Rapid Endothelialization of Off-the-Shelf Small Diameter Silk Vascular Grafts

On-Demand Bioactivation of Inert Materials With Plasma-Polymerized Nanoparticles

Conventional gas plasma treatments are crucial for functionalizing materials in biomedical applications, but have limitations hindering their broader use. These methods require exposure to reactive media under vacuum conditions, rendering them unsuitable for substrates that demand aqueous environments, such as proteins and hydrogels. In addition, complex geometries are difficult to treat, necessitating extensive customization for each material and shape. To address these constraints, an innovative approach employing plasma polymer nanoparticles (PPN) as a versatile functionalization tool is proposed. PPN share similarities with traditional plasma polymer coatings (PPC) but offer unique advantages: compatibility with aqueous systems, the ability to modify complex geometries, and availability as off-the-shelf products. Robust immobilization of PPN on various substrates, including synthetic polymers, proteins, and complex hydrogel structures is demonstrated in this study. This results in substantial improvements in surface hydrophilicity. Materials functionalization with arginylglycylaspartic acid (RGD)-loaded PPN significantly enhances cell attachment, spreading, and substrate coverage on inert scaffolds compared to passive RGD coatings. Improved adhesion to complex geometries and subsequent differentiation following growth factor exposure is also demonstrated. This research introduces a novel substrate functionalization approach that mimics the outcomes of plasma coating technology but vastly expands its applicability, promising advancements in biomedical materials and devices.

Silk fibroin increases the elasticity of alginate-gelatin hydrogels and regulates cardiac cell contractile function in cardiac bioinks

Silk fibroin (SF) is a natural protein extracted fromBombyx morisilkworm thread. From its common use in the textile industry, it emerged as a biomaterial with promising biochemical and mechanical properties for applications in the field of tissue engineering and regenerative medicine. In this study, we evaluate for the first time the effects of SF on cardiac bioink formulations containing cardiac spheroids (CSs). First, we evaluate if the SF addition plays a role in the structural and elastic properties of hydrogels containing alginate (Alg) and gelatin (Gel). Then, we test the printability and durability of bioprinted SF-containing hydrogels. Finally, we evaluate whether the addition of SF controls cell viability and function of CSs in Alg-Gel hydrogels. Our findings show that the addition of 1% (w/v) SF to Alg-Gel hydrogels makes them more elastic without affecting cell viability. However, fractional shortening (FS%) of CSs in SF-Alg-Gel hydrogels increases without affecting their contraction frequency, suggesting an improvement in contractile function in the 3D cultures. Altogether, our findings support a promising pathway to bioengineer bioinks containing SF for cardiac applications, with the ability to control mechanical and cellular features in cardiac bioinks.

Fabric-Enhanced Vascular Graft with Hierarchical Structure for Promoting the Regeneration of Vascular Tissue

Natural blood vessels have completed functions, including elasticity, compliance, and excellent antithrombotic properties because of their mature structure. To replace damaged blood vessels, vascular grafts should perform these functions by simulating the natural vascular structures. Although the structures of natural blood vessels are thoroughly explored, constructing a small-diameter vascular graft that matches the mechanical and biological properties of natural blood vessels remains a challenge. A hierarchical vascular graft is fabricated by Electrospinning, Braiding, and Thermally induced phase separation (EBT) processes, which could simulate the structure of natural blood vessels. The internal electrospun structure facilitates the adhesion of endothelial cells, thereby accelerating endothelialization. The intermediate PLGA fabric exhibits excellent mechanical properties, which allow it to maintain its shape during long-term transplantation and prevent graft expansion. The external macroporous structure is beneficial for cell growth and infiltration. Blood vessel remodeling aims to combine a structure that promotes tissue regeneration with anti-inflammatory materials. The results in vitro demonstrated that it EBT vascular graft (EBTVG) has matched the mechanical properties, reliable cytocompatibility, and the strongest endothelialization in situ. The results in vitro and replacement of the resected artery in vivo suggest that the EBTVG combines different structural advantages with biomechanical properties and reliable biocompatibility, significantly promoting the stabilization and regeneration of vascular endothelial cells and vascular smooth muscle cells, as well as stabilizing the blood microenvironment.

Development of a Novel Hierarchically Biofabricated Blood Vessel Mimic Decorated with Three Vascular Cell Populations for the Reconstruction of Small-Diameter Arteries

The availability of grafts to replace small-diameter arteries remains an unmet clinical need. Here, the validated methodology is reported for a novel hybrid tissue-engineered vascular graft that aims to match the natural structure of small-size arteries. The blood vessel mimic (BVM) comprises an internal conduit of co-electrospun gelatin and polycaprolactone (PCL) nanofibers (corresponding to the tunica intima of an artery), reinforced by an additional layer of PCL aligned fibers (the internal elastic membrane). Endothelial cells are deposited onto the luminal surface using a rotative bioreactor. A bioprinting system extrudes two concentric cell-laden hydrogel layers containing respectively vascular smooth muscle cells and pericytes to create the tunica media and adventitia. The semi-automated cellularization process reduces the production and maturation time to 6 days. After the evaluation of mechanical properties, cellular viability, hemocompatibility, and suturability, the BVM is successfully implanted in the left pulmonary artery of swine. Here, the BVM showed good hemostatic properties, capability to withstand blood pressure, and patency at 5 weeks post-implantation. These promising data open a new avenue to developing an artery-like product for reconstructing small-diameter blood vessels.

