Comparison of adipose derived stromal cells cultured on fibroin scaffolds fabricated by salt-leaching and by freeze-thawing

Fibroin, the main structural protein of Bombyx mori silk, is known for its mechanical properties, its biocompatibility and degradation characteristics in vivo. Various studies investigate its uses as cell carrier and/or material for surgical implants. Multiple protocols have been established to isolate fibroin from silk fibers and to produce scaffolds and films from fibroin solution. There is only limited literature available on how fibroin scaffolds manufactured by different methods compare to each other in terms of performance as cell carriers. This study compares the behaviour of human adipose derived stromal cells (ADSC) seeded on fibroin scaffolds produced by (i) salt-leaching and (ii) freeze-thawing. One type of freeze-thawing scaffold (poresize ≪ 315 μm) and three types of salt-leaching scaffolds (poresize ranging from 315 μm to 1000 μm) were used for this comparison. Measuring the DNA concentration on the seeded scaffolds as well as the seeded cells metabolic activity, we were able to determine freeze-thawed scaffolds to be superior for cell-seeding. ADSC seeded on salt-leaching scaffolds displayed a stronger downregulation of serum deprivation response gene than cells seeded on freeze-thaw scaffolds. In sum, our findings show that salt-leaching scaffolds offering different pore sizes differed much less among each other than salt-leaching from freeze-thawing scaffolds in terms of cell accommodation. Our work underlines the importance of physicochemical scaffold properties directly linked to different manufacturing methods and their influence on the cell seeding capacity of silk fibroin based carriers.

High Concentration Crystalline Silk Fibroin Solution for Silk-Based Materials

As a functional biomaterial, silk fibroin has been widely used in drug release, cell encapsulation and tissue regeneration. To meet the requirements of these applications, the properties of silk fibroin-based materials should be finely tunable. Many useful properties of biomaterials emerge from the collective interactions among ordered and disordered domains. Thus, increasing subtle control of silk hierarchical structures is required. As a characteristic of ordered silk fibroin, crystalline silk fibroin (CSF) is an important part of silk fibroin-based biomaterials, but the preparation of CSF solution, especially high concentration CSF solution, remains a challenge. Here, a solution composed of β-sheet-rich silk fibroin is reported. These CSF were obtained by the sonication of silk fibroin hydrogel, destroying the hydrogel network, and turning silk fibroin hydrogels into CSF solution. These β-sheet-rich CSF solutions were stable enough for several days or even weeks. In addition, they were typically ordered crystalline domains, which could be mixed with disordered domains and fabricated into porous scaffolds, films, hydrogels and other silk fibroin-based scaffolds with different properties.

Hierarchically 3-D Porous Structure of Silk Fibroin-Based Biocomposite Adsorbent for Water Pollutant Removal

This study explored the tunability of a 3-D porous network in a freeze-dried silk fibroin/soursop seed (SF:SS) polymer composite bioadsorbent. Morphological, physical, electronic, and thermal properties were assessed using scanning electron microscopy, the BET N2 adsorption-desorption test, Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). A control mechanism of pore opening–closing by tuning the SS fraction in SF:SS composite was found. The porous formation is apparently due to the amount of phytic acid as a natural cross-linker in SS. The result reveals that a large pore radius is formed using only 20% wt of SS in the composite, i.e., SF:SS (4:1), and the fibrous network closes the pore when the SS fraction increases up to 50%, i.e., SF:SS (1:1). The SF:SS (4:1) with the best physical and thermal properties shows an average pore diameter of 39.19 nm, specific surface area of 19.47 m2·g−1, and thermal stability up to ~450 °C. The removal of the organic molecule and the heavy metal was assessed using crystal violet (CV) dye and the Cu2+ adsorption test, respectively. The adsorption isotherm of both CV and Cu2+ on SF:SS (4:1) follows the Freundlich model, and the adsorption kinetic of CV follows the pseudo-first-order model. The adsorption test indicates that physisorption dominates the adsorption of either CV or Cu2+ on the SF:SS composites.

Functionalized Silk Vascular Grafts with Decellularized Human Wharton's Jelly Improves Remodeling via Immunomodulation in Rabbit Jugular Vein

Cell-free polymeric tissue-engineered vascular grafts (TEVGs) have shown great promise towards clinical translation; however, their limited bioactivity and remodeling ability challenge this cause. Here, a novel cell-free bioresorbable small diameter silk TEVG system functionalized with decellularized human Wharton's jelly (dWJ) matrix is developed and successfully implanted as interposition grafts into rabbit jugular vein. Implanted TEVGs remain patent for two months and integrate with host tissue, demonstrating neo-tissue formation and constructive remodeling. Mechanistic analysis reveals that dWJ matrix is a reservoir of various immunomodulatory cytokines (Interleukin-8, 6, 10, 4 and tumor necrosis factor alpha (TNF-α)), which aids in upregulating M2 macrophage-associated genes facilitating pro-remodeling behavior. Besides, dWJ treatment to human endothelial cells upregulates the expression of functional genes (cluster of differentiation 31 (CD31), endothelial nitric oxide synthase (eNOS), and vascular endothelial (VE)-cadherin), enables faster cell migration, and elevates nitric oxide (NO) production leading to the in situ development of endothelium. The dWJ functionalized silk TEVGs support increased host cell recruitment than control, including macrophages and vascular cells. It endows superior graft remodeling in terms of a dense medial layer comprising smooth muscle cells and elevates the production of extracellular matrix proteins (collagen and elastin). Altogether, these findings suggest that dWJ functionalization imitates the usefulness of cell seeding and enables graft remodeling.

