Electrospun scaffolds from silk fibroin and their cellular compatibility

Dry and wet wrinkling of a silk fibroin biopolymer by a shape-memory material with insight into mechanical effects on secondary structures in the silk network

Surface wrinkling provides an approach to modify the surfaces of biomedical devices to better mimic features of the extracellular matrix and guide cell attachment, proliferation, and differentiation. Biopolymer wrinkling on active materials holds promise but is poorly explored. Here we report a mechanically actuated assembly process to generate uniaxial micro-and nanosized silk fibroin (SF) wrinkles on a thermo-responsive shape-memory polymer (SMP) substrate, with wrinkling demonstrated under both dry and hydrated (cell compatible) conditions. By systematically investigating the influence of SMP programmed strain magnitude, film thickness, and aqueous media on wrinkle stability and morphology, we reveal how to control the wrinkle sizes on the micron and sub-micron length scale. Furthermore, as a parameter fundamental to SMPs, we demonstrate that the temperature during the recovery process can also affect the wrinkle characteristics and the secondary structures in the silk network. We find that with increasing SMP programmed strain magnitude, silk wrinkled topographies with increasing wavelengths and amplitudes are achieved. Furthermore, silk wrinkling is found to increase β-sheet content, with spectroscopic analysis suggesting that the effect may be due primarily to tensile (e.g., Poisson effect and high-curvature wrinkle) loading modes in the SF, despite the compressive bulk deformation (uniaxial contraction) used to produce wrinkles. Silk wrinkles fabricated from sufficiently thick films (roughly 250 nm) persist after 24 h in cell culture medium. Using a fibroblast cell line, analysis of cellular response to the wrinkled topographies reveals high viability and attachment. These findings demonstrate use of wrinkled SF films under physiologically relevant conditions and suggest the potential for biopolymer wrinkles on biomaterials surfaces to find application in cell mechanobiology, wound healing, and tissue engineering.

Facile In Situ Assembly of Nanofibers within Three-Dimensional Porous Matrices with Arbitrary Characteristics for Creating Biomimetic Architectures

It is challenging to recapitulate the natural extracellular matrix's hierarchical nano/microfibrous three-dimensional (3D) structure with multilevel pores, good mechanical and hydrophilic properties, and excellent bioactivity for designing and developing advanced biomimetic materials. This work reports a new facile strategy for the scalable manufacturing of such a 3D architecture. Natural polymers in an aqueous solution are interpenetrated into a 3D microfibrous matrix with arbitrary shapes and property characteristics to self-assemble in situ into a nanofibrous network. The collagen fiber-like hierarchical structure and interconnected multilevel pores are achieved by self-assembly of the formed nanofibers within the 3D matrix, triggered by a simple cross-linking treatment. The as-prepared alginate/polypropylene biomimetic matrices are bioactive and have a tunable mechanical property (compressive modulus from ∼17 to ∼24 kPa) and a tunable hydrophilicity (water contact angle from ∼94° to 63°). This facile and versatile strategy allows eco-friendly and scalable manufacturing of diverse biomimetic matrices or modification of any existing porous matrices using different polymers.

Nitric Oxide-Releasing Nanofibrous Scaffolds Based on Silk Fibroin and Zein with Enhanced Biodegradability and Antibacterial Properties

