English: Schematic depicts a patterned gecko surface (pattern of four setae represented as yellow pillars) filled with water (blue) nearly contacting a substrate (glass or PMMA). Using the tree frog adhesive system as a model, we describe the inter-setal distance or microchannel width as W and the height of the intervening water layer as h. During a step, where the gecko setae approach the substrate, h→0 and W>h (purple arrow), causing water to move out of the microchannels (small purple arrows). When the gecko removes the foot, using digital hyperextension, h increases and at h>W (red arrow), remaining water is moved back into the microchannels (small red arrows). The movement of water in and out of the microchannels is due to the pressure difference in the microchannels and thin water film.