Like a Snail

Photo: A snail on a road

The intestines are an extremely difficult area to navigate through with a medical device. Researchers have been trying to develop a small robot that moves independently through the intestinal tract. Up to now the robots ignore the layer of mucus which covers the inside of the large intestine and try to suck or grab on to the intestinal wall, which results in the walls being stretched and the patient feeling pain and discomfort.

A better method, according to Delft University of Technology researcher Dimitra Dodou, is in fact to use this layer of mucus and allow the robot to imitate the forward movement of a snail. A snail leaves a trail of slime behind it on the ground. This slimy material works simultaneously as a lubricant for gliding on and as a glue which the slug can grip onto.

An intestinal robot should also have a similar layer to use. To achieve this, an adhesive layer is added to the mucus-like properties, which allows the device to be stuck to the layer of mucus. The researchers discovered a group of polymers, so-called muco-adhesives that are suitable for this. Dodou used a pig's intestine to evaluate how this material worked. Her findings revealed that muco-adhesives in the form of films provided by far the highest degree of friction.

Despite this, there is nevertheless no possibility of movement. A snail uses the exertions of pressure to change the characteristics of the middle layer, and thus lower the degree of friction, in order to move. In the intestine, however, pressure cannot be exerted, because this would cause the intestine to become deformed. The solution then is found in using smaller and larger surfaces that slide over each other. If a large surface coated with muco-adhesive remains still, and a relatively small surface coated with muco-adhesive begins moving in relation to the larger surface, the smaller surface has less freedom of movement. One by one the small 'hands' of the robot move forward. After this, the entire robot can slide forward incrementally, whereupon the process of small surfaces shifting begins anew.

Additional experiments found that it is not only the size of the film surfaces, but also their shapes, which influence the degree of friction generated. The researchers are currently building a prototype that will be tested in living pigs.; Source: Technical University Delft