Clinical applications of scaffolds and implants have been associated with bacterial infection resulting in impaired tissue regeneration. Nanofibers provide a versatile structure for both antimicrobial molecule delivery and tissue engineering. In this study, the nitric oxide (NO) donor molecule S-nitrosoglutathione (GSNO) and the natural biodegradable polymer zein (ZN) were combined with silk fibroin (SF) to develop antibacterial and biodegradable nanofibrous scaffolds for tissue engineering applications. The compatibility and intermolecular interactions of SF and ZN were studied using differential scanning calorimetry and Fourier transform infrared spectroscopy. The incorporation of ZN increased the hydrophobicity of the fibers and resulted in a more controlled and prolonged NO release profile lasting for 48 h. Moreover, the degradation kinetics of the fibers was significantly improved after blending with ZN. The results of tensile testing indicated that the addition of ZN and GSNO had a positive effect on the strength and stretchability of SF fibers and did not adversely affect their mechanical properties. Finally, due to the antibacterial properties of both NO and ZN, the SF-ZN-GSNO fibers showed a synergistically high antibacterial efficacy with 91.6 ± 2.5% and 77.5 ± 3.1% reduction in viability of adhered Staphylococcus aureus and Escherichia coli after 24 h exposure, respectively. The developed NO-releasing fibers were not only antibacterial but also non-cytotoxic and successfully enhanced the proliferation and growth of fibroblast cells, which was quantitatively studied by a CCK-8 assay and visually observed through fluorescent staining. Overall, SF-ZN-GSNO fibers developed in this study were biodegradable and highly antibacterial and showed great cytocompatibility with fibroblasts, indicating their promising potential for a range of tissue engineering and medical device applications.

Exosomes of adult human fibroblasts cultured on 3D silk fibroin nonwovens intensely stimulate neoangiogenesis

Background

Bombyx mori silk fibroin is a biomacromolecule that allows the assembly of scaffolds for tissue engineering and regeneration purposes due to its cellular adhesiveness, high biocompatibility and low immunogenicity. Earlier work showed that two types of 3D silk fibroin nonwovens (3D-SFnws) implanted into mouse subcutaneous tissue were promptly vascularized via undefined molecular mechanisms. The present study used nontumorigenic adult human dermal fibroblasts (HDFs) adhering to a third type of 3D-SFnws to assess whether HDFs release exosomes whose contents promote neoangiogenesis.

Methods

Electron microscopy imaging and physical tests defined the features of the novel carded/hydroentangled 3D-SFnws. HDFs were cultured on 3D-SFnws and polystyrene plates in an exosome-depleted medium. DNA amounts and D-glucose consumption revealed the growth and metabolic activities of HDFs on 3D-SFnws. CD9-expressing total exosome fractions were from conditioned media of 3D-SFnws and 2D polystyrene plates HDF cultures. Angiogenic growth factors (AGFs) in equal amounts of the two groups of exosomal proteins were analysed via double-antibody arrays. A tube formation assay using human dermal microvascular endothelial cells (HDMVECs) was used to evaluate the exosomes’ angiogenic power.

Results

The novel features of the 3D-SFnws met the biomechanical requirements typical of human soft tissues. By experimental day 15, 3D-SFnws-adhering HDFs had increased 4.5-fold in numbers and metabolized 5.4-fold more D-glucose than at day 3 in vitro. Compared to polystyrene-stuck HDFs, exosomes from 3D-SFnws-adhering HDFs carried significantly higher amounts of AGFs, such as interleukin (IL)-1α, IL-4 and IL-8; angiopoietin-1 and angiopoietin-2; angiopoietin-1 receptor (or Tie-2); growth-regulated oncogene (GRO)-α, GRO-β and GRO-γ; matrix metalloproteinase-1; tissue inhibitor metalloproteinase-1; and urokinase-type plasminogen activator surface receptor, but lesser amounts of anti-angiogenic tissue inhibitor metalloproteinase-2 and pro-inflammatory monocyte chemoattractant protein-1. At concentrations from 0.62 to 10 μg/ml, the exosomes from 3D-SFnws-cultured HDFs proved their angiogenic power by inducing HDMVECs to form significant amounts of tubes in vitro.

Conclusions

The structural and mechanical properties of carded/hydroentangled 3D-SFnws proved their suitability for tissue engineering and regeneration applications. Consistent with our hypothesis, 3D-SFnws-adhering HDFs released exosomes carrying several AGFs that induced HDMVECs to promptly assemble vascular tubes in vitro. Hence, we posit that once implanted in vivo, the 3D-SFnws/HDFs interactions could promote the vascularization and repair of extended skin wounds due to burns or other noxious agents in human and veterinary clinical settings.

