Bio-inspired nacre-like zirconia/PMMA composite for dental CAD/CAM restoration
Huijun Sun^a, Parinaz Tabrizian^a, Aqsa Qambrani^b, Urangua Jargalsaikhan^b, Tan Sui^b, Tony Ireland^a, Bo Su^a
a Biomaterials Engineering Group, Bristol Dental School, University of Bristol, UK
^b School of Mechanical Engineering Sciences, University of Surrey, UK
 
Zirconia is a promising ceramic material used in dental CAD/CAM applications with outstanding strength. However, the mismatched hardness and modulus with the human tooth, pale appearance, and aging concern limit its clinical performance. Zirconia can be endowed with different properties by adding stabilisers such as yttria with different content. Among them, the 3 mol% yttria-stabilised tetragonal zirconia polycrystal (3Y-TZP) is characterized by outstanding mechanical properties, while the 5 mol% yttria partially stabilised zirconia (5Y-PSZ) presents better aesthetics and aging-resistance, but inferior mechanical properties(1,2).
On the other hand, the polymer-infiltrated-ceramic-network (PICN), with a sintered porous ceramic matrix infiltrated with polymer offers a way to achieve excellent strength, crack resistance, and tunable stiffness or hardness, potentially contributing to better long-term clinical performance. Moreover, efforts of bio-inspiration have been made to fabricate composite mimicking the ‘brick and mortar’ microstructure of nacre, which has both high strength and toughness. As a result, zirconia could be incorporated into PICN with a nacre-like microstructure to fabricate dental CAD/CAM restoration(3,4).
In this study, two types of bio-inspired nacre-like zirconia/PMMA composites, i.e., 3Y-TZP/PMMA and 5Y-PSZ/PMMA composites were fabricated via bi-directional freeze casting. The hierarchical structure is presented as a similar 'brick and mortar' structure of the nacre. Excellent flexural strength (up to 350 MPa and 230 MPa for 3Y-TZP/PMMA and 5Y-PSZ/PMMA composite, respectively), hardness and modulus within the range of those of enamel and dentine, and an increasing crack-resistance behaviour during crack propagation have been shown in both zirconia composites. Several extrinsic toughening mechanisms, such as crack deflection, ceramic bridges, and pull-out of the ceramic walls were characterised under in-situ SEM during the fracture toughness testing. In addition, the 5Y-PSZ/PMMA composite presented better total transmittance in the trans-layer direction than the 3Y-TZP/PMMA composite.
These two zirconia/PMMA composites exhibit the potential for dental CAD/CAM application as their mechanical properties are competent to be used as dental restorations and matched to those of human enamel. The nacre-mimetic-originated crack resistance could ensure better long-term clinical performance.

Reference 
1.         Zhang, F. et al. Strength, toughness and aging stability of highly-translucent Y-TZP ceramics for dental restorations. Dental Materials 32, e327–e337 (2016).
2.         Zhang, Y. & Lawn, B. R. Novel Zirconia Materials in Dentistry. J Dent Res 97, 140–147 (2018).
3.         Della Bona, A., Corazza, P. H. & Zhang, Y. Characterization of a polymer-infiltrated ceramic-network material. Dental Materials 30, 564–569 (2014).
4.         Tan, G. et al. Nature-Inspired Nacre-Like Composites Combining Human Tooth-Matching Elasticity and Hardness with Exceptional Damage Tolerance. Advanced Materials 31, 1–9 (2019).