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

University Of Delaware, Newark DE

Investigators

Abstract

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.