materials Communication Light-Driven Linear Inchworm Motor Based on Liquid Crystal Elastomer Actuators Fabricated with Rubbing Overwriting Mikolaj Rogó ˙ z 1, * , Jakub Haberko 2 and Piotr Wasylczyk 1   Citation: Rogó ˙ z, M.; Haberko, J.; Wasylczyk, P. Light-Driven Linear Inchworm Motor Based on Liquid Crystal Elastomer Actuators Fabricated with Rubbing Overwriting. Materials 2021, 14, 6688. https:// doi.org/10.3390/ma14216688 Academic Editor: Jianbo Yin Received: 28 September 2021 Accepted: 3 November 2021 Published: 6 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Photonic Nanostructure Facility, Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02-093 Warsaw, Poland; pwasylcz@fuw.edu.pl 2 Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Kraków, Poland; haberko@fis.agh.edu.pl * Correspondence: mikolajrogoz@uw.edu.pl Abstract: Linear displacement is used for positioning and scanning, e.g., in robotics at different scales or in scientific instrumentation. Most linear motors are either powered by rotary drives or are driven directly by pressure, electromagnetic forces or a shape change in a medium, such as piezoelectrics or shape-memory materials. Here, we present a centimeter-scale light-powered linear inchworm motor, driven by two liquid crystal elastomer (LCE) accordion-like actuators. The rubbing overwriting technique was used to fabricate the LCE actuators, made of elastomer film with patterned alignment. In the linear motor, a scanned green laser beam induces a sequence of travelling deformations in a pair of actuators that move a gripper, which couples to a shaft via friction moving it with an average speed in the order of millimeters per second. The prototype linear motor demonstrates how LCE light-driven actuators with a limited stroke can be used to drive more complex mechanisms, where large displacements can be achieved, defined only by the technical constrains (the shaft length in our case), and not by the limited strain of the material. Inchworm motors driven by LCE actuators may be scaled down to sub-millimeter size and can be used in applications where remote control and power supply with light, either delivered in free space beams or via fibers, is an advantage. Keywords: liquid crystal elastomer; linear motor; actuator; smart material; rubbing 1. Introduction Nature does not seem to use rotary motion often. We are aware of only three examples of rotary motion used in the animal kingdom: bacterial flagella are powered by a rotating motor built into the cell envelope, driven by the flow of protons or sodium ions; dung beetles use rolling to transport loads; and some caterpillars from the Crambidae family roll away when in danger. Our civilization, on the contrary, relies chiefly on rotary motion for energy conversion, transport and manufacturing. At the same time, linear motion is vital for many applications, e.g., in handling, positioning and metrology. There are a number of solutions for realizing linear displacement with rotary motors: via rack and pinion, wheel and belt or lead screw. It is also possible to generate linear displacement directly with either a pressurized medium acting on a piston, as in hydraulic and pneumatic actuators, or electromagnetic forces, as in voice coils and more complex linear electric motors. Another approach is based on direct material deformation, most notably in piezoelectric actuators, or in more exotic examples, based on shape memory alloys, or other shape memory material, such as liquid crystalline epoxy resins and composites [1,2], dielectric elastomers or liquid crystal networks [3,4]. Each of these solutions has certain advantages and limitations and several characteristics are taken into account when considering specific applications, among them: speed; stroke; maximum load that can be handled; maximum force that can be generated; accuracy; and locking force at rest with no power supply. In some applications, such as positioners for nuclear magnetic resonance (NMR) scanners, the materials involved in the actuator Materials 2021, 14, 6688. https://doi.org/10.3390/ma14216688 https://www.mdpi.com/journal/materials