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3D freeform printing of nanocomposite hydrogels
a printed hydrogel using their precursors. As proof- first material and ink change time. However, both
of-concept, two multi-material structural designs printed structures exhibited good mechanical
were created and printed with two nanocomposite stability and no delamination at the interface
hydrogels, HAc-Alg/15 wt% CaP and HAc-Alg/30 of the two materials. More importantly, in the
wt% CaP (Figure 8). freeform printing of hydrogel inks in aqueous
The mineral contents of HAc-Alg/CaP solutions, all of the printed gels were maintained
were varied by altering the phosphate ion in the swollen state and reacted in situ during
concentration in the HAc-Alg hydrogel inks, printing. Thus, the introduction of different
as illustrated in Figure 8A. As the precursors materials with various swelling behaviors did not
of CaP nanoparticles were ions, different induce any structural mismatch or distortion of
concentrations of these precursors did not the multi-material systems when the hydrogels
change the viscosity of the printing inks or were immersed in water.
gelatin-containing viscous medium. Two types The composite hydrogel systems can be applied
of HAc-Alg inks with different phosphate ion for scaffolds of interface tissue engineering (ITE),
concentrations were printed in a gelatin bath which aims to regenerate the native enthesis or
with excess CaCl using the same printing interface tissue between hard and soft tissues. Thus,
2
parameters. We had already confirmed that ITE requires multiphase and gradient biomaterials
the precipitated CaP nanocrystals distributed to engineer both types of tissue [46-51] . As gradient
uniformly within the hydrogel matrix regardless biomaterials often mimic the complex structures
of the precursor concentrations. During 3D and material properties of each soft and hard tissue,
printing, the printed inks contained the non- advanced micro- and nano-technologies such as
crosslinked GM-HAc liquid phase. Therefore, microfluidics, electrospinning, and bioprinting
sequential 3D printing of two different hydrogel have been introduced into the conventional
inks using only one printer head was feasible. fabrication process of biomaterials to capture
This avoided any interfacial problem because these dimensions . In particular, recent advances
[47]
the GM-HAc liquid diffused into the contact in 3D printing technologies have facilitated the
region around the joints of the printed filaments, development of composite biomaterials through
which promoted joint fusion and improved the ability to fabricate structurally, functionally,
bonding strength among the filaments of the and compositionally intricate constructs [1-4] . Thus,
adjacent layers. The two structural designs, our approach for multiphase composite scaffolds
vertical stacking and horizontal stacking of the can be applied for the fabrication of various
two composite hydrogels resulted in different functional or hybridized gradient biomaterials
printing sequence and interfaces between the with complex geometries for ITE scaffolds.
two materials with different mineral contents
(Figure 8B and C). In the case of vertical 4 Conclusion
stacking, the bottom layer of HAc-Alg/10 wt%
CaP could be deposited on the top layer of In this study, by introducing an in situ inorganic
HAc-Alg/30 wt% CaP, thus minimizing the time nanoparticle precipitation process to a 3D freeform
lapse between the printing of each material due printing system with a two-step crosslinking
to ink change. In contrast, horizontal stacking strategy, we successfully fabricated HAc-Alg/CaP
was achieved by printing the inner part with nanocomposite hydrogel scaffolds with various
HAc-Alg/10 wt% CaP, followed by printing the mineral contents and good structural integrity. The
external part of the structure with HAc-Alg/30 first ionic crosslinking of Alg provided structural
wt% CaP (Supplementary Figure 11). In this stability during printing, while the secondary
case, the time lapse between the first layers of the crosslinking of photo-curable GM-HAc improved
two materials was more than 5 min, which was the mechanical stability of the nanocomposite
equivalent to the summation of; printing time of the hydrogels by virtue of the superior bonding
46 International Journal of Bioprinting (2020)–Volume 6, Issue 2
