Three-Layered Silk Fibroin Tubular Scaffold for the Repair and Regeneration of Small Caliber Blood Vessels: From Design to in vivo Pilot Tests

Sutureless vascular anastomotic approaches and their potential impacts

Sutureless anastomotic devices present several advantages over traditional suture anastomosis, including expanded global access to microvascular surgery, shorter operation and ischemic times, and reduced costs. However, their adaptation for arterial use remains a challenge. This review aims to provide a comprehensive overview of sutureless anastomotic approaches that are either FDA-approved or under investigation. These approaches include extraluminal couplers, intraluminal devices, and methods assisted by lasers or vacuums, with a particular emphasis on tissue adhesives. We analyze these devices for artery compatibility, material composition, potential for intimal damage, risks of thrombosis and restenosis, and complications arising from their deployment and maintenance. Additionally, we discuss the challenges faced in the development and clinical application of sutureless anastomotic techniques. Ideally, a sutureless anastomotic device or technique should eliminate the need for vessel eversion, mitigate thrombosis through either biodegradation or the release of antithrombotic drugs, and be easily deployable for broad use. The transformative potential of sutureless anastomotic approaches in microvascular surgery highlights the necessity for ongoing innovation to expand their applications and maximize their benefits.

Silk fibroin-based scaffolds for tissue engineering

Silk fibroin is an important natural fibrous protein with excellent prospects for tissue engineering applications. With profound studies in recent years, its potential in tissue repair has been developed. A growing body of literature has investigated various fabricating methods of silk fibroin and their application in tissue repair. The purpose of this paper is to trace the latest developments of SF-based scaffolds for tissue engineering. In this review, we first presented the primary and secondary structures of silk fibroin. The processing methods of SF scaffolds were then summarized. Lastly, we examined the contribution of new studies applying SF as scaffolds in tissue regeneration applications. Overall, this review showed the latest progress in the fabrication and utilization of silk fibroin-based scaffolds.

Electrospun silk fibroin/fibrin vascular scaffold with superior mechanical properties and biocompatibility for applications in tissue engineering

Electrospun scaffolds play important roles in the fields of regenerative medicine and vascular tissue engineering. The aim of the research described here was to develop a vascular scaffold that mimics the structural and functional properties of natural vascular scaffolding. The mechanical properties of artificial vascular tissue represent a key issue for successful transplantation in small diameter engineering blood vessels. We blended silk fibroin (SF) and fibrin to fabricate a composite scaffold using electrospinning to overcome the shortcomings of fibrin with respect to its mechanical properties. Subsequently, we then carefully investigated the morphological, mechanical properties, hydrophilicity, hemocompatibility, degradation, cytocompatibility and biocompatibility of the SF/fibrin (0:100), SF/fibrin (15:85), SF/fibrin (25:75), and SF/fibrin (35:65) scaffolds. Based on these in vitro results, we implanted SF/fibrin (25:75) vascular scaffold subcutaneously and analyzed its in vivo degradation and histocompatibility. The fiber structure of the SF/fibrin hybrid scaffold was smooth and uniform, and its fiber diameters were relatively small. Compared with the fibrin scaffold, the SF/fibrin scaffold clearly displayed increased mechanical strength, but the hydrophilicity weakened correspondingly. All of the SF/fibrin scaffolds showed excellent blood compatibility and appropriate biodegradation rates. The SF/fibrin (25:75) scaffold increased the proliferation and adhesion of MSCs. The results of animal experiments confirmed that the degradation of the SF/fibrin (25:75) scaffold was faster than that of the SF scaffold and effectively promoted tissue regeneration and cell infiltration. All in all, the SF/fibrin (25:75) electrospun scaffold displayed balanced and controllable biomechanical properties, degradability, and good cell compatibility. Thus, this scaffold proved to be an ideal candidate material for artificial blood vessels.

