From Light-Powered Motors, to Micro-Grippers, to Crawling Caterpillars, Snails and Beyond-Light-Responsive Oriented Polymers in Action

Abstract

"How would you build a robot, the size of a bacteria, powered by light, that would swim towards the light source, escape from it, or could be controlled by means of different light colors, intensities or polarizations?" This was the question that Professor Diederik Wiersma asked PW on a sunny spring day in 2012, when they first met at LENS-the European Laboratory of Nonlinear Spectroscopy-in Sesto Fiorentino, just outside Florence in northern Italy. It was not just a vague question, as Prof. Wiersma, then the LENS director and leader of one of its research groups, already had an idea (and an ERC grant) about how to actually make such micro-robots, using a class of light-responsive oriented polymers, liquid crystal elastomers (LCEs), combined with the most advanced fabrication technique-two-photon 3D laser photolithography. Indeed, over the next few years, the LCE technology, successfully married with the so-called direct laser writing at LENS, resulted in a 60 micrometer long walker developed in Prof. Wiersma's group (as, surprisingly, walking at that stage proved to be easier than swimming). After completing his post-doc at LENS, PW returned to his home Faculty of Physics at the University of Warsaw, and started experimenting with LCE, both in micrometer and millimeter scales, in his newly established Photonic Nanostructure Facility. This paper is a review of how the ideas of using light-powered soft actuators in micromechanics and micro-robotics have been evolving in Warsaw over the last decade and what the outcomes have been so far.

Keywords: actuators; light-responsive materials; liquid crystal elastomers; micro-robotics; soft robotics.

Figure 1

Light-responsive LCE—from a simple contracting strip to films with arbitrary molecular alignment. (A) A piece of material with cross-linked polymer chains (olive) arranged in one direction undergoes a phase change in response to temperature increase, either in the entire volume (left) or locally, when illuminated with a green laser beam (right): the molecular order decreases, the polymer chains effectively shorten along the director and macroscopic deformation results. (B) A 10 mm long strip of LCE film heated in an oven from 29 °C to 150 °C shortens by approximately 35%. (C) A 22 × 22 mm piece of flat LCE film with azimuthal director orientation (the black dot marks the center of rotational symmetry) buckles when heated on a hot plate from 22 °C to 80 °C.

Figure 2

Rotary motor directly powered with light. (A) Schematic of the rotary piezoelectric (ultrasonic) motor. Travelling wave deformation generated in the rotor (orange) interacts via friction with the stator (blue) and sets the former in motion. (B) LCE rotary micro-motor, seen from the side. The LCE disc (rotor, orange) rotates with respect to the stator (rough solid surface, blue), around a steel axis. The white scale bar is 2 mm long. (C) The LCE rotor on a pencil tip for the scale demonstration (although, sadly, we must admit it cannot yet be used to sharpen pencils). (D) Numerical simulations of the LCE deformation upon illumination with a spatially modulated (rotating) laser beam. The top row is for azimuthal–radial, and the bottom row for azimuthal–azimuthal director orientation (see text for details). Adapted from [65].

Figure 3

Light-powered linear inchworm motor. (A) Accordion-like actuator heated in an oven exhibits very large contraction, with up to 80% stroke. (B) CAD rendering of a linear stepping motor with two actuators (orange), gripper (dark grey/yellow) and a sliding shaft (black, under the gripper). (C) The sequential action of the actuators, powered by a scanned green laser beam, results in orbital motion of the gripper that, in turn, moves the shaft. The black scale bar in (A) is 5 mm long. Adapted from [52].

Figure 4

Natural scale crawling caterpillar robot. (A) The caterpillar robot on the fingertip of one of its creators. (B) Schematic of the experimental setup—the green laser beam is scanned along the robot’s body with a galvo mirror driven by an asymmetric sawtooth signal. The beam was scanned at 0.4 Hz and had 2.5 W of power. (C) Snapshots of the video with the light-driven caterpillar crawling on a level surface. The white scale is 5 mm long. The laser light is filtered out with an orange optical filter. Adapted from [50].

Figure 5

Light-powered snail mimicking the adhesive locomotion of terrestrial gastropods. (A) A garden banded-snail Cepea hortensis meets the 10 mm long light-powered snail robot (having no shell, though). (B) In snails and slugs, pedal waves propagate along the ventral foot contact surface with a velocity V_S, propelling the animal with an average speed V_CM. In a similar way, the light-induced elastomer deformation moves along the robot’s soft body (yellow) covered with an artificial mucus layer (purple). In both cases, the deformations are in fact much smaller. (C) Snapshots from a video with the snail robot crawling on a horizontal glass plate topped with glycerine as an artificial mucus. The average speed is 1 mm/min, the black scale bar is 5 mm long. The material contraction was of the order of 0.1 mm. Adapted from [5].

Figure 6

From the pond to the lab (and back)—a light-powered natural-scale water strider robot with LCE muscle. (A) Water striders are common inhabitants of reservoirs and rivers in many climate zones. (B) First prototype of the light-powered robo-strider with the orange LCE bending actuator powering a pair of legs. (C) The latest generation of the robot body (here without the actuator installed) was 2D printed with a 3D extrusion printer and then hot-shaped to take the final form. Water strider photo by Schnobby licensed under CC BY-SA 3.0.

Figure 7

The ultimate walker: LCE ant with two degrees of freedom. (A) One of the models built to better understand six-legged locomotion. (B) Stripes of LCE with two different dyes, respond by bending to two high-power LEDs with 660 nm and 450 nm centered spectra, respectively. White light (top) and IR (bottom) images. (C) One of the concepts of the mechanical design of a six-legged walker with two tripods: one solid (blue) and one with lifting legs (yellow), shifted relative to each other with a bending strip (orange), performing a sequence of deformations that result in a step forward.

Figure 8

Fiber-grown microscale gripper. (A–C) Snapshots from the video show the bending actuator growth inside an LC-mixture-filled cell. Time in min: sec is shown in the bottom left corner of each frame. (D) Bending actuator in action, powered with green light delivered through the fiber (two photographs have been superimposed to show the magnitude of the deformation). (E) Two fibers with bending actuators make a light-powered gripper, here next to a Formica polyctena ant with its mandibles about ten times the size of our tool. Adapted from [93].