EAGER/Collaborative Research: Understanding How Enamel Prism Lattices Promote a Remarkable Combination of Fracture and Wear Resistance in Grazing Mammal Dentitions

断裂和磨损是工程中常见的问题，在某种程度上，术语“磨损”、“断裂”和“损坏”通常等同于设备的最终用途。在许多情况下，传统材料无法满足复杂的同时性能要求，这将是下一代工程系统的理想选择。食草动物牙齿的牙釉质代表了自然界最非凡的生物材料之一——一种类似陶瓷的复合材料，具有非凡的强度、韧性、耐磨性和减缓裂纹扩展的能力。这是一组重要的特性，对于这些动物的长期生存至关重要，因为它们需要有功能的牙齿来进食。这个探索性研究（EAGER）项目将结合进化生物学、生物力学和材料科学来研究这些耐损伤生物材料。这项研究的结果和方法将引起许多学科研究者的极大兴趣，包括工程学、材料科学、进化生物学、生态学、比较解剖学、哺乳动物学和古生物学。这项研究还将支持新型可持续材料的开发，这些材料可以改善磨损和断裂性能。研究生将参与到这个真正的跨学科项目中，学习各个领域如何协同工作来解决具有挑战性的问题。这项研究也将引入一个更有效的，进化的方法来探索自然的仿生例子。这项跨学科研究的目标是专门了解大型食草哺乳动物磨牙中被称为改良放射状牙釉质（MRE）的牙釉质晶格的生物力学形式、功能和性能。将从许多物种中获得样本，包括马（马）、牛科动物（如野牛和牛）和水生动物（如疣猪）。这项研究将特别关注这些动物的牙齿如何在咀嚼坚硬和磨蚀性植物时承受数千万到数亿次的高应力接触加载循环和冲击，例如根部充满坚硬、促进断裂的沉积物内含物的草。潜在的假设是，MRE是一种进化优化的折衷方案：1)由于棱柱结构可以局部损伤并战略性地控制裂纹方向，因此具有令人难以置信的抗断裂性；2)柔顺的蛋白棱柱鞘规避了羟基磷灰石固有的脆性，从而实现了意想不到的强度和韧性；3)通过坚硬的、超矿化的、定向的搪瓷棱柱传递耐磨性。该项目将通过两个目标来调查这一假设。首先，本研究将采用进化生物学方法鉴定马、牛科动物和疣猪独立进化的祖先珐琅织物特征状态。从这些信息中，可以很容易地确定珐琅织物的特定进化修饰，使研磨和识别可用于进行比较生物力学评估的活物种。其次，该项目将通过使用微观和纳米机械工具，光谱学和先进的电子显微镜综合表征材料特性，研究牙釉质在多个长度尺度上的结构-性能关系。磨牙种的MRE将与近亲保留祖先牙釉质结构的牙齿进行比较，从而揭示显著的解剖变化，使生物力学性能的优化组合成为可能。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Fracture and wear are common issues in engineering - to the extent that the terms "worn," "fractured," and "broken" are generally synonymous with the end-of-utility of devices. In many cases, traditional materials fail to meet the complex, simultaneous performance requirements that would be ideal for next generation engineering systems. The enamel of the teeth of grazing animals represents one of nature's most remarkable biological materials -- a ceramic-like composite showing exceptional strength, toughness, wear-resistance, and ability to slow crack propagation. This is an important set of properties for a structure that is key to long-term survival in these animals - as functional teeth are required for feeding. This EArly-Concept Grant for Exploratory Research (EAGER) project will study these damage-tolerant biomaterials using a combination of evolutionary biology, biomechanics, and materials science. Results and methods from this research will be of considerable interest to investigators in many disciplines, including engineering, materials science, evolutionary biology, ecology, comparative anatomy, mammalogy, and paleontology. The research will also support the development of novel, sustainable materials with improved wear and fracture behavior. Graduate students will be involved in this truly interdisciplinary project and learn how the various fields can work together to tackle challenging questions. This research will also introduce a more effective, evolutionary approach for exploring nature for biomimetic examples.The goal of this interdisciplinary research is to specifically understand the biomechanical form, function and performance of enamel lattices, known as Modified Radial Enamel (MRE), in the grinding teeth of large herbivorous mammals. Samples will be obtained from numerous species, including equines (horses), bovids (e.g. bison and cattle) and suids (e.g. warthogs). This study will specifically focus on how these animals' teeth endure tens to hundreds of millions of high stress contact loading cycles and impacts while chewing tough and abrasive plant matter, such as grasses whose roots are laden with hard, fracture-promoting sediment inclusions. The underlying hypothesis is that MRE is an evolutionarily optimized compromise for: 1) incredible fracture resistance due to prism arrangements that localize damage and strategically control crack direction; 2) unexpected strength and toughness made possible by compliant proteinaceous prism sheaths that circumvent hydroxyapatite's inherent brittleness; and 3) wear resistance conveyed through hard, hyper-mineralized, oriented enamel prisms. The project will investigate this hypothesis through two objectives. First, the study will use an evolutionary biology approach to identify the ancestral enamel fabric character states to MRE that independently evolved in horses, bovids and warthogs. From this information, it will be possible to readily identify the specific evolutionary modifications to the enamel fabrics that enabled grinding and identify living species that can be used to undertake comparative biomechanical assessment. Second, the project will investigate the structure-property relationships of the enamel across multiple length scales by comprehensively characterizing the material properties using micro-and nano-mechanical tools, spectroscopy, and advanced electron microscopy. Teeth of grinding species with MRE will be compared with close relatives that retain the ancestral enamel fabrics, thereby revealing the salient anatomical changes that enabled the optimized combination of biomechanical properties.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.