Vortex-induced injectable silk fibroin hydrogels

Highly elastomeric photocurable silk hydrogels

A photocurable silk fibroin hydrogel is prepared, for the first time, using natural silk protein fibroin and biophotosensitizer riboflavin. Riboflavin is excited by ultraviolet light to generate a triplet state which is transferred to produce active oxygen radicals with singlet oxygen as the main component. Active oxygen radicals can induce chemical cross-linking of amino-, phenol- and other groups in the silk fibroin macromolecules to form a photocurable hydrogel. The different biophysical characterizations of the gelation of this modified fibroin protein solution were studied by using Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, microplate reader and texture analyzer. The aggregate structures, surface morphologies, mechanical properties, light transmission and degradation properties of the gel were studied. The investigations showed that the silk fibroin/riboflavin hydrogels predominantly have random coils or alpha helix structures. These gels show resilience up to 90% after 80% compression and a light transmission of up to 97%. The cell culture experiment exhibits that the hydrogel has a satisfactory cytocompatibility.

Reverse-engineered silk hydrogels for cell and drug delivery

Silk is an important biopolymer for (bio)medical applications because of its unique and highly versatile structure and its robust clinical track record in human medicine. Silk can be processed into many material formats, including physically and chemically cross-linked hydrogels that have almost limitless applications ranging from tissue engineering to biomedical imaging and sensing. This concise review provides a detailed background of silk hydrogels, including silk structure-function relationships, biocompatibility and biodegradation, and it explores recent developments in silk hydrogel utilization, with specific reference to drug and cell delivery. We address common pitfalls and misconceptions while identifying emerging opportunities, including 3D printing.

Silk fibroin hydrogels from the Colombian silkworm Bombyx mori L: Evaluation of physicochemical properties

Hydrogel scaffolds are important materials in tissue engineering, and their characterization is essential to determine potential biomedical applications according to their mechanical and structural behavior. In this work, silk fibroin hydrogels were synthesized by two different methods (vortex and sonication), and agarose hydrogels were also obtained for comparison purposes. Samples were characterized by scanning electron microscopy, infrared analysis, thermo-gravimetrical analysis, confined compression test, and rheological test. The results indicate that nanofibers can be obtained via both silk fibroin and agarose hydrogels. The mechanical tests showed that the Young's modulus is similar to those found in the literature, with the highest value for agarose hydrogels. All the hydrogels showed a shear-thinning behavior. Additionally, the MTT test revealed that silk fibroin hydrogels had low cytotoxicity in THP-1 and HEK-293 cells, whereas the agarose hydrogels showed high toxicity for the THP-1 cell line. The results indicate that silk fibroin hydrogels obtained from a Colombian silkworm hybrid are suitable for the development of scaffolds, with potential applications in tissue engineering.

Controlling methacryloyl substitution of chondroitin sulfate: injectable hydrogels with tunable long-term drug release profiles

Drug delivery systems capable of local sustained release of small molecule therapeutics remain a critical need in many fields, including oncology. Here, a system to create tunable hydrogels capable of modulating the loading and release of cationic small molecule therapeutics was developed. Chondroitin sulfate (CS) is a sulfated glycosaminoglycan that has many promising properties, including biocompatibility, biodegradation and chemically modifiable groups for both covalent and non-covalent bonding. CS was covalently modified with photocrosslinkable methacryloyl groups (CSMA) to develop an injectable hydrogel fabrication. Utilizing anionic groups, cationic drugs can be adsorbed and released from the hydrogels. This study demonstrates the synthesis of CSMA with a varying degree of substitution (DS) to generate hydrogels with varying swelling properties, maximum injection force, and drug release kinetics. The DS of the synthesized CSMA ranged from 0.05 ± 0.02 (2 h reaction) to 0.28 ± 0.02 (24 h reaction) with a DS of 1 representing 100% modification. The altered DS resulted in changes in hydrogel properties with the swelling of 20% CSMA hydrogels ranging from 42 (2 h reaction) to 13 (24 h reaction) and injection forces ranging from 18 N (2 h reaction) to 94 N (24 h reaction). The release of sunitinib, an oncology therapeutic that inhibits intracellular signaling by targeting multiple receptor tyrosine kinases, ranged from 18 μg per day (2 h reaction) to 9 μg per day (24 h reaction). While decreasing the DS increased the hydrogel swelling and rate of therapeutic release, it also limited the hydrogel fabrication range to only those containing 10% or higher CSMA. Blended polymer systems with poly(vinyl alcohol)-methacrylate (PVAMA) were fabricated to stabilize the resulting hydrogels via attenuating the swelling properties. Release profiles previously unattainable with the pure CSMA hydrogels were achieved with the blended hydrogel formulations. Overall, these studies identify a method to formulate tunable CSMA and blended CSMA/PVAMA hydrogels capable of sustained release of cationic therapeutics over six weeks with applications in oncology therapeutics.

Regenerated Antheraea pernyi Silk Fibroin/Poly(N-isopropylacrylamide) Thermosensitive Composite Hydrogel with Improved Mechanical Strength

At present, Antheraea pernyi silk fibroin (ASF) has attracted research efforts to investigate it as a raw material for fabrication of biomedical devices because of its superior cytocompatibility. Nevertheless, native ASF is not easily processed into a hydrogel without any crosslinking agent, and a single hydrogel shows poor mechanical properties. In this paper, a series of ASF/poly (N-isopropylacrylamide) (PNIPAAm) composite hydrogels with different ASF contents were manufactured by a simple in situ polymerization method without any crosslinking agent. Meanwhile, the structures, morphologies and thermal properties of composite hydrogels were investigated by XRD, FTIR, SEM, DSC and TGA, respectively. The results indicate that the secondary structure of silk in the composite hydrogel can be controlled by changing the ASF content and the thermal stability of composite hydrogels is enhanced with an increase in crystalline structure. The composite hydrogels showed similar lower critical solution temperatures (LCST) at about 32 °C, which matched well with the LCST of PNIPAAm. Finally, the obtained thermosensitive composite hydrogels exhibited enhanced mechanical properties, which can be tuned by varying the content of ASF. This strategy to prepare an ASF-based responsive composite hydrogel with enhanced mechanical properties represents a valuable route for developing the fields of ASF, and, furthermore, their attractive applications can meet the needs of different biomaterial fields.

