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International Journal of Bioprinting 3D bioprinting techniques & hydrogels materials

bioink liquid and deposit it at the desired location. 83,84 resulting in the formation of intricate 3D structures
The printers typically comprise three layers: an energy- from a liquid bioresin. 101,102 Therefore, this 3D printing
absorbing layer, a ribbon layer consisting of bioinks, and technology is only compatible with photo-crosslinked
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a substrate layer. Since this printing method utilizes bioinks or resins. Over the past decade, several
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laser deposition of bioink, it eliminates the need for a new technologies have been developed depending on
nozzle, thereby significantly reducing shear stress on the type of light used, including stereolithography
the cells within the bioink. This guarantees that even (SLA), digital light processing (DLP), and two-photon
with highly viscous bioink, the printer can process it polymerization (2PP).
without clogging problems. Furthermore, high printing The SLA system uses a laser source to refract and scan
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accuracy and resolution are among the advantages of the material in a vat to cure the bioresin. Generally, when
this method. However, it is worth noting that the laser exposed to a light source, the photoinitiator generates
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produces a thermal effect, which can cause thermal free radicals or other reactive substances, initiating
damage to cells and affect cell survival. Its high cost limits the polymerization of the resin into a predetermined
its wide application as well. Recently, to fully leverage 2D shape via a digital micromirror device (DMD);
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the dynamic properties of cells and living biomaterials, this process occurs layer by layer. Depending on
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“time” has been considered the fourth dimension, the location of the laser source, the SLA system can be
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especially for soft tissue design. Through laser-assisted divided into top-down and bottom-up printing. Top-
printing, Douillet et al. successfully developed a new down printing, which is a traditional method, involves
model that can replicate the dynamic remodeling of printing a support structure and the target structure
fibroblasts in vitro, providing a new tool for the future together to form an accurately designed construct.
study of in vitro dermal tissue. 89 During bottom-up printing, the printer draws a section
of the component on the bottom layer of the resin, and as
2.4. Vat polymerization-based printing the building platform rises, the liquid resin flows beneath
Vat polymerization (VP)-based printing is a cutting- the previously cured layer, backfilling the resin layer. The
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edge 3D printing method that uses light of a specific inverted SLA system only requires a thin layer of bioresin;
wavelength to induce a photopolymerization reaction, thus, larger molding volumes can be printed without a

Table 2. Summary of the comparative characteristics of common printing techniques.

Printing method
Parameters Vat polymerization Tomographic Ref.
Inject printing Extrusion printing Laser-assisted printing volumetric
(VP)-based printing
printing
Materials Biomolecules; growth Hydrogel; ceramics Biomolecules; cells Liquid bioresin; Liquid bioresin; 90-92
factors hydrogel hydrogel
Resolution or ≈ 50 μm ≈ 200–1000 μm ≈ 10–100 μm ≈ 10–100 μm ≈ 80 μm 92-95
droplet size
Material 3.5–12 mPa/s 30 to 6 × 10 mPa/s 1–300 mPa/s Up to 5 Pa/s 4–93 Pa/s 91,92,96,97
7
viscosities
Print speed Fast (up to 10,000 Slow (10–50 μm/s) Fast (1.6 μm/s) Fast (>10 mm /h) Fast (>10 5 91,92,95
3
5
droplets per second) mm /h)
3
Advantages Wide availability; low High cell density; Low shear stress; wide High resolution; high High resolution; 96,98,99
cost; high resolution; high cell viability; viscosity; high printing printing speed; high high printing
high printing speed; wide viscosity speed; high resolution cell viability speed;
ability to introduce
concentration gradients
in 3D construct
Disadvantages Poor vertical structure Limited resolution; Low cell viability; Cumulative ultraviolet High cost; high 88,99,100
clogging characteristics; shear forces can relatively high cost light (UV) exposure; complexity
thermal and mechanical damage cells restrictive multi-
stress to cells; limited material functionality;
printable materials high cost; high
complexity

Volume 10 Issue 6 (2024) 71 doi: 10.36922/ijb.4472