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Bioprinting with collagen
intended . Nevertheless, the most important thing The majority of existing studies on 3D printing
[5]
here is that materials created using 3D bioprinting and bioprinting using collagen specify the main
and collagen have very high chances of clinical problem with collagen bioink – its low mechanical
success in the future, because collagen biomaterials properties [3,11] . All these studies were carried out
have been already been actively and successfully using collagen solutions of low concentration
used in clinical practice for a long time. This is – usually, not more than 5 mg/ml and rarely, 10
[12]
possible due to the unique properties of collagen mg/ml . This problem refers to not only in
– biocompatibility and low immunogenicity [6,7] . 3D bioprinting but also other sections of tissue
However, low immunogenicity of 3D constructs engineering. More than 90% of known studies were
can be achieved only with the use of high purity carried out using collagen hydrogels prepared from
[5]
collagen solutions without potential immunogenic solutions with not more than 10 mg/ml collagen .
admixtures . Thus, in this review, “collagen” will One of the possible approaches to overcome
[6]
be meant as a purified protein obtained through this limitation is the use of supportive hydrogels.
extraction from collagen-containing tissues and When using a supportive hydrogel for 3D
not a decellularized extracellular matrix of any bioprinting with a collagen bioink, the whole
tissue or organ containing a large amount of process occurs inside of the secondary hydrogel
collagen . The main barrier that prevents the (e.g., gelatin slurry), which in turn acts as
[8]
use of decellularized materials is immunological temporary thermo-reversible support (FRESH
rejection, which significantly limits the possibility technique – freeform reversible embedding of
of clinical use of such materials. suspended hydrogels) [13-15] . On the one hand, this
Therefore, the purpose of this review is to method allows printing complex structures using
conduct a comprehensive study of the use of collagen solutions of low concentrations with a
collagen-based bioinks for 3D bioprinting in polymerization period of 40 – 60 min. On the other
various fields of tissue engineering. The review hand, gelatin from the supportive hydrogel can
covers topics such as general limitations and diffuse inside the bioink during the polymerization
advantages of collagen and collagen-based bioinks period. This, in turn, will lead to a final construct
used in different areas and the main approaches that potentially contains gelatin. The effect of the
for collagen-based bioinks 3D printing. remaining FRESH gelatin in a final construct at
in vivo implantation is not fully studied.
2 Pure collagen bioink: Printability aspects Another approach to compensate for the low
mechanical properties of collagen hydrogels was
As it was already noted before, soft biomaterials proposed by Diamantides et al. . According to
[16]
loaded with living cells are called bioinks . The their study, the best way to improve the printability
[3]
basis of collagen bioink is a collagen hydrogel, of collagen bioinks is to increase the storage
physical properties of which represent its modulus of the ink before extrusion. This strategy
printability. The majority of collagen hydrogels was described more accurately by Osidak et al. ,
[17]
are produced from type I collagen, which makes up it was shown that collagen bioinks with a much
around 90% of the protein mass in the connective greater storage modulus than loss modulus are
tissues of mammals . Type I collagen belongs to suitable for direct extrusion bioprinting.
[9]
the group of fibril-forming collagens and consists The storage modulus of collagen solution
of three alpha-helices that form a triple-helical depends on the concentration of NaCl in the
structure [9,10] . Under physiological conditions solution , its temperature , and on collagen
[19]
[18]
(neutral pH and 37°C), collagen molecules start concentration [17,19,20] . The most effective method
to self-organize into fibrils, and collagen solution to increase the storage modulus is to increase
forms a hydrogel. The printability of collagen the collagen concentration in a solution .
[17]
bioink depends on the kinetics of this process – the Such highly concentrated collagen solutions of
higher the speed, the higher is printing accuracy. 80 mg/ml named Viscoll Bioink (viscous collagen

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