Bioinspired Three-Dimensional Human Neuromuscular Junction Development in Suspended Hydrogel Arrays

The physical connection between motoneurons and skeletal muscle targets is responsible for the creation of neuromuscular junctions (NMJs), which allow electrical signals to be translated to mechanical work. NMJ pathology contributes to the spectrum of neuromuscular, motoneuron, and dystrophic disease. Improving in vitro tools that allow for recapitulation of the physiology of the neuromuscular connection will enable researchers to better understand the development and maturation of NMJs, and will help to decipher mechanisms leading to NMJ degeneration. In this work, we first describe robust differentiation of bungarotoxin-positive human myotubes, as well as a reproducible method for encapsulating and aligning human myoblasts in three-dimensional (3D) suspended culture using bioprinted silk fibroin cantilevers as cell culture supports. Further analysis with coculture of motoneuron-like cells demonstrates feasibility of fully human coculture using two-dimensional and 2.5-dimensional culture methods, with appropriate differentiation of both cell types. Using these coculture differentiation conditions with motoneuron-like cells added to monocultures of 3D suspended human myotubes, we then demonstrate synaptic colocalization in coculture as well as acetylcholine and glutamic acid stimulation of human myocytes. This method represents a unique platform to coculture suspended human myoblast-seeded 3D hydrogels with integrated motoneuron-like cells derived from human induced neural stem cells. The platform described is fully customizable using 3D freeform printing into standard laboratory tissue culture materials, and allows for human myoblast alignment in 3D with precise motoneuron integration into preformed myotubes. The coculture method will ideally be useful in observation and analysis of neurite outgrowth and myogenic differentiation in 3D with quantification of several parameters of muscle innervation and function.

A silk fibroin/decellularized extract of Wharton's jelly hydrogel intended for cartilage tissue engineering

A hybrid hydrogel was obtained from decellularized extract from Wharton's jelly (DEWJ) and silk fibroin (SF) and characterized for cartilage tissue engineering. Wharton's jelly was used due to its similarity with articular cartilage in extracellular matrix composition. Also, silk fibroin has good mechanical properties which make this construct appropriate for cartilage repair. Decellularization of Wharton's jelly was verified by DAPI staining, DNA quantification, and PCR analysis. Then, the biochemical composition of DEWJ was determined by ELISA kits for total proteins, collagens, sulfated glycosaminoglycans (sGAG), and transforming growth factor β1 (TGF-β1). After fabricating pure SF and SF/DEWJ hybrid hydrogels, their physical and mechanical properties were characterized by FESEM, Fourier-transform infrared spectroscopy (FTIR) and rheological assays (amplitude and frequency sweeps). Furthermore, cell viability and proliferation were assessed by MTT assay. The results have shown that DEWJ in hybrid hydrogels enhances mechanical properties of the construct relative to pure SF hydrogels. Also, this extract at its 40% concentration in culture media and 20% or 40% concentrations in SF/DEWJ hybrid hydrogels significantly increases population of the cells compared to control and pure SF hydrogel after 7 days. In conclusion, this study proposes the potential of SF/DEWJ hybrid hydrogels for cartilage tissue engineering applications.

Biodegradation of injectable silk fibroin hydrogel prevents negative left ventricular remodeling after myocardial infarction

Random collagen fiber networks formed by a slowly degrading silk fibroin hydrogel injection prevented left ventricular enlargement after myocardial infarction.

The design and biomedical applications of self-assembled two-dimensional organic biomaterials

Self-assembling 2D organic biomaterials exhibit versatile abilities for structural and functional tailoring, as well as high potential for biomedical applications.

Design and performance of a poly(vinyl alcohol)/silk fibroin enzymatically crosslinked semi-interpenetrating hydrogel for a potential hydrophobic drug delivery

In this study, in order to obtain hydrogels with good properties for sustained release of hydrophobic drugs or for tissue engineering, poly(vinyl alcohol) (PVA)/silk fibroin (SF) semi-interpenetrating (semi-IPN) hydrogels with varied ratios of PVA/SF were enzymatically cross-linked using horseradish peroxidase. A vial inversion test determined approximate gelation times of PVA/SF hydrogels ranging from 5 to 10 min. The hydrogels with varied ratios showed differences in pore size and morphology. Mass loss rate of hydrogels increased from 15% to 58% with increasing PVA concentration. Stable hydrogels with PVA/SF at 0.5 : 1 w/w showed the best swelling ratio values in distilled water (7.36). FTIR analysis revealed that silk fibroin in these hydrogels exhibited the coexistence of amorphous and silk I crystalline structures and the SF and PVA molecules interacted with each other well. The mechanical properties of the composite hydrogels were controlled by the SF content. From the cell viability results, it was found that the hydrogels exerted very low cytotoxicity. Paeonol was chosen as the hydrophobic drug model for release studies from the hydrogels. Paeonol can be uniformly loaded into the composite hydrogels using the emulsifying property of PVA and paeonol release from the hydrogels was dependent on the PVA/SF ratio. This study applied a novel type of enzymatically crosslinked semi-IPN hydrogel that may have potential applications in drug delivery.