Bilayer vascular grafts with on-demand NO and H2S release capabilities

Nitric oxide (NO) and hydrogen sulfide (H2S) gasotransmitters exhibit potential therapeutic effects in the cardiovascular system. Herein, biomimicking multilayer structures of biological blood vessels, bilayer small-diameter vascular grafts (SDVGs) with on-demand NO and H2S release capabilities, were designed and fabricated. The keratin-based H2S donor (KTC) with good biocompatibility and high stability was first synthesized and then electrospun with poly (l-lactide-co-caprolactone) (PLCL) to be used as the outer layer of grafts. The electrospun poly (ε-caprolactone) (PCL) mats were aminolyzed and further chelated with copper (II) ions to construct glutathione peroxidase (GPx)-like structural surfaces for the catalytic generation of NO, which acted as the inner layer of grafts. The on-demand release of NO and H2S selectively and synergistically promoted the proliferation and migration of human umbilical vein endothelial cells (HUVECs) while inhibiting the proliferation and migration of human umbilical artery smooth muscle cells (HUASMCs). Dual releases of NO and H2S gasotransmitters could enhance their respective production, resulting in enhanced promotion of HUVECs and inhibition of HUASMCs owing to their combined actions. In addition, the bilayer grafts were conducive to forming endothelial cell layers under flow shear stress. In rat abdominal aorta replacement models, the grafts remained patency for 6 months. These grafts were capable of facilitating rapid endothelialization and alleviating neointimal hyperplasia without obvious injury, inflammation, or thrombosis. More importantly, the grafts were expected to avoid calcification with the degradation of the grafts. Taken together, these bilayer grafts will be greatly promising candidates for SDVGs with rapid endothelialization and anti-calcification properties.

Functional regeneration at the blood-biomaterial interface

The use of cardiovascular implants is commonplace in clinical practice. However, reproducing the key bioactive and adaptive properties of native cardiovascular tissues with an artificial replacement is highly challenging. Exciting new treatment strategies are under development to regenerate (parts of) cardiovascular tissues directly in situ using immunomodulatory biomaterials. Direct exposure to the bloodstream and hemodynamic loads is a particular challenge, given the risk of thrombosis and adverse remodeling that it brings. However, the blood is also a source of (immune) cells and proteins that dominantly contribute to functional tissue regeneration. This review explores the potential of the blood as a source for the complete or partial in situ regeneration of cardiovascular tissues, with a particular focus on the endothelium, being the natural blood-tissue barrier. We pinpoint the current scientific challenges to enable rational engineering and testing of blood-contacting implants to leverage the regenerative potential of the blood.

Epsin Mimetic UPI Peptide Delivery Strategies to Improve Endothelization of Vascular Grafts

Endothelialization of engineered vascular grafts for replacement of small-diameter coronary arteries remains a critical challenge. The ability for an acellular vascular graft to promote endothelial cell (EC) recruitment in the body would be very beneficial. This study investigated epsins as a target since they are involved in internalization of vascular endothelial growth factor receptor 2. Specifically, epsin-mimetic UPI peptides are delivered locally from vascular grafts to block epsin activity and promote endothelialization. The peptide delivery from fibrin coatings allowed for controlled loading and provided a significant improvement in EC attachment, migration, and growth in vitro. The peptides have even more important impacts after grafting into rat abdominal aortae. The peptides prevented graft thrombosis and failure that is observed with a fibrin coating alone. They also modulated the in vivo remodeling. The grafts are able to remodel without the formation of a thick fibrous capsule on the adventitia with the 100 µg mL-1 peptide-loaded condition, and this condition enabled the formation of a functional EC monolayer in the graft lumen after only 1 week. Overall, this study demonstrated that the local delivery of UPI peptides is a promising strategy to improve the performance of vascular grafts.

Development of a Silk Fibroin-Small Intestinal Submucosa Small-Diameter Vascular Graft with Sequential VEGF and TGF-β1 Inhibitor Delivery for In Situ Tissue Engineering

Proper endothelialization and limited collagen deposition on the luminal surface after graft implantation plays a crucial role to prevent the occurrence of stenosis. To achieve these conditions, a biodegradable graft with adequate mechanical properties and the ability to sequentially deliver therapeutic agents isfabricated. In this study, a dual-release system is constructed through coaxial electrospinning by incorporating recombinant human vascular endothelial growth factor (VEGF) and transforming growth factor β1 (TGF-β1) inhibitor into silk fibroin (SF) nanofibers to form a bioactive membrane. The functionalized SF membrane as the inner layer of the graft is characterized by the release profile, cell proliferation and protein expression. It presents excellent biocompatibility and biodegradation, facilitating cell attachment, proliferation, and infiltration. The core-shell structure enables rapid VEGF release within 10 days and sustained plasmid delivery for 21 days. A 2.0-mm-diameter vascular graft is fabricated by integrating the SF membrane with decellularized porcine small intestinal submucosa (SIS), aiming to facilitate the integration process under a stable extracellular matrix structure. The bioengineered graft is functionalized with the sequential administration of VEGF and TGF-β1, and with the reinforced and compatible mechanical properties, thereby offers an orchestrated solution for stenosis with potential for in situ vascular tissue engineering applications.

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.

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

Blood vessels not only transport oxygen and nutrients to each organ, but also play an important role in the regulation of tissue regeneration. Impaired or occluded vessels can result in ischemia, tissue necrosis, or even life-threatening events. Bioengineered vascular grafts have become a promising alternative treatment for damaged or occlusive vessels. Large-scale tubular grafts, which can match arteries, arterioles, and venules, as well as meso- and microscale vasculature to alleviate ischemia or prevascularized engineered tissues, have been developed. In this review, materials and techniques for engineering tubular scaffolds and vasculature at all levels are discussed. Examples of vascularized tissue engineering in bone, peripheral nerves, and the heart are also provided. Finally, the current challenges are discussed and the perspectives on future developments in biofunctional engineered vessels are delineated.

