Strips of paper embedded with a conducting polymer can perform a range of movements with electrical stimulation.
George Whitesides at Harvard University in Cambridge, Massachusetts, and his colleagues made paper actuators that expand and contract according to their water content. They added a conducting polymer that coated the fibres of the paper, and then applied Scotch tape to one side. When electrically activated, the paper heats up, dries out and contracts. When the electrical current is turned off, the paper absorbs water from the air and expands. The tape is not affected by heat or moisture, so directs the paper to bend in certain ways.
The authors made actuators of different shapes, including one that could curl up (pictured), and say that the devices could be used in lightweight micromachines.