Enzymatically cross-linked PVA/SF semi-IPN hydrogels with tunable pore structure have potential applications in sustained release of hydrophobic drug.

The regenerated silk fibroin hydrogel with designed architecture bioprinted by its microhydrogel

The regenerated silk fibroin microhydrogel with thixotropic property could be bioprinted and then ripened to a tough hydrogel because of the change in “the second network” of the microhydrogel.

In Situ Forming Injectable Silk Fibroin Hydrogel Promotes Skin Regeneration in Full Thickness Burn Wounds

Full-thickness skin wounds, associated with deep burns or chronic wounds pose a major clinical problem. Herein, the development of in situ forming hydrogel using a natural silk fibroin (SF) biomaterial for treating burn wounds is reported. Blends of SF solutions isolated from Bombyx mori and Antheraea assama show inherent self-assembly between silk proteins and lead to irreversible gelation at body temperature. Investigation of the gelation mechanism reveals crosslinking due to formation of β-sheet structures as examined by X-ray diffraction and Fourier transform infrared spectroscopy. The SF hydrogel supports proliferation of primary human dermal fibroblasts and migration of keratinocytes comparable to collagen gel (Col) as examined under in vitro conditions. The SF hydrogel also provides an instructive and supportive matrix to the full-thickness third-degree burn wounds in vivo. A 3-week comparative study with Col indicates that SF hydrogel not only promotes wound healing but also shows transitions from inflammation to proliferation stage as observed through the expression of TNF-α and CD163 genes. Further, deposition and remodeling of collagen type I and III fibers suggests an enhanced overall tissue regeneration. Comparable results with Col demonstrate the SF hydrogel as an effective and inexpensive formulation toward a potential therapeutic approach for burn wound treatment.

Modulatory effect of simultaneously released magnesium, strontium, and silicon ions on injectable silk hydrogels for bone regeneration

Injectable silk hydrogels are ideal carriers of therapeutic agents due to their biocompatibility and low immunogenicity. Injectable silk hydrogels for bone regeneration have been previously developed but often utilize expensive biologics. In this study, we have developed an injectable silk composite incorporated with a triphasic ceramic called MSM-10 (54 Mg₂SiO₄, 36 Si₃Sr₅ and 10 MgO (wt%)) capable of simultaneously releasing magnesium, silicon, and strontium ions into its environment. These ions have been previously reported to possess therapeutic effects for bone regeneration. MSM-10 particles were incorporated into the silk hydrogels at various weight percentages [0.1 (SMH-0.1), 0.6 (SMH-0.6), 1 (SMH-1) and 2 (SMH-2)]. The effects of the released ions on the physicochemical and biological properties of the silk hydrogel were comprehensively evaluated. Increased MSM-10 loading was found to hinder the gelation kinetics of the silk hydrogel through the reduction of beta-sheet phase formation, which in turn affected the required sonication time for gelation, compressive strength, force of injection, microstructure and in vitro degradation rate. Primary human osteoblasts seeded on SMH-0.6 demonstrated increased proliferation and early alkaline phosphatase activity, as well as enhanced osteogenic gene expression compared to pure silk hydrogel and SMH-0.1. In vivo results in subcutaneous mouse models showed both decreased fibrous capsule formation and increased number of new blood vessels around the injected SMH-0.1 and SMH-0.6 implants compared to pure silk hydrogels. The results in this study indicate that the ions released from MSM-10 is able to influence the physicochemical and biological properties of silk hydrogels, and SMH-0.6 in particular shows promising properties for bone regeneration.

Novel silk fibroin nanoparticles incorporated silk fibroin hydrogel for inhibition of cancer stem cells and tumor growth

Background

A multi-drug delivery platform is needed as the intra-tumoral heterogeneity of cancer leads to different drug susceptibility. Cancer stem cells (CSCs), a small population of tumor cells responsible for tumor seeding and recurrence, are considered chemotherapy-resistant and have been reported to be sensitive to salinomycin (Sal) instead of paclitaxel (Ptx). Here we report a novel silk fibroin (SF) hydrogel-loading Sal and Ptx by incorporating drug-loaded silk fibroin nanoparticles (SF-NPs) to simultaneously kill CSCs and non-CSCs.

Methods

Using the method we have previously reported to prepare Ptx-loaded SF-NPs (Ptx-SF-NPs), Sal-loaded SF-NPs (Sal-SF-NPs) were fabricated under mild and non-toxic conditions. The drug-loaded SF-NPs were dispersed in the ultrasound processed SF solution prior to gelation.

Results

The resulting SF hydrogel (Sal-Ptx-NP-Gel) retained its injectable properties, exhibited bio-degradability and demonstrated homogeneous drug distribution compared to the non-NP incorporated hydrogel. Sal-Ptx-NP-Gel showed superior inhibition of tumor growth compared to single drug-loaded hydrogel and systemic dual drug administration in the murine hepatic carcinoma H22 subcutaneous tumor model. Sal-Ptx-NP-Gel also significantly reduced CD44⁺CD133⁺ tumor cells and demonstrated the least tumor formation in the in vivo tumor seeding experiment, indicating superior inhibition of cancer stem cells.

Conclusion

These results suggest that SF-NPs incorporated SF hydrogel is a promising drug delivery platform, and Sal-Ptx-NP-Gel could be a novel and powerful locoregional tumor treatment regimen in the future.

