Modular Tissue Engineering: Engineering Biological Tissues from the Bottom Up

Abstract

Tissue engineering creates biological tissues that aim to improve the function of diseased or damaged tissues. To enhance the function of engineered tissues there is a need to generate structures that mimic the intricate architecture and complexity of native organs and tissues. With the desire to create more complex tissues with features such as developed and functional microvasculature, cell binding motifs and tissue specific morphology, tissue engineering techniques are beginning to focus on building modular microtissues with repeated functional units. The emerging field known as modular tissue engineering focuses on fabricating tissue building blocks with specific microarchitectural features and using these modular units to engineer biological tissues from the bottom up. In this review we will examine the promise and shortcomings of "bottom-up" approaches to creating engineered biological tissues. Specifically, we will survey the current techniques for controlling cell aggregation, proliferation and extracellular matrix deposition, as well as approaches to generating shape-controlled tissue modules. We will then highlight techniques utilized to create macroscale engineered biological tissues from modular microscale units.

Figure 1

Bottom-up & Top-down approaches to tissue engineering. In the bottom-up approach there are multiple methods for creating modular tissues, which are then assembled into engineered tissues with specific microarchitectural features. In the top-down approach, cells and biomaterial scaffolds are combined and cultured until the cells fill the support structure to create an engineered tissue.

Figure 2

Control of cell aggregation. Patterned stamps are used to create confined regions for cells to aggregate and interact (A, B, C), scale: 25 μm, reproduced with permission from Journal of Cell Science [29]. Use of larger geometric patterns such as connected tori (D) create modular tissues of specific cell and ECM architecture, scale = 400 μm [15]. Copyright 2007 by Fedn of Am Societies for Experimental Bio (FASEB). Reproduced with permission of Fedn of Am Societies for Experimental Bio (FASEB) in the format Journal via Copyright Clearance Center.

Figure 3

Modular cardiac tissue engineering. Native cardiomyocytes are mixed with ECM (collagen or Matrigel) in ring shaped molds. These rings are then assembled together on a cyclic stretching devices to condition the rings and allow them to integrate with the other rings creating an aligned tissue for implantation on the cardiac surface, scale: 10mm [5]. Adapted by permission from Macmillan Publishers Ltd: Nature Medicine, copyright 2006.

Figure 4

Hepatic tissue engineering using additive photopolymerization. Multiple layers are deposited sequentially (red, then green, then blue, A) to create multilayer hepatic tissues (B), scale: 1mm, (C), scale: 500 μm, (D) [25]. Copyright 2007 by Fedn of Am Societies for Experimental Bio (FASEB). Reproduced with permission of Fedn of Am Societies for Experimental Bio (FASEB) in the format Journal via Copyright Clearance Center.

Figure 5

Assembly of modular tissues in perfusable engineered tissues. HepG2 cells are encapsulated in collagen, cast into modular units then coated with a confluent layer of HUVEC cells and packed inside a perfused tube (A). The resultant tissue consists of interconnected cell modules (B–E) which are perfusable and non-coagulative due to the endothelial (HUVEC) cells [20].

Figure 6

Organ printing. The desired pattern is input into the computer attached to the cell printing device (A, D, E), individual cells and ECM are deposited (B) in larger geometrical patterns (C, F, G) [18]. Reprinted from TRENDS in Biotechnology, 21/4, Mironov, V., T. Boland, T. Trusk, G. Forgacs and R.R. Markwald, Organ printing: computer-aided jet-based 3D tissue engineering, Pages 157–61, Copyright (2003), with permission from Elsevier.

Figure 7

Directed assembly of modular tissues. Rectangular cell/hydrogel modules were created directly by photopolymerization using UV through a photomask, then allowed to aggregate and self-assemble in a hydrophobic media (mineral oil), scale: 200μm. After assembly a 2^nd UV polymerization solidified the structures, which varied in size and organization based on the module geometry [24].

Figure 8

Creation of human blood vessel from cell sheet technology. Fibroblast cells are harvested from the patient, expanded into cell sheets, wrapped and cultured around a cylindrical mandrel to create robust, blood vessels (A). After seeding w/harvested endothelial cells, the grafts were tested as AV shunts for dialysis patients (B, C), where they performed well through multiple puncture wounds [23, 62]. Adapted by permission from Macmillan Publishers Ltd: Nature Medicine, copyright 2006.