Silk fibroin-derived electrospun materials for biomedical applications: A review

Electrospinning is a versatile technique for fabricating polymeric fibers with diameters ranging from micro- to nanoscale, exhibiting multiple morphologies and arrangements. By combining silk fibroin (SF) with synthetic and/or natural polymers, electrospun materials with outstanding biological, chemical, electrical, physical, mechanical, and optical properties can be achieved, fulfilling the evolving biomedical demands. This review highlights the remarkable versatility of SF-derived electrospun materials, specifically focusing on their application in tissue regeneration (including cartilage, cornea, nerves, blood vessels, bones, and skin), disease treatment (such as cancer and diabetes), and the development of controlled drug delivery systems. Additionally, we explore the potential future trends in utilizing these nanofibrous materials for creating intelligent biomaterials, incorporating biosensors and wearable sensors for monitoring human health, and also discuss the bottlenecks for its widespread use. This comprehensive overview illuminates the significant impact and exciting prospects of SF-derived electrospun materials in advancing biomedical research and applications.

New Tissue-Engineered Vascular Matrix Based on Regenerated Silk Fibroin: in vitro Study

The aim of the study was to make a vascular patch based on regenerated silk fibroin (SF) and study its physical and mechanical characteristics, biocompatibility and matrix properties in comparison with polyhydroxybutyrate/valerate/polycaprolactone with incorporated vascular endothelial growth factor (PHBV/PCL/VEGF) and commercial bovine xenopericardium (XP) flap in experiments in vitro.

Materials and Methods

Tissue-engineered matrices were produced by electrospinning. The surface structure, physical and mechanical characteristics, hemocompatibility (erythrocyte hemolysis, aggregation, adhesion and activation of platelets after contact with the material) and matrix properties of vascular patches (adhesion, viability, metabolic activity of EA.hy926 cells on the material) were studied.

Results

The surface of SF-based matrices and PHBV/PCL/VEGF-based tissue engineered patches had a porous and fibrous structure compared to a denser and more uniform XP flap. The physical and mechanical characteristics of SF matrices were close to those of native vessels. Along with this, tissue-engineered patches demonstrated high hemocompatible properties, which do not differ from those for commercial XP flap. Adhesion, viability, and metabolic activity of EA.hy926 endothelial cells also corresponded to the previously developed PHBV/PCL/VEGF matrix and XP flap, which indicates the nontoxicity and biocompatibility of SF matrices.

Conclusion

Matrices produced from regenerated SF demonstrated satisfactory results, comparable to those for PHBV/PCL/VEGF and commercial XP flap, and in the case of platelet adhesion and activation, they outperformed these patches. In total, SF can be defined as material having sufficient biological compatibility, which makes it possible to consider a tissue-engineered matrix made from it as promising for implantation into the vascular wall.

Human Keratinocytes and Fibroblasts Co-Cultured on Silk Fibroin Scaffolds Exosomally Overrelease Angiogenic and Growth Factors

