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Osidak, et al.
bioink solution) are available on the market. When the ability to form gap junctions, was detected. In
mixed with a mammal cell suspension in a cultural another study of Koch et al. , similar bi-layered
[24]
medium and then heated to 37°C, Viscoll bioink constructs were created in in vitro conditions
quickly forms a stable cell-laden hydrogel. The and then implanted in vivo, employing the dorsal
survival rate of NIH 3T3 cells as a part of rigid skin-fold chamber in nude mice. It was found that
collagen hydrogels was approximately 90% after fibroblasts can migrate into a supportive scaffold.
printing and after a week of in vitro cultivation. Moreover, it was noted that the presence of several
Unfortunately, this is the only data on the behavior blood vessels in the wound bed could be observed
of cells during cultivation inside rigid 3D collagen after 11 days of transplantation.
[25]
hydrogels that are currently available. Shi et al. have printed six-layered cellular
structures using extrusion-based bioprinter. They
3 Tissue engineering applications of used three types of cells: Human melanocytes (HEM),
collagen- based bioinks HaCat, and human dermal fibroblasts (HDF). As a
material for bioink, they used a mixture of GeIMA
Due to the prevalence of collagen-based bioinks and collagen. In addition, I-2959 photoinitiator and
with a low protein concentration usage in various tyrosinases were added to the obtained mixture. The
fields of tissue engineering, collagen is mixed with biocompatibility of created designs was evaluated
various materials to improve the manufacturing in vitro and in vivo through implantation of these
process and the final characteristics of the printed structures without cells into a full-thickness wound
construct [21,22] . There are only few studies, where model of Sprague-Dawley rat. The viability of these
collagen bioinks were used as a pure substance three cell lines during 14 days of cultivation was
without any additives. These works are listed below. above 90%. In vivo tests have shown that healing
Currently, there are two general methods for rates of the wound can be accelerated when treated
creating tissue-engineering designs – in vitro with the tyrosinase doped bioinks.
bioprinting and in situ bioprinting. In the case of Another study worth noting was made up by
in vitro bioprinting, the printing of design is carried Yoon et al. . To create 3D skin substitutes, they
[12]
out in the laboratory environment. After printing, used pure (single-component) collagen bioinks.
the design is either implanted into a laboratory Primary human epidermal keratinocytes (HEK) and
animal or cultivated for a specific period for cell HDF were used to fabricate cell-laden 3D scaffolds.
behavior study. In the case of in situ bioprinting, Cell-laden 3D scaffolds were created through
printing is carried out directly onto the defective extrusion bioprinting and were composed of four
area of a laboratory animal. layers. The top-level contained keratinocytes and

3.1 Skin the other three layers had fibroblasts. According to
the results of the study, cell-laden 3D scaffolds in
Koch et al. in their work have printed a construct a 1 × 1 cm full-thickness excision mouse model
[23]
2
with the use of laser-assisted bioprinter onto the have successfully demonstrated their efficiency.
surface of a supportive scaffold – decellularized After 1 week, the damaged skin almost completely
dermal matrix (Matriderm). The printing process and clearly regenerated. The hair follicles on the
was carried out in two stages – 20 layers of fibroblast wound bed also regenerated almost perfectly.
(murine NIH 3T3) were applied onto the surface, In the work of Skardal et al. , amniotic fluid-
[26]
which was followed by 20 layers of keratinocyte derived stem cells (AFSC) and mesenchymal
(human HaCaT), embedded into collagen hydrogel stem cells (MSC) were separately suspended in
(3 mg/ml). As a result, it was shown that a bi-layered the fibrinogen/collagen solution. They used a
construct that generates dermis and epidermis has bioprinter to directly print two layers of a fibrin-
been successfully created. After 10 days of cell collagen gel by depositing a layer of thrombin,
cultivation inside of the construct, the presence a layer of fibrinogen/collagen, another layer of
of Connexin 43 in the epidermis, which showed thrombin, another layer of fibrinogen/collagen,

International Journal of Bioprinting (2020)–Volume 6, Issue 3 19