), with a 3HV of 25 wt.%, were used for the preparation of porous biopolymeric scaffolds. Solvent casting with particulate leaching (SCPL) and emulsion templating were evaluated to process these biopolymers in porous scaffolds. SCPL scaffolds were highly hydrophilic (>170% swelling in water) but fragile, probably due to the increase of the polymer’s polydispersity index and its high porosity (>50%). In contrast, the emulsion templating technique resulted in scaffolds with a good compromise between porosity (27–49% porosity) and hydrophilicity (>30% water swelling) and without impairing their mechanical properties (3.18–3.35 MPa tensile strength and 0.07–0.11 MPa Young’s Modulus). These specifications are in the same range compared to other polymer-based scaffolds developed for tissue engineering. P(3HB-co-3HV) displayed the best overall properties, namely, lower crystallinity (11.3%) and higher flexibility (14.8% elongation at break. Our findings highlight the potency of our natural biopolyesters for the future development of novel porous scaffolds in tissue engineering, thanks also to their safety and biodegradability.

Application of an ultrasound semi-quantitative assessment in the degradation of silk fibroin scaffolds in vivo

BACKGROUND

Research on the degradation of silk fibroin (SF) scaffolds in vivo lacks uniform and effective standards and experimental evaluation methods. This study aims to evaluate the application of ultrasound in assessing the degradation of SF scaffolds.

METHODS

Two groups of three-dimensional regenerated SF scaffolds (3D RSFs) were implanted subcutaneously into the backs of Sprague-Dawley rats. B-mode ultrasound and hematoxylin and eosin (HE) staining were performed on days 3, 7, 14, 28, 56, 84, 112, 140, and 196. The cross-sectional areas for two groups of 3D RSFs that were obtained using these methods were semi-quantitatively analyzed and compared to evaluate the biodegradation of the implanted RSFs.

RESULTS

The 3D RSFs in the SF-A group were wholly degraded at the 28th week after implantation. In contrast, the 3D RSFs in the SF-B group were completely degraded at the 16th week. Ultrasonic examination showed that the echoes of 3D RSFs in both groups gradually decreased with the increase of the implantation time. In the early stages of degradation, the echoes of the samples were higher than the echo of the muscle. In the middle of degeneration, the echoes were equal to the echo of the muscle. In the later stage, the echoes of the samples were lower than that of the muscle. The above changes in the SF-B group were earlier than those in the SF-A group. Semi-quantitative analysis of the cross-sectional areas detected using B-mode ultrasound revealed that the degradations of the two 3D RSF groups were significantly different. The degradation rate of the SF-B group was found to be higher than that of the SF-A group. This was consistent with the semi-quantitative detection results for HE staining. Regression analysis showed that the results of the B-mode ultrasound and HE staining were correlated in both groups, indicating that B-mode ultrasound is a reliable method to evaluate the SF scaffold degradation in vivo.

CONCLUSIONS

This study suggests that B-mode ultrasound can clearly display the implanted SF scaffolds non-invasively and monitor the degradation of the different SF scaffolds after implantation in living organisms in real-time.

Optimizing interactions between soluble silk fibroin and capryl glucoside for design of a natural and high-performance co-surfactant system

OBJECTIVES

Because of the strong consumer driver towards more natural or higher sustainability cosmetic products, silk fibroin was evaluated to help develop a formulation with natural and effective ingredients for personal care. In order to exploit the physical properties of silk fibroin, it was evaluated to maximize the surfactant properties of other commercial ingredients to lower surface tension and build up viscosity. A synergistic effect was seen between silk fibroin and capryl glucoside, a sugar surfactant which exhibited a natural and effective co-surfactant system. This system demonstrated better surface tension properties than sodium laureth sulphate (SLES), cocamidopropyl betaine (CAPB), rhamnolipids and sophorolipids, which led to greater foamability and cleansing properties. This system proved to also be compatible with polysaccharide viscosity modifiers to enhance the viscosity of the system. The present study comprises a systematic exploration of natural formulation development of silk proteins and other natural ingredients, which result in high performance such as enhanced foam quality, foam stability and enhanced sebum removal. All of these properties are desirable and may utilized when formulating cleaners and shampoos.

METHODS

A force tensiometer, Attension Sigma 701, was used to measure the surface tension of the silk protein and its various combinations with biosurfactants and biopolymers. To measure bulk rheology, a traditional mechanical rheometer TA DHR-3 was utilized. Foaming tests and sebum removal assays were also carried out to evaluate the performance of the samples.

