Page 124 - IJB-10-2

International Journal of Bioprinting 3D bioprinting for corneal regeneration

This resin is composed of two special components common form is sodium alginate, the sodium salt of
(a positive and negative tone resin). Printing is alginic acid. Cross-linking of alginate is typically achieved
accomplished using a femtosecond near-infrared (NIR) with divalent cations such as calcium (Ca ), magnesium
2+
laser source and a glass plate situated at the bottom of (Mg ), or barium (Ba ). This binding method allows
2+
2+
the resin bath. In contrast to cellular printing, laser reversible cross-linking, as the cations are released from the
techniques are primarily used for scaffolds, given that cross-linked alginate in a cross-linker-free environment.
the unbound resin from the printed sample is dissolved In addition to cationic cross-linking, alginic acid exhibits
using solvents that can be harmful to cells. However, the excellent cross-linking through enzymatic and photoactive
advantage lies in the capability of these techniques to compounds. While alginate-bound cells can be efficiently
swiftly and economically produce substantial quantities recovered, and cells remain viable for weeks in alginate
of constructs. 36,52,54-57 scaffolds, there are disadvantages. Cells enclosed in
In addition to potentially harmful solvent and alginate hydrogel tend to maintain a spherical shape due
resin-based techniques, many water-soluble, less, to encapsulation, and they may exhibit lower proliferation
or non-cytotoxic photoinitiators can be used in and differentiation rates. 38,44,62
corneal bioprinting with common hydrogels. For 7.2. Gelatin
instance, Irgacure and LAP (lithium phenyl-2,4,6- Gelatin, derived from collagen through the partial
trimethylbenzoylphosphinate) are commonly used hydrolysis of its tertiary structure, is a protein that
in the photocuring of acrylate and methacrylate- originates from various sources, such as pork, calf, or
based hydrogels. Studies have demonstrated that fish, each with slightly different properties. Gelatin-
a low concentration of LAP exhibits enhanced based hydrogels are renowned for their excellent
cytocompatibility and improved physio-mechanical biocompatibility and biodegradability, making them
properties in printed constructs compared to Irgacure a commonly used hydrogel component in 3D printing.
2959. Cytotoxicity tends to escalate with the initiator Gelatin-based hydrogels maintain cell viability and
58
concentration and exposure to cross-linking UV light. differentiation potential, which are important factors
59
Barroso et al. also used LAP as a photoinitiator for when working with stem cells. One disadvantage
60
printing a methacrylated silk fibroin-based bioink of using gelatin lies in its thermosensitive property,
(SilkMA) in artificial corneal research. Their study wherein its cross-linked structure becomes unstable
revealed that LAP-cured SilkMA exhibited good and liquefies under physiological conditions above
viability, and metabolic activity increased over the 14- 20°C, such as the standard human body temperature
day observation period. The prepared hydrogel could of 37°C. 38,44 To address this, improving the mechanical
be cured with neutral pH and low-energy UV light or properties of gelatin-based hydrogels is necessary and
through lithography-based printing. In another study, can be achieved by incorporating other polymers,
He et al. prepared PEGDA-GelMA hydrogel with LAP such as chitosan, collagen, or alginate, into the gelatin
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as a photoinitiator, successfully printed with cells using matrix. 38,63 Alternatively, chemical modifications on
the DLP method. Cells demonstrated proliferation in gelatin, such as methacrylation, result in the formation
the hydrogel 6 days after printing with approximately of gelatin methacrylamide (GelMA). The addition of a
90% viability. Additionally, numerous photoinitiators photoinitiator to GelMA allows photopolymerized using
can facilitate cross-linking via visible light, such as Eosin UV light. 64
Y, riboflavin, or ruthenium (as mentioned in Zhang et
al. ). These methods represent viable alternatives in 7.3. Collagen
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light-based bioprinting with high biocompatibility, as Collagen, an important component of ECM, is widely
measured using MTT or Live/Dead assay. 58 used in clinical settings as a tissue replacement and
regenerative material, as well as in 3D bioprinting due to
7. Hydrogels and scaffolds for its excellent mechanical and degradable properties. This
corneal bioprinting ECM protein is abundantly present in the connective
tissues of many organisms, with the primary sources for
7.1. Alginic acid
Alginic acid, commonly known as alginate, is a laboratory uses being animals like calves, pigs (skin and
65
polysaccharide extracted from brown algae. Its versatile bone), or marine animals.
applications extend across industries, including textiles, In the context of tissue printing, collagen stands out as
pharmaceuticals, and food, where it serves as a thickener, an excellent hydrogel material due to its scaffold structure.
gelling agent, and emulsifier marked as E400. Another The porous structure of collagen facilitates the diffusion

Volume 10 Issue 2 (2024) 116 doi: 10.36922/ijb.1669