Three-dimensional scaffolds for tissue engineering applications: role of porosity and pore size

Abstract

Tissue engineering applications commonly encompass the use of three-dimensional (3D) scaffolds to provide a suitable microenvironment for the incorporation of cells or growth factors to regenerate damaged tissues or organs. These scaffolds serve to mimic the actual in vivo microenvironment where cells interact and behave according to the mechanical cues obtained from the surrounding 3D environment. Hence, the material properties of the scaffolds are vital in determining cellular response and fate. These 3D scaffolds are generally highly porous with interconnected pore networks to facilitate nutrient and oxygen diffusion and waste removal. This review focuses on the various fabrication techniques (e.g., conventional and rapid prototyping methods) that have been employed to fabricate 3D scaffolds of different pore sizes and porosity. The different pore size and porosity measurement methods will also be discussed. Scaffolds with graded porosity have also been studied for their ability to better represent the actual in vivo situation where cells are exposed to layers of different tissues with varying properties. In addition, the ability of pore size and porosity of scaffolds to direct cellular responses and alter the mechanical properties of scaffolds will be reviewed, followed by a look at nature's own scaffold, the extracellular matrix. Overall, the limitations of current scaffold fabrication approaches for tissue engineering applications and some novel and promising alternatives will be highlighted.

FIG. 1.

Images of three-dimensional (3D) (A–C) scaffolds, (D, E) hydrogels, and (F) microcarriers of various geometry, size and morphology used in tissue engineering applications. Color images available online at www.liebertpub.com/teb

FIG. 2.

Scanning electron microscopy (SEM) images of (A, B) porous scaffolds and (C, D) human tissues with interconnected pores. Tissue engineered scaffolds should ideally mimic the porosity, pore size, and function of native human tissues.

FIG. 3.

Schematic illustrations of some conventional fabrication methods: (A) gas foaming/particulate leaching, (B) thermally induced phase separation, and (C) electrospinning used to obtain porous scaffolds. Images adapted and reproduced by permission of Elsevier and The Royal Society of Chemistry (http://dx.doi.org/10.1039/C2JM31290E).^– Color images available online at www.liebertpub.com/teb

FIG. 4.

Schematic illustrations and images of 3D scaffolds fabricated from rapid prototyping methods: (A) 3D printing, (B) selective laser sintering, (C) stereolithography, and (D) fused deposition modeling. Images adapted and reproduced by permission of Elsevier.^– Color images available online at www.liebertpub.com/teb

FIG. 5.

Schematic illustration of cell culture for nonporous hydrogels (NPHs) and superporous hydrogels (SPHs). Cell seeding onto NPHs will result in cells proliferating on the surface, while cell encapsulation entraps cells within the scaffold. For SPHs, cells will proliferate between the pores when they are seeded onto the scaffolds, or entrapped in the interior during the encapsulation process. Color images available online at www.liebertpub.com/teb

FIG. 6.

Schematic illustration of (A) mercury porosimetry, (B) three-step liquid displacement process, (C) SEM imaging technique, and (D) microcomputed tomography imaging used for porosity or pore size measurement of scaffolds. Images adapted and reproduced by permission of Elsevier and IOP Publishing Ltd. (DOI: 10.1088/1758-5082/3/3/034114).^, Color images available online at www.liebertpub.com/teb

FIG. 7.

Effect of (A) pore size and (B) matrix stiffness on MSC behavior. Actin cyoskeleton and nucleus of MSCs were stained with rhodamine–phalloidin (red) and DAPI (blue). MSCs were observed to be flattened and grow on the wall of the scaffold with large pores (>100 μm). For collagen–glycosaminoglycan scaffold (red) scaffolds with small pores (<50 μm), cells may attach in three-dimensions and differentiate (green nuclei) due to smaller forces present (smaller arrows). When cultured on microenvironments with elasticity of elasticity of 0.1, 11, and 34 kPa, MSCs showed to become neuron-like, myocyte-like, and osteoblasts-like respectively. Images adapted and reproduced by permission of Elsevier. Color images available online at www.liebertpub.com/teb