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International Journal of Bioprinting 3D bone: Current & future

specific cells can be incorporated into a particular location. 4.1.1. Material extrusion-based bioprinting
Therefore, 3D-bioprinted tissues are commonly used in Material extrusion-based bioprinting is mainly performed
in vitro testing and clinical implantations. Nonetheless, with either fused deposition modeling (FDM) or direct
the technology faces limitations and challenges, such as ink writing (DIW) (Figure 8). With FDM, thermoplastic
the intricacies of printing vascularized tissue, selecting materials can be printed for daily and industrial purposes,
and engineering suitable hydrogels, and maintaining cell but this technique is used for fast prototyping and
viability while printing larger tissue constructs. To achieve bioprinting hard scaffolds. In contrast, bioinks are printed
the most biologically similar artificial tissue, the initial with DIW printers, which apply pneumatic (compressed
stages of the 3D bioprinting process focus on precisely air flow) or mechanical (axial pistons or screws) pressure
designing and evaluating hydrogels, cells, and 3D models. or a solenoid system to eject the bioink through a nozzle or
In the design process, desired structures are conceptualized needle into one or more cartridges fixed to the print head.
using 3D computer-aided design (CAD) design software Most extrusion-based bioprinters are cartesian-type
and then translated into G-code (the computer numerical 3D printers, i.e., assigning the movements according to the
control (CNC) programming language for 3D printers) X, Y, and Z axes. Usually, the print head moves in the X and
via cutting programs. Likewise, the selection of hydrogel Y directions, while the bed or build plate moves in the Z
components should consider the ECM composition of the direction. Most printers have temperature-controlled print
target tissue, the biocompatibility of the hydrogel with the heads and platforms to maintain an optimal temperature
cells, and its printability. After printing, the cell-hydrogel for the cells and the hydrogel. Extrusion-based bioprinters
construct should be cultured accordingly to promote the have some drawbacks, such as poor cell viability, low
development of functional tissues. resolution, and slow printing speed. Nonetheless, these
bioprinters are the most widely used as they can handle
4.1. Types of 3D bioprinting techniques various hydrogels, have high cell density, are cost-effective,
In 3D printing, structures are built layer-wise from a wide and are easy to operate (Table 4). 35-37
array of printing techniques. According to ISO/ASTM
52900:2021, additive manufacturing can be classified into 4.1.2. Material jetting-based bioprinting
six categories: material extrusion, direct energy deposition, Material jetting-based bioprinting refers to droplet-based
material jetting, sheet lamination, powder bed fusion, bioprinting, whereby the hydrogel is ejected dropwise
and vat photopolymerization. Among these, three main to build the final construct. All material jetting-based
categories are relevant to 3D bioprinting: material extrusion, bioprinters are drop-on-demand printers, where only
material jetting, and vat polymerization (Figure 7). one drop of material leaves the printer head at a time.
Additionally, the powder bed fusion technique can also be Based on their droplet formation mechanism, material
used to print rigid scaffolds. 34 jetting-based bioprinting can be categorized into inkjet,

Figure 7. Classification of 3D printing techniques. Green boxes indicate the techniques used in bioprinting. This figure was created using BioRender.

Volume 10 Issue 3 (2024) 154 doi: 10.36922/ijb.2056