Immune response profiles induced by silk-based biomaterials: a journey from 'immunogenicity' towards 'immuno-compatibility

Silk is a widely accepted biomaterial for tissue regeneration owing to its tunable biomechanical properties and ease of chemical modification. However, a number of aspects associated with its clinical use are still debated. Indeed, to achieve clinical success, a biomaterial must favorably interact with host tissues without evoking local or systemic immuno-inflammatory responses. The analysis of immune responses associated with silk under in vitro and in vivo conditions provides useful insights, improving the understanding of the functional characteristics of silk biomaterials and further promoting their clinical application. Silk evokes moderate immune responses upon implantation in vivo, depending on the material structure, fabrication method, degradation time, and implantation in soft or hard tissue sites, which rapidly subside within a few days/weeks. In vitro studies indicate that its immune-stimulatory properties are largely due to inherent protein conformation and differential processing parameters. Strategically controlled levels of immune responses in vivo with marginal immunogenicity of silk-based biomaterials may contribute to matrix remodeling and replacement by native tissue matrix around the implanted site. Therefore, immunomodulatory strategies should be developed to promote the use of silk-based biomaterials as promising candidates for numerous clinical applications.

Comparative Analysis of Silk Fibroin Membranes across Cross-Linking Methods: Processing and Characterization

Silk fibroin (SF) extracted from silkworm silk can be transformed into transparent membranes with well-suited physical properties for ophthalmic applications. There is ample literature on the fabrication and characterization of SF-based membranes; however, the use of diverse SF extraction protocols and characterization methods or their settings makes it difficult to compare different silk membrane properties across studies. In this work, we fabricated 10 families of SF-based membranes by physical cross-linking and one non-cross-linked as a control. We evaluated transparency (ranging from 84.5 to 95.3% in the visible spectrum), enzyme stability (from 24 h to 200 days in protease XIV), decomposition temperature (280-290 °C), water uptake (40-60%), Young's modulus (8-30 MPa), roughness (1.6-22.7 nm), and FTIR spectra for the secondary structure. We found correlation between water uptake and the Young's modulus (the lower the water uptake, the higher the Young's modulus) and a relationship between membrane stability in protease XIV and the secondary structure of the proteins. Higher surface roughness and faster degradation were found in membranes cross-linked with polyethylene glycol, and conversely, lower roughness and lower degradation were found in methanol, ethanol, or isopropanol crossed-link membranes. This ample compilation of materials and their characterization will aid in the selection of a SF-based material according to the needs of the application.

Silk wastes and autoclaved degumming as an alternative for a sustainable silk process

Silk degumming is considered the first point in the preparation of silk-based materials since this process could modify the silk fiber and the properties of its related products. This study evaluated the differences in morphology, secondary structure, amino acid content, thermal stability, and mechanical properties of two types of raw materials, defective cocoons (DC) and silk fibrous waste (SW), degummed by chemical (C) and autoclaving (A) methods. Subsequently, silk fibroin films were prepared by dissolving each type of degummed fibers, and thermal and structural films properties were determined. The findings demonstrated that autoclaving is an efficient alternative to remove silk sericin, as the resulting fibers presented improved structural, thermal, and mechanical properties compared to those obtained by the chemical method. For films preparation, autoclave resulted in a good option, but dissolution parameters need to be adjusted for defective cocoons. Furthermore, similarities between the physicochemical properties of fibers and films from both fibrous wastes suggest that SW is a promising raw material for producing fibrous resources and regenerated silk fibroin materials. Overall, these findings suggest new recycling methods for fibrous waste and by-products generated in the silk textile production process.

Onto the differences in formulating micro-/nanoparticulate drug delivery system from Thai silk and Vietnamese silk: A critical comparison

Silk fibroin is a natural polymer with physicochemical properties heavily dependent on its silkworm sources and cultivation conditions. Hence, this study critically compared the characteristics and capacity to generate micro-/nanoparticles of fibroin extracted from the Thai silk and Vietnamese silk. Both Thai fibroin (SFT) and Vietnamese fibroin (SFV) were extracted and fabricated into micro-/nanoparticles using the same methods of desalination and condensation, respectively. Firstly, the amino acid compositions of SFT and SFV were determined and found to be similar, suggesting that the different cultivation conditions did not alter the fibroin chemical contents. Secondly, utilizing various analytical techniques, the SFT structure revealed less heavy chains, more light chains and P-25 glycoproteins, and lower crystallinity than those of SFV. Accordingly, compared to the particles formed by SFT, the SFV-based particles were significantly bigger (∼1700 nm vs. ∼150 nm), and possessed less drug (Amphotericin B) entrapment efficiency (64.3 ± 4.4% vs. 79.3 ± 5.1%), higher hemototoxicity, and less biostability in the blood. Conclusively, these differences add more insights for the appropriate applications of each fibroin kind to best promote its qualities and effectiveness.

Biomedical applications of silk and its role for intervertebral disc repair

Intervertebral disc (IVD) degeneration (IDD) is the main contributor to chronic low back pain. To date, the present therapies mainly focus on treating the symptoms caused by IDD rather than addressing the problem itself. For this reason, researchers have searched for a suitable biomaterial to repair and/or regenerate the IVD. A promising candidate to fill this gap is silk, which has already been used as a biomaterial for many years. Therefore, this review aims first to elaborate on the different origins from which silk is harvested, the individual composition, and the characteristics of each silk type. Another goal is to enlighten why silk is so suitable as a biomaterial, discuss its functionalization, and how it could be used for tissue engineering purposes. The second part of this review aims to provide an overview of preclinical studies using silk-based biomaterials to repair the inner region of the IVD, the nucleus pulposus (NP), and the IVD's outer area, the annulus fibrosus (AF). Since the NP and the AF differ fundamentally in their structure, different therapeutic approaches are required. Consequently, silk-containing hydrogels have been used mainly to repair the NP, and silk-based scaffolds have been used for the AF. Although most preclinical studies have shown promising results in IVD-related repair and regeneration, their clinical transition is yet to come.

