Characterising the Deformation Behaviour of Human Tooth Enamel at the Microscale
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Edith Cowan University Research Online Theses: Doctorates and Masters Theses 2013 Characterising the deformation behaviour of human tooth enamel at the microscale Simona O'Brien Edith Cowan University Follow this and additional works at: https://ro.ecu.edu.au/theses Part of the Dental Materials Commons, and the Nanoscience and Nanotechnology Commons Recommended Citation O'Brien, S. (2013). Characterising the deformation behaviour of human tooth enamel at the microscale. https://ro.ecu.edu.au/theses/566 This Thesis is posted at Research Online. https://ro.ecu.edu.au/theses/566 Edith Cowan University Research Online Theses: Doctorates and Masters Theses 2013 Characterising the deformation behaviour of human tooth enamel at the microscale Simona O'Brien Edith Cowan University Recommended Citation O'Brien, S. (2013). Characterising the deformation behaviour of human tooth enamel at the microscale. Retrieved from http://ro.ecu.edu.au/theses/566 This Thesis is posted at Research Online. http://ro.ecu.edu.au/theses/566 Edith Cowan University Copyright Warning You may print or download ONE copy of this document for the purpose of your own research or study. The University does not authorize you to copy, communicate or otherwise make available electronically to any other person any copyright material contained on this site. You are reminded of the following: Copyright owners are entitled to take legal action against persons who infringe their copyright. A reproduction of material that is protected by copyright may be a copyright infringement. Where the reproduction of such material is done without attribution of authorship, with false attribution of authorship or the authorship is treated in a derogatory manner, this may be a breach of the author’s moral rights contained in Part IX of the Copyright Act 1968 (Cth). Courts have the power to impose a wide range of civil and criminal sanctions for infringement of copyright, infringement of moral rights and other offences under the Copyright Act 1968 (Cth). Higher penalties may apply, and higher damages may be awarded, for offences and infringements involving the conversion of material into digital or electronic form. Characterising the Deformation Behaviour of Human Tooth Enamel at the Microscale Simona O’Brien, MEng Sc, MIEI TITLE PAGE A thesis presented for the degree of Doctor of Philosophy School of Engineering Faculty of Computing, Health and Science Edith Cowan University Western Australia 14th May 2013 i USE OF THESIS The Use of Thesis statement is not included in this version of the thesis. ABSTRACT Enamel plays an important role in tooth function. Optimal combinations of composition and structure endow enamel with unique mechanical properties that remain largely unexplored. Specifically, more detailed understanding of the load- bearing ability of enamel is needed to mimic it synthetically and to design next generation biocomposite materials. This research investigates the variables that influence deformation behaviour of tooth enamel in relation to its hierarchical structure. Initially, a new method was developed for preparing flat, finely polished tooth samples that were maintained in their normal hydrated state for nanoindentation testing. In contrast to conventional methods, which commonly utilise either inappropriate or excessive drying and/or chemically based embedding media (i.e., resins, glues), a novel embedding process was developed using an aqueous putty compound. Additionally, a custom-designed holder was manufactured for mounting wet tooth specimens on the nanoindentation stage that eliminated the need for hot wax or glue during testing. Considering that enamel is a functionally graded material that has different values of Young’s modulus (E) and hardness (H) over the enamel thickness, a new approach of data analysis was developed for interpreting the mechanical properties of enamel at a range of fixed constant indentation depths. Resultant functions were used for predictive purposes. The values of E and H obtained from the nanoindentation instrument demonstrated a well-known decreasing gradient from the enamel occlusal surface towards the enamel-dentine junction (EDJ). In contrast to studies using conventional methods, this research showed that both properties also decreased with increasing depths at fixed locations. Furthermore, experimental results showed that resin embedding had detrimental effects on the E and H of enamel (i.e., both properties decreased with increasing depth), but had positive effects on both mild and severe wear resistance parameters (i.e., both parameters increased with increasing depth). When contrasted against the mechanical properties of enamel samples prepared using conventional protocols, this study postulates that the new iii hydrated method has, for the first time, revealed the genuine E and H properties of this tissue. The effects of sample preparation methods on tooth microstructure, especially along the EDJ, were investigated with optical microscopy and scanning electron microscopy (SEM). The new method of sample preparation combined with a careful dehydration process maintained the integrity of the EDJ interface even after applying multiple Berkovich indents up to maximum load of 400 mN. In contrast, the EDJ and the enamel surface were commonly separated and fractured in teeth that had been resin-embedded. Accordingly, the new method of sample preparation proved to be reliable for investigating the genuine microstructural characteristics of teeth. The behaviour of the elastic region in tooth enamel was investigated with analytical and finite element models. The models were fitted into experimental values of E obtained from nanoindentation tests with a Berkovich indenter to identify a relationship between the mechanical responses of enamel under different loading conditions and microstructure. The decrease in E for enamel with increasing indentation depth was related to its enhanced load-bearing ability. The change of E was directly linked to the microstructural evolution (i.e., the rotation of mineral crystals) of enamel. The effective crystal orientation angle was found to be between 44o and 48o for indentation depths from 0.8 and 2.4 μm below according to the analytical model. The range of angles facilitated the shear sliding of mineral crystals and reduced the stress level as well as the volume of material under higher loads. The behaviour of the plastic region in healthy enamel was investigated with finite element models fitted to nanoindentation data obtained with a Berkovich indenter to determine deformation mechanisms that result in excellent mechanical responses for tooth enamel during loading. When nanoindentation was conducted with increasingly applied loads but at a fixed location, the values of H decreased with increasing indentation depth. The decreasing trend in H was simulated by finite element models and showed a reduction in stress level and yield strength with increasing load. This key mechanism of the loading iv dependence of mechanical properties resulted in remarkable enamel resilience and was related to the change of effective crystal orientation angle within the enamel microstructure. The mechanical behaviour of enamel with respect to its microstructure was also investigated on teeth exposed to commercially available whitening treatments (tooth bleaching). Enamels exposed to a 6% bleaching treatment exhibited degraded mechanical properties (E and H) compared to unbleached controls. Furthermore, the creep and recovery responses of bleached enamel were also significantly reduced compared to controls. To determine the variables regulating tooth enamel deformation mechanisms during whitening treatments, analytical models were fitted to stress-strain curves. The effective crystal orientation angle of healthy enamel and the protein shear stress, τc, were identified as 50o and 2.5 % of the transverse stiffness of a staggered composite (E2), respectively. After the bleaching treatment, the effective crystal orientation o angle of enamel increased to 54 for τc = 1.5 % of E2. Notably, bleaching reduced shear (τc) by 40 % compared to normal readings for unbleached controls. The changes in mechanical responses of bleached enamel were linked to the decrease of the shear bearing ability of protein components in the enamel microstructure. It is envisaged that these findings will provide new perspectives on applications of bleaching treatments and lead to the development of bleaching agents with less damaging effects to healthy enamel. This work should stimulate new interest in understanding the deformation behaviour of tooth enamel at small scales, and offer new methods for the collection and analysis of data from samples prepared close to their native state, upon which novel and biologically relevant high-performance biocomposite materials can be engineered. v DECLARATION I certify that this thesis does not, to the best of my knowledge and belief: (i) incorporate without acknowledgment any material previously submitted for a degree or diploma in any institution of higher education; (ii) contain any material previously published or written by another person except where due reference is made in the text; or (iii) contain any defamatory material I also grant permission for the Library at Edith Cowan University to make duplicate copies of my thesis as required. Simona O’Brien, 14th May 2013 vi ACKNOWLEDGEMENTS Appreciation is expressed to those who have made contribution to this thesis and who have made my PhD study