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.

Simultaneous control of the mechanical properties and adhesion of human umbilical vein endothelial cells to suppress platelet adhesion on a supramolecular substrate

The demand for artificial blood vessels to treat vascular disease will continue to increase in the future. To expand the application of blood-compatible poly(2-methoxyethyl acrylate) (pMEA) to artificial blood vessels, control of the mechanical properties of pMEA is established using supramolecular cross-links based on inclusion complexation of acetylated cyclodextrin. The mechanical properties, such as Young's modulus and toughness, of these pMEA-based elastomers change with the amount of cross-links, maintaining tissue-like behavior (J-shaped stress–strain curve). Regardless of the cross-links, the pMEA-based elastomers exhibit low platelet adhesion properties (approximately 3% platelet adherence) compared with those of poly(ethylene terephthalate), which is one of the commercialized materials for artificial blood vessels. Contact angle measurements imply a shift of supramolecular cross-links in response to the surrounding environment. When immersed in water, hydrophobic supramolecular cross-links are buried within the interior of the materials, thereby exposing pMEA chains to the aqueous environment; this is why supramolecular cross-links do not affect the platelet adhesion properties. In addition, the elastomers exhibit stable adhesion to human umbilical vein endothelial cells. This report shows the potential of combining supramolecular cross-links and pMEA.

Supramolecular cross-links in poly(2-methoxyethyl acrylate) enhanced mechanical properties of the polymers maintaining high blood compatibility. The high blood compatibility suggests a potential for artificial blood vessel.

Preparation and Biocompatibility Characterization of Silk Fibroin 3D Scaffolds

In this paper, three different mass fractions of sodium carbonate were used for degumming to obtain different degrees of damaged silk fibroin fibers, which were then treated with formic acid to shrink and bond them into 3D scaffolds. The structure and performance of silk fibroin fibers and silk fibroin 3D scaffolds were characterized by scanning electron microscopy, infrared spectroscopy, X-ray diffraction, a differential thermal scanner, a universal materials testing machine, and laser confocal microscopy, and the degradation performance was tested by protease degradation. The results showed that an excessive mass fraction of sodium carbonate would cause partial hydrolysis of fibroin fibers, decrease the mechanical properties of fibroin fiber, increase the surface roughness of fibroin fibers, and make mouse embryonic fibroblasts easier to adhere and grow. Silk fibroin fibers were slightly dissolved, shrunk, and dispersed in formic acid. The mass fraction of sodium carbonate can adjust the enzymatic degradation rate of the silk fibroin 3D scaffolds. With the extension of the degradation time, minerals will be deposited on the surface of the scaffolds. The results show that the silk fibroin 3D scaffolds have biocompatibility, mechanical properties, and degradability, which provides a good material for a barrier biofilm in the future.

Evaluation of silk-based bioink during pre and post 3D bioprinting: A review

During past few decades, the demand for the replacement of damaged organs is increasing consistently. This is due to the advancement in tissue engineering, which opens the possibility of regeneration of damaged organs or tissues into functional parts with the help of 3D bioprinting. Bioprinting technology presents an excellent potential to develop complex structures with precise control over cell suspension and structure. A brief description of different types of 3D bioprinting techniques, including inkjet-based, laser-based, and extrusion-based bioprinting is presented here. Due to innate advantageous features like tunable biodegradability, biocompatibility, elasticity and mechanical robustness, silk has carved a niche in the realm of tissue engineering. In this review article, the focus is to highlight the possible approach of exploring silk as bioink for fabrication of bioprinted implants using 3D bioprinting. This review discusses different type of degumming, dissolution techniques for extraction of proteins from different sources of silk. Different recently reported 3D bioprinting techniques suitable for silk-based bioink are further elaborated. Postprinting characterization of resultant scaffolds are also describe here. However, there is an astounding progress in 3D bioprinting technology, still there is a need to develop further the current bioprinting technology to make it suitable for generation of heterogeneous tissue construct. The possibility of utilizing the adhesive property of sericin to consider it as bioink is elaborated.