Application of decellularized vascular matrix in small-diameter vascular grafts

Coronary artery bypass grafting (CABG) remains the most common procedure used in cardiovascular surgery for the treatment of severe coronary atherosclerotic heart disease. In coronary artery bypass grafting, small-diameter vascular grafts can potentially replace the vessels of the patient. The complete retention of the extracellular matrix, superior biocompatibility, and non-immunogenicity of the decellularized vascular matrix are unique advantages of small-diameter tissue-engineered vascular grafts. However, after vascular implantation, the decellularized vascular matrix is also subject to thrombosis and neoplastic endothelial hyperplasia, the two major problems that hinder its clinical application. The keys to improving the long-term patency of the decellularized matrix as vascular grafts include facilitating early endothelialization and avoiding intravascular thrombosis. This review article sequentially introduces six aspects of the decellularized vascular matrix as follows: design criteria of vascular grafts, components of the decellularized vascular matrix, the changing sources of the decellularized vascular matrix, the advantages and shortcomings of decellularization technologies, modification methods and the commercialization progress as well as the application prospects in small-diameter vascular grafts.

Construction of spider silk protein small-caliber tissue engineering vascular grafts based on dynamic culture and its performance evaluation

Tissue engineering is an alternative method for preparing small-caliber (<6 mm) vascular grafts. Dynamic mechanical conditioning is being researched as a method to improve mechanical properties of tissue engineered blood vessels. This method attempts to induce unique reaction in implanted cells that regenerate the matrix around them, thereby improving the overall mechanical stability of the grafts. In this study, we used a bioreactor to seed endothelial cells and smooth muscle cells into the inner and outer layers of the electrospun spider silk protein scaffold respectively to construct vascular grafts. The cell proliferation, mechanical properties, blood compatibility and other indicators of the vascular grafts were characterized in vitro. Furthermore, the vascular grafts were implanted in Sprague Dawley rats, and the vascular grafts' patency, extracellular matrix formation, and inflammatory response were evaluated in vivo. We aimed to construct spider silk protein vascular grafts with the potential for in vivo implantation by using a pulsating flow bioreactor. The results showed that, when compared with the static culture condition, the dynamic culture condition improved cell proliferation on vascular scaffolds and enhanced mechanical function of vascular scaffolds. In vivo experiments also showed that the dynamic culture of vascular grafts was more beneficial for the extracellular matrix deposition and anti-thrombogenesis, as well as reducing the inflammatory response of vascular grafts. In conclusion, dynamic mechanical conditioning aid in the resolution of challenges impeding the application of electrospun scaffolds and have the potential to construct small-caliber blood vessels with regenerative function for cardiovascular tissue repair.

Monocyte Recruitment for Vascular Tissue Regeneration

A strategy to recruit monocytes (MCs) from blood to regenerate vascular tissue from unseeded (cell-free) tissue engineered vascular grafts is presented. When immobilized on the surface of vascular grafts, the fusion protein, H2R5 can capture blood-derived MC under static or flow conditions in a shear stress dependent manner. The bound MC turns into macrophages (Mϕ) expressing both M1 and M2 phenotype specific genes. When H2R5 functionalized acellular-tissue engineered vessels (A-TEVs) are implanted into the mouse aorta, they remain patent and form a continuous endothelium expressing both endothelial cell (EC) and MC specific proteins. Underneath the EC layer, multiple cells layers are formed coexpressing both smooth muscle cell (SMC) and MC specific markers. Lineage tracing analysis using a novel CX3CR1-confetti mouse model demonstrates that fluorescently labeled MC populates the graft lumen by two and four weeks postimplantation, providing direct evidence in support of MC/Mϕ recruitment to the graft lumen. Given their abundance in the blood, circulating MCs may be a great source of cells that contribute directly to the endothelialization and vascular wall formation of acellular vascular grafts under the right chemical and biomechanical cues.

Development of a decellularized human amniotic membrane-based electrospun vascular graft capable of rapid remodeling for small-diameter vascular applications

The performance of small-diameter vascular grafts adapted to vascular replacement is commonly hindered by stenosis. To address this issue, a graft featuring rapid remodeling with degradation is warranted. In this work, a 1.8-mm-diameter graft was constructed by fabricating a decellularized human amniotic membrane (HAM) with polycaprolactone (PCL)/silk fibroin (SF) around it through electrospinning, namely, an HPS graft, and applied in a rat aortic grafting model for comparison to a decellularized porcine small intestinal submucosa (SIS)-integrated PCL/SF (SPS) graft and an autologous aorta. In vitro studies demonstrated that HAM provided a bioactive milieu for rapid endothelial cell proliferation and resisting fibroblast-induced collagen secretion. PCL/SF provides a biocompatible microenvironment for cellular infiltration with mechanical properties resembling those of the rat aorta. In vivo studies showed that the HPS graft induced functional endothelialization more rapidly, along with less intensive ECM deposition than the SPS graft upon the histologically weaker inflammatory response and foreign body reaction 4 weeks after implantation, and maintained patency by progressively stabilizing the remodeling structure approximating the native counterparts over 24 weeks. The bioengineered graft expands the applicability of allogeneic matrices with degradable electrospun polymers for long-term in situ vascular applications. STATEMENT OF SIGNIFICANCE: An orchestrated remodeling of the vascular graft, featuring rapid endothelialization and resisting extracellular matrix (ECM) deposition on the luminal surface, with a mechanically stable structure, is requisite for long-term vascular patency. Nevertheless, off-the-shelf grafts might not fulfil the criteria under abdominal aortic pressure. Herein, we fabricated a 1.8-mm-diameter vascular graft through the integration of a decellularized human amniotic membrane (HAM) with electrospun polycaprolactone (PCL)/silk fibroin (SF). In a rat aortic grafting model, the graft is capable of rapid endothelialization and resisting collagen deposition and provides a native-like mechanical structure for stabilizing the remodeling process towards that of the native aorta. This bioengineered graft has potential for small-diameter vascular regeneration, and provides advanced strategies to facilitate full-remodeling tissue applications.

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.

