Light‐Fueled Microscopic Walkers

摘要

The first microscopic artificial walker equipped with liquid-crystalline elastomer muscle is reported. The walker is fabricated by direct laser writing, is smaller than any known living terrestrial creatures, and is capable of several autonomous locomotions on different surfaces. Nature provides a valuable source of inspiration for many fields of research. From optical effects to engineering problems, organisms have often adapted elaborate solutions and strategies, being the result of many years of natural selection.1, 2 Within this context, there is a lot of interest in creating artificial structures (robots) that can walk,3 swim,4 and perform various tasks.5 Micrometer scale robots, in particular, have been proposed for applications, including drug delivery,6 biosensing,7 and microsurgery.8 Based on the development of artificial muscles, a variety of soft robots that mimic natural creatures have been designed.9-12 Such artificial creatures rely on power delivered from outside, while the entire mechanical actuation is due to the inner stress of the muscle. Differently from external field driven devices,6, 7 they approach naturally occurring living systems and are considered of broad scientific interest and technological value. Terrestrial soft robots have been realized with walking,9, 10 gripping,11 and camouflage12 functionalities, but still remain in the centimeter scale. Working in the microscopic scale, strong adhesion arising from the surface related forces (e.g., van der Waals and the capillary forces) is the limit of existing examples. Inspiringly, it has been found that in nature van der Waals and the capillary forces dominate the reversible adhesion between terrestrial living organisms and the surfaces in contact.13-16 Typical natural adhesion force per unit area has been experimentally measured to be 0.3–1.3 MPa17, 18 (on gecko foot spatula), while a typical muscle stress in nature is between 0.01 and 0.2 MPa (peak 0.8 MPa).19 While the entire creature size shrinks down to micrometers (muscle cross section getting close to body-environment contact area), the adhesion becomes comparable with the muscle force, thus posing a great difficulty for any movement. The balance between achievable muscle stress and adhesive forces thus sets the lower limit for the size of terrestrial free moving organisms, being of the order of 140 μm.20 Similarly, microscopic artificial creatures equipped with artificial muscles are struggling to overcome the same adhesion hurdle as their natural relatives. The different physics ruling on microscopic length scales (including domination of adhesion forces) is worth exploring in micrometer-sized robotic systems. In addition, the realization of large numbers of autonomous robots can allow the study of cooperative effects, and open up new strategies for self-assembly. Muscles (soft sections of the creatures) have always been well isolated from the environment, while hard crust and hairy-like surfaces often assist to reduce the surface contact zone and hence minimize the biological adhesion. Liquid crystalline elastomers (LCEs), that combine elastomeric properties with liquid crystalline orientational order, can reversibly deform in response to external stimuli with dramatic contraction (up to 400%) and with a stress compatible to natural muscles.21 Therefore, LCEs have been considered as artificial muscles with great potential for creating biomimetic soft robot.22 Recently developed technologies enable to control molecular order in the microscopic and macroscopic scale23, 24 (instinct related to actuation) and fabricate patterned actuator with micrometer-sized complex shapes.25 However, such soft materials with typical elastic moduli E in the MPa range,21 behave extremely sticky. All approaches to these artificial muscles in the microscopic scale have failed, as the natural adhesion poses a huge hurdle for robot's locomotion. To isolate the LCE muscle from the environment, and then solve the natu