Gecko foot adhesion to walls, ceilings, and other surfaces is largely guided by the hierarchical fibril structure made from beta keratin proteins, ranging from micrometer setae to nanometer spatulae. It is hypothesized in the present investigation that the lack of crack formation and propagation leads to a discrepancy of pull-off force magnitudes between experimental results and values from pull-off simulations in periodic boundary conditions, particularly when accounting for the spatula adhesive area. To this end, an ultra-coarse-grained model of an entire gecko spatula being pulled from a surface was designed to be studied alongside united-atom models on a smaller scale. The forcefield was parameterized to reflect experimental Young’s Modulus and Poisson’s Ratio values of 4.5 GPa and 0.4, respectively, so the model exhibits realistic mechanical properties that are also in agreement with the united-atom model. After comparing pull-off pressures between the spatula and united-atom models to obtain parameters for nonbonding interactions between the spatula and surfaces, a series of steered molecular dynamics simulations using GROMACS were performed to mimic a realistic pull-off mechanism. Although pull-off simulations of the full-size spatula model are still being computed, the subsequent analysis is expected to be completed in the following weeks. Additionally, the effect of surface roughness on pull-off force will be investigated with the ultra-course-grained model.

Ultra-course-grained model of gecko foot spatula for molecular dynamics simulations of pull-off forces

ACS Spring 2022

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