Double-Network Hydrogel Armored Decellularized Porcine Pericardium as Durable Bioprosthetic Heart Valves

Heart valves have extraordinary fatigue resistance which beat ≈3 billion times in a lifetime. Bioprosthetic heart valves (BHVs) made from fixed heteroplasm that are incrementally used in heart valve replacement fail to sustain the expected durability due to thrombosis, poor endothelialization, inflammation, calcification, and especially mechanical damage induced biocompatibility change. No effective strategy has been reported to conserve the biological properties of BHV after long-term fatigue test. Here, a double-network tough hydrogel is introduced, which interpenetrate and anchor into the matrix of decellularized porcine pericardium (dCell-PP) to form robust and stable conformal coatings and reduce immunogenicity. The ionic crosslinked hyaluronic acid (HA) network mimics the glycocalyx on endothelium which improves antithrombosis and accelerates endothelialization; the chemical crosslinked hydrophilic polyacrylamide (PAAm) network further enhances antifouling properties and strengthens the shielding hydrogels and their interaction with dCell-PP. In vitro and rabbit ex vivo shunt assay demonstrate great hemocompatibility of polyacrylamide/HA hydrogel hybrid PP (P/H-PP). Cell experiments and rat subcutaneous implantation confirm satisfactory endothelialization, biocompatibility, and anticalcification properties. For hydrodynamic experiment, P/H-PP gains full mark at different flow conditions and sustains excellent biomechanical and biological properties after 200 000 000 cycles. P/H double-network hydrogel armoring dCell-PP is a promising progress to extend BHV durability for clinical implantation therapy.

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.

Novel magnetic silk fibroin scaffolds with delayed degradation for potential long-distance vascular repair

Although with the good biological properties, silk fibroin (SF) is immensely restrained in long-distance vascular defect repair due to its relatively fast degradation and inferior mechanical properties. It is necessary to construct a multifunctional composite scaffold based on SF. In this study, a novel magnetic SF scaffold (MSFCs) was prepared by an improved infiltration method. Compared with SF scaffold (SFC), MSFCs were found to have better crystallinity, magnetocaloric properties, and mechanical strength, which was ascribed to the rational introduction of iron-based magnetic nanoparticles (MNPs). Moreover, in vivo and in vitro experiments demonstrated that the degradation of MSFCs was significantly extended. The mechanism of delayed degradation was correlated with the dual effect that was the newly formed hydrogen bonds between SFC and MNPs and the complexing to tyrosine (Try) to inhibit hydrolase by internal iron atoms. Besides, the β-crystallization of protein in MSFCs was increased with the rise of iron concentration, proving the beneficial effect after MNPS doped. Furthermore, although macrophages could phagocytose the released MNPs, it did not affect their function, and even a reasonable level might cause some cytokines to be upregulated. Finally, in vitro and in vivo studies demonstrated that MSFCs showed excellent biocompatibility and the growth promotion effect on CD34-labeled vascular endothelial cells (VECs). In conclusion, we confirm that the doping of MNPs can significantly reduce the degradation of SFC and thus provide an innovative perspective of multifunctional biocomposites for tissue engineering.

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.

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.

Biomimetic silk biomaterials: Perlecan-functionalized silk fibroin for use in blood-contacting devices

Blood compatible materials are required for the development of therapeutic and diagnostic blood contacting devices as blood-material interactions are a key factor dictating device functionality. In this work, we explored biofunctionalization of silk biomaterials with a recombinantly expressed domain V of the human basement membrane proteoglycan perlecan (rDV) towards the development of blood compatible surfaces. Perlecan and rDV are of interest in vascular device development as they uniquely support endothelial cell, while inhibiting smooth muscle cell and platelet interactions. rDV was covalently immobilized on silk biomaterials using plasma immersion ion implantation (PIII), a new method of immobilizing proteins on silk biomaterials that does not rely on modification of specific amino acids in the silk protein chain, and compared to physisorbed and carbodiimide immobilized rDV. Untreated and treated silk biomaterials were examined for interactions with blood components with varying degrees of complexity, including isolated platelets, platelet rich plasma, blood plasma, and whole blood, both under agitated and flow conditions. rDV-biofunctionalized silk biomaterials were shown to be blood compatible in terms of platelet and whole blood interactions and the PIII treatment was shown to be an effective and efficient means of covalently immobilizing rDV in its bioactive form. These biomimetic silk biomaterials are a promising platform toward development of silk-based blood-contacting devices for therapeutic, diagnostic, and research applications. STATEMENT OF SIGNIFICANCE: Blood compatible materials are required for the development of therapeutic and diagnostic blood contacting devices as blood-material interactions are a key factor dictating device functionality. In this work, we explored biofunctionalization of silk biomaterials with a recombinantly expressed domain V (rDV) of the human basement membrane proteoglycan perlecan towards the development of blood compatible surfaces. Perlecan and rDV are of interest in vascular device development as they uniquely support endothelial cell, while inhibiting smooth muscle cell and platelet interactions. rDV was covalently immobilized on silk biomaterials using plasma immersion ion implantation (PIII), a new method of immobilizing proteins on silk biomaterials that does not rely on modification of specific amino acids in the silk protein chain. These biomimetic silk biomaterials are a promising platform toward development of silk-based blood-contacting devices for therapeutic, diagnostic, and research applications.

