In Vitro Study of the Proteolytic Degradation ofAntheraea pernyiSilk Fibroin

Preparation of fibre-reinforced PLA-collagen@PLA-PCL@PCL-gelatin three-layer vascular graft by EDC/NHS cross-linking and its performance study

In this study, a three-layer small diameter artificial vascular graft with a structure similar to that of natural blood vessels was first constructed by triple-step electrospinning technology, in which polylactic acid (PLA) and collagen (COL) were used for the inner layer, polylactic acid and polycaprolactone (PCL) was used for the middle layer and polycaprolactone and gelatin was used for the outer layer. The properties of the artificial vascular graft were adjusted by the EDC/NHS cross-linking agent through the reaction between the collagen or gelatine and EDC/NHS. The mechanical and hydrophilic properties of the cross-linked artificial vessels were substantially enhanced, with a maximum stress of 9.56 MPa in the axial direction and 9.31 MPa in the radial direction for the P/C (4:1) vascular graft, which exceeded that of many textile-based and natural vascular grafts. The increased hydrophilicity of the inner layer of the vessel before crosslinking was due to the addition of COL, and the inner layer of the artificial vessel after crosslinking had a substantial increase in hydrophilicity due to the production of a more hydrophilic urea derivative. The increased hydrophilicity led to easier cell adhesion to the inner layer of the artificial vessel, especially for the P/C (2:1) vascular graft, where the cell proliferation rate and adhesion were high due to COL incorporation and cross-linking. The three-layer vascular grafts studied did not lead to haemolysis. Therefore, the EDC/NHS cross-linked three-layer vascular graft had good mechanical properties, hydrophilicity, anticoagulation and could enhance cell adhesion and proliferation.

Vibrational Study on the Structure, Bioactivity, and Silver Adsorption of Silk Fibroin Fibers Grafted with Methacrylonitrile

Natural fibers have received increasing attention as starting materials for innovative applications in many research fields, from biomedicine to engineering. Bombyx mori silk fibroin has become a material of choice in the development of many biomedical devices. Grafting represents a good strategy to improve the material properties according to the desired function. In the present study, Bombyx mori silk fibroin fibers were grafted with methacrylonitrile (MAN) with different weight gains. The potential interest in biomedical applications of MAN functionalization relies on the presence of the nitrile group, which is an acceptor of H bonds and can bind metals. IR and Raman spectroscopy were used to characterize the grafted samples and the possible structural changes induced by grafting. Afterward, the same techniques were used to study the bioactivity (i.e., the calcium phosphate nucleation ability) of MAN-grafted silk fibroins after ageing in simulated body fluid (SBF) for possible application in bone tissue engineering, and their interaction with Ag+ ions, for the development of biomaterials with enhanced anti-microbial properties. MAN was found to efficiently polymerize on silk fibroin through polar amino acids (i.e., serine and tryptophan), inducing an enrichment in silk fibroin-ordered domains. IR spectroscopy allowed us to detect the nucleation of a thin calcium phosphate layer and the uptake of Ag+ ions through the nitrile group, which may foster the application of these grafted materials in biomedical applications.

Robust Nanofiber Mats Exfoliated From Tussah Silk for Potential Biomedical Applications

Nanofibers as elements for bioscaffolds are pushing the development of tissue engineering. In this study, tussah silk was mechanically disintegrated into nanofibers dispersed in aqueous solution which was cast to generate tussah silk fibroin (TSF) nanofiber mats. The effect of treatment time on the morphology, structure, and mechanical properties of nanofiber mats was examined. SEM indicated decreasing diameter of the nanofiber with shearing time, and the diameter of the nanofiber was 139.7 nm after 30 min treatment. These nanofiber mats exhibited excellent mechanical properties; the breaking strength increased from 26.31 to 72.68 MPa with the decrease of fiber diameter from 196.5 to 139.7 nm. The particulate debris was observed on protease XIV degraded nanofiber mats, and the weight loss was greater than 10% after 30 days in vitro degradation. The cell compatibility experiment confirmed adhesion and spreading of NIH-3T3 cells and enhanced cell proliferation on TSF nanofiber mats compared to that on Bombyx mori silk nanofiber mats. In conclusion, results indicate that TSF nanofiber mats prepared in this study are mechanically robust, slow biodegradable, and biocompatible materials, and have promising application in regenerative medicine.