Objectives: The optimal healing of skin wounds, deep burns, and chronic ulcers is an important clinical problem. Attempts to solve it have been driving the search for skin equivalents based on synthetic or natural polymers. Methods: Consistent with this endeavor, we used regenerated silk fibroin (SF) from Bombyx mori to produce a novel compound scaffold by welding a 3D carded/hydroentangled SF-microfiber-based nonwoven layer (C/H-3D-SFnw; to support dermis engineering) to an electrospun 2D SF nanofiber layer (ESFN; a basal lamina surrogate). Next, we assessed-via scanning electron microscopy, attenuated total reflectance Fourier transform infrared spectroscopy, differential scanning calorimetry, mono- and co-cultures of HaCaT keratinocytes and adult human dermal fibroblasts (HDFs), dsDNA assays, exosome isolation, double-antibody arrays, and angiogenesis assays-whether the C/H-3D-SFnws/ESFNs would allow the reconstitution of a functional human skin analog in vitro. Results: Physical analyses proved that the C/H-3D-SFnws/ESFNs met the requirements for human soft-tissue-like implants. dsDNA assays revealed that co-cultures of HaCaTs (on the 2D ESFN surface) and HDFs (inside the 3D C/H-3D-SFnws) grew more intensely than did the respective monocultures. Double-antibody arrays showed that the CD9⁺/CD81⁺ exosomes isolated from the 14-day pooled growth media of HDF and/or HaCaT mono- or co-cultures conveyed 35 distinct angiogenic/growth factors (AGFs). However, versus monocultures' exosomes, HaCaT/HDF co-cultures' exosomes (i) transported larger amounts of 15 AGFs, i.e., PIGF, ANGPT-1, bFGF, Tie-2, Angiogenin, VEGF-A, VEGF-D, TIMP-1/-2, GRO-α/-β/-γ, IL-1β, IL-6, IL-8, MMP-9, and MCP-1, and (ii) significantly more strongly stimulated human dermal microvascular endothelial cells to migrate and assemble tubes/nodes in vitro. Conclusions: Our results showed that both cell-cell and cell-SF interactions boosted the exosomal release of AGFs from HaCaTs/HDFs co-cultured on C/H-3D-SFnws/ESFNs. Hence, such exosomes are an asset for prospective clinical applications as they advance cell growth and neoangiogenesis and consequently graft take and skin healing. Moreover, this new integument analog could be instrumental in preclinical and translational studies on human skin pathophysiology and regeneration.

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.

Development of Silk Fibroin Scaffolds for Vascular Repair

Silk fibroin (SF) is a biocompatible natural protein with excellent mechanical characteristics. SF-based biomaterials can be structured using a number of techniques, allowing the tuning of materials for specific biomedical applications. In this study, SF films, porous membranes, and electrospun membranes were produced using solvent-casting, salt-leaching, and electrospinning methodologies, respectively. SF-based materials were subjected to physicochemical and biological characterizations to determine their suitability for tissue regeneration applications. Mechanical analysis showed stress-strain curves of brittle materials in films and porous membranes, while electrospun membranes featured stress-strain curves typical of ductile materials. All samples showed similar chemical composition, melting transition, hydrophobic behavior, and low cytotoxicity levels, regardless of their architecture. Finally, all of the SF-based materials promote the proliferation of human umbilical vein endothelial cells (HUVECs). These findings demonstrate the different relationship between HUVEC behavior and the SF sample's topography, which can be taken advantage of for the design of vascular implants.

Silk fibroin scaffolds: A promising candidate for bone regeneration

It remains a big challenge in clinical practice to repair large-sized bone defects and many factors limit the application of autografts and allografts, The application of exogenous scaffolds is an alternate strategy for bone regeneration, among which the silk fibroin (SF) scaffold is a promising candidate. Due to the advantages of excellent biocompatibility, satisfying mechanical property, controllable biodegradability and structural adjustability, SF scaffolds exhibit great potential in bone regeneration with the help of well-designed structures, bioactive components and functional surface modification. This review will summarize the cell and tissue interaction with SF scaffolds, techniques to fabricate SF-based scaffolds and modifications of SF scaffolds to enhance osteogenesis, which will provide a deep and comprehensive insight into SF scaffolds and inspire the design and fabrication of novel SF scaffolds for superior osteogenic performance. However, there still needs more comprehensive efforts to promote better clinical translation of SF scaffolds, including more experiments in big animal models and clinical trials. Furthermore, deeper investigations are also in demand to reveal the degradation and clearing mechanisms of SF scaffolds and evaluate the influence of degradation products.

Biomaterials for Regenerative Medicine in Italy: Brief State of the Art of the Principal Research Centers

Regenerative medicine is the branch of medicine that effectively uses stem cell therapy and tissue engineering strategies to guide the healing or replacement of damaged tissues or organs. A crucial element is undoubtedly the biomaterial that guides biological events to restore tissue continuity. The polymers, natural or synthetic, find wide application thanks to their great adaptability. In fact, they can be used as principal components, coatings or vehicles to functionalize several biomaterials. There are many leading centers for the research and development of biomaterials in Italy. The aim of this review is to provide an overview of the current state of the art on polymer research for regenerative medicine purposes. The last five years of scientific production of the main Italian research centers has been screened to analyze the current advancement in tissue engineering in order to highlight inputs for the development of novel biomaterials and strategies.