Hemocompatibility Evaluation of Thai Bombyx mori Silk Fibroin and Its Improvement with Low Molecular Weight Heparin Immobilization

Bombyx mori silk fibroin (SF), from Nangnoi Srisaket 1 Thai strain, has shown potential for various biomedical applications such as wound dressing, a vascular patch, bone substitutes, and controlled release systems. The hemocompatibility of this SF is one of the important characteristics that have impacts on such applications. In this study, the hemocompatibility of Thai SF was investigated and its improvement by low molecular weight heparin (LMWH) immobilization was demonstrated. Endothelial cell proliferation on the SF and LMWH immobilized SF (Hep/SF) samples with or without fibroblast growth factor-2 (FGF-2) was also evaluated. According to hemocompatibility evaluation, Thai SF did not accelerate clotting time, excess stimulate complement and leukocyte activation, and was considered a non-hemolysis material compared to the negative control PTFE sheet. Platelet adhesion of SF film was comparable to that of the PTFE sheet. For hemocompatibility enhancement, LMWH was immobilized successfully and could improve the surface hydrophilicity of SF films. The Hep/SF films demonstrated prolonged clotting time and slightly lower complement and leukocyte activation. However, the Hep/SF films could not suppress platelet adhesion. The Hep/SF films demonstrated endothelial cell proliferation enhancement, particularly with FGF-2 addition. This study provides fundamental information for the further development of Thai SF as a hemocompatible biomaterial.

Macrophage Polarization Profiling on Native and Regenerated Silk Biomaterials

We investigated the plasticity and polarization of THP-1 cells on native and regenerated silk-based biomaterials to address the basic paradigm of immune response. Here, we report redox kinetics, adhesion morphology, and nitric oxide release patterns to identify specific subtypes of macrophages at different time points. Water-annealed silk film and native fibrous braids from Bombyx mori silkworms showed higher anti-inflammatory cytokine profiles or M2 subtypes (as evidenced by the enhanced expression of interleukin-10, interleukin-13, and interleukin-4). Ethanol-treated Bombyx mori silk films and Antheraea mylitta braids exhibited higher levels of pro-inflammatory cytokines or the M1 subtype (as evidenced by enhanced expression of interleukin-1, interleukin-6, interleukin-8, interferon-γ, TNF-α, and GM-CSF) in contact with healthy THP monocytes for 14 days; such a long study is unprecedented. Cytokine microarray analysis revealed the transition (M0-M1, M1-M2), plasticity, and stable phenotype of THP-1 cells in a variable stage in contact with different physicochemical properties of silk-based biomaterials. The detailed immunogenicity in the context of the physicochemical properties of native and regenerative silk-based biomaterials will enable us to accurately predict the possibility of a pro-/anti-inflammatory response. It will helps to predict the in vivo reprogramming and avoid fibrosis formation to enhance their clinical translational potential.

The innate immune response of self-assembling silk fibroin hydrogels

Silk has a long track record of use in humans, and recent advances in silk fibroin processing have opened up new material formats. However, these new formats and their applications have subsequently created a need to ascertain their biocompatibility. Therefore, the present aim was to quantify the haemocompatibility and inflammatory response of silk fibroin hydrogels. This work demonstrated that self-assembled silk fibroin hydrogels, as one of the most clinically relevant new formats, induced very low blood coagulation and platelet activation but elevated the inflammatory response of human whole blood in vitro. In vivo bioluminescence imaging of neutrophils and macrophages showed an acute, but mild, local inflammatory response which was lower than or similar to that induced by polyethylene glycol, a benchmark material. The time-dependent local immune response in vivo was corroborated by histology, immunofluorescence and murine whole blood analyses. Overall, this study confirms that silk fibroin hydrogels induce a similar immune response to that of PEG hydrogels, while also demonstrating the power of non-invasive bioluminescence imaging for monitoring tissue responses.

Silk biomaterials for vascular tissue engineering applications

Vascular tissue engineering is a rapidly growing field of regenerative medicine, which strives to find innovative solutions for vascular reconstruction. Considering the limited success of synthetic grafts, research impetus in the field is now shifted towards finding biologically active vascular substitutes bestowing in situ growth potential. In this regard, silk biomaterials have shown remarkable potential owing to their favorable inherent biological and mechanical properties. This review provides a comprehensive overview of the progressive development of silk-based small diameter (<6 mm) tissue-engineered vascular grafts (TEVGs), emphasizing their pre-clinical implications. Herein, we first discuss the molecular structure of various mulberry and non-mulberry silkworm silk and identify their favorable properties at the onset of vascular regeneration. The emergence of various state-of-the-art fabrication methodologies for the advancement of silk TEVGs is rationally appraised in terms of their in vivo performance considering the following parameters: ease of handling, long-term patency, resistance to acute thrombosis, stenosis and aneurysm formation, immune reaction, neo-tissue formation, and overall remodeling. Finally, we provide an update on the pre-clinical status of silk-based TEVGs, followed by current challenges and future prospects. STATEMENT OF SIGNIFICANCE: Limited availability of healthy autologous blood vessels to replace their diseased counterpart is concerning and demands other artificial substitutes. Currently available synthetic grafts are not suitable for small diameter blood vessels owing to frequent blockage. Tissue-engineered biological grafts tend to integrate well with the native tissue via remodeling and have lately witnessed remarkable success. Silk fibroin is a natural biomaterial, which has long been used as medical sutures. This review aims to identify several favorable properties of silk enabling vascular regeneration. Furthermore, various methodologies to fabricate tubular grafts are discussed and highlight their performance in animal models. An overview of our understanding to rationally improve the biological activity fostering the clinical success of silk-based grafts is finally discussed.

