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International Journal of Bioprinting 3D bioprinting for vascularized skin tissue engineering

using ECs from placental pericytes and cord blood to skin transplants accelerated wound healing with sufficient
print pre-vascularized biofabricated structures. In their neovascularization within the skin graft. Utilizing 3D
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latest vascularized skin grafts, Baltazar et al. 132 integrated bioprinting, Ng et al. 134 developed pigmented human skin
keratinocytes with vascularized dermal fibroblasts, ECs, by incorporating ECs with other skin cells to promote
and pericytes using type I collagen bioink solutions. This vascularization. After 4 weeks of air–liquid interface (ALI)
resulted in the production of vasculature-like structures in culture, they combined the manual casting process with a
vitro by ECs and pericytes, while keratinocytes produced combined 3D bioprinting process to develop a functional
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stratified structures with multiple layers. Yanez et al. 126 skin substitute, as shown in Figure 8A. The pre-
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developed a skin graft by incorporating microvascular ECs vascularized 3D skin substitutes were treated with excisional
into dermal compartments containing fibroblasts. To create full-thickness wounds in immunodeficient rats. However,
a skin graft, keratinocytes were printed on top of dermal wounds without blood vessels were less transparent, and
compartments and vascularized through the incorporation treatments with pre-vascularized substitutes showed host
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of microvasculature via 3D printing. 126 tissue-integrated blood vessels. As seen in Figure 8B,
histological analysis after 7 days showed blood-perfused
Kolesky et al. reported that approximately 1-cm-thick blood vessels in pre-vascularized substitute wounds, which
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human vascularized and perfusable tissue was biofabricated were however absent in the control groups.
using 3D bioprinting methods with different materials
within 6 weeks. This tissue replaces the engineered ECM In this in vivo grafting model, under physical
that is encapsulated in a variety of human cells, including examination, the areas treated with control substitutes
hMSCs and human neonatal dermal fibroblasts (hNDFs). exhibited drying and detachment from the injured site by
Additionally, HUVECs have been used to enhance normal day 14. Among the 15 mice that received non-vascularized
endothelial-lining function. Initially, scientists created control substitutes, 11 exhibited skin blistering by day 7
printable inks containing cells and a castable ECM using after grafting. However, both the control-treated wounds
gelatin and fibrinogen. These materials were combined and the pre-vascularized substitutes fully re-established
for the 3D bioprinting of tissue constructs. This approach the outer layer of the skin (re-epithelialization) by day
satisfies important criteria such as ease of handling, ability 14. In contrast, pre-vascularized substitutes adhered to
to incorporate different components, and compatibility the wound tissue, resulting in remarkable healing with
with living systems. minimal wound contraction. Masson’s trichrome staining
revealed the presence of mature collagen fibers (bright
5.2. Enhancement of in vivo skin tissue regeneration blue and densely packed) within the pre-vascularized
Several vascularization strategies have been studied to substitutes, with a collagen content similar to that of the
develop tissue-engineered constructs with stable and host tissues. In contrast, wounds treated with control
functionally perfusable microvessels in vivo. When substitutes exhibited immature collagen (light blue
the construct is transplanted into the host, it gradually and loosely packed) surrounding the fibroblasts. As
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vascularizes as the body responds to foreign substances. demonstrated in Figure 8C, human HLA-ABC expression
One strategy for expediting vascular development in in artificial human blood vessels and the Malpighian layer
constructs is to optimize the scaffold characteristics attests to the successful transplantation of human 3D skin
and incorporate growth factor delivery systems to substitutes in mice. A quantitative assessment of blood
enhance wound healing by promoting blood vessel vessel density showed that the pre-vascularized substitutes
regeneration before transplantation into the host. had a higher vessel density compared with the control
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However, this approach relies on the host response to group. Figure 8C shows angiogenesis in both control
rapid microvascular development and may face challenges and pre-vascularized substitute-treated wounds, with both
in achieving prompt vascularization. Pre-vascularization human- and host-derived endothelium present.
of constructs, as reported by Heller et al., is a potential Several major bottlenecks exist in the tissue engineering
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method for promoting blood vessel development in of permanent skin substitutes, including insufficient
vivo after transplantation. For instance, Yanez et al. 126 induction of angiogenesis, ineffective graft–host
developed artificial skin using inkjet printing technology anastomoses, and an inadequate vascular architecture.
with HUVECs as seed cells for bioprinting. Type I collagen Ma et al. 135 synthesized homogeneous strontium silicate
and combined fibrinogen were employed as bioinks for micropillars as stable angiogenesis-inducing factors in
building a supportive structure to enhance vascularization biomaterial inks. They employed these micropillars to
in bioprinted skin tissues. This 3D-bioprinted skin graft print functional skin substitutes using multicellular cellular
was implanted on the backs of nude mice in the absence systems that were induced for angiogenesis. Strontium
of the thymus. Two weeks after surgery, the 3D-bioprinted silicate microcylinders were incorporated into the bioink

Volume 10 Issue 3 (2024) 103 doi: 10.36922/ijb.1727