Review of Polymeric Biomimetic Small-Diameter Vascular Grafts to Tackle Intimal Hyperplasia

Small-diameter artificial vascular grafts (SDAVG) are used to bypass blood flow in arterial occlusive diseases such as coronary heart or peripheral arterial disease. However, SDAVGs are plagued by restenosis after a short while due to thrombosis and the thickening of the neointimal wall known as intimal hyperplasia (IH). The specific causes of IH have not yet been deduced; however, thrombosis formation due to bioincompatibility as well as a mismatch between the biomechanical properties of the SDAVG and the native artery has been attributed to its initiation. The main challenges that have been faced in fabricating SDAVGs are facilitating rapid re-endothelialization of the luminal surface of the SDAVG and replicating the complex viscoelastic behavior of the arteries. Recent strategies to combat IH formation have been mostly based on imitating the natural structure and function of the native artery (biomimicry). Thus, most recently, developed grafts contain a multilayered structure with a designated function for each layer. This paper reviews the current polymeric, biomimetic SDAVGs in preventing the formation of IH. The materials used in fabrication, challenges, and strategies employed to tackle IH are summarized and discussed, and we focus on the multilayered structure of current SDAVGs. Additionally, the future aspects in this area are pointed out for researchers to consider in their endeavor.

Silk Vascular Grafts with Optimized Mechanical Properties for the Repair and Regeneration of Small Caliber Blood Vessels

As the incidence of cardiovascular diseases has been growing in recent years, the need for small-diameter vascular grafts is increasing. Considering the limited success of synthetic grafts, vascular tissue engineering/repair/regeneration aim to find novel solutions. Silk fibroin (SF) has been widely investigated for the development of vascular grafts, due to its good biocompatibility, tailorable biodegradability, excellent mechanical properties, and minimal inflammatory reactions. In this study, a new generation of three-layered SF vascular scaffolds has been produced and optimized. Four designs of the SILKGraft vascular prosthesis have been developed with the aim of improving kink resistance and mechanical strength, without compromising the compliance with native vessels and the proven biocompatibility. A more compact arrangement of the textile layer allowed for the increase in the mechanical properties along the longitudinal and circumferential directions and the improvement of the compliance value, which approached that reported for the saphenous and umbilical veins. The higher braid density slightly affected the grafts’ morphology, increasing surface roughness, but the novel design mimicked the corrugation approach used for synthetic grafts, causing significant improvements in kink resistance.

Synthetic Vascular Graft with Spatially Distinct Architecture for Rapid Biomimetic Cell Organisation in a Perfusion Bioreactor

Access to lab-grown fully functional blood vessels would provide an invaluable resource to vascular medicine. The complex architecture and cellular makeup of native vessels, however, makes this extremely challenging to reproduce in vitro. Bioreactor systems have helped advanced research in this area by replicating many of the physiological conditions necessary for full-scale tissue growth outside of the body. A key element underpinning these technologies are 3D vascular graft templates which serve as temporary scaffolds to direct cell growth into similar cellular architectures observed in native vessels. Grafts further engineered with appropriate physical cues to accommodate the multiple cell types that reside within native vessels may help improve the production efficiency and physiological accuracy of bioreactor-grown vessel substitutes. Here, we engineered two distinct scaffold architectures into an electrospun vascular graft aiming to encourage the spatial organisation of human vascular endothelial cells (hCAECs) in a continuous luminal monolayer, co-cultured with human fibroblasts (hFBs) populating the graft wall. Using an electrospun composite of polycaprolactone (PCL) and gelatin, we evaluated physical parameters including fibre thickness, fibre alignment, and porosity, that best mimicked the spatial composition and growth of hCAECs and hFBs in native vessels. Upon identifying the optimal scaffold architectures for each cell type, we constructed a custom-designed mandrel that combined these distinct architectures into a single vascular graft during a single electrospinning processing run. When connected to a perfusion bioreactor system, the dual architecture graft spatially oriented hCAECs and hFBs into the graft wall and lumen, respectively, directly from circulation. This biomimetic cell organisation was consistent with positive graft remodelling with significant collagen deposition in the graft wall. These findings demonstrate the influence of architectural cues to direct cell growth within vascular graft templates and the future potential of these approaches to more accurately and efficiency produce blood vessel substitutes in bioreactor systems.