Injectable hydrogels: a new paradigm for osteochondral tissue engineering

Osteochondral tissue engineering has become a promising strategy for repairing focal chondral lesions and early osteoarthritis (OA), which account for progressive joint pain and disability in millions of people worldwide. Towards improving osteochondral tissue repair, injectable hydrogels have emerged as promising matrices due to their wider range of properties such as their high water content and porous framework, similarity to the natural extracellular matrix (ECM), ability to encapsulate cells within the matrix and ability to provide biological cues for cellular differentiation. Further, their properties such as those that facilitate minimally invasive deployment or delivery, and their ability to repair geometrically complex irregular defects have been critical for their success. In this review, we provide an overview of innovative approaches to engineer injectable hydrogels towards improved osteochondral tissue repair. Herein, we focus on understanding the biology of osteochondral tissue and osteoarthritis along with the need for injectable hydrogels in osteochondral tissue engineering. Furthermore, we discuss in detail different biomaterials (natural and synthetic) and various advanced fabrication methods being employed for the development of injectable hydrogels in osteochondral repair. In addition, in vitro and in vivo applications of developed injectable hydrogels for osteochondral tissue engineering are also reviewed. Finally, conclusions and future perspectives of using injectable hydrogels in osteochondral tissue engineering are provided.

Simultaneous structuring and mineralization of silk fibroin scaffolds

Silk fibroin is commonly used as scaffold material for tissue engineering applications. In combination with a mineralization with different calcium phosphate phases, it can also be applied as material for bone regeneration. Here, we present a study which was performed to produce mineralized silk fibroin scaffolds with controlled macroporosity. In contrast to former studies, our approach focused on a simultaneous gelation and mineralization of silk fibroin by immersion of frozen silk fibroin monoliths in acidic calcium phosphate solutions. This was achieved by thawing frozen silk fibroin monoliths in acidic calcium phosphate solution, leading to the precipitation of monocalcium phosphate within the silk fibroin matrix. In the second approach, a conversion of incorporated β-tricalcium phosphate particles into brushite was successfully achieved. Furthermore, a controlled cryostructuring process of silk fibroin scaffolds was carried out leading to the formation of parallel-oriented pores with diameters of 30–50 µm.

Thai silk fibroin gelation process enhancing by monohydric and polyhydric alcohols

Silk fibroin hydrogel is an interesting natural material in various biomedical applications. However, the self-assembled gelation takes a long time. In this work, different alcohol types are used as gelation enhancers for aqueous silk fibroin solution. Monohydric alcohols having carbon chain length from C1 to C4 and polyhydric alcohols with the number of mono- to tri- hydroxyl groups were used as the enhancers which are effective for rapid gelation. The addition of monohydric alcohol distinctively reduced the gelation time, comparing to the polyhydric alcohol. The gelation process is directly dependent on the polarity of alcohol and hydrophobicity. The alcohol mediated gelation imparts strong viscoelastic property and enhanced compressive modulus of resulting hydrogels. This is due to the effective formation of self-assembled beta sheet network of the silk fibroin chains facilitates the gelation process.

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.

Combinatory approach for developing silk fibroin scaffolds for cartilage regeneration

Several processing technologies and engineering strategies have been combined to create scaffolds with superior performance for efficient tissue regeneration. Cartilage tissue is a good example of that, presenting limited self-healing capacity together with a high elasticity and load-bearing properties. In this work, novel porous silk fibroin (SF) scaffolds derived from horseradish peroxidase (HRP)-mediated crosslinking of highly concentrated aqueous SF solution (16 wt%) in combination with salt-leaching and freeze-drying methodologies were developed for articular cartilage tissue engineering (TE) applications. The HRP-crosslinked SF scaffolds presented high porosity (89.3 ± 0.6%), wide pore distribution and high interconnectivity (95.9 ± 0.8%). Moreover, a large swelling capacity and favorable degradation rate were observed up to 30 days, maintaining the porous-like structure and β-sheet conformational integrity obtained with salt-leaching and freeze-drying processing. The in vitro studies supported human adipose-derived stem cells (hASCs) adhesion, proliferation, and high glycosaminoglycans (GAGs) synthesis under chondrogenic culture conditions. Furthermore, the chondrogenic differentiation of hASCs was assessed by the expression of chondrogenic-related markers (collagen type II, Sox-9 and Aggrecan) and deposition of cartilage-specific extracellular matrix for up to 28 days. The cartilage engineered constructs also presented structural integrity as their mechanical properties were improved after chondrogenic culturing. Subcutaneous implantation of the scaffolds in CD-1 mice demonstrated no necrosis or calcification, and deeply tissue ingrowth. Collectively, the structural properties and biological performance of these porous HRP-crosslinked SF scaffolds make them promising candidates for cartilage regeneration.

STATEMENT OF SIGNIFICANCE

In cartilage tissue engineering (TE), several processing technologies have been combined to create scaffolds for efficient tissue repair. In our study, we propose novel silk fibroin (SF) scaffolds derived from enzymatically crosslinked SF hydrogels processed by salt-leaching and freeze-drying technologies, for articular cartilage applications. Though these scaffolds, we were able to combine the elastic properties of hydrogel-based systems, with the stability, resilience and controlled porosity of scaffolds processed via salt-leaching and freeze-drying technologies. SF protein has been extensively explored for TE applications, as a result of its mechanical strength, elasticity, biocompatibility, and biodegradability. Thus, the structural, mechanical and biological performance of the proposed scaffolds potentiates their use as three-dimensional matrices for cartilage regeneration.

