Multimaterial, heterogeneous, and multicellular three-dimensional bioprinting
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Introduction In the context of tissue engineering and regenerative medicine (TERM), bioprinting technologies emerged as an enabling tool for creating three-dimensional (3D) functional tissue constructs with tailored biological and mechanical properties (see the Introduction article in this issue).1,2 Bioprinting has been defined as “the use of computer-aided transfer processes for patterning and assembling living and non-living materials with a prescribed two-dimensional or 3D organization in order to produce bioengineered structures serving in TERM, pharmacokinetic, and basic cell-biology studies.”3 Together with bioassembly, bioprinting is one of the two main approaches of biofabrication in TERM.4 Different technologies have been proposed for patterning, depositing, and 3D shaping bioinks, which can be grouped into (micro)extrusion,5 inkjet,6 and photopolymerization.7 The simultaneous printing of biomaterials and cells allowed the achievement of several milestones, such as increased seeding efficiency and avoidance of nonhomogeneous cell distribution due to postfabrication seeding.8 Extrusion-based technologies are currently targeted as promising for building clinically relevant constructs.9 Although they have limited resolution, several companies are investing in this growing market,10 mainly proposing extrusion-based bioprinters with related bioinks.11
However, recapitulating in vitro the 3D multiscale microarchitecture with multiple cell types as well as the extracellular matrix (ECM) physicochemical cues of living tissues have remained unsolved challenges.12 The diffusion of nutrients and waste products above a distance of 150–200 μm is no longer efficient, compromising cellular viability and function in a short time.13 Growth factors are specifically located in the ECM to guide tissue development,14 and different types of cells are in close contact and in continuous cross-talking.15 Bioprinting a single cell type together with a single biomaterial cannot, therefore, bring further advancement. This article describes recent advances in controlling spatial heterogeneity of chemical and physical properties of scaffolds using bioprinting technologies. We also discuss how such bioprinted artificial tissues have a prescribed cellular composition and spatial arrangement. As different bioprinting technologies have been reviewed elsewhere,9–11,16 we focus on the advantages and constraints for multimaterial processing, by analyzing attempts to merge different bioprinting technologies.
General considerations for multicellular and multimaterial bioprinting Regardless of the chosen technology and the well-established issues regarding pores (i.e., pore size and shape, interconnectivity, and total porosity) and mechanical properties that
Carmelo De Maria, Department of Ingegneria dell’Informazione and Research Center E. Piaggio, University of Pisa, Italy; [email protected] Giovanni Vozzi, Department of Ingegneria dell’Informazione and Research Center E. Piaggio, University of Pisa, Italy; g.vozzi@ing
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