Self-Healing, Self-Adhesive Silk Fibroin Conductive Hydrogel as a Flexible Strain Sensor.

Flexible and wearable hydrogel strain sensors have attracted tremendous attention for applications in human motion and physiological signal monitoring. However, it is still a great challenge to develop a hydrogel strain sensor with certain mechanical properties and tensile deformation capabilities, which can be in conformal contact with the target organ and also have self-healing properties, self-adhesive capability, biocompatibility, antibacterial properties, high strain sensitivity, and stable electrical performance. In this paper, an ionic conductive hydrogel (named PBST) is rationally designed by proportionally mixing polyvinyl alcohol (PVA), borax, silk fibroin (SF), and tannic acid (TA). SF can not only be a reinforcement to introduce an energy dissipation mechanism into the dynamically cross-linked hydrogel network to stabilize the non-Newtonian behavior of PVA and borax but it can also act as a cross-linking agent to combine with TA to reduce the dissociation of TA on the hydrogel network, improving the mechanical properties and viscoelasticity of the hydrogel. The combination of SF and TA can improve the self-healing ability of the hydrogel and realize the adjustable viscoelasticity of the hydrogel without sacrificing other properties. The obtained hydrogel has excellent stretchability (strain > 1000%) and shows good conformal contact with human skin. When the hydrogel is damaged by external strain, it can rapidly self-repair (mechanical and electrical properties) without external stimuli. It shows adhesiveness and repeatable adhesiveness to different materials (steel, wood, PTFE, glass, iron, and cotton fabric) and biological tissues (pigskin) and is easy to peel off without residue. The obtained PBST conductive hydrogel also has a wide strain-sensing range (>650%) and reliable stability. The hydrogel adhered to the skin surface can monitor large strain movements such as in finger joints, wrist joints, knee joints, and so on and detect swallowing, smiling, facial bulging and calming, and other micro-deformation behaviors. It can also distinguish physical signals such as light smile, big laugh, fast and slow breathing, and deep and shallow breathing. Therefore, the PBST conductive hydrogel material with multiple synergistic functions has great potential as a flexible wearable strain sensor. The PBST hydrogel has antibacterial properties and good biocompatibility at the same time, which provides a safety guarantee for it as a flexible wearable strain sensor. This work is expected to provide a new way for people to develop ideal wearable strain sensors.

Facile preparation of a strong chitosan-silk biocomposite film.

Chitosan-silk biocomposite films with nanofibrous structures have been prepared by facile solution casting of chitosan and silk co-dissolved in formic acid. The morphology, structure and mechanical properties of the chitosan-silk biocomposite were characterized by SEM, FTIR, TG-DSC, and mechanical testing. The results demonstrate that the prepared biocomposite films with a chitosan-silk ratio of 3:1 shows a high tensile strength of 97.8 MPa, a strain at break of 10.8% and a Young's modulus of 3.5 GPa, indicating its high strength and elasticity. Also, the preliminary cell culture experiment demonstrated the ideal biocompatibility of chitosan-silk composite films. As a result the superior mechanical properties of this composite film can be attributed to the silk nanofibrils and chitosan self-assembled nanofibers, and the strong hydrogen bonding interaction between the silk nanofibril and chitosan nanofibers. The specific nanostructure, enhanced mechanical properties, and biocompatibility make the biocomposite films a promising material for applications in biomedical devices.

Different angiogenesis modes and endothelial responses in implanted porous biomaterials.

An in vivo experimental model based on implanting porous biomaterials to study angiogenesis was proposed. In the implanted porous polyvinyl alcohol, three major modes of angiogenesis, sprouting, intussusception and splitting, were found. By electron microscopy and three-dimensional simulation of the angiogenic vessels, we investigated the morphological characteristics of the three modes and paid special attention to the initial morphological difference between intussusception and splitting, and it was confirmed that the endothelial abluminal invagination and intraluminal protrusion are pre-representations of intussusception and splitting, respectively. Based on immunohistochemical analysis of HIF-1α, VEGF and Flt-1 expressions, it was demonstrated that the dominant mode of angiogenesis is related to the local hypoxic condition, and that there is difference in the response of endothelial cells to hypoxia-induced VEGF between sprouting and splitting. Specifically, in the biomaterials implanted for 3 days, the higher expression and gradient of VEGF induced by severe hypoxia in the avascular area caused sprouting of the peripheral capillaries, and in the biomaterial implanted for 9 days, with moderate hypoxia, splitting became a dominant mode. Whether on day 3 or day 9, Flt-1 expression in sprouting endothelia was significantly higher than that in splitting endothelia, which indicates that sprouting is caused by the strong response of endothelial cells to VEGF, while splitting is associated with their weaker response. As a typical experimental example, these results show the effectiveness of the porous biomaterial implantation model for studying angiogenesis, which is expected to become a new general model.

