EAGER: Bone Intrinsic Toughening by Stress Whitening is a Fundamental Molecular Mechanism of Collagen I

骨骼、肌腱、韧带和软骨承受很大的力。组织的柔软部分是纤维分子(主要是胶原蛋白)，适合扮演这一角色。这个研究项目的基本想法是，胶原蛋白具有一种分子水平的机制，可以帮助这些组织变得强壮和坚韧。正常、坚韧的骨骼在拉伸状态下的一个特性是，它在断裂前会变白。相比之下，脆骨在断裂前不会明显变白。先前的研究表明，美白是由于骨骼中的胶原蛋白分子发生了变化。这个早期概念的探索性研究拨款(AGERGE)项目的假设是，随着这些分子被拉伸，它会导致脱水，在分子之间产生强烈的结合，使它们变得更白，并延缓破裂。理论上，脱水的原因是与聚合物溶液中常见的相同的机理(称为调幅节点分解)，在聚合物溶液中，拉伸溶液会使溶解的聚合物分离成单独的固体形式。力学结果是，组织中的纤维从相对分散、柔软和坚韧转变为密集、僵硬和坚固，直到张力释放。美白是一种保护组织不断裂的方法，如果缺失，就会导致骨骼脆化。纤维组织存在于从海绵到大象的各种动物体内。证明存在潜在的分子水平的、动态的、机械的增韧机制，有助于理解许多动物的负荷支持。它还可以解释一些骨骼疾病。如果机械脱水可以部分解释胶原蛋白的韧性，这一知识可能会指导未来的医学研究，使骨骼随着年龄的增长而变得更坚韧。这项研究的结果将包括在骨骼组织力学和生物材料课程中。美白以前被归因于骨骼矿物成分中的微裂纹引起的光散射，但PI实验室的研究表明，美白是胶原基质的一种特性。对结缔组织和聚合物力学文献的回顾表明，白化的可能机制是拉伸应力诱导的胶原分子脱水，称为结晶。这可能是由于胶原-细胞外液溶液的伸展应变相关的调幅分解所致。有两个研究目标。第一个是确定压力美白和溶剂美白是否是脱水效应。第二个目的是研究使用高分子聚乙二醇溶液通过非渗透渗透压对胶原原纤维进行侧向压缩的部分脱水是否具有下列任何效果：a)减少引起美白所需的应变；b)降低断裂应变和断裂模数(表明组织更脆)；c)增加弹性模量和极限应力；以及d)压缩后的应变至美白的(降低的)应变至白化是否将预测与在具有相同应变至美白的较低Hansen系数的溶剂中的未压缩试件相同的最终应力(及其他机械性能)。在脱钙骨标本的力学测试过程中，将使用实时拉曼光谱监测胶原分子的侧向堆积和其他分子构型的变化。与侧向分子堆积相关的拉曼光谱预计将在张力达到临界值时显示逐步变化。该奖项反映了NSF的法定使命，并已通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Bone, tendon, ligament, and cartilage carry large forces. The soft parts of the tissues are fibrous molecules (mostly collagens) that are suited for this role. The underlying idea of this research project is that collagen has a molecular-level mechanism that helps these tissues to be strong and tough. A property of normal, tough bone under tension is that it "whitens" just before it breaks. In contrast, brittle bone doesn't whiten noticeably before it breaks. Previous research has shown that whitening results from changes to the collagen molecules in bone. The hypothesis of this EArly-concept Grant For Exploratory Research (EAGER) project is that as these molecules are stretched it results in dehydration that creates strong attachments between the molecules, making them whiter and also postponing breakage. It is theorized that the cause of the dehydration is the same mechanism (called "spinodal decomposition") that is common in polymer solutions, where stretching a solution causes the dissolved polymer to separate into a separate, solid form. The mechanical result is that the fibers in the tissue change from being relatively distributed, flexible, and tough to being dense, stiff, and strong until the tension is released. Whitening is a way to protect the tissue from breaking that, if missing, will result in a brittle bone. Fibrous tissues are found in animals that range from sponges to elephants. Demonstrating existence of an underlying molecular-level, dynamic, mechanical toughening mechanism could help in understanding load support in many animals. It also could explain some bone diseases. If mechanical dehydration explains part of collagen toughness, the knowledge could guide future medical research to make bone tougher as we age. Results of this research will be included in courses on Skeletal Tissue Mechanics and Biomaterials. Whitening was previously attributed to light scattering caused by microcracking in the mineral component of bone, but research from the PI's laboratory has demonstrated that whitening is a property of the collagenous matrix. Review of connective tissue and polymer mechanics literature indicates that the likely mechanism for whitening is tensile stress-induced dehydration of the collagen molecules, referred to as crystallization. This may be caused by extensional strain-dependent spinodal decomposition of the collagen-extracellular fluid solution. There are two research goals. The first is to determine whether stress- and solvent- whitening are dehydration effects. The second is to investigate whether partial dehydration by lateral compression of the collagen fibrils, via non-penetrating osmotic pressure using high molecular weight polyethylene glycol solutions has any of the following effects: a) decreases the strain needed to cause whitening; b) decreases both rupture strain and modulus of rupture (indicating a more brittle tissue); c) increases modulus and ultimate stress; and, finally, d) if the (decreased) strain-to-whitening after compression will predict the same ultimate stress (and other mechanical properties) as for an uncompressed specimen in a solvent of lower Hansen's coefficient that has the same strain-to-whitening. The lateral packing of the collagen molecules and other changes in molecular configuration during mechanical testing of decalcified bone specimens will be monitored using real-time Raman spectroscopy. Raman spectra correlated with lateral molecular packing are expected to show step-wise changes when tension reaches a critical value.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.