Intimal hyperplasia in vascular grafts

Lower Extremity Bypass for Occlusive Disease: A Brief History

BACKGROUND

This is a narrative review that aims to highlight key advancements that led to the current state of lower extremity bypass surgery. It focuses on key contributors during the last century who have driven the standardization of surgical treatment of peripheral arterial occlusive disease.

METHODS

A narrative review was conducted utilizing available resources in the scientific and historical literature to track landmark achievements in the development of modern lower extremity bypass surgery for occlusive disease, focusing primarily on the last century of advancement.

RESULTS

Several critical conceptual, technological, and technical landmarks were identified as critical components of modern lower extremity bypass surgery. This includes fundamental developments in the techniques of vascular anastomosis led by Carrel and others, a developing understanding of vascular occlusive disease as a localized and segmental process with broad implementation of the techniques of arteriography, and the development of safe thromboendarterectomy aided by the development and utilization of heparin for anticoagulation. These factors led to the first femoral-to-popliteal artery bypass by Jean Kunlin in 1948. From here, advances in vascular prosthetic material pioneered by Voorhees and others, alternative vascular conduits, increasing acceptance of tibial revascularization, and dispelling the myth of diabetic "small vessel" disease broadened revascularization options for patients with complex patterns of occlusive disease and those who have limited conduit availability.

CONCLUSIONS

Modern lower extremity bypass surgery for occlusive disease arose steadily over a course of a century, driven by complex problem-solving in the pathophysiological understanding of atherosclerosis, technical developments in vascular anastomosis and arteriography, and evolution of conduit materials and pharmacologic therapy. Future advancements in bypass surgery are targeted at solving the complex problems of anastomotic intimal hyperplasia, expanding technology for alternative vascular conduits, ongoing optimization of risk factors, and scrutinizing of outcomes to make patient-centered, evidence-based decisions regarding revascularization strategy.

The effect of cilostazol on the platelet-derived growth factor-beta/beta isoform reduction on venous hyperplasia in an experimental balloon-induced injury model

BACKGROUND

Intimal hyperplasia is the response to endothelial injury. Platelet-derived growth factor is released early and favors the formation of intimal hyperplasia. Although multiple treatments, from open surgery to endovascular techniques, have been used they remain controversial. There is currently interest in developing pharmacological strategies to address this pathology. Local vascular inflammation induced by vessel barotrauma generates intimal hyperplasia due to mechanical stress over the venous endothelium. Cilostazol is a selective phosphodiesterase type 3 (PDE3) selective inhibitor with a regulatory effect over intimal hyperplasia. The objective was to investigate cilostazol's role in inhibiting smooth muscle cell proliferation due to changes in the expression and release of PDGF-BB isoform and the effect on developing IH using an experimental model of vascular barotrauma (balloon-induced injury model).

METHODS

We included 12 New Zealand rabbits. The balloon-induced injury model (BIIM) and experimental group cilostazol (20 mg/kg/day) included 6 rabbits each. Contralateral veins from 6 rabbits used in BIIM model has been taken as control group. We measured and compared the expression of PDGF-BB and the development of IH. A pathologist board chooses a PDGFRα antibody to localized its expression by immunohistochemistry analysis. Subsequently, using an automated immunohistochemical staining machine, the PDGFR expression was evaluated using a Zeiss Primo Star 4 light microscope.

RESULTS

The measurement obtained in the intimal layer was: 126.12 μm2 in the CG, 232 μm2 in the BIIM group, and 178 μm2 in the EG. A statistically significant difference was observed. Baseline serum concentrations of PDGF-BB in the BIIM group were 0.22 pg/mL. At 12 h 0.42 pg/mL, and 0.17 pg/mL at seven days. In the experimental group, the basal levels were 0.33 pg/mL. With the use of cilostazol, a lower peak was obtained at 12 h (0.08 pg/mL). This difference was statistically significant.

CONCLUSIONS

Cilostazol induced a significant reduction of IH caused by barotrauma in the venous endothelium, which correlates with decrease in the PDGF-BB in serum. This could be attributed to the pharmacologic effect on PDGFR expression.

Macrophage-Biomimetic Nanoplatform-Based Therapy for Inflammation-Associated Diseases

Inflammation-associated diseases are very common clinically with a high incidence; however, there is still a lack of effective treatments. Cell-biomimetic nanoplatforms have led to many breakthroughs in the field of biomedicine, significantly improving the efficiency of drug delivery and its therapeutic implications especially for inflammation-associated diseases. Macrophages are an important component of immune cells and play a critical role in the occurrence and progression of inflammation-associated diseases while simultaneously maintaining homeostasis and modulating immune responses. Therefore, macrophage-biomimetic nanoplatforms not only inherit the functions of macrophages including the inflammation tropism effect for targeted delivery of drugs and the neutralization effect of pro-inflammatory cytokines and toxins via membrane surface receptors or proteins, but also maintain the functions of the inner nanoparticles. Macrophage-biomimetic nanoplatforms are shown to have remarkable therapeutic efficacy and excellent application potential in inflammation-associated diseases. In this review, inflammation-associated diseases, the physiological functions of macrophages, and the classification and construction of macrophage-biomimetic nanoplatforms are first introduced. Next, the latest applications of different macrophage-biomimetic nanoplatforms for the treatment of inflammation-associated diseases are summarized. Finally, challenges and opportunities for future biomedical applications are discussed. It is hoped that the review will provide new ideas for the further development of macrophage-biomimetic nanoplatforms.

Off-the-Shelf Synthetic Biodegradable Grafts Transform In Situ into a Living Arteriovenous Fistula in a Large Animal Model

Current vascular access options require frequent interventions. In situ tissue engineering (TE) may overcome these limitations by combining the initial success of synthetic grafts with long-term advantages of autologous vessels by using biodegradable grafts that transform into autologous vascular tissue at the site of implantation. Scaffolds (6 mm-Ø) made of supramolecular polycarbonate-bisurea (PC-BU), with a polycaprolactone (PCL) anti-kinking-coil, are implanted between the carotid artery and jugular vein in goats. A subset is bio-functionalized using bisurea-modified-Stromal cell-derived factor-1α (SDF1α) derived peptides and ePTFE grafts as controls. Grafts are explanted after 1 and 3 months, and evaluated for material degradation, tissue formation, compliance, and patency. At 3 months, the scaffold is resorbed and replaced by vascular neo-tissue, including elastin, contractile markers, and endothelial lining. No dilations, ruptures, or aneurysms are observed and grafts are successfully cannulated at termination. SDF-1α-peptide-biofunctionalization does not influence outcomes. Patency is lower in TE grafts (50%) compared to controls (100% patency), predominantly caused by intimal hyperplasia. Rapid remodeling of a synthetic, biodegradable vascular scaffold into a living, compliant arteriovenous fistula is demonstrated in a large animal model. Despite lower patency compared to ePTFE, transformation into autologous and compliant living tissue with self-healing capacity may have long-term advantages.

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.

Challenges and Strategies for Endothelializing Decellularized Small-Diameter Tissue-Engineered Vessel Grafts

Vascular diseases are the leading cause of ischemic necrosis in tissues and organs, necessitating using vascular grafts to restore blood supply. Currently, small vessels for coronary artery bypass grafts are unavailable in clinical settings. Decellularized small-diameter tissue-engineered vessel grafts (SD-TEVGs) hold significant potential. However, they face challenges, as simple implantation of decellularized SD-TEVGs in animals leads to thrombosis and calcification due to incomplete endothelialization. Consequently, research and development focus has shifted toward enhancing the endothelialization process of decellularized SD-TEVGs. This paper reviews preclinical studies involving decellularized SD-TEVGs, highlighting different strategies and their advantages and disadvantages for achieving rapid endothelialization of these vascular grafts. Methods are analyzed to improve the process while addressing potential shortcomings. This paper aims to contribute to the future commercial viability of decellularized SD-TEVGs.

