Multi-scale analysis of the influence of filler shapes on the mechanical performance of resin composites using high resolution nano-CT images
Introduction
Posterior restorations have been increasingly performed with resin composites (RCs) to satisfy the aesthetic demands of patients [1]. However, the mechanical properties of RCs have some limitations compared with metal and ceramics [2]. RCs with universal shade, consisting of a resin matrix and glass filler, can be color matched to the tooth being restored, providing an aesthetic advantage [3,4]. Newer compositions of these RC materials have been improved to withstand stress and wear [5]. RCs have been invented to closely mimic the aesthetics and function of natural tooth tissue, and their longevity in the oral environment depends largely on their fatigue or wear properties [6]. Identification of the mechanical properties of commercially-available RCs is difficult because of the variety of synthesis conditions. Clinical reviews have shown that the wear of RCs largely depend on the particle size and volume of filler or the resin matrix [7,8]. However, other factors such as filler shape, filler distribution, coupling agent between filler and monomer, and filler/monomer composition ratio have not been considered.
Although the impact of the size and content of filler on the mechanical properties of RCs has been reported [9], the relationship between the mechanical properties and the RC micro-structure has not been clarified because the above other factors are influenced by one another and are not controllable as an independent parameter in vitro. Also, distinguishing nano-hybrids from micro-hybrids is difficult and their mechanical properties, such as flexural strength and modulus, tend to be similar [10]. In this study, the mechanical properties of RCs at the micro- and macro-scale were evaluated using in silico models with precise simulation of the micro-structure reconstructed based on high-resolution nano-CT images. Multiple factors, such as the amount of unpolymerized methacrylate monomer, the degree of hydrolysis at the interface between the resin matrix and filler [11], and temperature [12], are considered in an in vitro test. However, a method identifying the fracture initiation of materials has not been established and details of the 3D shape/structure of filler has not been investigated because of their small diameter.
In silico multi-scale analysis is a method for analyzing physical properties and their behavior among different size scales, which can be used to address issues in the characterization of heterogeneous substances by considering the material properties at the micro-scale [13]. In silico multi-scale analysis has been further enhanced to solve issues with the analysis of the mechanical properties of RCs [14] and the influence of the physical properties of an implant on cortical bone resorption [15].
The finite element analysis used in the in silico multi-scale analysis is a numerical approach to investigate the mechanical properties, and the maximum principal strain is used as a fracture criterion to predict the flexural strength of RCs [16,17]. The utility of the maximum principal strain at the micro-scale has not been reported because of the limited resolution of three-dimensional imaging technologies such as micro-CT to construct numerical model reflect to real morphology of fillers [9].
The aim of this study was to investigate the fracture criteria for predicting the fracture initiation of RCs at the micro-scale and assess the influence of filler shapes on the flexural properties of RCs by combining nano-CT imaging and in silico multi-scale analysis.
Section snippets
Sample preparation
An experimental RC containing irregular-shaped (IS) silica filler and 2,2-bis[4-(2-hydroxy-3-methacrylyloxypropoxy)-phenyl]-propane (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) was prepared. The content (monomer/filler) of the experimental RC was 68.8/31.2 vol% (50.0/50.0 wt%). The monomer compositions were Bis-GMA (75 wt%), TEGDMA (25 wt%), benzoyl peroxide (BPO) (1.4 wt%), dl-camphorquinone (CQ) (0.7 wt%), and aldehyde compound with dibutylhydroxytoluene (2.17 wt%). IS silica glass
Homogenization analysis
In the X, Y, and Z directions, significantly greater elastic modulus values were obtained for the RC models containing IS filler than with RC models containing SS filler (p < 0.05) (Fig. 3a). In the Y and Z directions, significantly smaller Poisson’s ratios were obtained for RC models containing IS filler than with RC models containing SS filler (p < 0.05) (Fig. 3b).
In silico three-point bending test
The flexural strength of the RC models containing IS filler was significantly greater than RC models containing SS filler (p <
Discussion
We established an in silico multi-scale analysis of RCs with different filler shapes (irregular/sphere). The multi-scale analysis clarifies the influence of the filler shape on the flexural properties of RCs.
Few systematic investigations have reported the effect of particle size and shape on RCs [[18], [19], [20], [21], [22]]. In the previous studies, the impact of the filler shape on the mechanical properties of RC were investigated using in vitro testing of various commercially-available
Conclusions
The in silico multi-scale analysis demonstrated that RC models containing irregular-shaped filler had greater flexural strength than RC models loaded with sphere-shaped filler, suggesting that the mechanical strength of RCs can be improved by optimizing the shape of silica filler.
Acknowledgements
This research was supported by a Grant-in-Aid for Scientific Research (No. JP 19K10244) from the Japan Society for the Promotion of Science(JSPS). The authors are grateful to Kuraray Noritake Dental for kindly providing some of the materials used. We thank Ashleigh Cooper, PhD, from Edanz Group (https://en-author-services.edanzgroup.com/) for editing a draft of this manuscript.
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2023, Ceramics InternationalCitation Excerpt :Takahiko et al. combined nano-CT imaging and in silico multi-scale analysis and showed that optimizing the shape of silica fillers improved the mechanical strength of resin composites (RCs). Therefore, the criteria for predicting the fracture initiation of RCs at the micro-scale could be obtained [40]. Gaku et al. used synchronous X-ray nano-CT to investigate the microstructure evolution during co-sintering of multi-layer ceramic capacitors (MLCC) and presented a model that explained defect formation from heterogeneous particles packed in an electrode layer [41].
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2022, Journal of DentistryCitation Excerpt :Thermal cycling is defined as the in vitro procedure that involves subjecting dental materials to temperature extremes similar to those found in the oral cavity [43]. The difference in thermal expansion between the resin matrix and the filler particles will cause internal strains within the material [44,45]. For example, glass and ceramic fillers have a thermal coefficient of expansion of 8 to 12 × 10−6 /°C, while acrylic resin has a thermal coefficient of expansion of 76 × 10−6 /°C [46].
Long-term mechanical stability and light transmission characteristics of one shade resin-based composites
2022, Journal of DentistryCitation Excerpt :While these particles may help to match the relative refractive indices of both matrix and filler system [45], the importance of larger fillers or filler agglomerates as well as filler shape for mechanical stability cannot be understated. [43, 46] In a thorough analysis of spherical and irregular filler shapes Sakai et al. have demonstrated that irregular shaped fillers, as present in VD and VP, yield greater flexural strength than their spherical counterparts, due to the higher amount of strain present adjacent to the latter, [47] which could serve as a possible explanation for the superior mechanical properties of VD and VP. Another consideration must be made, when recognizing the fact that OC allegedly possesses the highest filler fraction (68 vol.-%) out of all three RBCs in question.
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2021, Journal of DentistryCitation Excerpt :The mechanical properties of RBCs depend on the character of all their components. In the case of fillers, this is related to differences in their chemical composition, the volume fraction [3,4], and their size [5,6], shape and orientation [7]. With modern, clinically frequently used light-curing RBCs, increased flexural strength and modulus of elasticity are observed up to a filler volume fraction of approx. 60% [3].