EAGER: Slick Yet Stuck? Using Adhesion to Expose the Conditions of the Buried Cartilage Interface

在整个动物王国中，已经进化出了非凡的粘附机制，以帮助跳蛛附着着陆，章鱼捕捉猎物，壁虎在垂直表面上移动，以及粘鱼抓住光滑的生物污染岩石。在每一个被充分研究的独特案例中，黏附和生存之间都有很强的联系，从而推动了进化过程。这个研究小组最近发现，软骨，一种以极低的粘合摩擦而闻名的材料，产生了非常强的粘合应力——与壁虎、章鱼和粘鱼相当。这一发现是自相矛盾的，因为摩擦和附着力在根本上是有联系的。此外，与其他自然强附着力的例子不同，强关节附着力没有明显的进化压力。软骨同时表现出“超润滑性”和“超粘附性”，这一事实违背了长期以来关于软骨润滑的假设，同时暴露了不寻常的潜在力学的关键线索。这个探索性研究（EAGER）项目旨在解决这个“软骨润滑-粘附”悖论。研究结果将：(1)阐明关节中拉力是如何产生的，以及这对关节润滑的意义；(2)为生物粘附提供原创性见解，为生物灵感和材料发现提供新方向；(3)深入了解“关节开裂”的原因和后果，这是一种常见的，临床上重要的，但鲜为人知的现象。如果对软骨功能力学的理解浓缩成一个单一的框架，它将为保护、治疗或替换受损关节提供实质性的改进策略。该项目将使用新的界面张力测量来量化界面渗透率，通过实验分离粘附（壁虎）和吸力（粘鱼）机制的贡献，并系统地评估软骨摩擦和粘附在长度尺度上的交集。使用微力显微镜，将在实验压头上量化法向和摩擦力引起的梁位移。软骨样本（骨软骨核心）将从牛关节中获得。已经制定了三个目标。第一种方法是通过改变抽离前的停留时间来系统地改变间隙液损失，从而分离出渗出液对粘附性、吸力和界面间隙的影响。第二个目标将通过消除吸力的贡献来隔离和量化粘附力，使用准静态速率进行拉离测试。第三个目标将通过使用不同的探针半径来量化长度尺度上的界面特性（例如滑动摩擦、有效接触模量、平衡模量和流体载荷分数），这将对吸力（取决于半径）和附着力（与半径无关）产生不同的影响。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。

Remarkable adhesive mechanisms have evolved across the animal kingdom to help jumping spiders stick a landing, octopi catch prey, geckos locomote on vertical surfaces, and clingfish grip slippery, bio-fouled rocks. In each of these distinct and well-studied cases, there were strong connections between adhesion and survival to drive the evolutionary process. This research team has recently discovered that cartilage, a material well-known for exceptionally low adhesive friction, generates exceptionally strong adhesive stresses -- comparable to those of the gecko, octopus, and clingfish. This finding is paradoxical, because friction and adhesion are fundamentally linked. Additionally, unlike other natural examples of strong adhesion, there is no obvious evolutionary pressure for strong joint adhesion. The fact that cartilage simultaneously exhibits both "super-lubricity" and "super-adhesion" violates long-standing assumptions about cartilage lubrication while exposing critical clues about the unusual underlying mechanics. This EArly-Concept Grant for Exploratory Research (EAGER) project seeks to resolve this "cartilage lubrication-adhesion" paradox. The results will: (1) clarify how tensile forces are generated in the joint and what that implies about joint lubrication; (2) provide original insights into bio-adhesion while exposing new directions for bio-inspiration and materials discovery; (3) lend insight into the causes and consequences of "joint cracking", a common, clinically important, and poorly understood phenomenon. If the understanding of cartilage functional mechanics are condensed into a single framework, it will inform substantially improved strategies to protect, treat, or replace compromised joints.The project will use novel interfacial tension measurements to quantify interfacial permeability, experimentally separate the contributions from adhesion (seen in the gecko) and suction (seen in the clingfish) mechanisms, and systematically assess the intersection between cartilage friction and adhesion across length scales. Using micro force microscopy, beam displacements from normal and friction forces will be quantified on an experimental indenter. Cartilage samples (osteochondral cores) will be obtained from bovine joints. Three aims have been developed. The first will isolate the effect of exudation on adhesion, suction, and interfacial gaps by varying the dwell time prior to pull-off to systematically vary interstitial fluid loss. The second aim will isolate and quantify adhesive forces by eliminating the contribution of suction, using quasistatic rates for the pull-off tests. The third aim will quantify the interfacial properties (e.g. sliding friction, effective contact modulus, equilibrium modulus, and fluid load fraction) across length scales by using varying probe radii, which will have varying effects on suction forces (dependent on radius) and adhesion forces (independent of radius).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.