Characterising the deformation behaviour of human tooth enamel at the microscale

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 loadbearing 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 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 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 angle of enamel increased to 54o 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

SolidWorks Secondary Development with Visual Basic 6 for an Automated Modular Fixture Assembly Approach

Modular fixtures (MFs) play an important role in terms of cost and production time reduction in manufacturing processes. In this paper, the authors illustrate an automated approach for MFs design and assembly. This approach is based on the secondary development of SolidWorks integrating with Visual Basic (VB) 6 programing language. SolidWorks API (Application programming interface) functions were applied in order to control SolidWorks commands and assembly operations. An ActiveX DLL project was created in VB 6 and a plug-in file in .dll format was generated. The outcomes were creating new menus in SolidWorks environment for selecting, inserting, and assembling MFs elements. The approach was applied for a side clamping procedure and for a semi-circular workpiece

Revealing the structural and mechanical characteristics of ovine teeth

The survival and function of dentition over the lifetime of an animal depends upon the ability of the teeth to resist wear and chemical erosion, and to withstand occlusal loading conditions without suffering debilitating fracture. Understanding how geometrical factors (radius, height, enamel thickness) and mechanical properties of the dental tissues (Young\u27s modulus E, hardness H and toughness KIC of enamel and dentin) combine to ensure the survival of an animal\u27s teeth can provide great insight into the evolutionary history of the animal and its dietary adaptation. While the geometrical factors are beginning to be understood, the range of animals for which measurements of dental tissue properties are available is very narrow, being restricted almost entirely to humans and other primates. The absence of comparative data across a broader range of species makes it impossible to draw conclusions with any certainty. The present study expands knowledge of mammalian dental tissue properties by reporting the Young\u27s modulus and hardness of ovine (sheep) enamel and dentin measured using nano-indentation.We found that sheep molar enamel Young\u27s modulus and hardness are both lower than those of human enamel, by approximately 30%, and 9% respectively, while the properties of dentin are similar. The combination of E and H makes the ovine enamel approximately 30% more resistant to wear than human enamel, which is an imperative in ruminant dentition. The results of this study are interpreted in terms of the ovine feeding ecology, and the structure of the ovine molar and its occlusal surface

Size dependent elastic modulus and mechanical resilience of dental enamel

Human tooth enamel exhibits a unique microstructure able to sustain repeated mechanical loading during dental function. Although notable advances have been made towards understanding the mechanical characteristics of enamel, challenges remain in the testing and interpretation of its mechanical properties. For example, enamel was often tested under dry conditions, significantly different from its native environment. In addition, constant load, rather than indentation depth, has been used when mapping the mechanical properties of enamel. In this work, tooth specimens are prepared under hydrated conditions and their stiffnesses are measured by depth control across the thickness of enamel. Crystal arrangement is postulated, among other factors, to be responsible for the size dependent indentation modulus of enamel. Supported by a simple structure model, effective crystal orientation angle is calculated and found to facilitate shear sliding in enamel under mechanical contact. In doing so, the stress build-up is eased and structural integrity is maintained