Vascularization in Bioartificial Parenchymal Tissue: Bioink and Bioprinting Strategies

Abstract

Among advanced therapy medicinal products, tissue-engineered products have the potential to address the current critical shortage of donor organs and provide future alternative options in organ replacement therapy. The clinically available tissue-engineered products comprise bradytrophic tissue such as skin, cornea, and cartilage. A sufficient macro- and microvascular network to support the viability and function of effector cells has been identified as one of the main challenges in developing bioartificial parenchymal tissue. Three-dimensional bioprinting is an emerging technology that might overcome this challenge by precise spatial bioink deposition for the generation of a predefined architecture. Bioinks are printing substrates that may contain cells, matrix compounds, and signaling molecules within support materials such as hydrogels. Bioinks can provide cues to promote vascularization, including proangiogenic signaling molecules and cocultured cells. Both of these strategies are reported to enhance vascularization. We review pre-, intra-, and postprinting strategies such as bioink composition, bioprinting platforms, and material deposition strategies for building vascularized tissue. In addition, bioconvergence approaches such as computer simulation and artificial intelligence can support current experimental designs. Imaging-derived vascular trees can serve as blueprints. While acknowledging that a lack of structured evidence inhibits further meta-analysis, this review discusses an end-to-end process for the fabrication of vascularized, parenchymal tissue.

Keywords: additive manufacturing; bioartificial organs; bioink; biomaterial; bioprinting; regenerative medicine; tissue engineering; vascularization.

Figure 1

Overview of bioink elements and common bioprinting platforms. (a) Bioink composition. The main elements of bioinks are cells, support materials—mostly hydrogels—signaling molecules, and matrix compounds. (b) Jet-based drop-on-demand bioprinting enables droplet-based bioink deposition. Droplet formation and deposition can be induced, for example, thermally induced air pressure or piezoelectric pressure. (c) Microextrusion bioprinting is based on bioink filament extrusion to fabricate constructs in a layer-by-layer fashion. Bioink extrusion is facilitated by pneumatic or mechanical forces (piston or screw system). (d) Laser-assisted bioprinting enables bioink deposition on a receiving substrate layer by pulsed-laser-induced forward transfer (arrow) via an energy-absorbing layer.

Figure 2

Overview of hierarchical vascular tree and vascular development mechanisms. (a) Partial representation of arterial vascular tree with dichotomous branching. Arteries and their smaller downstream branches, the arterioles, present a layered wall structure. Generally, the media containing smooth muscle cells is more pronounced in arterial vessels. Via the arterioles, blood is transported into the capillary bed, capable of gas and nutrient exchange. Capillaries consist of a single endothelial layer enabling permeability. (b) The mechanism of vasculogenesis describes the formation of a primitive vascular network by endothelial progenitor cells, usually during embryogenesis. (c) Angiogenesis describes a growth of vessels from the existing vasculature, e.g., by sprouting. Angiogenesis can be a physiological or pathological process.

Figure 3

Bioprinting strategies to fabricate vascularization. (a) Embedded bioprinting into a (functional) viscoplastic support bath. Viscoplastic microsphere fluids present self-healing properties with fluid-like regions and shear-thinning effects directly along the moving microextrusion nozzle. Direct printing and subsequent in situ crosslinking of vascular bioink enables fabrication of free-form structures with high shape fidelity. Crosslinking can be initiated by the support bath, e.g., chemically. The microextrusion and subsequent dissolution of sacrificial bioinks after previous curing of the functional support bath can create hollow channel networks. Such networks can be perfused with endothelial cell solution for cell adherence and formation of an endothelial lining. (b) Coaxial microextrusion bioprinting using concentric nozzles enables simultaneous printing with different bioinks with a core–shell or layered cross-section. (c) Multinozzle bioprinting technology with fast high-frequency switching can fabricate continuous filaments from multiple materials. (d) Self-assembly of capillaries from bioprinted vessel-like structures. The cultivation of bioprinted tissue constructs can provide an environment that further enhances vascularization by self-assembly. Therefore, relevant angiogenic signals might need to be integrated in bioinks before printing.

Figure 4

Bioprinting technologies categorized regarding their applicability for fabrication of macro- or microvascularization. Limited printability can be due to lack of resolution, low printing precision, and/or a long duration of the printing procedure. Small-scale capillaries cannot yet be fabricated by 3D-bioprinting technologies and are subject to self-assembly strategies.

Figure 5

End-to-end manufacturing concept based on imaging data. Discussion of a concept for 3D bioprinting of vascularized parenchymal tissue. Process model for fabrication of vascularized parenchymal tissue potentially integrating several printing technologies in one fabrication process (business process model and notation [BPMN] 2.0).

Figure 6

Decision model for bioink development. Evidence-driven or rather structured data-driven decision model and translation to an end-to-end process by decision model notation (DMN) for development of bioink for vascularization.