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Review
. 2019 Sep 20;12(19):3065.
doi: 10.3390/ma12193065.

Contactless Manipulation of Soft Robots

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Free PMC article
Review

Contactless Manipulation of Soft Robots

Jae Gwang Kim et al. Materials (Basel). .
Free PMC article

Abstract

In recent years, jointless soft robots have demonstrated various curvilinear motions unlike conventional robotic systems requiring complex mechanical joints and electrical design principles. The materials employed to construct soft robots are mainly programmable anisotropic polymeric materials to achieve contactless manipulation of miniaturized and lightweight soft robots through their anisotropic strain responsivity to external stimuli. Although reviews on soft actuators are extensive, those on untethered soft robots are scant. In this study, we focus on the recent progress in the manipulation of untethered soft robots upon receiving external stimuli such as magnetic fields, light, humidity, and organic solvents. For each external stimulus, we provide an overview of the working principles along with the characteristics of programmable anisotropic materials and polymeric composites used in soft robotic systems. In addition, potential applications for untethered soft robots are discussed based on the physicochemical properties of programmable anisotropic materials for the given external stimuli.

Keywords: anisotropic materials; hydrogels; liquid crystalline polymers; magnetic composites; soft robots.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Soft robots (from meter to sub-mm). (a) Vascular-system-adapted swimming soft robot. (b) Chemical-reaction-fueled pneumatic octobot. (c) Multi-modal magnetic soft robot. (d) Underwater swimmer with a photoactive liquid crystal polymer. (e) Walking liquid crystal polymeric soft robot. (f) Pneumatic gripper. (g) Biomimetic three-dimensional (3D)-printed hydrogel flower. (h) Helical photomobile liquid crystal polymeric soft robot. (i) Electro-active walking soft robot. (j) Magnetic swimmer. (a) Reproduced with permission [55]. Copyright 2019, Nature Publishing Group. (b) Reproduced with permission [43]. Copyright 2016, Nature Publishing Group. (c) Reproduced with permission [9]. Copyright 2018, Nature Publishing Group. (d) Reproduced with permission [26]. Copyright 2016, Nature Publishing Group. (e) Reproduced with permission [19]. Copyright 2015, Wiley-VCH. (f) Reproduced with permission [56]. Copyright 2011, Wiley-VCH. (g) Reproduced with permission [30]. Copyright 2016, Nature Publishing Group. (i) Reproduced with permission [40]. Copyright 2014, Royal Society of Chemistry. (j) Reproduced with permission [57]. Copyright 2018, Wiley-VCH.
Figure 2
Figure 2
Classification of magnetic materials.
Figure 3
Figure 3
Geometry-induced locomotion of magnetic soft robots. (a) Helical soft robots produced using the dynamic light writing process when a photoresist polymer is used, followed by Ni/Ti bilayer coating: (i) scanning electron microscopy (SEM) image of helical micromachines with a microholder (scale bar: 10 μm), and (ii) principle of corkscrew motion. (b) Fabrication scheme for molding a helical geometry via (i) two-step polymerization, and (ii) ascending performance on uphill obstacles, and (iii) discrete walls. (c) (i) Preparation of multi-legged soft robots, (ii) loading performance at 100 times its own weight and when overcoming an obstacle. (a) was reproduced with permission [1]. Copyright 2012, Wiley-VCH.
Figure 4
Figure 4
Directional motion induced by aligning magnetic components within a UV-curable polymer of soft robots. (a) (i) Schematic depiction of crawling motion, and (ii) optical snapshot images, including differently programmed head, tail (vertical alignments), and bodies (horizontal alignments) (scale bar: 100 μm). (b) (i) Preparation strategies for folding of a bilayer hydrogel film including two orthogonal magneto-alignments, and (ii) a time-lapse image of its swimming motility (scale bar: 2 mm). (c) (i) Tumbling locomotion of axial alignments of the x-axis, and (ii) z-axis for lengthwise tumbling (LT) and sideways tumbling (ST), respectively (scale bar: 300 μm). (a) Reproduced with permission [5]. Copyright 2011, Nature Publishing Group.
Figure 5
Figure 5
Multimodal magneto-actuation through multiple magneto-alignments in single soft robots. (a) (i) Reconfigurable soft body embedded with a counterclockwise alignment of polarity. (ii) Navigation across a synthetic stomach phantom and cargo transportation through multimodal locomotion (scale bars: 1 mm). (b) (i) 3D printing process using composite inks composed of uniaxial reoriented magnetic particles under a magnetic field. (ii) Printing design of magnetic polarity with a top-down view and actuation of the printed hexapedal and auxetic structure. The height of the auxetic 3D structure is 5 mm at the bottom of the panel. (iii) Carrying a pharmaceutical pill with a six-legged structure. (a) was used with permission [9]. Copyright 2018, Nature Publishing Group. (b) Reproduced with permission [10]. Copyright 2018, Nature Publishing Group.
Figure 6
Figure 6
Principles of shape morphing for liquid crystalline polymers. (a) Schematic of the light-induced shape change of a liquid crystalline polymer resulting from the order–disorder transition. (b) Representation of the dimension changes in liquid crystalline polymers through order–disorder transition upon exposure to external stimuli: (i) uniaxial, (ii) cholesteric, (iii) twisted nematic, and (iv) hybrid geometry for liquid crystal polymers. (c) Molecular alignment-dependent actuations: (i) Bending and twisting depends on offsetting (Φc) the nematic director of the molecule at midplane (red arrow) and the cutting direction (dotted line), (ii) 3D image of a dynamic fingerprint under UV exposure with a homeotropic to planar alignment. (b) Reproduced with permission [54]. Copyright 2015, Nature Publishing Group. (c) (i) Reproduced with permission [70]. Copyright 2014, Nature Publishing Group. (ii) Reproduced with permission [71]. Copyright 2014, Nature Publishing Group.
