Controlled release from multilayer silk biomaterial coatings to modulate vascular cell responses

Evaluation of new robust silk fibroin hydrogels for posterior scleral reinforcement in rabbits

Background: Currently, there is no ideal material available for posterior scleral reinforcement (PSR) to prevent the progression of high myopia. In this study, we investigated robust regenerated silk fibroin (RSF) hydrogels as potential grafts for PSR in animal experiments to evaluate their safety and biological reactions. Methods: PSR surgery was performed on the right eye of twenty-eight adult New Zealand white rabbits, with the left eye serving as a self-control. Ten rabbits were observed for 3 months, while 18 rabbits were observed for 6 months. The rabbits were evaluated using intraocular pressure (IOP), anterior segment and fundus photography, A- and B-ultrasound, optical coherence tomography (OCT), histology, and biomechanical tests. Results: No complications such as significant IOP fluctuation, anterior chamber inflammation, vitreous opacity, retinal lesion, infection, or material exposure were observed. Furthermore, no evidence of pathological changes in the optic nerve and retina, or structural abnormalities on OCT, were found. The RSF grafts were appropriately located at the posterior sclera and enclosed in fibrous capsules. The scleral thickness and collagen fiber content of the treated eyes increased after surgery. The ultimate stress of the reinforced sclera increased by 30.7%, and the elastic modulus increased by 33.0% compared to those of the control eyes at 6 months after surgery. Conclusion: Robust RSF hydrogels exhibited good biocompatibility and promoted the formation of fibrous capsules at the posterior sclera in vivo. The biomechanical properties of the reinforced sclera were strengthened. These findings suggest that RSF hydrogel is a potential material for PSR.

In-vivo evaluation of silk fibroin small-diameter vascular grafts: state of art of preclinical studies and animal models

Autologous vein and artery remains the first choice for vascular grafting procedures in small-diameter vessels such as coronary and lower limb districts. Unfortunately, these vessels are often found to be unsuitable in atherosclerotic patients due to the presence of calcifications or to insufficient size. Synthetic grafts composed of materials such as expanded polytetrafluoroethylene (ePTFE) are frequently employed as second choice, because of their widespread availability and success in the reconstruction of larger arteries. However, ePTFE grafts with small diameter are plagued by poor patency rates due to surface thrombogenicity and intimal hyperplasia, caused by the bioinertness of the synthetic material and aggravated by low flow conditions. Several bioresorbable and biodegradable polymers have been developed and tested to exploit such issues for their potential stimulation to endothelialization and cell infiltration. Among these, silk fibroin (SF) has shown promising pre-clinical results as material for small-diameter vascular grafts (SDVGs) because of its favorable mechanical and biological properties. A putative advantage in graft infection in comparison with synthetic materials is plausible, although it remains to be demonstrated. Our literature review will focus on the performance of SF-SDVGs in vivo, as evaluated by studies performing vascular anastomosis and interposition procedures, within small and large animal models and different arterial districts. Efficiency under conditions that more accurately mime the human body will provide encouraging evidence towards future clinical applications.

Delivering on the promise of recombinant silk-inspired proteins for drug delivery

Effective drug delivery is essential for the success of a medical treatment. Polymeric drug delivery systems (DDSs) are preferred over systemic administration of drugs due to their protection capacity, directed release, and reduced side effects. Among the numerous polymer sources, silks and recombinant silks have drawn significant attention over the past decade as DDSs. Native silk is produced from a variety of organisms, which are then used as sources or guides of genetic material for heterologous expression or engineered designs. Recombinant silks bear the outstanding properties of natural silk, such as processability in aqueous solution, self-assembly, drug loading capacity, drug stabilization/protection, and degradability, while incorporating specific properties beneficial for their success as DDS, such as monodispersity and tailored physicochemical properties. Moreover, the on-demand inclusion of sequences that customize the DDS for the specific application enhances efficiency. Often, inclusion of a drug into a DDS is achieved by simple mixing or diffusion and stabilized by non-specific molecular interactions; however, these interactions can be improved by the incorporation of drug-binding peptide sequences. In this review we provide an overview of native sources for silks and silk sequences, as well as the design and formulation of recombinant silk biomaterials as drug delivery systems in a variety of formats, such as films, hydrogels, porous sponges, or particles.

