Pavlov's plastic


Inanimate material can be ‘trained’ to behave like small artificial muscles, a new study suggests. 


Finnish researchers have trained pieces of plastic to "walk" under the command of light.
CREDIT: Zeng and Zhang et al / matter

By Ian Connellan

Picture a robot and images of futuristic humanoids of metal and code come to mind. However, researchers in Finland are creating a tunable “micro-robot” that could become an important asset for biomedical procedures.

It’s made from liquid crystal polymer networks (the base for most plastics) layered with a coat of dye that responds to heat. It converts energy into a bending motion, much like a human finger curls, thereby “walking” at roughly the speed of a snail.

The method, published in the journal Matter, represents the first time an inanimate material has “learnt” an action. Researchers used conditioning with heat and its association with light to produce a response – something like training a pet.

“Our research is essentially asking the question if an inanimate material can somehow learn in a very simplistic sense,” says senior author Arri Priimägi, from Tampere University of Applied Sciences.

Learning can be considered a sequence of processes through which a biological system or organism modifies its behaviour based upon past experiences.

The full complexity of learning is unknown, and involves perception, memory, motor functions, consciousness, and reward-seeking, many of which have been connected solely to living organisms.

However, simple organisms learn by fundamental learning forms such as habituation, sensitisation and classical conditioning.

In the micro-robot, classical conditioning was used to prompt a response – bending – to an initially neutral stimulus: light.

“My colleague, Professor Olli Ikkala from Aalto University, posed the question: ‘Can materials learn, and what does it mean if materials would learn?’” says Priimägi.

“We then joined forces in this research to make robots that would somehow learn new tricks.”

The conditioning process to associate light with heat includes turning the material blue as the dye on the surface diffuses throughout it. This increases the overall light absorption, which boosts the photothermal effect (an increase in energy in atoms caused by the absorption of a particle of light) and raises the micro-robot’s temperature.

It then “learns” to bend when exposed to light as it self-heats.

Besides walking, the material can recognise and respond to different light wavelengths that correspond to the coating of its dye. This characteristic makes the material a tunable soft micro-robot that can be remotely controlled – an ideal material for biomedical applications.

“I think there's a lot of cool aspects there,” says Priimägi.

“These remotely controlled liquid crystal networks behave like small artificial muscles. I hope and believe there are many ways that they can benefit the biomedical field, among other fields such as photonics, in the future.”

Priimägi says the study was inspired by Pavlov’s dog experiment, where a dog was conditioned to associate food with the ringing of a bell, and thus began salivating at the sound of it.

“If you think about our system, heat corresponds to the food, and the light would correspond to the bell in Pavlov’s experiment,” he says.

“Many will say that we are pushing this analogy too far. In some sense, those people are right because compared to biological systems, the material we studied is very simple and limited. But under right circumstances, the analogy holds.”

The next step for the team is to increase the level of complexity and controllability of the systems.

“We aim at asking questions which maybe allow us to look at inanimate materials from a new light,” Priimägi concludes.

  1. https://dx.doi.org/10.1016/j.matt.2019.10.019
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