A Fast and Reliable Process to Fabricate Regenerated Silk Fibroin Solution from Degummed Silk in 4 Hours

Silk fibroin has a high potential for use in several approaches for technological and biomedical applications. However, industrial production has been difficult to date due to the lengthy manufacturing process. Thus, this work investigates a novel procedure for the isolation of non-degraded regenerated silk fibroin that significantly reduces the processing time from 52 h for the standard methods to only 4 h. The replacement of the standard degumming protocol by repeated short-term microwave treatments enabled the generation of non-degraded degummed silk fibroin. Subsequently, a ZnCl₂ solution was used to completely solubilize the degummed fibroin at only 45 °C with an incubation time of only 1 h. Desalting was performed by gel filtration. Based on these modifications, it was possible to generate a cytocompatible aqueous silk fibroin solution from degummed silk within only 4 h, thus shortening the total process time by 48 h without degrading the quality of the isolated silk fibroin solution.

Molecularly Imprinted Silk Fibroin Nanoparticles

Nanosized biomimetics prepared by the strategy of molecular imprinting, that is, the stamping of recognition sites by means of a template-assisted synthesis, are demonstrating potential as plastic antibodies in medicine, proving effective for cell imaging and targeted therapies. Most molecularly imprinted nanoparticles (MIP-NPs) are currently made of soft matter, such as polyacrylamide and derivatives. Yet, MIP-NPs biocompatibility is crucial for their effective translation into clinical uses. Here, we propose the original idea to synthesize fully biocompatible molecularly imprinted nanoparticles starting from the natural polymer silk fibroin (MIP SF-NPs), which is nontoxic and highly biocompatible. The conditions to produce MIP SF-NPs of different sizes (d_mean ∼ 50 nm; d_mean ∼ 100 nm) were set using the response surface method. The stamping of a single, high affinity (K_D = 57 × 10^-9 M), and selective recognition site per silk fibroin nanoparticle was demonstrated, together with the confirmation of nontoxicity. Additionally, MIP SF-NPs were used to decorate silk microfibers and silk nanofibers, providing a general means to add entailed biofunctionalities to materials.

MS1-type bioengineered spider silk nanoparticles do not exhibit toxicity in an in vivo mouse model

Background: Due to factors such as silk sequence, purification, degradation, morphology and functionalization, each silk variant should be individually tested for potential toxicity. Aim:  In vivo toxicological evaluation of the previously characterized bioengineered H2.1MS1 spider silk particles that can deliver chemotherapeutics to human epidermal growth factor receptor 2-positive breast cancer. Materials & methods: Silk nanoparticles (H2.1MS1 and control MS1) were administered intravenously to mice, and then the organismal response was assessed. Several parameters of acute and subchronic toxicity were analyzed, including animal mortality and behavior, nanosphere biodistribution, and histopathological analysis of internal organs. Also, the complete blood count, as well as the concentration of biochemical parameters and cytokines in the serum, were examined. Results & conclusion: No toxicity of the systemically administrated silk nanosphere was observed, indicating their potential application in biomedicine.

Polymeric hydrogel as a vitreous substitute: current research, challenges, and future directions

Vitreoretinal surgery is an essential approach to treat proliferative diabetic vitreopathy, retinal detachment, retinal tear, ocular trauma, and macular holes. The removal of the natural vitreous and the replacement with substitutes are critical steps for retina reattachment. Vitreous substitutes including silicone oil (SiO), air, sulfur hexafluoride (SF₆), and perfluoropropane (C₃F₈), have been widely applied in clinical practice. However, these substitutes are reported to cause complications such as emulsification, high intraocular pressure, and lens opacification. Polymeric hydrogels are a kind of material with favorable physical, mechanical properties, and adaptable biocompatibility, thus being highly expected to be ideal vitreous substitutes. Despite years of research, very few polymeric hydrogels can be applied practically in the vitreous cavity. In this review, we focus on the development of polymeric natural-based hydrogels and synthetic hydrogels. Particularly, we pay attention to recent advances in the novel stimuli-response and self-assembly supramolecular hydrogels. Characterized by easy injectability and long residence time, this kind of hydrogel becomes the potentially promising candidates for ideal vitreous substitutes. Finally, we evaluate the current challenges and provide the future directions of vitreous substitutes.

Co-delivery of inhalable therapies: Controlling active ingredients spatial distribution and temporal release

The management of respiratory diseases relies on the daily administration of multiple active pharmaceutical ingredients (APIs), leading to a lack of patient compliance and impaired quality of life. The frequency and dosage of the APIs result in increased side effects that further worsens the overall patient condition. Here, the manufacture of polymer-polymer core-shell microparticles for the sequential delivery of multiple APIs by inhalation delivery is reported. The microparticles, composed of biodegradable polymers silk fibroin (shell) and poly(L-lactic acid) (core), incorporating ciprofloxacin in the silk layer and ibuprofen (PLLA core) as the antibiotic and anti-inflammatory model APIs, respectively. The polymer-polymer core-shell structure and the spatial distribution of the APIs have been characterized using cutting-edge synchrotron macro ATR-FTIR technique, which was correlated with the respective API sequential release profiles. The APIs microparticles had a suitable size and aerosol properties for inhalation therapies (≤4.94 ± 0.21μm), with low cytotoxicity and immunogenicity in healthy lung epithelial cells. The APIs compartmentalization obtained by the microparticles not only could inhibit potential actives interactions but can provide modulation of the APIs release profiles via an inhalable single administration.