Cytotoxicity Assessment of a New Design for a Biodegradable Ureteral Mitomycin Drug-Eluting Stent in Urothelial Carcinoma Cell Culture

Urothelial tumour of the upper urinary tract is a rare neoplasm, but unfortunately, it has a high recurrence rate. The reduction of these tumour recurrences could be achieved by the intracavitary instillation of adjuvant chemotherapy after nephron-sparing treatment in selected patients, but current instillation methods are ineffective. Therefore, the aim of this in vitro study is to evaluate the cytotoxic capacity of a new instillation technology through a biodegradable ureteral stent/scaffold coated with a silk fibroin matrix for the controlled release of mitomycin C as an anti-cancer drug. Through a comparative study, we assessed, in urothelial carcinoma cells in a human cancer T24 cell culture for 3 and 6 h, the cytotoxic capacity of mitomycin C by viability assay using the CCK-8 test (Cell counting Kit-8). Cell viability studies in the urothelial carcinoma cell line confirm that mitomycin C embedded in the polymeric matrix does not alter its cytotoxic properties and causes a significant decrease in cell viability at 6 h versus in the control groups. These findings have a clear biomedical application and could be of great use to decrease the recurrence rate in patients with upper tract urothelial carcinomas by increasing the dwell time of anti-cancer drugs.

Structural design and mechanical performance of composite vascular grafts

This study reviews the state of the art in structural design and the corresponding mechanical behaviours of composite vascular grafts. We critically analyse surface and matrix designs composed of layered, embedded, and hybrid structures along the radial and longitudinal directions; materials and manufacturing techniques, such as tissue engineering and the use of textiles or their combinations; and the corresponding mechanical behaviours of composite vascular grafts in terms of their physical–mechanical properties, especially their stress–strain relationships and elastic recovery. The role of computational studies is discussed with respect to optimizing the geometrics designs and the corresponding mechanical behaviours to satisfy specialized applications, such as those for the aorta and its subparts. Natural and synthetic endothelial materials yield improvements in the mechanical and biological compliance of composite graft surfaces with host arteries. Moreover, the diameter, wall thickness, stiffness, compliance, tensile strength, elasticity, and burst strength of the graft matrix are determined depending on the application and the patient. For composite vascular grafts, hybrid architectures are recommended featuring multiple layers, dimensions, and materials to achieve the desired optimal flexibility and function for complying with user-specific requirements. Rapidly emerging artificial intelligence and big data techniques for diagnostics and the three-dimensional (3D) manufacturing of vascular grafts will likely yield highly compliant, subject-specific, long-lasting, and economical vascular grafts in the near-future.

Graphic abstract

Assessment of a Coated Mitomycin-Releasing Biodegradable Ureteral Stent as an Adjuvant Therapy in Upper Urothelial Carcinoma: A Comparative In Vitro Study

A major limitation of the treatment of low-grade upper tract urothelial carcinoma is the difficulty of intracavitary instillation of adjuvant therapy. Therefore, the aim of this in vitro study was to develop and to assess a new design of biodegradable ureteral stent coated with a silk fibroin matrix for the controlled release of mitomycin C as a chemotherapeutic drug. For this purpose, we assessed the coating of a biodegradable ureteral stent, BraidStent^®, with silk fibroin and subsequently loaded the polymeric matrix with two formulations of mitomycin to evaluate its degradation rate, the concentration of mitomycin released, and changes in the pH and the weight of the stent. Our results confirm that the silk fibroin matrix is able to coat the biodegradable stent and release mitomycin for between 6 and 12 h in the urinary environment. There was a significant delay in the degradation rate of silk fibroin and mitomycin-coated stents compared to bare biodegradable stents, from 6–7 weeks to 13–14 weeks. The present study has shown the feasibility of using mitomycin C-loaded silk fibroin for the coating of biodegradable urinary stents. The addition of mitomycin C to the coating of silk fibroin biodegradable stents could be an attractive approach for intracavitary instillation in the upper urinary tract.

Anti-Coagulant and Antimicrobial Recombinant Heparin-Binding Major Ampullate Spidroin 2 (MaSp2) Silk Protein

Governed by established structure–property relationships, peptide motifs comprising major ampullate spider silk confer a balance of strength and extensibility. Other biologically inspired small peptide motifs correlated to specific functionalities can be combined within these units to create designer silk materials with new hybrid properties. In this study, a small basic peptide, (ARKKAAKA) known to both bind heparin and mimic an antimicrobial peptide, was genetically linked to a protease-resistant, mechanically robust silk-like peptide, MaSp2. Purified fusion proteins (four silk domains and four heparin-binding peptide repeats) were expressed in E. coli. Successful fusion of a MaSp2 spider silk peptide with the heparin-binding motif was shown using a variety of analytical assays. The ability of the fusion peptide to bind heparin was assessed with ELISA and was further tested for its anticoagulant property using aPTT assay. Its intrinsic property to inhibit bacterial growth was evaluated using zone of inhibition and crystal violet (CV) assays. Using this strategy, we were able to link the two types of genetic motifs to create a designer silk-like protein with improved hemocompatibility and antimicrobial properties.