Silk Fibroin as a Green Material

Silk fibroin has been explored as a suitable biomaterial due to its biocompatibility, tunable degradability, low toxicity, and mechanical properties. To harness silk fibroin's innate properties, it is purified from native silkworm cocoons by removing proteins and debris that have the potential to cause inflammatory responses. Typically, within the purification and fabrication steps, chemical solvents, energy-intensive equipment, and large quantities of water are used to reverse engineer silk fibroin into an aqueous solution and then process into the final material format. Gentler, green methods for extraction and fabrication have been developed that reduce or remove the need for harmful chemical additives and energy-inefficient equipment while still producing mechanically robust biomaterials. This review will focus on the alternative green processing and fabrication methods that have proven useful in creating silk fibroin materials for a range of applications including consumer and medical materials.

Tanshinone IIA Delivery Silk Fibroin Scaffolds Significantly Enhance Articular Cartilage Defect Repairing via Promoting Cartilage Regeneration

Cartilage tissue engineering is a promising approach for repairing articular cartilage defects and requires proper scaffolds and necessary growth factors. Herein, tanshinone IIA (TAN) delivery silk fibroin scaffolds were prepared for efficient cartilage defect repair by bioactivities of TAN. By incubating with the TAN delivery silk fibroin scaffold, the transcription of the chondrocytic activity-related genes was enhanced in chondrocytes, and it also can inhibit cell apoptosis and reduce the oxidative stress by regulating the transcription of related genes, indicating that these scaffolds may promote cartilage regeneration. TAN10 delivery silk fibroin scaffolds, in which the concentration of TAN is 10 μg/mL, significantly promotes chondrocytes to generate the cartilage-specific extracellular matrix and tissue both in vitro and in vivo, compared with silk fibroin scaffolds. By treating rabbit articular cartilage defects with TAN10 delivery silk fibroin scaffolds, cartilage defects were filled with hyaline-cartilage-like tissue that integrated with the surrounding cartilage perfectly and displayed strong mechanical properties and higher extracellular matrix content. Hence, TAN facilitates cartilage regeneration, and TAN delivery silk fibroin scaffolds can be potentially applied in the clinics treating cartilage defects in the future.

Freezing-induced silk I crystallization of silk fibroin

Water-insoluble silk fibroin materials with the silk I structure can be prepared by a simple and green freezing–annealing treatment.

Soft freezing-induced self-assembly of silk fibroin for tunable gelation

Silk fibroin (SF) hydrogel is a promising candidate in biomaterial field; however its application is quite limited by long-gelation time. In the present study, we developed a novel strategy named soft freezing to accelerate the process and control the sol-gel transition of SF protein. SF protein was induced to self-assembly by soft freezing process for achieving the reconstructed SF solution with metastable structure. It was found that the soft freezing process triggers the structural transition from random structure to ordered structure-rich conformation. Gelation kinetics showed that the gelation time of SF protein could be regulated by changing freezing time and initial concentration. The reconstructed SF solution allowed enhanced sol-gel transition within 6 hours, even at extremely low concentration. The attractive features of the method described here include the accelerated gelation, free of chemical agents, and reducing processing complexity. The SF solution with short gelation time will be applicable as cell encapsulation and injectable applications for tissue engineering and regenerative medicine, which greatly expand the applications of SF hydrogels.

Biomimetic Silk Scaffolds with an Amorphous Structure for Soft Tissue Engineering

Fine tuning physical cues of silk fibroin (SF) biomaterials to match specific requirements for different soft tissues would be advantageous. Here, amorphous SF nanofibers were used to fabricate scaffolds with better hierarchical extracellular matrix (ECM) mimetic microstructures than previous silk scaffolds. Kinetic control was introduced into the scaffold forming process, resulting in the direct production of water-stable scaffolds with tunable secondary structures and thus mechanical properties. These biomaterials remained with amorphous structures, offering softer properties than prior scaffolds. The fine mechanical tunability of these systems provides a feasible way to optimize physical cues for improved cell proliferation and enhanced neovascularization in vivo. Multiple physical cues, such as partly ECM mimetic structures and optimized stiffness, provided suitable microenvironments for tissue ingrowth, suggesting the possibility of actively designing bioactive SF biomaterials. These systems suggest a promising strategy to develop novel SF biomaterials for soft tissue repair and regenerative medicine.