Bioengineered Vascular Grafts

Initial History

The origin of vascular surgery is frequently traced back to the development of the triangulated vascular anastomosis in the early 20th century by Alexis Carrel, a French surgeon who received the Nobel Prize in Physiology or Medicine in 1912 for this advancement. Carrel would later partner with aviator and engineer, Charles Lindberg, to develop perfusion pumps and incubators that would allow maintaining of organs for prolonged periods of time, paving the way for organ culture that would anticipate many modern approaches in tissue engineering. Although venous interposition grafts had been used for trauma and aneurysmal disease, it was not until 1948 that saphenous vein was used successfully as a bypass graft for atherosclerotic vascular disease. In the 1950s Michael DeBakey, Denton Cooley, and colleagues reported the use of Dacron grafts for aortic reconstruction. Shortly thereafter, in the 1960s, the field of nephrology was revolutionized when Michael Brescia and James Cimino developed the radiocephalic arteriovenous fistula as an alternative to the Scribner shunt. ^, By the 1970s, the earlier pioneering investigations into the use of synthetic vascular conduits progressed to using polytetrafluoroethylene (PTFE) and expanded PTFE (ePTFE) as replacements for small arteries and as hemodialysis access. ^,

Somewhat surprisingly, the first investigations into the use of bioengineered blood vessels were conducted in the 1940s when Robert Gross and colleagues experimented with aortic transplantation in dogs. This work demonstrated that, although preservation of graft vessels by freezing alone carried unacceptable rates of thrombosis and hemorrhage, biological modification of donor aortas by storage in a nutrient solution at temperatures slightly above freezing resulted in long-term patency even after several weeks of extracorporeal storage. Those experiments led to the successful translation of these techniques to human tissue. Cadaveric human arteries were successfully treated and used both for aortic reconstruction in pediatric patients with aortic coarctation and as shunt material in patients with tetralogy of Fallot. This type of tissue preservation strategy, although successful, was quite volatile and required significant improvement in order to produce a therapy that was reliable and readily available.

Intact Isolated Blood Vessels

Gross’ work spurred the earliest examples of bioengineered blood vessels: intact blood vessels isolated from either animals (xenogeneic grafts) or later from humans (allogeneic grafts). While intact isolated blood vessels come from donor cells, they can be considered bioengineered grafts because they undergo an extensive process to biologically modify the tissue so that it can be implanted without producing an immune-mediated response. A common way to prepare xenogeneic grafts for use is by treating the tissue with a chemical decellularization process and then fixation of the remaining collagen, connective tissue proteins, and cells with glutaraldehyde to reduce antigenicity. This technique fixes or “tans” the tissue, cross-linking proteins to prevent an immunogenic reaction. Unfortunately, variations in xenogeneic graft fixation and inability of the chemically cross-linked vascular matrix to become repopulated with host cells ^, introduce the potential for aneurismal degradation over time.

The use of isolated intact blood vessels, first reported in the 1960s, persists in several grafts commercially available today.

Luminal Modification (Biohybrids)

The endothelium is a dynamic environment, not simply an inert blood-contacting surface. The endothelium, and particularly vascular endothelial cells (ECs), play a central role in regulating cardiovascular physiology. ECs regulate vascular tone and thus modulate hemodynamics. In addition to providing an antithrombotic surface for blood flow under normal conditions, ECs control platelet activation and adhesion as well as the adhesion and migration of both leukocytes and vascular smooth muscle cells in response to pathological stimuli. The benefits of an intact and functional endothelium are clear, leading several investigators to develop “biohybrid” grafts by modifying the luminal surfaces of conduits in order to recreate the physiologic function of the native endothelium.

The concept of transplanting autogenous ECs into grafts emerged in the late 1970s. This can be done as either a single stage or two stage method. In one stage methods, ECs are both harvested and seeded at the time of surgery. Two stage methods require seeding of the recipient graft with ECs via serial passaging in a bioreactor and incubation for several weeks prior to implant. In the 1990s, Zilla, Deutsch and colleagues in Europe and South Africa developed means of seeding autologous endothelium into the lumen of ePTFE conduits, where ECs derived from small vein biopsies were expanded and cultured on ePTFE grafts over approximately 4 weeks. These endothelialized grafts were then implanted as infrainguinal femoropopliteal grafts in more than 300 patients with severe peripheral arterial disease. Primary and secondary patency rates were successfully comparable to those of native saphenous vein. However, the need for harvesting suitable numbers of autologous ECs from vein segments, and challenges with long-term EC adherence to artificial surfaces, has prevented widespread adoption of this technology.

Later investigators were able to improve cell retention as well as cell growth structure and adaption, by exposing the EC-treated conduit in vitro to hydrostatic pressure, laminar shear stresses, and circumferential and longitudinal stretching. ^, Indeed, some suggest that shear stresses from prolonged laminar flow (as opposed to turbulent flow) play a crucial role in the signal transduction, gene expression, cell proliferation, and cell survival of both ECs and vascular smooth muscle cells (VSMC). This is hypothesized to reduce the incidence of both thrombotic complications and the development of atherosclerotic lesions in vivo .

Tissue-Engineered Vascular Grafts

Tissue-engineered vascular grafts (TEVGs) are cellular or tissue-based vascular conduits designed to have any synthetic components degrade over time with progressive replacement by autogenous tissues resulting in a functional blood vessel with an intact endothelium and a capacity to respond appropriately to physiologic stimuli. This technology has been made possible only recently by groundbreaking advances in the field of tissue engineering, although it truly represents the extension of half a century of continuous exploration and improvement.

TEVGs may be classified by the method used to construct the conduit. At present there are three main categories of TEVGs: grafts created in vivo by “bioreactors,” scaffold-based TEVGs, and sheet-based TEVGs.