Scientists have developed a low-cost technology that allows a simple piece of paper to spring to life - bending, folding or flattening itself on command. A thin layer of conducting thermoplastic, applied to common paper with an inexpensive 3D printer or even painted by hand, serves as a low-cost, reversible actuator. When an electrical current is applied, the thermoplastic heats and expands, causing the paper to bend or fold; when the current is removed, the paper returns to a predetermined shape. "Actuation truly turns paper into another medium, one that has both artistic and practical uses," said Lining Yao, assistant professor at Carnegie Mellon University (CMU) in the US. Researchers have designed basic types of actuators, including some based on origami and kirigami forms. These enable the creation of structures that can turn themselves into balls or cylinders. They can also be used to construct more elaborate objects, such as a lampshade that changes its shape and the amount of light it emits, or an artificial mimosa plant with leaf petals that sequentially open when one is touched. In June, over 50 students in a workshop at Zhejiang University in China used the paper actuation technology to create elaborate pop-up books, including interpretations of famous artworks, such as Van Gogh's Starry Night and Sunflowers."Most robots - even those that are made of paper - require an external motor," said Guanyun Wang, a post-doctoral researcher at CMU."Ours do not, which creates new opportunities, not just for robotics, but for interactive art, entertainment and home applications," said Wang. Creating a paper actuator is a relatively simple process, Cheng said. It employs the least expensive type of 3D printer, a so-called FDM printer that lays down a continuous filament of melted thermoplastic. The researchers use an off-the-shelf printing filament - graphene polylactide composite - that conducts electricity. The thermoplastic actuator is printed on plain copy paper in a thin layer, just half a millimetre thick. The actuator is then heated in an oven or with a heat gun and the paper is bent or folded into the desired shape and allowed to cool. This will be the default shape of the paper. Electrical leads can then be attached to the actuator; applying electrical current heats the actuator, causing the thermoplastic to expand and thus straighten the paper. When the current is removed, the paper automatically returns to its default shape. Researchers are refining this method, changing the printing speed or the width of the line of thermoplastic to achieve different folding or bending effects. They have also developed methods for printing touch sensors, finger sliding sensors and bending angle detectors that can control the paper actuators.