Printing of Three-Dimensional Tissue Analogs for Regenerative Medicine

Abstract

Three-dimensional (3-D) cell printing, which can accurately deposit cells, biomaterial scaffolds and growth factors in precisely defined spatial patterns to form biomimetic tissue structures, has emerged as a powerful enabling technology to create live tissue and organ structures for drug discovery and tissue engineering applications. Unlike traditional 3-D printing that uses metals, plastics and polymers as the printing materials, cell printing has to be compatible with living cells and biological matrix. It is also required that the printing process preserves the biological functions of the cells and extracellular matrix, and to mimic the cell-matrix architectures and mechanical properties of the native tissues. Therefore, there are significant challenges in order to translate the technologies of traditional 3-D printing to cell printing, and ultimately achieve functional outcomes in the printed tissues. So it is essential to develop new technologies specially designed for cell printing and in-depth basic research in the bioprinted tissues, such as developing novel biomaterials specifically for cell printing applications, understanding the complex cell-matrix remodeling for the desired mechanical properties and functional outcomes, establishing proper vascular perfusion in bioprinted tissues, etc. In recent years, many exciting research progresses have been made in the 3-D cell printing technology and its application in engineering live tissue constructs. This review paper summarized the current development in 3-D cell printing technologies; focus on the outcomes of the live printed tissues and their potential applications in drug discovery and regenerative medicine. Current challenges and limitations are highlighted, and future directions of 3-D cell printing technology are also discussed.

Keywords: 3-D tissue model; Cell printing; Regenerative medicine; Tissue engineering.

Figure 1

Printing mechanisms of major cell printing techniques. A. Micro-extrusion based cell printer uses computer controlled piston or pneumatic pressure to extrude the materials out of a syringe needle. B. Inkjet printer uses several mechanisms (thermal bubble, piezoelectric or electromechanical valve) to create droplets out of liquid solution. C. Laser direct writing uses the energy of the focused laser beam to generate localized heat to form liquid droplet. Adapted from Malda et al.

Figure 2

Applications of cell printing: A. pattern the cell-cell interactions; B. generate cell spheroids to induce cell fusion for organoid culture; C. create 3-D tissue construct by integrating biomaterial hydrogels.

Figure 3

Examples of tissue constructs created by cell printing. (A) 3-D tumor models were created using HeLa Cells printed in gelatin/alginate/fibrinogen hydrogels, shown here is the top view of 3D HeLa/hydrogel constructs on day 0, day 5 and day 8. Scale bar, 5 mm. (B) A porous constructs containing osteogenic and endothelial progenitor cells were printed, shown here is the printed graft: 10×20×1mm; (left) endothelial progenitor cell-laden Matrigel part, (right) multipotent stromal cell-laden Matrigel part with added biphasic calcium phosphate. (C) Bioprinting a cartilage structure, combining inkjet printing with a poly(ethylene glycol) dimethacrylate (PEGDMA) solution containing cells in suspension with a simultaneous photo-polymerization process. (lower panel) Light microscopy image of cell-containing polyethylene hydrogel printed into a defect formed in an osteochondral plug (scale bar, 2 mm). (D) 3-D structure of bioprinted multi-layered skin structure consists of fibroblasts layer and keratinocyte layer, shown here is the volume rendered immunofluorescent images of multi-layered printing of skin. (E) Multi-scale vascular network was created within 3-D hydrogel using cell printing. GFP–HUVECs were embedded within fibrin part for microvascularization. RFP–HUVECs were seeded on the two fluidic channels to form vasculature with mm-scale of lumen size. (F) Bioprinting of heart valve conduit with encapsulation of human aortic interstitial cells within the leaflets: (left) heart valve model designed by Solidworks, (right) printed valve conduit. (G) Multicellular spheroids assembled into tubular structures, shown here is the fused branched construct after 6 days of deposition.