Therapeutic strategies based on non-ionizing radiation to prevent venous neointimal hyperplasia: the relevance for stenosed arteriovenous fistula, and the role of vascular compliance

We have reviewed the development and current status of therapies based on exposure to non-ionizing radiation (with a photon energy less than 10 eV) aimed at suppressing the venous neointimal hyperplasia, and consequentially at avoiding stenosis in arteriovenous grafts. Due to the drawbacks associated with the medical use of ionizing radiation, prominently the radiation-induced cardiovascular disease, the availability of procedures using non-ionizing radiation is becoming a noteworthy objective for the current research. Further, the focus of the review was the use of such procedures for improving the vascular access function and assuring the clinical success of arteriovenous fistulae in hemodialysis patients. Following a brief discussion of the physical principles underlying radiotherapy, the current methods based on non-ionizing radiation, either in use or under development, were described in detail. There are currently five such techniques, including photodynamic therapy (PDT), far-infrared therapy, photochemical tissue passivation (PTP), Alucent vascular scaffolding, and adventitial photocrosslinking. The last three are contingent on the mechanical stiffening achievable by the exogenous photochemical crosslinking of tissular collagen, a process that leads to the decrease of venous compliance. As there are conflicting opinions on the role of compliance mismatch between arterial and venous conduits in a graft, this aspect was also considered in our review.

Techniques and Technologies to Improve Vein Graft Patency in Coronary Surgery

Vein grafts are the most used conduits in coronary artery bypass grafting (CABG), even though many studies have suggested their lower patency compared to arterial alternatives. We have reviewed the techniques and technologies that have been investigated over the years with the aim of improving the quality of these conduits. We found that preoperative and postoperative optimal medical therapy and no-touch harvesting techniques have the strongest evidence for optimizing vein graft patency. On the other hand, the use of venous external support, endoscopic harvesting, vein preservation solution and anastomosis, and graft configuration need further investigation. We have also analyzed strategies to treat vein graft failure: when feasible, re-doing the CABG and native vessel primary coronary intervention (PCI) are the best options, followed by percutaneous procedures targeting the failed grafts.

Perivascular CLICK-gelatin delivery of thrombospondin-2 small interfering RNA decreases development of intimal hyperplasia after arterial injury

Bypass graft failure occurs in 20%-50% of coronary and lower extremity bypasses within the first-year due to intimal hyperplasia (IH). TSP-2 is a key regulatory protein that has been implicated in the development of IH following vessel injury. In this study, we developed a biodegradable CLICK-chemistry gelatin-based hydrogel to achieve sustained perivascular delivery of TSP-2 siRNA to rat carotid arteries following endothelial denudation injury. At 21 days, perivascular application of TSP-2 siRNA embedded hydrogels significantly downregulated TSP-2 gene expression, cellular proliferation, as well as other associated mediators of IH including MMP-9 and VEGF-R2, ultimately resulting in a significant decrease in IH. Our data illustrates the ability of perivascular CLICK-gelatin delivery of TSP-2 siRNA to mitigate IH following arterial injury.

Methyl donor diet attenuates intimal hyperplasia after vascular injury in rats

Environmental factors, particularly dietary habits, play an important role in cardiovascular disease susceptibility and progression through epigenetic modification. Previous studies have shown that hyperplastic vascular intima after endarterectomy is characterized by genome-wide hypomethylation. The purpose of this study was to investigate whether methyl donor diet affects intimal hyperplasia and the possible mechanisms involved. Intimal hyperplasia was induced in SD rats by carotid artery balloon injury. From 8 d before surgery to 28 d after surgery, the animals were fed a normal diet (ND) or a methyl donor diet (MD) supplemented with folic acid, vitamin B12, choline, betaine, and zinc. Carotid artery intimal hyperplasia was observed by histology, the effect of MD on carotid protein expression was analyzed by proteomics, functional clustering, signaling pathway, and upstream-downstream relationship of differentially expressed proteins were analyzed by bioinformatics. Results showed that MD attenuated balloon injury-induced intimal hyperplasia in rat carotid arteries. Proteomic analysis showed that there were many differentially expressed proteins in the common carotid arteries of rats fed with two different diets. The differentially expressed proteins are mainly related to the composition and function of the extracellular matrix (EMC), and changes in the EMC can lead to vascular remodeling by affecting fibrosis and stiffness of the blood vessel wall. Changes in the levels of vasculotropic proteins such as S100A9, ILF3, Serpinh1, Fbln5, LOX, HSPG2, and Fmod may be the reason why MD attenuates intimal hyperplasia. Supplementation with methyl donor nutrients may be a beneficial measure to prevent pathological vascular remodeling after injury.

Epoxy- versus Glutaraldehyde-Treated Bovine Jugular Vein Conduit for Pulmonary Valve Replacement: A Comparison of Morphological Changes in a Pig Model

Valved conduits are often required to replace pulmonary arteries (PA). A widely used Contegra device is made of bovine jugular vein (BJV), preserved with glutaraldehyde (GA) and iso-propanol. However, it has several drawbacks that may be attributed to its chemical treatment. We hypothesized that the use of an alternative preservation compound may significantly improve BJV conduit performance. This study aimed to compare the macroscopic and microscopic properties of the BJV treated with diepoxide (DE) and GA in a porcine model. Twelve DE-BJVs and four Contegra conduits were used for PA replacement in minipigs. To assess the isolated influence of GA, we included an additional control group-BJV treated with 0.625% GA (n = 4). The animals were withdrawn after 6 months of follow-up and the conduits were examined. Explanted DE-BJV had a soft elastic wall with no signs of thrombosis or calcification and good conduit integration, including myofibroblast germination, an ingrowth of soft connective tissue formations and remarkable neoangiogenesis. The inner surface of DE-BJVs was covered by a thin neointimal layer with a solid endothelium. Contegra grafts had a stiffer wall with thrombosis on the leaflets. Calcified foci, chondroid metaplasia, and hyalinosis were observed within the wall. The distal anastomotic sites had hyperplastic neointima, partially covered with the endothelium. The wall of GA-BJV was stiff and rigid with degenerative changes, a substantial amount of calcium deposits and dense fibrotic formations in adventitia. An irregular neointimal layer was presented in the anastomotic sites without endothelial cover in the GA BJV wall. These results demonstrate that DE treatment improves conduit integration and the endothelialization of the inner surface while preventing the mineralization of the BJV, which may reduce the risk of early conduit dysfunction.

Histopathological Validation of Microvascular Anastomosis using Two-Throw Reef Knots - An Experimental Study

Background

Knot configuration is an important but relatively neglected topic in microvascular anastomosis literature.

Objective

To study the differences between end-to-end microvascular anastomosis performed with two-throw reef knots as compared to traditional three-throw knots in a rat femoral artery model at the histological level.

Material and Methods

Sprague Dawley rats underwent end-to-end microvascular anastomosis of the right femoral artery (one-way-up method). The rats were divided into two groups: two-throw reef knots versus traditional three-throw knots. The patency was checked by the standard empty refill method. After 2 weeks, the rats underwent re-exploration. An anastomotic segment was sent for histological analysis. Histological alterations including luminal patency and changes in Tunica intima, Tunica media, and Tunica adventitia were compared between the two groups.

Results

Twenty-nine rats were operated on by the senior author (17 by three-throw and 12 by two-throw reef knots). In the two-throw reef knot group versus the traditional three-throw knot group, the immediate patency rates were 100% versus 82.4%, and the delayed patency rates were 90.9% versus 62.5%, respectively. The histopathological patency rates were concordant with delayed patency rates. Subintimal proliferation and fibrosis were comparable in both groups. Adventitial granulomas were noted in all, irrespective of the knotting technique. Tunica media preservation rates for the two-throw reef knot versus the traditional three-throw knot group were 63.6% versus 0%. Five rats were operated by the beginner in the field, all by two-throw reef knots (to assess the safety of this new method in the hands of a beginner).