Cooperative Assembly of a Peptide Gelator and Silk Fibroin Afford an Injectable Hydrogel for Tissue Engineering

Silk fibroin (SF) from Bombyx mori has received increasing interest in biomedical fields, because of its slow biodegradability, good biocompatibility, and low immunogenicity. Although SF-based hydrogels have been studied intensively as a potential matrix for tissue engineering, weak gelation performance and low mechanical strength are major limitations that hamper their widespread applicability. Therefore, searching for new strategies to improve the SF gelation property is highly desirable in tissue engineering research. Herein, we report a facile approach to induce rapid gelation of SF by a small peptide gelator (e.g., NapFF). Following the simple mixing of SF and NapFF in water, a stable hydrogel of SF was obtained in a short time period at physiological pH, and the minimum gelation concentration of SF can reach as low as 0.1%. In this process of gelation, NapFF not only can behave itself as a gelator for supramolecular self-assembly, but also can trigger the conformational transition of the SF molecule from random coil to β-sheet structure via hydrophobic and hydrogen-bonding interactions. More importantly, for the generation of a scaffold with favorable cell-surface interactions, a new peptide gelator (NapFFRGD) with Arg-Gly-Asp (RGD) domain was applied to functionalize SF hydrogel with improved bioactivity for cell adhesion and growth. Following encapsulating the vascular endothelial growth factor (VEGF), the SF gel was subcutaneously injected in mice, and served as an effective matrix to trigger the generation of new blood capillaries in vivo.

Silk fibroin preserves beta cell function under inflammatory stress while stimulating islet cell surface GLUT2 expression

Silk fibroin is a novel biomaterial for enhancing transplanted islet cell function and survival. This study investigated whether silk fibroin may have unique properties that improve islet function in the face of inflammatory-mediated stress during transplantation. Murine islet function was tested in vitro with either silk fibroin or alginate and challenged with inflammatory cytokines. The glucose-stimulated insulin secretion index for all conditions decreased with inflammatory cytokines, but was better preserved for islets exposed to silk compared to those exposed to alginate or medium. GLUT2 transporter expression on the cell surface of islets exposed to silk was increased compared to alginate or medium alone. Upon cytokine stress, a greater percentage of islet cells exposed to silk expressed GLUT2 on their surface. We conclude that preconditioning islets with silk fibroin stimulates islet cell surface GLUT2 expression, an increase, which persists under inflammatory stress, and may improve islet engraftment and function after transplantation.

Rapidly responsive silk fibroin hydrogels as an artificial matrix for the programmed tumor cells death

Timely and spatially-regulated injectable hydrogels, able to suppress growing tumors in response to conformational transitions of proteins, are of great interest in cancer research and treatment. Herein, we report rapidly responsive silk fibroin (SF) hydrogels formed by a horseradish peroxidase (HRP) crosslinking reaction at physiological conditions, and demonstrate their use as an artificial biomimetic three-dimensional (3D) matrix. The proposed SF hydrogels presented a viscoelastic nature of injectable hydrogels and spontaneous conformational changes from random coil to β-sheet conformation under physiological conditions. A human neuronal glioblastoma (U251) cell line was used for screening cell encapsulation and in vitro evaluation within the SF hydrogels. The transparent random coil SF hydrogels promoted cell viability and proliferation up to 10 days of culturing, while the crystalline SF hydrogels converted into β-sheet structure induced the formation of TUNEL-positive apoptotic cells. Therefore, this work provides a powerful tool for the investigation of the microenvironment on the programed tumor cells death, by using rapidly responsive SF hydrogels as 3D in vitro tumor models.

Sodium oleate induced rapid gelation of silk fibroin

Silk fibroin has acquired increasing interest in the last years for application in medicine and namely in tissue engineering. Several methods have been developed to process fibroin and for the fabrication of nets, sponges, films and gels. This paper deals with the fabrication and characterization of fibroin hydrogels obtained by using sodium oleate as gelation agent. Gels have been prepared by mixing Silk fibroin (SF) and Sodium oleate (SO) water solutions in different concentrations, and a quite wide frame of compositions have been explored. Rheological tests have been performed to determine the gelation times, scanning electron microscopies have been made to evaluate morphologies, FTIR analysis has been done to determine the conformation of the starting materials and of the resulting gels, water content has been measured and cytotoxicity tests have been performed to validate the potential biomedical use of the hydrogels. Depending on the SF and SO different gelation times have been obtained thanks to the formation of intermolecular bonds between the fibroin chains. The obtained fastest gelation of about 80 s could make this specific formulation compatible with in situ gelation. By changing composition, gels with different morphologies, rheological properties and water contents have been prepared.

Hydrogel matrices based on elastin and alginate for tissue engineering applications

Hydrogels from natural polymers are widely used in tissue engineering due to their unique properties, especially when regarding the cell environment and their morphological similarity to the extracellular matrix (ECM) of native tissues. In this study, we describe the production and characterization of novel hybrid hydrogels composed of alginate blended with elastin from bovine neck ligament. The properties of elastin as a component of the native ECM were combined with the excellent chemical and mechanical stability as well as biocompatibility of alginate to produce two hybrid hydrogels geometries, namely 2D films obtained using sonication treatment and 3D microcapsules produced by pressure-driven extrusion. The resulting blend hydrogels were submitted to an extensive physico-chemical characterization. Furthermore, the biological compatibility of these materials was assessed using normal human dermal fibroblasts, indicating the suitability of this blend for soft tissue engineering.

