Photoresponsive hydrogel-based soft robot: A review

Abstract

Soft robots have received a lot of attention because of their great human-robot interaction and environmental adaptability. Most soft robots are currently limited in their applications due to wired drives. Photoresponsive soft robotics is one of the most effective ways to promote wireless soft drives. Among the many soft robotics materials, photoresponsive hydrogels have received a lot of attention due to their good biocompatibility, ductility, and excellent photoresponse properties. This paper visualizes and analyzes the research hotspots in the field of hydrogels using the literature analysis tool Citespace, demonstrating that photoresponsive hydrogel technology is currently a key research direction. Therefore, this paper summarizes the current state of research on photoresponsive hydrogels in terms of photochemical and photothermal response mechanisms. The progress of the application of photoresponsive hydrogels in soft robots is highlighted based on bilayer, gradient, orientation, and patterned structures. Finally, the main factors influencing its application at this stage are discussed, including the development directions and insights. Advancement in photoresponsive hydrogel technology is crucial for its application in the field of soft robotics. The advantages and disadvantages of different preparation methods and structures should be considered in different application scenarios to select the best design scheme.

Keywords: Actuators; Citespace; Photoresponsive hydrogels; Response mechanisms; Soft robots; Structural forms.

Graphical abstract

Fig. 1

Keywords co-occurrence network map of photoresponsive hydrogel.

Fig. 2

Keywords with the strongest citation bursts.

Fig. 3

a) Photoisomerization of Azobenzene [20] and Spiropyran. Reproduced with permission. Copyright 2011, Elsevier. b) Microgel particles converted to cis-structure under UV light and trans-structure under blue light [29]. Reproduced with permission. Copyright 2012, Wiley-VCH. c) Chemical structures and isomerizations of spirobenzopyran isomers [31]. Reproduced with permission. Copyright 2011, Royal Society of Chemistry. d)Schematic illustration of photo-crosslinking reaction among PVMAA and the structures of the PVMAA micelles and those photo-crosslinked [37]. Reproduced with permission. Copyright 2014, American Chemical Society. e) Volume deformation of MNPs-containing hydrogels under blue light irradiation [42]. Reproduced with permission. Copyright 2015, Springer Nature. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

a) Schematic diagram of bending of PNIPAM layer/P(AAc-co-AAm) layer [49]. Reproduced with permission. Copyright 2018, Royal Society of Chemistry. b) Schematic of the static bending motion of the bilayer actuator [54]. Reproduced with permission. Copyright 2016, American Chemical Society. c) The design and deformation of the 2D actuator [57]. d) The design and deformation of the 3D actuator [57]. Reproduced with permission. Copyright 2021, Wiley-VCH. e) Significant diagram of White Whale Software Robot Movement [64]. Reproduced with permission. Copyright 2019, American Chemical Society. f) Magnetic parcel micro transport robot [61]. Reproduced with permission. Copyright 2013, Wiley-VCH. g) One cycle locomotion of inchworm [63]. Reproduced with permission. Copyright 2019, IEEE.

Fig. 5

a) The actuator completes various deformations of “U" “J" “S" and "Ω" [71]. Reproduced with permission. Copyright 2021, Wiley-VCH. b1-b9) CGM actuator walks in near-infrared light [75]. Reproduced with permission. Copyright 2015, Wiley-VCH. c) Jellyfish Software Robot (JSMR) Swatching diagram [76]. Reproduced with permission. Copyright 2021, American Chemical Society. d1-d3) Software Swimming Robot (Oscibot) travel diagram [77]. Reproduced with permission. Copyright 2019, The American Association for the Advancement of Science.

Fig. 6

a) Preparation of hydrogel and seagull-like gliding motion [82]. Reproduced with permission. Copyright 2020, Wiley-VCH. b) Tins/AUNP hydrogel preparation and light-induced stretch [81]. c) Light response driver for the bionic earthworm [81]. Reproduced with permission. Copyright 2018, Wiley-VCH.

Fig. 7

a-c) Patterned hydrogel deformation under different shapes [86]; a：Tubular，b：Spiral，c：Curl. d) Patterned hydrogel sheet moves in the opposite direction of the scan under repeated light irradiation [86]. Reproduced with permission. Copyright 2020, Springer Nature.

Fig. 8

Discussion and future outlook.