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International Journal of Bioprinting 3D bioprinting for corneal regeneration

An important property of their hydrogels is the utilization of cell types. One solution to this issue is the utilization
visible light for cross-linking, thereby safeguarding cells from of 3D fabric printing. Currently, several 3D printing
damage inherent in common UV cross-linking methods. techniques are available in the market, such as methods
Moreover, the hydrogel can undergo gelation regardless of based on inkjet, extrusion, or light. The selection among
pH conditions, facilitating easier handling. The resultant these methods hinges on the specific characteristics of
scaffold exhibited commendable physio-mechanical the intended sample, considering both their advantages
properties, maintaining the printed shape of the cornea. and disadvantages. Depending on the type of printer
After 30 min of saline washing to eliminate the yellow color used, the scaffold or printing mold can have a positive or
of ruthenium, the scaffold retained 94% transparency and negative pattern, and it is even possible to print without
remained transparent after 10 days. Human trabecular a mold using materials with special properties, such as
meshwork stem cells (hTMSCs) mixed into the corneal poloxamer (Figure 2).
dECM hydrogel demonstrated 90% viability even 48 h 6.1. Material extrusion methods
after printing. Notably, cornea-specific gene upregulation
was observed in the cells, and immunostaining revealed Material extrusion-based printers can be categorized
collagen production. Zhang et al. utilized cornea-derived into two types based on the method used to dispense the
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dECM (CECM) and GelMA-based hydrogel for DLP- material for printing: pneumatic (utilizing compressed
printed cornea structures. In vitro employed human corneal air) and mechanical material extrusion. In both methods,
printing is executed by one or more fixed print heads
fibroblasts, while in vivo testing utilized rabbit models. positioned above a printing table movable in three
Rheological tests demonstrated excellent physio-mechanical dimensions (X, Y, and Z directions). Pneumatic systems
properties of their cornea scaffold, exhibiting stability and may exhibit less direct control over material flow due
resistance to various forces and near-complete transparency. to the delay introduced by gas volume compression.
The CECM/GelMA hydrogel exhibited only 17% water loss Conversely, mechanically operated systems employ a
after 4 h of air drying compared to pure GelMA (31%). screw-controlled piston in the syringes, making them
Cell viability gradually increased during the 14-day culture more suitable for printing high-viscosity hydrogels.
period, indicating cell proliferation within the hydrogel, Continuous material flow must be carefully maintained
with observed migration toward inner areas. The CECM/ in both types. Hydrogels used in this technique must
GelMA hydrogel provided an optimal microenvironment undergo cross-linking during or after printing, achieved
to the cells, leading to a gradual increase in collagen, through physical or chemical methods. This technique
lumican, and ALDH3A1 production, as observed through is versatile, allowing the printing of various tissues,
immunostaining. In an in vivo rabbit model, the implanted including the cornea. Material extrusion facilitates fast
CECM/GelMA hydrogel did not induce inflammation or and cost-effective printing. A range of starting materials,
rejection, and increased re-epithelialization was observed including hydrogel containing different cell types, dECM,
around the wound, resulting in a healing rate of 93.5% at 28 and synthetic polymer fibers, can be used for scaffolds.
days after surgery. However, material extrusion techniques have drawbacks.
Another significant category of polymers applicable Inadequate and excessive pressure application and
to tissue printing comprises synthetic polymers, often overly swift movement of the printing table can disrupt
preferred over biopolymers due to their mechanical the continuity of the print pattern, resulting in lower
strength and non-immunogenic properties. 38,44 Typically resolution and slower printing speeds compared to other
produced through chemical reactions, these materials are methods. Attention must be given to the viscosity of the
transformed into hydrogels using the inverse dispersion hydrogel, as excessive viscous hydrogels can lead to print
technique. However, a disadvantage of synthetic polymers head clogging. Moreover, cellular viability during cellular
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is their reliance on organic solvents and high temperatures printing using material extrusion may be lower due to the
for 3D printing, potentially compromising the biological high pressure and shearing forces applied to the cells, in
activity of cells and various active substances and factors contrast to other techniques. 6,36,47-50
incorporated into the hydrogels. 38,45,46 Consequently, 6.2. Inkjet printing
synthetic polymers find greater utility in constructing the Within the inkjet printing technique, six methods can
frames of printed structures.
be distinguished: piezoelectric, thermal, electrostatic,
6. Type of printing methods electrohydrodynamic, microvalve-based, and acoustic.
Material jetting offers the advantages of computer-
The primary challenge in constructing complex structures controlled droplet formation with high precision
lies in the necessity for scaffolds to incorporate multiple and resolution, enabling control over the placement

Volume 10 Issue 2 (2024) 114 doi: 10.36922/ijb.1669