Finnish scientists have taught a strip of plastic to move in response to light by causing it to associate one stimulus with another. Their efforts were inspired by Ivan Pavlov's famous experiments on dogs and blurring the boundaries between the biological and physical worlds. However, for those worried plastic is already taking over the world, (or anyone who saw Terminator II) this looks like a nightmare under development.

The plastics Tampere University's Professor Arri Priimägi is working with are a lot more sophisticated than something used to hold fizzy drinks. They are made from thermo-responsive liquid crystal polymer networks coated with dye and can turn energy into motion.

Initially, the plastic responded to heat, but not other forms of energy such as light. However, Priimägi and colleagues report in the journal Matter that they followed in Pavlov's footsteps, conditioning pieces of this plastic to associate light with heat.

Where Pavlov taught his dogs to associate bells with food, to the point where they salivated on the sound alone, Priimägi exposed the plastic strips to heat and light together, after which the light alone was enough to generate the same response as heat. Conditioned strips curl up when exposed to light, and uncurl in the dark. By shaping the plastic suitably this curling and uncurling can be converted into forward motion, rather than simply staying on the spot.

The authors refer to this as “walking” but looks more like an inchworm's crawl. Existing speeds are about 1 millimeter per second (2 inches a minute), so we don't have to worry just yet about such materials outrunning us, but efforts to clean the oceans could soon face a new challenge.

The work began when co-author Professor Olli Ikkala of Aalto University pondered if it was possible for materials to learn. We know computers can do so, but what about something much simpler? In a previous study, the same team proved the answer was yes by conditioning a gel to melt on exposure to light.

For the new paper, they turned to a material whose flexibility changes depending on the relative positioning of internal molecules. "For material to learn, it must have a memory. When the material is heated, the dye originally spread on the surface of the liquid crystal polymer penetrates into the material, thus forming the memory," Ikkala said. 

The authors even taught the plastics to grab hold of objects and have plans to tune responses to specific colors of light, using the fact some dyes respond to narrow wavelength bands. Potentially this could enable the creation of exceptionally lightweight robots that don't need to carry their own power supply, and are instead controlled at a distance using light.

Priimägi acknowledged critics may consider the analogy with Pavlov's dog to be overblown given the simplicity of the system compared to something biological. However, he sees the work as a stepping stone to introducing ever greater complexity.