Tissue Engineering at the Blood-Contacting Surface: A Review of Challenges and Strategies in Vascular Graft Development

Transformation of metallo-elastomer grafts in a carotid artery interposition model over a year

Current vascular grafts, primarily Gore-Tex® and Dacron®, don't integrate with the host and have low patency in small-diameter vessels (<6 mm). Biomaterials that possess appropriate viscoelasticity, compliance, and high biocompatibility are essential for their application in small blood vessels. We have developed metal ion crosslinked poly(propanediol-co-(hydroxyphenyl methylene)amino-propanediol sebacate) (M-PAS), a biodegradable elastomer with a wide range of mechanical properties. We call these materials metallo-elastomers. An initial test on Zn-, Fe-, and Cu-PAS grafts reveals that Cu-PAS is the most suitable because of its excellent elastic recoil and well-balanced polymer degradation/tissue regeneration rate. Here we report host remodeling of Cu-PAS vascular grafts in rats over one year. 76 % of the grafts remain patent and >90 % of the synthetic polymer is degraded by 12 months. Extensive cell infiltration leads to a positive host remodeling. The remodeled grafts feature a fully endothelialized lumen. Circumferentially organized smooth muscle cells, elastin fibers, and widespread mature collagen give the neoarteries mechanical properties similar to native arteries. Proteomic analysis further reveals the presence of important vascular proteins in the neoarteries. Evidence suggests that Cu-PAS is a promising material for engineering small blood vessels.

Low concentrations of TNF-α in vitro transform the phenotype of vascular smooth muscle cells and enhance their survival in a three-dimensional culture system

BACKGROUND

Vascular smooth muscle cells (VSMCs) are commonly used as seed cells in tissue-engineered vascular constructions. However, their variable phenotypes and difficult to control functions pose challenges. This study aimed to overcome these obstacles using a three-dimensional culture system.

METHODS

Calf VSMCs were administered tumor necrosis factor-alpha (TNF-α) before culturing in two- and three-dimensional well plates and polyglycolic acid (PGA) scaffolds, respectively. The phenotypic markers of VSMCs were detected by immunofluorescence staining and western blotting, and the proliferation and migration abilities of VSMCs were detected by CCK-8, EDU, cell counting, scratch, and Transwell assays.

RESULTS

TNF-α rapidly decreased the contractile phenotypic markers and elevated the synthetic phenotypic markers of VSMCs, as well as markedly increasing the proliferation and migration ability of VSMCs under two- and three-dimensional culture conditions.

CONCLUSIONS

TNF-α can rapidly induce a phenotypic shift in VSMCs and change their viability on PGA scaffolds.

Endothelial progenitor cells for fabrication of engineered vascular units and angiogenesis induction

The promotion of vascularization and angiogenesis in the grafts is a crucial phenomenon in the healing process and tissue engineering. It has been shown that stem cells, especially endothelial progenitor cells (EPCs), can stimulate blood vessel formation inside the engineered hydrogels after being transplanted into the target sites. The incorporation of EPCs into the hydrogel can last the retention time, long-term survival, on-target delivery effects, migration and differentiation into mature endothelial cells. Despite these advantages, further modifications are mandatory to increase the dynamic growth and angiogenesis potential of EPCs in in vitro and in vivo conditions. Chemical modifications of distinct composites with distinct physical properties can yield better regenerative potential and angiogenesis during several pathologies. Here, we aimed to collect recent findings related to the application of EPCs in engineered vascular grafts and/or hydrogels for improving vascularization in the grafts. Data from the present article can help us in the application of EPCs as valid cell sources in the tissue engineering of several ischemic tissues.

Identification of a subpopulation of highly adherent endothelial cells for seeding synthetic vascular grafts

OBJECTIVE

There is an unmet clinical need for alternatives to autologous vessel grafts. Small-diameter (<6 mm) synthetic vascular grafts are not suitable because of unacceptable patency rates. This mainly occurs due to the lack of an endothelial cell (EC) monolayer to prevent platelet activation, thrombosis, and intimal hyperplasia. There are no reliable methods to endothelialize small-diameter grafts because most seeded ECs are lost due to exposure to fluid shear stress after implantation. The goal of this work is to determine if EC loss is a random process or if it is possible to predict which cells are more likely to remain adherent.

METHODS

In initial studies, we sorted ECs using fluid shear stress and identified a subpopulation of ECs that are more likely to resist detachment. We use RNA sequencing to examine gene expression of adherent ECs compared with the whole population. Using fluorescence activated cell sorting, we sorted ECs based on the expression level of a candidate marker and studied their retention in small-diameter vascular grafts in vitro.

RESULTS

Transcriptomic analysis revealed that fibronectin leucine rich transmembrane protein 2 (FLRT2), encoding protein FLRT2, is downregulated in the ECs that are more likely to resist detachment. When seeded onto vascular grafts and exposed to shear stress, ECs expressing low levels of FLRT2 exhibit 59.2% ± 7.4% retention compared with 24.5% ± 6.1% retention for the remainder of the EC population.

CONCLUSIONS

For the first time, we show EC detachment is not an entirely random process. This provides validation for the concept that we can seed small-diameter vascular grafts only with highly adherent ECs to maintain a stable endothelium and improve graft patency rates.

The Effects of Biomimetic Surface Topography on Vascular Cells: Implications for Vascular Conduits

Cardiovascular diseases (CVDs) are the leading cause of mortality worldwide and represent a pressing clinical need. Vascular occlusions are the predominant cause of CVD and necessitate surgical interventions such as bypass graft surgery to replace the damaged or obstructed blood vessel with a synthetic conduit. Synthetic small-diameter vascular grafts (sSDVGs) are desired to bypass blood vessels with an inner diameter <6 mm yet have limited use due to unacceptable patency rates. The incorporation of biophysical cues such as topography onto the sSDVG biointerface can be used to mimic the cellular microenvironment and improve outcomes. In this review, the utility of surface topography in sSDVG design is discussed. First, the primary challenges that sSDVGs face and the rationale for utilizing biomimetic topography are introduced. The current literature surrounding the effects of topographical cues on vascular cell behavior in vitro is reviewed, providing insight into which features are optimal for application in sSDVGs. The results of studies that have utilized topographically-enhanced sSDVGs in vivo are evaluated. Current challenges and barriers to clinical translation are discussed. Based on the wealth of evidence detailed here, substrate topography offers enormous potential to improve the outcome of sSDVGs and provide therapeutic solutions for CVDs.

Protein adsorption on blood-contacting surfaces: A thermodynamic perspective to guide the design of antithrombogenic polymer coatings

Blood-contacting medical devices often succumb to thrombosis, limiting their durability and safety in clinical applications. Thrombosis is fundamentally initiated by the nonspecific adsorption of proteins to the material surface, which is strongly governed by thermodynamic factors established by the nature of the interaction between the material surface, surrounding water molecules, and the protein itself. Along these lines, different surface materials (such as polymeric, metallic, ceramic, or composite) induce different entropic and enthalpic changes at the surface-protein interface, with material wettability significantly impacting this behavior. Consequently, protein adsorption on medical devices can be modulated by altering their wettability and surface energy. A plethora of polymeric coating modifications have been utilized for this purpose; hydrophobic modifications may promote or inhibit protein adsorption determined by van der Waals forces, while hydrophilic materials achieve this by mainly relying on hydrogen bonding, or unbalanced/balanced electrostatic interactions. This review offers a cohesive understanding of the thermodynamics governing these phenomena, to specifically aid in the design and selection of hemocompatible polymeric coatings for biomedical applications. STATEMENT OF SIGNIFICANCE: Blood-contacting medical devices often succumb to thrombosis, limiting their durability and safety in clinical applications. A plethora of polymeric coating modifications have been utilized for addressing this issue. This review offers a cohesive understanding of the thermodynamics governing these phenomena, to specifically aid in the design and selection of hemocompatible polymeric coatings for biomedical applications.

