Page 81 - MSAM-3-1

Materials Science in Additive Manufacturing Bioactive hydrogels for 3D bioprinting

processing by automated biofabrication technologies. face technical challenges in 3D bioprinting, including:
Aqueous formulations of polymers or hydrogel precursors (i) temperature-dependent extrudability, where the
containing biological factors are categorized as biomaterial viscosity of the hydrogel changes with temperature, affecting
inks that can become bioinks upon the addition of cells to extrudability; (ii) poor dimensional accuracy due to surface
the formulation. 3 tension and interfacial forces, leading to layer fusion,
By employing hydrogels as the primary bioink material, particularly exacerbation in smaller parts; (iii) shrinkage
extrusion-based 3D bioprinting, the most commonly used and swelling that occur in response to crosslinking and
type of 3D printing in tissue engineering applications, environmental changes, altering construct dimensions
enables the placement of biological materials typically and other properties; and (iv) poor shape fidelity after 3D
printing.
Beyond hydrogel’s inherent limitations, the
6,14-20
within a supportive matrix, mimicking the environment current practices for evaluating printing outcomes often rely
of native tissues for a variety of applications. 1,4-6 Hydrogels on subjective visual inspection of printability. Researchers
6
are hydrophilic polymer materials with several advantages are actively addressing these challenges through various
for 3D bioprinting, including biocompatibility, tunable strategies, including modifying hydrogel composition with
extrudability and printability, biodegradability, and the reinforcing agents, optimizing crosslinking methods, and
ability to encapsulate and deliver bioactive molecules and applying post-printing treatments. 5,21-28
living cells. These features make hydrogels ideal for creating
functional biomimetic constructs that promote cellular Gelatin is a heterogeneous mixture of polypeptides
response, tissue regeneration, and specific functions obtained by controlled hydrolysis of collagen with cell-
aligned with the intended objectives. Hydrogels are adhesive ligands such as the tripeptide Arg-Gly-Asp (RGD)
7,8
often characterized by their high percentage of water sequence. Gelatin is a low-cost biodegradable protein with
29
content, which contributes to their softness and flexibility. molecular weight ranging from 15 to 400 kDa. It exhibits
While this property makes them suitable for 3D printing, shear thinning behavior, and its viscosity is dependent on
encapsulation, tissue regeneration, and angiogenesis, it temperature because the hydrogen bonds that hold the
compromises their mechanical strength and stiffness, triple-helix conformation of gelatin together are weakened
9,10
biodegradation rate, and dimensional accuracy. Figure 1 by increased temperature. It undergoes physical gelation
illustrates the advantages and disadvantages associated below room temperature, which can restore collagen-
with hydrogel materials. like triple helix structures at 25 – 35°C, and above this
range, the triple helix dissociates, allowing the solvation
Most physically crosslinked hydrogels exhibit shear of gelatin chains. 30-32 Various gelatin-based bioinks have
thinning behavior, where their viscosity decreases under been formulated to improve biocompatibility and enable
increased shear rate, allowing them to flow smoothly the thermal crosslinking of the compound at room
through bioprinting nozzles and deposit onto the print temperature. 31,33
surface. The presence of hydrophilic moieties such as
carboxyl, amide, amino, and hydroxyl groups contributes Alginate is a naturally occurring, low-cost, non-toxic,
to the high hydrophilicity of hydrogels and absorbing and biodegradable linear anionic heteropolysaccharides
relatively large amounts of fluids. This high water content composed of (1 – 4)-linked β-d-mannuronic (M) and α-l-
results in a low density of polymer chains per unit volume, guluronic (G) acids, arranged in the homogeneous (MM
which leads to weak interactions between adjacent chains or GG) and heterogeneous (MG or GM) blocks. The ionic
and ultimately results in poor mechanical properties in crosslinking of alginate occurs when calcium ions interact
hydrogels. In particular, the weak physical interactions in with the carboxyl groups (COO-) present in both G and M
physically crosslinked hydrogels result in poor resistance blocks. Calcium ions form bridges between the carboxyl
against gravitational sagging and poor stabilization of the groups of adjacent G blocks, linking them together to form
ink after dispensing. 10-13 The obtained filaments spread a 3D network structure, also known as the egg-box mode
fairly easily in 3D-bioprinted constructs composed of these mechanism. The aqueous solution of sodium alginate
hydrogels, leading to poor shape retention. This affects can be easily extruded and, afterward, form hydrogels
the subsequently printed layers and, consequently, the when crosslinked with calcium ions to immobilize the
whole structure, as the first few printed layers are prone hydrogel and improve its mechanical properties at room
34-37
to collapse or deformation under the weight of the upper temperature.
layers. Shear-thinning hydrogels can be extruded at room Although extensive research has demonstrated the
temperature, minimizing harm to encapsulated cells and biocompatibility and performance of 3D-bioprinted
preserving temperature-sensitive biomolecules. However, hydrogels, the incorporation of additional components
most physically crosslinked hydrogels, such as gelatin, can further enhance their properties, broadening their

Volume 3 Issue 1 (2024) 2 https://doi.org/10.36922/msam.2845