Page 126 - IJB-8-2

Microstructured Calcium Phosphate Ceramics Scaffolds by Material Extrusion

  a −τ y τ 1 the representative platelet orientation α of each segment;
γ a =   (3.6.) we can visualize the gradient platelet orientation in a
n
 K  typical filament. At the bottom of the filament, CaP
Platelets in the core experienced γ  γ <  a in the platelets were oriented close to 0°, almost parallel to
nozzle and hence remained disordered upon extrusion, the print substrate (Figure 6E). The angle α increased
sharply to 45° before decreasing gradually to ~0° near the


while platelets in the shell experienced γ γ > a and top. Indeed, the platelets preferentially aligned with the
preferentially aligned to the nozzle wall. Indeed, our extrusion direction, especially near the nozzle wall where
observations of the misalignment ratio at different print the shear stresses are highest. Platelets at the bottommost
parameters obey the model described by Equation 3.4 and topmost segments also displayed the most uniform
(see Supplementary File: Section 1.2 and 3.2 for details alignment (Figure 6F). Filaments extruded at different
on the model). Furthermore, our experimental results print parameters are expected to exhibit similar trends
showed that t ∝ 1/d (see Supplementary File: Section since their misalignment ratio is about the same.
a
3.2) and we therefore obtain the following relation: Having studied the microstructure within one single
 1 printed filament, we now look at complex 3D prints
γ a ∝ (3.7)
t × d to show how our 3D printing approach can be used to
a
Although changing the print parameters such as flow construct multi-layered bulk or porous, overhanging
structures, while retaining the core-shell microstructure.
rate multiplier f or the nozzle diameter d changes the volume
flow rate Q and shear stress, the misalignment ratio remains 3.4. Building microstructured multi-layered
constant since t changes at the same time. For example, structures
a
when f is increased from 500% to 800% (Figure 6C), the
increase in Q leads to the higher shear stress and shear rate (1) Buildability of microstructured CaP
experienced by the platelets in the nozzle, so the required Moving from single filaments, we explored the capability
alignment time t is reduced. At the same time, the increased of the printing strategy to build microstructured multi-
a
ink extrusion rate provides less time for platelets to rotate layered structures. Thanks to the drying method, good
and align to the equilibrium orientation, resulting in an buildability of multi-layered structures from 21 vol%
unchanged misalignment ratio when f increased. Therefore, brushite ink was possible at room temperature. Figure 7A
the filament microstructure is unaffected by printing shows that multi-layered parts have the core-shell
parameters and is expected to depend solely on the ink’s microstructure like in single filaments (Figure 5A). The
rheology. For a given ink, this enables design flexibility at post-calcination layer height is ~0.5 mm using the nozzle
the macro-level (print resolution and print speed) without of 0.58 mm diameter at f = 500% (Figure 7A).
affecting the core-shell microstructure. Furthermore, with Since solvent removal by capillary action starts at
the help of the model and the possibility to relate the the substrate surface, it can be expected that there is a
rheological properties to the microstructure, it should be
possible to design inks to obtain specific microstructures. maximum height above which the drying takes too long
to build more layers with good filament shape fidelity.
For example, to tune the misalignment ratio, the viscosity In practice, this issue can be circumvented through
could be modified by fine tuning the concentration in PVP combination with evaporative drying using a fan, heating
in the ink, without modifying the concentration of CaP
microplatelets. gun, infrared lamp, or in an environment with controlled
To confirm the platelet orientation inside a printed temperature. Figure 7B shows a thin wall 6 layers tall
filament, SEM micrographs of the x-z cross-section of printed at room temperature at v = 5 mm/s. By heating
those filaments were examined (Figure 6D). Gradient gypsum on a hot plate, a 20-layer tall thin wall can be
changes in the platelet orientation from the bottom built at v = 1 mm/s (Figure 7C). In Figure 7D, a small
of the filament to the middle and from the middle to jigsaw piece ten layers thick was printed on heated
the top were observed, as expected from the gradient gypsum, showing the construction of intricate parts for
increase in shear stress towards the filament edges due custom repairs. Indeed, print fidelity on heated gypsum
to the laminar flow. To quantify the change in platelet enables near net shape fabrication of ceramics.
orientation, the electron micrographs of each filament (2) Porosity and print resolution
were divided into six segments of equal height from
bottom to top, and the platelet orientation and the degree β-CPP has similar osteoinductivity as hydroxyapatite,
of alignment in each segment were quantified by Fast and a biodegradability rate between hydroxyapatite and
Fourier Transformation (FFT). The FFT results for β-tricalcium phosphate (β-TCP and Ca PO ) . The
[38]
3
4
filaments extruded at d = 0.58, f = 500%, v = 5 mm/s in resorption rate is dependent on the crystallographic
the +x direction are presented in Figures 6D-F. Taking structure which could be altered by sterilization and other
118 International Journal of Bioprinting (2022)–Volume 8, Issue 2