Breaking the Limit of Cardiovascular Regenerative Medicine: Successful 6-Month Goat Implant in World's First Ascending Aortic Replacement Using Biotube Blood Vessels

This study investigated six-month outcomes of first models of ascending aortic replacement. The molds used to produce the Biotube were implanted subcutaneously in goats. After 2-3 months, the molds were explanted to obtain the Biotubes (inner diameter, 12 mm; wall thickness, 1.5 mm). Next, we performed ascending aortic replacement using the Biotube in five allogenic goats. At 6 months, the animals underwent computed tomography (CT) and histologic evaluation. As a comparison, we performed similar surgeries using glutaraldehyde-fixed autologous pericardial rolls or pig-derived heterogenous Biotubes. At 6 months, CT revealed no aneurysmalization of the Biotube or pseudoaneurysm formation. The histologic evaluation showed development of endothelial cells, smooth muscle cells, and elastic fibers along the Biotube. In the autologous pericardium group, there was no evidence of new cell development, but there was calcification. The histologic changes observed in the heterologous Biotube group were similar to those in the allogenic Biotube group. However, there was inflammatory cell infiltration in some heterologous Biotubes. Based on the above, we could successfully create the world's first Biotube-based ascending aortic replacement models. The present results indicate that the Biotube may serve as a scaffold for aortic tissue regeneration.

Long-term observation of polycaprolactone small-diameter vascular grafts with thickened outer layer and heparinized inner layer in rabbit carotid arteries

In our previous study, the pristine bilayer small-diameterin situtissue engineered vascular grafts (pTEVGs) were electrospun from a heparinized polycaprolactone (PCL45k) as an inner layer and a non-heparinized PCL80k as an outer layer in the thickness of about 131 μm and 202 μm, respectively. However, the hydrophilic enhancement of inner layer stemmed from the heparinization accelerated the degradation of grafts leading to the early formation of arterial aneurysms in a period of 3 months, severely hindering the perennial observation of the neo-tissue regeneration, host cell infiltration and graft remodeling in those implanted pTEVGs. Herein to address this drawback, the thickness of the outer layers was increased with PCL80k to around 268 μm, while the inner layer remained unchangeable. The thickened TEVGs named as tTEVGs were evaluated in six rabbits via a carotid artery interpositional model for a period of 9 months. All the animals kept alive and the grafts remained patent until explantation except for one whose one side of arterial blood vessels was occluded after an aneurysm occurred at 6 months. Although a significant degradation was observed in the implanted grafts at 9 month, the occurrence of aneurysms was obviously delayed compared to pTEVGs. The tissue stainings indicated that the endothelial cell remodeling was substantially completed by 3 months, while the regeneration of elastin and collagen remained smaller and unevenly distributed in comparison to autologous vessels. Additionally, the proliferation of macrophages and smooth muscle cells reached the maximum by 3 months. These tTEVGs possessing a heparinized inner layer and a thickened outer layer exhibited good patency and significantly delayed onset time of aneurysms.

Amnion-derived hydrogels as a versatile platform for regenerative therapy: from lab to market

In recent years, the amnion (AM) has emerged as a versatile tool for stimulating tissue regeneration and has been of immense interest for clinical applications. AM is an abundant and cost-effective tissue source that does not face strict ethical issues for biomedical applications. The outstanding biological attributes of AM, including side-dependent angiogenesis, low immunogenicity, anti-inflammatory, anti-fibrotic, and antibacterial properties facilitate its usage for tissue engineering and regenerative medicine. However, the clinical usage of thin AM sheets is accompanied by some limitations, such as handling without folding or tearing and the necessity for sutures to keep the material over the wound, which requires additional considerations. Therefore, processing the decellularized AM (dAM) tissue into a temperature-sensitive hydrogel has expanded its processability and applicability as an injectable hydrogel for minimally invasive therapies and a source of bioink for the fabrication of biomimetic tissue constructs by recapitulating desired biochemical cues or pre-defined architectural design. This article reviews the multi-functionality of dAM hydrogels for various biomedical applications, including skin repair, heart treatment, cartilage regeneration, endometrium regeneration, vascular graft, dental pulp regeneration, and cell culture/carrier platform. Not only recent and cutting-edge research is reviewed but also available commercial products are introduced and their main features and shortcomings are elaborated. Besides the great potential of AM-derived hydrogels for regenerative therapy, intensive interdisciplinary studies are still required to modify their mechanical and biological properties in order to broaden their therapeutic benefits and biomedical applications. Employing additive manufacturing techniques (e.g., bioprinting), nanotechnology approaches (e.g., inclusion of various bioactive nanoparticles), and biochemical alterations (e.g., modification of dAM matrix with photo-sensitive molecules) are of particular interest. This review article aims to discuss the current function of dAM hydrogels for the repair of target tissues and identifies innovative methods for broadening their potential applications for nanomedicine and healthcare.

Pair of Functional Polyesters That Are Photo-Cross-Linkable and Electrospinnable to Engineer Elastomeric Scaffolds with Tunable Structure and Properties

A pair of alkyne- and thiol-functionalized polyesters are designed to engineer elastomeric scaffolds with a wide range of tunable material properties (e.g., thermal, degradation, and mechanical properties) for different tissues, given their different host responses, mechanics, and regenerative capacities. The two prepolymers are quickly photo-cross-linkable through thiol-yne click chemistry to form robust elastomers with small permanent deformations. The elastic moduli can be easily tuned between 0.96 ± 0.18 and 7.5 ± 2.0 MPa, and in vitro degradation is mediated from hours up to days by adjusting the prepolymer weight ratios. These elastomers bear free hydroxyl and thiol groups with a water contact angle of less than 85.6 ± 3.58 degrees, indicating a hydrophilic nature. The elastomer is compatible with NIH/3T3 fibroblast cells with cell viability reaching 88 ± 8.7% relative to the TCPS control at 48 h incubation. Differing from prior soft elastomers, a mixture of the two prepolymers without a carrying polymer is electrospinnable and UV-cross-linkable to fabricate elastic fibrous scaffolds for soft tissues. The designed prepolymer pair can thus ease the fabrication of elastic fibrous conduits, leading to potential use as a resorbable synthetic graft. The elastomers could find use in other tissue engineering applications as well.

