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

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

Regenerated cellulose nanofibers from cellulose acetate: Incorporating hydroxyapatite (HAp) and silver (Ag) nanoparticles (NPs), as a scaffold for tissue engineering applications

Cellulose nanofibers, which are troublesome to spin into fibers, can be easily fabricated by post-regeneration of its acetate-derived threads. Cellulose is a natural polymer; it enjoys better biocompatibility, cellular mimicking, and hydrophilic properties than its proportionate analog. Herein, we regenerated acetate-free nanofibers by alkaline de-acetylation of as-spun nanofibers. The resultant cellulose nanofibers previously loaded with hydroxyapatite (HAp) were immobilized using silver (Ag) nanoparticles (NPs) by reduction of adsorbed Ag ions on using sodium borohydride. These amalgamated nanofibers were characterized for SEM, EDX, TEM, FTIR, and hydrophilicity tests revealing the existence of both HAp and Ag NPs in/on the nanofiber scaffolds. The de-acetylation of composite nanofibers resulted in spontaneous hydrophilicity. These nanofibers were cytocompatible, as resolved by MTT assay conducted on chicken embryo fibroblasts. The SEM of the samples after cell culture revealed that these composites allowed a proliferation of the fibroblasts over and within the nanofiber network, and increased concentration of HAp levitated the excessive of apatite formation as well as increased cell growth. The antimicrobial activity of these nanofibers was assessed on E. coli (BL21) and S. aureus, suggesting the potential of de-acetylated nanofibers to restrain bacterial growth. The degradation study for 10, 30, and 60 days indicated degradation of the fibers much is faster in enzymes as compared to degradation in PBS. The results certify that these nanofibers possess enormous potential for soft and hard tissue engineering besides their antimicrobial properties.

Engineering the hard-soft tissue interface with random-to-aligned nanofiber scaffolds

Tendon injuries can be difficult to heal and have high rates of relapse due to stress concentrations caused by scar formation and the sutures used in surgical repair. Regeneration of the tendon/ligament-to-bone interface is critical to provide functional graft integration after injury. The objective of this study is to recreate the tendon-to-bone interface using a gradient scaffold which is fabricated by a one-station electrospinning process. Two cell phenotypes were grown on a poly-ε-caprolactone nanofiber scaffold which possesses a gradual transition from random to aligned nanofiber patterns. We assessed the effects of the polymer concentration, tip-to-collector distance, and electrospinning time on the microfiber diameter and density. Osteosarcoma and fibroblast cells were seeded on the random and aligned sections of scaffolds, respectively. A random-to-aligned cocultured tissue interface which mimicked the native transition in composition of enthesis was created after 96 h culturing. The results showed that the microstructure gradient influenced the cell morphology, tissue topology, and promoted enthesis formation. This study demonstrates a heterogeneous nanofiber scaffold strategy for interfacial tissue regeneration. It provides a potential solution for mimicking transitional interface between distinct tissues, and can be further developed as a heterogeneous cellular composition platform to facilitate the formation of multi-tissue complex systems.

NF-κB signaling is key in the wound healing processes of silk fibroin

Silk fibroin (SF) is a well-studied biomaterial for tissue engineering applications including wound healing. However, the signaling mechanisms underlying the impact of SF on this phenomenon have not been determined. In this study, through microarray analysis, regulatory genes of NF-ĸB signaling were activated in SF-treated NIH3T3 cells along with other genes. Immunoblot analysis confirmed the activation of the NF-ĸB signaling pathway as SF induced protein expression levels of IKKα, IKKβ, p65, and the degradation of IκBα. The treatment of NIH3T3 cells with SF also increased the expression of cyclin D1, vimentin, fibronectin, and vascular endothelial growth factor (VEGF). The expression of these factors by SF treatment was abrogated when NF-ĸB was inhibited by a pharmacological inhibitor Bay 11-7082. Knockdown of NF-ĸB using siRNA of IKKα and IKKβ also inhibited the SF-induced wound healing response of the NIH3T3 cells in a wound scratch assay. Collectively, these results indicated that SF-induced wound healing through the canonical NF-κB signaling pathway via regulation of the expression of cyclin D1, vimentin, fibronectin, and VEGF by NIH3T3 cells. Using an in vivo study with a partial-thickness excision wound in rats we demonstrated that SF-induced wound healing via NF-κB regulated proteins including cyclin D1, fibronectin, and VEGF. The in vitro and in vivo data suggested that SF induced wound healing via modulation of NF-ĸB signaling regulated proteins.