Silk fibroin/sodium alginate fibrous hydrogels regulated hydroxyapatite crystal growth.

Use of organic templates for controlling the growth of inorganic crystals is one of the research topics in biomimetic field. In particular, oriented growth of hydroxyapatite (HAp) in organic fibrous matrix is provided a new view angle to study biomineralization of bone and its potential biomedical applications. The crystallization of HAp in fibrous hydrogels could mimic such biomineralization. In this paper, we report HAp nanorod crystal synthesized successfully by a biomimetic method using calcium chloride and ammonium dihydrogen phosphate as reagents in the presence of silk fibroin/sodium alginate (SF/SA) fibrous hydrogels. The effects of influence factors such as mineral times, pH, and temperature on controlling HAp nanorod crystals are discussed. The elongated HAp nanorods with rectangular column are grown with the increase of mineral times in biomimetic process. By changing pH, HAp nanorod crystals are obtained at alkaline condition in fibrous hydrogels. Moreover, compared to other temperatures, rod-shaped HAp crystals were formed at 20°C. The results imply this to be an effective method for preparing HAp crystals with controllable morphology for bone repair application.

Silk fibroin/copolymer composite hydrogels for the controlled and sustained release of hydrophobic/hydrophilic drugs.

In the present study, a composite system for the controlled and sustained release of hydrophobic/hydrophilic drugs is described. Composite hydrogels were prepared by blending silk fibroin (SF) with PLA-PEG-PLA copolymer under mild aqueous condition. Aspirin and indomethacin were incorporated into SF/Copolymer hydrogels as two model drugs with different water-solubility. The degradation of composite hydrogels during the drug release was mainly caused by the hydrolysis of copolymers. SF with stable ß-sheet-rich structure was not easily degraded which maintained the mechanical integrity of composite hydrogel. The hydrophobic/hydrophilic interactions of copolymers with model drugs would significantly alter the morphological features of composite hydrogels. Various parameters such as drug load, concentration ratio, and composition of copolymer were considered in vitro drug release. Aspirin as a hydrophilic drug could be controlled release from composite hydrogel at a constant rate for 5 days. Its release was mainly driven by diffusion-based mechanism. Hydrophobic indomethacin could be encapsulated in copolymer nanoparticles distributing in the composite hydrogel. Its sustained release was mainly degradation controlled which could last up to two weeks. SF/Copolymer hydrogel has potential as a useful composite system widely applying for controlled and sustained release of various drugs.

Facile fabrication of robust silk nanofibril films via direct dissolution of silk in CaCl2-formic acid solution.

In this study, we report for the first time a novel silk fibroin (SF) nanofibrous films with robust mechanical properties that was fabricated by directly dissolving silk in CaCl2-formic acid solution. CaCl2-FA dissolved silk rapidly at room temperature, and more importantly, it disintegrated silk into nanofibrils instead of separate molecules. The morphology of nanofibrils crucially depended on CaCl2 concentrations, which resulted in different aggregation nanostructure in SF films. The SF film after drawing had maximum elastic modulus, ultimate tensile strength, and strain at break reaching 4 GPa, 106 MPa, and 29%, respectively, in dry state and 206 MPa, 28 MPa, and 188%, respectively, in wet state. Moreover, multiple yielding phenomena and substantially strain-hardening behavior was also observed in the stretched films, indicating the important role played by preparation method in regulating the mechanical properties of SF films. These exceptional and unique mechanical properties were suggested to be caused by preserving silk nanofibril during dissolution and stretching to align these nanofibrils. Furthermore, the SF films exhibit excellent biocompatibility, supporting marrow stromal cells adhesion and proliferation. The film preparation was facile, and the resulting SF films manifested enhanced mechanical properties, unique nanofibrous structures, and good biocompability.

[Application of silk fibroin scaffold in bone tissue engineering].

OBJECTIVE: To review the application of silk fibroin scaffold in bone tissue engineering. METHODS: The related literature about the application of silk fibroin scaffold in bone tissue engineering was reviewed, analyzed, and summarized. RESULTS: Silk fibroin can be manufactured into many types, such as hydrogel, film, nano-fiber, and three-dimensional scaffold, which have superior biocompatibility, slow biodegradability, nontoxic degradation products, and excellent mechanical strength. Meanwhile these silk fibroin biomaterials can be chemically modified and can be used to carry stem cells, growth factors, and compound inorganic matter. CONCLUSION: Silk fibroin scaffolds can be widely used in bone tissue engineering. But it still needs further study to prepare the scaffold in accordance with the requirement of tissue engineering.

Evaluation of electronspun silk fibroin-based transplants used for facial nerve repair.

