3D bioprinting for engineering complex tissues

Abstract

Bioprinting is a 3D fabrication technology used to precisely dispense cell-laden biomaterials for the construction of complex 3D functional living tissues or artificial organs. While still in its early stages, bioprinting strategies have demonstrated their potential use in regenerative medicine to generate a variety of transplantable tissues, including skin, cartilage, and bone. However, current bioprinting approaches still have technical challenges in terms of high-resolution cell deposition, controlled cell distributions, vascularization, and innervation within complex 3D tissues. While no one-size-fits-all approach to bioprinting has emerged, it remains an on-demand, versatile fabrication technique that may address the growing organ shortage as well as provide a high-throughput method for cell patterning at the micrometer scale for broad biomedical engineering applications. In this review, we introduce the basic principles, materials, integration strategies and applications of bioprinting. We also discuss the recent developments, current challenges and future prospects of 3D bioprinting for engineering complex tissues. Combined with recent advances in human pluripotent stem cell technologies, 3D-bioprinted tissue models could serve as an enabling platform for high-throughput predictive drug screening and more effective regenerative therapies.

Keywords: 3D printing; Bioink; Bioprinting; Drug screening; Hydrogel; Regenerative medicine; Tissue engineering.

Figure 1. Bioprinting process, techniques, and applications

(A) For human therapeutic applications, the typical workflow of bioprinting would involve the isolation and expansion of human cells prior to printing the desired cell-laden scaffold. These scaffolds could then ultimately be used as therapeutic devices themselves, as a testing platform for drug screening and discovery, or as an in vitro model system for disease. (B) Inkjet printers eject small droplets of cells and hydrogel sequentially to build up tissues. (C) Laser bioprinters use a laser to vaporize a region in the donor layer (top) forming a bubble that propels a suspended bioink to fall onto the substrate. (D) Extrusion bioprinters use pneumatics or manual force to continuously extrude a liquid cell-hydrogel solution. (E) Stereolithographic printers use a digital light projector to selectively crosslink bioinks plane-by-plane. In (C) and (E), colored arrows represent a laser pulse or projected light, respectively.

Figure 2. Stereolithographic bioprinting

(A) Schematic illustration of stereolithography system (Gauvin et al. 2012). (B)–(E) The side view of woodpile (B) and hexagonal (D), as well as the top view of woodpile (C) and hexagonal (E) structures generated by their stereolithography system. (F)–(H) 3D confocal images showing the proliferation of encapsulated HUVEC cells in day 1 (F), day 2 (G) and day 4 (H). (Scale bar: 100 μm)

Figure 3. Bioprinting strategies for vascularization

(A) Fabrication of long (> 1 meter) vascular conduits using a coaxial nozzle system yielding internal lumen diameters below 1 mm (Dolati et al. 2014). (B) Pluronic F127 as a sacrificial bioink to form open lumens (red) while concurrently printing encapsulated cells around the vessels (green) (Kolesky et al. 2014). (C) Carbohydrate glass to cast vascular features into a variety of hydrogels, forming perfusable vessels that support cell growth (Miller et al. 2012). Figure adapted from (Miller et al. 2012; Dolati et al. 2014; Kolesky et al. 2014).

Figure 4. Examples of bioprinted tissues and organs

(A) Printed ear-shaped PCL and alginate scaffolds with bioinks localized to certain tissue regions (Lee et al. 2014). (B) Cartilaginous ear scaffolds printed using a novel nanocellulose-alginate bioink supported human chondrocytes (Markstedt et al. 2015). (C) Fabrication of a synthetic nerve graft by printing cell-dense tubes of Schwann cells and BSMC (Owens et al. 2013). (D) Demonstration of the feasibility of printing mouse ganglion and glial cells (Lorber et al. 2014). (E) Printed PEG-based guidance conduits for nerve repair studies, showing their biocompatablity and efficacy (Pateman et al. 2015). Figure adapted from (Owens et al. 2013; Lee et al. 2014; Lorber et al. 2014; Markstedt et al. 2015; Pateman et al. 2015).