Controlled Release of Molecular Intercalants from Two-Dimensional Nanosheet Films

Solution co-deposition of two-dimensional (2D) nanosheets with chemical solutes yields nanosheet-molecular heterostructures. A feature of these macroscopic layered hybrids is their ability to release the intercalated molecular agent to express chemical functionality on their surfaces or in their near surroundings. Systematic design methods are needed to control this molecular release to match the demand for rate and lifetime in specific applications. We hypothesize that release kinetics are controlled by transport processes within the layered solids, which primarily involve confined molecular diffusion through nanochannels formed by intersheet van der Waals gaps. Here a variety of graphene oxide (GO)/molecular hybrids are fabricated and subject to transient experiments to characterize release kinetics, locations, and mechanisms. The measured release rate profiles can be successfully described by a numerical model of internal transport processes, and the results used to extract effective Z-directional diffusion coefficients for various film types. The diffusion coefficients are found to be 8 orders of magnitude lower than those in free solution due to nanochannel confinement and serpentine path effects, and this retardation underlies the ability of 2D materials to control and extend release over useful time scales. In-plane texturing of the heterostructured films by compressive wrinkling or crumpling is shown to be a useful design tool to control the release rate for a given film type and molecular intercalant. The potential of this approach is demonstrated through case studies on the controlled release of chemical virucidal agents.

Advances in coatings on magnesium alloys for cardiovascular stents - A review

Magnesium (Mg) and its alloys, as potential biodegradable materials, have drawn wide attention in the cardiovascular stent field because of their appropriate mechanical properties and biocompatibility. Nevertheless, the occurrence of thrombosis, inflammation, and restenosis of implanted Mg alloy stents caused by their poor corrosion resistance and insufficient endothelialization restrains their anticipated clinical applications. Numerous surface treatment tactics have mainly striven to modify the Mg alloy for inhibiting its degradation rate and enduing it with biological functionality. This review focuses on highlighting and summarizing the latest research progress in functionalized coatings on Mg alloys for cardiovascular stents over the last decade, regarding preparation strategies for metal oxide, metal hydroxide, inorganic nonmetallic, polymer, and their composite coatings; and the performance of these strategies in regulating degradation behavior and biofunction. Potential research direction is also concisely discussed to help guide biological functionalized strategies and inspire further innovations. It is hoped that this review can give assistance to the surface modification of cardiovascular Mg-based stents and promote future advancements in this emerging research field.

Enhancing the Carboxylation Efficiency of Silk Fibroin through the Disruption of Noncovalent Interactions

Silk fibroin is a semicrystalline protein used as a renewable polymer source and as a biomaterial platform, but existing methods to synthetically modify fibroin suffer from low efficiencies that can limit the protein's utility. This work reports on a mild synthesis that results in a 2-fold increase in carboxylation through the disruption of noncovalent interactions during the reaction. Importantly, silk fibroin maintains its ability to form β-sheets that are critical for tailoring mechanical and degradation properties, as well as for rendering solid constructs (e.g., films and scaffolds) insoluble in water. Increasing carboxyl functionalization affords control over protein charge, which permits tailoring the loading and release of small molecules using electrostatic interactions. Disruption of noncovalent interactions during aqueous carbodiimide coupling also significantly enhances conjugation efficiency of molecules containing primary amine groups, thus enabling high degrees of functionalization with biological molecules, such as proteins and peptides, for biomaterial applications.

Silk Fibroin as a Coating Polymer for Sirolimus-Eluting Magnesium Alloy Stents

Magnesium (Mg) alloy-based, bioresorbable scaffolding is a promising candidate for next-generation stents. Rapid corrosion of Mg alloy in the physiological environment, however, hinders its clinical application. Hydrofluoric acid (HF) treatment and biodegradable polymer coating have been widely reported to enhance corrosion resistance of the Mg alloy. Poor biocompatibility of biodegradable polymers, however, is known to promote adverse events such as intimal hyperplasia and thrombosis. We selected silk fibroin (SF) as the polymer for stent coating and evaluated drug release from the SF layer, corrosion resistance of the Mg alloy, and biocompatibility. After the stent was coated with SF, ethanol treatment of the SF layer enriched the β-sheet content. Release of sirolimus (SRL), a drug that prevents intimal hyperplasia, from the SF layer was slower than that with a poly(ε-caprolactone), the conventional biodegradable polymer used on medical devices. Ethanol treatment of the SF-coated stent further slowed SRL release from the SF layer. Crystalline domains in SF formed by the β-sheet structure could contribute to the slow release of SRL. The SF coating suppressed local and deep corrosion of the Mg alloy stent, although total corrosion remained unaffected. Uniform corrosion without local or deep corrosion prolongs the stent's radial strength. The SF coating showed excellent biocompatibility with human umbilical vein endothelial cells and minimal platelet adhesion. SF is expected to replace traditional biodegradable polymers for use on bioresorbable stents.

