Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W18: Bio-Mineralization, Hydration & Hardening in Colloidal Gels: Building Solid Materials Out of Soft GelsInvited
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Sponsoring Units: DSOFT Room: 205 |
Friday, March 6, 2020 8:00AM - 8:36AM |
W18.00001: Compositional and structural gradients in dental enamel: from nano- to microscale Invited Speaker: Derk Joester Dental enamel has evolved to bear large masticatory forces, resist mechanical fatigue, and withstand wear over decades of use. Functional impairment or loss, as a consequence of developmental defects or tooth decay, has a dramatic impact on health and quality of life. While the last decade has seen great progress in our understanding of enamel formation and the functional properties of mature enamel, attempts to repair enamel lesions or synthesize enamel in vitro have had limited success. This is partly due to the highly hierarchical structure of enamel and the additional complexities arising from chemical gradients that we are only beginning to understand. Herein we show, using atomic-scale quantitative imaging and correlative spectroscopies, that human enamel is comprised of crystalline apatite and a Mg-rich amorphous intergranular phase. Individual crystallites have core-shell structure. The core is comprised of two thin layers enriched in Mg flanking a region that is poor in Mg and enriched in Na. Fluoride is often also present in layers. The sandwich core is surrounded by a shell largely free of substitutional defects. A mechanical model of coherent crystallites based on DFT calculations predicts that significant residual stresses, with important implications for enamel dissolution, crystallite and tissue mechanical properties, and crystal growth processes during amelogenesis. In addition to these gradients at length scales from single digit to 10s of nanometers, we will report on systematic changes in average lattice parameters and coherence length across single enamel rods, i.e. length scales on the order of 1-10 µm. |
Friday, March 6, 2020 8:36AM - 9:12AM |
W18.00002: Hydrogel Mineralization via Amorphous Precursors Invited Speaker: Rosa Espinosa-Marzal Marine organisms exploit varied physical and chemical interactions between calcium carbonate and organic additives to yield mineral-organic complexes that are not only visually stunning but also highly functional. Despite recent leaps in the understanding of these systems, knowledge of the relation between kinetics of mineral growth, microstructure of the composite and mechanical response is still lacking. Our aim is to mechanistically elucidate potential pathways that afford control of the mineralization pathway of hydrogels and of their microstructure, and yield composites with tunable mechanical response. Our previous studies demonstrated that the formation of amorphous calcium carbonate precursors (ACC) throughout agarose hydrogels is a diffusion-limited process, and therefore, it is strongly affected by the solution composition and by the hydrogel composition. In contrast, amorphous precursors in the hydrogel control tightly the inclusion of the polymer into calcite, and crystal morphology, as well as the rate of crystal growth, affording a uniform crystal growth throughout the hydrogels, in the absence of concentration gradients, over a wide range of solution conditions and hydrogel compositions. Based on these findings, we have extended these studies to phosphates, and show that amorphous precursors also modulate the mineralization kinetics quite tightly. Furthermore, the phosphate-mineralized hydrogels exhibit key advantages with regard to microstructure -as provided by hydroxyapatite nanosheets- and mechanical response, compared to the mineralized hydrogels in the absence of phosphates. The results of this work not only reveal an important mechanism underlying (bio)mineralization but it can also inspire new avenues to craft biomimetic materials. |
Friday, March 6, 2020 9:12AM - 9:48AM |
W18.00003: Christian Hellmich Invited Talk
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Friday, March 6, 2020 9:48AM - 10:24AM |
W18.00004: Nacre toughening due to cooperative plastic deformation of co-oriented aragonite columns Invited Speaker: Hyun-Chae Loh Nacre’s structure-property relationships have been a source of inspiration for designing advanced functional materials with both high strength and toughness. These outstanding mechanical properties have been mostly attributed to the interplay between aragonite platelets and organic matrix in the typical brick-and-mortar structure. Here, we show that crystallographically co-oriented stacks of aragonite platelets—aragonite columns—define another hierarchical level that contributes to the toughening of nacre. By correlating piezo-Raman and micro-indentation results, we quantify the residual strain energy associated with strain hardening capacity. Our findings suggest that aragonite columns with characteristic dimensions of around 20 µm in height effectively store energy through cooperative plastic deformation. The existence of a larger length scale beyond the brick-and-mortar structure offers an opportunity for a more efficient implementation of biomimetic strategies. |
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