Functional Protein-Based Bioinspired Nanomaterials: From Coupled Proteins, Synthetic Approaches, Nanostructures to Applications

Protein-based bioinspired nanomaterials (PBNs) combines the advantage of the size, shape, and surface chemistry of nanomaterials, the morphology and functions of natural materials, and the physical and chemical properties of various proteins. Recently, there are many exciting developments on biomimetic nanomaterials using proteins for different applications including, tissue engineering, drug delivery, diagnosis and therapy, smart materials and structures, and water collection and separation. Protein-based biomaterials with high biocompatibility and biodegradability could be modified to obtain the healing effects of natural organisms after injury by mimicking the extracellular matrix. For cancer and other diseases that are difficult to cure now, new therapeutic methods involving different kinds of biomaterials are studied. The nanomaterials with surface modification, which can achieve high drug loading, can be used as drug carriers to enhance target and trigger deliveries. For environment protection and the sustainability of the world, protein-based nanomaterials are also applied for water treatment. A wide range of contaminants from natural water source, such as organic dyes, oil substances, and multiple heavy ions, could be absorbed by protein-based nanomaterials. This review summarizes the formation and application of functional PBNs, and the details of their nanostructures, the proteins involved, and the synthetic approaches are addressed.

A tranilast and BMP-2 based functional bilayer membrane is effective for the prevention of epidural fibrosis during spinal lamina reconstruction

Failed Back Surgery Syndrome (FBSS) is a common complication of lumbar surgery.

Local biological effects of adipose stromal vascular fraction delivery systems after subcutaneous implantation in a murine model

The aim of this study was to test alginate beads and silk fibroin non-woven mats as stromal vascular fraction delivery systems to support cell implantation for tissue repair and regeneration, through trophic and immunomodulant paracrine signaling. Furthermore, in vivo scaffold biocompatibility was histologically analyzed in a murine model at different time endpoints, with particular focus on construct-induced vascularization and neoangiogenesis. The fibroin mat induced a typical foreign body reaction, recruiting macrophages and giant cells and concurrently promoted neovascularization of the implanted construct. Conversely, alginate beads triggered a more circumscribed, chronic inflammatory reaction, which decreased over time. The combined in vivo implantation of alginate beads and fibroin mat with stromal vascular fraction promoted vascularization and integration of scaffolds into the surrounding subcutaneous environment. The new blood vessel ingrowth should, hopefully, support engineered cell viability and functionality, as well as the transport of soluble bioactive molecules. Due to their neovascularization properties, stromal vascular fraction administration, using alginate or fibroin scaffolds, is a new, promising, cost-effective tissue engineering approach.

Development of advanced antimicrobial and sterilized plasma polypropylene grafted muga (Antheraea assama) silk as suture biomaterial

Surface modification of silk fibroin (SF) materials using environmentally friendly and non-hazardous process to tailor them for specific application as biomaterials has drawn a great deal of interest in the field of biomedical research. To further explore this area of research, in this report, polypropylene (PP) grafted muga (Antheraea assama) SF (PP-AASF) suture is developed using plasma treatment and plasma graft polymerization process. For this purpose, AASF is first sterilized in argon (Ar) plasma treatment followed by grafting PP onto its surface. AASF is a non-mulberry variety having superior qualities to mulberry SF and is still unexplored in the context of suture biomaterial. AASF, Ar plasma treated AASF (AASFAr) and PP-AASF are subjected to various characterization techniques for better comparison and the results are attempted to correlate with their observed properties. Excellent mechanical strength, hydrophobicity, antibacterial behavior, and remarkable wound healing activity of PP-AASF over AASF and AASFAr make it a promising candidate for application as sterilized suture biomaterial.