Silk fibroin/collagen protein hybrid cell-encapsulating hydrogels with tunable gelation and improved physical and biological properties

Cell encapsulating hydrogels with tunable mechanical and biological properties are of special importance for cell delivery and tissue engineering. Silk fibroin and collagen, two typical important biological proteins, are considered potential as cell culture hydrogels. However, both have been used individually, with limited properties (e.g., collagen has poor mechanical properties and cell-mediated shrinkage, and silk fibroin from Bombyx mori (mulberry) lacks cell adhesion motifs). Therefore, the combination of them is considered to achieve improved mechanical and biological properties with respect to individual hydrogels. Here, we show that the cell-encapsulating hydrogels of mulberry silk fibroin / collagen are implementable over a wide range of compositions, enabled simply by combining the different gelation mechanisms. Not only the gelation reaction but also the structural characteristics, consequently, the mechanical properties and cellular behaviors are accelerated significantly by the silk fibroin / collagen hybrid hydrogel approach. Of note, the mechanical and biological properties are tunable to represent the combined merits of individual proteins. The shear storage modulus is tailored to range from 0.1 to 20 kPa along the iso-compositional line, which is considered to cover the matrix stiffness of soft-to-hard tissues. In particular, the silk fibroin / collagen hydrogels are highly elastic, exhibiting excellent resistance to permanent deformation under different modes of stress; without being collapsed or water-squeezed out (vs. not possible in individual proteins) - which results from the mechanical synergism of interpenetrating networks of both proteins. Furthermore, the role of collagen protein component in the hybrid hydrogels provides adhesive sites to cells, stimulating anchorage and spreading significantly with respect to mulberry silk fibroin gel, which lacks cell adhesion motifs. The silk fibroin / collagen hydrogels can encapsulate cells while preserving the viability and growth over a long 3D culture period. Our findings demonstrate that the silk / collagen hydrogels possess physical and biological properties tunable and significantly improved (vs. the individual protein gels), implying their potential uses for cell delivery and tissue engineering.

STATEMENT OF SIGNIFICANCE

Development of cell encapsulating hydrogels with excellent physical and biological properties is important for the cell delivery and cell-based tissue engineering. Here we communicate for the first time the novel protein composite hydrogels comprised of 'Silk' and 'Collagen' and report their outstanding physical, mechanical and biological properties that are not readily achievable with individual protein hydrogels. The properties include i) gelation accelerated over a wide range of compositions, ii) stiffness levels covering 0.1 kPa to 20 kPa that mimic those of soft-to-hard tissues, iii) excellent elastic behaviors under various stress modes (bending, twisting, stretching, and compression), iv) high resistance to cell-mediated gel contraction, v) rapid anchorage and spreading of cells, and vi) cell encapsulation ability with a long-term survivability. These results come from the synergism of individual proteins of alpha-helix and beta-sheet structured networks. We consider the current elastic cell-encapsulating hydrogels of silk-collagen can be potentially useful for the cell delivery and tissue engineering in a wide spectrum of soft-to-hard tissues.

Silk Fibroin-Based Scaffold for Bone Tissue Engineering

Regeneration of diseased or damaged skeletal tissues is one of the challenge that needs to be solved. Although there have been many bone tissue engineering developed, scaffold-based tissue engineering complement the conventional treatment for large bone by completing biological and functional environment. Among many materials, silk fibroin (SF) is one of the favorable material for applications in bone tissue engineering scaffolding. SF is a fibrous protein mainly extracted from Bombyx mori. and spiders. SF has been used as a biomaterial for bone graft by its unique mechanical properties, controllable biodegradation rate and high biocompatibility. Moreover, SF can be processed using conventional and advanced biofabrication methods to form various scaffold types such as sponges, mats, hydrogels and films. This review discusses about recent application and advancement of SF as a biomaterial.

[The prospects of hydrogels usage as a basis for curable osteoplastic materials]

The article deals with the main types of the polymers used in hydrogel preparation. Their biological, physical and chemical properties was compared. Ways of polymers hardening and prospects of medical application were considered. The prospect of use of chitosan hydrogels activated by osteoinductors as a material for bone augmentation were concluded.

Silk fibroin-chondroitin sulfate scaffold with immuno-inhibition property for articular cartilage repair

The demand of favorable scaffolds has increased for the emerging cartilage tissue engineering. Chondroitin sulfate (CS) and silk fibroin have been investigated and reported with safety and excellent biocompatibility as tissue engineering scaffolds. However, the rapid degradation rate of pure CS scaffolds presents a challenge to effectively recreate neo-tissue similar to natural articular cartilage. Meanwhile the silk fibroin is well used as a structural constituent material because its remarkable mechanical properties, long-lasting in vivo stability and hypoimmunity. The application of composite silk fibroin and CS scaffolds for joint cartilage repair has not been well studied. Here we report that the combination of silk fibroin and CS could synergistically promote articular cartilage defect repair. The silk fibroin (silk) and silk fibroin/CS (silk-CS) scaffolds were fabricated with salt-leaching, freeze-drying and crosslinking methodologies. The biocompatibility of the scaffolds was investigated in vitro by cell adhesion, proliferation and migration with human articular chondrocytes. We found that silk-CS scaffold maintained better chondrocyte phenotype than silk scaffold; moreover, the silk-CS scaffolds reduced chondrocyte inflammatory response that was induced by interleukin (IL)-1β, which is in consistent with the well-documented anti-inflammatory activities of CS. The in vivo cartilage repair was evaluated with a rabbit osteochondral defect model. Silk-CS scaffold induced more neo-tissue formation and better structural restoration than silk scaffold after 6 and 12weeks of implantation in ICRS histological evaluations. In conclusion, we have developed a silk fibroin/ chondroitin sulfate scaffold for cartilage tissue engineering that exhibits immuno-inhibition property and can improve the self-repair capacity of cartilage.

STATEMENT OF SIGNIFICANCE

Severe cartilage defect such as osteoarthritis (OA) is difficult to self-repair because of its avascular, aneural and alymphatic nature. Current scaffolds often focus on providing sufficient mechanical support or bio-mimetic structure to promote cartilage repair. Thus, silk has been adopted and investigated broadly. However, inflammation is one of the most important factors in OA. But few scaffolds for cartilage repair reported anti-inflammation property. Meanwhile, chondroitin sulfate (CS) is a glycosaminoglycan present in the natural cartilage ECM, and has exhibited a number of useful biological properties including anti-inflammatory activity. Thus, we designed this silk-CS scaffold and proved that this scaffold exhibited good anti-inflammatory effects both in vitro and in vivo, promoted the repair of articular cartilage defect in animal model.