Vibrational Study on Structure and Bioactivity of Protein Fibers Grafted with Phosphorylated Methacrylates

In the last decades, silk fibroin and wool keratin have been considered functional materials for biomedical applications. In this study, fabrics containing silk fibers from Bombyx mori and Tussah silk fibers from Antheraea pernyi, as well as wool keratin fabrics, were grafted with phosmer CL and phosmer M (commercial names, i.e., methacrylate monomers containing phosphate groups in the molecular side chain) with different weight gains. Both phosmers were recently proposed as flame retarding agents, and their chemical composition suggested a possible application in bone tissue engineering. IR and Raman spectroscopy were used to disclose the possible structural changes induced by grafting and identify the most reactive amino acids towards the phosmers. The same techniques were used to investigate the nucleation of a calcium phosphate phase on the surface of the samples (i.e., bioactivity) after ageing in simulated body fluid (SBF). The phosmers were found to polymerize onto the biopolymers efficiently, and tyrosine and serine underwent phosphorylation (monitored through the strengthening of the Raman band at 1600 cm-1 and the weakening of the Raman band at 1400 cm-1, respectively). In grafted wool keratin, cysteic acid and other oxidation products of disulphide bridges were detected together with sulphated residues. Only slight conformational changes were observed upon grafting, generally towards an enrichment in ordered domains, suggesting that the amorphous regions were more prone to react (and, sometimes, degrade). All samples were shown to be bioactive, with a weight gain of up to 8%. The most bioactive samples contained the highest phosmers amounts, i.e., the highest amounts of phosphate nucleating sites. The sulphate/sulphonate groups present in grafted wool samples appeared to increase bioactivity, as shown by the five-fold increase of the IR phosphate band at 1040 cm-1.

Preclinical Validation of SilkBridgeTM for Peripheral Nerve Regeneration

Silk fibroin (Bombyx mori) was used to manufacture a nerve conduit (SilkBridge^TM) characterized by a novel 3D architecture. The wall of the conduit consists of two electrospun layers (inner and outer) and one textile layer (middle), perfectly integrated at the structural and functional level. The manufacturing technology conferred high compression strength on the device, thus meeting clinical requirements for physiological and pathological compressive stresses. As demonstrated in a previous work, the silk material has proven to be able to provide a valid substrate for cells to grow on, differentiate and start the fundamental cellular regenerative activities in vitro and, in vivo, at the short time point of 2 weeks, to allow the starting of regenerative processes in terms of good integration with the surrounding tissues and colonization of the wall layers and of the lumen with several cell types. In the present study, a 10 mm long gap in the median nerve was repaired with 12 mm SilkBridge^TM conduit and evaluated at middle (4 weeks) and at longer time points (12 and 24 weeks). The SilkBridge^TM conduit led to a very good functional and morphological recovery of the median nerve, similar to that observed with the reference autograft nerve reconstruction procedure. Taken together, all these results demonstrated that SilkBridge^TM has an optimized balance of biomechanical and biological properties, which allowed proceeding with a first-in-human clinical study aimed at evaluating safety and effectiveness of using the device for the reconstruction of digital nerve defects in humans.

Silk fibroin for skin injury repair: Where do things stand?

