Figure 1. Schematic venn diagram illustrating the interrelation between Biofabrication, Additive Manufacturing, and the related fields of Tissue Engineering (TE) and Regenerative Medicine (RM). Size of the areas and the overlaps does not represent the scale of the research fields
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With the classical TE approach, cells are seeded onto a prefabricated scaffold, typically in conjunction with the delivery of bioactive factors that ensure maintenance of cellular phenotype and appropriate extracellular matrix formation. This is achieved through in vitro maturation followed by subsequent implantation, with the aim to functionally regenerate tissue . An alternative concept envisions the development of in vitro 3D tissue models, that exhibit functional features of native tissues, for application in an in vitro testing or screening system.
A more recent approach, in line with the strategy of RM, is so-called in situ TE or in situ tissue regeneration, which omits in vitro cell culture and/or in vitro tissue maturation steps. This approach aims instead to design materials and/or exploit the use of chemokines to recruit resident (stem-) cells to the scaffold and modulate the local immune response, inducing regenerative mechanisms in situ . It is within this context that Additive Manufacturing has been, and still is, used to generate biomedical implants and scaffolds for seeding with cells in a classical TE approach.
Figure 2. The term Biofabrication is used to describe natural processes such as biomineralization and technological processes in various disciplines such as catalysis, biotechnology, sensing, synthetic biology, and especially TE and RM. For application within TE and RM, Biofabrication can generally be divided into two distinct approaches, Bioprinting and Bioassembly.
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From a research strategy perspective, Biofabrication within TE and RM aims at exploiting automated processes, for the most part Additive Manufacturing techniques, to generate cell-biomaterial constructs that, through their internal and external spatial arrangement may mature into functional tissue equivalents. Accordingly, these constructs are produced with a structural organization that induces or modulates the host response after implantation through paracrine effects. When living single cells, bioactive molecules, biomaterials, or cell-aggregates small enough to be printed are used for fabrication, the mentioned constructs can be achieved by Bioprinting, which is one of the two main strategies of Biofabrication.
We note that Additive Manufacturing of 3D scaffolds followed by seeding with cells complies with this strategy when the subsequent maturation process yields a structural biologically functional construct. This can for example be achieved by the scaffold instructing or inducing the cells to develop into a tissue mimetic or tissue analogue structure, for example, through distinctive cell interaction, hierarchical induction of differentiation or functional evolution of the manufactured scaffold.
Our strategy for fabricating such constructs is to work with larger pre-formed multicellular fabrication units in the form of cell aggregates, cell fibers, cell sheets or more complex structures, such as organoids or microtissues, comprising cells and their extracellular matrix.
These more complex building blocks are formed through cell-driven self-organization in 3D or lower dimension culture, i.e. through a bottom-up approach, often through the use of enabling technologies, such as microfabricated molds or microfluidics, followed by tissue fusion and maturation.
Thus, we consider the general area of Biofabrication within the context of TE and RM should be distinguished from its use in the area of natural processes, such as biomineralisation and be confined to manufacturing processes as discussed above. Within this technology space, we understand that there are two distinct methodologies that can be distinguished by the length scale of the minimum fabrication unit (pixel or voxel). These are 'Bioprinting' and 'Bioassembly'. For Bioprinting, the minimum fabrication unit is down to molecular level. In the case of Bioassembly, the minimum fabrication units are pre-formed cell containing building blocks with sizes large enough so that automated assembly can technologically be achieved. Figure 2 shows the interrelation between these terms in a schematic diagram. We note that the aim of Biofabrication is to generate a construct with biological function. Hence, in most cases neither Bioprinting nor Bioassembly fully describe the complete Biofabrication process, which is usually dependent on a maturation phase for the constructed pre-tissue assembly to allow it to develop a continuous and coherent functional structure. This maturation process may occur either in vitro with a culture phase—typically adopting the use of bioreactor technology—or conceivably in vivo after transplantation.