Silk Fibroin Scaffold Architecture Regulates Inflammatory Responses and Engraftment of Bone Marrow-Mononuclear Cells

Despite being one of the most clinically trialed cell therapies, bone marrow-mononuclear cell (BM-MNC) infusion has largely failed to fulfill its clinical promise. Implanting biomimetic scaffolds at sites of injury prior to BM-MNC infusion is a promising approach to enhance BM-MNC engraftment and therapeutic function. Here, it is demonstrated that scaffold architecture can be leveraged to regulate the immune responses that drive BM-MNC engraftment. Silk scaffolds with thin fibers and low porosity (LP) impairs immune activation in vitro compared with thicker fiber, high porosity (HP) scaffolds. Using the authors' established in vivo bioluminescent BM-MNC tracking model, they showed that BM-MNCs home to and engraft in greater numbers in HP scaffolds over 14 days. Histological analysis reveals thicker fibrous capsule formation, with enhanced collagen deposition in HP compared to LP scaffolds consistent with substantially more native CD68⁺ macrophages and CD4⁺ T cells, driven by their elevated pro-inflammatory M1 and Th1 phenotypes, respectively. These results suggest that implant architecture impacts local inflammation that drives differential engraftment and remodeling behavior of infused BM-MNC. These findings inform the future design of biomimetic scaffolds that may better enhance the clinical effectiveness of BM-MNC infusion therapy.

Characterization of a Water-Dispersed Biodegradable Polyurethane-Silk Composite Sponge Using 13C Solid-State Nuclear Magnetic Resonance as Coating Material for Silk Vascular Grafts with Small Diameters

Recently, Bombyx mori silk fibroin (SF) has been shown to be a suitable material for vascular prostheses for small arteries. In this study, we developed a softer SF graft by coating water-dispersed biodegradable polyurethane (PU) based on polycaprolactone and an SF composite sponge on the knitted SF vascular graft. Three kinds of 13C solid-state nuclear magnetic resonance (NMR), namely carbon-13 (13C) cross-polarization/magic angle spinning (MAS), 13C dipolar decoupled MAS, and 13C refocused insensitive nuclei enhanced by polarization transfer (r-INEPT) NMR, were used to characterize the PU-SF coating sponge. Especially the 13C r-INEPT NMR spectrum of water-dispersed biodegradable PU showed that both main components of the non-crystalline domain of PU and amorphous domain of SF were highly mobile in the hydrated state. Then, the small-diameter SF artificial vascular grafts coated with this sponge were evaluated through implantation experiments with rats. The implanted PU-SF-coated SF grafts showed a high patency rate. It was confirmed that the inside of the SF grafts was covered with vascular endothelial cells 4 weeks after implantation. These results showed that the water-dispersed biodegradable PU-SF-coated SF graft created in this study could be a strong candidate for small-diameter artificial vascular graft.

A modified 3D printer as a hybrid bioprinting-electrospinning system for use in vascular tissue engineering applications

There is a high demand for small diameter vascular grafts having mechanical and biological properties similar to that of living tissues. Tissue-engineered vascular grafts using current methods have often failed due to the mismatch of mechanical properties between the implanted graft and living tissues. To address this limitation, a hybrid bioprinting-electrospinning system is developed for vascular tissue engineering applications. The setup is capable of producing layered structure from electrospun fibres and cell-laden hydrogel. A Creality3D Ender 3D printer has been modified into a hybrid setup having one bioprinting head and two electrospinning heads. Fortus 250mc and Flashforge Creator Pro 3D printers were used to print parts using acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) polymers. An Arduino mega 2560 and a Ramps 1.4 controller board were selected to control the functions of the hybrid bioprinting setup. The setup was tested successfully to print a tubular construct around a rotating needle.

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.

Evaluation of small-diameter silk vascular grafts implanted in dogs

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Vascular Tissue Engineering: Polymers and Methodologies for Small Caliber Vascular Grafts

Cardiovascular disease is the most common cause of death in the world. In severe cases, replacement or revascularization using vascular grafts are the treatment options. While several synthetic vascular grafts are clinically used with common approval for medium to large-caliber vessels, autologous vascular grafts are the only options clinically approved for small-caliber revascularizations. Autologous grafts have, however, some limitations in quantity and quality, and cause an invasiveness to patients when harvested. Therefore, the development of small-caliber synthetic vascular grafts (<5 mm) has been urged. Since small-caliber synthetic grafts made from the same materials as middle and large-caliber grafts have poor patency rates due to thrombus formation and intimal hyperplasia within the graft, newly innovative methodologies with vascular tissue engineering such as electrospinning, decellularization, lyophilization, and 3D printing, and novel polymers have been developed. This review article represents topics on the methodologies used in the development of scaffold-based vascular grafts and the polymers used in vitro and in vivo.

Digital 3D Topographic Microscopy: Bridging the Gaps Between Macroscopy, Microscopy and Scanning Electron Microscopy

Re-endothelialization of vascular lumen after endovascular procedures is a critical healing milestone and is subjected to routine pathological evaluation during preclinical safety assessment of new cardiovascular devices. Gross evaluation, microscopic evaluation, and scanning electron microscopy (SEM) are the methods of choice for evaluation of vascular surfaces. In this article, we present a new digital imaging approach of surface topography herein referred to as topographical digital microscopy (TDM) that is able to meet the objectives of endovascular healing assessment in a single instrumental platform combined with the same sample preparation techniques as for histology or SEM. This platform is taking advantage of digitally managed illumination, X-Y stitching, and Z-stacking to enable direct optical imaging of tissue surfaces at levels of details ranging from the macroscopic to the cellular level. This technique is enabled by advances in digital optical microscopy and provides images in color and 3 dimensions that can help in the analysis, especially in distinguishing biologically meaningful observations from technical preparation artifacts and in visualizing surface topography.

