Biological mechanisms influencing prosthetic bypass graft patency: possible targets for modern graft design

Current Status of Bioprinting Using Polymer Hydrogels for the Production of Vascular Grafts

Cardiovascular disease is one of the leading causes of death and serious illness in Europe and worldwide. Conventional treatment-replacing the damaged blood vessel with an autologous graft-is not always affordable for the patient, so alternative approaches are being sought. One such approach is patient-specific tissue bioprinting, which allows for precise distribution of cells, material, and biochemical signals. With further developmental support, a functional replacement tissue or vessel can be created. This review provides an overview of the current state of bioprinting for vascular graft manufacturing and summarizes the hydrogels used as bioinks, the material of carriers, and the current methods of fabrication used, especially for vessels smaller than 6 mm, which are the most challenging for cardiovascular replacements. The fabrication methods are divided into several sections-self-supporting grafts based on simple 3D bioprinting and bioprinting of bioinks on scaffolds made of decellularized or nanofibrous material.

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.

A Systematic Review and Meta-Analysis of Heparin-Bonded Expanded Polytetrafluoroethylene Grafts for Below-The-Knee Femoral Bypass Surgery

BACKGROUND

Heparin-bonded expanded polytetrafluoroethylene (hb-ePTFE) synthetic grafts are an alternative to autologous vein grafts (AVG) for surgical bypass interventions in lower limb peripheral arterial disease (LLPAD). However, the clinical benefits of hb-ePTFE grafts have not been reviewed systematically for patients undergoing below-the-knee (BK) surgical bypass. This study aimed to meta-analyze available data on the utility of hb-ePTFE in patients undergoing BK surgical bypass.

METHODS

Medline, Embase, and Cochrane databases were searched, restricted to material in English with no date restriction. In addition, proceedings from relevant congresses were screened going back 2 years. The search was performed in December 2021. Eligible studies included prospective or retrospective comparative studies or prospective single-arm cohorts with an hb-ePTFE arm. Methodological quality was assessed with the ROBINS-I criteria. Outcomes included primary patency, amputation/limb salvage, and overall survival. Clinical outcomes were expressed as event rates. Studies were compared using meta-analysis to generate a standardized mean event rate for each outcome, with its 95% confidence interval (95% CI), using a random-effects model.

RESULTS

Following deduplication, 10,263 records were identified and 261 were assessed as full texts. No prospective comparative studies were identified. The level of evidence was uniformly low. Seventeen publications describing data from 9 individual patient cohorts met the inclusion criteria. These cohorts included a total of 1,452 patients undergoing BK surgical bypass with hb-ePTFE. The primary patency rate was 78.9% [95% CI: 72.2-85.7%] at 1 year, 68.2% [95% CI: 62.8-73.6%] at 2 years, decreasing to 48.0% [95% CI: 27.3-68.7%] at 5 years. The secondary patency rate was 84.8% [95% CI: 77.0-92.5%] at 1 year and 68.9% [95% CI: 43.0-94.9%] at 3 years; the 1-year limb salvage rate was 88.3% [95% CI: 79.6-97.1%] at 1 year and 79.0% [95% CI: 56.7-100%] at 3 years.

CONCLUSIONS

In patients undergoing BK bypass surgery, hb-ePTFE synthetic grafts, compared to uncoated grafts, perform well for patency and limb salvage. However, the quality of the evidence is low, and well-performed randomized clinical trials are needed to inform clinical decision-making on the choice of synthetic graft.

Femoro-popliteal bypass versus remote endarterectomy: a propensity matched analysis

BACKGROUND

The purpose of this study was to compare femoropopliteal bypass (FPB) and remote endarterectomy (RE) for long femoropopliteal lesions.

METHODS

Single center retrospective propensity matching analysis of the symptomatic patients with long occlusion of the femoro-popliteal segment (>250 mm), who underwent femoro-popliteal bypass above the knee or remote endarterectomy from 2014 to 2020. Primary endpoints: primary patency (PP), secondary patency (SP), target lesion revascularization (TLR). Secondary endpoints: MALE, MACE, clinical improvement and survival.

