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International Journal of Bioprinting GradGelMA 3D-bioprinted vascular skin

efficient transport of nutrients and oxygen while providing such as secreting extracellular matrix and growth factors,
sufficient space for cell attachment and extension, enabling and is often used to study the role of fibroblasts in tissue
them to perform their biological functions. Human repair and regeneration. HFF cells have a high survival rate
fibroblasts and vascular endothelial cells are relatively at 5%, 10%, 15%, and 20% (w/v) GelMA (Figure 5A and C).
large, necessitating the selection of hydrogels with larger On the first day, HFF cells in the 5% (w/v) GelMA group
pores, i.e., lower-concentration GelMA hydrogels, as had slightly extended. By the seventh day, almost all cells in
carriers for printing. the 5% (w/v) GelMA group had extended and multiplied
exponentially. In the 10% (w/v) GelMA group, only
The rheological properties of hydrogels directly
influence the material flow and molding quality during some cell extension and a small amount of proliferation
were observed, while cells in other groups remained
the printing process. Rheology refers to the property of spherical. When HFF cells in the 5% (w/v) GelMA group
hydrogels to deform and recover under external force. were cultured until the tenth day, the cells extended into
Figure 3F indicates that for GelMA of all concentrations, long, spindle-shaped structures, and the extension length
both G´ and G˝ decrease as the temperature rises, but the exceeded 200 μm (Figure 5B). Type I collagen is a fibrous
decrease rate of G’ is greater. The intersection point of G’ collagen that maintains tissue elasticity in the human
and G’’ represents the gel point. When the temperature dermis and is mainly secreted by fibroblasts. Fibrinogen,
is lower than the gel-point temperature, G´ > G˝ for the as a natural biomaterial with excellent biocompatibility,
hydrogel shows a solid state; conversely, above the gel point, is a key material for manufacturing skin substitutes in
where G˝ > G´, it shows a liquid state. Figure 3G shows traditional tissue engineering. According to the numerical
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the “shear-thinning” phenomenon of GelMA hydrogels, analysis of the gray value, the content of type I collagen in
where the viscosity decreases as the shear rate increases. the 5% (w/v) GelMA group is significantly higher than that
This may be because when subjected to a large shear force, in the high-concentration groups but slightly lower than
the entanglement points of the polymer chains inside that in the fibrinogen group (Figure 5D). Therefore, among
the hydrogel are opened, and the structure undergoes GelMA groups with different concentrations, 5% (w/v)
directional stretching, resulting in a decrease in viscosity. GelMA is more suitable for the functional performance of
Kim et al. and Won-woo et al. have demonstrated the human skin fibroblasts.
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shear-thinning behavior of porous hydrogels.
The human skin epidermis mainly comprises
3.3. Printability and biocompatibility keratinocytes, melanocytes, and Langerhans cells.
The printing test results are shown in Figure 4A. All GelMA Keratinocytes are derived from the ectoderm and account
hydrogels of different concentrations have a printable for over 90% of human epidermal cells. Scholars have
interval and can form a stable and regular two-layer conducted extensive research on the culture methods
hydrogel grid structure. When the concentration of GelMA of keratinocytes. Research shows that keratinocytes
hydrogel increases, its suitable printing pressure and must be cultured under air–liquid interface conditions
temperature will also increase. For a single concentration of to differentiate into a multi-layer epidermal structure
GelMA, if the printing pressure or temperature is too low, in vitro. 40-42 Keratinocytes can attach and proliferate on
over-gelling and irregular extrusion will occur. Conversely, the surfaces of 5%, 10%, 15%, and 20% (w/v) GelMA
under-gelling and excessive softening extrusion will occur solidified through manual dropping (Figure 5E). On Day
if the printing pressure or temperature is too high. 2, HaCaT cells in the 20% (w/v) group grew in a sheet-like
Cell viability staining (Figure 4B) shows that HFF cells’ manner. By Day 5, HaCaT cells in the 20% (w/v) group
survival rates for both mesh and membrane structures formed a film and covered the hydrogel surface, which was
on Day 0 and Day 7 are greater than 90%, and the cell significantly better than the other three groups. Moreover,
viability increased with prolonged culture time. This higher concentration results in better attachment and
indicates that GelMA is a hydrogel bio-ink with excellent proliferation of HaCaT cells throughout the growth and
biocompatibility, allowing cells to maintain good vitality. reproduction cycle. By Day 10, there was an obvious basal-
Song et al. demonstrated that bio-inks made from GelMA epidermal layer interface in the 20% (w/v) GelMA group
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exhibit cell morphology on the first day of cell seeding on and the collagen group, with clear epidermal structures,
the GelMA bio-ink, with cell confluence exceeding 90% by and the thicknesses were 53.63 ± 11.27 and 52.26 ± 26.33
the seventh day. μm, respectively. By Day 28, a relatively thick epidermal
layer structure could be seen in both groups of samples,
3.4. Skin cells adapted to composite ink with thicknesses of 130.61 ± 8.84 and 119.29 ± 18.87 μm,
The HFF cell line is derived from human neonatal foreskin respectively, both approaching the 200 μm thickness of
tissue. It has the normal functions of human fibroblasts, the human epidermis. However, the printed GelMA group

Volume 11 Issue 4 (2025) 337 doi: 10.36922/IJB025090069