Conclusion

Microvascular anastomosis performed with two-throw reef knots appears not only feasible but better in terms of anastomosis patency. Histological superiority in terms of Tunica media preservation further validates the technique.

Fluid-structure interactions of peripheral arteries using a coupled in silico and in vitro approach

Vascular compliance is considered both a cause and a consequence of cardiovascular disease and a significant factor in the mid- and long-term patency of vascular grafts. However, the biomechanical effects of localised changes in compliance cannot be satisfactorily studied with the available medical imaging technologies or surgical simulation materials. To address this unmet need, we developed a coupled silico-vitro platform which allows for the validation of numerical fluid-structure interaction results as a numerical model and physical prototype. This numerical one-way and two-way fluid-structure interaction study is based on a three-dimensional computer model of an idealised femoral artery which is validated against patient measurements derived from the literature. The numerical results are then compared with experimental values collected from compliant arterial phantoms via direct pressurisation and ring tensile testing. Phantoms within a compliance range of 1.4-68.0%/100 mmHg were fabricated via additive manufacturing and silicone casting, then mechanically characterised via ring tensile testing and optical analysis under direct pressurisation with moderately statistically significant differences in measured compliance ranging between 10 and 20% for the two methods. One-way fluid-structure interaction coupling underestimated arterial wall compliance by up to 14.7% compared with two-way coupled models. Overall, Solaris™ (Smooth-On) matched the compliance range of the numerical and in vivo patient models most closely out of the tested silicone materials. Our approach is promising for vascular applications where mechanical compliance is especially important, such as the study of diseases which commonly affect arterial wall stiffness, such as atherosclerosis, and the model-based design, surgical training, and optimisation of vascular prostheses.

Acute Left Ventricular Assist Device Failure From Outflow Graft Dissection Flap Successfully Treated With Stent Placement

Outflow graft obstruction (OGO) is a potentially fatal complication of left ventricular assist device (LVAD) implantation. Common causes include graft kinking, luminal stenosis, and graft thrombosis. We describe a case of a 51-year-old female who presented with acute LVAD failure and cardiogenic shock. Initial imaging revealed OGO without luminal stenosis following which she was she was empirically treated for graft thrombosis with anticoagulation and catheter-based thrombolytic therapy with no success. Further imaging in the catheterization laboratory revealed a potential neointimal flap dissection acting as a one-way valve. A stent was successfully placed across the outflow graft with immediate resolution of cardiogenic shock. The case emphasizes the importance of including neointimal flap as a differential for OGO in acute LVAD failure, especially in cases where thrombolytic therapy fails to correct hemodynamic compromise.

In vivo evaluation of compliance mismatch on intimal hyperplasia formation in small diameter vascular grafts

Small diameter synthetic vascular grafts have high failure rate due to the thrombosis and intimal hyperplasia formation. Compliance mismatch between the synthetic graft and native artery has been speculated to be one of the main causes of intimal hyperplasia. However, changing the compliance of synthetic materials without altering material chemistry remains a challenge. Here, we used poly(vinyl alcohol) (PVA) hydrogel as a graft material due to its biocompatibility and tunable mechanical properties to investigate the role of graft compliance in the development of intimal hyperplasia and in vivo patency. Two groups of PVA small diameter grafts with low compliance and high compliance were fabricated by dip casting method and implanted in a rabbit carotid artery end-to-side anastomosis model for 4 weeks. We demonstrated that the grafts with compliance that more closely matched with rabbit carotid artery had lower anastomotic intimal hyperplasia formation and higher graft patency compared to low compliance grafts. Overall, this study suggested that reducing the compliance mismatch between the native artery and vascular grafts is beneficial for reducing intimal hyperplasia formation.

An optical coherence tomography and endothelial shear stress study of a novel bioresorbable bypass graft

Endothelial shear stress (ESS) plays a key role in the clinical outcomes in native and stented segments; however, their implications in bypass grafts and especially in a synthetic biorestorative coronary artery bypass graft are yet unclear. This report aims to examine the interplay between ESS and the morphological alterations of a biorestorative coronary bypass graft in an animal model. Computational fluid dynamics (CFD) simulation derived from the fusion of angiography and optical coherence tomography (OCT) imaging was used to reconstruct data on the luminal anatomy of a bioresorbable coronary bypass graft with an endoluminal "flap" identified during OCT acquisition. The "flap" compromised the smooth lumen surface and considerably disturbed the local flow, leading to abnormally low ESS and high oscillatory shear stress (OSI) in the vicinity of the "flap". In the presence of the catheter, the flow is more stable (median OSI 0.02384 versus 0.02635, p < 0.0001; maximum OSI 0.4612 versus 0.4837). Conversely, OSI increased as the catheter was withdrawn which can potentially cause back-and-forth motions of the "flap", triggering tissue fatigue failure. CFD analysis in this report provided sophisticated physiological information that complements the anatomic assessment from imaging enabling a complete understanding of biorestorative graft pathophysiology.

Development of a dual-component infection-resistant arterial replacement for small-caliber reconstructions: A proof-of-concept study

Introduction: Synthetic vascular grafts perform poorly in small-caliber (<6mm) anastomoses, due to intimal hyperplasia and thrombosis, whereas homografts are associated with limited availability and immunogenicity, and bioprostheses are prone to aneurysmal degeneration and calcification. Infection is another important limitation with vascular grafting. This study developed a dual-component graft for small-caliber reconstructions, comprising a decellularized tibial artery scaffold and an antibiotic-releasing, electrospun polycaprolactone (PCL)/polyethylene glycol (PEG) blend sleeve. Methods: The study investigated the effect of nucleases, as part of the decellularization technique, and two sterilization methods (peracetic acid and γ-irradiation), on the scaffold's biological and biomechanical integrity. It also investigated the effect of different PCL/PEG ratios on the antimicrobial, biological and biomechanical properties of the sleeves. Tibial arteries were decellularized using Triton X-100 and sodium-dodecyl-sulfate. Results: The scaffolds retained the general native histoarchitecture and biomechanics but were depleted of glycosaminoglycans. Sterilization with peracetic acid depleted collagen IV and produced ultrastructural changes in the collagen and elastic fibers. The two PCL/PEG ratios used (150:50 and 100:50) demonstrated differences in the structural, biomechanical and antimicrobial properties of the sleeves. Differences in the antimicrobial activity were also found between sleeves fabricated with antibiotics supplemented in the electrospinning solution, and sleeves soaked in antibiotics. Discussion: The study demonstrated the feasibility of fabricating a dual-component small-caliber graft, comprising a scaffold with sufficient biological and biomechanical functionality, and an electrospun PCL/PEG sleeve with tailored biomechanics and antibiotic release.

Personalized tissue-engineered arteries as vascular graft transplants: A safety study in sheep

Patients with cardiovascular disease often need replacement or bypass of a diseased blood vessel. With disadvantages of both autologous blood vessels and synthetic grafts, tissue engineering is emerging as a promising alternative of advanced therapy medicinal products for individualized blood vessels. By reconditioning of a decellularized blood vessel with the recipient’s own peripheral blood, we have been able to prevent rejection without using immunosuppressants and prime grafts for efficient recellularization in vivo. Recently, decellularized veins reconditioned with autologous peripheral blood were shown to be safe and functional in a porcine in vivo study as a potential alternative for vein grafting. In this study, personalized tissue engineered arteries (P-TEA) were developed using the same methodology and evaluated for safety in a sheep in vivo model of carotid artery transplantation. Five personalized arteries were transplanted to carotid arteries and analyzed for safety and patency as well as with histology after four months in vivo. All grafts were fully patent without any occlusion or stenosis. The tissue was well cellularized with a continuous endothelial cell layer covering the luminal surface, revascularized adventitia with capillaries and no sign of rejection or infection. In summary, the results indicate that P-TEA is safe to use and has potential as clinical grafts.