RESULTS

Four hundred patients were divided into two groups: 200 in the FPB group and 200 in the RE group. As a result of propensity score matching, 110 (FPB) and 109 (RE) patients remained. Three-year primary patency rates were 62% for FPB vs. 53% for RE, P=0.16. Secondary patency rates were 84% for FPB vs. 75% for RE, P=0.10. Freedom from TLR were 61% for FPB vs. 71% for RE P=0.21. Survival and amputation-free survival (AFS) also did not differ (93% vs. 94%, P=0.81 and 87% vs. 92%, P=0.19 respectively). Primary patency of the GSV higher than RE (P=0.00) and PTFE (P=0.00). It was established statistically advantages of RE and great saphenous vein (GSV) bypass over a PTFE bypass in SP (P=0.01 P=0.03), TLR (P=0.02 P=0.00) and AFS (P=0.03 P=0.01).

CONCLUSIONS

Surgical treatment of long femoropopliteal occlusions with an autovenous bypass or remote endarterectomy showed significantly better results in secondary patency, TLR and AFS than the use of PTFE prostheses. GSV remains the gold standard for femoropopliteal bypass surgery.

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.

Development, comparative structural analysis, and first in vivo evaluation of acellular implanted highly compacted fibrin tubes for arterial bypass grafting

The generation of small-caliber vascular grafts remains a significant challenge within the field of tissue engineering. In pursuit of this objective, fibrin has emerged as a promising scaffold material. However, its lack of biomechanical strength has limited its utility in the construction of tissue engineered vascular grafts. We have previously reported about the implementation of centrifugal casting molding to generate compacted fibrin tubes with a highly increased biomechanical strength. In this study, we conducted a structural analysis of compacted fibrin tubes using the open-source software Fiji/BoneJ. The primary aim was to validate the hypothesis that the compaction of fibrin leads to a more complex structure characterized by increased crosslinking of fibrin fibers. Structural analysis revealed a strong correlation between fibrin's structure and its biomechanical strength. Moreover, we enhanced fibrin compaction in a subsequent dehydration process, leading to a significant increase of biomechanical strength. Thus, the presented method in combination with an adequate imaging, e.g., micro-CT, has substantial potential as a powerful tool for quality assurance in the development of fibrin-based vascular grafts. To validate this concept, acellular highly compacted fibrin tubes were implanted as substitutes of a segment of the carotid artery in a sheep model (n = 4). After 6 months explanted segments exhibited distinct remodeling, transitioning into newly formed arteries.

Bioadhesive Perivascular Microparticle-Gel Drug Delivery System for Intimal Hyperplasia Prevention: In Vitro Evaluation and Preliminary Biocompatibility Assessment

Intimal hyperplasia (IH) is an undesirable pathology occurring after peripheral or coronary bypass surgery. It involves the proliferation and migration of vascular smooth muscle cells, leading to a reduction in the diameter of the vascular lumen, which can lead to stenosis and graft failure. Topically applied atorvastatin (ATV) has been shown to slow down this process. To be effective, the drug delivery system should remain at the perivascular site for 5-8 weeks, corresponding to the progression of IH, and be capable of releasing an initial dose of the drug followed by a sustained release. Ideally, bioadhesion would anchor the gel to the application site. To meet these needs, we encapsulated ATV in a 2-component system: a hyaluronic acid-dopamine bioadhesive gel for rapid release and biodegradable microparticles for sustained release. The system was characterized by scanning electron microscopy, rheology, bioadhesion on porcine arteries, and a release profile. The rheological properties were adequate for perivascular application, and we demonstrated superior bioadhesion and cohesion compared to the control HA formulations. The release profile showed a burst, generated by free ATV, followed by sustained release over 8 weeks. A preliminary evaluation of subcutaneous biocompatibility in rats showed good tolerance of the gel. These results offer new perspectives on the perivascular application towards an effective solution for the prevention of IH.

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.

