Thermally Actuated Soft Robotics

Abstract

Soft robots with exceptional adaptability and versatility have opened new possibilities for applications in complex and dynamic environments. Thermal actuation has emerged as a promising method among various actuation strategieis, offering distinct advantages such as programmability, light weight, low actuation voltage, and untethered operation. This review provides a comprehensive overview of soft thermal actuators, focusing on their heating mechanisms, material innovations, structural designs, and emerging applications. Heat generation mechanisms including Joule heating, electromagnetic induction, and electromagnetic radiation and heat transfer mechanisms such as fluid convection are discussed. Advances in materials are grouped into two areas: heating materials, primarily based on nanomaterials, and thermally responsive materials including hydrogels, liquid crystal elastomers, and shape-memory polymers. Structural designs, such as extension, bending, twisting, and 3D deformable configurations, are explored for enabling complex and precise movements. Applications of soft thermal actuators span environmental exploration, gripping and manipulation, biomedical devices for rehabilitation and surgery, and interactive systems for virtual/augmented reality and therapy. The review concludes with an outlook on challenges and future directions, emphasizing the need for further improvement in speed, energy efficiency, and intelligent soft robotic systems. By bridging fundamental principles with cutting-edge applications, this review aims to inspire further advancements in the field of thermally actuated soft robotics.

Keywords: bioinspiration; soft actuators; soft robots; thermal actuation.

Figure 1

Overview of the thermal heating mechanisms and materials selection.

Figure 2

Four different heating mechanisms for thermal actuation. a) Schematic of conductive patterns, the approximate areas that will be heated upon actuation, and the resulting shape change of a bimorph bending actuator (left to right) (Reproduced with permission.^{[ 69 ]} Copyright 2021, Royal Society of Chemistry). b) A thermal bimorph actuator for a Kresling Origami unit: temperature as a function of time for different currents applied from 0.05 to 0.125 A, a photograph and an IR image of the patterned heater (Reproduced with permission.^{[ 20 ]} Copyright 2023, American Association for the Advancement of Science). c) Simulation and experimental images of LCE‐LM actuator with different EMI frequencies and magnitudes (Reproduced with permission.^{[ 52 ]} Copyright 2023, Wiley‐VCH). d) Schematic of a SMA‐based quadruped crawler actuated by microwave radiation (Reproduced with permission.^{[ 70 ]} Copyright 2022, Wiley‐VCH). e) Schematic of a photothermal LCE actuator with the actuation behavior dependent on the applied wavelengths. NIR causes reversible local deformation, UV creates permanent bending, and visible light reverses the bending induced by the UV light (Reproduced with permission.^{[ 71 ]} Copyright 2018, Wiley‐VCH). f) LCE‐based actuator with an applied heating and cooling cycle lifting and releasing a 40 g weight, respectively. g) Schematic of an LCE‐based actuator with fluid convection‐based heating and cooling cycles (f and g: Reproduced with permission.^{[ 72 ]} Copyright 2020, American Chemical Society).

Figure 3

Thermal management for soft thermal actuators. a) A soft bimorph actuator with self‐sensing function. The real time temperature feedback facilitates the high frequency snap‐through and snap‐back (Reproduced with permission.^{[ 19 ]} Copyright 2022, Mary Ann Liebert Inc). b) A bioinspired untethered soft robot employing pumpless phase change soft actuators powered by two‐way thermoelectrics (Reproduced with permission.^{[ 84 ]} Copyright 2023, Elsevier). c) A photothermal bimorph actuator equipped with an in‐built cooling layer (Reproduced with permission.^{[ 48 ]} Copyright 2019, Wiley‐VCH). d) Active‐cooling‐in‐the‐loop controller for an SMA‐driven soft robotic tentacle (Reproduced with permission.^{[ 85 ]} Copyright 2023, IEEE).

