Biology subjects

Walter Federle

Publications and source records attributed to Walter Federle.

2 recordsLinked to original sources

bioRxiv2015

Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing.

Organismal functions are size-dependent whenever body surfaces supply body volumes. Larger organisms can develop strongly folded internal surfaces for enhanced diffusion, but in many cases areas cannot be folded so that their enlargement is constrained by anatomy, presenting a problem for larger animals. Here, we study the allometry of adhesive pad area in 225 climbing animal species, covering more than seven orders of magnitude in weight. Across all taxa, adhesive pad area showed extreme positive allometry and scaled with weight, implying a 200-fold increase of relative pad area from mites to geckos. However, allometric scaling coefficients for pad area systematically decreased with taxonomic level, and were close to isometry when evolutionary history was accounted for, indicating that the substantial anatomical changes required to achieve increases in relative pad area are limited by phylogenetic constraints. Using a comparative phylogenetic approach, we found that the departure from isometry is almost exclusively caused by large differences in size-corrected pad area between arthropods and vertebrates. To mitigate the expected decrease of weight-specific adhesion within closely related taxa where pad area scaled close to isometry, data for several taxa suggest that the pads adhesive strength increased for larger animals. The combination of adjustments in relative pad area for distantly related taxa and changes in adhesive strength for closely related groups helps explain how climbing with adhesive pads has evolved in animals varying over seven orders of magnitude in body weight. Our results illustrate the size limits of adhesion-based climbing, with profound implications for large-scale bio-inspired adhesives.

Evolutionary Biology↗

bioRxiv2015

Biomechanics of shear-sensitive adhesion in climbing animals: peeling, pre-tension and sliding-induced changes in interface strength

Rapid control of adhesive forces is one of the key benchmarks where footpads of climbing animals outperform conventional adhesives, promising novel bio-inspired attachment systems. All climbing animals use shear forces to switch rapidly between firm attachment and easy detachment, but the detailed mechanisms underlying shear-sensitive adhesion have remained unclear. Here, we show that attachment forces of stick insects follow classic peeling theory when shear forces are small, but strongly exceed predictions as soon as their pads start to slide due to high shear forces. Pad sliding dramatically increases the critical peel force via a combination of two distinct mechanisms. First, partial sliding will pre-stretch the pads, so that they are effectively stiffer upon detachment and peel increasingly like inextensible tape. We demonstrate how this effect can be directly related to peeling theories which account for frictional dissipation. Second, pad sliding reduces the thickness of the secretion layer in the contact zone, thereby decreasing the interfacial mobility, and increasing the stress levels required for peeling. The approximately linear increase of adhesion with friction results in a sharp increase of adhesion at peel angles less than ca. 30{degrees}, allowing rapid switching between attachment and detachment during locomotion. Our results may apply to diverse climbing animals independent of pad morphology and adhesive mechanism, and highlight that control of adhesion is not solely achieved by direction-dependence and morphological anisotropy, suggesting promising new routes for the development of bio-inspired adhesives.

Zoology↗