Tissue engineering applications of recombinant human collagen: a review of recent progress

With the rapid development of synthetic biology, recombinant human collagen has emerged as a cutting-edge biological material globally. Its innovative applications in the fields of material science and medicine have opened new horizons in biomedical research. Recombinant human collagen stands out as a highly promising biomaterial, playing a pivotal role in crucial areas such as wound healing, stroma regeneration, and orthopedics. However, realizing its full potential by efficiently delivering it for optimal therapeutic outcomes remains a formidable challenge. This review provides a comprehensive overview of the applications of recombinant human collagen in biomedical systems, focusing on resolving this crucial issue. Additionally, it encompasses the exploration of 3D printing technologies incorporating recombinant collagen to address some urgent clinical challenges in regenerative repair in the future. The primary aim of this review also is to spotlight the advancements in the realm of biomaterials utilizing recombinant collagen, with the intention of fostering additional innovation and making significant contributions to the enhancement of regenerative biomaterials, therapeutic methodologies, and overall patient outcomes.

A strategy for mechanically integrating robust hydrogel-tissue hybrid to promote the anti-calcification and endothelialization of bioprosthetic heart valve

Bioprosthetic heart valve (BHV) replacement has been the predominant treatment for severe heart valve diseases over decades. Most clinically available BHVs are crosslinked by glutaraldehyde (GLUT), while the high toxicity of residual GLUT could initiate calcification, severe thrombosis, and delayed endothelialization. Here, we construed a mechanically integrating robust hydrogel-tissue hybrid to improve the performance of BHVs. In particular, recombinant humanized collagen type III (rhCOLIII), which was precisely customized with anti-coagulant and pro-endothelialization bioactivity, was first incorporated into the polyvinyl alcohol (PVA)-based hydrogel via hydrogen bond interactions. Then, tannic acid was introduced to enhance the mechanical performance of PVA-based hydrogel and interfacial bonding between the hydrogel layer and bio-derived tissue due to the strong affinity for a wide range of substrates. In vitro and in vivo experimental results confirmed that the GLUT-crosslinked BHVs modified by the robust PVA-based hydrogel embedded rhCOLIII and TA possessed long-term anti-coagulant, accelerated endothelialization, mild inflammatory response and anti-calcification properties. Therefore, our mechanically integrating robust hydrogel-tissue hybrid strategy showed the potential to enhance the service function and prolong the service life of the BHVs after implantation.

Durable Immunomodulatory Nanofiber Niche for the Functional Remodeling of Cardiovascular Tissue

Functional remodeling and prolonged anti-inflammatory responses are both vital for repairing damage in the cardiovascular system. Although these aspects have each been studied extensively alone, attempts to fabricate scaffolds that combine these effects have seen limited success. In this study, we synthesized salvianic acid A (SA, danshensu) blocked biodegradable polyurethane (PCHU-D) and enclosed it within electrospun nanofibers to synthesize a durable immunomodulatory nanofiber niche (DINN), which provided sustained SA release during inflammation. Given its excellent processability, mechanical properties, and shape memory function, we developed two variants of the DINN as vascular scaffolds and heart patches. Both these variants exhibited outstanding therapeutic effects in in vivo experiments. The DINN was expertly designed such that it gradually decomposes along with SA release, substantially facilitating cellular infiltration and tissue remodeling. Therefore, the DINN effectively inhibited the migration and chemotaxis of inflammatory cells, while also increasing the expression of angiogenic genes. As a result, it promoted the recovery of myocardial function after myocardial infarction and induced rapid reendothelialization following arterial orthotopic transplantation repair. These excellent characteristics indicate that the DINN holds great potential as a multifunctional agent for repairing cardiovascular tissues.

Nanomaterials for small diameter vascular grafts: overview and outlook

Small-diameter vascular grafts (SDVGs) cannot meet current clinical demands owing to their suboptimal long-term patency rate. Various materials have been employed to address this issue, including nanomaterials (NMs), which have demonstrated exceptional capabilities and promising application potentials. In this review, the utilization of NMs in different forms, including nanoparticles, nanofibers, and nanofilms, in the SDVG field is discussed, and future perspectives for the development of NM-loading SDVGs are highlighted. It is expected that this review will provide helpful information to scholars in the innovative interdiscipline of cardiovascular disease treatment and NM.

Biofunctionalized Electrospun Vascular Scaffolds for Enhanced Antithrombotic Properties and In Situ Endothelialization

The development of innovative vascular substitutes has become increasingly significant due to the prevalence of vascular diseases. In this study, we designed a biofunctionalized electrospun vascular scaffold by chemically conjugating heparin molecules as an antithrombotic agent with an endothelial cell (EC)-specific antibody to promote in situ endothelialization. To optimize this biofunctionalized electrospun vascular scaffolding system, we examined various parameters, including material compositions, cross-linker concentrations, and cross-linking and conjugation processes. The findings revealed that a higher degree of heparin conjugation onto the vascular scaffold resulted in improved antithrombotic properties, as confirmed by the platelet adhesion test. Additionally, the flow chamber study demonstrated that the EC-specific antibody immobilization enhanced the scaffold's EC-capturing capability compared to a nonconjugated vascular scaffold. The optimized biofunctionalized vascular scaffolds also displayed exceptional mechanical properties, such as suture retention strength and tensile properties. Our research demonstrated that the biofunctionalized vascular scaffolds and the directed immobilization of bioactive molecules could provide the necessary elements for successful acellular vascular tissue engineering applications.

A synergistic strategy of dual-crosslinking and loading intelligent nanogels for enhancing anti-coagulation, pro-endothelialization and anti-calcification properties in bioprosthetic heart valves

Currently, glutaraldehyde (GA)-crosslinked bioprosthetic heart valves (BHVs) still do not guarantee good biocompatibility and long-term effective durability for clinical application due to their subacute thrombus, inflammation, calcification, tearing and limited durability. In this study, double-modified xanthan gum (oxidized/vinylated xanthan gum (O2CXG)) was acquired from xanthan gum for subsequent double crosslinking and modification platform construction. Sulfonic acid groups with anticoagulant properties were also introduced through the free radical polymerization of vinyl sulfonate (VS) and vinyl on O2CXG. Taking advantage of the drug-loading function of xanthan gum, the treated pericardium was further loaded with inflammation-triggered dual drug-loaded nanogel (heparin (Hep) and atorvastatin (Ator)). Mechanical properties of O2CXG-crosslinked porcine pericardium (O2CXG-PP) were significantly improved via the first network formed by Schiff base bonds and the second C-C bonds network. Due to the presence of sulfonic acid groups as well as the dual drug release from nanogels under the stimulation of H2O2, the hemocompatibility, anti-inflammatory, pro-endothelialization and anti-calcification properties of the crosslinked pericardium modified with nanogels loaded with Hep and Ator (O2CXG+VS+(Hep+Ator) nanogel-PP) was significantly better than that of GA-crosslinked PP (GA-PP). The collaborative strategy of double crosslinking and sequential release of anticoagulant/endothelium-promoting drugs triggered by inflammation could effectively meet the requirement of enhanced multiple performance and long-term durability of bioprosthetic heart valves and provide a valuable pattern for multi-functionalization of blood contacting materials. STATEMENT OF SIGNIFICANCE: Currently, glutaraldehyde-crosslinked bioprosthetic heart valves (BHVs) are subject to subacute thrombus, inflammation, calcification and tearing, which would not guarantee good biocompatibility and long-term effective durability. We developed a cooperative strategy of double crosslinking and surface modification in which double-modified xanthan gum plays a cornerstone. The mechanical properties of this BHV were significantly improved via the first network formed by Schiff base bonds and the second C-C bonds network. Inflammation-triggered combination delivery of heparin and atorvastatin has been demonstrated to enhance anticoagulation, anti-inflammatory and pro-endothelialization of BHVs by utilizing local inflammatory response. The collaborative strategy could effectively meet the requirement of enhanced multiple performance and long-term durability of BHVs and provide a valuable pattern for the multi-functionalization of blood-contacting materials.