Several synthetic and natural materials are used in soft tissue engineering and regenerative medicine with varying degrees of success. Among them, silkworm silk protein fibroin, a naturally occurring protein-based biomaterial, exhibits many promising characteristics such as biocompatibility, controllable biodegradability, tunable mechanical properties, aqueous preparation, minimal inflammation in host tissue, low cost and ease of use. Silk fibroin is often used alone or in combination with other materials in various formats and is also a promising delivery system for bioactive compounds as part of such repair scenarios. These properties make silk fibroin an excellent biomaterial for skin tissue engineering and repair applications. This review focuses on the promising characteristics and recent advances in the use of silk fibroin for skin wound healing and/or soft-tissue repair applications. The benefits and limitations of silk fibroin as a scaffolding biomaterial in this context are also discussed. STATEMENT OF SIGNIFICANCE: Silk protein fibroin is a natural biomaterial with important biological and mechanical properties for soft tissue engineering applications. Silk fibroin is obtained from silkworms and can be purified using alkali or enzyme based degumming (removal of glue protein sericin) procedures. Fibroin is used alone or in combination with other materials in different scaffold forms, such as nanofibrous mats, hydrogels, sponges or films tailored for specific applications. The investigations carried out using silk fibroin or its blends in skin tissue engineering have increased dramatically in recent years due to the advantages of this unique biomaterial. This review focuses on the promising characteristics of silk fibroin for skin wound healing and/or soft-tissue repair applications.

Preparation of natural amphoteric silk nanofibers by acid hydrolysis

Direct extraction of silk nanofibers (SNs) from natural silk fibers was developed via a low-intensity ultrasonic-assisted sulfuric acid hydrolysis process.

A Biodegradable Stent with Surface Functionalization of Combined-Therapy Drugs for Colorectal Cancer

In-stent restenosis caused by tumor ingrowth is a major problem for patients undergoing stent placement because conventional stents often lack sustainable antitumor capabilities. The aim of this work is to develop a silk fibroin (SF)-based nanofibrous membrane that is loaded with combined-therapy drugs by using electrospinning technologies, which is further coated on a polydioxanone (PDO) stent and used for the treatment of colorectal cancer (CRC). In order to improve treatment effectiveness, a combination of therapeutic drugs, i.e., curcumin (CUR) and 5-fluorouracil (5-FU), is dissolved into SF solution and then eletrospun onto the surface of the PDO stent. The morphology, secondary structure, and in vitro drug release profiles of the membranes are characterized. The antitumor efficacy is assessed in vitro and in vivo using a human CRC cell line and normal cells, and tumor-bearing nude mice. In vitro and in vivo studies on the nanofibrous memembrane-coating demonstrate improved antitumor effects for the CUR/5-FU dual drug system which can be attributed to cell cycle arrest in the S phase in association with induced apoptosis in tumor cells by blocking signal transducer and activator of transcription3 (Stat3) and nuclear factor kappa beta (NF-kB) signaling pathways, suggesting potential in the treatment of CRC in the future.

Preparation and Characterization of Antheraea pernyi Silk Fibroin Based Nanohydroxyapatite Composites

In the present study, Antheraea pernyi silk fibroin (Ap-SF) is employed to regulate the mineralization of hydroxyapatite (HAp) nanocrystals. Calcium phosphate crystals precipitate in the aqueous solution of regenerated Ap-SF at pH 7.4 and room temperature. The samples are charaterized with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Based on the XRD results, the inorganic phase in the mineralized Ap-SF composites is HAp as small particles with low crystallinity. The FTIR analysis reveals that the HAp crystals are carbonate-substituted HAp and compounded with Ap-SF. The TEM images show mineralized nanofibrils in the composites as rod-like shapes 4—5 nm in diameter. Electron diffraction (SAED) patterns of selected areas of the composites exhibit polycrystalline rings that are ascribed to HAp. SEM images show mineralized Ap-SF composites with several mineralized rods 49—74 nm in diameter.