Evaluation of a simple off-the-shelf bi-layered vascular scaffold based on poly(L-lactide-co-ε-caprolactone)/silk fibroin in vitro and in vivo

Purpose: In the field of small-caliber vascular scaffold research, excellent vascular remodeling is the key to ensuring anticoagulant function. We prepared an off-the-shelf bi-layered vascular scaffold with a dense inner layer and a loose outer layer and evaluated its remodeling capabilities by in vivo transplantation.

Materials and Methods: Based on poly(L-lactide-co-ε-caprolactone) (PLCL), silk fibroin(SF), and heparin (Hep), PLCL/SF/Hep bi-layered scaffolds and PLCL/Hep bi-layered scaffolds were prepared by electrospinning. The inner layer was a PLCL/SF/Hep or PLCL/Hep nanofiber membrane, and the outer layer was PLCL/SF nano yarn. The in vitro tests included a hydrophilicity test, mechanical properties test, and blood and cell compatibility evaluation. The in vivo evaluation was conducted via single rabbit carotid artery replacement and subsequent examinations, including ultrasound imaging, immunoglobulin assays, and tissue section staining.

Results: Compared to the PLCL/Hep nanofiber membrane, the hydrophilicity of the PLCL/SF/Hep nanofiber membrane was significantly improved. The mechanical strength met application requirements. Both the blood and cell compatibility were optimal. Most importantly, the PLCL/SF/Hep scaffolds maintained lumen patency for 3 months after carotid artery transplantation in live rabbits. At the same time, CD31 and α-SMA immunofluorescence staining confirmed bionic endothelial and smooth muscle layers remodeling.

Conclusion: Using this hybrid strategy, PLCL and SF were combined to manufacture bi-layered small-caliber vascular scaffolds; these PLCL/SF/Hep scaffolds showed satisfactory vascular remodeling.

Silk fibroin/poly(L-lactic acid-co-ε-caprolactone) electrospun nanofibrous scaffolds exert a protective effect following myocardial infarction

Electrospinning using biocompatible polymer scaffolds, seeded with or without stem cells, is considered a promising technique for producing fibrous scaffolds with therapeutic possibilities for ischemic heart disease. However, no optimal scaffolds for treating ischemic heart disease have been identified thus far. In the present study, it was evaluated whether electrospun silk fibroin (SF)-blended poly(L-lactic acid-co-ε-caprolactone) [P(LLA-CL)] scaffolds that were seeded with cluster of differentiation 117 (c-kit)⁺ bone marrow (BM) cells may serve a protective role in cardiac remodeling following myocardial infarction (MI). Mechanical characteristics and cytocompatibility were compared between SF/P(LLA-CL) and P(LLA-CL) electrospun nanofibrous scaffolds in vitro. It was observed that MI led to a significant increase of the c-kit⁺ BM cell subpopulation in mice. Magnetic activated cell sorting was performed to harvest the c-kit⁺ cell population from the BM of mice following MI. c-kit⁺ BM cells were seeded on SF/P(LLA-CL) and P(LLA-CL) electrospun nanofibrous scaffolds. Results indicated that SF/P(LLA-CL) electrospun nanofibrous scaffolds were superior to P(LLA-CL) electrospun nanofibrous scaffolds in improving c-kit⁺ BM cell proliferation. Additionally, compared with pure SF/P(LLA-CL) electrospun nanofibrous scaffolds, SF/P(LLA-CL) scaffolds seeded with c-kit⁺ BM cells resulted in lower levels of MI markers and reduced infarct size, leading to greater global heart function improvement in vivo. The findings of the present study indicated that SF/P(LLA-CL) electrospun nanofibrous scaffolds seeded with c-kit⁺ BM cells exert a protective effect against MI and may be a promising approach for cardiac regeneration after ischemic heart disease.

