Die-grown reinforced arterial grafts: observations on long-term animal grafts and clinical experience

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

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

Bioengineered human blood vessels

Since the advent of the vascular anastomosis by Alexis Carrel in the early 20th century, the repair and replacement of blood vessels have been key to treating acute injuries, as well as chronic atherosclerotic disease. Arteries serve diverse mechanical and biological functions, such as conducting blood to tissues, interacting with the coagulation system, and modulating resistance to blood flow. Early approaches for arterial replacement used artificial materials, which were supplanted by polymer fabrics in recent decades. With recent advances in the engineering of connective tissues, including arteries, we are on the cusp of seeing engineered human arteries become mainstays of surgical therapy for vascular disease. Progress in our understanding of physiology, cell biology, and biomanufacturing over the past several decades has made these advances possible.

Progressive Reinvention or Destination Lost? Half a Century of Cardiovascular Tissue Engineering

The concept of tissue engineering evolved long before the phrase was forged, driven by the thromboembolic complications associated with the early total artificial heart programs of the 1960s. Yet more than half a century of dedicated research has not fulfilled the promise of successful broad clinical implementation. A historical account outlines reasons for this scientific impasse. For one, there was a disconnect between distinct eras each characterized by different clinical needs and different advocates. Initiated by the pioneers of cardiac surgery attempting to create neointimas on total artificial hearts, tissue engineering became fashionable when vascular surgeons pursued the endothelialisation of vascular grafts in the late 1970s. A decade later, it were cardiac surgeons again who strived to improve the longevity of tissue heart valves, and lastly, cardiologists entered the fray pursuing myocardial regeneration. Each of these disciplines and eras started with immense enthusiasm but were only remotely aware of the preceding efforts. Over the decades, the growing complexity of cellular and molecular biology as well as polymer sciences have led to surgeons gradually being replaced by scientists as the champions of tissue engineering. Together with a widening chasm between clinical purpose, human pathobiology and laboratory-based solutions, clinical implementation increasingly faded away as the singular endpoint of all strategies. Moreover, a loss of insight into the healing of cardiovascular prostheses in humans resulted in the acceptance of misleading animal models compromising the translation from laboratory to clinical reality. This was most evident in vascular graft healing, where the two main impediments to the in-situ generation of functional tissue in humans remained unheeded–the trans-anastomotic outgrowth stoppage of endothelium and the build-up of an impenetrable surface thrombus. To overcome this dead-lock, research focus needs to shift from a biologically possible tissue regeneration response to one that is feasible at the intended site and in the intended host environment of patients. Equipped with an impressive toolbox of modern biomaterials and deep insight into cues for facilitated healing, reconnecting to the “user needs” of patients would bring one of the most exciting concepts of cardiovascular medicine closer to clinical reality.

Biomaterial-driven in situ cardiovascular tissue engineering-a multi-disciplinary perspective

There is a persistent and growing clinical need for readily-available substitutes for heart valves and small-diameter blood vessels. In situ tissue engineering is emerging as a disruptive new technology, providing ready-to-use biodegradable, cell-free constructs which are designed to induce regeneration upon implantation, directly in the functional site. The induced regenerative process hinges around the host response to the implanted biomaterial and the interplay between immune cells, stem/progenitor cell and tissue cells in the microenvironment provided by the scaffold in the hemodynamic environment. Recapitulating the complex tissue microstructure and function of cardiovascular tissues is a highly challenging target. Therein the scaffold plays an instructive role, providing the microenvironment that attracts and harbors host cells, modulating the inflammatory response, and acting as a temporal roadmap for new tissue to be formed. Moreover, the biomechanical loads imposed by the hemodynamic environment play a pivotal role. Here, we provide a multidisciplinary view on in situ cardiovascular tissue engineering using synthetic scaffolds; starting from the state-of-the art, the principles of the biomaterial-driven host response and wound healing and the cellular players involved, toward the impact of the biomechanical, physical, and biochemical microenvironmental cues that are given by the scaffold design. To conclude, we pinpoint and further address the main current challenges for in situ cardiovascular regeneration, namely the achievement of tissue homeostasis, the development of predictive models for long-term performances of the implanted grafts, and the necessity for stratification for successful clinical translation.