Review: Tissue Engineering of Small-Diameter Vascular Grafts and Their In Vivo Evaluation in Large Animals and Humans

To date, a wide range of materials, from synthetic to natural or a mixture of these, has been explored, modified, and examined as small-diameter tissue-engineered vascular grafts (SD-TEVGs) for tissue regeneration either in vitro or in vivo. However, very limited success has been achieved due to mechanical failure, thrombogenicity or intimal hyperplasia, and improvements of the SD-TEVG design are thus required. Here, in vivo studies investigating novel and relative long (10 times of the inner diameter) SD-TEVGs in large animal models and humans are identified and discussed, with emphasis on graft outcome based on model- and graft-related conditions. Only a few types of synthetic polymer-based SD-TEVGs have been evaluated in large-animal models and reflect limited success. However, some polymers, such as polycaprolactone (PCL), show favorable biocompatibility and potential to be further modified and improved in the form of hybrid grafts. Natural polymer- and cell-secreted extracellular matrix (ECM)-based SD-TEVGs tested in large animals still fail due to a weak strength or thrombogenicity. Similarly, native ECM-based SD-TEVGs and in-vitro-developed hybrid SD-TEVGs that contain xenogeneic molecules or matrix seem related to a harmful graft outcome. In contrast, allogeneic native ECM-based SD-TEVGs, in-vitro-developed hybrid SD-TEVGs with allogeneic banked human cells or isolated autologous stem cells, and in-body tissue architecture (IBTA)-based SD-TEVGs seem to be promising for the future, since they are suitable in dimension, mechanical strength, biocompatibility, and availability.

Selection of different endothelialization modes and different seed cells for tissue-engineered vascular graft

Tissue-engineered vascular grafts (TEVGs) have enormous potential for vascular replacement therapy. However, thrombosis and intimal hyperplasia are important problems associated with TEVGs especially small diameter TEVGs (<6 mm) after transplantation. Endothelialization of TEVGs is a key point to prevent thrombosis. Here, we discuss different types of endothelialization and different seed cells of tissue-engineered vascular grafts. Meanwhile, endothelial heterogeneity is also discussed. Based on it, we provide a new perspective for selecting suitable types of endothelialization and suitable seed cells to improve the long-term patency rate of tissue-engineered vascular grafts with different diameters and lengths.

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The material, diameter and length of tissue-engineered vascular graft are all key factors affecting its long-term patency.

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Endothelialization strategies should consider the different diameters and lengths of tissue-engineered vascular grafts.

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Cell heterogeneity and tissue heterogeneity should be considered in the application of seed cells.

Delivery of targeted gene therapies using a hybrid cryogel-coated prosthetic vascular graft

OBJECTIVES

The success of prosthetic vascular grafts in the management of peripheral arterial disease is frequently limited by the development of anastomotic neointimal hyperplasia (ANIH), with the host response to prosthetic grafts beginning soon after implantation. To address this, we combine a platform of polyethylene terephthalate (PET) fabric with an applied cryogel layer containing biologic agents to create a bioactive prosthetic graft system, with the ability to deliver therapeutics targeting modulators of the ANIH-associated transcriptome response, along with antithrombotic agents.

METHODS

Hybrid graft materials were synthesized by cryopolymerization of methacrylated alginate and heparin onto electrospun (ePET), knitted PET (kPET), or woven PET (wPET). Arg-Gly-Asp (RGD) peptides were added to increase cell adhesion. Scanning electron microscopy (SEM) was used to study the microstructure at 1 day, and 2, 4, and 8 weeks. Physical properties such as swelling ratio, pore connectivity, shape recovery, and stiffness were evaluated. Human aortic endothelial cell (HAoEC) adherence was visualized using confocal microscopy after 24 hours and proliferation was evaluated with a resazurin-based assay for 7 days. Confocal microscopy was used to assess delivery of adeno-associated virus (AAV-GFP) after incubation of hybrid grafts with HAoECs. Heparin activity of the materials was measured using an anti-Xa assay.

