Effects of progressive and degressive elastic elements in load-sharing. Effects of progressive and degressive elastic elements in load-sharing. (a) Stress–strain curve of the skin of the tokay gecko [19], with a piecewise linear fit shown in dashed red. The ratio of k2 to k1 is 5, corresponding to a progressive elastic element. (b) Stress–strain curve of a strand of extruded nylon, with a k2 to k1 ratio of 3, used as a progressive element for testing in the synthetic device. (c) Stress–strain curve of a superelastic nitinol wire, with ratio of k2 to k1 of 0.07, corresponding to a degressive elastic element. (d) A simple model based on identical nonlinear springs loaded in parallel with different initial displacements predicts the behaviour of synthetic progressive springs (dashed red) and synthetic degressive springs (solid blue) when used to load 24 adhesive patches. The initial displacement offsets were measured from the synthetic adhesion system (appendix A.2). At the maximum displacement x = 4.6 mm, the spread in forces is predicted to be much larger for the progressive springs than for the degressive nitinol springs. The average forces Favg are 74% and 97% of the maximum force Fmax for the two cases. (e) Force and displacement data from the synthetic adhesion system with progressive nylon springs (dotted red) and degressive nitinol springs (solid blue) (electronic supplementary material, Methods, force and displacement sensor). At the displacement x = 4.6 mm, the average force Favg for the progressive springs was 70% of the maximum force Fmax, but 94% for the degressive nitinol springs. Elliot W. Hawkes et al. J. R. Soc. Interface 2015;12:20140675 © 2014 The Author(s) Published by the Royal Society. All rights reserved.