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Safety and functionality evaluation of a tissue engineered ATMP in a sheep model of carotid transplantation.

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Efficient cellularization of a personalized tissue engineered artery in vivo.

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Personalized tissue engineered artery fully patent after four months in vivo.

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

Electrospun POSS integrated poly(carbonate-urea)urethane provides appropriate surface and mechanical properties for the fabrication of small-diameter vascular grafts

This study developed a platform for fabricating small-diameter vascular grafts using electrospun poly(carbonate-urea)urethane bonded with different concentrations of POSS nanocage. The characteristics of electrospun POSS-PCUUs were investigated by ATR-FTIR, 1HNMR, EDS, SEM, AFM, WCA, and DSC analyses. Besides, mechanical attributes such as tensile strength, modulus, elastic recovery, and inelastic behaviors were monitored. The survival rate and cellular attachment capacity were studied using human endothelial cells during a 7-day culture period. The results showed that electrospun nanofibers with 6 wt.% POSS-PCUU had better surface properties in terms of richness of POSS nanocage with notable improved mechanical strength and hysteresis loss properties (p < 0.05). The surface roughness of electrospun 6 wt.% POSS-PCUU reached 646 ± 10 nm with statistically significant differences compared to the control PCUU and groups containing 2, 4 wt.% POSS-PCUU (p < 0.05). The addition of 6 wt.% POSS increased the ultimate mechanical strength of nanofibers related to control PCUU and other groups (p < 0.05). The expansion of human endothelial cells on the 6 wt.% POSS-PCUU surface increased the viability reaching maximum levels on day 7 (p < 0.05). Immunofluorescence imaging using DAPI staining displayed the formation single-layer endothelial barrier at the luminal surface, indicating an appropriate cell-to-cell interaction.

Soft Biomimetic 3D Free-Form Artificial Vascular Graft Using a Highly Uniform Microspherical Porous Structure

This study presents a biomimetic 3D customizable artificial vascular graft with a highly porous and uniform microscale structure. The structural features were obtained by dip coating of a highly close-packed microsphere suspension on a 3D printed sacrificial template. Considering the structured arrangement of microspherical porogens in the coating layer, the microsphere-leached constructs showed higher uniformity and porosity than the conventionally particulate-leached structures, leading to ultrasoft mechanical compliance. Considering biomechanical compatibility, the resulting elastic moduli were at the sub-MPa level, comparable with those of native vascular tissues. In addition, the developed porous graft was reinforced selectively at the edge regions using a nonporous coating to secure its practical sutureability for clinical use. The sufficiently low cytotoxicity was clinically confirmed to alleviate the stiffness mismatch issues at the anastomotic interface between the native tissue and the artificial graft, thus overcoming the relevant clinical complications. Furthermore, the overall superior properties could be implemented on the 3D printed template for patient-specific medicare, thus implying the manufacturability of patient-specific vascular grafts.

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.

Endothelial Cell TGF-β (Transforming Growth Factor-Beta) Signaling Regulates Venous Adaptive Remodeling to Improve Arteriovenous Fistula Patency

BACKGROUND

Arteriovenous fistulae (AVF) are the gold standard for vascular access for hemodialysis. Although the vein must thicken and dilate for successful hemodialysis, excessive wall thickness leads to stenosis causing AVF failure. Since TGF-β (transforming growth factor-beta) regulates ECM (extracellular matrix) deposition and smooth muscle cell (SMC) proliferation-critical components of wall thickness-we hypothesized that disruption of TGF-β signaling prevents excessive wall thickening during venous remodeling.

METHODS

A mouse aortocaval fistula model was used. SB431542-an inhibitor of TGF-β receptor I-was encapsulated in nanoparticles and applied to the AVF adventitia in C57BL/6J mice. Alternatively, AVFs were created in mice with conditional disruption of TGF-β receptors in either SMCs or endothelial cells. Doppler ultrasound was performed serially to confirm patency and to measure vessel diameters. AVFs were harvested at predetermined time points for histological and immunofluorescence analyses.

RESULTS

Inhibition of TGF-β signaling with SB431542-containing nanoparticles significantly reduced p-Smad2-positive cells in the AVF wall during the early maturation phase (days 7-21) and was associated with decreased AVF wall thickness that showed both decreased collagen density and decreased SMC proliferation. SMC-specific TGF-β signaling disruption decreased collagen density but not SMC proliferation or wall thickness. Endothelial cell-specific TGF-β signaling disruption decreased both collagen density and SMC proliferation in the AVF wall and was associated with reduced wall thickness, increased outward remodeling, and improved AVF patency.

CONCLUSIONS

Endothelial cell-targeted TGF-β inhibition may be a translational strategy to improve AVF patency.

Impact of extra-anatomical bypass on coarctation fluid dynamics using patient-specific lumped parameter and Lattice Boltzmann modeling

Accurate hemodynamic analysis is not only crucial for successful diagnosis of coarctation of the aorta (COA), but intervention decisions also rely on the hemodynamics assessment in both pre and post intervention states to minimize patient risks. Despite ongoing advances in surgical techniques for COA treatments, the impacts of extra-anatomic bypass grafting, a surgical technique to treat COA, on the aorta are not always benign. Our objective was to investigate the impact of bypass grafting on aortic hemodynamics. We investigated the impact of bypass grafting on aortic hemodynamics using a patient-specific computational-mechanics framework in three patients with COA who underwent bypass grafting. Our results describe that bypass grafting improved some hemodynamic metrics while worsened the others: (1) Doppler pressure gradient improved (decreased) in all patients; (2) Bypass graft did not reduce the flow rate substantially through the COA; (3) Systemic arterial compliance increased in patients #1 and 3 and didn't change (improve) in patient 3; (4) Hypertension got worse in all patients; (5) The flow velocity magnitude improved (reduced) in patient 2 and 3 but did not improve significantly in patient 1; (6) There were elevated velocity magnitude, persistence of vortical flow structure, elevated turbulence characteristics, and elevated wall shear stress at the bypass graft junctions in all patients. We concluded that bypass graft may lead to pseudoaneurysm formation and potential aortic rupture as well as intimal hyperplasia due to the persistent abnormal and irregular aortic hemodynamics in some patients. Moreover, post-intervention, exposures of endothelial cells to high shear stress may lead to arterial remodeling, aneurysm, and rupture.

Extravascular rapamycin film inhibits the endothelial-to-mesenchymal transition through the autophagy pathway to prevent vein graft restenosis

Following vein grafting, the vein must adapt to arterial hemodynamics, which can lead to intimal hyperplasia (IH) and restenosis. Moreover, endothelial-to-mesenchymal transition (EndMT) components are highly associated with IH. Therefore, in this study, we aimed to design an extravascular film loaded with rapamycin (extravascular rapamycin film [ERF]) to limit vein graft stenosis. The film exhibited stable physicochemical properties as well as in vivo and in vitro biocompatibility. In vivo, the film inhibited the EndMT by activating the autophagy pathway. Moreover, rapamycin enhanced this biological effect. Collectively, these findings highlighted the applicability of ERF as a new therapeutic target for preventing vein graft restenosis.