Anti-thrombotic poly(AAm-co-NaAMPS)-xanthan hydrogel-expanded polytetrafluoroethylene (ePTFE) vascular grafts with enhanced endothelialization and hemocompatibility properties

Cardiovascular diseases (CVDs) are the leading cause of death among all non-communicable diseases globally. Although expanded polytetrafluoroethylene (ePTFE) has been widely used for larger-diameter vascular graft transplantation, the persistent thrombus formation and intimal hyperplasia of small-diameter vascular grafts (SDVGs) made of ePTFE to treat severe CVDs remain the biggest challenges due to lack of biocompatibility and endothelium. In this study, bi-layered poly(acrylamide-co-2-Acrylamido-2-methyl-1-propanesulfonic acid sodium)-xanthan hydrogel-ePTFE (poly(AAm-co-NaAMPS)-xanthan hydrogel-ePTFE) vascular grafts capable of promoting endothelialization and prohibiting thrombosis were synthesized and fabricated. While the external ePTFE layer of the vascular grafts provided the mechanical stability, the inner hydrogel layer offered much-needed cytocompatibility, hemocompatibility, and endothelialization functions. The interface morphology between the inner hydrogel layer and the outer ePTFE layer was observed by scanning electron microscope (SEM), which revealed that the hydrogel was well attached to the porous ePTFE through mechanical interlocking. Among all the hydrogel compositions tested with cell culture using human umbilical vein endothelial cells (HUVECs), the hydrogel with the molar ratio of 40:60 (NaAMPS/AAm) composition (i.e., Hydrogel 40:60) exhibited the best endothelialization function, as it produced the largest endothelialization area that was three times more than of that of plain ePTFE on day 14, maintained the highest average cell viability, and had the best cell morphology. Hydrogel 40:60 also showed excellent hemocompatibility, prolonged activated partial thromboplastin time (aPTT), and good mechanical properties. Overall, bi-layered poly(AAm-co-NaAMPS)-xanthan hydrogel-ePTFE vascular grafts with the Hydrogel 40:60 composition could potentially solve the critical challenge of thrombus formation in vascular graft transplantation applications.

Manufacturing the multiscale vascular hierarchy: progress toward solving the grand challenge of tissue engineering

In human vascular anatomy, blood flows from the heart to organs and tissues through a hierarchical vascular tree, comprising large arteries that branch into arterioles and further into capillaries, where gas and nutrient exchange occur. Engineering a complete, integrated vascular hierarchy with vessels large enough to suture, strong enough to withstand hemodynamic forces, and a branching structure to permit immediate perfusion of a fluidic circuit across scales would be transformative for regenerative medicine (RM), enabling the translation of engineered tissues of clinically relevant size, and perhaps whole organs. How close are we to solving this biological plumbing problem? In this review, we highlight advances in engineered vasculature at individual scales and focus on recent strategies to integrate across scales.

Preclinical in vivo assessment of a cell-free multi-layered scaffold prepared by 3D printing and electrospinning for small-diameter blood vessel tissue engineering in a canine model

Tissue-engineered vascular grafts (TEVGs) are promising alternatives to existing prosthetic grafts. The objective of this study is to evaluate the clinical feasibility of a novel multi-layered small-diameter vascular graft that has a hierarchical structure. Vascular grafts with elaborately designed composition and architecture were prepared by 3D printing and electrospinning and were implanted into the femoral artery of 5 dogs. The patency of the grafts was assessed using Doppler ultrasonography. After 6 months, the grafts were retrieved and histological and SEM examinations were conducted. During implantation, the grafts exhibited resistance to kinking and no blood seepage thanks to the helical structure of the innermost and outermost layers. The grafts showed a high patency rate and remodelling ability. At 6 months post-implantation, the lumen was endothelialized and middle layers were regenerated by infiltration of smooth muscle cells (SMCs) and deposition of extracellular matrix (ECM). These results suggest that the multi-layered vascular graft may be a promising candidate for small-diameter blood vessel tissue engineering in clinical practice.

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.

Research progress of implantation materials and its biological evaluation

With the development of modern material science, life science and medical science, implantation materials are widely employed in clinical fields. In recent years, these materials have also evolved from inert supports or functional substitutes to bioactive materials able to trigger or promote the regenerative potential of tissues. Reasonable biological evaluation of implantation materials is the premise to make sure their safe application in clinical practice. With the continual development of implantation materials and the emergence of new implantation materials, new challenges to biological evaluation have been presented. In this paper, the research progress of implantation materials, the progress of biological evaluation methods, and also the characteristics of biocompatibility evaluation for novel implantation materials, like animal-derived implantation materials, nerve contact implantation materials, nanomaterials and tissue-engineered medical products were reviewed in order to provide references for the rational biological evaluation of implantable materials.

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.

Effects of different preservation methods of human iliac veins

With the progress of vascular anastomosis technology, the radical resection surgery of cancer combining with vascular resection and reconstruction has been focused by surgeon. As a natural substitute material for blood vessel, vascular allografts have good vascular compliance and histocompatibility. Generally, the donated veins could not be used immediately, and need to be well preserved. So, it is greatly significant to do research in the preservation effects of different preservation methods on veins. In this study, the effects of different preservative methods of human iliac veins were compared and analyzed in terms of cell viability, vascular wall structure and tension resistance. The donated human iliac veins were randomly divided into three groups: Cold Storage Group (4 °C) (CSG), Frozen Storage Group (-186 °C) (FSG)and Fresh Control Group (FCG). Six detection time-points of preservation for 1, 3, 5, 7, 14, 28 days were set respectively. There are ten samples in each group and each time-point separately. Survival and apoptosis of vascular cell were evaluated by MTT assay and Tunel fluorescence staining. Tensile test was used to evaluate mechanical properties of vessels. The changes of vascular endothelial cells, smooth muscle cells, collagen fibers and elastic fibers were evaluated by HE staining, Masson staining and EVG staining. Furthermore, the changes of organelles were observed by transmission electron microscope. With the extension of preservation period, the vascular cell viability and tension resistance of two groups decreased, and the apoptotic cells increased gradually. The apoptosis index of CSG was higher than FSG at each time point (P < 0.05). In terms of cell viability, CSG was higher within 3 days (P < 0.05), both groups were same between 3 and 14 days, and then CSG lower than FSG after 14 days (P < 0.05). In terms of tension resistance, CSG was stronger than FSG (P < 0.05) in first 7 days, both groups were same in 2nd week, and then CSG was weaker in 4th week (P < 0.05). In terms of vascular wall structure, in CSG, vascular endothelial cells were damaged and shed, smooth muscle cells were edema after 14 days, but the cell membrane and intercellular connection were still intact. In 4th week, endothelial cells were completely damaged and shed, the boundary of smooth muscle cell membrane was unclear, intercellular connection was damaged. Moreover, organelles were destroyed and disappeared, perinuclear condensation of chromatin was observed, and some cells had incomplete nuclear membrane or nuclear fragmentation; However, there were no obvious changes in the FSG within 28 days. Finally, local exfoliation and destruction of endothelial cells and edema-like changes of organelles were observed; the collagen fibers and elastic fibers of blood vessels in the two groups had no obvious damage and change within 28 days. For excised human iliac vein, cold and frozen storage can effectively preserve the cell viability, wall structure and tension resistance of blood vessels. With the extension of preservation time, the related performance of vessels declined in varying degrees. Within first week, the effect of cold storage is better than frozen storage, but frozen storage is significantly better than cold storage after 2 weeks.

