Collaborative Research: Revealing Strengthening and Toughening Mechanisms in Coconut Endocarp through Integrated Multiscale Modeling and Characterization

椰子坚硬的外壳，称为内果皮，是一种重量轻的材料，具有令人印象深刻的强度、韧性和硬度。与许多生物材料一样，这种杰出的行为是由于其高度复杂的结构。当在增加放大倍率下研究时，内果皮在每个放大倍率水平上显示不同的结构。在最大的层面上，可以看到一个多孔网络，由成束的空心通道组成。放大后可以看到一个分级的细胞结构，较大的细胞向内，较小的细胞向外。细胞本身的壁由许多层组成，每一层都由微小的原纤维组成。全面了解所有这些元素是如何共同作用使椰子如此坚固和坚韧是一项重大挑战，特别是因为它们的大小不一。该项目将开发新的计算机模拟技术，能够同时处理相关尺寸的这些不同元素。该项目还将开发新的实验技术，直接测量和可视化椰子内果皮内不同元素如何相互作用，以测试和校准计算机模型。这种综合计算和实验方法将为椰子的结构如何产生其卓越性能提供前所未有的见解。这些见解和方法可用于设计受椰子启发的轻质应用程序，这些应用程序坚固耐用，例如改进头盔。本项目将为本科生提供研究机会。例如，计算和实验训练系列将在夏季提供给本科生。包括女性和少数民族学生在内的代表性不足的学生将参与本研究项目。该项目还将为残疾学生提供远程计算建模的机会。为吸引初高中学生参与生物材料研究，将向他们提供演讲和研讨会。技术概述：椰子的内果皮比木材更坚固、更硬，尽管它们的主要成分相同：纤维素、半纤维素和木质素。这种令人印象深刻的机械性能的关键是一个复杂的结构，在分子尺度和宏观尺度之间有许多层次结构。该项目的目标是通过整合新颖的计算和实验技术的多尺度努力，对内果皮的结构/性质关系进行严格的理解。并发原子连续体（CAC）计算工具将自然地跨越该材料系统的所有相关长度尺度。这种方法将克服当前计算方法的局限性，在当前计算方法中，不同的长度尺度用需要接口的概念上不同的模型来处理。CAC方法将首次用于分层材料，代表了材料科学的一个改变游戏规则的发展。实验工作将反映计算工作，并提供所有长度尺度的表征。扫描探针技术将在表征内果皮纳米和微尺度组分的结构和力学性能以及它们之间的相互作用方面发挥重要作用。结果将是一个强大的模型，可以在多个长度尺度上进行校准和验证。受椰子内果皮的启发，该模型可以作为建立具有优异机械性能的合成细胞轻质材料自下而上分层设计指南的基础。该项目由材料研究（DMR）部门的生物材料计划（BMAT）和促进竞争研究的既定计划（EPSCoR）共同资助。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。

Non-Technical Summary:The hard shell of the coconut, called endocarp, is a lightweight material with impressive strength, toughness, and hardness. As with many biological materials, this outstanding behavior is due to a highly complex structure. When studied at increasing magnifications, the endocarp reveals different structures at each magnification level. At the largest level, a porous network can be seen, consisting of bundles of hollow channels. Larger magnifications reveal a graded cellular structure, where larger cells are found toward the inside of the coconut, and smaller cells toward the outside. The cells themselves feature walls consisting of many layers, and each of these layers consists of tiny fibrils. Understanding comprehensively how all of these elements work together to make the coconut so strong and tough is a significant challenge, especially because of their disparity in size. This project will develop novel computer simulation techniques with the capability of treating these different elements simultaneously at the relevant sizes. This project will also develop new experimental techniques to measure and visualize directly how the different elements inside the coconut endocarp interact, to test and calibrate the computer models. This integrated computational and experimental approach will provide unprecedented insights into how the coconut’s structure gives rise to its outstanding performance. These insights and methods can then be used to engineer coconut-inspired lightweight applications that are strong and tough, for instance to improve helmets. This project will provide research opportunities to undergraduate students. For instance, computational and experimental training series will be offered to undergraduate students during the summer. Underrepresented students including female and minority students will participate in this research project. This project will also provide opportunities to students with disabilities to work on computational modeling remotely. Presentations and seminar talks will be offered to middle and high school students to attract them to participate into biomaterial research. Technical Summary:Coconut endocarp is substantially stronger and stiffer than wood, despite sharing the same major ingredients: cellulose, hemi-cellulose, and lignin. The key to this impressive mechanical performance is a sophisticated structure with many levels of structural hierarchy between the molecular scale and the macroscale. This project’s goal is to develop a rigorous understanding of the endocarp’s structure/property relationships by means of a multi-scale effort integrating novel computational and experimental techniques. A concurrent atomic-continuum (CAC) computational tool will be developed to span all relevant length scales of this materials system naturally. This approach will overcome limitations of current computational approaches, where different length scales are treated with conceptually different models that need to be interfaced. The CAC approach will be used on hierarchical materials for the first time, representing a game changing development for the materials sciences. The experimental efforts will mirror the computational work and provide characterization across all length scales. Scanning probe techniques will play a crucial role in characterizing not only the structure and mechanical properties of nano- and microscale constituents of the endocarp, but also their interfacial interactions. The outcome will be a powerful model that is calibrated and verified across multiple length scales. This model can serve as a basis to establish guidelines for bottom-up hierarchical design of synthetic cellular lightweight materials with outstanding mechanical performance, inspired by the coconut endocarp.This project is jointly funded by the Biomaterials progam (BMAT) in the division of materials research (DMR) and the Established Program to Stimulate Competitive Research (EPSCoR).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.