Figure 4

Heating materials for thermally actuated soft robots. a) Scanning electron microscope (SEM) image of AuNPs coated with shells (Reproduced with permission.^{[ 87 ]} Copyright 2021, Wiley‐VCH). The scale bar is 20 microns. b) Schematic representation of CB‐based actuators with a sandwich structure under UV light (Reproduced with permission.^{[ 88 ]} Copyright 2022, Elsevier). c) Actuation mechanism of a GO‐polydopamine/AuNPs bilayer structure, which integrates thermal expansion and desorption during light exposure (Reproduced with permission.^{[ 79 ]} Copyright 2020, Wiley‐VCH). d) SEM image of AgNW network (Reproduced with permission.^{[ 89 ]} Copyright 2024, Royal Society of Chemistry). e) CuNWs patterned by UV laser ablation method on top of polyethylene terephthalate (Reproduced with permission.^{[ 90 ]} Copyright 2022, Wiley‐VCH). f) SEM image of the CNT film (Reproduced with permission.^{[ 91 ]} Copyright 2023, American Chemical Society). g) SEM images of the AgNW/CNT‐coated conductive fabric (Reproduced with permission.^{[ 92 ]} Copyright 2020, Wiley‐VCH). h) The cross‐section SEM image of LIG based actuator (Reproduced with permission.^{[ 93 ]} Copyright 2020, Wiley‐VCH). i) Composite film was fabricated with GO‐polydopamine /reduced graphene oxide layer (Reproduced with permission.^{[ 94 ]} Copyright 2019, Wiley‐VCH). j) SEM image showing the cross‐sectional view of the graphite enabled actuator (Reproduced with permission.^{[ 95 ]} Copyright 2024, Wiley‐VCH). k) Cross‐sectional SEM images of MXene/LDPE bilayer film (Reproduced with permission.^{[ 96 ]} Copyright 2021, American Chemical Society). l) 4D printed LM/LCE in a spiral pattern (Reproduced with permission.^{[ 97 ]} Copyright 2020, American Chemical Society). Scale bar is 5 mm. (m) Schematic of printed LM film under EMI heating (Reproduced with permission.^{[ 52 ]} Copyright 2023, Wiley‐VCH). n) A robotic hand with five fingers made of PEDOT: PSS/PDMS actuators (Reproduced with permission.^{[ 98 ]} Copyright 2018, Mary Ann Liebert Inc). o) A group of LCE actuators with running hot/cold water in the built‐in micro channels (Reproduced with permission.^{[ 72 ]} Copyright 2020, American Chemical Society).

Figure 5

Thermally responsive materials for thermally actuated soft robots. a) Photographs of PNIPAM‐based hydrogels triggered by 808 nm laser or 60 °C water (Reproduced with permission.^{[ 123 ]} Copyright 2024, Elsevier). b) SEM image of SEM images of double network PNIPam hydrogel at a swollen state and shrunken state. Scale bar is 10 µm (Reproduced with permission.^{[ 124 ]} Copyright 2025, Wiley‐VCH). c, d, e) RM 257‐based LCE schematic illustrations of mesogens, spacers, crosslinkers, and experimental images. (c) Initial polydomain structure stage, post‐fabrication, pre‐stretch. (d) Temporary monodomain structure, post‐stretch along vertical axis, pre‐photo crosslink. (e) Permanent monodomain structure, post‐stretch, post‐photopolymerization under UV (c, d, and e: Reproduced with permission.^{[ 125 ]} Copyright 2015, Royal Society of Chemistry). f) Schematic illustration of the preparation of single‐phase two‐way shape actuator (Reproduced with permission.^{[ 126 ]} Copyright 2019, American Chemical Society). g) Multi‐shape memory stages of commercially available POEs (Reproduced with permission.^{[ 127 ]} Copyright 2014, American Chemical Society). h) Two‐way reversible shape memory demonstrated by end‐capped poly(octylene adipate) (Reproduced with permission.^{[ 128 ]} Copyright 2015, American Chemical Society). i) Schematic of focused ultrasound phase transition‐driven elongation actuator which would deform directionally via vaporization of NOVEC 7000. (j) Sequential thermal actuation and rapid liquid cargo release are induced by localized ultrasound (i and j: Reproduced with permission.^{[ 129 ]} Copyright 2024, Nature Publishing Group).