Bovine carotid artery (Artegraft) as a hemodialysis access conduit in patients who are poor candidates for native arteriovenous fistulae

Our experience with bovine carotid artery graft (BCAG) for hemodialysis access (Artegraft, North Brunswick, New Jersey) is presented. A review of all patients who underwent placement of BCAG for hemodialysis access at our institution was performed. Between January 2012 and June 2013, 17 BCAGs were placed in 17 patients. Indications included skin compromise, recurrent expanded polytetrafluoroethylene (ePTFE) and catheter infections, immunosuppression, groin placement, and surgeon's choice. Actuarial primary, primary-assisted, and secondary patency rates at 18 months were 73.3%, 67%, and 89%, respectively. One immunosuppressed patient developed a vancomycin-resistant enterococcus graft infection and required removal 2 months following the initial procedure. We conclude that BCAG can be used as an alternative to ePTFE for angioaccess in patients with no available superficial vein in high-risk patients with low morbidity and good functional patency. Our 1-year patency rates were superior to ePTFE as reported in the contemporary peer-reviewed literature.

History of vascular access for haemodialysis

The history of vascular access is a history of vascular surgery as well as a history of dialysis therapy. This survey is a personal view on the history of vascular access without the ambition to cover every detail, but with an effort to mention the major steps in a fascinating panorama.

Advances in vascular tissue engineering

Coronary and peripheral artery bypass grafting is commonly used to relieve the symptoms of vascular deficiencies, but the supply of autologous artery or vein may not be sufficient or suitable for multiple bypass or repeat procedures, necessitating the use of other materials. Synthetic materials are suitable for large bore arteries but often thrombose when used in smaller arteries. Suitable replacement grafts must have appropriate characteristics, including resistance to infection, low immunogenicity and good biocompatability and thromboresistance, with appropriate mechanical and physiological properties and cheap and fast manufacture. Current avenues of graft development include coating synthetic grafts with either biological chemicals or cells with anticoagulatory properties. Matrix templates or acellular tubes of extracellular matrix (such as collagen) may be coated or infiltrated with cultured cells. Once placed into the artery, these grafts may become colonised by host cells and gain many of the properties of normal artery. "Tissue-engineered blood vessels" may also be formed from layers of human vascular cells grown in culture. These engineered vessels have many of the characteristics of arteries formed in vivo. "Artificial arteries" may be also be derived from peritoneal granulation tissue in body "bioreactors" by adapting the body's natural wound healing response to produce a hollow tube.

Enhanced in vitro fibrinolytic activity of immobilized plasmin on collagen beads

Plasmin was immobilized on collagenous substrates using carbodiimide as a linking agent. The kinetics of soluble and immobilized plasmin were monitored by reacting them with the chromogenic substrate S-2251 (H-D-Val-Leu-Lys-pNA) in the presence and absence of a2-antiplasmin (a2-PI). The ability of immobilized plasmin to lyse synthetic clots formed from fibrinogen and thrombin was determined by detecting the formation of fibrin degradation products (FDP). The activity of immobilized plasmin was 0.02 casein units (CU)/mg of collagen. The kinetic analysis of soluble and immobilized plasmin in the presence and absence of a2-PI shows that while soluble plasmin activity was inhibited by the presence of a2-PI, the plasmin inhibitor did not interfere with the ability of immobilized plasmin to attack fibrin. In the absence of a2-PI, the ability of the immobilized plasmin to lyse synthetic clots was the same as that of soluble plasmin. In the presence of a2-PI, immobilized plasmin produced twice the amount of FDP as did soluble plasmin. The immobilized plasmin activity was stable for a period of at least 3 months.