RESULTS

Silk fibroin was evaluated to maximize the surfactant properties of other commercial systems to develop a formulation containing natural and effective ingredients for personal care. The surface activity of silk proteins was seen to be synergistically enhanced in the presence of sugar surfactants such as capryl glucoside, resulting in a surface tension at the air-water interface which is lower than either that of pure silk fibroin or pure capryl glucoside. This surface tension value is additionally lower than that obtained from currently utilized synthetic surfactants like sodium laureth sulphate (SLES) and cocamidopropyl betaine (CAPB). This reduction in surface tension demonstrated greater foamability and cleansing properties than that of the commercial systems. The very low surface tension values obtained through combinations of silk proteins and glucoside resulted in a natural and effective co-surfactant system by forming high-quality stable foams and enhancing sebum removal. The rheological performance of the silk proteins was impacted through microstructure modifications as a result of interactions with biopolymers like carrageenan. This shows that this system is compatible with polysaccharide viscosity modifiers. It was observed that both the flow curve and the absolute viscosity values were significantly impacted in the presence of carrageenan, with higher viscosity generation and significant non-Newtonian/shear thinning behaviour evolution. These results indicate that the silk fibroin can be utilized to build a high-performance natural product and significantly enhance the performance of other natural/sustainable cosmetic formulations through building synergistic interactions with other natural ingredients such as sugar surfactants and biopolymers. These properties exhibited by this system are all desirable for cleansers and shampoos within the cosmetic industry.

CONCLUSION

Silk fibroin in combination with capryl glucoside outperforms other commercial surfactants that are commonly used in the industry because of its surface-active behaviour and synergy. This system is then enhanced further with polysaccharide rheological modifiers, carrageen and xanthan gum to help build up viscosity. The complex mixture of silk fibroin, sugar surfactant and biopolymer results in a formulation that is all natural, while still having high performance by achieving great foamability and enhanced sebum removal. The mixture can further be used to formulate a fully natural product such as a cleanser or shampoo while still having the same or greater effectiveness as synthetic surfactants and ingredients typically used in cosmetic formulations.

A silk-based high impact composite for the core decompression of the femoral head

Core decompression of the femoral head is a recommended head-conserving strategy for early-stage osteonecrosis of the femoral head. However, no ideal filling material has been found so far. In this study, we fabricated a "solid core-porous coating" composite scaffold, which is a silk fibroin/hydroxypropyl methylcellulose (SF/HPMC) scaffold, by a "two-step" process. The solid core scaffold possesses a sufficient compression modulus (860 MPa) for support, while the porous coating scaffold with controllable pore size and porosity provides a suitable microenvironment for the osteoblast cell to adhere and proliferate. Moreover, the porous coating scaffold was mineralized by adding different contents of hydroxyapatite crystal to further enhance its osteoinductivity, according to the simulated body fluid (SBF) biomineralization assay. To demonstrate the biocompatibility and osteoinductivity of such composite scaffolds, a series of in vitro experiments were performed, indicating the MC3T3-E1 pre-osteoblast cells grew and differentiated well on the mineralized porous coating scaffolds. The mechanical testing results also proved that the mechanical property of the solid core scaffold varied (230-1600 MPa) with different solid contents of SF/HPMC, as expected. Furthermore, the rabbit femoral head core decompression model was adopted and confirmed the excellent mechanical performance of the solid core scaffolds, as well as the satisfied osteoinductivity of the porous coating scaffold, by inserting the composite scaffolds into the bone tunnel in vivo. All of the preliminary results implied that the novel biodegradable composite scaffold has an outstanding prospective for the clinical use of core decompression of the femoral head.

Porous Three-Dimensional Silk Fibroin Scaffolds for Tracheal Epithelial Regeneration in Vitro and in Vivo

The regeneration of functional epithelial lining is critical for artificial grafts to repair tracheal defects. Although silk fibroin (SF) scaffolds have been widely studied for biomedical application (e.g., artificial skin), its potential for tracheal substitute and epithelial regeneration is still unknown. In this study, we fabricated porous three-dimensional (3D) silk fibroin scaffolds and cocultured them with primary human tracheobronchial epithelial cells (HBECs) for 21 days in vitro. Examined by scanning electronic microscopy (SEM) and calcein-AM staining with inverted phase contrast microscopy, the SF scaffolds showed excellent properties of promoting cell growth and proliferation for at least 21 days with good viability. In vivo, the porous 3D SF scaffolds (n = 18) were applied to repair a rabbit anterior tracheal defect. In the control group (n = 18), rabbit autologous pedicled trachea wall without epithelium, an ideal tracheal substitute, was implanted in situ. Observing by endoscopy and computed tomography (CT) scan, the repaired airway segment showed no wall collapse, granuloma formation, or stenosis during an 8-week interval in both groups. SEM and histological examination confirmed the airway epithelial growth on the surface of porous SF scaffolds. Both the epithelium repair speed and the epithelial cell differentiation degree in the SF scaffold group were comparable to those in the control group. Neither severe inflammation nor excessive fibrosis occurred in both groups. In summary, the porous 3D SF scaffold is a promising biomaterial for tracheal repair by successfully supporting tracheal wall contour and promoting tracheal epithelial regeneration.