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Dental restorations of resin-based composites mainly fail due to secondary caries and bulk fracture. Self-healing strategies in polymeric materials have been shown to enhance the mechanical properties with the ability of self-repair and crack inhibition, suggesting a prolonged life for dental composite restorations. The systematic review on self-healing dental composites (SHDCs) concluded that a healing performance of 25-80 % recovery rate of the virgin fracture toughness can been achieved based on current literature. The self-healing systems used were PUF microcapsules of DCPD and TEGDMA-DHEPT or silica microcapsules of water/polyacid healing agent. TEGDMA-DHEPT microcapsules were synthesised as they have been proven previously to be biocompatible for dental materials. Microcapsules were prepared by in situ emulsion polymerisation of PUF shells (average diameter 150-300 µm). The experimental SHDC included: Bis-GMA:TEGDMA (1:1), 1 wt% BAPO, 0.5 wt% BPO catalyst, 20 wt% SiO2 (15 nm), and (0, 2.5, 5, 7.5, 10 wt%) of microcapsules. SEM imaging of the capsular shell revealed a smooth outer surface with deposits of PUF nanoparticles which offers a rough surface that may improve resin matrix retention. FT-IR showed that microcapsules crushed with BPO catalyst had DC of up to 60.3 %. The DC of SHDC after 24 h polymerisation was 73-76 % (P > 0.05), micro-hardness 22-26 VHN (P > 0.05), however, the flexural strength was reduced significantly from 80 to 55 MPa with increasing microcapsules to 10 wt% in composites (P < 0.05). Microencapsulation parameters that can affect the microcapsule properties in the material performance were explored. Stirring speed and ratio of core:shell materials influenced: size/polydispersity of microcapsules 130-300 µm; microcapsule fill content 90-98 %; encapsulation efficiency 37-78 %; encapsulation yield 33-59 %; capsule permeability 6-25 % (leaking in 28 days). Composites containing 10 wt% microcapsules showed self-healing performance reached up to 36 % (recovery rate) in dental composite. The model SHDC may have the capability for crack-repair and prevention of catastrophic failure of dental restorations.

Over the past few decades, various types of filler materials have been employed to develop the advanced resin-based dental composites, enhancing the lifetime of the restorations. However, further effort in the research on the multi-functional composite that is comparable to dental tissue in mechanical strength, as well as offering the improved antibacterial function and the better aesthetics, is continuously required. In this thesis, micro-sized short S-glass fibres and halloysite nanotubes (HNTs) are employed to serve as excellent load-carrying filler members and antibacterial agent in the dental composites. The mechanical reinforcement mechanism and the interfacial behaviours between filler and resin matrix have been precisely investigated through the multiscale analysis from atomistic to macro by utilising the combined experimental, theoretical, and computational methods. The surface modification process on the short S-glass fibres, named selective atomic-level metal etching, has been developed, which enables to strengthen the interfacial bond between resin matrix and glass fibre by increasing the surface roughness and reactive sites on the fibre. The influence of the surface treatment on the interfacial strength and mechanical properties of the resulted composites were examined through the single-fibre pull-out tests. Also, the modified Lewis-Nielsen model has been developed, where the effective fibre length factor is applied to accurately predict the modulus of the short fibre reinforced composites. For better understanding of the atomistic interfacial bonding and fracture behaviours between glass fibre and resin matrix, molecular dynamics simulations were conducted. The numerical results of the single fibre pull-out and the uniaxial composite tension simulations were validated with the experimental findings. The optimised computational design and analysis methods were established for developing new dental and bio-composites with the accurate prediction on the mechanical performances. The surface modification process on the HNTs was developed to promote the mechanical reinforcement effect and to add an antimicrobial functionality in the composites. The composite reinforced with 2.0 wt.% of chitosan grafted HNTs showed an increased efficacy in flexural strength and modulus up to 8.1% and 14.1%, respectively, and exhibited an improved antibacterial functionality against S. mutans with 39% reduction, making it a desirable dental material.

Better tools to study the interfacial integrity of dental composites and the factors associated with its eventual breakdown are needed. The initial work focused on the optimization of techniques to study leakage and bond strength. We developed a method for 3D quantification of leakage around dental restorations by using silver nitrate infiltration, micro-CT and image subtraction, overcoming previous limitations of this technique. Also, a new variant of the Brazilian Disk Test specimen was developed for testing dentin-composite bond strength. This novel test provided several advantages including: zero premature failure, simpler testing procedures, a consistent failure mode involving the adhesive interface, and reduced variation in the measurements. We also developed a model to investigate the role of oral biofilms on the degradation of the adhesive interface. Our results suggested that oral biofilms in the presence of sucrose led to active degradation of the adhesive interface. Furthermore, we studied the combined effect of mechanical and biofilm challenge on the interfacial integrity of dental restorations. The results showed that when specimens are subjected to both challenges, a greater reduction in fracture strength is seen compared to the effects of fatigue and biofilm alone, demonstrating the additive effect of both. Again, the presence of biofilm showed a substantial effect in the degradation of the adhesive interface, which was significant in the presence of sucrose. The effect of fatigue was much less pronounced. Antimicrobial containing adhesives are used to prevent the breakdown of the adhesive interface. In the final part of this project, a multispecies oral biofilm/culture model was developed to test the effectiveness of commercially available MDPB-containing adhesive system. We found that the polymerized antimicrobial adhesive reduced biofilm formation and metabolic activity when tested in a closed multispecies biofilm model; however complete inhibition of oral biofilm formation was not observed. The presence of sucrose in the media abolished the antimicrobial effect seen under basal growing conditions. In addition, the polymerized MDPB-containing adhesive failed to inhibit multispecies oral biofilm formation in the absence and presence of sucrose when tested in a Drip Flow Reactor.