A novel small diameter nanotextile arterial graft is associated with surgical feasibility and safety and increased transmural endothelial ingrowth in pig

Globally, millions of patients are affected by myocardial infarction or lower limb gangrene/amputation due to atherosclerosis. Available surgical treatment based on vein and synthetic grafts provides sub-optimal benefits. We engineered a highly flexible and mechanically robust nanotextile-based vascular graft (NanoGraft) by interweaving nanofibrous threads of poly-L-lactic acid to address the unmet need. The NanoGrafts were rendered impervious with selective fibrin deposition in the micropores by pre-clotting. The pre-clotted NanoGrafts (4 mm diameter) and ePTFE were implanted in a porcine carotid artery replacement model. The fibrin-laden porous milieu facilitated rapid endothelization by the transmural angiogenesis in the NanoGraft. In-vivo patency of NanoGrafts was 100% at 2- and 4-weeks, with no changes over time in lumen size, flow velocities, and minimal foreign-body inflammatory reaction. However, the patency of ePTFE at 2-week was 66% and showed marked infiltration, neointimal thickening, and poor host tissue integration. The study demonstrates the in-vivo feasibility and safety of a thin-layered vascular prosthesis, viz., NanoGraft, and its potential superiority over the commercial ePTFE.

A critical review of surgical strategies to minimise venous stenosis in arteriovenous grafts

It is inevitable that complications arising from surgical procedures are ascribed to surgical technique, and this applies to venous stenosis (VS) in arteriovenous grafts. However, despite a wide range of cellular studies, computer modelling, observational series and clinical trials, there remains uncertainty on whether surgical technique contributes to VS. This article reviews evidence from basic science, fluid dynamics and clinical data to try and rationalise the main surgical options to modify the occurrence of venous stenosis. There is sufficient data from diverse sources to make recommendations on clinical practice (size of target vein, shape of anastomosis, angle of approach, distance from venous needling, trauma to the target vein) whilst at the same time this emphasises the need to carefully report the practical aspects of surgical technique in future clinical trials.

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.

Tissue engineered bovine saphenous vein extracellular matrix scaffolds produced via antigen removal achieve high in vivo patency rates

Diseases of small diameter blood vessels encompass the largest portion of cardiovascular diseases, with over 4.2 million people undergoing autologous vascular grafting every year. However, approximately one third of patients are ineligible for autologous vascular grafting due to lack of suitable donor vasculature. Acellular extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissue have potential to serve as ideal biomaterials for production of off-the-shelf vascular grafts capable of eliminating the need for autologous vessel harvest. A modified antigen removal (AR) tissue process, employing aminosulfabetaine-16 (ASB-16) was used to create off-the-shelf small diameter (< 3 mm) vascular graft from bovine saphenous vein ECM scaffolds with significantly reduced antigenic content, while retaining native vascular ECM protein structure and function. Elimination of native tissue antigen content conferred graft-specific adaptive immune avoidance, while retention of native ECM protein macromolecular structure resulted in pro-regenerative cellular infiltration, ECM turnover and innate immune self-recognition in a rabbit subpannicular model. Finally, retention of the delicate vascular basement membrane protein integrity conferred endothelial cell repopulation and 100% patency rate in a rabbit jugular interposition model, comparable only to Autograft implants. Alternatively, the lack of these important basement membrane proteins in otherwise identical scaffolds yielded a patency rate of only 20%. We conclude that acellular antigen removed bovine saphenous vein ECM scaffolds have potential to serve as ideal off-the-shelf small diameter vascular scaffolds with high in vivo patency rates due to their low antigen content, retained native tissue basement membrane integrity and preserved native ECM structure, composition and functional properties. STATEMENT OF SIGNIFICANCE: The use of autologous vessels for the treatment of small diameter vascular diseases is common practice. However, the use of autologous tissue poses significant complications due to tissue harvest and limited availability. Developing an alternative vessel for use for the treatment of small diameter vessel diseases can potentially increase the success rate of autologous vascular grafting by eliminating complications related to the use of autologous vessel and increased availability. This manuscript demonstrates the potential of non-antigenic extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissue as off-the-shelf vascular grafts for the treatment of small diameter vascular diseases.

Hydraulic expansion facilitates remodeling of arteriovenous fistulas without increasing venous intimal hyperplasia in rabbits

Background

An arteriovenous fistula (AVF) is considered essential for chronic hemodialysis.

Objective

To determine the effects of hydraulic expansion on the intimal hyperplasia of an AVF.

Methods

We divided 12 healthy male New Zealand white rabbits into a control group (vein without special handling and direct anastomosis with an artery, n = 6) and a hydraulic expansion group (vein dilated by hydraulic pressure before anastomosis, n = 6). Histopathomorphology was examined with hematoxylin and eosin staining and immunohistochemistry. Analysis of covariance (ANCOVA) was used to compare the data between the groups.

Results

Immediately and 1 day after surgery, the diameter of the fistula vein in rabbits in the hydraulic expansion group was significantly larger than it was in the control group (P = 0.02 and 0.03 respectively), but not on subsequent days. After hydraulic expansion and before construction of the fistula, the wall of vein was noticeably thinner on macroscopic observation, and the anterior and posterior walls were indistinguishable. At 3 weeks after surgery in the hydraulic expansion group, cells in the vein wall were disordered, there were fewer elastic fibers, tissues from the endothelium to tunica externa were less dense, and there was less extracellular matrix than in the control group. Expression of connective tissue growth factor in the hydraulic expansion group was significantly less than that in the control group (P = 0.01). No differences were found in intimal thickness or immunohistochemistry scores for transforming growth factor-β1 between the groups.

Conclusion

Hydraulic expansion did not increase intimal hyperplasia of an AVF, but facilitates remodeling of AVFs in rabbits.

Tissue-Engineered Vascular Graft with Co-Culture of Smooth Muscle Cells and Human Endothelial Vein Cells on an Electrospun Poly(lactic-co-glycolic acid) Microtube Array Membrane

Coronary artery disease is one of the major diseases that plagues today’s modern society. Conventional treatments utilize synthetic vascular grafts such as Dacron^® and Teflon^® in bypass graft surgery. Despite the wide adaptation, these synthetic grafts are often plagued with weaknesses such as low hemocompatibility, thrombosis, intimal hyperplasia, and risks of graft infection. More importantly, these synthetic grafts are not available at diameters of less than 6 mm. In view of these challenges, we strived to develop and adapt the electrospun Poly Lactic-co-Glycolic Acid (PLGA) Microtube Array Membrane (MTAM) vascular graft for applications smaller than 6 mm in diameter. Homogenously porous PLGA MTAMs were successfully electrospun at 5.5–8.5 kV under ambient conditions. Mechanically, the PLGA MTAMs registered a maximum tensile strength of 5.57 ± 0.85 MPa and Young’s modulus value of 1.134 ± 0.01 MPa; while MTT assay revealed that seven-day Smooth Muscle Cells (SMCs) and Human Umbilical Vein Endothelial Cells (HUVECs) registered a 6 times and 2.4 times higher cell viability when cultured in a co-culture setting in medium containing α-1 haptaglobulin. When rolled into a vascular graft, the PLGA MTAMs registered an overall degradation of 82% after 60 days of cell co-culture. After eight weeks of culturing, immunohistochemistry staining revealed the formation of a monolayer of HUVECs with tight junctions on the surface of the PLGA MTAM, and as for the SMCs housed within the lumens of the PLGA MTAMs, a monolayer with high degree of orientation was observed. The PLGA MTAM registered a burst pressure of 1092.2 ± 175.3 mmHg, which was sufficient for applications such as small diameter blood vessels. Potentially, the PLGA MTAM could be used as a suitable substrate for vascular engineering.

