Vascularization strategies for tissue engineering

Abstract

Tissue engineering is currently limited by the inability to adequately vascularize tissues in vitro or in vivo. Issues of nutrient perfusion and mass transport limitations, especially oxygen diffusion, restrict construct development to smaller than clinically relevant dimensions and limit the ability for in vivo integration. There is much interest in the field as researchers have undertaken a variety of approaches to vascularization, including material functionalization, scaffold design, microfabrication, bioreactor development, endothelial cell seeding, modular assembly, and in vivo systems. Efforts to model and measure oxygen diffusion and consumption within these engineered tissues have sought to quantitatively assess and improve these design strategies. This review assesses the current state of the field by outlining the prevailing approaches taken toward producing vascularized tissues and highlighting their strengths and weaknesses.

FIG. 1.

Schematic diagrams of different vascularization approaches. (A) Scaffold functionalization. Tissue engineering scaffolds may be loaded or chemically coupled with angiogenic factors, including vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and platelet-derived growth factor (PDGF), among others. They may also be designed and engineered to have microchannels or porous microgrooves to improve oxygen/nutrient perfusion or vascular cell alignment, respectively. (B) Cell-based techniques. Multi-cellular spheroid cultures are used to generate capillary-like sprouts when embedded within a biological matrix. Other cell-based techniques include the use of transfected cells to secrete angiogenic factors within a scaffold to induce blood vessel formation. (C) Bioreactor designs. Rotating bioreactors or perfusion bioreactors are used to overcome issues of mass transport in culture, with perfusion bioreactors being particularly useful for forming functional arteries in vitro. (D) Microelectromechanical systems–related approaches. Microfluidic systems are used to form a vascular tree–like organization within a synthetic or biodegradable polymer. These systems can subsequently be seeded with endothelial cells to form a rudimentary vasculature. (E) Modular assembly. Microtissues composed of cell-embedded hydrogels covered with a confluent endothelial cell layer have been combined together to form a macrotissue under perfusion, with the endothelial cell layer acting as an antithrombogenic surface. Vessel-embedded hydrogel systems are used to quantitatively measure nutrient and oxygen permeability before translating that knowledge into forming critically sized multi-vascular modules with one inlet and outlet. (F) In vivo systems. Confluent cell sheets are stacked and vascularized upon implantation adjacent to arteries and veins before building tissue thickness through the addition of more acellular layers for vascularization. Arteriovenous (AV) loops are used to vascularize tissues in vivo within a chamber housing the arteriovenous loop with or without an extracellular matrix (ECM) scaffold or cells. Color images available online at www.liebertonline.com/ten.

FIG. 2.

Basic geometries for modeling oxygen diffusion and consumption. (A) Static cell-seeded scaffold or hydrogel with uni-axial diffusion of oxygen. Oxygen diffusion is characterized by Fick's Law with oxygen gradients within the scaffold at steady state controlled by the concentration of cells and their corresponding metabolic rate, described by Michaelis-Menten kinetics. (B) Perfused cell-seeded scaffold or hydrogel with diffusion of oxygen from multiple boundaries. Oxygen diffusion and consumption may be described using Fick's Law and Michaelis-Menten kinetics, respectively, with the flow of media described by the Navier-Stokes equations. These more complex geometries require careful consideration of boundary conditions, diffusion coefficients, and metabolic rates to accurately model oxygen gradients within the engineered tissue. Color images available online at www.liebertonline.com/ten.