Seoul National University College of Engineering (Dean Byung-ho Lee) announced that a joint research team of Professor Sung-hoon Ahn and Professor Hee-jae Park (researchers Min-soo Kim, Young-jun Jeon, Hye-seong Lee, Young-kyun Cho, Jae-kyung Huh, and Seung-woo Lee) of the Department of Mechanical Engineering had developed a light-sensitive artificial muscle that mimics the muscle of the iris that contracts when stimulated by light.

The pupil reflex, often used to check a person's consciousness, occurs when the sensory organ senses a light stimulus, and the iris muscle contracts in response to the central nervous system. On the other hand, the pupil reflex of an animal with a relatively less developed central nervous system, such as a mouse, can be achieved by the iris muscle directly sensing light and responding immediately. Therefore, there is no need for a complicated central nervous system, and it can operate with a small and simple structure.

But, what if there was an artificial muscle that responds immediately to the color of light, in other words, the wavelength of light? Various changes can be made according to the stimulation of light.



▲ (From left) Professor Sung-hoon Ahn and Hee-jae Park, Department of Mechanical Engineering, Seoul National University





▲ Participating researchers (from left): First authors, Dr. Min-soo Kim, Researcher Hye-sung Lee, Researcher Young-kyun Cho, Researcher Jae-kyung Huh, Researcher Seung-woo Lee, and Dr. Young-jun Jeon

Based these expectations, our research team developed a miniature metal artificial muscle that responds to laser light. By introducing a meta surface to a nickel-titanium shape memory alloy having a size of 60 x 5 x 5 micrometers, it was designed to absorb 355 nm wavelength ultraviolet and 785 nm wavelength infrared laser differently. Since this actuator is small in size, when light is illuminated without an electric circuit, the temperature of the material rises and the shape memory effect appears, it generates a large force compared to its size, and it has the advantage of being able to implement various deformations using light of different wavelengths.

The meta-surface shape memory alloy reacts differently depending the wavelength of light, which can be explained by the principle that determines the color of an object. The lattice patterns of several hundred nanometers (about 1/1000 the thickness of a hair) that make up the meta surface generate a plasmonic phenomenon in which light resonates the surface when light is irradiated. By using the plasmonic phenomenon, the intrinsic light absorption/reflection characteristics of an object can be changed. For example, adding a metasurface to gold, which is originally golden, reflects more red wavelengths of light, making it appear reddish. By adding a meta surface to an ordinary shape memory alloy with a low absorption rate of infrared light, it can be changed to absorb infrared light well.

To realize this, the research team designed the optical properties of the shape memory alloy metasurface based experiments and computer simulations, and implemented the nanopatterns the surface using the focused ion beam process. By adjusting the spacing of the nanopatterns, the degree of shrinkage of the shape memory alloy when it receives light can be controlled.

Corresponding author Professor Sung-hoon Ahn said, “This micro artificial muscle has the characteristic of having different movements depending the wavelength of light by adding the principle of 'meta surface' to the basic principle of shape memory alloy that responds to temperature.” If we develop this further, we expect to be able to create a new physical artificial intelligence system that automatically responds to light without an electronic control circuit or controls movement by a difference in light wavelength, such as the example of a mouse iris that operates without a central nervous system.”

Meanwhile, this research result was published as a cover paper March 8 in Advanced Optical Materials, a world-renowned international academic journal.



<Meta surface and cross-section with increased infrared laser absorption of shape memory alloy>



<Difference in driving characteristics according to the presence or absence of a metasurface: When there is a metasurface, when the same amount of light energy is irradiated, more driving displacement is shown>



<Example of applying a normal surface and a metasurface to a microactuator at the same time: When heating with an infrared laser, the upper actuator with a metasurface works, and when heating with an ultraviolet laser, the lower actuator works>

<Text>Surface Nanopatterned Shape Memory Alloy (SMA)‐Based Photosensitive Artificial Muscle (wiley.com)
<Journal Site>
https://onlinelibrary.wiley.com/toc/21951071/2022/10/5

[Inquiry]
Sung-hoon Ahn, Professor, Department of Mechanical Engineering, Seoul National University / ahnsh@snu.ac.kr