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Dee, et al.
thickness (see Supplementary file: Section 1.2). These viscosity of the inks increased with the concentration of
dimensions were chosen because microplatelets with brushite. In particular, the ink with 27 vol% brushite was
nanoscopic thickness are thought to be optimal to achieve paste-like, with an increased viscosity at rest of 600 Pa∙s
platelet pull-out instead of brittle fracture in bioinspired (see Supplementary file: Section 2). Furthermore, the
microstructured composites . The ink solvent was water ink gradually shifted from a zero-shear viscosity profile at
[3]
to allow good dispersion and high concentration of lower solid loadings to a yield stress profile at the highest
brushite microplatelets. An anionic surfactant was used as solid loading of Φ = 27 vol% (Figure 3A). The inks
dispersant and PVP was used as the binder to strengthen were modeled as Herschel-Bulkley fluids and their flow
−1
−1
the green part after drying. The printed parts were dried profiles from 10 s to 100 s were fitted with:
overnight before calcination at 900°C for a dwell time of τ =τ y + Kγ n  (3.1)
6 h to yield a stiff, consolidated part. The arrangement of
microplatelets within the printed filaments remained intact where τ is the shear stress (Pa), τ the yield stress
y
−1
as the low calcination temperature of 900°C prevented (Pa), the shear rate (s ), K the consistency index and
[28,33]
necking and shrinkage, while ensuring complete phase n the flow index . All inks had a flow index n < 1,
transformation into β-calcium pyrophosphate (β-CPP, confirming their shear-thinning properties, which is
Ca P O ) (Supplementary file: Section 1.1). After necessary for the extrusion (Figure 3B). Also as expected,
[32]
the consistency increased with solid loading, denoting
2 2
7
printing, drying, and calcinating, the CaP material has a less fluidity as more brushite particles are loaded in the
relative density of about 23 vol%. ink. These data suggest that all inks can be extruded
The key to enabling extrusion at the nozzle,
microstructuring and high relative density in the final through a nozzle. However, a yield stress of 100 ~ 800 Pa
is typically reported as one of the printability criteria for
material resides in the optimization of the brushite solid extruded filaments to support their own weight and avoid
content in the ink, whereas the realization of complex, sagging . From Equation 3.1, only the ink containing 27
[34]
3D shapes is governed by the fast drying of the ink on the vol% CaP microplatelets exhibits a yield stress. Another
porous substrate. How to determine the ink composition important rheological property is the stiffness of the ink
and its printability are detailed in the following section. at rest indicated by G’ , which is the plateau value of
eq
3.2. Rheology and printability of brushite inks the storage modulus before the yield point [28,34] . Printable
ceramic inks typically have G’ at rest ranging from 25
eq
The rheological properties of inks were tested to determine to 200 kPa and a damping factor at rest G” /G’ ranging
eq
eq
the required brushite solid loading Φ (Figure 3). The from 0.1 to 0.3 . These properties are key to realizing
[34]
primary property that had to be achieved is flowability to shape fidelity. The viscoelastic properties of brushite inks
enable extrusion through a nozzle. To this aim, brushite at various solid loadings were thus measured. In all of our
inks with solid loadings from 16 to 27 vol% were tested brushite inks, liquid-like behavior generally dominated
in viscometry (Figure 3A). As expected, the apparent with damping factor G”/G’ > 1, even for 27 vol% which

Figure 2. Schematic of the 3D printing strategy: scalable synthesis of brushite microplatelets of high aspect ratio loaded into an aqueous-
based ink. The ink is 3D printed by direct extrusion onto porous substrate, before drying and calcination to yield a solid, microstructured part.

International Journal of Bioprinting (2022)–Volume 8, Issue 2 113