A Review for the Synthesis of Silk Fibroin Nanoparticles with Different Techniques and Their Ability to be Used for Drug Delivery

Background:

Silk fibroin is the main protein of silk, and it has recently been evaluated for drug delivery applications due to its excellent properties. Specifically, silk fibroin exhibits good biocompatibility, biodegradability and low immunogenicity. Fibroin nanoparticles have attracted attention due to their high binding ability to different drugs as well as their ability for controlled drug release. The improvement of the therapeutic efficiency of drug encapsulation is important and depends on the particle size, the chemical structure and the properties of the silk fibroin nanoparticles.

Methods:

There is a variety of methods for the preparation of fibroin nanoparticles such as (i) electrospraying and desolvation method, (ii) supercritical fluid technologies, (iii) capillary-microdot technique, (iv) salting out etc. Furthermore, various techniques have been used for the characterization of nanoparticles such as SEM (scanning electron microscopy), TEM (transmission electron microscopy), DLS (dynamic light scattering), Zeta-potential and FTIR (Fourier transform infrared spectroscopy). Different drugs (paclitaxel, curcumin, 5-fluorouracil etc) have been encapsulated in fibroin nanoparticles.

Results:

Each separated synthesis method has different advantages such as (i) high yield, (ii) avoid use of toxic solvents, (iii) low cost, (iv) controllable particle size, (v) no organic solvent residue, (vi) simplicity of operation, (vii) small particles size, (viii) homeliness of operation, (ix) restrainable particle size, (x) easy and safe to operate, (xi) no use of organic solvent. Moreover, some major drugs studied are Floxuridine, Fluorouracil, Curcumin, Doxorubicin, Metotrexate, Paclitaxel and Doxorubicin, Horseradish peroxidase. All the above combinations (preparation method-drug) are studied in detail.

Conclusion:

Various drugs have been encapsulated successfully in silk fibroin and all of them exhibit a significant release rate. Finally, the encapsulation efficiency and release rate depend on the molecular weight of the drugs and it can be adjusted by controlling the crystallinity and concentration of silk fibroin.

Extended release formulations using silk proteins for controlled delivery of therapeutics

INTRODUCTION

Silk is a promising biomaterial for controlled delivery of therapeutics and has a unique protein chemistry that can be tuned to form different carrier formats. The protein has been studied for sustained release depot systems for the targeted or localized delivery of drugs.

AREAS COVERED

An overview of natural silk proteins for controlled delivery of therapeutics is provided, with a focus on the features of silk proteins that allow them to be useful tools for controlled delivery. Recent applications of natural silk proteins as controlled delivery systems are also summarized.

EXPERT OPINION

The versatility of silk proteins makes them desirable biomaterials for a broad range of applications for controlled delivery of both small and large molecules. Further, the degradation profile leading to peptides and amino acids provides compatibility with pH-sensitive therapeutics. While silk sericin and spider silks are under study, silk fibroin extracted from silkworms (e.g. Bombyx mori) dominates pharmaceutical studies with silk. Silk fibroin can be formed into drug delivery tools for systemic or local injections, topical and transdermal applications, and implantation; depending on the target disease and therapeutic molecule. In vitro to in vivo correlations and scale-up needs are the next steps towards clinical applications.

Thermal annealed silk fibroin membranes for periodontal guided tissue regeneration

Guided tissue regeneration (GTR) is a surgical procedure applied in the reconstruction of periodontal defects, where an occlusive membrane is used to prevent the fast-growing connective tissue from migrating into the defect. In this work, silk fibroin (SF) membranes were developed for periodontal guided tissue regeneration. Solutions of SF with glycerol (GLY) or polyvinyl alcohol (PVA) where prepared at several weight ratios up to 30%, followed by solvent casting and thermal annealing at 85 °C for periods of 6 and 12 h to produce high flexible and stable membranes. These were characterized in terms of their morphology, physical integrity, chemical structure, mechanical and thermal properties, swelling capability and in vitro degradation behavior. The developed blended membranes exhibited high ductility, which is particular relevant considering the need for physical handling and adaptability to the defect. Moreover, the membranes were cultured with human periodontal ligament fibroblast cells (hPDLs) up to 7 days. Also, the higher hydrophilicity and consequent in vitro proteolytic degradability of these blends was superior to pure silk fibroin membranes. In particular SF/GLY blends demonstrated to support high cell adhesion and viability with an adequate hPDLs' morphology, make them excellent candidates for applications in periodontal regeneration.