Figure 7
Figure 7
Crawling of liquid crystalline polymeric soft robots. (a) Photo-induced inchworm walking of the liquid crystalline polymer film by alternate irradiation with UV (366 nm, 240 mW∙cm−2) and visible light (>540 nm, 129 mW∙cm−2) at room temperature. (b) A light-driven caterpillar-inspired soft robot with alternated hybrid alignment (inset image). The initial bent shape deforms to flat geometry under visible light (448 nm, 150 mW∙cm−2). (c) Near-infrared (NIR)-light-driven walking carbon nanotube-liquid crystalline elastomer (CNT-LCE)/silicon bilayer soft robot. (d) Climbing a hill using the crawling motion of the soft robot. (e) Crawling of the wrinkle-shaped soft robot against the laser scanning direction (NIR, 980 nm, 4.5 W∙cm−2). The top of film was alternately crosslinked/decrosslinked, whereas the bottom of the film was fully decrosslinked. (a) Reproduced with permission [12]. Copyright 2009, Royal Society of Chemistry. (b) Reproduced with permission [13]. Copyright 2017, Wiley-VCH. (c) Reproduced with permission [20]. Copyright 2013, Wiley-VCH. (d) Reproduced with permission [21]. Copyright 2016, Wiley-VCH. (e) Reproduced with permission [22]. Copyright 2019, Wiley-VCH.
Figure 8
Figure 8
Swimming motion of liquid crystalline polymeric soft robots. (a) Photo-induced actuation of liquid crystal elastomer (LCE) on the liquid surface: (i) disk-shaped (5 mm in diameter, 0.32 mm in thickness) LCE on water swimming away from light (Ar+ ion laser, 495 nm, 1.1 W cm−2), (ii) rectangular-shaped LCE in ethylene glycol that folds and then swims away from the light. (b) Soft robotic swimmer based on polydopamine-coated LCE. Schematics of the bending/unbending of the soft robotic swimmer (up) and swimming motion of the soft robot near the air–water interface (down). (c) Swimming motion of an azobenzene-containing liquid crystal network (LCN)-Kapton bilayer swimmer. (d) A dynamic light field from a digital micromirror device is projected onto the liquid crystal polymeric soft robot (left), which propels itself in liquid through traveling-wave deformation (right). (a) Reproduced with permission [23]. Copyright 2004, Nature Publishing Group. (b) Reproduced with permission [24]. Copyright 2018, American Chemical Society. (c) Reproduced with permission [25]. Copyright 2019, Wiley-VCH. (d) Reproduced with permission [26]. Copyright 2016, Nature Publishing Group.
Figure 9
Figure 9
Rolling motions of liquid crystalline polymeric soft robots. (a) Photo-induced rolling motion of a ring-shaped LCE film. The LCE ring was irradiated through UV (366 nm, 200 mW∙cm−2) and visible light (>500 nm, 120 mW∙cm−2). (b) Rapid translation of a helical liquid crystalline polymer under continuous light illumination. (c) Light-driven forward movement of a wheel supported LCE-Polypropylene (PP) vehicle. The LCE-PP helical ribbon was configured with a spring-like motor. (d) Rolling motion of an LCE-carbon nanotube (CNT) rod: (i) rolling of the LCE-CNT rod on a hot surface, (ii) light-induced reverse rolling of an LCE-CNT composite rod on a flat hot surface. (a) Reproduced with permission [27]. Copyright 2008, Wiley-VCH. (c) Reproduced with permission [15]. Copyright 2017, Wiley-VCH. (d) Reproduced with permission [16]. Copyright 2018, American Chemical Society.
Figure 10
Figure 10
(a) Making waves in LCN films. (b) Jumping of CNT-LCE composites with magnets on both ends. (a) Reproduced with permission [17]. Copyright 2017, Nature Publishing Group. (b) Reproduced with permission [18]. Copyright 2019, Wiley-VCH.
Figure 11
Figure 11
Actuation of hydrogel-based soft robots. (a) (i) Schematic of the ideal processing mechanism of anisotropic thermal expansion of a hydrogel-based soft robot (5 mm thick) from electrostatic repulsion of titanium nanosheet (TiNS) in dehydrated hydrogel, (ii) thermo-responsive actuation of an electrostatically anisotropic hydrogel soft robot. Alternate heating and cooling was conducted between 25 °C and 45 °C at a rate of 0.1 °C/s. (b) (i) Scheme of the bending direction of electro-active materials; acrylamide (AAm)/sodium acrylated (NaAc) copolymer (light gray) and AAm/quaternized dimethylaminoethyl methacrylate (DMAEMA-Q) copolymer (blue), (ii) electro-actuated hydrogel walker in 0.01 M NaCl composed of 50% NaAc and 30% DMAEMA-Q legs and an applied field of 5 V/cm (scale bar: 5 mm). (a) Reproduced with permission [28]. Copyright 2015, Nature Publishing Group. (b) Reproduced with permission [40]. Copyright 2014, Royal Society of Chemical.
Figure 12
Figure 12
(a) (i) Schematic of moisture adsorption- and desorption-driven actuation of π-stacked carbon nitride polymer (CNP) film, (ii) scheme and high-speed snapshot of the jumping motion of a CNP film under UV (365 nm) irradiation. (b) (i) Scheme of the octobot system. Oxygen propellant from the decomposition of the H2O2 fuel induced pneumatic actuation of the octobot, (ii) monopropellant decomposition-powered actuation of the octobot (scale bar: 10 mm). (a) Reproduced with permission [37]. Copyright 2016, Nature Publishing Group. (b) Reproduced with permission [43]. Copyright 2016, Nature Publishing Group.

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