Silk fibroin safety in the eye: a review that highlights a concern

The biomedical use of silk as a suture dates back to antiquity. Fibroin is the structural element that determines the strength of silk and here we consider the safety of fibroin in its role in ophthalmology. The high mechanical strength of silk meant sufficiently thin threads could be made for eye microsurgery, but such usage was all but superseded by synthetic polymer sutures, primarily because silk in its entirety was more inflammatory. Significant immunological response can normally be avoided by careful manufacturing to provide high purity fibroin, and it has been utilised in this form for tissue engineering an array of fibre and film substrata deployed in research with cells of the eye. Films of fibroin can also be made transparent, which is a required property in the visual pathway. Transparent layers of corneal epithelial, stromal and endothelial cells have all been demonstrated with maintenance of phenotype, as have constructs supporting retinal cells. Fibroin has a lack of demonstrable infectious agent transfer, an ability to be sterilised and prepared with minimal contamination, long-term predictable degradation and low direct cytotoxicity. However, there remains a known ability to be involved in amyloid formation and potential amyloidosis which, without further examination, is enough to currently question whether fibroin should be employed in the eye given its innervation into the brain.

3D printing of silk microparticle reinforced polycaprolactone scaffolds for tissue engineering applications

Polycaprolactone (PCL) scaffolds have been widely investigated for tissue engineering applications, however, they exhibit poor cell adhesion and mechanical properties. Subsequently, PCL composites have been produced to improve the material properties. This study utilises a natural material, Bombyx mori silk microparticles (SMP) prepared by milling silk fibre, to produce a composite to enhance the scaffolds properties. Silk is biocompatible and biodegradable with excellent mechanical properties. However, there are no studies using SMPs as a reinforcing agent in a 3D printed thermoplastic polymer scaffold. PCL/SMP (10, 20, 30 wt%) composites were prepared by melt blending. Rheological analysis showed that SMP loading increased the shear thinning and storage modulus of the material. Scaffolds were fabricated using a screw-assisted extrusion-based additive manufacturing system. Scanning electron microscopy and X-ray microtomography was used to determine scaffold morphology. The scaffolds had high interconnectivity with regular printed fibres and pore morphologies within the designed parameters. Compressive mechanical testing showed that the addition of SMP significantly improved the compressive Young's modulus of the scaffolds. The scaffolds were more hydrophobic with the inclusion of SMP which was linked to a decrease in total protein adsorption. Cell behaviour was assessed using human adipose derived mesenchymal stem cells. A cytotoxic effect was observed at higher particle loading (30 wt%) after 7 days of culture. By day 21, 10 wt% loading showed significantly higher cell metabolic activity and proliferation, high cell viability, and cell migration throughout the scaffold. Calcium mineral deposition was observed on the scaffolds during cell culture. Large calcium mineral deposits were observed at 30 wt% and smaller calcium deposits were observed at 10 wt%. This study demonstrates that SMPs incorporated into a PCL scaffold provided effective mechanical reinforcement, improved the rate of degradation, and increased cell proliferation, demonstrating potential suitability for bone tissue engineering applications.

Natural Fibrous Protein for Advanced Tissue Engineering Applications: Focusing on Silk Fibroin and Keratin

As one of the important branches of natural biopolymer, natural fibrous protein has a lot of advantages including good mechanical properties, excellent biocompatibility, controllable biodegradability, renewability, abundant sources, and so on. Moreover, natural fibrous protein is also a protein that could only be used for structure supporting without any bioactivities, which attracts a lot of attentions in the field of tissue engineering scaffold. This chapter is taking silk fibroin and keratin as model materials of natural fibrous protein, focusing on their protein structure, chemical compositions, processing and extraction methods, chemical modification methods, and their applications in tissue engineering through advanced manufacturing.

Electrospun silk fibroin fibers for storage and controlled release of human platelet lysate

Human platelet lysate (hPL) is a pool of growth factors and cytokines able to induce regeneration of different tissues. Despite its good potentiality as therapeutic tool for regenerative medicine applications, hPL has been only moderately exploited in this field. A more widespread adoption has been limited because of its rapid degradation at room temperature that decreases its functionality. Another limiting factor for its extensive use is the difficulty of handling the hPL gels. In this work, silk fibroin-based patches were developed to address several points: improving the handling of hPL, enabling their delivery in a controlled manner and facilitating their storage by creating a device ready to use with expanded shelf life. Patches of fibroin loaded with hPL were synthesized by electrospinning to take advantage of the fibrous morphology. The release kinetics of the material was characterized and tuned through the control of fibroin crystallinity. Cell viability assays, performed with primary human dermal fibroblasts, demonstrated that fibroin is able to preserve the hPL biological activity and prolong its shelf-life. The strategy of storing and preserving small active molecules within a naturally-derived, protein-based fibrous scaffold was successfully implemented, leading to the design of a biocompatible device, which can potentially simplify the storage and the application of the hPL on a human patient, undergoing medical procedures such as surgery and wound care.

STATEMENT OF SIGNIFICANCE

Human platelets lysate (hPL) is a mixture of growth factors and cytokines able to induce the regeneration of damaged tissues. This study aims at enclosing hPL in a silk fibroin electrospun matrix to expand its utilization. Silk fibroin showed the ability to preserve the hPL activity at temperature up to 60 °C and the manipulation of fibroin's crystallinity provided a tool to modulate the hPL release kinetic. This entails the possibility to fabricate the hPL silk fibroin patches in advance and store them, resulting in an easy and fast accessibility and an expanded use of hPL for wound healing.