Silk nanofibril self-assembly versus electrospinning

Natural silk fibers represent one of the most advanced blueprints for (bio)polymer scientists, displaying highly optimized mechanical properties due to their hierarchical structures. Biotechnological production of silk proteins and implementation of advanced processing methods enabled harnessing the potential of these biopolymer not just based on the mechanical properties. In addition to fibers, diverse morphologies can be produced, such as nonwoven meshes, films, hydrogels, foams, capsules and particles. Among them, nanoscale fibrils and fibers are particularly interesting concerning medical and technical applications due to their biocompatibility, environmental and mechanical robustness as well as high surface-to-volume ratio. Therefore, we introduce here self-assembly of silk proteins into hierarchically organized structures such as supramolecular nanofibrils and fabricated materials based thereon. As an alternative to self-assembly, we also present electrospinning a technique to produce nanofibers and nanofibrous mats. Accordingly, we introduce a broad range of silk-based dopes, used in self-assembly and electrospinning: natural silk proteins originating from natural spinning glands, natural silk protein solutions reconstituted from fibers, engineered recombinant silk proteins designed from natural blueprints, genetic fusions of recombinant silk proteins with other structural or functional peptides and moieties, as well as hybrids of recombinant silk proteins chemically conjugated with nonproteinaceous biotic or abiotic molecules. We highlight the advantages but also point out drawbacks of each particular production route. The scope includes studies of the natural self-assembly mechanism during natural silk spinning, production of silk fibrils as new nanostructured non-native scaffolds allowing dynamic morphological switches, as well as studying potential applications. This article is categorized under:  Biology-Inspired Nanomaterials > Peptide-Based Structures  Nanotechnology Approaches to Biology > Nanoscale Systems in Biology  Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Characterization of Cell Scaffolds by Atomic Force Microscopy

This review reports on the use of the atomic force microscopy (AFM) in the investigation of cell scaffolds in recent years. It is shown how the technique is able to deliver information about the scaffold surface properties (e.g., topography), as well as about its mechanical behavior (Young's modulus, viscosity, and adhesion). In addition, this short review also points out the utilization of the atomic force microscope technique beyond its usual employment in order to investigate another type of basic questions related to materials physics, chemistry, and biology. The final section discusses in detail the novel uses that those alternative measuring modes can bring to this field in the future.

Fenugreek Incorporated Silk Fibroin Nanofibers-A Potential Antioxidant Scaffold for Enhanced Wound Healing

Free radicals are generated by various biochemical pathways in the living system, causing severe oxidative damage to the biomolecules leading to adverse disease conditions. Hence, there is an increasing interest in antioxidant studies for preventing the effects of these free radicals. Herein, we propose a novel electrospun scaffold with antioxidant properties that can be used as wound healing material. Fenugreek, a natural antioxidant incorporated silk fibroin nanofiber, was prepared in four different ratios by the co-electrospinning method. The biocompatibility of the nanofibers and its antioxidant activity were evaluated through 3-(4, 5-dimethylthiazol-2-yl)-diphenyltetrazolium bromide (MTT) assay and 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging assay, respectively. The experimental observations indicate that the incorporation of fenugreek increases the thermal and mechanical properties of silk fibroin nanofibers. DPPH assay proves that the antioxidant property is enhanced with increasing concentration of fenugreek in nanofiber mats, and the Swiss albino 3T6 fibroblasts show better proliferation on the nanofibrous scaffolds. Further, the wound healing efficiency of fenugreek incorporated silk fibroin nanofibrous scaffolds was evaluated using full thickness excisional wounds in rat model. Wound healing was accelerated in silk fibroin-fenugreek nanofibers treated wounds with complete re-epithelialization and enhanced collagen deposition. The present study validates the use of fenugreek incorporated silk fibroin nanofiber mats as antioxidant scaffolds in wound healing applications.