IN VITRO AND IN VIVO DEGRADATION OF A TWISTED SILK FIBROIN–POLY(LACTIC-CO-GLYCOLIC ACID) FIBER COMPOSITE ROPE-LIKE SCAFFOLD AND CHANGES IN ITS MECHANICAL PROPERTIES

Natural silk fibroin fiber is slowly degraded, which makes it difficult to be replaced quickly by regenerating tissues of tissue engineering. We used poly(lactic-co-glycolic acid) (PLGA, lactic acid:glycolic acid [Formula: see text] 10:90) fibers to adjust the overall degradation rate of the scaffolds. This study fabricated a three-strand helical composite rope-like scaffold from silk fibroin and PLGA fibers (silk fibroin:PLGA [Formula: see text] 36:64) using a twisting method. In vitro and in vivo degradation experiments were performed over 16 weeks. Results suggest that the in vitro and in vivo degradation tendencies of the scaffold were similar, with mass loss lagging behind mechanical property loss. The speed of degradation in vivo was faster than that in vitro. Mechanical property loss of the scaffold was fast during the first three weeks, when mass loss was slow. Mass loss rate accelerated from weeks 3 to 8. The mass and mechanical properties were relatively stable from 8 to 16 weeks. After 16 weeks of degradation, the scaffold still had considerably strong mechanical properties. The scaffold showed a reasonable and suitable degradation speed with good histocompatibility for ligament tissue engineering.

Conducting polymer-silk biocomposites for flexible and biodegradable electrochemical sensors

Approaches to form flexible biosensors require strategies to tune materials for various biomedical applications. We report a facile approach using photolithography to fabricate poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (

PEDOT

PSS) sensors on a fully biodegradable and flexible silk protein fibroin support. A benchtop photolithographic setup is used to fabricate high fidelity and high resolution

PEDOT

PSS microstructures over a large (cm) area using only water as the solvent. Using the conductive micropatterns as working electrodes, we demonstrate biosensors with excellent electrochemical activity and stability over a number of days. The fabricated biosensors display excellent nonspecific detection of dopamine and ascorbic acid with high sensitivity. These devices are mechanically flexible, optically transparent, electroactive, cytocompatible and biodegradable. The benign fabrication protocol allows the conducting ink to function as a matrix for enzymes as shown by a highly sensitive detection of glucose. These sensors can retain their properties under repeated mechanical deformations, but are completely degradable under enzymatic action. The reported technique is scalable and can be used to develop sensitive, robust, and inexpensive biosensors with controllable biodegradability, leading to applications in transient or implantable bioelectronics and optoelectronics.

Identification and classification of silks using infrared spectroscopy

Lepidopteran silks number in the thousands and display a vast diversity of structures, properties and industrial potential. To map this remarkable biochemical diversity, we present an identification and screening method based on the infrared spectra of native silk feedstock and cocoons. Multivariate analysis of over 1214 infrared spectra obtained from 35 species allowed us to group silks into distinct hierarchies and a classification that agrees well with current phylogenetic data and taxonomies. This approach also provides information on the relative content of sericin, calcium oxalate, phenolic compounds, poly-alanine and poly(alanine-glycine) β-sheets. It emerged that the domesticated mulberry silkmoth Bombyx mori represents an outlier compared with other silkmoth taxa in terms of spectral properties. Interestingly, Epiphora bauhiniae was found to contain the highest amount of β-sheets reported to date for any wild silkmoth. We conclude that our approach provides a new route to determine cocoon chemical composition and in turn a novel, biological as well as material, classification of silks.

Acylation Modification ofAntheraea pernyiSilk Fibroin Using Succinic Anhydride and Its Effects on Enzymatic Degradation Behavior

The degradation rate of tissue engineering scaffolds should match the regeneration rate of new tissues. Controlling the degradation behavior of silk fibroin is an important subject for silk-based tissue engineering scaffolds. In this study,Antheraea pernyisilk fibroin was successfully modified with succinic anhydride and then characterized by zeta potential, ninhydrin method, and FTIR.In vitro, three-dimensional scaffolds prepared with modified silk fibroin were incubated in collagenase IA solution for 18 days to evaluate the impact of acylation on the degradation behavior. The results demonstrated that the degradation rate of modified silk fibroin scaffolds was more rapid than unmodified ones. The content of theβ-sheet structure in silk fibroin obviously decreased after acylation, resulting in a high degradation rate. Above all, the degradation behavior of silk fibroin scaffolds could be regulated by acylation to match the requirements of various tissues regeneration.