Preparation and characterization of silk fibroin hydrogel as injectable implants for sustained release of Risperidone

The principal objective of the present study is to achieve a depot formulation of Risperidone by gelation of silk fibroin (SF). For this purpose, hydrochloric acid (HCl)/acetone-based and methanol-based hydrogels were prepared with different drug/polymer ratios (1:3, 1:6, and 1:15). For all the drug-loaded methanol-based hydrogels, gel transition of SF solutions occurred immediately and the gelation time was 1 min, while the gelation time of HCL/acetone-based hydrogels was around 360 min. According to the results obtined from Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) spectra, solvent systems and Risperidone could induce β-sheet structure, but HCL/acetone system had the lowest effect on induction of β-sheets. The crystallinity was increased by increasing the amount of Risperidone, and drug to polymer ratio of 1:3 possessed the highest crystallinity. Thermogravimetric analysis (TGA) indicated that increasing the amount of drug in formulation increased the stability of hydrogels, and methanol-based hydrogel with a ratio of 1:3 had the most stable structure. The release rate of Risperidone from methanol-based hydrogel at ratio of 1:3 was lower than that for HCl/acetone-based one, and it decreased by increasing the amount of Risperidone. The release of Risperidone from methanol hydrogel at ratios 1:3 and 1:6 continued up to 25 d which is acceptable for depot form of Risperidone and shows that the extended release of Risperidone was achieved successfully. In conclusion, SF hydrogel with the ability to respond to the environmental stimuli is an excellent candidate for injectable implants for extended release of Risperidone.

Intrinsic 3D Prestressing: A New Route for Increasing Strength and Improving Toughness of Hybrid Inorganic Biocements

Cement is the most consumed resource and is the most widely used material globally. The ability to extrinsically prestress cementitious materials with tendons usually made from steel allows the creation of high-strength bridges and floors from this otherwise brittle material. Here, a dual setting cement system based on the combination of hydraulic cement powder with an aqueous silk fibroin solution that intrinsically generates a 3D prestressing during setting, dramatically toughening the cement to the point it can be cut with scissors, is reported. Changes of both ionic concentration and pH during cement setting are shown to create an interpenetrating silk fibroin inorganic composite with the combined properties of the elastic polymer and the rigid cement. These hybrid cements are self-densifying and show typical ductile fracture behavior when dry and a high elasticity under wet conditions with mechanical properties (bending and compressive strength) nearly an order of magnitude higher than the fibroin-free cement reference.

Applicability of biotechnologically produced insect silks

Silks are structural proteins produced by arthropods. Besides the well-known cocoon silk, which is produced by larvae of the silk moth Bombyx mori to undergo metamorphosis inside their silken shelter (and which is also used for textile production by men since millennia), numerous further less known silk-producing animals exist. The ability to produce silk evolved multiple independent times during evolution, and the fact that silk was subject to convergent evolution gave rise to an abundant natural diversity of silk proteins. Silks are used in air, under water, or like honey bee silk in the hydrophobic, waxen environment of the bee hive. The good mechanical properties of insect silk fibres together with their non-toxic, biocompatible, and biodegradable nature renders these materials appealing for both technical and biomedical applications. Although nature provides a great diversity of material properties, the variation in quality inherent in materials from natural sources together with low availability (except from silkworm silk) impeded the development of applications of silks. To overcome these two drawbacks, in recent years, recombinant silks gained more and more interest, as the biotechnological production of silk proteins allows for a scalable production at constant quality. This review summarises recent developments in recombinant silk production as well as technical procedures to process recombinant silk proteins into fibres, films, and hydrogels.

Cell-laden hydrogels for osteochondral and cartilage tissue engineering

Despite tremendous advances in the field of regenerative medicine, it still remains challenging to repair the osteochondral interface and full-thickness articular cartilage defects. This inefficiency largely originates from the lack of appropriate tissue-engineered artificial matrices that can replace the damaged regions and promote tissue regeneration. Hydrogels are emerging as a promising class of biomaterials for both soft and hard tissue regeneration. Many critical properties of hydrogels, such as mechanical stiffness, elasticity, water content, bioactivity, and degradation, can be rationally designed and conveniently tuned by proper selection of the material and chemistry. Particularly, advances in the development of cell-laden hydrogels have opened up new possibilities for cell therapy. In this article, we describe the problems encountered in this field and review recent progress in designing cell-hydrogel hybrid constructs for promoting the reestablishment of osteochondral/cartilage tissues. Our focus centers on the effects of hydrogel type, cell type, and growth factor delivery on achieving efficient chondrogenesis and osteogenesis. We give our perspective on developing next-generation matrices with improved physical and biological properties for osteochondral/cartilage tissue engineering. We also highlight recent advances in biomanufacturing technologies (e.g. molding, bioprinting, and assembly) for fabrication of hydrogel-based osteochondral and cartilage constructs with complex compositions and microarchitectures to mimic their native counterparts.

STATEMENT OF SIGNIFICANCE

Despite tremendous advances in the field of regenerative medicine, it still remains challenging to repair the osteochondral interface and full-thickness articular cartilage defects. This inefficiency largely originates from the lack of appropriate tissue-engineered biomaterials that replace the damaged regions and promote tissue regeneration. Cell-laden hydrogel systems have emerged as a promising tissue-engineering platform to address this issue. In this article, we describe the fundamental problems encountered in this field and review recent progress in designing cell-hydrogel constructs for promoting the reestablishment of osteochondral/cartilage tissues. Our focus centers on the effects of hydrogel composition, cell type, and growth factor delivery on achieving efficient chondrogenesis and osteogenesis. We give our perspective on developing next-generation hydrogel/inorganic particle/stem cell hybrid composites with improved physical and biological properties for osteochondral/cartilage tissue engineering. We also highlight recent advances in biomanufacturing and bioengineering technologies (e.g. 3D bioprinting) for fabrication of hydrogel-based osteochondral and cartilage constructs.