RESULTS

SEM demonstrated large interconnected pores throughout the entire structure for all graft types, with minimal degradation of the cryogel after 8 weeks. Hybrid materials showed a trend towards increased shape recovery, increased stiffness, decreased swelling ratio, and no difference in pore connectivity. HAoECs incorporated, adhered, and proliferated over 7 days on all materials. HAoECs were successfully transduced with AAV-GFP from the hybrid graft materials. Anti-Xa assay confirmed continued activity of heparin from all materials for over 7 days.

CONCLUSIONS

We have developed a bioactive prosthetic graft system with a cryogel coating capable of delivering biologic agents with antithrombotic activity. By applying the cryogel and selected agents onto PET prior to graft implantation, this study sets the stage for the system to be individualized and tailored to the patient, with bioengineering and targeted gene therapy strategies dovetailing to create an improved prosthetic graft adaptable to emerging knowledge and technologies.

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

Tissue engineered vascular grafts (TEVGs) are beginning to achieve clinical success and hold promise as a source of grafting material when donor grafts are unsuitable or unavailable. Significant technological advances have generated small-diameter TEVGs that are mechanically stable and promote functional remodeling by regenerating host cells. However, developing a biocompatible blood-contacting surface remains a major challenge. The TEVG luminal surface must avoid negative inflammatory responses and thrombogenesis immediately upon implantation and promote endothelialization. The surface has therefore become a primary focus for research and development efforts. The current state of TEVGs is herein reviewed with an emphasis on the blood-contacting surface. General vascular physiology and developmental challenges and strategies are briefly described, followed by an overview of the materials currently employed in TEVGs. The use of biodegradable materials and stem cells requires careful control of graft composition, degradation behavior, and cell recruitment ability to ensure that a physiologically relevant vessel structure is ultimately achieved. The establishment of a stable monolayer of endothelial cells and the quiescence of smooth muscle cells are critical to the maintenance of patency. Several strategies to modify blood-contacting surfaces to resist thrombosis and control cellular recruitment are reviewed, including coatings of biomimetic peptides and heparin.

The Tissue-Engineered Vascular Graft-Past, Present, and Future

Cardiovascular disease is the leading cause of death worldwide, with this trend predicted to continue for the foreseeable future. Common disorders are associated with the stenosis or occlusion of blood vessels. The preferred treatment for the long-term revascularization of occluded vessels is surgery utilizing vascular grafts, such as coronary artery bypass grafting and peripheral artery bypass grafting. Currently, autologous vessels such as the saphenous vein and internal thoracic artery represent the gold standard grafts for small-diameter vessels (<6 mm), outperforming synthetic alternatives. However, these vessels are of limited availability, require invasive harvest, and are often unsuitable for use. To address this, the development of a tissue-engineered vascular graft (TEVG) has been rigorously pursued. This article reviews the current state of the art of TEVGs. The various approaches being explored to generate TEVGs are described, including scaffold-based methods (using synthetic and natural polymers), the use of decellularized natural matrices, and tissue self-assembly processes, with the results of various in vivo studies, including clinical trials, highlighted. A discussion of the key areas for further investigation, including graft cell source, mechanical properties, hemodynamics, integration, and assessment in animal models, is then presented.

Aortic arch redo surgery for endograft migration and premature thrombosis of debranching graft

Background Thoracic endovascular aortic repair (TEVAR) has become the treatment of choice in the management of the aortic arch and descending aorta diseases. Thrombosis is a common reason for vascular graft failure, but there is no consensus on the anticoagulation management after placement of vascular graft.

Case Description A 21-year-old patient with traumatic rupture of aortic isthmus underwent redo open surgery for two successive complications: stent-graft migration and premature debranching prosthesis thrombosis.

Conclusion Open surgery remains an efficient approach when TEVAR is contraindicated or failed. Postoperatively pharmacological prophylaxis against vascular grafts' thrombosis should be emphasized.

Advanced treatment of acute femoropopliteal bypass graft occlusion with Fogarty catheter guidance

PURPOSE

The guiding role of the Fogarty catheter was investigated among patients suffering from limb ischemia due to acute femoropopliteal bypass graft occlusion.