Textile cell-free scaffolds for in situ tissue engineering applications

In this article, the benefits offered by micro-fibrous scaffold architectures fabricated by textile manufacturing techniques are discussed: How can established and novel fiber-processing techniques be exploited in order to generate templates matching the demands of the target cell niche? The problems related to the development of biomaterial fibers (especially from nature-derived materials) ready for textile manufacturing are addressed. Attention is also paid on how biological cues may be incorporated into micro-fibrous scaffold architectures by hybrid manufacturing approaches (e.g. nanofiber or hydrogel functionalization). After a critical review of exemplary recent research works on cell-free fiber based scaffolds for in situ TE, including clinical studies, we conclude that in order to make use of the whole range of favors which may be provided by engineered fibrous scaffold systems, there are four main issues which need to be addressed: (1) Logical combination of manufacturing techniques and materials. (2) Biomaterial fiber development. (3) Adaption of textile manufacturing techniques to the demands of scaffolds for regenerative medicine. (4) Incorporation of biological cues (e.g. stem cell homing factors).

Molecular architecture of silk fibroin of Indian golden silkmoth, Antheraea assama

The golden silk spun by Indian golden silkmoth Antheraea assama, is regarded for its shimmering golden luster, tenacity and value as biomaterial. This report describes the gene coding for golden silk H-fibroin (AaFhc), its expression, full-length sequence and structurally important motifs discerning the underlying genetic and biochemical factors responsible for its much sought-after properties. The coding region, with biased isocodons, encodes highly repetitious crystalline core, flanked by a pair of 5′ and 3′ non-repetitious ends. AaFhc mRNA expression is strictly territorial, confined to the posterior silk gland, encoding a protein of size 230 kDa, which makes homodimers making the elementary structural units of the fibrous core of the golden silk. Characteristic polyalanine repeats that make tight β-sheet crystals alternate with non-polyalanine repeats that make less orderly antiparallel β-sheets, β-turns and partial α-helices. Phylogenetic analysis of the conserved N-terminal amorphous motif and the comparative analysis of the crystalline region with other saturniid H-fibroins reveal that AaFhc has longer, numerous and relatively uniform repeat motifs with lower serine content that assume tighter β-crystals and denser packing, which are speculated to be responsible for its acclaimed properties of higher tensile strength and higher refractive index responsible for golden luster.

The in vitro characterization of a gelatin scaffold, prepared by cryogelation and assessed in vivo as a dermal replacement in wound repair

A sheet gelatin scaffold with attached silicone pseudoepidermal layer for wound repair purposes was produced by a cryogelation technique. The resulting scaffold possessed an interconnected macroporous structure with a pore size distribution of 131 ± 17 μm at one surface decreasing to 30 ± 8 μm at the attached silicone surface. The dynamic storage modulus (G') and mechanical stability were comparable to the clinical gold standard dermal regeneration template, Integra®. The scaffolds were seeded in vitro with human primary dermal fibroblasts. The gelatin based material was not only non-cytotoxic, but over a 28 day culture period also demonstrated advantages in cell migration, proliferation and distribution within the matrix when compared with Integra®. When seeded with human keratinocytes, the neoepidermal layer that formed over the cryogel scaffold appeared to be more advanced and mature when compared with that formed over Integra®. The in vivo application of the gelatin scaffold in a porcine wound healing model showed that the material supports wound healing by allowing host cellular infiltration, biointegration and remodelling. The results of our in vitro and in vivo studies suggest that the gelatin based scaffold produced by a cryogelation technique is a promising material for dermal substitution, wound healing and other potential biomedical applications.