OBJECTIVE: To study whether regenerated electrospun silk fibroin (SF) nanofibers as nerve conduits could improve nerve regeneration microenvironment and induce the facial nerve regeneration of Sprague-Dawley rats. DESIGN: Electrospun SF nanofibers were prepared to bridge a 5-mm facial nerve defect in Sprague-Dawley rats. Three months after implantation, a comprehensive morphologic and functional evaluation was performed by electrophysiology, histology, Fluorogold retrograde tracing, and transmission electron micrograph. RESULTS: The SF nanofiber tube exhibited good biocompatibility in vivo, and no distinct regional inflammation response and scar formation was observed. After 3 months of operation, the morphologic and functional investigation has shown a positive evaluation on the nerve repair outcome elicited by SF nanofiber graft and autograft. CONCLUSION: Electrospun SF grafts could promote nerve regeneration after facial nerve injury and become a potential possibility of newly developed nerve grafts as an alternative of autografts to peripheral nerve regeneration.

Electrospun silk fibroin nanofibers in different diameters support neurite outgrowth and promote astrocyte migration.

Nerve tissue engineering has been one of the promising strategies for regenerative treatment in patients suffering from neural tissue loss, but considerable challenges remain before it is able to progress toward clinical application. It has been demonstrated that transplantation of cells in combination with physically or chemically modified biomaterials provides better environments for neurite outgrowth and further promotes axonal regeneration in animal models of spinal cord injury. In this study, neurons and astrocytes were incorporated into 400-nm, 800-nm, and 1200-nm electrospun Bombyx mori silk fibroin (SF) materials to investigate the effects of scaffold-diameter in regulating and directing cell behaviors. ß-III-tubulin immunofluorescence analyses reveal that SF nanofibers with smaller diameters are more favorable to the development and maturation of subventricular zone-derived neurons than 1200-nm SF scaffolds. In addition, astrocytes exhibited well-arranged glial fibrillary acidic protein (GFAP) expression on SF scaffolds, and a significant increase in cell-spreading area was observed on 400-nm but not 1200-nm SF scaffolds. Moreover, a significantly enhanced migration efficiency of astrocytes grown on SF scaffolds was verified, which highlights the guiding roles of SF nanofibers to the migratory cells. Overall, our results may provide valuable information to develop effective tissue remodeling substrates and to optimize existing biomaterials for neural tissue engineering applications.

Studies of in situ-forming hydrogels by blending PLA-PEG-PLA copolymer with silk fibroin solution.

Hydrogels had been prepared by blending PLA-PEG-PLA copolymer with Bombyx mori silk fibroin (SF) solution. Copolymers were synthesized by ring opening polymerization of L-lactide in the presence of dihydroxyl PEG with molar mass of 400 and 1000, and characterized by using (1)H NMR and DSC. Hydrogels formed leaf-like lamellar structures with many nanoglobules which may reserve drugs or growth factors more effectively. Rheological measurements indicated that the adding of copolymer significantly accelerated the hydrogelation of silk fibroin solution which leads to orders-of-magnitude increase in the complex shear modulus to form rigid hydrogel. Hydrogelation kinetics could be controlled easily by changing the concentration ratio, kinds of copolymer and hydrogelation temperature, suggesting the hydrogels could be formed in situ under physiological conditions with suitable mechanical properties. Furthermore, Fourier transform infrared, X-ray diffraction, and differential thermal analysis were employed to study the structure of hydrogels. The copolymer and SF in blend hydrogels were phase separation. There was an increase of ß-sheet content and formation of silk II structure during hydrogelation. These results may indicate that copolymer/SF hydrogels could be a valuable candidate scaffold as in situ-forming hydrogels for drug/growth factor release in tissue engineering.

The effects of electrospun TSF nanofiber diameter and alignment on neuronal differentiation of human embryonic stem cells.

Although transplantation of human embryonic stem cells (hESCs)-derived neural precursors (NPs) has been demonstrated with some success for nervous repair in small animal model, control of the survival, and directional differentiation of these cells is still challenging. Meanwhile, the notion that using suitable scaffolding materials to control the growth and differentiation of grafted hESC-derived NPs raises the hope for better clinical nervous repair. In this study, we cultured hESC-derived NPs on Tussah silk fibroin (TSF)-scaffold of different diameter (i.e., 400 and 800 nm) and orientation (i.e., random and aligned) to analyze the effect of fiber diameter and alignment on the cell viability, neuronal differentiation, and neurite outgrowth of hESC-derived NPs. The results show that TSF-scaffold supports the survival, migration, and differentiation of hESC-derived NPs. Aligned TSF-scaffold significantly promotes the neuronal differentiation and neurite outgrowth of hESC-derived neurons compared with random TSF-scaffold. Moreover, on aligned 400 nm fibers cell viability, neuronal differentiation and neurite outgrowth are greater than that on aligned 800 nm fibers. Together, these results demonstrate that aligned 400 nm TSF-scaffold is more suitable for the development of hESC-derived NPs, which shed light on optimization of the therapeutic potential of hESCs to be employed for neural regeneration.