Coating small-diameter ePTFE vascular grafts with tunable poly(diol-co-citrate-co-ascorbate) elastomers to reduce neointimal hyperplasia

Lack of long-term patency has hindered the clinical use of small-diameter prosthetic vascular grafts with the majority of these failures due to the development of neointimal hyperplasia. Previous studies by our laboratory revealed that small-diameter expanded polytetrafluoroethylene (ePTFE) grafts coated with antioxidant elastomers are a promising localized therapy to inhibit neointimal hyperplasia. This work is focused on the development of poly(diol-co-citrate-co-ascorbate) (POCA) elastomers with tunable properties for coating ePTFE vascular grafts. A bioactive POCA elastomer (@20 : 20 : 8, [citrate] : [diol] : [ascorbate]) coating was applied on a 1.5 mm diameter ePTFE vascular graft as the most promising therapeutic candidate for reducing neointimal hyperplasia. Surface ascorbate density on the POCA elastomer was increased to 67.5 ± 7.3 ng mg-1 cm-2. The mechanical, antioxidant, biodegradable, and biocompatible properties of POCA demonstrated desirable performance for in vivo use, inhibiting human aortic smooth muscle cell proliferation, while supporting human aortic endothelial cells. POCA elastomer coating number was adjusted by a modified spin-coating method to prepare small-diameter ePTFE vascular grafts similar to natural vessels. A significant reduction in neointimal hyperplasia was observed after implanting POCA-coated ePTFE vascular grafts in a guinea pig aortic interposition bypass graft model. POCA elastomer thus offers a new avenue that shows promise for use in vascular engineering to improve long-term patency rates by coating small-diameter ePTFE vascular grafts.

Coatings in Decellularized Vascular Scaffolds for the Establishment of a Functional Endothelium: A Scoping Review of Vascular Graft Refinement

Developments in tissue engineering techniques have allowed for the creation of biocompatible, non-immunogenic alternative vascular grafts through the decellularization of existing tissues. With an ever-growing number of patients requiring life-saving vascular bypass grafting surgeries, the production of functional small diameter decellularized vascular scaffolds has never been more important. However, current implementations of small diameter decellularized vascular grafts face numerous clinical challenges attributed to premature graft failure as a consequence of common failure mechanisms such as acute thrombogenesis and intimal hyperplasia resulting from insufficient endothelial coverage on the graft lumen. This review summarizes some of the surface modifying coating agents currently used to improve the re-endothelialization efficiency and endothelial cell persistence in decellularized vascular scaffolds that could be applied in producing a better patency small diameter vascular graft. A comprehensive search yielding 192 publications was conducted in the PubMed, Scopus, Web of Science, and Ovid electronic databases. Careful screening and removal of unrelated publications and duplicate entries resulted in a total of 16 publications, which were discussed in this review. Selected publications demonstrate that the utilization of surface coating agents can induce endothelial cell adhesion, migration, and proliferation therefore leads to increased re-endothelialization efficiency. Unfortunately, the large variance in methodologies complicates comparison of coating effects between studies. Thus far, coating decellularized tissue gave encouraging results. These developments in re-endothelialization could be incorporated in the fabrication of functional, off-the-shelf alternative small diameter vascular scaffolds.

Graphene Oxide Coating Improves the Mechanical and Biological Properties of Decellularized Umbilical Cord Arteries

The lack of small-diameter vascular grafts (inner diameter <5 mm) to substitute autologous grafts in arterial bypass surgeries has a massive impact on the prognosis and progression of cardiovascular diseases, the leading cause of death globally. Decellularized arteries from different sources have been proposed as an alternative, but their poor mechanical performance and high collagen exposure, which promotes platelet and bacteria adhesion, limit their successful application. In this study, these limitations were surpassed for decellularized umbilical cord arteries through the coating of their lumen with graphene oxide (GO). Placental and umbilical cord arteries were decellularized and perfused with a suspension of GO (C/O ratio 2:1) with ∼1.5 μm lateral size. A homogeneous GO coating that completely covered the collagen fibers was obtained for both arteries, with improvement of mechanical properties being achieved for umbilical cord decellularized arteries. GO coating increased the maximum force in 27%, the burst pressure in 29%, the strain in 25%, and the compliance in 10%, compared to umbilical cord decellularized arteries. The achieved theoretical burst pressure (1960 mmHg) and compliance (13.9%/100 mmHg) are similar to the human saphenous vein and mammary artery, respectively, which are used nowadays as the gold standard in coronary and peripheral artery bypass surgeries. Furthermore, and very importantly, coatings with GO did not compromise the endothelial cell adhesion but decreased platelet and bacteria adhesion to decellularized arteries, which will impact on the prevention of thrombosis and infection, until full re-endothetialization is achieved. Overall, our results reveal that GO coating has an effective role in the improvement of decellularized umbilical cord artery performance, which is a huge step toward their application as a small-diameter vascular graft.

Histopathology of Microvascular Anastomosis—Comparison of Patent and Nonpatent Anastomosis: An Experimental Study

Objective To compare the histopathology of patent and nonpatent microvascular anastomosis using rat femoral artery end-to-end anastomosis model.

Materials and Methods In 15 Sprague–Dawley rats, end-to-end anastomosis was performed on the right femoral artery. The classical method was used in four cases and one-way up method in 11 cases. The animals were sacrificed after 2 weeks and the anastomosis was subjected to histopathology. The pathological changes in patent and nonpatent cases were compared.

Results The immediate patency rate and delayed patency (after 2 weeks) rate was 86.7% and 66.7%, respectively. The mean follow-up was 3 months. At sacrifice, 5/15 anastomosis were not patent. Marked subintimal thickening was noted in ⅘ (80%) of the nonpatent group, which was absent in the patent group. Severe loss or fibrosis of tunica media and marked adventitial inflammation were noted in all nonpatent cases (5/5, 100%). As much as ⅘ of the nonpatent cases had poor or indeterminate apposition; in contrast, good apposition was seen in 6/10 (60%) of the patent group. The mean clamp time and mean suturing time were significantly longer in the nonpatent group (69.2 minutes and 53.8 minutes, respectively) as compared with the patent group (48.8 minutes and 31.8 minutes, respectively). A single case that was initially nonpatent was found to have recanalized at 6 months.

Conclusion Minimal intimal injury and reaction, minimal thinning of tunica media, mild-to-moderate adventitial changes, good apposition, and equidistant sutures were associated with a successful microvascular anastomosis. Short duration of vessel clamping time and suturing comes with experience and dedicated practice in a skills laboratory.

Mechanically tuned vascular graft demonstrates rapid endothelialization and integration into the porcine iliac artery wall

Mechanical properties of vascular grafts likely play important roles in healing and tissue regeneration. Healthy arteries are compliant at low pressures but stiffen rapidly with increasing load, ensuring sufficient volumetric expansion without overstretching the vessel. Commercial synthetic vascular grafts are stiff and unable to expand under physiologic loads, which may result in altered hemodynamics, deleterious cellular responses, and compromised clinical performance. The goal of this study was to develop an Elastomeric Nanofibrillar Graft (ENG) with artery-tuned nonlinear compliance and compare its healing responses to conventional expanded polytetrafluoroethylene (ePTFE) grafts in a porcine iliac artery model. Human and porcine iliac arteries were mechanically characterized, and an ENG with similar properties was created by utilizing residual strains within electrospun nanofibers. The ENG was tested for implantation suitability and implanted onto n = 5 domestic swine iliac arteries, with control ePTFE grafts implanted onto the contralateral iliac arteries. After two weeks in vivo, all iliac arteries and grafts remained patent with no signs of thrombosis or dilation. The mechanically tuned ENG implants exhibited a more confluent CD31-positive cell monolayer (1.53 ± 0.73 µm²/mm vs 0.52 ± 0.55 µm²/mm, p = 0.042) on the graft lumenal surface and a higher fraction of αSMA-positive cells (16.2 ± 8.6% vs 1.4 ± 0.7%, p = 0.018) within the graft wall than the ePTFE controls. Despite heavy cellular infiltration, the ENG retained its artery-like mechanical characteristics after two weeks in vivo. These short-term results demonstrate potential advantages of mechanically tuned biomimetic vascular grafts over standard ePTFE grafts. STATEMENT OF SIGNIFICANCE: Off-the-shelf synthetic vascular grafts are often the only option available for treating advanced stages of vascular disease. Despite significant efforts devoted to improving their biochemical characteristics, synthetic peripheral arterial grafts continue to demonstrate poor clinical outcomes leading to costly reinterventions. Here, we hypothesized that a synthetic vascular graft with elastomeric mechanical properties tuned to a healthy peripheral artery promotes better healing responses than a synthetic stiff graft. To test this hypothesis, we developed an Elastomeric Nanofibrillar Graft (ENG) with artery-tuned mechanical properties and compared its performance to a commercial ePTFE graft in a preclinical porcine iliac artery model. Our results suggest that mechanically tuned ENGs can offer better healing responses, potentially leading to better clinical outcomes for peripheral arterial repairs.

Saphenous veins in coronary artery bypass grafting need external support

The saphenous vein is the most commonly used conduit for coronary artery bypass grafting. Arterial grafts are harvested with the outer pedicle intact whereas saphenous veins are harvested with the pedicle removed in the conventional graft harvesting technique. This conventional procedure causes considerable vascular damage. One strategy to improve vein graft patency has been to provide external support. Ongoing studies show that fitting a metal external support improves conventionally harvested saphenous vein graft patency. On the other hand, the no-touch technique of harvesting the saphenous vein provides an improved graft with long-term patency comparable to that of the internal mammary artery. This improvement is suggested to be due to preservation of vessel structures. Interestingly, many of the mechanisms proposed to be associated with the beneficial actions of an artificial external support on saphenous vein graft patency are similar to those underlying the beneficial effect of no-touch saphenous vein grafts where the intact outer layer acts as a natural support. Additional actions of external supports have been advocated, including promotion of angiogenesis, increased production of vascular-protective factors, and protection of endothelial cells. Using no-touch harvesting, normal vascular architecture is maintained, tissue and cell damage is minimized, and factors beneficial for graft patency are preserved. In this review, the significance of external support of saphenous vein grafts in coronary artery bypass grafting is discussed.

Saphenous Vein Vasa Vasorum as a Potential Target for Perivascular Fat-Derived Factors

Perivascular adipose tissue (PVAT) is a source of factors affecting vasomotor tone with the potential to play a role in the performance of saphenous vein (SV) bypass grafts. As these factors have been described as having constrictor or relaxant effects, they may be considered either beneficial or detrimental. The close proximity of PVAT to the adventitia provides an environment whereby adipose tissue-derived factors may affect the vasa vasorum, a microvascular network providing the vessel wall with oxygen and nutrients. Since medial ischaemia promotes aspects of graft occlusion the involvement of the PVAT/vasa vasorum axis in vein graft patency should be considered.

Tapered and non-tapered prosthetic grafts in upper extremity dialysis access: A systematic review and meta-analysis

OBJECTIVE

It is unclear whether tapered arteriovenous grafts (AVGs) are superior to non-tapered AVGs when it comes to preventing upper extremity ischemic steal syndrome. We aimed to evaluate the outcomes of tapered and non-tapered AVGs using systematic review and meta-analysis.

METHODS

A literature search was systemically performed to identify all English publications from 1999 to 2019 that directly compared the outcomes of upper extremity tapered and non-tapered AVGs. Outcomes evaluated were the primary patency at 1-year (number of studies (n) = 4), secondary patency at 1-year (n = 3), and risk of ischemic steal (n = 5) and infection (n = 4). Effect sizes of individual studies were pooled using random-effects model, and between-study variability was assessed using the I² statistic.

RESULTS

Of 5808 studies screened, five studies involving 4397 patients have met the inclusion criteria and included in the analysis. Meta-analyses revealed no significant difference for the risk of ischemic steal syndrome (pooled odds ratio (OR) 0.92, 95% Confidence Incidence (CI) 0.29-2.91, p = 0.89, I² = 48%) between the tapered and non-tapered upper extremity AVG. The primary patency (OR 1.33, 95% CI 0.93-1.90, p = 0.12, I² = 10%) and secondary patency at 1-year (OR 1.49, 95% CI 0.84-2.63, p = 0.17, I² = 13%), and rate of infection (OR 0.62, 95% CI 0.30-1.27, p = 0.19, I² = 29%) were also similar between the tapered and non-tapered AVG.

CONCLUSIONS

The risk of ischemic steal syndrome and patency rate are comparable for upper extremity tapered and non-tapered AVGs. This meta-analysis does not support the routine use of tapered graft over non-tapered graft to prevent ischemic steal syndrome in upper extremity dialysis access. However, due to small number of studies and sample sizes as well as limited stratification of outcomes based on risk factors, future studies should take such limitations into account while designing more robust protocols to elucidate this issue.

The Effects of Pro-Inflammatory and Anti-Inflammatory Agents for the Suppression of Intimal Hyperplasia: An Evidence-Based Review

Anti-atherogenic therapy is crucial in halting the progression of inflammation-induced intimal hyperplasia. The aim of this concise review was to methodically assess the recent findings of the different approaches, mainly on the recruitment of chemokines and/or cytokine and its effects in combating the intimal hyperplasia caused by various risk factors. Pubmed and Scopus databases were searched, followed by article selection based on pre-set inclusion and exclusion criteria. The combination of keywords used were monocyte chemoattractant protein-1 OR MCP-1 OR TNF-alpha OR TNF-α AND hyperplasia OR intimal hyperplasia OR neointimal hyperplasia AND in vitro. These keywords combination was incorporated in the study and had successfully identified 77 articles, with 22 articles were acquired from Pubmed, whereas 55 articles were obtained from Scopus. However, after title screening, only twelve articles meet the requirements of defined inclusion criteria. We classified the data into 4 different approaches, i.e., utilisation of natural product, genetic manipulation and protein inhibition, targeted drugs in clinical setting, and chemokine and cytokines induction. Most of the articles are working on genetic manipulation targeted on specific pathway to inhibit the pro-inflammatory factors expression. We also found that the utilisation of chemokine- and cytokine-related treatments are emerging throughout the years. However, there is no study utilising the combination of approaches that might give a better outcome in combating intimal hyperplasia. Hopefully, this concise review will provide an insight regarding the usage of different novel approaches in halting the progression of intimal hyperplasia, which serves as a key factor for the development of atherosclerosis in cardiovascular disease.

Current understanding of intimal hyperplasia and effect of compliance in synthetic small diameter vascular grafts

Despite much effort, synthetic small diameter vascular grafts still face limited success due to vascular wall thickening known as intimal hyperplasia (IH). Compliance mismatch between graft and native vessels has been proposed to be one of a key mechanical factors of synthetic vascular grafts that could contribute to the formation of IH. While many methods have been developed to determine compliance both in vivo and in vitro, the effects of compliance mismatch still remain uncertain. This review aims to explain the biomechanical factors that are responsible for the formation and development of IH and their relationship with compliance mismatch. Furthermore, this review will address the current methods used to measure compliance both in vitro and in vivo. Lastly, current limitations in understanding the connection between the compliance of vascular grafts and the role it plays in the development and progression of IH will be discussed.

Hemodialysis fistula stenosis caused by extrinsic vascular compression: The May-Thurner of fistulas

Significant venous outflow stenosis of a forearm arteriovenous fistulae caused by an extrinsic vascular compression is a very rare entity (no reported cases in the literature). We report a case of high venous pressures during dialysis, secondary to an extrinsic compression of the underlying brachial artery that was diagnosed with Doppler ultrasound. The stenosis localization resembles the typical and frequent juxta-anastomotic venous outflow stenosis. This similarity can be a confounding factor and can lead to failure of conventional treatments. Careful surgical technique of arteriovenous graft creation and Doppler ultrasound mapping can prevent this entity.

A novel method of vein cuff creation for below-knee femoropopliteal bypass with a prosthetic graft

Vein cuff anastomosis is beneficial in cases in which a prosthetic graft is anastomosed to a small and thickened peripheral artery. Various types of vein cuff are currently in use, although their design is insufficient when a size discrepancy exists between the two vessels and the angle of anastomosis requires adjustment. We report a case of a patient who underwent below-knee femoropopliteal bypass using a new design of vein cuff (boat-form vein cuff) that increases the ease of cuff creation and enables surgeons to adjust the anastomotic size and angle.

Polylysine Enriched Matrices: A Promising Approach for Vascular Grafts

Cardiovascular diseases represent the leading cause of death in developed countries. Modern surgical methods show poor efficiency in the substitution of small-diameter arteries (<6 mm). Due to the difference in mechanical properties between the native artery and the substitute, the behavior of the vessel wall is a major cause of inefficient substitutions. The use of decellularized scaffolds has shown optimal prospects in different applications for regenerative medicine. The purpose of this work was to obtain polylysine-enriched vascular substitutes, derived from decellularized porcine femoral and carotid arteries. Polylysine acts as a matrix cross-linker, increasing the mechanical resistance of the scaffold with respect to decellularized vessels, without altering the native biocompatibility and hemocompatibility properties. The biological characterization showed an excellent biocompatibility, while mechanical tests displayed that the Young's modulus of the polylysine-enriched matrix was comparable to native vessel. Burst pressure test demonstrated strengthening of the polylysine-enriched matrix, which can resist to higher pressures with respect to native vessel. Mechanical analyses also show that polylysine-enriched vessels presented minimal degradation compared to native. Concerning hemocompatibility, the performed analyses show that polylysine-enriched matrices increase coagulation time, with respect to commercial Dacron vascular substitutes. Based on these findings, polylysine-enriched decellularized vessels resulted in a promising approach for vascular substitution.

Evaluation of 1-mm-diameter endothelialized dense collagen tubes in vascular microsurgery

Although much progress has been made in engineering vascular grafts for large- and small-diameter arterial repair or bypass, the extension of these results to the microsurgical size scale has been challenging. Here, we evaluated the use of dense collagen tubes (outer diameter 1 mm, inner diameter 0.5 mm) for vascular microsurgery as interpositional grafts to the femoral artery of Lewis rats. These tubes were formed by dehydrating tubular collagen gels around a mandrel, crosslinking them with genipin, seeding with syngeneic endothelial cells, and culturing before implantation by suture anastomosis. The retention of a confluent endothelial lining inside the tubes after mock surgical handling depended strongly on the crosslinker concentration and culture time. Optimized preparation conditions enabled retention of endothelium after mock surgical handling in ~80% of tubes and maintenance of patency 7 days after implantation in ~40% of grafts. Histological analysis showed the development of granulation tissue and the presence of CD31-positive structures on the inner and outer surfaces of implants. This study provides a proof-of-principle demonstration that endothelialized dense collagen tubes can remain patent for up to 7 days after vascular microsurgery, and points to the importance of mild scaffold crosslinking for maintaining firm endothelial adhesion.

Small Diameter Xenogeneic Extracellular Matrix Scaffolds for Vascular Applications

Currently, despite the success of percutaneous coronary intervention (PCI), coronary artery bypass graft (CABG) remains among the most commonly performed cardiac surgical procedures in the United States. Unfortunately, the use of autologous grafts in CABG presents a major clinical challenge as complications due to autologous vessel harvest and limited vessel availability pose a significant setback in the success rate of CABG surgeries. Acellular extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissues have the potential to overcome these challenges, as they offer unlimited availability and sufficient length to serve as "off-the-shelf" CABGs. Unfortunately, regardless of numerous efforts to produce a fully functional small diameter xenogeneic ECM scaffold, the combination of factors required to overcome all failure mechanisms in a single graft remains elusive. This article covers the major failure mechanisms of current xenogeneic small diameter vessel ECM scaffolds, and reviews the recent advances in the field to overcome these failure mechanisms and ultimately develop a small diameter ECM xenogeneic scaffold for CABG. Impact Statement Currently, the use of autologous vessel in coronary artery bypass graft (CABG) is common practice. However, the use of autologous tissue poses significant complications due to tissue harvest and limited availability. Developing an alternative vessel for use in CABG can potentially increase the success rate of CABG surgery by eliminating complications related to the use of autologous vessel. However, this development has been hindered by an array of failure mechanisms that currently have not been overcome. This article describes the currently identified failure mechanisms of small diameter vascular xenogeneic extracellular matrix scaffolds and reviews current research targeted to overcoming these failure mechanisms toward ensuring long-term graft patency.

Assessing the Disturbed Flow and the Transition to Turbulence in the Arteriovenous Fistula

The arteriovenous fistula (AVF) is the main form of vascular access for hemodialysis patients, but its maintenance is very challenging. Its failure is mainly related to intimal hyperplasia (IH), leading to stenosis. The aim of this work was twofold: (i) to perform a computational study for the comparison of the disturbed blood dynamics in different configurations of AVF and (ii) to assess the amount of transition to turbulence developed by the specific geometric configuration of AVF. For this aim, we reconstructed realistic three-dimensional (3D) geometries of two patients with a side-to-end AVF, performing a parametric study by changing the angle of incidence at the anastomosis. We solved the incompressible Navier–Stokes equations modeling the blood as an incompressible and Newtonian fluid. Large eddy simulations (LES) were considered to capture the transition to turbulence developed at the anastomosis. The values of prescribed boundary conditions are obtained from clinical echo-color Doppler (ECD) measurements. To assess the disturbed flow, we considered hemodynamic quantities such as the velocity field, the pressure distribution, and wall shear stresses (WSS) derived quantities, whereas to quantify the transition to turbulence, we computed the standard deviation of the velocity field among different heartbeats and the turbulent kinetic energy.

Elucidating the role of graft compliance mismatch on intimal hyperplasia using an ex vivo organ culture model

There is a growing clinical need to address high failure rates of small diameter (<6 mm) synthetic vascular grafts. Although there is a strong empirical correlation between low patency rates and low compliance of synthetic grafts, the mechanism by which compliance mismatch leads to intimal hyperplasia is poorly understood. To elucidate this relationship, synthetic vascular grafts were fabricated that varied compliance independent of other graft variables. A computational model was then used to estimate changes in fluid flow and wall shear stress as a function of graft compliance. The effect of compliance on arterial remodeling in an ex vivo organ culture model was then examined to identify early markers of intimal hyperplasia. The computational model prediction of low wall shear stress of low compliance grafts and clinical control correlated well with alterations in arterial smooth muscle cell marker, extracellular matrix, and inflammatory marker staining patterns at the distal anastomoses. Conversely, high compliance grafts displayed minimal changes in fluid flow and arterial remodeling, similar to the sham control. Overall, this work supports the intrinsic link between compliance mismatch and intimal hyperplasia and highlights the utility of this ex vivo organ culture model for rapid screening of small diameter vascular grafts. STATEMENT OF SIGNIFICANCE: We present an ex vivo organ culture model as a means to screen vascular grafts for early markers of intimal hyperplasia, a leading cause of small diameter vascular graft failure. Furthermore, a computational model was used to predict the effect of graft compliance on wall shear stress and then correlate these values to changes in arterial remodeling in the organ culture model. Combined, the ex vivo bioreactor system and computational model provide insight into the mechanistic relationship between graft-arterial compliance mismatch and the onset of intimal hyperplasia.