Heparin functionalization increases retention of TGF-β2 and GDF5 on biphasic silk fibroin scaffolds for tendon/ligament-to-bone tissue engineering

The tendon/ligament-to-bone transition (enthesis) is a highly specialized interphase tissue with structural gradients of extracellular matrix composition, collagen molecule alignment and mineralization. These structural features are essential for enthesis function, but are often not regenerated after injury. Tissue engineering is a promising strategy for enthesis repair. Engineering of complex tissue interphases such as the enthesis is likely to require a combination of biophysical, biological and chemical cues to achieve functional tissue regeneration. In this study, we cultured human primary adipose-derived mesenchymal stem cells (AdMCs) on biphasic silk fibroin scaffolds with integrated anisotropic (tendon/ligament-like) and isotropic (bone/cartilage like) pore alignment. We functionalized those scaffolds with heparin and explored their ability to deliver transforming growth factor β2 (TGF-β2) and growth/differentiation factor 5 (GDF5). Heparin functionalization increased the amount of TGF-β2 and GDF5 remaining attached to the scaffold matrix and resulted in biological effects at low growth factor doses. We analyzed the combined impact of pore alignment and growth factors on AdMSCs. TGF-β2 and pore anisotropy synergistically increased the expression of tendon/ligament markers and collagen I protein content. In addition, the combined delivery of TGF-β2 and GDF5 enhanced the expression of cartilage markers and collagen II protein content on substrates with isotropic porosity, whereas enthesis markers were enhanced in areas of mixed anisotropic/isotropic porosity. Altogether, the data obtained in this study improves current understanding on the combined effects of biological and structural cues on stem cell fate and presents a promising strategy for tendon/ligament-to-bone regeneration.

STATEMENT OF SIGNIFICANCE

Regeneration of the tendon/ligament-to-bone interphase (enthesis) is of significance in the repair of ruptured tendons/ligaments to bone to improve implant integration and clinical outcome. This study proposes a novel approach for enthesis regeneration based on a biomimetic and integrated tendon/ligament-to-bone construct, stem cells and heparin-based delivery of growth factors. We show that heparin can keep growth factors local and biologically active at low doses, which is critical to avoid supraphysiological doses and associated side effects. In addition, we identify synergistic effects of biological (growth factors) and structural (pore alignment) cues on stem cells. These results improve current understanding on the combined impact of biological and structural cues on the multi-lineage differentiation capacity of stem cells for regenerating complex tissue interphases.

Silk fibroin porous scaffolds by N2O foaming

Silk fibroin has acquired increasing interest for biomedical applications, and namely for the fabrication of scaffolds for tissue engineering, because of its highly positive biological interaction and the possibility to adapt the material to several application requirements by adopting different fabrication methods, in order to make films, sponges, fibers, nets or gels with predictable degradation times. For tissue engineering, in most cases porous scaffolds are required, in some cases possibly in situ forming and therefore fabricated in mild body-compatible conditions. In this work, we present a novel one-step method for the preparation of silk fibroin foams starting from water solutions and using low-pressure nitrous oxide gas as foaming agent. This foaming technique allows preparing fibroin porous scaffolds with easily tunable porosity, in mild processing conditions with the use of a relatively inert foaming agent saturating a fibroin water solution, that could be occasionally injected through a thin needle in the implantation site where expansion and foaming would occur. Optimal foaming processing conditions have been investigated, and the prepared foams have been characterized with Fourier Transform Infrared Spectroscopy (FTIR) compressive mechanical and rheological properties measurements, and by scanning electron microscopy and microCT.

* Fabrication and Characterization of Biphasic Silk Fibroin Scaffolds for Tendon/Ligament-to-Bone Tissue Engineering

Tissue engineering is an attractive strategy for tendon/ligament-to-bone interface repair. The structure and extracellular matrix composition of the interface are complex and allow for a gradual mechanical stress transfer between tendons/ligaments and bone. Thus, scaffolds mimicking the structural features of the native interface may be able to better support functional tissue regeneration. In this study, we fabricated biphasic silk fibroin scaffolds designed to mimic the gradient in collagen molecule alignment present at the interface. The scaffolds had two different pore alignments: anisotropic at the tendon/ligament side and isotropic at the bone side. Total porosity ranged from 50% to 80% and the majority of pores (80-90%) were <100-300 μm. Young's modulus varied from 689 to 1322 kPa depending on the type of construct. In addition, human adipose-derived mesenchymal stem cells were cultured on the scaffolds to evaluate the effect of pore morphology on cell proliferation and gene expression. Biphasic scaffolds supported cell attachment and influenced cytoskeleton organization depending on pore alignment. In addition, the gene expression of tendon/ligament, enthesis, and cartilage markers significantly changed depending on pore alignment in each region of the scaffolds. In conclusion, the biphasic scaffolds fabricated in this study show promising features for tendon/ligament-to-bone tissue engineering.

Silk/agarose scaffolds with tunable properties via SDS assisted rapid gelation

We developed a simple approach to fabricate silk/agarose scaffolds with tunable properties via controlling the gelation degree of silk fibroin.

Biomimetic Designing of Functional Silk Nanotopography Using Self-assembly

In nature inorganic-organic building units create multifunctional hierarchical architectures. Organic silk protein is particularly attractive in this respect because of its micro-nanoscale structural blocks that are attributed to sophisticated hierarchical assembly imparting flexibility and compressibility to designed biohybrid materials. In the present study, aqueous silk fibroin is assembled to form nano/microtopography on inorganic silica surface via a facile diffusion-limited aggregation process. This process is driven by electrostatic interaction and only possible at a specified aminated surface chemistry. The self-assembled topography depends on the age and concentration of protein solution as well as on the surface charge distribution of the template. The self-assembled silk trails closely resemble natural cypress leaf architecture, which is considered a structural analogue of neuronal cortex. This assembled surface significantly enhances anchorage of neuronal cell and cytoskeletal extensions, providing an effective nano/microtopographical cue for cellular recognition and guidance.

A novel low-profile thin-film nitinol/silk endograft for treating small vascular diseases

Since the introduction of various endovascular graft materials such as expanded polytetrafluoroethylene (e-PTFE) and Dacron^® polyester, they have been rapidly applied in endovascular devices for treating a variety of clinical situations. While present endovascular grafts have been successful in treating large blood vessels, there are still significant challenges and limitations for small and tortuous vessels to their use. Recently, our group has demonstrated the potential to use thin-film nitinol (TFN) as a novel material to develop endografts used in the treatment of a wide range of small vascular diseases because TFN is ultralow profile (that is, a few micrometers thick), relatively thromboresistant, and superelastic. While TFN has shown superior thromboresistance, its surface endothelialization is not rapid and sufficient. Therefore, our laboratory has been exploring the feasibility of using thin-film silk as a novel coating for facilitating rapid and confluent endothelial cell growth. The purpose of this study is to fabricate a low-profile composite endograft using thin layers of nitinol and silk, and to evaluate both thrombogenicity as well as endothelial cell and smooth muscle cell responses. This study also evaluates the functionality of the composite endograft using an in vitro blood circulation model. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 575-584, 2017.

Comprehensive characterization of well-defined silk fibroin surfaces: Toward multitechnique studies of surface modification effects

The study aims at a comprehensive surface characterization of untreated and oxygen plasma-treated silk fibroin with a particular focus on phenomena relevant to biointeraction and cell adhesion. For that purpose, a range of advanced surface diagnostic techniques is employed to thoroughly investigate well-defined and especially clean silk fibroin samples in a comparable setting. This includes surface chemistry and surface charges as factors, which control protein adsorption, but also hydration and swelling of the material as important parameters, which govern the mechanical stiffness at the interface with aqueous media. Oxygen plasma exposure of silk fibroin surfaces reveals that material ablation strongly predominates over the introduction of functional groups even for mild plasma conditions. A substantial increase in mechanical stiffness is identified as the most prominent effect upon this kind of plasma treatment. Regarding the experimental approach and the choice of techniques, the work goes beyond previous studies in this field and paves the way for well-founded investigations of other surface-selective modification procedures that enhance the applicability of silk fibroin in biomedical applications.

Silk-based biomaterials for sustained drug delivery

Silk presents a rare combination of desirable properties for sustained drug delivery, including aqueous-based purification and processing options without chemical cross-linkers, compatibility with common sterilization methods, controllable and surface-mediated biodegradation into non-inflammatory by-products, biocompatibility, utility in drug stabilization, and robust mechanical properties. A versatile silk-based toolkit is currently available for sustained drug delivery formulations of small molecule through macromolecular drugs, with a promise to mitigate several drawbacks associated with other degradable sustained delivery technologies in the market. Silk-based formulations utilize silk's well-defined nano- through microscale structural hierarchy, stimuli-responsive self-assembly pathways and crystal polymorphism, as well as sequence and genetic modification options towards targeted pharmaceutical outcomes. Furthermore, by manipulating the interactions between silk and drug molecules, near-zero order sustained release may be achieved through diffusion- and degradation-based release mechanisms. Because of these desirable properties, there has been increasing industrial interest in silk-based drug delivery systems currently at various stages of the developmental pipeline from pre-clinical to FDA-approved products. Here, we discuss the unique aspects of silk technology as a sustained drug delivery platform and highlight the current state of the art in silk-based drug delivery. We also offer a potential early development pathway for silk-based sustained delivery products.

Silk/chitosan biohybrid hydrogels and scaffolds via green technology

A green technology approach towards conjugation of biopolymers for designing biohybrid silk/chitosan based biomaterials for therapeutic applications.

Silk hydrogels from non-mulberry and mulberry silkworm cocoons processed with ionic liquids

Matrices based on silk fibroin from the non-mulberry silkworm Antheraea mylitta and the mulberry silkworm Bombyx mori have demonstrated good applicability in regenerative medicine. However, the cocoons of A. mylitta are underutilized in part due to their lack of solubility in traditional organic solvents. Therefore, the present work investigates the solubilization and processing of degummed fibers obtained from the cocoons of both silkworm species into hydrogels using ionic liquids (ILs). The developed hydrogels exhibited a rubbery consistency, viscoelastic behavior and rapid degradation in the presence of protease XIV. Scanning electron and confocal microscopy images suggest that human adipose stem cells (hASCs) are able to adhere to and migrate at different levels within the hydrogel structures. Moreover, the MTS assay demonstrated the maintenance of cell metabolic activity for up to 28 days, while DNA quantification showed that hASCs were able to proliferate on the seeded hydrogels. The findings indicate that complete IL removal from the fabricated hydrogels results in a positive hASCs cellular response. Thus the present approach provides a unique opportunity to broaden the processability and application of silk fibroin obtained from A. mylitta cocoons for regenerative medicine, namely cartilage regeneration.

Multifunctional silk-heparin biomaterials for vascular tissue engineering applications

Over the past 30 years, silk has been proposed for numerous biomedical applications that go beyond its traditional use as a suture material. Silk sutures are well tolerated in humans, but the use of silk for vascular engineering applications still requires extensive biocompatibility testing. Some studies have indicated a need to modify silk to yield a hemocompatible surface. This study examined the potential of low molecular weight heparin as a material for refining silk properties by acting as a carrier for vascular endothelial growth factor (VEGF) and improving silk hemocompatibility. Heparinized silk showed a controlled VEGF release over 6 days; the released VEGF was bioactive and supported the growth of human endothelial cells. Silk samples were then assessed using a humanized hemocompatibility system that employs whole blood and endothelial cells. The overall thrombogenic response for silk was very low and similar to the clinical reference material polytetrafluoroethylene. Despite an initial inflammatory response to silk, apparent as complement and leukocyte activation, the endothelium was maintained in a resting, anticoagulant state. The low thrombogenic response and the ability to control VEGF release support the further development of silk for vascular applications.

The optimization of a scaffold for cartilage regeneration

Natural polymers offer various advantages in cartilage tissue engineering applications, thanks to their intrinsic bioactivity and adaptability, which can be exploited for the optimization of scaffold properties. In particular, silk fibroin has multifunctional features driven by the self-assembly of molecular subunits in appropriate environmental conditions. For these reasons, it was used in combination with hyaluronic acid to produce porous sponges for cartilage regeneration. The added amount of hyaluronic acid and the cross-linking with genipin modulated scaffold properties in a synergistic way, showing a strong inter-correlation among macroscopic and microscopic characteristics. Interestingly, hyaluronic acid affected silk fibroin conformation and induced a physical separation between the two material components in absence of genipin. Instead, this was prevented by the cross-linking reaction, resulting in a more interspersed network of protein and polysaccharide molecules partially resembling the structure of cartilage extracellular matrix. In addition, the systematic evaluation of sponge properties and how they can be modulated will represent a significant starting point for the interpretation of the complex outcomes driven by the scaffold in vitro and in vivo.

Blood compatibility of polymers derived from natural materials

Several polymers derived from natural materials are effective for tissue engineering or drug delivery applications, due to specific properties, such as biocompatibility, biodegradability, and structural activity. Their blood compatibility needs to be carefully evaluated to avoid thrombosis and other material-related adverse events in the hematic environment. We compared the surface properties and blood compatibility of protein and polysaccharide polymers, including fibroin, gelatin, and chitosan. Both fibroin and chitosan showed good hemocompatibility, with low platelet adhesion and spreading. Chitosan induced strong interactions with plasma proteins, especially with albumin. It was hypothesized that surface passivation by albumin inhibited the adsorption of other procoagulant and proadhesive proteins on chitosan and fibroin films, which limited platelet spreading. However, the significant and rapid polymer swelling encouraged protein entrapment within the soft, gelatin films, inducing higher platelet adhesion and activation. Thrombin generation assay confirmed the higher blood compatibility of chitosan and fibroin with regard to clotting.

Fibroin scaffold repairs critical-size bone defects in vivo supported by human amniotic fluid and dental pulp stem cells

The main aim of this study was the comparative evaluation of fibroin scaffolds combined with human stem cells, such as dental pulp stem cells (hDPSCs) and amniotic fluid stem cells (hAFSCs), used to repair critical-size cranial bone defects in immunocompromised rats. Two symmetric full-thickness cranial defects on each parietal region of rats have been replenished with silk fibroin scaffolds with or without preseeded stem cells addressed toward osteogenic lineage in vitro. Animals were euthanized after 4 weeks postoperatively and cranial tissue samples were taken for histological analysis. The presence of human cells in the new-formed bone was confirmed by confocal analysis with an antibody directed to a human mitochondrial protein. Fibroin scaffolds induced mature bone formation and defect correction, with higher bone amount produced by hAFSC-seeded scaffolds. Our findings demonstrated the strong potential of stem cells/fibroin bioengineered constructs for correcting large cranial defects in animal model and is likely a promising approach for the reconstruction of human large skeletal defects in craniofacial surgery.

Impact of processing parameters on the haemocompatibility of Bombyx mori silk films

Silk has traditionally been used for surgical sutures due to its lasting strength and durability; however, the use of purified silk proteins as a scaffold material for vascular tissue engineering goes beyond traditional use and requires application-orientated biocompatibility testing. For this study, a library of Bombyx mori silk films was generated and exposed to various solvents and treatment conditions to reflect current silk processing techniques. The films, along with clinically relevant reference materials, were exposed to human whole blood to determine silk blood compatibility. All substrates showed an initial inflammatory response comparable to polylactide-co-glycolide (PLGA), and a low to moderate haemostasis response similar to polytetrafluoroethylene (PTFE) substrates. In particular, samples that were water annealed at 25 °C for 6 h demonstrated the best blood compatibility based on haemostasis parameters (e.g. platelet decay, thrombin-antithrombin complex, platelet factor 4, granulocytes-platelet conjugates) and inflammatory parameters (e.g. C3b, C5a, CD11b, surface-associated leukocytes). Multiple factors such as treatment temperature and solvent influenced the biological response, though no single physical parameter such as β-sheet content, isoelectric point or contact angle accurately predicted blood compatibility. These findings, when combined with prior in vivo data on silk, support a viable future for silk-based vascular grafts.

The influence of elasticity and surface roughness on myogenic and osteogenic-differentiation of cells on silk-elastin biomaterials

The interactions of C2C12 myoblasts and human bone marrow stem cells (hMSCs) with silk-tropoelastin biomaterials, and the capacity of each to promote attachment, proliferation, and either myogenic- or osteogenic-differentiation were investigated. Temperature-controlled water vapor annealing was used to control beta-sheet crystal formation to generate insoluble silk-tropoelastin biomaterial matrices at defined ratios of the two proteins. These ratios controlled surface roughness and micro/nano-scale topological patterns, and elastic modulus, stiffness, yield stress, and tensile strength. A combination of low surface roughness and high stiffness in the silk-tropoelastin materials promoted proliferation and myogenic-differentiation of C2C12 cells. In contrast, high surface roughness with micro/nano-scale surface patterns was favored by hMSCs. Increasing the content of human tropoelastin in the silk-tropoelastin materials enhanced the proliferation and osteogenic-differentiation of hMSCs. We conclude that the silk-tropoelastin composition facilitates fine tuning of the growth and differentiation of these cells.

Regulation of silk material structure by temperature-controlled water vapor annealing

We present a simple and effective method to obtain refined control of the molecular structure of silk biomaterials through physical temperature-controlled water vapor annealing (TCWVA). The silk materials can be prepared with control of crystallinity, from a low content using conditions at 4 °C (α helix dominated silk I structure), to highest content of ∼60% crystallinity at 100 °C (β-sheet dominated silk II structure). This new physical approach covers the range of structures previously reported to govern crystallization during the fabrication of silk materials, yet offers a simpler, green chemistry, approach with tight control of reproducibility. The transition kinetics, thermal, mechanical, and biodegradation properties of the silk films prepared at different temperatures were investigated and compared by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), uniaxial tensile studies, and enzymatic degradation studies. The results revealed that this new physical processing method accurately controls structure, in turn providing control of mechanical properties, thermal stability, enzyme degradation rate, and human mesenchymal stem cell interactions. The mechanistic basis for the control is through the temperature-controlled regulation of water vapor to control crystallization. Control of silk structure via TCWVA represents a significant improvement in the fabrication of silk-based biomaterials, where control of structure-property relationships is key to regulating material properties. This new approach to control crystallization also provides an entirely new green approach, avoiding common methods that use organic solvents (methanol, ethanol) or organic acids. The method described here for silk proteins would also be universal for many other structural proteins (and likely other biopolymers), where water controls chain interactions related to material properties.

Stabilization of Bombyx mori silk fibroin/sericin films by crosslinking with PEG-DE 600 and genipin

Silk fibroin and sericin have been widely used as a biomaterial for many different purposes. In this study, the properties of crosslinked fibroin/sericin 90/10 films have been investigated using two different crosslinkers poly(ethylene glycol) diglycidyl ether 600 (PEG-DE) and genipin. The films were characterized by molecular weight, fibroin conformation, crystallinity, and thermal stability. Both PEG-DE and genipin cross-linkers stabilized the random/α-helix structures in the fibroin and prevented phase separation between fibroin and sericin that occurs with non-crosslinked materials. The genipin crosslinked film displayed highest elongation at break and tensile strength. The in vitro cytotoxicity of the materials was assessed by lactate dehydrogenase activity and the viability was based on the tetrazolium salt and WST-1 tests. All films exhibited good biocompatibility and supported cell adhesion and proliferation.

Biomaterials derived from silk-tropoelastin protein systems

A structural protein blend system based on silkworm silk fibroin and recombinant human tropoelastin is described. Silk fibroin, a semicrystalline fibrous protein with beta-sheet crystals provides mechanical strength and controllable biodegradation, while tropoelastin, a noncrystallizable elastic protein provides elasticity. Differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC) indicated that silk becomes miscible with tropoelastin at different blend ratios, without macrophase separation. Fourier transform infrared spectroscopy (FTIR) revealed secondary structural changes of the blend system (beta-sheet content) before and after methanol treatment. Atomic Force Microscopy (AFM) nano-indentation demonstrated that blending silk and tropoelastin at different ratios resulted in modification of mechanical features, with resilience from approximately 68%- approximately 97%, and elastic modulus between 2 and 9 Mpa, depending on the ratio of the two polymers. Some of these values are close to those of native aortic elastin or elastin-like polypeptides. Significantly, during blending and drying silk-tropoelastin form micro- and nano-scale porous morphologies which promote human mesenchymal stem cell attachment and proliferation. These blends offer a new protein biomaterial system for cell support and tailored biomaterial properties to match mechanical needs.

The synergistic effects of 3-D porous silk fibroin matrix scaffold properties and hydrodynamic environment in cartilage tissue regeneration

Autologous cell-based tissue engineering using three-dimensional porous scaffolds has provided a good option for the repair of cartilage defects. Silk fibroin-based scaffolds are naturally degradable materials with excellent biocompatibility and robust mechanical properties, indicating potential applications in cartilage tissue engineering. In this study, silk fibroin scaffolds prepared by freeze-drying (FD) and salt-leaching (SL300 and SL500) were fully characterized and used to study the effects of silk fibroin scaffold properties on chondrocyte attachment, proliferation and differentiation. The synergistic effects of scaffold properties and hydrodynamic environment generated by in vitro rocking culture were also investigated using static cultures as control. FD scaffolds with small pore size and lower porosity increased cell attachment but inhibited cell penetration and limited cell proliferation and differentiation. In contrast, SL scaffolds displaying a bigger pore size, higher porosity and crystallinity resulted in homogenous cell distribution, increasing cell proliferation and advanced chondrocyte differentiation in terms of their spherical morphology, predominant chondrogenic gene expression and abundant cartilaginous extracellular matrix production. A hydrodynamic environment was beneficial to chondrocyte proliferation, differentiation, and integrin gene expression in a pore size dependent manner with superior cartilage matrix production but limited hypertrophic differentiation obtained using chondrocyte-seeded SL500 scaffolds. Integrin alpha5beta1 might mediate these effects. Chondrocyte/SL500 silk fibroin constructs obtained under in vitro rocking culture might serve as an excellent implant for in vivo cartilage defect reparation.