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.

Functionalization of in vivo tissue-engineered living biotubes enhance patency and endothelization without the requirement of systemic anticoagulant administration

Vascular regeneration and patency maintenance, without anticoagulant administration, represent key developmental trends to enhance small-diameter vascular grafts (SDVG) performance. In vivo engineered autologous biotubes have emerged as SDVG candidates with pro-regenerative properties. However, mechanical failure coupled with thrombus formation hinder translational prospects of biotubes as SDVGs. Previously fabricated poly(ε-caprolactone) skeleton-reinforced biotubes (PBs) circumvented mechanical issues and achieved vascular regeneration, but orally administered anticoagulants were required. Here, highly efficient and biocompatible functional modifications were introduced to living cells on PB lumens. The 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-N-methoxy (DMPE)-PEG-conjugated anti-coagulant bivalirudin (DPB) and DMPE-PEG-conjugated endothelial progenitor cell (EPC)-binding TPS-peptide (DPT) modifications possessed functionality conducive to promoting vascular graft patency. Co-modification of DPB and DPT swiftly attained luminal saturation without influencing cell viability. DPB repellent of non-specific proteins, DPB inhibition of thrombus formation, and DPB protection against functional masking of DPT's EPC-capture by blood components, which promoted patency and rapid endothelialization in rat and canine artery implantation models without anticoagulant administration. This strategy offers a safe, facile, and fast technical approach to convey additional functionalization to living cells within tissue-engineered constructs.

Interpenetrating network design of bioactive hydrogel coatings with enhanced damage resistance

Bioactive hydrogel coatings offer a promising route to introduce sustained thromboresistance to cardiovascular devices without compromising bulk mechanical properties. Poly(ethylene glycol)-based hydrogels provide antifouling properties to limit acute thromobosis and incorporation of adhesive ligands can be used to promote endothelialization. However, conventional PEG-based hydrogels at stiffnesses that promote cell attachment can be brittle and prone to damage in a surgical setting, limiting their utility in clinical applications. In this work, we developed a durable hydrogel coating using interpenetrating networks of polyether urethane diacrylamide (PEUDAm) and poly(N-acryloyl glycinamide) (pNAGA). First, diffusion-mediated redox initiation of PEUDAm was used to coat electrospun polyurethane fiber meshes with coating thickness controlled by the immersion time. The second network of pNAGA was then introduced to enhance damage resistance of the hydrogel coating. The durability, thromboresistance, and bioactivity of the resulting multilayer grafts were then assessed. The IPN hydrogel coatings displayed resistance to surgically-associated damage mechanisms and retained the anti-fouling nature of PEG-based hydrogels as indicated by reduced protein adsorption and platelet attachment. Moreover, incorporation of functionalized collagen into the IPN hydrogel coating conferred bioactivity that supported endothelial cell adhesion. Overall, this conformable and durable hydrogel coating provides an improved approach for cardiovascular device fabrication with targeted biological activity.

Dialdehyde xanthan gum and curcumin synergistically crosslinked bioprosthetic valve leaflets with anti-thrombotic, anti-inflammatory and anti-calcification properties

Currently commercial glutaraldehyde (GA)-crosslinked bioprosthetic valve leaflets (BVLs) suffer from thromboembolic complications, calcification, and limited durability, which are the major stumbling block to wider clinical application of BVLs. Thus, developing new-style BVLs will be an urgent need to enhance the durability of BVLs and alleviate thromboembolic complications. In this study, a quick and effective collaborative strategy of the double crosslinking agents (oxidized polysaccharide and natural active crosslinking agent) was reported to realize enhanced mechanical, and structural stability, excellent hemocompatibility and anti-calcification properties of BVLs. Dialdehyde xanthan gum (AXG) exhibiting excellent stability to heat, acid-base, salt, and enzymatic hydrolysis was first introduced to crosslink decellularized porcine pericardium (D-PP) and then curcumin with good properties of anti-inflammatory, anti-coagulation, anti-liver fibrosis, and anti-atherosclerosis was used to synergistically crosslink and multi-functionalize D-PP to obtain AXG + Cur-PP. A comprehensive evaluation of structural characterization, hemocompatibility, endothelialization potential, mechanical properties and component stability showed that AXG + Cur-PP exhibited better anti-thrombotic properties and endothelialization potential, milder immune responses, excellent anti-calcification properties and enhanced mechanical properties compared with GA-crosslinked PP. Overall, this cooperative crosslinking strategy provides a novel solution to achieve BVLs with enhanced mechanical properties and excellent anti-coagulation, anti-inflammatory, anti-calcification, and the ability to promote endothelial cell proliferation.

Engineered Vasculature for Cancer Research and Regenerative Medicine

Engineered human tissues created by three-dimensional cell culture of human cells in a hydrogel are becoming emerging model systems for cancer drug discovery and regenerative medicine. Complex functional engineered tissues can also assist in the regeneration, repair, or replacement of human tissues. However, one of the main hurdles for tissue engineering, three-dimensional cell culture, and regenerative medicine is the capability of delivering nutrients and oxygen to cells through the vasculatures. Several studies have investigated different strategies to create a functional vascular system in engineered tissues and organ-on-a-chips. Engineered vasculatures have been used for the studies of angiogenesis, vasculogenesis, as well as drug and cell transports across the endothelium. Moreover, vascular engineering allows the creation of large functional vascular conduits for regenerative medicine purposes. However, there are still many challenges in the creation of vascularized tissue constructs and their biological applications. This review will summarize the latest efforts to create vasculatures and vascularized tissues for cancer research and regenerative medicine.

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.

Promoting Angiogenesis Using Immune Cells for Tissue-Engineered Vascular Grafts

Implantable tissue-engineered vascular grafts (TEVGs) usually trigger the host reaction which is inextricably linked with the immune system, including blood-material interaction, protein absorption, inflammation, foreign body reaction, and so on. With remarkable progress, the immune response is no longer considered to be entirely harmful to TEVGs, but its therapeutic and impaired effects on angiogenesis and tissue regeneration are parallel. Although the implicated immune mechanisms remain elusive, it is certainly worthwhile to gain detailed knowledge about the function of the individual immune components during angiogenesis and vascular remodeling. This review provides a general overview of immune cells with an emphasis on macrophages in light of the current literature. To the extent possible, we summarize state-of-the-art approaches to immune cell regulation of the vasculature and suggest that future studies are needed to better define the timing of the activity of each cell subpopulation and to further reveal key regulatory switches.

