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International Journal of Bioprinting Liver printing: from structure to application

Figure 5. The 3D bioprinting process. The figure was created using BioRender (BioRender.com). Abbreviations: CAD, computer-aided design; CT,
computed tomography; MRI, magnetic resonance imaging.

4.1. Bioinks viscoelasticity of a material is also crucial for protecting
Bioink is fundamental to 3D bioprinting, ideally requiring cells from the effects of high-shear stress. Viscoelasticity is
three key qualities: printability (mechanical properties), a time-dependent mechanical property that combines the
cell viability maintenance, and guidance of cell behavior characteristics of an elastic solid and a viscous liquid, and it
(biological properties). This necessitates balancing is a nearly universal characteristic of the ECM of biological
mechanical and biological performance 104,105 (Figure 6A). tissues. Viscoelastic materials exhibit an instantaneous
Mechanical properties refer to the ability of the printing elastic response to applied stress, store energy, and have a
material to form a stable 3D structure with high structural time-dependent viscous response that dissipates energy.
integrity and fidelity after printing, while also ensuring that Additionally, these materials can also exhibit distinct
the viability and performance of cells are not negatively features, such as stress relaxation (decay of stress under
affected by the bioprinting process, such as the high constant strain) and creep (increased deformation of
shear forces in extrusion-based printing. In extrusion- an object under constant applied stress). 108,109 Rizwan
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based printing, bioink is extruded and deposited onto a et al. designed a viscoelastic hydrogel using hyaluronan
collection platform in a controlled manner in filaments, and laminin via an adjustable stress relaxation rate. This
thus emphasizing the viscosity demands of the bioprinting hydrogel could mimic the stress relaxation rate of the liver
process. Viscosity refers to the degree of stickiness; under physiological conditions and support the growth of
generally, substances with higher viscosity correspond cholangiocyte organoids. 110
to less flow, while those with lower viscosity would flow
more easily. Lower viscosity is required during printing Biological performance generally requires bioinks
to facilitate material extrusion from the printhead, while to have low cytotoxicity and support cell adhesion,
higher viscosity is required post-extrusion to maintain proliferation, and migration, thereby facilitating cell
the stability of the printed structure. Hence, materials functionality. In particular, they also support cell-mediated
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with shear-thinning characteristics (i.e., non-Newtonian matrix remodeling, which is crucial for regulating ECM
behavior where viscosity decreases with increasing shear production and remodeling. The advancement in tissue
rate) are widely used in many 3D bioprinting applications. engineering techniques has redefined the biocompatibility
When subjected to high shear stress, shear-thinning of implanted materials not only to coexist with endogenous
hydrogels exhibit low viscosity and return to their tissues without adverse immune reactions but also to
original viscosity after the shear stress is removed. The actively modulate inflammatory responses, support

Volume 10 Issue 5 (2024) 129 doi: 10.36922/ijb.3819