Building Scaffolds for Tubular Tissue Engineering

Abstract

Hollow organs and tissue systems drive various functions in the body. Many of these hollow or tubular systems, such as vasculature, the intestines, and the trachea, are common targets for tissue engineering, given their relevance to numerous diseases and body functions. As the field of tissue engineering has developed, numerous benchtop models have been produced as platforms for basic science and drug testing. Production of tubular scaffolds for different tissue engineering applications possesses many commonalities, such as the necessity for producing an intact tubular opening and for formation of semi-permeable epithelia or endothelia. As such, the field has converged on a series of manufacturing techniques for producing these structures. In this review, we discuss some of the most common tissue engineered applications within the context of tubular tissues and the methods by which these structures can be produced. We provide an overview of the general structure and anatomy for these tissue systems along with a series of general design criteria for tubular tissue engineering. We categorize methods for manufacturing tubular scaffolds as follows: casting, electrospinning, rolling, 3D printing, and decellularization. We discuss state-of-the-art models within the context of vascular, intestinal, and tracheal tissue engineering. Finally, we conclude with a discussion of the future for these fields.

Keywords: 3D printing; biomaterials; decellularization; electrospinning; intestine; lumen; trachea; vascular.

FIGURE 1

(A) Structure of native tubular tissue systems. Specifically, this schematic highlights the structure of vasculature, the intestines, and the trachea. (B) Typical structure for tissue engineered tubular systems corresponding the native systems in (A).

FIGURE 2

Stresses present for dynamic flow of a fluid through a tube. (A) Pumping a liquid or gaseous medium through a tube will generate a pressure on the tube walls. The highest stress resultant from this pressurization is the hoop stress (σ_h). Any scaffold must possess sufficient strength to compensate for this hoop stress. (B) Fluid movement will also result in a shear stress (τ_s) on the walls of the tube. Generally, these factors (σ_h and τ_s) can be calculated based on the geometry of the tube, i.e., wall thickness (t) and inner radius (r), the pressure (p) on the tube walls, the flow rate of the fluid (v), and the viscosity of the fluid. However, these calculations are complicated by scaffold porosity and the potential for effects of cell growth on the scaffold over the course of an experiment.

FIGURE 3

Example of tubular scaffold for intestinal tissue engineering constructed through the casting process (Chen et al., 2015). (A) Schematic showing scaffold production. Silk is lyophilized in a mold to create a porous scaffold with a central lumen. Cells are seeded into the scaffold and into the interior of the lumen. (B) Image showing scaffold lumen. Scale bar is 4 mm. (C) Immunostain for ZO-1 and cell nuclei (DAPI). Scale bar is 100 μm. (D) Scanning electron microscopic (SEM) image of epithelial lining. Scale bar is 1 μm. (E) Alkaline phosphatase (ALP) staining for ALP enzymatic activity on interior lining of lumen. Scale bar is 200 μm. (F) Confocal z-stack of cells in scaffold immunostained for SM22a, a marker for myofibroblasts. Scale bar is 50 μm. Images were reordered from multiple panels, and lettering has been relabeled for consistency as part of this review article (Chen et al., 2015). These images are reprinted under Creative Commons Attribution 4.0 International License, available at http://creativecommons.org/licenses/by/4.0/.

FIGURE 4

Example of electrospinning process for manufacturing a tubular scaffold for vascular tissue engineering (Lee et al., 2007). (A) Schematic of electrospinning process. (B) Schematic of spinning mandrel for tubular scaffold production. (C) SEM image of bulk scaffold, produced from collagen, elastin, and polymeric mixture, at 18x magnification. (D) SEM image of scaffold from inset in (C) at 500x magnification. (E) Hematoxylin and eosin histological stain of scaffold seeded with smooth muscle cells. Images were reordered from multiple panels, and lettering has been relabeled for consistency as part of this review article (Lee et al., 2007).

FIGURE 5

Example of rolling process for a vascular tissue engineered tubular construct (Cheng et al., 2017). (A–I) Schematic detailing process for producing cell-seeded, stress-induced rolling membrane (SIRM). (A–C) Poly(dimethyl siloxane) (PDMS) substrate is coated with poly(DL-lactide-co-glycolide) (PLGA) and poly(ε-caprolactone) (PCL) through electrospinning (ES) under high voltage (HV). (D–F) Resultant substrate is seeded with endothelial cells (ECs), smooth muscle cells (SMCs), and fibroblasts. (G–I) Scaffold is released from pre-stressed substrate, causing rolling. (J,K) Resultant stratified cell layers in rolled substrate: ECs are shown in green, SMCs are shown in blue, and fibroblasts are shown in magenta. Images were reordered from multiple panels, and lettering has been relabeled for consistency as part of this review article (Cheng et al., 2017).

FIGURE 6

Example of 3D printing process for construction of tissue engineered trachea (Park et al., 2019). (A) Image showing 3D printing of alginate into a cell-compatible hydrogel. (B) Image of 3D printed trachea. Scaffold consisted of 5 layers (innermost to outermost): gridded pattern of poly(capralactone), alginate hydrogel containing primary nasal epithelial cells, cylindrical pattern of poly(capralactone), alginate hydrogel containing primary auricular chondrocytes, gridded pattern of poly(capralactone). (C) SEM image showing lamellar structure of scaffold. (D–F) Fluorescence microscopy images showing stratified cell layers in scaffold cross-section. Epithelial cells are shown in green, and chondrocytes are shown in red. Images were reordered from multiple panels, and lettering has been relabeled for consistency as part of this review article (Park et al., 2019). These images are reprinted under Creative Commons Attribution 4.0 International License, available at http://creativecommons.org/licenses/by/4.0/.

FIGURE 7

Example of engineered decellularized scaffold for vascular engineering (Quint et al., 2011). Scaffolds were produced by seeding smooth muscle cells onto a poly(glycolic acid) mesh. The scaffolds were cultured until the cells produced a contiguous ECM throughout the mesh and the mesh was almost entirely degraded. At this point, the scaffolds were decellularized, resulting in a cell-derived ECM tubular scaffold. Ultimately, scaffolds were re-seeded with endothelial cells and implanted in a porcine model. (A) Image of decellularized tubular scaffold. (B) Hematoxylin and eosin histological staining of cross-section of scaffold. Hematoxylin and eosin staining of scaffold microstructure (C) before decellularization and (D) after decellularization. Scale bars are 500 μm. Images were reordered from multiple panels, and lettering has been relabeled for consistency as part of this review article (Quint et al., 2011).