Anti-Inflammatory and Anti-Thrombogenic Properties of Arterial Elastic Laminae

Elastic laminae, an elastin-based, layered extracellular matrix structure in the media of arteries, can inhibit leukocyte adhesion and vascular smooth muscle cell proliferation and migration, exhibiting anti-inflammatory and anti-thrombogenic properties. These properties prevent inflammatory and thrombogenic activities in the arterial media, constituting a mechanism for the maintenance of the structural integrity of the arterial wall in vascular disorders. The biological basis for these properties is the elastin-induced activation of inhibitory signaling pathways, involving the inhibitory cell receptor signal regulatory protein α (SIRPα) and Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1). The activation of these molecules causes deactivation of cell adhesion- and proliferation-regulatory signaling mechanisms. Given such anti-inflammatory and anti-thrombogenic properties, elastic laminae and elastin-based materials have potential for use in vascular reconstruction.

Cellular energy supply for promoting vascular remodeling of small-diameter vascular grafts: a preliminary study of a new strategy for vascular graft development

Rapid endothelialization is extremely essential for the success of small-diameter tissue-engineered vascular graft (TEVG) (<6 mm) transplantation. However, severe inflammation in situ often causes cellular energy decline of endothelial cells. The cellular energy supply involved in vascular graft therapy remains unclear, and whether promoting energy supply would be helpful in the regeneration of vascular grafts needs to be established. In our work, we generated an AMPK activator (5-aminoimidazole-4-carboxamide ribonucleotide, AICAR) immobilized vascular graft. AICAR-modified vascular grafts were successfully generated by the co-electrospinning technique. In vitro results indicated that AICAR could upregulate energy supply in endothelial cells and reprogram macrophages (MΦ) to assume an anti-inflammatory phenotype. Furthermore, endothelial cells (ECs) co-cultured with AICAR achieved higher survival rates, better migration, and angiogenic capacity than the controls. Concurrently, a rabbit carotid artery transplantation model was used to investigate AICAR-modified vascular grafts at different time points. The results showed that AICAR-modified vascular grafts had higher patency rates (92.9% and 85.7% at 6 and 12 weeks, respectively) than those of the untreated group (11.1% and 0%). In conclusion, AICAR strengthened the cellular energy state and attenuated the adverse effects of inflammation. AICAR-modified vascular grafts achieved better vascular remodeling. This study provides a new perspective on promoting the regeneration of small-diameter vascular grafts.

A textile-reinforced composite vascular graft that modulates macrophage polarization and enhances endothelial cell migration, adhesion and proliferation in vitro

At the present time, there is no successful off-the-shelf small-caliber vascular graft (<6 mm) for the repair or bypass of the coronary or carotid arteries. In this study, we engineer a textile-reinforced hydrogel vascular graft. The textile fibers are circularly knitted into a flexible yet robust conduit to serve as the backbone of the composite vascular graft and provide the primary mechanical support. It is embedded in the hydrogel matrix which seals the open structure of the knitted reinforcement and mediates cellular response toward a faster reendothelialization. The mechanical properties of the composite vascular graft, including bursting strength, suture retention strength and radial compliance, significantly surpass the requirement for the vascular graft application and can be adjusted by altering the structure of the textile reinforcement. The addition of hydrogel matrix, on the other hand, improves the survival, adhesion and proliferation of endothelial cells in vitro. The composite vascular graft also enhances macrophage activation and upregulates M1 and M2 related gene expression, which further improves the endothelial cell migration that might favor the reendothelialization of the vascular graft. Taken together, the textile-reinforced hydrogel shows it potential to be a promising scaffold material to fabricate a tissue engineered vascular graft.

Strategies to counteract adverse remodeling of vascular graft: A 3D view of current graft innovations

Vascular grafts are widely used for vascular surgeries, to bypass a diseased artery or function as a vascular access for hemodialysis. Bioengineered or tissue-engineered vascular grafts have long been envisioned to take the place of bioinert synthetic grafts and even vein grafts under certain clinical circumstances. However, host responses to a graft device induce adverse remodeling, to varied degrees depending on the graft property and host's developmental and health conditions. This in turn leads to invention or failure. Herein, we have mapped out the relationship between the design constraints and outcomes for vascular grafts, by analyzing impairment factors involved in the adverse graft remodeling. Strategies to tackle these impairment factors and counteract adverse healing are then summarized by outlining the research landscape of graft innovations in three dimensions-cell technology, scaffold technology and graft translation. Such a comprehensive view of cell and scaffold technological innovations in the translational context may benefit the future advancements in vascular grafts. From this perspective, we conclude the review with recommendations for future design endeavors.

A small-diameter vascular graft immobilized peptides for capturing endothelial colony-forming cells

Combining synthetic polymers and biomacromolecules prevents the occurrence of thrombogenicity and intimal hyperplasia in small-diameter vascular grafts (SDVGs). In the present study, an electrospinning poly (L)-lactic acid (PLLA) bilayered scaffold is developed to prevent thrombosis after implantation by promoting the capture and differentiation of endothelial colony-forming cells (ECFCs). The scaffold consists of an outer PLLA scaffold and an inner porous PLLA biomimetic membrane combined with heparin (Hep), peptide Gly-Gly-Gly-Arg-Glu-Asp-Val (GGG-REDV), and vascular endothelial growth factor (VEGF). Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle goniometry were performed to determine successful synthesis. The tensile strength of the outer layer was obtained using the recorded stress/strain curves, and hemocompatibility was evaluated using the blood clotting test. The proliferation, function, and differentiation properties of ECFCs were measured on various surfaces. Scanning electronic microscopy (SEM) was used to observe the morphology of ECFCs on the surface. The outer layer of scaffolds exhibited a similar strain and stress performance as the human saphenous vein via the tensile experiment. The contact angle decreased continuously until it reached 56° after REDV/VEGF modification, and SEM images of platelet adhesion showed a better hemocompatibility surface after modification. The ECFCs were captured using the REDV + VEGF + surface successfully under flow conditions. The expression of mature ECs was constantly increased with the culture of ECFCs on REDV + VEGF + surfaces. SEM images showed that the ECFCs captured by the REDV + VEGF + surface formed capillary-like structures after 4 weeks of culture. The SDVGs modified by REDV combined with VEGF promoted ECFC capture and rapid differentiation into ECs, forming capillary-like structures in vitro. The bilayered SDVGs could be used as vascular devices that achieved a high patency rate and rapid re-endothelialization.

Biological properties of self-assembled nanofibers of elastin-like block polypeptides for tissue-engineered vascular grafts: platelet inhibition, endothelial cell activation and smooth muscle cell maintenance

Strategic materials design is essential for the development of small-diameter, tissue-engineered vascular grafts. Self-assembled nanofibers of elastin-like polypeptides represent promising vascular graft components as they replicate the organized elastin structure of native blood vessels. Further, the bioactivity of nanofibers can be modified by the addition of functional peptide motifs. In the present study, we describe the development of a novel nanofiber-forming elastin-like polypeptide (ELP) with an arginine-glutamic acid-aspartic acid-valine (REDV) sequence. The biological characteristics of the REDV-modified ELP nanofibers relevant to applications in vascular grafting were compared to ELP without ligands for integrin, ELP with arginine-glycine-aspartic acid (RGD) sequence, collagen and cell culture glass. Among them, REDV-modified ELP nanofibers met the preferred biological properties for vascular graft materials, i.e. (i) inhibition of platelet adhesion and activation, (ii) endothelial cell adhesion and proliferation and (iii) maintenance of smooth muscle cells in a contractile phenotype to prevent cell overgrowth. The results indicate that REDV-modified ELP nanofibers represent promising candidates for the further development of small-diameter vascular grafts.

