Toward Application of Liquid Crystalline Elastomer for Smart Robotics: State of the Art and Challenges

Abstract

Liquid crystalline elastomers (LCEs) are lightly crosslinked polymers that combine liquid crystalline order and rubber elasticity. Owing to their unique anisotropic behavior and reversible shape responses to external stimulation (temperature, light, etc.), LCEs have emerged as preferred candidates for actuators, artificial muscles, sensors, smart robots, or other intelligent devices. Herein, we discuss the basic action, control mechanisms, phase transitions, and the structure-property correlation of LCEs; this review provides a comprehensive overview of LCEs for applications in actuators and other smart devices. Furthermore, the synthesis and processing of liquid crystal elastomer are briefly discussed, and the current challenges and future opportunities are prospected. With all recent progress pertaining to material design, sophisticated manipulation, and advanced applications presented, a vision for the application of LCEs in the next generation smart robots or automatic action systems is outlined.

Keywords: liquid crystal; liquid crystalline elastomer; smart robots; soft robots.

Figure 1

Applications of liquid crystalline elastomers in optical device [11] (Reproduced with permission from López-Valdeolivas, M.; Liu, D.; Broer, D.J.; Sánchez-Somolinos, C., Macromol. Rapid Commun.; Copyright 2018 John Wiley and Sons.), smart switches [34] (Reproduced with permission from Sánchez-Ferrer, A.; Fischl, T.; Stubenrauch, M.; Wurmus, H.; Hoffmann, M.; Finkelmann, H, Macromol. Chem. Phys.; Copyright 2009 John Wiley and Sons.), artificial muscles [35] (Reproduced with permission from He, Q.; Wang, Z.; Song, Z.; Cai, S., Adv. Mater. Technol.; Copyright 2019 John Wiley and Sons.), micro-robots [36] (Reproduced with permission from Ahn, C.; Liang, X.; Cai, S., Adv. Mater. Technol.; Copyright 2019 John Wiley and Sons.), soft grippers [37] (Reproduced with permission from He, Q.; Wang, Z.; Wang, Y.; Minori, A.; Tolley, M.T.; Cai, S., Science Advances; Copyright 2019 American Association for the Advancement of Science.), intelligent skins [38] (Adapted from Ref. [38] with permission from the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry.), and antennas [39] (Adapted with permission from Kim, H.; Gibson, J.; Maeng, J.; Saed, M.O.; Pimentel, K.; Rihani, R.T.; Pancrazio, J.J.; Georgakopoulos, S.V.; Ware, T.H. Responsive, 3D Electronics Enabled by Liquid Crystal Elastomer Substrates. ACS Appl. Mater. Interfaces 2019, 11, 19506–19513, doi:10.1021/acsami.9b04189. Copyright (2019) American Chemical Society).

Figure 2

Notional properties and molecular configurations of LCPs (a), LCNs (b), and LCEs (c).

Figure 3

(a) A printed strip exhibits reversible linear actuation until the ends are welded together at 60 °C via dynamic bond exchange to form a reversibly actuating Möbius strip [91] (Reproduced with permission from Davidson, E.C.; Kotikian, A.; Li, S.; Aizenberg, J.; Lewis, J.A., Adv. Mater.; Copyright 2020 John Wiley and Sons.), (b) Printing schematic and photographs of a disk printed with two different actuation temperatures of LCE materials in a +1 defect print pattern [92] (Reproduced with permission from Saed, M.O.; Ambulo, C.P.; Kim, H.; De, R.; Raval, V.; Searles, K.; Siddiqui, D.A.; Cue, J.M.O.; Stefan, M.C.; Shankar, M.R.; et al., Adv. Funct. Mater.; Copyright 2019 John Wiley and Sons.), (c) Schematic of LCE oligomer ink DIW 3D printing [25] (Reproduced with permission from Roach, D.J.; Kuang, X.; Yuan, C.; Chen, K.; Qi, H.J., Smart Mater. Struct.; ©2018 IOP Publishing. Reproduced with permission. All rights reserved.), (d) Schematics and photographs of locally programmed popping-up and oscillating behaviors of LCE actuators [101] (Adapted with permission from Ren, L.; Li, B.; He, Y.; Song, Z.; Zhou, X.; Liu, Q.; Ren, L. Programming Shape-Morphing Behavior of Liquid Crystal Elastomers via Parameter-Encoded 4D Printing. ACS Appl. Mater. Interfaces 2020, 11. Copyright (2020) American Chemical Society.), (e) Schematic of femtosecond DLW of 3D microstructures in the cell via two-photon polymerization [102], (f) SEM images of the LCE 3D microstructures by using DLW [102] (Adapted with permission from Chen, L.; Dong, Y.; Tang, C.-Y.; Zhong, L.; Law, W.-C.; Tsui, G.C.P.; Yang, Y.; Xie, X. Development of Direct-Laser-Printable Light-Powered Nanocomposites. ACS Appl. Mater. Interfaces 2019, 11, 19541–19553, doi:10.1021/acsami.9b05871. Copyright (2019) American Chemical Society.), (g) Photographs of the microscopic artificial walker [103] (Reproduced with permission from Zeng, H.; Wasylczyk, P.; Parmeggiani, C.; Martella, D.; Burresi, M.; Wiersma, D.S., Adv. Mater.; Copyright 2015 John Wiley and Sons).