Microcryogels as injectable 3-D cellular microniches for site-directed and augmented cell delivery

The success of cell therapy for tissue repair and regeneration demands efficient and reliable cell delivery methods. Here we established a novel microengineered cryogel (microcryogel) array chip containing microcryogels with predefined size and shape as injectable cell delivery vehicles. The microscale macroporous cryogels enabled automatic and homogeneous loading of tailored cellular niches (e.g. cells, matrices, bioactive factors) and could be easily harvested from the ready-to-use array chip. In contrast to microscale hydrogels, microcryogels exhibited excellent elasticity and could retain their shape and integrity after injection through the microsyringe routinely used for cell therapy. Human mesenchymal stromal cells loaded within microcryogels could be shielded from the mechanical insult and necrosis caused by during direct cell injection. After subcutaneous injection to the mice, cell-loaded microcryogels exhibited concentrated localization and enhanced retention at the injection site compared to dissociated cells. To demonstrate the potential therapeutic application for ischemic diseases, site-directed induction of angiogenesis was achieved subcutaneously in mice 2weeks after injection of NIH/3T3 fibroblast-loaded microcryogels, indicating long-term engraftment, accumulative paracrine stimulation and augmented host tissue integration. Our results convincingly showed the great promise of microcryogels as 3-D cellular microniches and injectable cell delivery vehicles to tackle major challenges faced by cell therapy-based regenerative medicine including shear-induced damages, uncontrolled localization, poor retention, limited cellular survival and functionalities in vivo.

Biocompatibility study of a silk fibroin-chitosan scaffold with adipose tissue-derived stem cells in vitro

The use of tissue engineering technology in the repair of spinal cord injury (SCI) is a topic of current interest. The success of the repair of the SCI is directly affected by the selection of suitable seed cells and scaffold materials with an acceptable biocompatibility. In this study, adipose tissue-derived stem cells (ADSCs) were incorporated into a silk fibroin-chitosan scaffold (SFCS), which was constructed using a freeze-drying method, in order to assess the biocompatibility of the ADSCs and the SFCS and to provide a foundation for the use of tissue engineering technology in the repair of SCI. Following the seeding of the cells onto the scaffold, the adhesion characteristics of the ADSCs and the SFCS were assessed. A significant difference was observed between the experimental group (a composite of the ADSCs with the SFCS) and the control group (ADSCs without the scaffold) following a culture period of 6 h (P<0.05). The differences in the results at the following time-points were statistically insignificant (P>0.05). The use of an MTT assay to assess the proliferation of the cells on the scaffold revealed that there were significant differences between the experimental and control groups following culture periods of 2 and 4 days (P<0.05). However, the results at the subsequent time-points were not statistically significantly different (P>0.05). Scanning electron microscopy (SEM), using hematoxylin and eosin (H&E) staining, was used to observe the cellular morphology following seeding, and this revealed that the cells displayed the desired morphology. The results indicate that ADSCs have a good biocompatibility with a SFCS and thus provide a foundation for further studies using tissue engineering methods for the repair of SCI.

Silk fibroin in tissue engineering

Tissue engineering (TE) is a multidisciplinary field that aims at the in vitro engineering of tissues and organs by integrating science and technology of cells, materials and biochemical factors. Mimicking the natural extracellular matrix is one of the critical and challenging technological barriers, for which scaffold engineering has become a prime focus of research within the field of TE. Amongst the variety of materials tested, silk fibroin (SF) is increasingly being recognized as a promising material for scaffold fabrication. Ease of processing, excellent biocompatibility, remarkable mechanical properties and tailorable degradability of SF has been explored for fabrication of various articles such as films, porous matrices, hydrogels, nonwoven mats, etc., and has been investigated for use in various TE applications, including bone, tendon, ligament, cartilage, skin, liver, trachea, nerve, cornea, eardrum, dental, bladder, etc. The current review extensively covers the progress made in the SF-based in vitro engineering and regeneration of various human tissues and identifies opportunities for further development of this field.

Use of nitrocellulose membranes as a scaffold in cell culture

Nitrocellulose membranes, one of the most important and oldest cellulose derivatives, are commonly used for nucleic acid and protein detection in research and diagnostic applications. However, a limited number of studies have explored whether they can act as scaffolds for cell growth. In this study, we investigated this polymeric material for its ability to support the growth of human cells. Eight established cell lines were examined for adherence, growth, spread, and survival on nitrocellulose membranes by optical microscopy after hematoxylin and eosin and/or immunocytochemical staining and by scanning electron microscopy. Apoptosis and leakage of lactate dehydrogenase (LDH) were also assessed. All cells readily adhered to and spread on the surface of nitrocellulose membranes as well as coverslips, and the cells maintained the expression of digestive system-specific genes. No significant change was detected in apoptosis or leakage of LDH from cells grown on nitrocellulose membranes. These results suggested that nitrocellulose membranes have a suitable cytocompatibility towards human cells and that they might be used for tissue-engineering scaffolds. Moreover, we demonstrate an additional and underused property of nitrocellulose of specific relevance to microscopic imaging, as it can be rendered virtually transparent, thus the cells growing on such membranes can be observed directly under an optical microscope after staining.

Biomaterials to prevascularize engineered tissues

Tissue engineering promises to restore tissue and organ function following injury or failure by creating functional and transplantable artificial tissues. The development of artificial tissues with dimensions that exceed the diffusion limit (1-2 mm) will require nutrients and oxygen to be delivered via perfusion (or convection) rather than diffusion alone. One strategy of perfusion is to prevascularize tissues; that is, a network of blood vessels is created within the tissue construct prior to implantation, which has the potential to significantly shorten the time of functional vascular perfusion from the host. The prevascularized network of vessels requires an extracellular matrix or scaffold for 3D support, which can be either natural or synthetic. This review surveys the commonly used biomaterials for prevascularizing 3D tissue engineering constructs.

Vascularization is the key challenge in tissue engineering

The main limitation in engineering in vitro tissues is the lack of a sufficient blood vessel system - the vascularization. In vivo almost all tissues are supplied by these endothelial cell coated tubular networks. Current strategies to create vascularized tissues are discussed in this review. The first strategy is based on the endothelial cells and their ability to form new vessels known as neoangiogenesis. Herein prevascularization techniques are compared to approaches in which biomolecules, such as growth factors, cytokines, peptides and proteins as well as cells are applied to generate new vessels. The second strategy is focused on scaffold-based techniques. Naturally-derived scaffolds, which contain vessels, are distinguished from synthetically manufactured matrices. Advantages and pitfalls of the approaches to create vascularized tissues in vitro are outlined and feasible future strategies are discussed.

Structural analysis and biomedical applications of dextran produced by a new isolate Pediococcus pentosaceus screened from biodiversity hot spot Assam

Dextran produced by a natural isolate of Pediococcus pentosaceus, screened from Assam, in the Northeastern region of India, was estimated, purified, structure characterised and functionality analysed. The dextran concentration in the cell free supernatant of the isolate P. pentosaceus was 10.2mg/ml. FT-IR analysis revealed the hydroxyl and carboxyl functional groups present in the dextran. (1)H NMR and (13)C NMR spectral data revealed that the dextran has a linear backbone of alpha-(1-->6) linked D-glucose residues. The decrease in viscosity of dextran solution with the increase in shear rate, threw light on its typical non-Newtonian pseudoplastic behaviour. The cytotoxicity tests on human cervical cancer (HeLa) cell line was studied which showed the dextran is non-toxic and biocompatible, rendering it safe for drug delivery, tissue engineering and various other biomedical applications.

Tissue Response to Subcutaneously Implanted Recombinant Spider Silk: An in Vivo Study

Spider silk is an interesting biomaterial for medical applications. Recently, a method for production of recombinant spider silk protein (4RepCT) that forms macroscopic fibres in physiological solution was developed. Herein, 4RepCT and Mersilk^TM (control) fibres were implanted subcutaneously in rats for seven days, without any negative systemic or local reactions. The tissue response, characterised by infiltration of macrophages and multinucleated cells, was similar with both fibres, while only the 4RepCT-fibres supported ingrowth of fibroblasts and newly formed capillaries. This in vivo study indicates that 4RepCT-fibres are well tolerated and could be used for medical applications, e.g., tissue engineering.