Vascular endothelial growth factor attenuates neointimal hyperplasia of decellularized small-diameter vascular grafts by modulating the local inflammatory response

Small-diameter vascular grafts (diameter <6 mm) are in high demand in clinical practice. Neointimal hyperplasia, a common complication after implantation of small-diameter vascular grafts, is one of the common causes of graft failure. Modulation of local inflammatory responses is a promising strategy to attenuates neointimal hyperplasia. Vascular endothelial growth factor (VEGF) is an angiogenesis stimulator that also induces macrophage polarization and modulates inflammatory responses. In the present study, we evaluated the effect of VEGF on the neointima hyperplasia and local inflammatory responses of decellularized vascular grafts. In the presence of rhVEGF-165 in RAW264.6 macrophage culture, rhVEGF-165 induces RAW264.6 macrophage polarization to M2 phenotype. Decellularized bovine internal mammary arteries were implanted into the subcutaneous and infrarenal abdominal aorta of New Zealand rabbits, with rhVEGF-165 applied locally to the adventitial of the grafts. The vascular grafts were removed en-bloc and submitted to histological and immunofluorescence analyses on days 7 and 28 following implantation. The thickness of the fibrous capsule and neointima was thinner in the VEGF group than that in the control group. In the immunofluorescence analysis, the number of M2 macrophages and the ratio of M2/M1 macrophages in vascular grafts in the VEGF group were higher than those in the control group, and the proinflammatory factor IL-1 was expressed less than in the control group, but the anti-inflammatory factor IL-10 was expressed more. In conclusion, local VEGF administration attenuates neointimal hyperplasia in decellularized small-diameter vascular grafts by inducing macrophage M2 polarization and modulating the inflammatory response.

Nanofibrous polytetrafluoroethylene/poly(ε-caprolactone) membrane with hierarchical structures for vascular patch

With the prevalence of cardiovascular diseases, developing cardiovascular supplements is becoming increasingly urgent. The ability of cells to rapidly adhere and proliferate to achieve endothelialization is extremely important for vascular grafts. In this work, we electrospun polytetrafluoroethylene (PTFE) nanofibrous membranes and used induced crystallization to manufacture poly(ε-caprolactone) (PCL) shish-kebab microstructures on PTFE nanofibers to overcome the inertness of PTFE, and promote cell adhesion and proliferation. PCL lamella periodically grew on the surface of PTFE nanofibers yielding a hierarchical structure, which improved the biocompatibility and mechanical properties of the PTFE nanofibrous membrane. The deposition of PCL lamella improved the hydrophilicity of electrospun PTFE nanofibers membrane, leading to good cell proliferation and adhesion. Also, due to the surface inertness of the substrate material PTFE, this PTFE/PCL composite film has good anti-platelet adhesion properties. Furthermore, cell proliferation could be regulated by controlling the integrity of the PCL crystal network. The vascular patch showed similar mechanical properties to natural blood vessels, providing a new strategy for vascular tissue engineering.

An Investigation of the Constructional Design Components Affecting the Mechanical Response and Cellular Activity of Electrospun Vascular Grafts

Cardiovascular disease is anticipated to remain the leading cause of death globally. Due to the current problems connected with using autologous arteries for bypass surgery, researchers are developing tissue-engineered vascular grafts (TEVGs). The major goal of vascular tissue engineering is to construct prostheses that closely resemble native blood vessels in terms of morphological, mechanical, and biological features so that these scaffolds can satisfy the functional requirements of the native tissue. In this setting, morphology and cellular investigation are usually prioritized, while mechanical qualities are generally addressed superficially. However, producing grafts with good mechanical properties similar to native vessels is crucial for enhancing the clinical performance of vascular grafts, exposing physiological forces, and preventing graft failure caused by intimal hyperplasia, thrombosis, aneurysm, blood leakage, and occlusion. The scaffold's design and composition play a significant role in determining its mechanical characteristics, including suturability, compliance, tensile strength, burst pressure, and blood permeability. Electrospun prostheses offer various models that can be customized to resemble the extracellular matrix. This review aims to provide a comprehensive and comparative review of recent studies on the mechanical properties of fibrous vascular grafts, emphasizing the influence of structural parameters on mechanical behavior. Additionally, this review provides an overview of permeability and cell growth in electrospun membranes for vascular grafts. This work intends to shed light on the design parameters required to maintain the mechanical stability of vascular grafts placed in the body to produce a temporary backbone and to be biodegraded when necessary, allowing an autologous vessel to take its place.

The path to a hemocompatible cardiovascular implant: Advances and challenges of current endothelialization strategies

Cardiovascular (CV) implants are still associated with thrombogenicity due to insufficient hemocompatibility. Endothelialization of their luminal surface is a promising strategy to increase their hemocompatibility. In this review, we provide a collection of research studies and review articles aiming to summarize the recent efforts on surface modifications of CV implants, including stents, grafts, valves, and ventricular assist devises. We focus in particular on the implementation of micrometer or nanoscale surface modifications, physical characteristics of known biomaterials (such as wetness and stiffness), and surface morphological features (such as gratings, fibers, pores, and pits). We also review how biomechanical signals originating from the endothelial cell for surface interaction can be directed by topography engineering approaches toward the survival of the endothelium and its long-term adaptation. Finally, we summarize the regulatory and economic challenges that may prevent clinical implementation of endothelialized CV implants.

A high-hydrophilic Cu2O-TiO2/Ti2O3/TiO coating on Ti-5Cu alloy: Perfect antibacterial property and rapid endothelialization potential

In order to make novel antibacterial Ti-Cu alloy more suitable for cardiovascular implant application, a Cu-containing oxide coating was manufactured on Ti-Cu alloy by plasma-enhanced oxidation deposition in plasma enhanced chemical vapor deposition (PECVD) equipment to further improve the antibacterial ability and the surface bioactivity. The results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and water contact angle indicated that a sustainably high-hydrophilic Cu₂O-TiO₂/Ti₂O₃/TiO coating with nano-morphology on Ti-5Cu was successfully constructed. The corrosion performance results showed that the coating enhanced the corrosion resistance while releasing more Cu²⁺, compared with Ti-5Cu. Antibacterial tests confirmed the perfect antibacterial property of the coating (R ≥ 99.9 %), superior to Ti-Cu alloy (R > 90 %). More delightfully, it was observed by phalloidin-FITC and DAPI staining that the coating improved the early adhesion of HUVEC cells mainly due to strong hydrophilicity and nano-morphology. It was demonstrated that the extract of the coated sample significantly promoted proliferation (RGR = 112 %-138 % after cultivation for 1 to 3 days) and migration of HUVEC cells due to the appropriate Cu²⁺ release concentration. Hemolysis assay and platelet adhesion results showed that the coating had excellent blood compatibility. All results suggested that the coating on Ti-Cu alloy might be a promising surface with the perfect antibacterial ability, blood compatibility and evident promoting endothelialization ability for the cardiovascular application.

