Bio-inspired nacre-like ceramic/polymer composites for dentistry

Abstract

Despite its extensive use in dentistry, zirconia still has some drawbacks. These include
increased wear on opposing teeth, poor machinability due to its high hardness, and potential
long-term ageing and aesthetics issues. Recent advances in the manufacture of polymerinfiltrated ceramic network (PICN) materials, which combine the benefits of both ceramic
and polymer to achieve tooth-matching mechanical properties, present a promising
alternative to pure zirconia, balancing aesthetics and machinability. To further enhance the
performance of PICN materials, biomimicry has been employed to improve the fracture
toughness of these composites. Inspired by nacre, known for its excellent combination of
strength and toughness, artificial nacre-like composites have been developed with a ‘brick
and mortar’ lamellar structure that mimics natural nacre. These composites show
simultaneously improved strength and fracture toughness. However, research into developing
such nacre-like composites specifically for dental applications remains limited.
In this study, bioinspired nacre-like ceramic/polymer composites were fabricated,
including 3 mol% yttria-stabilised tetragonal zirconia polycrystal (3Y-TZP)/polymethyl
methacrylate (PMMA), 5 mol% yttria partially stabilised zirconia (5Y-PSZ)/PMMA,
zirconia-toughened alumina (ZTA)/PMMA, and glass flake/PMMA. These composites were
fabricated using bi-directional freeze casting and self-assembly via vacuum filtration to
produce ceramic or glass scaffolds with lamellar microstructures. PMMA was then
incorporated through in-situ thermal polymerisation of methyl methacrylate (MMA)
monomers. The resulting composites exhibited a ‘brick and mortar’ structure similar to
natural nacre. These nacre-like composites demonstrated suitable flexural strength for dental
applications (up to 400 MPa, 290 MPa, 370 MPa, and 150 MPa for 3Y-TZP/PMMA, 5Y-PSZ/PMMA, ZTA/PMMA, and glass flake/PMMA composites, respectively), along with
tooth-mimicking hardness and elastic modulus. Such properties would contribute to their
clinical safety, reducing the risk of unwanted tooth wear. Additionally, they showed
exceptional crack resistance under load, with fracture toughness reach up to 13-18 MPa.m1/2
for 3Y-TZP/PMMA and ZTA/PMMA composites. Their superior fracture toughness could
provide enhanced performance and durability in the oral cavity under repetitive chewing
stresses.
In addition to their mechanical performance, these composites offered several functional
benefits. The machinability of the 3Y-TZP/PMMA composite was comparable to the current
benchmark product, Vita Enamic ((Vita Zahnfabrik, Bad Säckingen, Germany), making it
suitable for chairside computer aided design and computer aided manufacture (CAD/CAM)
dental restorations. To assess ageing resistance, hydrothermal ageing tests were conducted.
The results demonstrated that ZTA/PMMA exhibited enhanced ageing resistance compared to
3Y-TZP/PMMA, exhibiting a slower rate of tetragonal to monoclinic phase transformation
and more stable mechanical properties throughout the ageing process. Additionally, an
antibacterial nacre-like glass flake/polymer composite with a translucent appearance has been
developed. This composite is suitable for aesthetic restorations such as veneers and
orthodontic brackets. Its antibacterial properties may help lower the risk of secondary caries
and white spot lesions (WSLs) caused by cariogenic bacteria in both applications.
In summary, these nacre-like ceramic/polymer composites with layered microstructures
exhibit excellent mechanical properties and exceptional crack resistance, making them
suitable for various dental applications. Incorporating functional properties into these
composites could further improve their clinical performance and broaden their potential uses
in dentistry.

Date of Award	4 Feb 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Bo Su (Supervisor) & Tony Ireland (Supervisor)