Figure 4

(a) Schematic representation of thermal-induced order-disorder phase transition in LCEs. (b) Reversible trans-cis photoisomerization of azobenzenes. (c) Schematic representation of photo-induced order-disorder phase transition in LCEs. (d) Schematic representation of photothermal-induced order-disorder phase transition in LCEs. (e) A light-driven plastic motor with the azobenzene-containing LCE laminated film [112] (Reproduced with permission from Yamada, M.; Kondo, M.; Mamiya, J.; Yu, Y.; Kinoshita, M.; Barrett, C.J.; Ikeda, T., Angewandte Chemie International Edition; Copyright 2008 John Wiley and Sons.) (f) Photothermal bending actuation triggered by sunlight (100 mW cm⁻²) and the light emitting diode on a mobile phone (150 mW cm⁻²) [86] (Reproduced with permission from Kim, H.; Lee, J.A.; Ambulo, C.P.; Lee, H.B.; Kim, S.H.; Naik, V.V.; Haines, C.S.; Aliev, A.E.; Ovalle-Robles, R.; Baughman, R.H.; et al., Adv. Funct. Mater.; Copyright 2019 John Wiley and Sons).

Figure 5

Grippers made by LCEs: (a) Multifunctional soft gripper with twisting and grasping functions [37] (Reproduced with permission from He, Q.; Wang, Z.; Wang, Y.; Minori, A.; Tolley, M.T.; Cai, S., Science Advances; Copyright 2019 American Association for the Advancement of Science.) (b) 4D printed four hinges soft robotic gripper picking and placing a ping pong ball [25] (Reproduced with permission from Roach, D.J.; Kuang, X.; Yuan, C.; Chen, K.; Qi, H.J., Smart Mater. Struct.; ©2018 IOP Publishing. Reproduced with permission. All rights reserved.) (c) A temperature-sensitive gripper. At a lower temperature (70 °C), the gripper’s lower-temperature response layer deforms to grab and lift the object; at a higher temperature (140 °C), the gripper’s higher-temperature response layer deforms and lowers the object [92] (Reproduced with permission from Davidson, E.C.; Kotikian, A.; Li, S.; Aizenberg, J.; Lewis, J.A., Adv. Mater.; Copyright 2020 John Wiley and Sons).

Figure 6

Worm-inspired LCE actuators: (a) Caterpillar robot motion on the nail of a human finger under the irradiation of 488 nm laser [85] (Reproduced with permission from Zeng, H.; Wani, O.M.; Wasylczyk, P.; Priimagi, Macromol. Rapid Commun.; Copyright 2018 John Wiley and Sons.), (b) The images of the serpentine robot moving under NIR light [156] (Reproduced from Ref. [156] with permission from The Royal Society of Chemistry.), (c) LCE capillary placed in a glass tube moving when tube heating [84] (Adapted from [84] with permission from The Royal Society of Chemistry.), (d) Snail-inspired LCE actuator climbing over an obstacle [29] (Reproduced with permission from Rogóż, M.; Dradrach, K.; Xuan, C.; Wasylczyk, P., Macromol. Rapid Commun.; Copyright 2019 John Wiley and Sons.), (e) Schematic of the LCE crawler made of three Sarrus, the robot crawling diagram modules, and representative experimental images of the caterpillar locomotion actuator [158] (Adapted with permission from Minori, A.F.; Fernandes, A.; He, Q.; Glick, P.; Adibnazari, I.; Stopol, A.; Cai, S.; Tolley, M.T., Smart Materials and Structures.; ©2020 IOP Publishing. Reproduced with permission. All rights reserved).

Figure 7

(a) Schematic illustration of the arch shape soft robot moving and crawling and jumping motion, and video frames of crawling, jumping, squeezing, and second jumping of the arch shape soft robot powered by light [36] (Reproduced with permission from Ahn, C.; Liang, X.; Cai, S., Adv. Mater. Technol.; Copyright 2019 John Wiley and Sons.), (b) Schematic illustration of the soft locomoting robot with automatic and programmable control, and images of the soft robot during moving [47] (Reprinted with permission from He, Q.; Wang, Z.; Wang, Y.; Song, Z.; Cai, S. Recyclable and Self-Repairable Fluid-Driven Liquid Crystal Elastomer Actuator. ACS Appl. Mater. Interfaces 2020, acsami.0c10021, doi:10.1021/acsami.0c10021. Copyright (2020) American Chemical Society.), (c) Schematic illustration and Photographs of the LCE walker robot parallel parking [155] (Reproduced with permission from Zuo, B.; Wang, M.; Lin, B.-P.; Yang, H.,Nat Commun; published by 2019 Springer Nature Limited).

Figure 8

(a) LM–LCE composites function as a conductive wire to run current through an LED, as a transducer to sense touch, and as a Joule-heated actuator to lift a weight. An LED turns on when the sensing composite responds to touch, and internal Joule-heated actuation is activated [166] (Reproduced with permission from Ford, M.J.; Ambulo, C.P.; Kent, T.A.; Markvicka, E.J.; Pan, C.; Malen, J.; Ware, T.H.; Majidi, C. A, Proc Natl Acad Sci USA; published by 2019 National Academy of Sciences.) (b) Photographs of LCE composite antenna shape changes at different temperatures. (c) Measured reflection coefficient of the LCE antenna as temperature decreases(left); and measured frequency response of the reflection coefficient from 0 to 100 cycles(right) [39] (Adapted with permission from Kim, H.; Gibson, J.; Maeng, J.; Saed, M.O.; Pimentel, K.; Rihani, R.T.; Pancrazio, J.J.; Georgakopoulos, S.V.; Ware, T.H. Responsive, 3D Electronics Enabled by Liquid Crystal Elastomer Substrates. ACS Appl. Mater. Interfaces 2019, 11, 19506–19513, doi:10.1021/acsami.9b04189. Copyright (2019) American Chemical Society.) (d) Photographs of the Ag NPs/LCE nanocomposite actuator driving the electric circuit at different view angles and temperatures [38] (Adapted from Ref. [38] with permission from the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry).