Page 119 - IJB-8-2

Dee, et al.
forces. The CaP parts could later be consolidated with the 2.2. Preparation and characterization of the
microstructure retained by calcination at a temperature brushite ink
lower than the sintering temperature. First, we study
the rheological properties of aqueous inks at various The aqueous brushite ink consisted of various volume
solid loadings of brushite and assess the printability fraction Φ of brushite microplatelets, an anionic surfactant
to obtain a good print resolution. Then, we analyze the Dolapix CE 64 (Zschimmer & Schwarz, Germany) as the
microstructure of extruded filaments and use a simple dispersant, and polyvinylpyrrolidone (PVP) of average
model to relate the microstructure to the rheology of the molecular weight ~360,000 (Sigma-Aldrich) as the
ink. Finally, we demonstrate that complex, porous or bulk binder. Synthesized brushite powder was slowly added to
3D shapes can be obtained with our approach, such as deionized water containing 0.1 wt% Dolapix CE 64 with
scaffolds or bone plates. This work is anticipated to be of respect to brushite, frequently mixing with vortex and
interest for fabricating patient-adapted implants with more probe sonicator (Bandelin), resting 2 s between every 1 s
biomimetic microstructure to enable enhanced properties. of low amplitude ultrasonic pulse to prevent overheating.
Future work could tackle infiltration of the 3D printed CaP 20 w/v% PVP stock solution was added to the suspension
scaffolds with a biomimetic polymeric matrix to study the to give a final concentration of 7 w/v% PVP with respect
mechanical properties and the cellular response. to water. The brushite ink was homogenized by mixing
with vortex and probe sonicator before use.
2. Materials and methods The rheological properties of the inks were measured
using a rheometer (Bohlin, Malvern Instruments) and
2.1. Synthesis and characterization of brushite 15 mm serrated plates. All measurements were performed
microplatelets at room temperature with a gap of 1000 µm. An amplitude
sweep from 0.1 Pa to 1000 Pa was carried out in oscillatory
Brushite microplatelets were prepared using a method mode at 1 s . Rotational measurements were carried out
−1
described by Gao et al. and Mandel and Tas . Briefly, from 0.01 s to 100 s . All measurements were repeated
[22]
[31]
−1
−1
6.062 g of calcium chloride (CaCl ) (anhydrous, ≥97%, at least three times.
2
Sigma) dissolved in 0.4 L of deionized water was rapidly
added to a beaker with 1.650 g of potassium dihydrogen 2.3. Drying of brushite ink
phosphate (KH PO ) (anhydrous, ≥99% ACS, VWR
4
2
Chemicals, Singapore) and 6.026 g of disodium hydrogen The possibility of drying extruded brushite inks by
phosphate (Na HPO ) (anhydrous, ≥99% ACS, VWR solvent removal through a water-absorbent print substrate
4
2
Chemicals, Singapore) dissolved in 1.4 L of deionized was studied using gypsum, also known as plaster of
water. After stirring at room temperature for 1 h, the white Paris. Gypsum is an abundant, safe, and easily moldable
precipitate was collected by vacuum filtration and dried in material. Gypsum was prepared by mixing 6 parts water
a 45°C oven for 2 days. 6.062 g CaCl in 1.8 L reaction to every 10 parts gypsum casting powder (Cera-Mix
2
mixture produced a yield of ~7.0 g brushite. The synthesis Standard Plaster Casting Compound) by weight. To
is easily scalable, with a yield of ~10.6 g brushite collected obtain a flat gypsum substrate, the gypsum slurry was
from 2.7 L reaction mixture using 9.093 g CaCl . The cast onto a non-stick surface and mechanically smoothed
2
powder was stored in a dry box until use. at the top by pressing with a flat surface while wet, then
The synthesized powder was sputtered with gold left overnight to set.
and examined under a field emission scanning electron A 5 µL droplet of 21 vol% brushite ink was
microscope (SEM) (JEOL 6340F) to determine its deposited on the gypsum slab with a micropipette.
dimensions. The powder was grinded using mortar and A digital microscope (Dino-Lite Edge AM7915MZTL)
pestle and characterized using powder X-ray diffraction was used to record videos of the droplet drying on the
(XRD) with Bragg-Brentano geometry (Panalytical gypsum substrate. The droplet size and contact angle
X’Pert Pro, Cu K 0.1541874 nm, scanning range 2θ were measured from the video snapshots using ImageJ.
α
from 10° to 60°, step size 0.017°). The scanned material The experiment took place at an ambient temperature
was checked against the PDF-4+ database (2021) using of ~22°C and relative humidity ~74%.
Match! (version 3.6, Crystal Impact). Reference CIF files 2.4. Direct material extrusion of brushite
obtained from the Inorganic Crystal Structure Database
(ICSD, FIZ Karlsruhe) was used for analysis in TOPAS For 3D printing, the brushite ink was filled into a 10 mL
(version 6, Bruker AXS). Energy dispersive spectroscopy Luer-lock syringe (Terumo) of inner diameter 14.3 mm.
(EDS) was performed on coated powder in a thermionic Cone-shaped polyethylene nozzles (Nordson EFD
SEM (JEOL 5500LV) with Oxford Inca 200 detector, Optimum SmoothFlow) were used. The syringe was
20 kV accelerating voltage) to measure the calcium/ fitted to a clay printer (3D Potterbot Micro 8, USA)
phosphorus atomic ratio. using a custom adapter to enable mechanical extrusion of

International Journal of Bioprinting (2022)–Volume 8, Issue 2 111