History, progress and future challenges of artificial blood vessels: a narrative review

Cardiovascular disease serves as the leading cause of death worldwide, with stenosis, occlusion, or severe dysfunction of blood vessels being its pathophysiological mechanism. Vascular replacement is the preferred surgical option for treating obstructed vascular structures. Due to the limited availability of healthy autologous vessels as well as the incidence of postoperative complications, there is an increasing demand for artificial blood vessels. From synthetic to natural, or a mixture of these components, numerous materials have been used to prepare artificial vascular grafts. Although synthetic grafts are more appropriate for use in medium to large-diameter vessels, they fail when replacing small-diameter vessels. Tissue-engineered vascular grafts are very likely to be an ideal alternative to autologous grafts in small-diameter vessels and are worthy of further investigation. However, a multitude of problems remain that must be resolved before they can be used in biomedical applications. Accordingly, this review attempts to describe these problems and provide a discussion of the generation of artificial blood vessels. In addition, we deliberate on current state-of-the-art technologies for creating artificial blood vessels, including advances in materials, fabrication techniques, various methods of surface modification, as well as preclinical and clinical applications. Furthermore, the evaluation of grafts both in vivo and in vitro, mechanical properties, challenges, and directions for further research are also discussed.

Adult Human Vascular Smooth Muscle Cells on 3D Silk Fibroin Nonwovens Release Exosomes Enriched in Angiogenic and Growth-Promoting Factors

Background. Our earlier works showed the quick vascularization of mouse skin grafted Bombyx mori 3D silk fibroin nonwoven scaffolds (3D-SFnws) and the release of exosomes enriched in angiogenic/growth factors (AGFs) from in vitro 3D-SFnws-stuck human dermal fibroblasts (HDFs). Here, we explored whether coronary artery adult human smooth muscle cells (AHSMCs) also release AGFs-enriched exosomes when cultured on 3D-SFnws in vitro. Methods. Media with exosome-depleted FBS served for AHSMCs and human endothelial cells (HECs) cultures on 3D-SFnws or polystyrene. Biochemical methods and double-antibody arrays assessed cell growth, metabolism, and intracellular TGF-β and NF-κB signalling pathways activation. AGFs conveyed by CD9⁺/CD81⁺ exosomes released from AHSMCs were double-antibody array analysed and their angiogenic power evaluated on HECs in vitro. Results. AHSMCs grew and consumed D-glucose more intensely and showed a stronger phosphorylation/activation of TAK-1, SMAD-1/-2/-4/-5, ATF-2, c-JUN, ATM, CREB, and an IκBα phosphorylation/inactivation on SFnws vs. polystyrene, consistent overall with a proliferative/secretory phenotype. SFnws-stuck AHSMCs also released exosomes richer in IL-1α/-2/-4/-6/-8; bFGF; GM-CSF; and GRO-α/-β/-γ, which strongly stimulated HECs’ growth, migration, and tubes/nodes assembly in vitro. Conclusions. Altogether, the intensified AGFs exosomal release from 3D-SFnws-attached AHSMCs and HDFs could advance grafts’ colonization, vascularization, and take in vivo—noteworthy assets for prospective clinical applications.

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.

