Simple Vibrating Bot Climbs Tubes With Ease:

Its simple motor turns an unbalanced mass at a uniform velocity. As the mass swings around, it causes the robot to bounce back and forth between the tube walls. Two rubber o-rings let the researches specify the exact contact points and increase friction with the walls.

Carnegie Mellon University researchers have developed a robot that can climb three-dimensional tubes.

Source: http://spectrum.ieee.org/automaton/robotics/medical-robots/tube-climbing-robot