The effects of electrospun TSF nanofiber diameter and alignment on neuronal differentiation of human embryonic stem cells

Although transplantation of human embryonic stem cells (hESCs)-derived neural precursors (NPs) has been demonstrated with some success for nervous repair in small animal model, control of the survival, and directional differentiation of these cells is still challenging. Meanwhile, the notion that using suitable scaffolding materials to control the growth and differentiation of grafted hESC-derived NPs raises the hope for better clinical nervous repair. In this study, we cultured hESC-derived NPs on Tussah silk fibroin (TSF)-scaffold of different diameter (i.e., 400 and 800 nm) and orientation (i.e., random and aligned) to analyze the effect of fiber diameter and alignment on the cell viability, neuronal differentiation, and neurite outgrowth of hESC-derived NPs. The results show that TSF-scaffold supports the survival, migration, and differentiation of hESC-derived NPs. Aligned TSF-scaffold significantly promotes the neuronal differentiation and neurite outgrowth of hESC-derived neurons compared with random TSF-scaffold. Moreover, on aligned 400 nm fibers cell viability, neuronal differentiation and neurite outgrowth are greater than that on aligned 800 nm fibers. Together, these results demonstrate that aligned 400 nm TSF-scaffold is more suitable for the development of hESC-derived NPs, which shed light on optimization of the therapeutic potential of hESCs to be employed for neural regeneration.

Synchrotron FTIR microspectroscopy of single natural silk fibers

Synchrotron FTIR (S-FTIR) microspectroscopy was used to monitor the silk protein conformation in a range of single natural silk fibers (domestic and wild silkworm and spider dragline silk). With the selection of suitable aperture size, we obtained high-resolution S-FTIR spectra capable of semiquantitative analysis of protein secondary structures. For the first time, we have determined from S-FTIR the β-sheet content in a range of natural single silk fibers, 28 ± 4, 23 ± 2, and 17 ± 4% in Bombyx mori, Antheraea pernyi, and Nephila edulis silks, respectively. The trend of β-sheet content in different silk fibers from the current study accords quite well with published data determined by XRD, Raman, and (13)C NMR. Our results indicate that the S-FTIR microspectroscopy method has considerable potential for the study of single natural silk fibers.

Degradation mechanism and control of silk fibroin

Controlling the degradation process of silk is an important and interesting subject in the field of biomaterials. In the present study, silk fibroin films with different secondary conformations and nanostructures were used to study degradation behavior in buffered protease XIV solution. Different from previous studies, silk fibroin films with highest β-sheet content achieved the highest degradation rate in our research. A new degradation mechanism revealed that degradation behavior of silk fibroin was related to not only crystal content but also hydrophilic interaction and then crystal-noncrystal alternate nanostructures. First, hydrophilic blocks of silk fibroin were degraded. Then, hydrophobic crystal blocks that were formerly surrounded and immobilized by hydrophilic blocks became free particles and moved into solution. Therefore, on the basis of the mechanism, which enables the process to be more controllable and flexible, controlling the degradation behavior of silk fibroin without affecting other performances such as its mechanical or hydrophilic properties becomes feasible, and this would greatly expand the applications of silk as a biomedical material.

Skeletal tissue engineering using silk biomaterials

Silks have been proposed as potential scaffold materials for tissue engineering, mainly because of their physical properties. They are stable at physiological temperatures, flexible and resist tensile and compressive forces. Bombyx mori (silkworm) cocoon silk has been used as a suture material for over a century, and has proved to be biocompatible once the immunogenic sericin coating is removed. Spider silks have a similar structure to silkworm silk but do not have a sericin coating. This paper provides a general overview on the use of silk protein in biomaterials, with a focus on skeletal tissue engineering.