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Materials Science in Additive Manufacturing Hydrogels in mandibular reconstruction

the mandible’s intricate anatomical structure. This high – due to reversible state changes under altered physical
level of customization provides an effective treatment conditions – limits their application in high mechanical
strategy for bone tissue regeneration, as the scaffolds can load scenarios.
be designed for patient-specific defects with remarkable In contrast, chemically crosslinked hydrogels rely on
precision, offering a highly personalized and efficient irreversible covalent bonds formed through mechanisms
solution for clinical applications. such as Schiff base crosslinking, Diels–Alder (DA)
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2.1. Natural and synthetic hydrogels reactions, or radical polymerization. These hydrogels
achieve mechanical strength on the order of MPa,
Hydrogels can be categorized into natural and synthetic meeting the mechanical demands of bone tissue repair.
types based on polymer substrate origins. Natural Nevertheless, challenges such as residual crosslinking
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hydrogels include polysaccharides (chitosan, agarose, agents or overly dense networks may lead to cytotoxicity.
hyaluronic acid, alginate, cellulose, etc.) and polypeptides
(gelatin, collagen, poly-L-glutamic acid, poly-L-lysine, Hydrogels with greater complexity can be synthesized
etc.). Their composition resembles the ECM, granting through the integration of multiple crosslinking
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them superior biocompatibility and bioactivity. For techniques. Even when composed of identical materials,
instance, chitosan exhibits antibacterial properties, while hydrogels produced through distinct crosslinking
collagen’s RGD sequences enhance osteoblast adhesion. approaches develop varied network architectures,
Their non-toxic degradation byproducts make them ultimately leading to differences in their physical and
suitable for carrying stem cells or growth factors to chemical properties. 14-16 For instance, double-network
promote localized regeneration. However, low mechanical hydrogels utilize covalent crosslinking to provide
strength and unpredictable degradation rates limit their mechanical support, combined with physical crosslinking
use in large-scale defects (like post-tumor resection), to enable self-healing capabilities. Alternatively, they
often necessitating reinforcement with hydroxyapatite or may employ dynamic covalent bonds to balance
crosslinking modifications. reconfigurability and stability, significantly enhancing the
material’s strength and toughness. In addition, Chuang
Synthetic hydrogels (polyethylene glycol [PEG], et al. analyzed collagen hydrogels with comparable
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polyacrylic acid, poly [lactic-co-glycolic acid], etc.) chemical compositions and physical attributes but
allow precise control over mechanical properties, pore prepared using distinct crosslinking techniques. Their
structures, and degradation timelines through molecular study revealed that variations in crosslinking bonds
design. Photocurable PEG hydrogels adapt to complex between the two hydrogel types resulted in disparities in
defect geometries, while thermosensitive poly(N- permeability, microstructure, and mechanical strength.
isopropylacrylamide) gels enable minimally invasive filling Specifically, covalently bonded hydrogels exhibited lower
of irregular defect areas with robust enzymatic resistance permeability, higher density, and enhanced mechanical
for long-term support. Nevertheless, their inherent stability due to their tightly interconnected networks.
bioinertia requires modification with bioactive peptides
to enhance cellular interaction, and residual monomers or 2.3. Stimulus-responsive hydrogel
acidic degradation products may trigger inflammation. Smart hydrogels are fabricated by incorporating specific
chemical structures and additives during or after the
2.2. Physically, chemically, and multi-crosslinked polymerization process, enabling them to exhibit stimuli-
hydrogels
responsive properties. These hydrogels can dynamically
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The crosslinking mechanism of hydrogels is a pivotal factor respond to external triggers such as pH levels, magnet,
in regulating their network structures and functional temperature, light, or electricity (Figure 3B). By leveraging
properties. Based on crosslinking mechanism, hydrogels mechanisms like controlled drug release, they adapt to the
can be categorized into physically crosslinked, chemically unique microenvironments of diseased tissues, thereby
crosslinked, and multi-crosslinked systems (Figure 3A). 10,11 enhancing therapeutic efficacy.
Physically crosslinked hydrogels form reversible Bone tissue, as a mechanosensitive tissue, relies on
networks through dynamic non-covalent interactions mechanical stimulation to maintain its structure and
such as ionic interactions, complementary base pairing, function. This is particularly evident in the jawbone,
hydrogen bonding, or hydrophobic associations. These which experiences frequent occlusal forces, resulting in a
hydrogels exhibit environmental responsiveness, making significantly higher metabolic rate and bone remodeling
them suitable for minimally invasive injection and activity compared to other skeletal regions. To enhance bone
controlled drug release. However, their inherent instability regeneration, researchers have developed mechanically

Volume 4 Issue 2 (2025) 4 doi: 10.36922/MSAM025070006