Bioengineering artificial blood vessels from natural materials

Bioengineering an effective, small diameter (<6 mm) artificial vascular graft for use in bypass surgery when autologous grafts are unavailable remains a persistent challenge. Commercially available grafts are typically made from plastics, which have high strength but lack elasticity and present a foreign surface that triggers undesirable biological responses. Tissue engineered grafts, leveraging decellularized animal vessels or derived de novo from long-term cell culture, have dominated recent research, but failed to meet clinical expectations. More effective constructs that are readily translatable are urgently needed. Recent advances in natural materials have made the production of robust acellular conduits feasible and their use increasingly attractive. Here, we identify a subset of natural materials with potential to generate durable, small diameter vascular grafts.

Systematic Review of Tissue-Engineered Vascular Grafts

Pathologies related to the cardiovascular system are the leading causes of death worldwide. One of the main treatments is conventional surgery with autologous transplants. Although donor grafts are often unavailable, tissue-engineered vascular grafts (TEVGs) show promise for clinical treatments. A systematic review of the recent scientific literature was performed using PubMed (Medline) and Web of Science databases to provide an overview of the state-of-the-art in TEVG development. The use of TEVG in human patients remains quite restricted owing to the presence of vascular stenosis, existence of thrombi, and poor graft patency. A total of 92 original articles involving human patients and animal models were analyzed. A meta-analysis of the influence of the vascular graft diameter on the occurrence of thrombosis and graft patency was performed for the different models analyzed. Although there is no ideal animal model for TEVG research, the murine model is the most extensively used. Hybrid grafting, electrospinning, and cell seeding are currently the most promising technologies. The results showed that there is a tendency for thrombosis and non-patency in small-diameter grafts. TEVGs are under constant development, and research is oriented towards the search for safe devices.

Bioengineering silk into blood vessels

The rising incidence of cardiovascular disease has increased the demand for small diameter (<6 mm) synthetic vascular grafts for use in bypass surgery. Clinically available synthetic grafts (polyethylene terephthalate and expanded polytetrafluorethylene) are incredibly strong, but also highly hydrophobic and inelastic, leading to high rates of failure when used for small diameter bypass. The poor clinical outcomes of commercial synthetic grafts in this setting have driven significant research in search of new materials that retain favourable mechanical properties but offer improved biocompatibility. Over the last several decades, silk fibroin derived from Bombyx mori silkworms has emerged as a promising biomaterial for use in vascular applications. Progress has been driven by advances in silk manufacturing practices which have allowed unprecedented control over silk strength, architecture, and the ensuing biological response. Silk can now be manufactured to mimic the mechanical properties of native arteries, rapidly recover the native endothelial cell layer lining vessels, and direct positive vascular remodelling through the regulation of local inflammatory responses. This review summarises the advances in silk purification, processing and functionalisation which have allowed the production of robust vascular grafts with promise for future clinical application.

Silk biomaterials for vascular tissue engineering applications

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

Electrospun nanofiber scaffold for vascular tissue engineering

Due to the prevalence of cardiovascular diseases, there is a large need for small diameter vascular grafts that cannot be fulfilled using autologous vessels. Although medium to large diameter synthetic vessels are in use, no suitable small diameter vascular graft has been developed due to the unique dynamic environment that exists in small vessels. To achieve long term patency, a successful tissue engineered vascular graft would need to closely match the mechanical properties of native tissue, be non-thrombotic and non-immunogenic, and elicit the proper healing response and undergo remodeling to incorporate into the native vasculature. Electrospinning presents a promising approach to the development of a suitable tissue engineered vascular graft. This review provides a comprehensive overview of the different polymers, techniques, and functionalization approaches that have been used to develop an electrospun tissue engineered vascular graft.

Improved Repopulation Efficacy of Decellularized Small Diameter Vascular Grafts Utilizing the Cord Blood Platelet Lysate

BACKGROUND

The development of functional bioengineered small-diameter vascular grafts (SDVGs), represents a major challenge of tissue engineering. This study aimed to evaluate the repopulation efficacy of biological vessels, utilizing the cord blood platelet lysate (CBPL).