Present and future of tissue engineering scaffolds for dentin-pulp complex regeneration

More than two thirds of the global population suffers from tooth decay, which results in cavities with various levels of lesion severity. Clinical interventions to treat tooth decay range from simple coronal fillings to invasive root canal treatment. Pulp capping is the only available clinical option to maintain the pulp vitality in deep lesions, but irreversible pulp inflammation and reinfection are frequent outcomes for this treatment. When affected pulp involvement is beyond repair, the dentist has to perform endodontic therapy leaving the tooth non-vital and brittle. On-going research strategies have failed to overcome the limitations of existing pulp capping materials so that healthy and progressive regeneration of the injured tissues is attained. Preserving pulp vitality is crucial for tooth homeostasis and durability, and thus, there is a critical need for clinical interventions that enable regeneration of the dentin-pulp complex to rescue millions of teeth annually. The identification and development of appropriate biomaterials for dentin-pulp scaffolds are necessary to optimize clinical approaches to regenerate these hybrid dental tissues. Likewise, a deep understanding of the interactions between the micro-environment, growth factors, and progenitor cells will provide design basis for the most fitting scaffolds for this purpose. In this review, we first introduce the long-lasting clinical dental problem of rescuing diseased tooth vitality, the limitations of current clinical therapies and interventions to restore the damaged tissues, and the need for new strategies to fully revitalize the tooth. Then, we comprehensively report on the characteristics of the main materials of naturally-derived and synthetically-engineered polymers, ceramics, and composite scaffolds as well as their use in dentin-pulp complex regeneration strategies. Finally, we present a series of innovative smart polymeric biomaterials with potential to overcome dentin-pulp complex regeneration challenges.

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

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

Advances in Protein-Based Materials: From Origin to Novel Biomaterials

Biomaterials play a very important role in biomedicine and tissue engineering where they directly affect the cellular activities and their microenvironment . Myriad of techniques have been employed to fabricate a vast number natural, artificial and recombinant polymer s in order to harness these biomaterials in tissue regene ration , drug delivery and various other applications. Despite of tremendous efforts made in this field during last few decades, advanced and new generation biomaterials are still lacking. Protein based biomaterials have emerged as an attractive alternatives due to their intrinsic properties like cell to cell interaction , structural support and cellular communications. Several protein based biomaterials like, collagen , keratin , elastin , silk protein and more recently recombinant protein s are being utilized in a number of biomedical and biotechnological processes. These protein-based biomaterials have enormous capabilities, which can completely revolutionize the biomaterial world. In this review, we address an up-to date review on the novel, protein-based biomaterials used for biomedical field including tissue engineering, medical science, regenerative medicine as well as drug delivery. Further, we have also emphasized the novel fabrication techniques associated with protein-based materials and implication of these biomaterials in the domain of biomedical engineering .

The penetration and phenotype modulation of smooth muscle cells on surface heparin modified poly(ɛ-caprolactone) vascular scaffold

The tubular porous poly(ɛ-caprolactone) (PCL) scaffold was fabricated by electrospinning. After then, the scaffold's surface was firstly eroded by hexyldiamine to endow amine group, and heparin was covalently grafted to the surface to get surface heparin modified scaffold (ShPCL scaffold). It was found that ShPCL scaffold can induce smooth muscle cells (SMCs) to penetrate the scaffold surface, while the SMCs cannot penetrate the surface of PCL scaffold. Subsequently, the rabbit SMCs were seeded on the ShPCL scaffold and cultured for 14 days. It was found the expression of α-smooth muscle actin in ShPCL scaffold maintained much higher level than that in culture plate, which implied the SMC differentiation in ShPCL scaffold. Furthermore, the immunefluorescence staining of the cross-sections of ShPCL scaffold exhibited the expression of calponin in ShPCL scaffold can be detected after 7 and 14 days, whereas the expression of smooth muscle myosin heavy chain can also be detected at 14 days. These results proved that penetrated SMCs preferably differentiated in to contractile phenotype. The successful SMC penetration and the contractile phenotype expression implied ShPCL scaffold is a suitable candidate for regenerating smooth muscle layer in vascular tissue engineering. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2806-2815, 2017.

Biodegradable Porous Silk Microtubes for Tissue Vascularization

Cardiovascular diseases are the leading cause of mortality around the globe, and microvasculature replacements to help stem these diseases are not available. Additionally, some vascular surgeries needing small diameter vascular grafts present different performance requirements. In this work silk fibroin scaffolds based on silk/polyethylene oxide blends were developed as microtubes for vasculature needs and for different tissue regeneration times, mechanical properties and structural designs. Systems with 13, 14 and 15% silk alone or blended with 1 or 2% of polyethylene oxide (PEO) were used to generate porous microtubes using gel-spinning. Microtubes with inner diameters (ID) of 150-300 μm and 100 μm wall thickness were fabricated. The systems were assessed for porosity, mechanical properties, enzymatic degradability, and in vitro vascular endothelial cell attachment and metabolic activity. After 14 days all tubes supported the proliferation of cells and cell attachment increased with porosity. The silk tubes with PEO had similar crystallinity but higher elastic modulus compared with the systems without PEO. The silk (13%)/PEO (1%) system showed the highest porosity (20 μm pore diameters on average), highest cell attachment and fastest degradation profile. There was a good correlation between these parameters with silk concentration and the presence of PEO. The results demonstrate the ability to generate versatile and tunable tubular biomaterials based on silk-PEO-blends with potential for microvascular grafts.

