Three-dimensional bioprinting of volumetric tissues and organs
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oduction Inclusion of live cells in additive manufacturing processes has seen tremendous progress in the last few years. Mammalian cells need to be kept in a soft, aqueous environment when embedded in a biomaterial matrix—this does not allow for the fabrication of structurally well-defined volumetric cell-laden constructs by means of three-dimensional (3D) bioprinting.1 Thus, creating macroscopic objects was found to be more complicated than initially expected. Simple upscaling is not possible due to the mismatch between the mechanical properties needed for cell embedding and manufacturing of 3D objects with high shape fidelity, hence several strategies have been investigated to overcome this problem. Two main technologies can be distinguished—inkjet- and extrusion-based. Figure 1a shows schematic representations of both technologies and the respective terminology. For bioprinting with inkjet-like printers, either single-cell suspensions (not suitable for manufacturing of volumetric structures), cell aggregates or cells encapsulated in hydrogel beads can be utilized as building blocks. This technology allows for achieving high cell densities, and hence it is advantageous for bioprinting of artificial organs in which close cell–cell contacts are crucial for proper function. The disadvantage of utilizing cell aggregates as building blocks is the need to produce large
numbers of cells and assemble them into spherical aggregates; both are cost-intensive and time-consuming procedures. For this type of cell-printing, the term “bioassembly” has been suggested to distinguish it from extrusion-based bioprinting.2 Norotte and co-workers used the bioassembly technology, for example, to create hollow, blood vessel-like morphologies by arranging spherical cell aggregates in 2009.3 Utilizing extrusion-based bioprinting where cells (or small cell aggregates) are typically suspended in (bio)polymer hydrogels has made the manufacturing of volumetric structures easier and less expensive. Since the applicability of biomaterials in extrusion-based 3D printing is actually limited only by their viscosity, a wide range of materials can be utilized. In addition, 3D printers for this technology are already commercially available for less than USD$10,000, and construction kits are available for even less. Combinations of hydrogels and cells are called bioinks. As previously mentioned, after the extrusion process, bioinks must form soft hydrogels to support cell survival and maintenance over longer cultivation periods. In Figure 1b, hydrogel formation from a cell-laden bioink is shown schematically. Low-viscosity materials are not suitable for the manufacturing of large, and still structurally well defined, constructs and novel strategies had to be developed to enable both—successful bioprinting and fabrication of volumetric
David Kilian, Center for Translational Bone, Joint and Soft Tissue Research, Technische Universität Dresden, Germany; [email protected] Tilman Ahlfeld, Center for Translational Bone, Joint and Soft Tissue Resear
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