Oscillating Magnetic Field Regulates Cell Adherence and Endothelialization Based on Magnetic Nanoparticle-Modified Bacterial Cellulose

Despite the widely explored biomaterial scaffolds in vascular tissue engineering applications lately, no ideal platform has been provided for small diameter synthetic vascular grafts mainly due to the thrombosis issue. Endothelium is the only known completely non-thrombogenic material; so, functional endothelialization onto vascular biomaterials is critical in maintaining the patency of vascular networks. Bacterial cellulose (BC) is a natural biomaterial with superior biocompatibility and appropriate hydrophilicity as potential vascular grafts. In previous studies, surface modification of active peptides such as Arg-Gly-Asp (RGD) sequences onto biomaterials has been proven to achieve accelerated and selective endothelial cell (EC) adhesion. In our study, we demonstrated a new strategy to remotely regulate the adhesion of endothelial cells based on an oscillating magnetic field and achieve successful endothelialization on the modified BC membranes. In details, we synthesized bacterial cellulose (BC), magnetic BC (MBC), and RGD peptide-grafted magnetic BC (RMBC), modified with the HOOC-PEG-COOH-coated iron oxide nanoparticles (PEG-IONs). The endothelial cells were cultured on the three materials under different frequencies of an oscillating magnetic field, including "stationary" (0 Hz), "slow" (0.1 Hz), and "fast" (2 Hz) groups. Compared to BC and MBC membranes, the cells on RMBC membranes generally show better adhesion and proliferation. Meanwhile, the "slow" frequency of a magnetic field promotes this phenomenon on RMBC and achieves endothelialization after culture for 4 days, whereas "fast" inhibits the cellular attachment. Overall, we demonstrate a non-invasive and convenient method to regulate the endothelialization process, with promising applications in vascular tissue engineering.

Slow degrading poly(glycerol sebacate) derivatives improve vascular graft remodeling in a rat carotid artery interposition model

Porous synthetic grafts made of poly (glycerol sebacate) (PGS) can transform into autologous vascular conduits in vivo upon degradation of PGS. A long-held doctrine in tissue engineering is the necessity to match degradation of the scaffolds to tissue regeneration. Here, we tested the impact of degradation of PGS and its derivative in an interposition model of rat common carotid artery (CCA). Previous work indicates a complete degradation of PGS within approximately 2 weeks, likely at the fast end of the spectrum. Thus, the derivation of PGS focuses on delay degradation by conjugating the free hydroxy groups in PGS with a long chain carboxylic acid: palmitic acid, one of the most common lipid components. We evaluated two of the resultant palmitate-PGS (PPGS) in this study: one containing 9% palmitate (9-PPGS) and the other16% palmitate (16-PPGS). 16-PPGS grafts had the highest patency. Ultrasound imaging showed that the lumens of 16-PPGS grafts were similar to CCA and smaller than 9-PPGS and PGS grafts 12 weeks post-operation. Immunohistological and histological examination showed an endothelialized lumens in all three types of grafts within 4 weeks. Inflammatory responses to 16-PPGS grafts were limited to the adventitial space in contrast to a more diffusive infiltration in 9-PPGS and PGS grafts in week 4. Examination of calponin+ and αSMA+ cells revealed that 16-PPGS grafts remodeled into a distinctive bi-layered wall, while the walls of 9-PPGS grafts and PGS grafts only had one thick layer of smooth muscle-like cells. Correspondingly, the expression of collagen III and elastin displayed an identical layered structure in the remodeled 16-PPGS grafts, in contrast to a more spread distribution in 9-PPGS and PGS grafts. All the three types of grafts exhibited the same collagen content and burst pressure after 12 weeks of host remodeling. However, the compliance and elastin content of 16-PPGS grafts in week 12 were closest to those of CCA. Overall, placing the degradation of PGS derived elastomer to a window of 4-12 weeks results in vascular conduits closer to arteries in a rat carotid artery interposition model over a 12-week observation period.

From arteries to capillaries: approaches to engineering human vasculature

From micro-scaled capillaries to millimeter-sized arteries and veins, human vasculature spans multiple scales and cell types. The convergence of bioengineering, materials science, and stem cell biology has enabled tissue engineers to recreate the structure and function of different hierarchical levels of the vascular tree. Engineering large-scale vessels has been pursued over the past thirty years to replace or bypass damaged arteries, arterioles, and venules, and their routine application in the clinic may become a reality in the near future. Strategies to engineer meso- and microvasculature have been extensively explored to generate models to study vascular biology, drug transport, and disease progression, as well as for vascularizing engineered tissues for regenerative medicine. However, bioengineering of large-scale tissues and whole organs for transplantation, have failed to result in clinical translation due to the lack of proper integrated vasculature for effective oxygen and nutrient delivery. The development of strategies to generate multi-scale vascular networks and their direct anastomosis to host vasculature would greatly benefit this formidable goal. In this review, we discuss design considerations and technologies for engineering millimeter-, meso-, and micro-scale vessels. We further provide examples of recent state-of-the-art strategies to engineer multi-scale vasculature. Finally, we identify key challenges limiting the translation of vascularized tissues and offer our perspective on future directions for exploration.

In vitro comparison of three percutaneous atrial septal defect closure devices for endothelialisation and haemocompatibility

BACKGROUND

Percutaneous device closure of atrial septal defect (ASD) is the gold-standard treatment, but several delayed complications may occur as a result of incomplete device endothelialisation.

AIMS

In this in vitro study, we compared three ASD closure devices [Nit-Occlud® ASD-R (device 1); Hyperion™ ASDO (device 2); and Amplatzer™ Septal Occluder (device 3)] in terms of the endothelialisation process, using human endothelial progenitors cells (EPCs), and haemocompatibility.

METHODS

EPCs from umbilical cord blood were extracted, cultured and characterised. Device samples were seeded with 100,000 cells/cm². EPC adhesion was investigated at 3 and 24hours, and EPC proliferation was monitored, which allowed longitudinal follow-up (days 1-12). Haemocompatibility of device samples was assessed using a complement C3a assay and platelet and coagulation activation.

RESULTS

With regard to EPC adhesion and proliferation, no statistically significant differences were found between the three devices. We observed for each device a significant time-dependent EPC proliferation, appearing at day 8 for devices 2 and 3 and day 10 for device 1. No complement or platelet activation occurred within 15minutes of contact with devices. However, there was minimal activation of coagulation for the three devices.