Effect of fibroin sponge coating on in vivo performance of knitted silk small diameter vascular grafts

Vascular grafts under 5 mm or less in diameter are not developed due to a problem caused by early thrombus formation, neointimal hyperplasia, etc. Bombyx mori silk fibroin (SF) which has biodegradability and tissue infiltration is focused as tube and coating material of vascular grafts. Coating is an important factor to maintain the strength of the anastomotic region of vascular grafts, and to prevent the blood leak from the vascular grafts after implantation. Therefore, in this research, we focused on the SF concentration of the coating solution, and tissue infiltration and remodeling were compared among each SF concentration. Silk poly (-ethylene) glycol diglycidyl ether (PGDE) coating with concentrations of 1.0%, 2.5%, 5.0%, and 7.5% SF were applied for the double-raschel knitted small-sized vessel with 1.5 mm diameter and 1cm in length. The grafts were implanted in the rat abdominal aorta and removed after 3 weeks or 3 months. Vascular grafts patency was monitored by ultrasound, and morphological evaluation was performed by histopathological examination. SF concentration had no significant effects on the patency rate. However, tissue infiltration was significantly higher in the sample of 2.5% SF in 3 weeks, and 1.0% and 2.5% SF in 3 months. Also, in comparison of length inside of the graft, stenosis were not found in 3 weeks, however, found with 5.0% and 7.5% in 3 months. From these results, it is clear that 2.5% SF coating is the most suitable concentration, based on the characteristics of less stenosis, early tissue infiltration, and less neointimal hyperplasia.

Silk fibroin as biomaterial for bone tissue engineering

Silk fibroin (SF) is a fibrous protein which is produced mainly by silkworms and spiders. Its unique mechanical properties, tunable biodegradation rate and the ability to support the differentiation of mesenchymal stem cells along the osteogenic lineage, have made SF a favorable scaffold material for bone tissue engineering. SF can be processed into various scaffold forms, combined synergistically with other biomaterials to form composites and chemically modified, which provides an impressive toolbox and allows SF scaffolds to be tailored to specific applications. This review discusses and summarizes recent advancements in processing SF, focusing on different fabrication and functionalization methods and their application to grow bone tissue in vitro and in vivo. Potential areas for future research, current challenges, uncertainties and gaps in knowledge are highlighted.

STATEMENT OF SIGNIFICANCE

Silk fibroin is a natural biomaterial with remarkable biomedical and mechanical properties which make it favorable for a broad range of bone tissue engineering applications. It can be processed into different scaffold forms, combined synergistically with other biomaterials to form composites and chemically modified which provides a unique toolbox and allows silk fibroin scaffolds to be tailored to specific applications. This review discusses and summarizes recent advancements in processing silk fibroin, focusing on different fabrication and functionalization methods and their application to grow bone tissue in vitro and in vivo. Potential areas for future research, current challenges, uncertainties and gaps in knowledge are highlighted.

Clinical translation of controlled protein delivery systems for tissue engineering

Strategies that utilize controlled release of drugs and proteins for tissue engineering have enormous potential to regenerate damaged organs and tissues. The multiple advantages of controlled release strategies merit overcoming the significant challenges to translation, including high costs and long, difficult regulatory pathways. This review highlights the potential of controlled release of proteins for tissue engineering and regenerative medicine. We specifically discuss treatment modalities that have reached preclinical and clinical trials, with emphasis on controlled release systems for bone tissue engineering, the most advanced application with several products already in clinic. Possible strategies to address translational and regulatory concerns are also discussed.

Preparation of silk fibroin carriers for controlled release

Silk fibroin provides a new option for controlled release systems as a result of its excellent biodegradability, biocompatibility, and mechanical properties. As the core material, silk fibroin can be designed and widely used in drug/gene delivery, regenerative medicine, and other biomedical fields. This review focuses on the preparation methods, loading molecules, and applications of silk fibroin-based controlled release systems including microspheres, microcapsules, films, microparticles, microneedles, liposomes, and hydrogels. These systems provide numerous advantages such as high encapsulation efficiency, avoiding loss of bioactivity and maintaining desirable range without peaks and valleys in comparison to the traditional administration approaches. Microsc. Res. Tech. 80:312-320, 2017. © 2015 Wiley Periodicals, Inc.