METHODS

Human umbilical arteries (hUAs, n = 10) were submitted to decellularization. Then, an evaluation of decellularized hUAs, involving histological, biochemical and biomechanical analysis, was performed. Wharton's Jelly (WJ) Mesenchymal Stromal Cells (MSCs) were isolated and characterized for their properties. Then, WJ-MSCs (1.5 × 10⁶ cells) were seeded on decellularized hUAs (n = 5) and cultivated with (Group A) or without the presence of the CBPL, (Group B) for 30 days. Histological analysis involving immunohistochemistry (against Ki67, for determination of cell proliferation) and indirect immunofluorescence (against activated MAP kinase, additional marker for cell growth and proliferation) was performed.

RESULTS

The decellularized hUAs retained their initial vessel's properties, in terms of key-specific proteins, the biochemical and biomechanical characteristics were preserved. The evaluation of the repopulation process indicated a more uniform distribution of WJ-MSCs in group A compared to group B. The repopulated vascular grafts of group B were characterized by greater Ki67 and MAP kinase expression compared to group A.

CONCLUSION

The results of this study indicated that the CBPL may improve the repopulation efficacy, thus bringing the biological SDVGs one step closer to clinical application.

Biofabrication of tissue engineering vascular systems

Cardiovascular disease (CVD) is the leading cause of death among persons aged 65 and older in the United States and many other developed countries. Tissue engineered vascular systems (TEVS) can serve as grafts for CVD treatment and be used as in vitro model systems to examine the role of various genetic factors during the CVD progressions. Current focus in the field is to fabricate TEVS that more closely resembles the mechanical properties and extracellular matrix environment of native vessels, which depends heavily on the advance in biofabrication techniques and discovery of novel biomaterials. In this review, we outline the mechanical and biological design requirements of TEVS and explore the history and recent advances in biofabrication methods and biomaterials for tissue engineered blood vessels and microvascular systems with special focus on in vitro applications. In vitro applications of TEVS for disease modeling are discussed.

Future Perspectives in Small-Diameter Vascular Graft Engineering

The increased demands of small-diameter vascular grafts (SDVGs) globally has forced the scientific society to explore alternative strategies utilizing the tissue engineering approaches. Cardiovascular disease (CVD) comprises one of the most lethal groups of non-communicable disorders worldwide. It has been estimated that in Europe, the healthcare cost for the administration of CVD is more than 169 billion €. Common manifestations involve the narrowing or occlusion of blood vessels. The replacement of damaged vessels with autologous grafts represents one of the applied therapeutic approaches in CVD. However, significant drawbacks are accompanying the above procedure; therefore, the exploration of alternative vessel sources must be performed. Engineered SDVGs can be produced through the utilization of non-degradable/degradable and naturally derived materials. Decellularized vessels represent also an alternative valuable source for the development of SDVGs. In this review, a great number of SDVG engineering approaches will be highlighted. Importantly, the state-of-the-art methodologies, which are currently employed, will be comprehensively presented. A discussion summarizing the key marks and the future perspectives of SDVG engineering will be included in this review. Taking into consideration the increased number of patients with CVD, SDVG engineering may assist significantly in cardiovascular reconstructive surgery and, therefore, the overall improvement of patients' life.

Silk Polymers and Nanoparticles: A Powerful Combination for the Design of Versatile Biomaterials

Silk fibroin (SF) is a natural protein largely used in the textile industry but also in biomedicine, catalysis, and other materials applications. SF is biocompatible, biodegradable, and possesses high tensile strength. Moreover, it is a versatile compound that can be formed into different materials at the macro, micro- and nano-scales, such as nanofibers, nanoparticles, hydrogels, microspheres, and other formats. Silk can be further integrated into emerging and promising additive manufacturing techniques like bioprinting, stereolithography or digital light processing 3D printing. As such, the development of methodologies for the functionalization of silk materials provide added value. Inorganic nanoparticles (INPs) have interesting and unexpected properties differing from bulk materials. These properties include better catalysis efficiency (better surface/volume ratio and consequently decreased quantify of catalyst), antibacterial activity, fluorescence properties, and UV-radiation protection or superparamagnetic behavior depending on the metal used. Given the promising results and performance of INPs, their use in many different procedures has been growing. Therefore, combining the useful properties of silk fibroin materials with those from INPs is increasingly relevant in many applications. Two main methodologies have been used in the literature to form silk-based bionanocomposites: in situ synthesis of INPs in silk materials, or the addition of preformed INPs to silk materials. This work presents an overview of current silk nanocomposites developed by these two main methodologies. An evaluation of overall INP characteristics and their distribution within the material is presented for each approach. Finally, an outlook is provided about the potential applications of these resultant nanocomposite materials.