CONCLUSIONS

In this in vitro study we showed that, despite the three ASD occluders having different device designs and coatings, adhesion and proliferation of human endothelial cells was similar for all devices. This should be further confirmed by similar studies including shear stress forces and anti-thrombotic treatments.

Bioresorbable silk grafts for small diameter vascular tissue engineering applications: In vitro and in vivo functional analysis

The success of tissue-engineered vascular graft (TEVG) predominantly relies on the selection of a suitable biomaterial and graft design. Natural biopolymer silk has shown great promise for various tissue-engineering applications. This study is the first to investigate Indian endemic non-mulberry silk (Antheraea assama-AA) - which inherits naturally superior mechanical and biological traits (e.g., RGD motifs) compared to Bombyx mori-BM silk, for TEVG applications. We designed bi-layered biomimetic small diameter AA-BM silk TEVGs adopting a new fabrication methodology. The inner layer showed ideally sized (~40 µm) pores with interconnectivity to allow cellular infiltration, and an outer dense electrospun layer that confers mechanical resilience. Biodegradation of silk TEVGs into amino acids as resorbable byproducts corroborates their in vivo remodeling ability. Following our previous reports, we surgically implanted human adipose tissue-derived stromal vascular fraction (SVF) seeded silk TEVGs in Lewis rats as abdominal aortic interposition grafts for 8 weeks. Adequate suture retention strength (0.45 ± 0.1 N) without any blood seepage post-implantation substantiate the grafts' viability. AA silk-based TEVGs showed superior animal survival and graft patency compared to BM silk TEVGs. Histological analysis revealed neo-tissue formation, host cell infiltration and graft remodeling in terms of extracellular matrix turnover. Altogether, this study demonstrates promising aspects of AA silk TEVGs for vascular tissue engineering applications. STATEMENT OF SIGNIFICANCE: Clinical 'off the shelf' implementation of tissue-engineered vascular grafts (TEVGs) remains a challenge. Achieving optimal blood vessel regeneration requires the use of bioresorbable materials having suitable degradation rates while producing minimal or no toxic byproducts. Host cell recruitment and preventing acute thrombosis are other pre-requisites for successful graft remodeling. In this study, for the first time we explored the use of naturally derived Indian endemic non-mulberry Antheraea assama silk in combination with Bombyx mori silk for TEVG applications by adopting a new biomimetic approach. Our bi-layered silk TEVGs were optimally porous, mechanically resilient and biodegradable. In vivo implantation in rat aorta showed long-term patency and graft remodeling by host cell infiltration and extracellular matrix deposition corroborating their clinical feasibility.

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.

Rapid Photocrosslinking of Silk Hydrogels with High Cell Density and Enhanced Shape Fidelity

Silk fibroin hydrogels crosslinked through di-tyrosine bonds are clear, elastomeric constructs with immense potential in regenerative medicine applications. In this study, demonstrated is a new visible light-mediated photoredox system for di-tyrosine bond formation in silk fibroin that overcomes major limitations of current conventional enzymatic-based crosslinking. This photomediated system rapidly crosslinks silk fibroin (<1 min), allowing encapsulation of cells at significantly higher cell densities (15 million cells mL^-1 ) while retaining high cell viability (>80%). The photocrosslinked silk hydrogels present more stable mechanical properties which do not undergo spontaneous transition to stiff, β-sheet-rich networks typically seen for enzymatically crosslinked systems. These hydrogels also support long-term culture of human articular chondrocytes, with excellent cartilage tissue formation. This system also facilitates the first demonstration of biofabrication of silk fibroin constructs in the absence of chemical modification of the protein structure or rheological additives. Cell-laden constructs with complex, ordered, graduated architectures, and high resolution (40 µm) are fabricated using the photocrosslinking system, which cannot be achieved using the enzymatic crosslinking system. Taken together, this work demonstrates the immense potential of a new crosslinking approach for fabrication of elastomeric silk hydrogels with applications in biofabrication and tissue regeneration.

Immobilized Macrophage Colony-Stimulating Factor (M-CSF) Regulates the Foreign Body Response to Implanted Materials

The functionality and durability of implanted biomaterials are often compromised by an exaggerated foreign body reaction (FBR). M1/M2 polarization of macrophages is a critical regulator of scaffold-induced FBR. Macrophage colony-stimulating factor (M-CSF), a hematopoietic growth factor, induces macrophages into an M2-like polarized state, leading to immunoregulation and promoting tissue repair. In the present study, we explored the immunomodulatory effects of surface bound M-CSF on poly-l-lactic acid (PLLA)-induced FBR. M-CSF was immobilized on the surface of PLLA via plasma immersion ion implantation (PIII). M-CSF functionalized PLLA, PLLA-only, and PLLA+PIII were assessed in an IL-1β luciferase reporter mouse to detect real-time levels of IL-1β expression, reflecting acute inflammation in vivo. Additionally, these different treated scaffolds were implanted subcutaneously into wild-type mice to explore the effect of M-CSF in polarization of M2-like macrophages (CD68⁺/CD206⁺), related cytokines (pro-inflammatory: IL-1β, TNF and MCP-1; anti-inflammatory: IL-10 and TGF-β), and angiogenesis (CD31) by immunofluorescent staining. Our data demonstrated that IL-1β activity in M-CSF functionalized scaffolds was ∼50% reduced compared to PLLA-only at day 1 (p < 0.01) and day 2 (p < 0.05) post-implantation. There were >2.6-fold more CD206+ macrophages in M-CSF functionalized PLLA compared to PLLA-only at day 7 (p < 0.001), along with higher levels of IL-10 at both day 7 (p < 0.05) and day 14 (p < 0.01), and TGF-β at day 3 (p < 0.05), day 7 (p < 0.05), and day 14 (p < 0.001). Lower levels of pro-inflammatory cytokines were also detected in M-CSF functionalized PLLA in the early phase of the immune response compared to PLLA-only: a ∼58% decrease at day 3 in IL-1β; a ∼91% decrease at day 3 and a ∼66% decrease at day 7 in TNF; and a ∼60% decrease at day 7 in MCP-1. Moreover, enhanced angiogenesis inside and on/near the scaffold was observed in M-CSF functionalized PLLA compared to PLLA-only at day 3 (p < 0.05) and day 7 (p < 0.05), respectively. Overall, M-CSF functionalized PLLA enhanced CD206+ macrophage polarization and angiogenesis, consistent with lower levels of pro-inflammatory cytokines and higher levels of anti-inflammatory cytokines in early stages of the host response, indicating potential immunoregulatory functions on the local environment.

