Asa H. Barber

Human teeth are mechanically robust through a complex structural composite organisation of materials and morphology. Efforts to replicate mechanical function in artificial teeth (typodont teeth), such as in dental training applications, attempt to replicate the structure and morphology of real teeth but lack tactile similarities during mechanical cutting of the teeth. In this study, biomimetic typodont teeth, with morphology derived from X-ray microtomography scans of extracted teeth, were 3D printed using an approach to develop novel composites. These composites with a range of glass, hydroxyapatite and porcelain reinforcements within a methacrylate-based photopolymer resin were compared to six commercial artificial typodont teeth. Mechanical performance of the extracted human teeth and 3D printed typodont teeth were evaluated using a haptic approach of measuring applied cutting forces. Results indicate 3D printed typodont teeth replicating enamel and dentine can be mechanically comparable to extracted human teeth despite the material compositions differing from the materials found in human teeth. A multiple parameter variable of material elastic modulus and hardness is shown to describe the haptic response when cutting through both human and biomimetic, highlighting a critical dependence between the ratio of material mechanical properties and not absolute material properties in determining tooth mechanical performance under the action of cutting forces.

Osteoregenerative biomaterials for the treatment of bone defects are under much development, with the aim of favouring osteointegration up to complete bone regeneration. A detailed investigation of bone-biomaterial integration is vital to understanding and predicting the ability of such materials to promote bone formation, preventing further bone damage and supporting load-bearing regions. This study aims to characterise the ex vivo micromechanics and microdamage evolution of bone-biomaterial systems at the tissue level, combining high resolution synchrotron micro-computed tomography, in situ mechanics and digital volume correlation. Results showed that the main microfailure events were localised close to or within the newly formed bone tissue, in proximity to the bone-biomaterial interface. The apparent nominal compressive load applied to the composite structures resulted in a complex loading scenario, mainly due to the higher heterogeneity but also to the different biomaterial degradation mechanisms. The full-field strain distribution allowed characterisation of microdamage initiation and progression. The findings reported in this study provide a deeper insight into bone-biomaterial integration and micromechanics in relation to the osteoregeneration achieved in vivo, for a variety of biomaterials. This could ultimately be used to improve bone tissue regeneration strategies.

Digital volume correlation (DVC), combined with in situ synchrotron microcomputed tomography (SR-microCT) mechanics, allows for 3D full-field strain measurement in bone at the tissue level. However, long exposures to SR radiation are known to induce bone damage, and reliable experimental protocols able to preserve tissue properties are still lacking. This study aims to propose a proof-of-concept methodology to retain bone tissue integrity, based on residual strain determination using DVC, by decreasing the environmental temperature during in situ SR-microCT testing. Compact and trabecular bone specimens underwent five consecutive full tomographic data collections either at room temperature or 0 °C. Lowering the temperature seemed to reduce microdamage in trabecular bone but had minimal effect on compact bone. A consistent temperature gradient was measured at each exposure period, and its prolonged effect over time may induce localised collagen denaturation and subsequent damage. DVC provided useful information on irradiation-induced microcrack initiation and propagation. Future work is necessary to apply these findings to in situ SR-microCT mechanical tests, and to establish protocols aiming to minimise the SR irradiation-induced damage of bone.

Teeth are complex biological structures consisting primarily of enamel and dentin. The mechanical loading of teeth due to the action of brushing provides a complex interaction between applied forces and the biological structure, resulting in volume loss of dental tissue. The volume loss as a resulting of using an ex situ brushing of teeth was measured using x-ray microcomputed tomography (XCT) and corresponding image analyses. Three-dimension reconstructions of teeth reveal a considerably loss of dentin at a rate of over 30 times that of enamel during brushing, as would be expected by the consideration of the corresponding structures. The approach here is flexible and can be applied to a range of brushing, teeth, and environmental conditions.

Summary: A micromechanical characterization of biomaterials for bone tissue engineering is essential to understand the quality of the newly regenerated bone, enabling the improvement of tissue regeneration strategies. A combination of microcomputed tomography in conjunction with in situ mechanical testing and digital volume correlation (DVC) has become a powerful technique to investigate the internal deformation of bone structure at a range of dimensional scales. However, in order to obtain accurate three‐dimensional strain measurement at tissue level, high‐resolution images must be acquired, and displacement/strain measurement uncertainties evaluated. The aim of this study was to optimize imaging parameters, image postprocessing and DVC settings to enhance computation based on ‘zero‐strain’ repeated high‐resolution synchrotron microCT scans of trabecular bone and bone‐biomaterial systems. Low exposures to SR X‐ray radiation were required to minimize irradiation‐induced tissue damage, resulting in the need of advanced three‐dimensional filters on the reconstructed images to reduce DVC‐measured strain errors. Furthermore, the computation of strain values only in the hard phase (i.e. bone, biomaterial) allowed the exclusion of large artefacts localized in the bone marrow. This study demonstrated the suitability of a local DVC approach based on synchrotron microCT images to investigate the micromechanics of trabecular bone and bone‐biomaterial composites at tissue level with a standard deviation of the errors in the region of 100 microstrain after a thorough optimization of DVC computation.Lay Description: Understanding the quality of newly regenerated bone after implantation of novel biomaterials is essential to improve bone tissue engineering strategies and formulation of biomaterials. The relationship between microstructure and mechanics of bone has been previously addressed combining microcomputed tomography with in situ mechanical testing. The addition of an image‐based experimental technique such as digital volume correlation (DVC) allows to characterize the deformation of materials in a three‐dimensional manner. However, in order to obtain accurate information at the micro‐scale, high‐resolution images, obtained for example by using synchrotron radiation microcomputed tomography, as well as optimization of the DVC computation are needed. This study presents the effect of different imaging parameters, image postprocessing and DVC settings for as accurate investigation of trabecular bone structure and bone‐biomaterial interfaces. The results showed that when appropriate image postprocessing and DVC settings are used DVC computation results in very low strain errors. This is of vital importance for a correct understanding of the deformation in bone‐biomaterial systems and the ability of such biomaterials in producing new bone comparable with the native tissue they are meant to replace.