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Fayyazbakhsh, et al.

Figure 5. Photographs of the 3D-printed dressings. G4-A4 and G6-A2 dressings showed the finest mesh structure and best shape fidelity.
The G8-A0 showed poor printability and inconsistent pore shape fidelity, while G2-A6 and G0-A8 samples were too viscous and difficult
to extrude with irregular pore shape and size.

blend denser and more consistent, as both gelatin and
alginate are semi-interpenetrating networks (semi-IPN),
as depicted in Figure 1. In semi-IPN hydrogel blends, a
linear or branched hydrogel is embedded within the other
hydrogel network with or without crosslinking, which
decreases the free volume. While in IPN hydrogels,
two or more hydrogels cross-link in the presence of
each other to form a 3D network with an increased free
volume . The lower free volume and higher consistency
[39]
are responsible for enhanced flowability in the hydrogel
mixture. Accordingly, the G6-A2 and G4-A4 hydrogel
samples exhibit a good balance between viscosity and
extrudability. Higher viscosity implies mixed effects on Figure 6. Young’s modulus of 3D-printed dressings (n = 3).
hydrogel printability, fror example, higher mechanical Mechanical stiffness is increased by alginate content; however,
stability but lower printability due to the higher extrusion only G6-A2 samples are in the same range as normal skin. The
[41]
pressure required. Furthermore, higher extrusion pressure Young’s Modulus of the normal skin is adopted from . Wound
increases shear stress during printing, which is invasive dressings need to have the stiffness matched with normal skin to
to the cells. Shear stresses have been shown to induce support body movement, non-adhesive coverage, and persistence
on the wound site. The mechanical properties of the plain gelatin
morphological changes, cytoskeleton reorganization, and dressing are not measurable.
generation of reactive oxygen species, and alter gene and
protein expression . bonds in post-printing alginate and (ii) a larger number
[37]
3.2. Mechanical properties of cross-links formed within the alginate network. It is
generally accepted that the stronger chemical bonds in
Many authors in the literature have discussed the effects of the hydrogel network result in higher mechanical strength
calcium ion exposure on sodium-alginate crosslinking and and lower permeability. However, only the 3D-printed
mechanical strength . In this research, we cross-linked G6-A2 dressings exhibited Young’s modulus in the
[25]
the 3D-printed dressings with different gelatin: alginate range of the normal skin. Burn wound dressings must
ratios to improve the mechanical strength. Figure 6 provide a non-adhesive surface that is elastic enough
shows Young’s modulus of the 3D-printed dressings. It to support body movement with no pain or trauma in
is important to highlight the fact that wound dressings are the wound site. Furthermore, an adequate mechanical
required to exhibit adequate tensile stiffness during the stiffness is required to maintain the dressings fixed on the
application, wearing, and removal to serve as a barrier wound without falling apart. Therefore, the samples with
against traumas and external pathogens. Furthermore, Young’s modulus matched with normal skin considered
dressings should be adequately elastic to adapt to the as best samples for further testing. It is notable that the
wound surrounding tissues and body movement, that is, dressings with lower stiffness will move on the wound
in the same range as normal skin . The Young’s modulus surface during body movement, while dressings with
[40]
(E) of normal skin fluctuates between 0.42 MPa and higher stiffness compared to the skin will limit body
0.85 MPa . The tensile testing results from this work movement causing stress shielding, secondary trauma,
[41]
strongly support the positive effect of alginate content and skin tear on the wound site and surrounding tissues.
on mechanical stiffness due to (i) stronger chemical The G8-A0 sample was excluded from experiment as the

International Journal of Bioprinting (2022)–Volume 8, Issue 4 281