Hierarchical porous silk fibroin/poly(L-lactic acid) fibrous membranes towards vascular scaffolds

Fibrous membranes played an important role to prepare tubular scaffolds for muscular artery regeneration. In this study, a strategy has been developed to combine silk fibroin (SF) with highly porous electrospun poly(L-lactic acid) (PLLA) fibrous membrane towards vascular scaffolds. After PLLA fibres were electrospun and collected, they were immersed into acetone to generate a porous structure with ultra-high surface area. While the pores on PLLA fibres were fulfilled with SF solution and dried, SF was coated uniformly and tightly on PLLA fibres. A multi-layer tubular structure of the tunica media was simulated by winding and stacking a strip of electrospun fibrous membrane. In vitro viability and morphology studies of A7r5 smooth muscle cells were undertaken for up to 14 days. Because the hydrophilicity of SF/PLLA composite fibres were improved dramatically, it had a positive effect on cell adhesion rate (97%) and proliferation (64.4%). Moreover, good cell morphology was observed via a multiphoton laser confocal microscope on SF/PLLA bioactive materials. These results demonstrated that the hierarchical porous SF/PLLA fibrous membranes are promising off-the-shelf scaffolds for muscular artery regeneration.

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.

Bioresorbable Polymeric Scaffold in Cardiovascular Applications

Advances in material science and innovative medical technologies have allowed the development of less invasive interventional procedures for deploying implant devices, including scaffolds for cardiac tissue engineering. Biodegradable materials (e.g., resorbable polymers) are employed in devices that are only needed for a transient period. In the case of coronary stents, the device is only required for 6-8 months before positive remodelling takes place. Hence, biodegradable polymeric stents have been considered to promote this positive remodelling and eliminate the issue of permanent caging of the vessel. In tissue engineering, the role of the scaffold is to support favourable cell-scaffold interaction to stimulate formation of functional tissue. The ideal outcome is for the cells to produce their own extracellular matrix over time and eventually replace the implanted scaffold or tissue engineered construct. Synthetic biodegradable polymers are the favoured candidates as scaffolds, because their degradation rates can be manipulated over a broad time scale, and they may be functionalised easily. This review presents an overview of coronary heart disease, the limitations of current interventions and how biomaterials can be used to potentially circumvent these shortcomings in bioresorbable stents, vascular grafts and cardiac patches. The material specifications, type of polymers used, current progress and future challenges for each application will be discussed in this manuscript.

Insight on the endothelialization of small silk-based tissue-engineered vascular grafts

Along with an increased incidence of cardiovascular diseases, there is a strong need for small-diameter vascular grafts. Silk has been investigated as a biomaterial to develop such grafts thanks to different processing options. Endothelialization was shown to be extremely important to ensure graft patency and there is ongoing research on the development and behavior of endothelial cells on vascular tissue-engineered scaffolds. This article reviews the endothelialization of silk-based scaffolds processed throughout the years as silk non-woven nets, films, gel spun, electrospun, or woven scaffolds. Encouraging results were reported with these scaffolds both in vitro and in vivo when implanted in small- to middle-sized animals. The use of coatings and heparin or sulfur to enhance, respectively, cell adhesion and scaffold hemocompatibility is further presented. Bioreactors also showed their interest to improve cell adhesion and thus promoting in vitro pre-endothelialization of grafts even though they are still not systematically used. Finally, the importance of the animal models used to study the right mechanism of endothelialization is discussed.