Challenges and Possibilities of Cell-Based Tissue-Engineered Vascular Grafts

Abstract

There is urgent demand for biologically compatible vascular grafts for both adult and pediatric patients. The utility of conventional nonbiodegradable materials is limited because of their thrombogenicity and inability to grow, while autologous vascular grafts involve considerable disadvantages, including the invasive procedures required to obtain these healthy vessels from patients and insufficient availability in patients with systemic atherosclerosis. All of these issues could be overcome by tissue-engineered vascular grafts (TEVGs). A large body of evidence has recently emerged in support of TEVG technologies, introducing diverse cell sources (e.g., somatic cells and stem cells) and novel fabrication methods (e.g., scaffold-guided and self-assembled approaches). Before TEVG can be applied in a clinical setting, however, several aspects of the technology must be improved, such as the feasibility of obtaining cells, their biocompatibility and mechanical properties, and the time needed for fabrication, while the safety of supplemented materials, the patency and nonthrombogenicity of TEVGs, their growth potential, and the long-term influence of implanted TEVGs in the body must be assessed. Although recent advances in TEVG fabrication have yielded promising results, more research is needed to achieve the most feasible methods for generating optimal TEVGs. This article reviews multiple aspects of TEVG fabrication, including mechanical requirements, extracellular matrix components, cell sources, and tissue engineering approaches. The potential of periodic hydrostatic pressurization in the production of scaffold-free TEVGs with optimal elasticity and stiffness is also discussed. In the future, the integration of multiple technologies is expected to enable improved TEVG performance.

Figure 1

Schematic representation of TEVG manufacturing methods. Four representative fabrication methods are depicted. TEVGs fabricated as depicted in the right (green) panels are self-assembled approaches, while TEVGs fabricated as depicted in the left (yellow) panels are scaffold-guided approaches. TEVGs fabricated according to these methods are intended as either large-diameter venous shunts in pediatric patients with congenital heart disease (placement shown in green in human schematic) or small-diameter arteriovenous shunts in adult patients with renal disease (placement shown in yellow). This review introduces the spheroid-based technique as a novel application of 3D bioprinting (lower right panel). Graphical objects were created with BioRender (https://biorender.com/).

Figure 2

A periodic hydrostatic pressurization system. Our experimental system was composed of a pressure chamber containing a cell culture dish, a compressor for collecting 37°C air containing 5% CO₂ from the incubator and transferring it to an air tank, a flow sensor and regulator for controlling the flow of pressurized air into the pressure chamber, and a computer for setting the desired magnitude and frequency of the periodic pressurization. Pressurization to 110-180 kPa at a frequency of 0.002 Hz is illustrated.

Figure 3

Fabrication procedure for SMC sheet containing layered elastic fibers. (a) Vascular SMCs were seeded on a cell culture disk to make the first SMC layer. Beginning 24 hours after seeding, SMCs were exposed to periodic hydrostatic pressure (PHP) for 24 hours. This PHP promoted SMC-derived fibronectin fibrillogenesis (illustrated as green color), which enabled the SMCs of the second layer to attach to the first layer. This procedure was repeated up to ten times to form a ten-layered SMC sheet. AP: atmospheric pressure. (b) A section of our ten-layered SMC sheet was stained with the Elastica van Gieson stain and the Masson trichrome stain to reveal the elastic fibers (deep purple color, upper panel) and collagens (blue color, lower panel), respectively. Scale bars: 100 μm. (c) Our SMC sheet was able to be stretched with a DMT560 tissue puller (Danish Myo Technology, Aarhus N, Denmark). Stretched and unstretched SMC sheets are shown.