STATEMENT OF SIGNIFICANCE

Silk fibroin has been effectively used as a dressing for wound treatment for more than a century. However, mechanistic insight into the basis for wound healing via silk fibroin has not been elucidated. Here we report a key mechanism involved in silk fibroin induced wound healing both in vitro and in vivo. Using genetic- and protein-level analyses, NF-κB signaling was found to regulate silk fibroin-induced wound healing by modulating target proteins. Thus, the NF-κB signaling pathway may be utilized as a therapeutic target during the formulation of silk fibroin-based biomaterials for wound healing and tissue engineering.

Enhanced osteogenesis on biofunctionalized poly(ɛ-caprolactone)/poly(m-anthranilic acid) nanofibers

Biofunctionalized nanofibers with a desired biological function can be used as a tissue engineering scaffold due to their small fiber diameters and porous structure. In the present study, poly(ɛ-caprolactone)/poly(m-anthranilic acid) nanofibers were biofunctionalized with covalent immobilization of bone morphogenetic protein-2 (BMP-2) through 1-ethyl-3-(dimethyl-aminopropyl) carbodiimide hydrochloride/N-hydroxysuccinimide activation. Fourier transform infrared analysis of the nanofiber surfaces confirmed the successful immobilization. The amount of immobilized BMP-2 was determined with bicinchoninic acid protein assay. The nanofibers before and after BMP-2 immobilization were non-cytotoxic and enhanced the attachment and proliferation of Saos-2 cells. Biofunctionalization of nanofibers with BMP-2 promoted in vitro osteogenic activity. The alkaline phosphatase activity and calcium mineralizatio of cells after 14 days of in vitro culture were enhanced on nanofibers with immobilized BMP-2.

Covalent streptavidin immobilization on electrospun poly(m-anthranilic acid)/polycaprolactone nanofibers and cytocompatibility

An electrospun poly( m-anthranilic acid)/poly(ε-caprolactone) nanofiber mat was fabricated with functionalization of the surface with streptavidin which can enhance the cell attachment and proliferation. Poly(ε-caprolactone) as biodegradable, biocompatible, and electrospinnable polymer was blended with poly( m-anthranilic acid) because of the carboxylic acid (–COOH) groups on its backbone which allow the covalent immobilization of streptavidin onto nanofibers. 1-Ethyl-3-(dimethyl-aminopropyl) carbodiimide hydrochloride/ N-hydroxyl succinimide coupling reaction was used for immobilization and the presence of bound protein was investigated by Fourier transform infrared–attenuated total reflection spectroscopy and electrochemical impedance spectroscopy, as well as the confocal microscopy. Human osteoblast-like cells (SaOS2) were cultured on poly( m-anthranilic acid)/poly(ε-caprolactone) and streptavidin-immobilized poly( m-anthranilic acid)/poly(ε-caprolactone) nanofibers to evaluate the in vitro biocompatibility of nanofibers. Fluorescence staining of F-actin was performed to observe the cell morphology. The results confirmed the successful immobilization of streptavidin on the nanofibers and streptavidin immobilization enhanced the cell attachment and proliferation onto the poly( m-anthranilic acid)/poly(ε-caprolactone) nanofibers.

Preparation and characterization of silk fibroin/poly(l-lactide-co-ε-caprolactone) nanofibrous membranes for tissue engineering applications

In recent years, silk fibroin has become one of the most promising tissue engineering materials because of its excellent cytocompatibility. Poly(l-lactide-co-ε-caprolactone), the copolymer of poly(l-lactide) and poly(ε-caprolactone), possesses good mechanical properties, and its degradation rates can be manipulated by varying the ratio of the constituent polymers. In this study, in order to combine their respective characteristics, silk fibroin/poly(l-lactide-co-ε-caprolactone) nanofibrous membranes were fabricated through electrospinning with different mass ratios of 100:0, 75:25, 50:50, 25:75, and 0:100. The surface properties, thermodynamic properties, mechanical properties, and cytocompatibility of silk fibroin/poly(l-lactide-co-ε-caprolactone) blended membranes were evaluated, and an optimal blending ratio was identified. The results showed that the silk fibroin/poly(l-lactide-co-ε-caprolactone) blended membranes containing 75% of silk fibroin and 25% of poly(l-lactide-co-ε-caprolactone) achieved the most improved performances compared with the single-component membranes or the blended membranes with other mixing ratios. The results from this study indicated that 75/25 silk fibroin/poly(l-lactide-co-ε-caprolactone) blended membranes which combined the advantages of poly(l-lactide-co-ε-caprolactone) and silk fibroin might be a suitable candidate material for use in tissue engineering.