Source of holding force

 As we saw in previous texts, snake hold pole during climbing. The source of holding force is skin structure and helical movement.

Yoo Sang Jun, Modular type robot for field moving and tree climbing, 2009

Snake's skin is covered by unidirectional scales like above picture. This scales generate low friction when snake moves forward and generate large friction when snake slip backward.

http://www.eplantscience.com/index/general_zoology/characteristics_of_reptiles_that_distinguish_them_from_amphibians.php

Scales make bumpy surface like upper picture to generate asymmetric friction coefficient. Asymmetric friction coefficient help climbing of snake.

First source of holding force, skin structure, can improves climbing ability of robot. But I can't find research team that study about snake skin for robots. Current research only focus on second source of holding force, helical motion.

Yoo Sang Jun, Modular type robot for field moving and tree climbing, 2009

When snake coil the vertical pole, we can model that situation like this picture. Every joint generate torque using their muscles and torque make stable contact between pole and snake. It also increase normal forces (N1, N2, N3, N4). It means snake get larger friction force from the pole. This phenomenon make snake climb up the vertical pole.