Reconfigurable photoactuator through synergistic use of photochemical and photothermal effects

​Experimental scientific research is a manifestation of successful team work in the lab, which at best leads to the happy event of publishing a scientific paper. Our recently published article in Nature Communications tells, in addition to the interesting science, a different story – a story about a quarrel between the members of the Smart Photonic Materials team at Tampere University of Technology.

Go to the profile of Arri Priimagi
Oct 08, 2018

The paper in Nature Communications is here:

A few years ago, we got into the business of making soft actuators for small-scale robotic devices. Soft robotics is dominated by researchers working on magnetically driven materials but we believe in light, since it provides an unprecedented degree of control over the material movements. Having worked on different aspects of light-responsive materials for a long time, photomobile micro-robotics was the way to go. Among the different classes of light-responsive materials, we choose so-called liquid crystalline networks (LCNs). Working with this unique class of materials has provided us with much fun, as exemplified by imprinting angry bird images into them, or making devices mimicking the action of human iris and the Venus flytrap plant. But an important question remained: why does the material move so efficiently upon light irradiation?

Often times chemists insist that photochemically induced isomerization of azobenzene molecules incorporated into the LCN is the key for efficient photoactuation. Physicists, in turn, believe that a simpler process, photothermal heating, does the job. Considering the composition of our team – half chemists and half physicists – we had a problem: how to reach a consensus which one is better? One day the physicists showed that adding any pigment into or onto the LCN made the material to deform under light irradiation. “Photoisomerization is redundant”, they happily concluded. The next day the chemists took over and showed that ultra-low-intensity UV light deforms the photochemically driven material. “No more need for dirty pigments or inelegant heating processes”, they concluded. Thus, a quarrel started and led to the rare and undesired event of professor entering the lab, suggesting a compromise: “Instead of searching for good and bad, why not simply combine the two approaches?” That’s how the story behind the material concept behind this article got started.

Our material works as expected when illuminated with different wavelengths: UV/blue light triggers photochemical actuation, while red light triggers photothermal actuation. The magic happens when both wavelengths are applied simultaneously, leading to markedly more efficient actuation. Why? Because photochemical isomerization serves to modulate the mechanical properties of the LCN and generate stress into the material, while photothermal heating efficiently releases the stress and deforms the material. Using patterned UV illumination, we realized a reconfigurable actuator where photochemistry is used for shape programming and photothermal effects for shape morphing. With the same principle, we were also able to make a light-fueled gripping device that can be commanded to either grip-and-release or grip-and-hold an object after ceasing the illumination. After devising a smart material whose functionality was synergistically driven by photochemical and photothermal effects, the Smart Photonic Materials team members started again working synergistically in the lab, and the undesired event of professor entering the lab was no longer needed.

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Go to the profile of Arri Priimagi

Arri Priimagi

Associate Professor, Tampere University of Technology

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