Multimodal Failure Mechanics in the Collagen Fibril

胶原蛋白的进化长寿是对其复杂的、自组装结构的生存价值的赞扬。虽然在光学显微镜水平上对结缔组织的弹性损伤和过载损伤进行了广泛的研究，但在纳米尺度上对机械载荷所承受的损伤的研究还不够深入。正是在那里，胶原蛋白的强度才得以实现，也许对生存更重要的是，它的韧性在这里得到了决定。在过去10年左右的时间里，我一直对两个基本问题感兴趣： (I)胶原蛋白的机械损伤在其最基本的水平上是什么样子的？ (Ii)在胶原蛋白中是否存在损伤的结构基序，从而激活生理上适当的细胞修复或替换？这些问题对于了解胶原纤维的生物力学演化，以及合理设计能够模拟天然韧性并调节炎症和愈合的加工胶原产品具有重要意义。 我们最近证实，肌腱胶原蛋白的过载会产生一种特征的、局部的、纳米级的胶原纤维“扭曲”(直径5200 nm)。纤维水平的损伤导致胶原分子的热力学不稳定，这与局部变性一致。超负荷肌腱的酶解和超高倍率扫描电子显微镜(50-70kX)显示，扭结区的一部分(仅)亚原纤维被破坏，而其他部分仍然存在。反复的塑料超载而没有破裂，会导致沿着单个受损纤维的扭结线性致密。因此，胶原纤维被揭示出出人意料的异质性：无论是直径还是长度都是如此。我们有一个工作理论，即局部“扭结”失效机制具有双向进化价值：使肌腱等组织变得坚韧，以防止灾难性的失效，同时提供结构线索，指导受损纤维的吸收和/或修复。我相信这些知识是可以利用的。 我建议进一步探索我们迄今为止工作中出现的基本结构--力学问题，应用扫描电子显微镜、低温电子显微镜和原子力显微镜加上更精细的、拉伸保留的样品来减少损伤的不均一性，从而在弹性回弹之前可视化扭结区域的破坏。我们还将生产实验室挤出的胶原纤维，以研究离散可塑性机制所需的异质纤维组装在多大程度上是胶原所固有的，以及必须采取哪些干预措施才能生产出具有技术价值的胶原纤维。最后，我们将研究在受损纤维中形成的连续扭曲区是周期性的还是随机确定的问题。有了这些知识，我们将探索扭结形成的纤维到纤维的传播，并试图产生一个模型，可以将纳米级的分子/纤维损伤与微米级的纤维破坏结合起来。

Collagen's evolutionary longevity is a tribute to the survival value of its complex, self--assembled structure. While both elasticity and overload damage have been studied extensively at the light microscope level in connective tissues, it is below that scalereally at the nanometer scalewhere mechanical load is borne. It is there that collagen's strength is achievedand perhaps more important for survival, where its toughness is determined. For the last 10 years or so, I have been interested in two fundamental questions: (i) What does mechanical damage in collagen look like at its most fundamental levels? (ii) Are there structural motifs for damage in collagen that activate physiologically appropriate cellular repair or replacement? These are questions of deep import for understanding the biomechanical evolution of the collagen fibril and for rational design of processed collagen products which can mimic native toughness and modulate inflammation and healing. We recently demonstrated that overloading of tendon collagen produces a characteristic, local, nano-scaled “kinking” of collagen fibrils (5200 nm dia.). Fibril-level damage leads to thermodynamic instability of the packed collagen molecules, consistent with local denaturation. Enzymolysis and very high magnification SEM (50-70kX) of overloaded tendons have shown that a sub-set (only) of the sub-fibrils at the kink zones are disrupted while others remain. Repeated plastic overload without rupture produces a linear densification of the kinks along individual damaged fibrils. The collagen fibril has thus been revealed to be unexpectedly heterogeneous: both across its diameter and along its length. We have a working theory that the local “kink” failure mechanism has 2--way evolutionary value: toughening tissues like tendons to prevent catastrophic failure while providing the structural cues that guide resorption and/or repair of damaged fibrils. I believe this knowledge can be used. I propose to further explore the fundamental structuro--mechanical questions which have emerged from our work to date, applying SEM, cryo-TEM, and AFM plus finer-scale, stretch-retained samples to reduce heterogeneity of damage, thereby visualizing kink zone failures before elastic rebound. We will also produce laboratory-extruded collagen fibres to study the extent to which the heterogeneous fibril assembly necessary to the discrete plasticity mechanism is innate to collagen, and what interventions must be applied to produce it for technological value. Finally, we will study the question of whether the serial kink zones which form in damaged fibrils are periodic in nature or stochastically determined. With that knowledge, we will explore fibril-to-fibril propagation of kink formation and seek to produce a model which can couple nanoscale molecular/fibril damage to micron-scale fibre failure.