Enzymatically crosslinked silk-hyaluronic acid hydrogels

In this study, silk fibroin and hyaluronic acid (HA) were enzymatically crosslinked to form biocompatible composite hydrogels with tunable mechanical properties similar to that of native tissues. The formation of di-tyrosine crosslinks between silk fibroin proteins via horseradish peroxidase has resulted in a highly elastic hydrogel but exhibits time-dependent stiffening related to silk self-assembly and crystallization. Utilizing the same method of crosslinking, tyramine-substituted HA forms hydrophilic and bioactive hydrogels that tend to have limited mechanics and degrade rapidly. To address the limitations of these singular component scaffolds, HA was covalently crosslinked with silk, forming a composite hydrogel that exhibited both mechanical integrity and hydrophilicity. The composite hydrogels were assessed using unconfined compression and infrared spectroscopy to reveal of the physical properties over time in relation to polymer concentration. In addition, the hydrogels were characterized by enzymatic degradation and for cytotoxicity. Results showed that increasing HA concentration, decreased gelation time, increased degradation rate, and reduced changes that were observed over time in mechanics, water retention, and crystallization. These hydrogel composites provide a biologically relevant system with controllable temporal stiffening and elasticity, thus offering enhanced tunable scaffolds for short or long term applications in tissue engineering.

Native extracellular matrix/fibroin hydrogels for adipose tissue engineering with enhanced vascularization

Adipose tissue engineering is a promising field for regeneration of soft tissue defects. However, vascularization is needed since nutrients and oxygen cannot reach cells in thick implants by diffusion. Obtaining a biocompatible scaffold with good mechanical properties is another problem. In this study, we aimed to develop thick and vascularized adipose tissue constructs supporting cell viability and adipose tissue regeneration. Hydrogels were prepared by mixing rat decellularized adipose tissue (DAT) and silk fibroin (Fib) at different v/v ratios (3:1, 1:1 and 1:3) and vortexing. Gelation times decreased with increasing fibroin ratio Among hydrogel groups 1:3-DAT:Fib ratio group showed similar mechanical properties with adipose tissue. Both pre-adipocytes and pre-endothelial cells, pre-differentiated from adipose derived stem cells (ASCs), were encapsulated in hydrogels at a 1: 3 ratio. In vitro analyses showed that hydrogels with 1:3 (v/v) DAT:Fib ratio supported better cell viability. Pre-adipocytes had lipid vesicles, and pre-endothelial cells formed tubular structures inside hydrogels only after 3 days in vitro. When endothelial and adipogenic pre-differentiated ASCs (for 7 days before encapsulation) were encapsulated together into 1:3-DAT:Fib hydrogels both cell types continued to differentiate into the committed cell lineage. Vascularization process in the hydrogels implanted with adipogenic and endothelial pre-differentiated ASCs took place between the first and second week after implantation which was faster than observed in the empty hydrogels. ASCs pre-differentiated towards adipogenic lineage inside hydrogels had begun to accumulate lipid vesicles after 1 week of subcutaneous implantation Based on these results, we suggest that 1:3-DAT:Fib hydrogels with enhanced vascularization hold promise for adipose tissue engineering.

A comparative investigation of Bombyx mori silk fibroin hydrogels generated by chemical and enzymatic cross-linking

Fibroin, the major proteinaceous component of the silk fiber produced by larvae of the domesticated silk moth (Bombyx mori), has been widely investigated as a biomaterial for potential applications in tissue engineering and regenerative medicine. Following sol-gel transition, silk fibroin solutions can generate hydrogels that present certain advantages when employed as biomaterials, especially if they are cross-linked. The subject of this study was the self-cross-linking of silk fibroin through a process induced by the enzyme horseradish peroxidase (HRP) in the presence of hydrogen peroxide, a method only recently proposed and scarcely reported. The hydrogels were prepared either by physical cross-linking, by cross-linking with a natural compound (genipin), or by enzymatic cross-linking. The products were comparatively characterized in regard to their synthesis and background chemical aspects, physical and optical properties, mechanical properties, secondary structure, swelling/deswelling behavior, enzymatic degradation, and compatibility as substrates for cell adhesion and proliferation. The study confirmed the advantages of the HRP-induced cross-linking, which included considerably shorter gelation times, enhanced elasticity of the resulting hydrogels, and improved cytocompatibility. Discrepancies between certain results of this investigation and those reported previously were discussed in detail.

A Review of Structure Construction of Silk Fibroin Biomaterials from Single Structures to Multi-Level Structures

The biological performance of artificial biomaterials is closely related to their structure characteristics. Cell adhesion, migration, proliferation, and differentiation are all strongly affected by the different scale structures of biomaterials. Silk fibroin (SF), extracted mainly from silkworms, has become a popular biomaterial due to its excellent biocompatibility, exceptional mechanical properties, tunable degradation, ease of processing, and sufficient supply. As a material with excellent processability, SF can be processed into various forms with different structures, including particulate, fiber, film, and three-dimensional (3D) porous scaffolds. This review discusses and summarizes the various constructions of SF-based materials, from single structures to multi-level structures, and their applications. In combination with single structures, new techniques for creating special multi-level structures of SF-based materials, such as micropatterning and 3D-printing, are also briefly addressed.