METHODS

A total of 27 patients with a history of femoropopliteal bypass operation who was admitted with acute limb ischemia were enrolled in this retrospective study. In cases in which the Fogarty catheter could not be passed through the popliteal anastomosis, the popliteal region was explored and a new bypass or patch plasty was performed for the distal anastomosis. The cases in which the blood circulation was observed in the graft, but in which the Fogarty catheter balloon was stuck in the native vessels on the proximal and distal side of the graft and the balloon could be withdrawn by deflation, were referred to conventional angiography. The stenosis observed in native vessels was managed by endovascular stent grafting and/or balloon dilatation.

FINDINGS

Graft patency was achieved in all patients. In 11 patients, conventional angiography was implemented following embolectomy. In these patients, all the occlusions found as significant on angiography were removed by percutaneous transluminal angioplasty.

CONCLUSION

Effective use of Fogarty catheter is safe in acute femoropopliteal bypass graft occlusions and in particular, in the planning of further treatment following thrombectomy.

Fabrication of small-diameter vascular scaffolds by heparin-bonded P(LLA-CL) composite nanofibers to improve graft patency

The poor patency rate following small-diameter vascular grafting remains a major hurdle for the widespread clinical application of artificial blood vessels to date. Our previous studies found that electrospun poly(L-lactide-co-epsilon-caprolactone) (P[LLA-CL]) nanofibers facilitated the attachment and growth of endothelial cells (EC), and heparin incorporated into P(LLA-CL) nanofibers was able to release in a controlled manner. Hence, we hypothesized that heparin-bonded P(LLA-CL) vascular scaffolds with autologous EC pre-endothelialization could significantly promote the graft patency rate. To construct a small-diameter vascular scaffold, the inner layer was fabricated by heparin-bonded P(LLA-CL) nanofibers through coaxial electrospinning, while the outer layer was woven by pure P(LLA-CL) nanofibers. Except dynamic compliance (5.4 1.7 versus 12.8 2.4 × 10⁻⁴/mmHg, P < 0.05), maximal tensile strength, burst pressure, and suture retention of the composite, scaffolds were comparable to those of canine femoral arteries. In vitro studies indicated that the scaffolds can continuously release heparin for at least 12 weeks and obtain desirable endothelialization through dynamic incubation, which was confirmed by EC viability and proliferation assay and scanning electronic microscopy. Furthermore, in vivo studies demonstrated that pre-endothelialization by autologous ECs provided a better effect on graft patency rate in comparison with heparin loading, and the united application of pre-endothelialization and heparin loading markedly promoted the 24 weeks patency rate of P(LLA-CL) scaffolds (88.9% versus 12.5% in the control group, P < 0.05) in the canine femoral artery replacement model. These results suggest that heparin-bonded P(LLA-CL) scaffolds have similar biomechanical properties to those of native arteries and possess a multiporous and biocompatible surface to achieve satisfactory endothelialization in vitro. Heparin-bonded P(LLA-CL) scaffolds with autologous EC pre-endothelialization have the potential to be substitutes for natural small-diameter vessels in planned vascular bypass surgery.

Nitric oxide donors for cardiovascular implant applications

In an era of increased cardiovascular disease burden in the ageing population, there is great demand for devices that come in to contact with the blood such as heart valves, stents, and bypass grafts that offer life saving treatments. Nitric oxide (NO) elution from healthy endothelial tissue that lines the vessels maintains haemostasis throughout the vasculature. Surgical devices that release NO are desirable treatment options and N-diazeniumdiolates and S-nitrosothiols are recognized as preferred donor molecules. There is a keen interest to investigate newer methods by which NO donors can be retained within biomaterials so that their release and kinetic profiles can be optimized. A range of polymeric scaffolds incorporating microparticles and nanomaterials are presenting solutions to current challenges, and have been investigated in a range of clinical applications. This review outlines the application of NO donors for cardiovascular therapy using biomaterials that release NO locally to prevent thrombosis and intimal hyperplasia (IH) and enhance endothelialization in the fabrication of next generation cardiovascular device technology.