Structural design and mechanical performance of composite vascular grafts

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

Graphic abstract

Cell Adhesion Strength Indicates the Antithrombogenicity of Poly(2-Methoxyethyl Acrylate) (PMEA): Potential Candidate for Artificial Small-Diameter Blood Vessel

Poly (2-methoxyethyl acrylate) (PMEA) is a US FDA-approved biocompatible polymer, although there is insufficient work on human umbilical vein endothelial cells (HUVECs) and platelet interaction analysis on PMEA-analogous polymers. In this study, we extensively investigated HUVEC–polymer and platelet–polymer interaction behavior by measuring the adhesion strength using single-cell force spectroscopy. Furthermore, the hydration layer of the polymer interface was observed using frequency-modulation atomic force microscopy. We found that endothelial cells can attach and spread on the PMEA surface with strong adhesion strength compared to other analogous polymers. We found that the hydration layers on the PMEA-analogous polymers were closely related to their weak platelet adhesion behavior. Based on our results, it can be concluded that PMEA is a promising candidate for the construction of artificial small-diameter blood vessels owing to the presence of IW and a hydration layer on the interface.

Hemocompatibility Evaluation of Thai Bombyx mori Silk Fibroin and Its Improvement with Low Molecular Weight Heparin Immobilization

Bombyx mori silk fibroin (SF), from Nangnoi Srisaket 1 Thai strain, has shown potential for various biomedical applications such as wound dressing, a vascular patch, bone substitutes, and controlled release systems. The hemocompatibility of this SF is one of the important characteristics that have impacts on such applications. In this study, the hemocompatibility of Thai SF was investigated and its improvement by low molecular weight heparin (LMWH) immobilization was demonstrated. Endothelial cell proliferation on the SF and LMWH immobilized SF (Hep/SF) samples with or without fibroblast growth factor-2 (FGF-2) was also evaluated. According to hemocompatibility evaluation, Thai SF did not accelerate clotting time, excess stimulate complement and leukocyte activation, and was considered a non-hemolysis material compared to the negative control PTFE sheet. Platelet adhesion of SF film was comparable to that of the PTFE sheet. For hemocompatibility enhancement, LMWH was immobilized successfully and could improve the surface hydrophilicity of SF films. The Hep/SF films demonstrated prolonged clotting time and slightly lower complement and leukocyte activation. However, the Hep/SF films could not suppress platelet adhesion. The Hep/SF films demonstrated endothelial cell proliferation enhancement, particularly with FGF-2 addition. This study provides fundamental information for the further development of Thai SF as a hemocompatible biomaterial.

Step-wise CAG@PLys@PDA-Cu2+ modification on micropatterned nanofibers for programmed endothelial healing

Native-like endothelium regeneration is a prerequisite for material-guided small-diameter vascular regeneration. In this study, a novel strategy is proposed to achieve phase-adjusted endothelial healing by step-wise modification of parallel-microgroove-patterned (i.e., micropatterned) nanofibers with polydopamine-copper ion (PDA-Cu²⁺) complexes, polylysine (PLys) molecules, and Cys-Ala-Gly (CAG) peptides (CAG@PLys@PDA-Cu²⁺). Using electrospun poly(l-lactide-co-caprolactone) random nanofibers as the demonstrating biomaterial, step-wise modification of CAG@PLys@PDA-Cu²⁺ significantly enhanced substrate wettability and protein adsorption, exhibited an excellent antithrombotic surface and outstanding phase-adjusted capacity of endothelium regeneration involving cell adhesion, endothelial monolayer formation, and the regenerated endothelium maturation. Upon in vivo implantation for segmental replacement of rabbit carotid arteries, CAG@PLys@PDA-Cu²⁺ modified grafts (2 mm inner diameter) with micropatterns on inner surface effectively accelerated native-like endothelium regeneration within 1 week, with less platelet aggregates and inflammatory response compared to those on non-modified grafts. Prolonged observations at 6- and 12-weeks post-implantation demonstrated a positive vascular remodeling with almost fully covered endothelium and mature smooth muscle layer in the modified vascular grafts, accompanied with well-organized extracellular matrix. By contrast, non-modified vascular grafts induced a disorganized tissue formation with a high risk of thrombogenesis. In summary, step-wise modification of CAG@PLys@PDA-Cu²⁺ on micropatterned nanofibers can significantly promote endothelial healing without inflicting thrombosis, thus confirming a novel strategy for developing functional vascular grafts or other blood-contacting materials/devices.

Peripheral blood derived endothelial colony forming cells as suitable cell source for pre-endothelialization of arterial vascular grafts under dynamic flow conditions

In regenerative medicine, autologous peripheral blood derived endothelial colony forming cells (PB-derived ECFC) represent a promising source of endothelial cells (EC) for pre-endothelialization of arterial tissue engineered vascular grafts (TEVG) since they are readily attainable, can easily be isolated and possess a high proliferation potential. The aim of this study was to compare the phenotype of PB-derived ECFC with arterial and venous model cells such as human aortic endothelial cells (HAEC) and human umbilical vein endothelial cells (HUVEC) under dynamic cell culture conditions to find a suitable cell source of EC for pre-endothelialization. In this study PB-derived ECFC were cultivated over 24 h under a high pulsatile shear stress (20 dyn/cm², 1 Hz) and subsequently analyzed. ECFC oriented and elongated in the direction of flow and expressed similar anti-thrombotic and endothelial differentiation markers compared to HAEC. There were significant differences observable in gene expression levels of CD31, CD34 and NOTCH4 between ECFC and HUVEC. These results therefore suggest an arterial phenotype for PB-derived ECFC both under static and flow conditions, and this was supported by NOTCH4 protein expression profiles. ECFC also significantly up-regulated gene expression levels of anti-thrombotic genes such as krueppel-like factor 2, endothelial nitric oxide synthase 3 and thrombomodulin under shear stress cultivation as compared to static conditions. Dynamically cultured PB-derived ECFC therefore may be a promising cell source for pre-endothelialization of arterial TEVGs.

Poly(2-Methoxyethyl Acrylate) (PMEA)-Coated Anti-Platelet Adhesive Surfaces to Mimic Native Blood Vessels through HUVECs Attachment, Migration, and Monolayer Formation

Confluent monolayers of human umbilical vein endothelial cells (HUVECs) on a poly(2-methoxyethyl acrylate) (PMEA) antithrombogenic surface play a major role in mimicking the inner surface of native blood vessels. In this study, we extensively investigated the behavior of cell–polymer and cell–cell interactions by measuring adhesion strength using single-cell force spectroscopy. In addition, the attachment and migration of HUVECs on PMEA-analogous substrates were detected, and the migration rate was estimated. Moreover, the bilateral migration of HUVECs between two adjacent surfaces was observed. Furthermore, the outer surface of HUVEC was examined using frequency-modulation atomic force microscopy (FM-AFM). Hydration was found to be an indication of a healthy glycocalyx layer. The results were compared with the hydration states of individual PMEA-analogous polymers to understand the adhesion mechanism between the cells and substrates in the interface region. HUVECs could attach and spread on the PMEA surface with stronger adhesion strength than self-adhesion strength, and migration occurred over the surface of analogue polymers. We confirmed that platelets could not adhere to HUVEC monolayers cultured on the PMEA surface. FM-AFM images revealed a hydration layer on the HUVEC surfaces, indicating the presence of components of the glycocalyx layer in the presence of intermediate water. Our findings show that PMEA can mimic original blood vessels through an antithrombogenic HUVEC monolayer and is thus suitable for the construction of artificial small-diameter blood vessels.