Silk-based biomaterials in biomedical textiles and fiber-based implants

Biomedical textiles and fiber-based implants (BTFIs) have been in routine clinical use to facilitate healing for nearly five decades. Amongst the variety of biomaterials used, silk-based biomaterials (SBBs) have been widely used clinically viz. sutures for centuries and are being increasingly recognized as a prospective material for biomedical textiles. The ease of processing, controllable degradability, remarkable mechanical properties and biocompatibility have prompted the use of SBBs for various BTFIs for extracorporeal implants, soft tissue repair, healthcare/hygiene products and related needs. The present Review focuses on BTFIs from the perspective of types and physical and biological properties, and this discussion is followed with an examination of the advantages and limitations of BTFIs from SBBs. The Review covers progress in surface coatings, physical and chemical modifications of SBBs for BTFIs and identifies future needs and opportunities for the further development for BTFIs using SBBs.

Absorbable biologically based internal fixation

Absorbable devices for use in internal fixation have advanced over the years to become reliable and cost-effective alternatives to metallic hardware. In the past, biodegradable fixation involved a laborious implantation process, and induced osteolysis and inflammatory reactions. Modern iterations exhibit increased strength, smoother resorption, and lower rates of reactivity. A newer generation manufactured from silk has emerged that may address existing limitations and provide a greater range of fixation 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.

Effect of solvent on drug release and a spray-coated matrix of a sirolimus-eluting stent coated with poly(lactic-co-glycolic acid)

Sirolimus (SRL) release from the biodegradable poly(l-lactic-co-glycolic acid) (PLGA) matrix was investigated for the application of drug-eluting stents (DES). In particular, this study focused on whether various organic solvents affect the interaction between SRL and PLGA and the formation of microstructures during ultrasonic coating. The SRL-loaded PLGA coated by tetrahydrofuran or acetone showed a significant initial burst, whereas that from acetonitrile was constantly released during a period of 21 days. On the basis of these results, the interactions at the molecular level of SRL with the polymer matrix were estimated according to various organic solvents. Although the topographies of the coated surface were obviously different, the correlation between surface roughness and SRL release was very poor. Irrespective of organic solvents, FT-IR data showed significantly weak SRL-PLGA interactions. From the result of wide-angle X-ray diffraction, it was confirmed that SRL was dispersed in an amorphous state in the polymer matrix after ultrasonic coating. The glass-transition temperature was also influenced by organic solvents, resulting in a plasticizing effect. The particle size of SRL appeared to determine the release profile from the PLGA matrix, which was the combination of diffusion and polymer degradation at an SRL size of more than 800 nm and the Fickian release at that of less than 300 nm. Therefore, organic solvents can lead to a heterogeneous microstructure in the SRL-loaded PLGA matrix, which is at or near the surface, consisting of aggregated drug- and polymer-rich regions. It is expected that the drug release can be controlled by physicochemical properties of organic solvents, and this study can be used effectively for localized drug release in biomedical devices such as drug-eluting stents.

Influence of Layer-by-Layer Polyelectrolyte Deposition and EDC/NHS Activated Heparin Immobilization onto Silk Fibroin Fabric

To enhance the hemocompatibility of silk fibroin fabric as biomedical material, polyelectrolytes architectures have been assembled through the layer-by-layer (LbL) technique on silk fibroin fabric (SFF). In particular, 1.5 and 2.5 bilayer of oppositely charged polyelectrolytes were assembled onto SFF using poly(allylamine hydrochloride) (PAH) as polycationic polymer and poly(acrylic acid) (PAA) as polyanionic polymer with PAH topmost. Low molecular weight heparin (LMWH) activated with 1-ethyl-3-(dimethylaminopropyl) carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) was then immobilized on its surface. Alcian Blue staining, toluidine blue assay and X-ray photoelectron spectroscopy (XPS) confirmed the presence of heparin on modified SFF surfaces. The surface morphology of the modified silk fibroin fabric surfaces was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), and obtained increased roughness. Negligible hemolytic effect and a higher concentration of free hemoglobin by a kinetic clotting time test ensured the improved biological performance of the modified fibroin fabric. Overall, the deposition of 2.5 bilayer was found effective in terms of biological and surface properties of the modified fibroin fabric compared to 1.5 bilayer self-assembly technique. Therefore, this novel approach to surface modification may demonstrate long term patency in future in vivo animal trials of small diameter silk fibroin vascular grafts.

Bilayered vascular grafts based on silk proteins

A major block in the development of small diameter vascular grafts is achieving suitable blood vessel regeneration while minimizing the risk of thrombosis, intimal hyperplasia, suture retention, and mechanical failure. Silk-based tubular vessels for tissue engineering have been prepared by molding, dipping, electrospinning, or gel spinning, however, further studies are needed to improve the mechanical and blood compatibility properties. In the present study a bilayered vascular graft based on silk fibroin (SF) was developed. The graft was composed of an inner silk fiber-reinforced SF tube containing heparin and a highly porous SF external layer. Compared with previously fabricated SF tubes the fiber-reinforcement provided a comparable or higher mechanical strength, burst pressure, and suture retention strength, as well as mechanical compliance, to saphenous veins for vascular grafts. Heparin release was sustained for at least 1 month, affording blood compatibility to the grafts. The outer layer of the grafts prepared through lyophilization had a highly porous structure in which the macropore walls were composed of nanofibers similar to extracellular matrix, which offered an excellent environment for cell growth. In vitro studies showed good cytocompatibility and hemocompatibility.

