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Nano-Hydroxyapatite Bone Scaffolds with Different Porous Structures Processed by Digital Light Processing 3D Printing
A B

C D

Figure 2. Slurry properties, thermogravimetric analysis of HA green part, and heating strategy. (A) Viscosity with different shear rates.
(B) The relationship between cure depth and energy doses. (C) TGA and derivative thermogravimetry (DTG) curve. (D) Debinding and
sintering strategy.

C = 229 8. * lnE ( 0 / 6 89. ) (3.3) for removing the organic phase of green parts. Besides,
d
the other heating stages were heated with 1°C/min up to
From Equation 3.3, the value of cure sensitivity 1250°C. After holding 3 h, the temperature in the furnace
reached 299.8, indicating high curing abilities and fast was gradually decreased to 800°C in 100 min to remove
photo-polymerization speed. the residual thermal stress of sintered scaffolds.
During the DLP printing, the cure depth should X-ray diffraction was performed to confirm the
exceed the printing layer thickness to guarantee a phase composition of the HA powder and sintered HA
sufficient interface combination between layers to avoid ceramic. The standard peaks of HA powders (~25.9°,
the delamination and laminar crack in the sintering . ~31.9°, ~33.0°, etc.) have an agreement with the HA
[31]
Therefore, considering both the cure depth of the slurry PDF card (PDF#73-1731) showing that the purity of the
and printer set-up, 30 µm was set as the printing layer HA powders, as shown in Figure 3. After sintering, the
thickness. ~10 mJ/cm of energy dose with cure depth of new peaks, such as ~31.1° and ~34.5°, were detected
2
~105 µm was correspondingly applied in the printing. demonstrating the phase transformation from HA to beta-
An optimized debinding strategy was beneficial tricalcium phosphate (β-TCP) occurred. However, the
for obtaining crack-free HA scaffold. The thermal strong peaks of HA verified that HA was still abundant in
decomposition of the green parts and the corresponding the sintered ceramics. The transformation was common,
debinding strategy is shown in Figure 2C and D. Before which was also published in many studies [33,34] . The
~300°C, there was rarely mass loss in the green parts . presence of β-TCP is beneficial for BTE applications as it
[32]
Most of the mass loss occurred in the temperature range may increase the dissolution abilities.
between ~360°C and ~480°C. The peak value of the
fastest mass loss occurred at ~430°C. Exceeding ~500°C, 3.2. Dimensional observation of scaffolds
the mass of green parts was stable at ~55 wt. %. The overall morphologies of the three types of scaffolds
According to the TGA result, the debinding strategy are shown in Figure 4A. The upper and lower rows of
was determined, as shown in Figure 2D. In the temperature scaffolds were the green parts and final sintered HA
range between ~270°C and ~500°C, the heating rate was ceramics, respectively. The dimensional difference
set as 0.15°C/min. Three hours were held at the highest revealed the linear shrinkage of the scaffolds during the
speed of the decomposition to provide sufficient time thermal treatment. Form the green part to the sintered part,

202 International Journal of Bioprinting (2022)–Volume 8, Issue 1