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Microstructured Calcium Phosphate Ceramics Scaffolds by Material Extrusion
found on their natural counterparts [10,11] . Developing 3D electrostatic interaction with the polymers and were
printed CaP scaffolds with a complex microstructure not cytotoxic to fibroblasts . Although there has been
[25]
could induce more biomimetic properties. For example, no 3D printed microstructured CaP scaffolds, 3D printing
the microstructure could be designed to increase crack of other microstructured ceramics have been reported,
resistance and impart anisotropic mechanical properties often using alumina microplatelets of high aspect ratio.
to CaP scaffolds. Therefore, new 3D printing strategies Digital light processing (DLP) was used to create various
are needed to allow the fabrication of CaP materials architectures from alumina microplatelets suspended
with controlled shape and design as well as an internal in a UV-curable resin using magnetic alignment . The
[26]
microstructure. alumina microplatelets were pre-coated with iron oxide
Conventional fabrication methods to synthesize nanoparticles so that the microplatelet orientation could be
microstructured CaP include biotemplating, freeze- controlled by magnetic fields applied during DLP. While
casting, and lamination. In biotemplating, natural porous the local control over the alumina microstructure was
microstructured materials such as cuttlefish bone or precise, the composite had a low mineral content of only
[12]
rattan wood [13-16] are decellularized then remineralized 15 vol%) . Alternatively, direct extrusion of alumina
[26]
with CaP by a hydrothermal treatment. Through this microplatelets was used to induce alignment by shear [27,28] .
way, the original porous hierarchical microstructure Alumina filaments with a core-shell microstructure
of the natural material is preserved in the synthetic were robocast to form a twisted plywood structure .
[27]
material. In freeze-casting, also called ice-templating, The pluronic-based gel containing 31 vol% alumina
the microstructure is induced by the directional growth microplatelets and microparticles produced green bodies
of ice dendrites during freezing [17-19] . When subjecting with 36 vol% residual porosity after drying and isostatic
slurries containing ceramic particles to freeze-casting, pressing . Higher sintering temperatures could further
[27]
the growing ice dendrites push and shear the particles, reduce the final porosity through templated grain growth,
leading to the increase in local packing of particles. whereby the surrounding alumina nanoparticles enhanced
When the particles are anisometric, self-assembly and grain growth and densification of the aligned alumina
particle alignment are also reported . After removing platelets without affecting the microstructure [28,29] . While
[20]
the ice dendrites by sublimation, a lamellar ceramic vat photopolymerization-based approaches such as DLP
scaffold with elongated pores oriented in desired achieve higher resolution and better surface finish than
directions is obtained. This method can be combined direct extrusion [10,30] , the volume fraction of particles which
with external fields or temperature gradients to control can be suspended in the photocurable resin is limited by
the microstructure and shape. For example, bidirectional the increase in viscosity. In vat photopolymerization, the
freezing of a hydroxyapatite suspension produced a low ceramic content makes printed parts prone to cracking
biomimetic, nacre-like microstructure with enhanced and shrinkage during debinding and sintering [10,30] .
mechanical properties . Besides, microstructured CaP Direct material extrusion, on the other hand, can yield
[21]
materials can be built using lamination from sheets higher concentrations of solid particles in the green
of CaP platelets or whiskers. For example, a nacre- body. Continuous filament extrusion could therefore be
like composite was made by laminating alginate films applied to an ink composition containing anisometric
containing CaP microplatelets . A twisted plywood CaP minerals with the microstructure controlled using
[22]
or Bouligand microstructure could also be made using shear stresses. Along with the microstructure, controllable
CaP microfibers, leading to significant enhancement in macroscopic porosity as found in conventional bone
the toughness of the composite . Unfortunately, these scaffolds would also be required in 3D printed structures
[23]
methods are not yet easily compatible with 3D printing, to allow vascularization and cell invasion. Such porous
which is required to provide patient-adapted shapes and structures have not been reported with the microstructured
tunable designs of internal pore structures [10,24] . alumina printing.
3D printed CaP scaffolds have demonstrated In this work, we 3D print microstructured CaP parts
promising osteogenic ability for bone tissue engineering by direct ink writing (DIW), based on line-by-line material
and research has been growing [10,24] . In particular, direct extrusion. The microstructure is created by orienting
material extrusion (or robocasting) and indirect vat synthesized brushite (CaHPO ·2H O) microplatelets
2
4
photopolymerization-based techniques have been the suspended in a water-based ink using shear stresses that
most extensively used for CaPs. In an interesting example develop in the printing nozzle. The ink is designed to
which applied photopolymerization after material allow simultaneously efficient particle alignment and high
extrusion with cell seeding, an ultraviolet (UV)-curable solid loading after printing, of about 23 vol%. The strategy
hydrogel scaffold was bioprinted with only up to 30 wt% to create complex shapes is to simultaneously dry while
nanocrystalline CaPs formed by in situ precipitation, which depositing the ink onto a porous gypsum substrate that
contributed to the scaffold’s compressive strength through sucks out water from the deposited ink through capillary

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