Evaluating medical device and material thrombosis under flow: current and emerging technologies

This review highlights the importance of flow in medical device thrombosis and explores current and emerging technologies to evaluate dynamic biomaterial Thrombosis in vitro.

Altered processing enhances the efficacy of small-diameter silk fibroin vascular grafts

Current synthetic vascular grafts are not suitable for use in low-diameter applications. Silk fibroin is a promising natural graft material which may be an effective alternative. In this study, we compared two electrospun silk grafts with different manufacturing processes, using either water or hexafluoroisopropanol (HFIP) as solvent. This resulted in markedly different Young's modulus, ultimate tensile strength and burst pressure, with HFIP spun grafts observed to have thicker fibres, and greater stiffness and strength relative to water spun. Assessment in a rat abdominal aorta grafting model showed significantly faster endothelialisation of the HFIP spun graft relative to water spun. Neointimal hyperplasia in the HFIP graft also stabilised significantly earlier, correlated with an earlier SMC phenotype switch from synthetic to contractile, increasing extracellular matrix protein density. An initial examination of the macrophage response showed that HFIP spun conduits promoted an anti-inflammatory M2 phenotype at early timepoints while reducing the pro-inflammatory M1 phenotype relative to water spun grafts. These observations demonstrate the important role of the manufacturing process and physical graft properties in determining the physiological response. Our study is the first to comprehensively study these differences for silk in a long-term rodent model.

Fabrication Techniques for Vascular and Vascularized Tissue Engineering

Impaired or damaged blood vessels can occur at all levels in the hierarchy of vascular systems from large vasculatures such as arteries and veins to meso- and microvasculatures such as arterioles, venules, and capillary networks. Vascular tissue engineering has become a promising approach for fabricating small-diameter vascular grafts for occlusive arteries. Vascularized tissue engineering aims to fabricate meso- and microvasculatures for the prevascularization of engineered tissues and organs. The ideal small-diameter vascular graft is biocompatible, bridgeable, and mechanically robust to maintain patency while promoting tissue remodeling. The desirable fabricated meso- and microvasculatures should rapidly integrate with the host blood vessels and allow nutrient and waste exchange throughout the construct after implantation. A number of techniques used, including engineering-based and cell-based approaches, to fabricate these synthetic vasculatures are herein explored, as well as the techniques developed to fabricate hierarchical structures that comprise multiple levels of vasculature.

Steady-State Behavior and Endothelialization of a Silk-Based Small-Caliber Scaffold In Vivo Transplantation

A silk-based small-caliber tubular scaffold (SFTS), which is fabricated using a regenerated silk fibroin porous scaffold embedding a silk fabric core layer, has been proved to possess good cell compatibility and mechanical properties in vitro. In this study, the endothelialization ability and the steady-state blood flow of SFTSs were evaluated in vivo by implanting and replacing a common carotid artery in a rabbit. The results of the color doppler ultrasound and angiographies showed that the blood flow was circulated in the grafts without aneurysmal dilations or significant stenoses at any time point, and ran stronger and close to the autologous blood vessel from one month after implantation. The SFTSs presented an initial tridimensionality without being distorted or squashed. SEM and immunohistochemistry results showed that a clear and discontinuous endodermis appeared after one month of implantation; when implanted for three months, an endothelial layer fully covered the inner surface of SFTSs. RT-PCR results indicated that the gene expression level of CD31 in SFTSs was 45.8% and 75.3% by that of autologous blood vessels at 3 months and 12 months, respectively. The VEGF gene showed a high expression level that continued to increase after implantation.

Advanced Silk Fibroin Biomaterials and Application to Small-Diameter Silk Vascular Grafts

As the incidences of cardiovascular diseases have been on the rise in recent years, the need for small-diameter artificial vascular grafts is increasing globally. Although synthetic polymers such as expanded polytetrafluoroethylene or poly(ethylene terephthalate) have been successfully used for artificial vascular grafts ≥6 mm in diameter, they fail at smaller diameters (<6 mm) due to thrombus formation and intimal hyperplasia. Thus, development of vascular grafts for small diameter vessel replacement that are <6 mm in diameter remains a major clinical challenge. Silk fibroin (SF) from Bombyx mori silkworm is well-known as an excellent textile and also has been used as suture material in surgery for more than 2000 years. Many attempts to develop small-diameter SF vascular grafts with <6 mm in diameter have been reported. Here, research and development in small-diameter vascular grafts with SF are reviewed as follows: (1) the heterogeneous structure of SF fiber (Silk II), including the packing arrangements and type II β-turn structure of SF (Silk I*) before spinning; (2) SF modified by transgenic silkworm, which is more suitable for vascular grafts; (3) preparation of small-diameter SF vascular grafts; (4) characterization of SF in the hydrated state, including dynamics of water molecules by nuclear magnetic resonance; and (5) evaluation of the SF grafts by in vivo implantation experiment. According to the findings, SF is a promising material for small-diameter vascular graft development.