Scientists have long had it in their mind to make a robot lizard  . They began with finding out why a lizard can hang on a wall. They noticed the lizard's toes were unique: They have suckers  , which enable it to hang on walls. They, therefore, made a robot with suckers on its hands and feet. The robot could hang on the wall but fell off when crawling. So, they went on researching. 6 years ago, scientists discovered that suckers only were not enough. It is the bristles   on each foot that adds friction   and static adsorption   that makes a lizard move on the smooth wall easily without falling down.  Then scientists made great efforts to fix thick bristles to the robot's hands and feet. However, the effect was not satisfactory. The robot still couldn't attach itself firmly to the wall.  Scientists got puzzled: How on earth can the lizard crawl on an extremely smooth wall or even on a ceiling without dropping off?  An accidental finding inspired them: One day a scientist happened to see an animal attack a lizard and bite off its tail. The lizard broke away from the animal's teeth and threw itself on a wall to escape, only to fall off heavily on the ground. The scientist wondered: Is it the tail that plays an important role in its travelling on the wall? He caught some lizards for an experiment. The result proved his assumption: A tailless lizard has no trouble walking on an ordinary wall but can't on a smooth one. A further study showed the lizard's tail can prevent it falling over backward and, what's more, that the tail acts as an additional leg while one of the lizard's legs leaves the wall, which is always the case while it is walking on the wall.  Thus, Tailbot, a super tailed robot, is born. The bristles function as something   _  .
to protect the lizard's toes from injury to produce friction and static adsorption to prevent a lizard falling over backward to help a lizard move about without falling