Effect of microtopographic structures of silk fibroin on endothelial cell behavior

Stent implantation has become the preferred revascularization treatment for occlusive blood vessel disease; however, there are occasionally complications resulting in re-narrowing of the treated artery. One approach to overcoming this problem is to establish a confluent monolayer of endothelial cells (ECs) on the stent, and a coating would facilitate the attachment of ECs. Silk fibroin was reported to be used as an ideal coating applied to stent for the culture of human ECs. The aim of the present study is to gain more insight into the influence of the internal microtopographical structure of silk fibroin on cell behavior, such as attachment and growth, and to further investigate its molecular mechanism using human umbilical vein ECs (HUVECs). Our results evaluated the effect of different microtopographical structures on cell behavior. In addition, we analyzed the cell cycle and investigated relevant molecules involved. The results indicated that the microtopographic structure of silk fibroin was associated with EC morphology, attachment and proliferation.

Stabilization of vaccines and antibiotics in silk and eliminating the cold chain

Sensitive biological compounds, such as vaccines and antibiotics, traditionally require a time-dependent "cold chain" to maximize therapeutic activity. This flawed process results in billions of dollars worth of viable drug loss during shipping and storage, and severely limits distribution to developing nations with limited infrastructure. To address these major limitations, we demonstrate self-standing silk protein biomaterial matrices capable of stabilizing labile vaccines and antibiotics, even at temperatures up to 60 °C over more than 6 months. Initial insight into the mechanistic basis for these findings is provided. Importantly, these findings suggest a transformative approach to the cold chain to revolutionize the way many labile therapeutic drugs are stored and utilized throughout the world.

Silk materials--a road to sustainable high technology

This review addresses the use of silk protein as a sustainable material in optics and photonics, electronics and optoelectronic applications. These options represent additional developments for this technology platform that compound the broad utility and impact of this material for medical needs that have been recently described in the literature. The favorable properties of the material certainly make a favorable case for the use of silk, yet serve as a broad inspiration to further develop biological foundries for both the synthesis and processing of Nature's materials for technological applications.

Natural and Genetically Engineered Proteins for Tissue Engineering

To overcome the limitations of traditionally used autografts, allografts and, to a lesser extent, synthetic materials, there is the need to develop a new generation of scaffolds with adequate mechanical and structural support, control of cell attachment, migration, proliferation and differentiation and with bio-resorbable features. This suite of properties would allow the body to heal itself at the same rate as implant degradation. Genetic engineering offers a route to this level of control of biomaterial systems. The possibility of expressing biological components in nature and to modify or bioengineer them further, offers a path towards multifunctional biomaterial systems. This includes opportunities to generate new protein sequences, new self-assembling peptides or fusions of different bioactive domains or protein motifs. New protein sequences with tunable properties can be generated that can be used as new biomaterials. In this review we address some of the most frequently used proteins for tissue engineering and biomedical applications and describe the techniques most commonly used to functionalize protein-based biomaterials by combining them with bioactive molecules to enhance biological performance. We also highlight the use of genetic engineering, for protein heterologous expression and the synthesis of new protein-based biopolymers, focusing the advantages of these functionalized biopolymers when compared with their counterparts extracted directly from nature and modified by techniques such as physical adsorption or chemical modification.

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.

Insect natural products and processes: new treatments for human disease

In this overview, some of the more significant recent developments in bioengineering natural products from insects with use or potential use in modern medicine are described, as well as in utilisation of insects as models for studying essential mammalian processes such as immune responses to pathogens. To date, insects have been relatively neglected as sources of modern drugs although they have provided valuable natural products, including honey and silk, for at least 4-7000 years, and have featured in folklore medicine for thousands of years. Particular examples of Insect Folk Medicines will briefly be described which have subsequently led through the application of molecular and bioengineering techniques to the development of bioactive compounds with great potential as pharmaceuticals in modern medicine. Insect products reviewed have been derived from honey, venom, silk, cantharidin, whole insect extracts, maggots, and blood-sucking arthropods. Drug activities detected include powerful antimicrobials against antibiotic-resistant bacteria and HIV, as well as anti-cancer, anti-angiogenesis and anti-coagulant factors and wound healing agents. Finally, the many problems in developing these insect products as human therapeutic drugs are considered and the possible solutions emerging to these problems are described.