Researchers in Spain have used a smart polymer to develop a robot finger that can feel the weight of what it’s pushing and adjust the energy it uses accordingly. The polypyrrole material expands in response to electric current and conducts differently in response to changes in pressure.

To convert the polymer’s shape change into a bending motion, scientists at the Polytechnic University of Cartagena stick two thin polypyrrole films to either side of an insulating plastic tape. When they apply a positive charge to one of the films and a negative charge to the other, the first contracts and the second swells.

These shape changes occur as electrons are pulled from or added to the chain-like polymer molecules in the films. An object in the path of the finger’s moving tip as it bends gets pushed away at the same speed, regardless of its weight. For heavier objects, the finger pushes with more force and uses more energy.