They started with the setae. The team designed polymer versions, making them 17 microns across and with ends shaped like mushroom caps. The setae are slightly flexible as well, just like the gecko's, to maximize the area that comes into contact with uneven surfaces. The more setae that contact the surface, the better it can hold the robot up.
But well-designed setae that have strong clinging power need some kind of force to peel them off. This is why Krahn and his team put the mushroom-capped setae on tank-like treads. Since treads pull away at an angle as they roll, there is no problem with the setae getting stuck.
Next was building a robot that would put enough pressure on the treads to get the sticking action. To do that, the scientists built two sections to the robot, so that as front half of the robot approaches a surface, it gets a little push from the rear half.
Since the setae and tread are clinging to the walls using molecular forces and not some kind of tacky, wet adhesive, such as glue, the machine is easier to keep clean. The treads can simply be rinsed off, which means they don't have to be replaced frequently. A wet adhesive that's picking up dust and dirt along the way would stop working -- just think of how a piece of tape gets after it's re-used many times. Geckos' feet self-clean, though nobody is quite sure how.