Treatment of Silk Fibroin with Poly(ethylene glycol) for the Enhancement of Corneal Epithelial Cell Growth

A silk protein, fibroin, was isolated from the cocoons of the domesticated silkworm (Bombyx mori) and cast into membranes to serve as freestanding templates for tissue-engineered corneal cell constructs to be used in ocular surface reconstruction. In this study, we sought to enhance the attachment and proliferation of corneal epithelial cells by increasing the permeability of the fibroin membranes and the topographic roughness of their surface. By mixing the fibroin solution with poly(ethylene glycol) (PEG) of molecular weight 300 Da, membranes were produced with increased permeability and with topographic patterns generated on their surface. In order to enhance their mechanical stability, some PEG-treated membranes were also crosslinked with genipin. The resulting membranes were thoroughly characterized and compared to the non-treated membranes. The PEG-treated membranes were similar in tensile strength to the non-treated ones, but their elastic modulus was higher and elongation lower, indicating enhanced rigidity. The crosslinking with genipin did not induce a significant improvement in mechanical properties. In cultures of a human-derived corneal epithelial cell line (HCE-T), the PEG treatment of the substratum did not improve the attachment of cells and it enhanced only slightly the cell proliferation in the longer term. Likewise, primary cultures of human limbal epithelial cells grew equally well on both non-treated and PEG-treated membranes, and the stratification of cultures was consistently improved in the presence of an underlying culture of irradiated 3T3 feeder cells, irrespectively of PEG-treatment. Nevertheless, the cultures grown on the PEG-treated membranes in the presence of feeder cells did display a higher nuclear-to-cytoplasmic ratio suggesting a more proliferative phenotype. We concluded that while the treatment with PEG had a significant effect on some structural properties of the B. mori silk fibroin (BMSF) membranes, there were minimal gains in the performance of these materials as a substratum for corneal epithelial cell growth. The reduced mechanical stability of freestanding PEG-treated membranes makes them a less viable choice than the non-treated membranes.

Effect of nanofiber content on bone regeneration of silk fibroin/poly(ε-caprolactone) nano/microfibrous composite scaffolds

The broad application of electrospun nanofibrous scaffolds in tissue engineering is limited by their small pore size, which has a negative influence on cell migration. This disadvantage could be significantly improved through the combination of nano- and microfibrous structure. To accomplish this, different nano/microfibrous scaffolds were produced by hybrid electrospinning, combining solution electrospinning with melt electrospinning, while varying the content of the nanofiber. The morphology of the silk fibroin (SF)/poly(ε-caprolactone) (PCL) nano/microfibrous composite scaffolds was investigated with field-emission scanning electron microscopy, while the mechanical and pore properties were assessed by measurement of tensile strength and mercury porosimetry. To assay cell proliferation, cell viability, and infiltration ability, human mesenchymal stem cells were seeded on the SF/PCL nano/microfibrous composite scaffolds. From in vivo tests, it was found that the bone-regenerating ability of SF/PCL nano/microfibrous composite scaffolds was closely associated with the nanofiber content in the composite scaffolds. In conclusion, this approach of controlling the nanofiber content in SF/PCL nano/microfibrous composite scaffolds could be useful in the design of novel scaffolds for tissue engineering.

Aligned natural-synthetic polyblend nanofibers for peripheral nerve regeneration

Peripheral nerve regeneration remains a significant clinical challenge to researchers. Progress in the design of tissue engineering scaffolds provides an alternative approach for neural regeneration. In this study aligned silk fibroin (SF) blended poly(L-lactic acid-co-ε-caprolactone) (P(LLA-CL)) nanofibrous scaffolds were fabricated by electrospinning methods and then reeled into aligned nerve guidance conduits (NGC) to promote nerve regeneration. The aligned SF/P(LLA-CL) NGC was used as a bridge implanted across a 10mm defect in the sciatic nerve of rats and the outcome in terms of of regenerated nerve at 4 and 8 weeks was evaluated by a combination of electrophysiological assessment and histological and immunohistological analysis, as well as electron microscopy. The electrophysiological examination showed that functional recovery of the regenerated nerve in the SF/P(LLA-CL) NGC group was superior to that in the P(LLA-CL) NGC group. The morphological analysis also indicated that the regenerated nerve in the SF/P(LLA-CL) NGC was more mature. All the results demonstrated that the aligned SF/P(LLA-CL) NGC promoted peripheral nerve regeneration significantly better in comparison with the aligned P(LLA-CL) NGC, thus suggesting a potential application in nerve regeneration.