Synthesis and Processing of a 3D-Printed Ultra-Translucent Zirconia Ink for Dental Prosthesis 

Edmara T.P. Bergamo*^1,2, Marly Valle², Edisa Souza³, Larissa M. Alves³, Ernesto B. Benalcazar Jalkh³, Tiago M. Campos, Estevam A. Bonfante³, James E. Smay⁴, Leila Jahangiri¹, Thomas Giugliano¹, Mijin Choi¹, Lukasz Witek^2,5,6.

Introduction. Synthesize an ultra-translucent zirconia colloidal gel for 3D printing (3DP), designed with different layer thicknesses, and evaluate the physical and mechanical properties relative to the conventional pressed counterpart.

Methods. A colloidal gel suspension was fabricated using an ultra-translucent zirconia powder (5Y-PSZ, Zpex Smile, Tosoh Corporation) mixed with predetermined volumes of distilled water, dispersant polymer (ammonium polyacrylate), viscoscifier (hydroxypropylmethylcellulose, and flocculant (polyethyleneimine). Disc-shaped specimens were designed and 3DP using direct-ink writing (DIW) at a print velocity of 2 mm/s and deposition rate of ~0.1 mm³/s. The 3D-printing parameters were configured to print layers with thicknesses of 250 and 500 μm. The control group comprised of ceramic discs fabricated by uniaxially pressing powder at 3,500 Kgf/cm² for 30 seconds using a metallic matrix. All discs were pre-sintered at 1000°C/1h and sintered at 1450°C/2h. Crystalline content was determined by X-ray diffraction (XRD) and the microstructure by scanning electron microscopy (SEM). The mechanical properties were assessed by piston-on-three-balls test to determine the flexural strength (ISO 6872:2015) and nanoindentation to determine the elastic modulus and hardness.

Results. XRD spectra showed a typical crystalline content, primarily consisting of cubic zirconia phase. SEM micrographs exhibited a dense and homogeneous polycrystalline matrix for both processing methods; however, pores/defects were more prominent for discs fabricated using the 3DP method. Weibull modulus ranged from 2.5 to 4.3 for all tested groups. Pressed zirconia discs demonstrated significantly higher characteristic strength [474 (426-529) MPa] relative to 3DP zirconia discs, with both 3DP-thickness discs showing no significant difference between them [250 μm: 340 (CI:316-398) MPa/ 500 μm: 355 (CI: 283-408) MPa]. Independent of fabrication technique (e.g., pressed or 3DP) zirconia presented high probability of survival for missions up to 100 MPa (>96%), with no significant difference between the two methods, which is equivalent to the indication of single-unit prostheses, according to the ISO 6872 requirements. 

Conclusion. The results of this study demonstrate that the ultra-translucent zirconia colloidal gel obtained for 3D printing was successfully synthesized and processed with different layer thicknesses, being suitable for the fabrication of dental crowns.