Peritoneovenous shunt occlusion. Etiology, diagnosis, therapy

Electronic pressure testing of every LeVeen valve has practically eliminated mechanical malfunction as a cause of shunt failure. Nonetheless, failures do occur and in a series of 240 cases, early or late shunt failure occurred in 29 patients. Thirty-five additional cases of failures were either referred by other physicians over a period of 6 years or information and x-rays were accumulated by direct contact. Shunt failure becomes manifest by a sudden reaccumulation of ascites in patients with a previously functioning shunt. In immediate failure, the ascites may fail to disappear after surgery or reaccumulate if removed. Ideally, caval clotting should be first excluded by x-ray visualization of the superior vena prior to injection of the shunt with contrast agent. Shuntograms are done with fine-bore needles. The venous pressure is also measured. The entry of contrast into the vena cava without pooling indicates a patent venous limb. The contrast will empty from the venous tubing with forceful inspiration if the entire system is patent. The venous tube will not clear if the valve or peritoneal collecting tubes are blocked. Only the valve and collecting tube need then be replaced if contrast enters the cava but does not leave the venous tubing. Occluded valves must not be flushed to restore patency since inflammatory exudate and cellular debris are erroneously identified as "fibrin flecks." Histology and culture are mandatory. Immediate and early failure are often caused by malposition of the venous tubing. Malplacements can often be diagnosed simply by chest x-rays. Intraoperative injection of methylene blue into the venous tubing establishes a satisfactory washout prior to wound closure. Fresh clots in the vena cava can be dissolved by the slow injection of streptokinase into the venous tubing. Other patent veins are chosen for access. Patients having repeat surgery after clotting must be heparinized to prevent a similar recurrence. Flushing blood clots from the cava can be fatal.

Clinical experience with expanded polytetrafluoroethylene (Gore-Tex) grafts for femoropopliteal arterial bypass

Thirty-seven grafts of expanded polytetrafluoroethylene were implanted in 28 patients in whom autogeneous saphenous vein was not available, either for symptoms of severe claudication or limb salvage. The length of follow-up ranges from 8 to 28 months . The patency rate is 86.9 percent for the patients with severe claudication and 71.4 percent in the limb salvage group; the overall patency rate is 81 percent. We believe that expanded polytetrafluoroethylene is a good prosthetic substitute when autogenous vein is unavailable.

Can a small blood vessel prosthesis be derived from heterologous foreign body reactive tissue?

This study investigated as a small diameter vascular replacement the tissue formed adjacent to an implanted cylindrical foreign body, heterologously transplanted. Grafts of 4 mm i.d. were grown in rabbits and transplanted to dogs as segmental carotid and femoral artery replacements. These maintained 50% patency after 3 weeks. Variables in the further development of this concept are discussed.

Comparison of long-term results of 364 femoropopliteal or femorotibial bypasses for revascularization of severely ischemic lower extremities

Successful revascularization of the severely ischemic lower extremity can be achieved by femorotibial as well as femoropopliteal bypass. The incidence of delayed graft occlusion after salvage of the severely ischemic lower extremity is low in patients with femorotibial or femoropopliteal bypass. Femorotibial bypass was performed in over one-third of patients undergoing bypass. Tibial bypasses resulted in effective prolonged revascularization of the severely ischemic lower extremity. An aggressive diagnostic and therapeutic approach to revascularization of the severely ischemic lower extremity can result in prolonged limb salvage by tibial or popliteal bypasses in lieu of primary amputation.

A small arterial substitute: expanded microporous polytetrafluoroethylene: patency versus porosity

Eighty-nine grafts of expanded microporous polytetrafluoroethylene (PTFE) with a diameter of 4 mm, were placed in the carotid and femoral arteries of dogs. The animals were sacrificed at varying intervals beginning three days after operation. Four animals remain alive with patent grafts 10 months post-operatively. Twenty-four of 89 grafts were occluded, an overall patency of 73%. Fibril length (pore size) of the graft material was varied from 4 to 110 microns. Average pore size ranged from 9 to 65 microns. Wall thickness varied from 0.3 to 0.75 mm. Density ranged from 0.24 to 0.35 g/ml. Tissue ingrowth, neointimization and patency rate as compared to pore size, wall-thickness and density of expanded PTFE were observed. Pore size is the primary determinant of tissue ingrowth, neointimization and patency. Of 51 grafts with an average pore size of 22 microns or less, there were 6 occlusions, an 88% patency rate. There were 38 grafts with an average pore size of 34 microns or greater. In these 38 grafts, 18 occlusions were observed, a 53% patency rate. Patent grafts demonstrated tissue ingrowth, capillary formation an a thin neointima. Using small pore grafts, patency rates of 90% can be anticipated in the dog. Expanded microporous PTFE with its ease of handling, strength and pliability may be the vascular prosthesis of choice in man.

Silicone mandril method for growing reinforced autogenous femoro-popliteal artery grafts in situ

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