manufacturing techniques, 3D bioprinting allows the free-form
fabrication of 3D shapes through the deposition of consecutive
layers of hydrogel strands that contain living cells [3].
To succeed in the biofabrication process, a key factor consists in
formulating a proper hydrogel precursor solution, the bioink [4]. In
fact, an ideal bioink should possess two fundamental features: first,
the bioink should instantaneously turn into a gel as soon as it is
deposited; second, it should support cell migration, proliferation,
and differentiation. Hence, formulating a bioink is an extremely
complex task as most natural biopolymers in their pristine version
(i.e., not chemically modified) do not undergo a fast sol-gel transition under mild and cell-friendly conditions.
To overcome this issue, a common strategy consists in tuning
the rheological properties of bioinks to have a pronounced
shear-thinning behavior [5–7]. In this way, bioinks can easily flow
(liquid-like behavior) under applied pressure and rapidly form a gel
(solid-like behavior) after extrusion when the force is removed.
However, this strategy is generally time-consuming and requires a
thorough rheological characterization and a fine optimization of all
bioprinting parameters.
Another approach consists in blending one or more biopolymers with an additional templating agent [8, 9]. The main role of
such compound consists in improving the processability and printability of other biopolymers in blend, without affecting cell
viability.
So far, the most common templating agent used in 3D bioprinting experiments is alginate. Alginate is a polysaccharide
extracted from brown algae that is capable of undergoing a fast,
reversible, and cell-friendly gelation in presence of divalent ions
(such as Ca
2+ , Sr
2+
, Ba
2+ ). This unique feature generally allows
the fabrication of high-resolution and high-shape fidelity constructs, with a cell viability above 80%. Additionally, alginate is
commercially available in a wide range of molecular weight values
ranging from few tens to hundreds of kDa, allowing researchers to
easily tune the rheological properties of the final bioinks.
Alginate-based bioinks have been deposited in 3D using several
strategies, including inkjet printing [10, 11], divalent ion spraying
[12, 13], printing in a coagulation bath [14, 15], pre-crosslinking
[16], and coaxial nozzle extrusion [17, 18]. Among these, coaxial
nozzle extrusion represents one of the most versatile and powerful
approaches. This approach consists of two coaxially mounted nozzles in which an alginate-based bioink and a calcium chloride
solution flow. Hydrogel fiber gelation takes place instantaneously
at the tip of the coaxial extruder where the two solutions meet,
allowing the fine deposition of hydrogel strands [19].
A unique advantage of coaxial systems consists in the possibility
to deposit either bulky or hollow (i.e., perfusable) tiny fibers down
to ~100 μm in diameter. This can be easily achieved by supplying
46
Marco Costantini et al.
fabrication of 3D shapes through the deposition of consecutive
layers of hydrogel strands that contain living cells [3].
To succeed in the biofabrication process, a key factor consists in
formulating a proper hydrogel precursor solution, the bioink [4]. In
fact, an ideal bioink should possess two fundamental features: first,
the bioink should instantaneously turn into a gel as soon as it is
deposited; second, it should support cell migration, proliferation,
and differentiation. Hence, formulating a bioink is an extremely
complex task as most natural biopolymers in their pristine version
(i.e., not chemically modified) do not undergo a fast sol-gel transition under mild and cell-friendly conditions.
To overcome this issue, a common strategy consists in tuning
the rheological properties of bioinks to have a pronounced
shear-thinning behavior [5–7]. In this way, bioinks can easily flow
(liquid-like behavior) under applied pressure and rapidly form a gel
(solid-like behavior) after extrusion when the force is removed.
However, this strategy is generally time-consuming and requires a
thorough rheological characterization and a fine optimization of all
bioprinting parameters.
Another approach consists in blending one or more biopolymers with an additional templating agent [8, 9]. The main role of
such compound consists in improving the processability and printability of other biopolymers in blend, without affecting cell
viability.
So far, the most common templating agent used in 3D bioprinting experiments is alginate. Alginate is a polysaccharide
extracted from brown algae that is capable of undergoing a fast,
reversible, and cell-friendly gelation in presence of divalent ions
(such as Ca
2+ , Sr
2+
, Ba
2+ ). This unique feature generally allows
the fabrication of high-resolution and high-shape fidelity constructs, with a cell viability above 80%. Additionally, alginate is
commercially available in a wide range of molecular weight values
ranging from few tens to hundreds of kDa, allowing researchers to
easily tune the rheological properties of the final bioinks.
Alginate-based bioinks have been deposited in 3D using several
strategies, including inkjet printing [10, 11], divalent ion spraying
[12, 13], printing in a coagulation bath [14, 15], pre-crosslinking
[16], and coaxial nozzle extrusion [17, 18]. Among these, coaxial
nozzle extrusion represents one of the most versatile and powerful
approaches. This approach consists of two coaxially mounted nozzles in which an alginate-based bioink and a calcium chloride
solution flow. Hydrogel fiber gelation takes place instantaneously
at the tip of the coaxial extruder where the two solutions meet,
allowing the fine deposition of hydrogel strands [19].
A unique advantage of coaxial systems consists in the possibility
to deposit either bulky or hollow (i.e., perfusable) tiny fibers down
to ~100 μm in diameter. This can be easily achieved by supplying
46
Marco Costantini et al.