Figure 6

Structural designs of soft thermal actuators. a) An electrothermal actuator enabled by a hollow LCE fiber filled with liquid metal (Reproduced with permission.^{[ 163 ]} Copyright 2024, Wiley‐VCH). b) A bimorph actuator with AgNW/ PDMS heater sandwiched between PDMS and PI (Reproduced with permission.^{[ 17 ]} Copyright 2017, Royal Society of Chemistry). c) A Soft crawling robot with distributed patterned heater to control its locomotion direction (Reproduced with permission.^{[ 20 ]} Copyright 2023, American Association for the Advancement of Science). d) Tubular LCE actuator with three serpentine shaped metallic wire heaters (Reproduced with permission.^{[ 18 ]} Copyright 2019, American Association for the Advancement of Science). e) LCE artificial tendril with asymmetric core‐sheath structure (Reproduced with permission.^{[ 164 ]} Copyright 2023, Wiley‐VCH). Scale bars: 200 µm. f) A phototunable self‐oscillating system powered by a self‐winding fiber actuator (Reproduced with permission.^{[ 165 ]} Copyright 2021, Nature Publishing Group). g) A fiber‐shaped artificial nylon muscles with spiral design (Reproduced with permission.^{[ 166 ]} Copyright 2016, PNAS). h) A modular Origami robot with integrated thermal actuators showing the contraction and bending motions (Reproduced with permission.^{[ 34 ]} Copyright 2024, PNAS). i) A self‐folding 32‐tile self‐folding Origami sheet transforming into a boat (Reproduced with permission.^{[ 167 ]} Copyright 2010, PNAS). j) Long‐strip kirigami LCE structure with 2 × 4 units (32 hinged squares) and its configuration snapshots at different temperatures (Reproduced with permission.^{[ 168 ]} Copyright 2021, Wiley‐VCH). k) A periodically programmed LCE film with nine cones arisen (Reproduced with permission.^{[ 169 ]} Copyright 2015, American Association for the Advancement of Science). l) Schematic and photograph of the direct ink writing enabled bi‐layer hydrogel architectures (Reproduced with permission.^{[ 170 ]} Copyright 2016, Nature Publishing Group). m) Illustration and microscope images of a braided thermal actuator (Reproduced with permission.^{[ 171 ]} Copyright 2023, Wiley‐VCH). n) A knitted LCE actuator with distributed knitting patterns (Reproduced with permission.^{[ 172 ]} Copyright 2023, Wiley‐VCH).

Figure 7

Instability design in soft thermal actuators. a) Screenshots of a three‐ribbon‐structured soft bimorph actuator with snap‐through instability (Reproduced with permission.^{[ 19 ]} Copyright 2022, Mary Ann Liebert). b) Schematics and photographs of an untethered soft swimming robot with SMP muscles and a group of bistable beam elements (Reproduced with permission.^{[ 178 ]} Copyright 2018, National Academy of Sciences). c) a high‐speed body temperature‐triggered soft ballon robot with mechanical instability (Reproduced with permission. (Reproduced with permission.^{[ 179 ]} Copyright 2022, Mary Ann Liebert). d) A 4D printed metal jumper inspired by biological insect jumping mechanisms (Reproduced with permission.^{[ 180 ]} Copyright 2024, Wiley‐VCH). e) A self‐sustained snapping ring structure with locomotion and load carrying capabilities (Reproduced with permission.^{[ 181 ]} Copyright 2023, Wiley‐VCH). f) A highly dynamic bistable soft actuator composed of a prestretched membrane sandwiched between two 3D printed frames with embedded SMA coils (Reproduced with permission.^{[ 182 ]} Copyright 2023, Wiley‐VCH).

Figure 8

Thermally actuated soft locomotion robots. a) An untethered multi‐gait walking robot enabled by tubular thermal actuators (Reproduced with permission.^{[ 18 ]} Copyright 2019, American Association for the Advancement of Science). b) A caterpillar inspired soft crawling robot with distributed and programmable heaters (Reproduced with permission.^{[ 20 ]} Copyright 2023, American Association for the Advancement of Science). c) Snapshots of a Kresling Origami structured modular crawling robot in the steering motion (Reproduced with permission.^{[ 34 ]} Copyright 2024, National Academy of Sciences). d) A light‐driven soft jumping robot based on a double‐folded LCE ribbon actuator with a monolithic three‐leaf panel fold structure (Reproduced with permission.^{[ 194 ]} Copyright 2023, Wiley‐VCH). e) A jellyfish‐like swimming robot actuated by SMA composite actuators (Reproduced with permission.^{[ 186 ]} Copyright 2011, IOP Publishing). f) A self‐rolling robot showing the snapping and reversed locomotion after encountering obstacles (Reproduced with permission.^{[ 196 ]} Copyright 2022, National Academy of Sciences). g) A climbing robot demonstrating locomotion on a glass ceiling, a PI cylinder (diameter: ≈10 mm), a glass sphere (diameter: ≈25 cm), and leaf of Epipremnum aureum (Reproduced with permission.^{[ 190 ]} Copyright 2022, National Academy of Sciences). The scale bar is 10 mm.

Figure 9

Thermally actuated soft grippers. a) An SMA‐based finger‐like soft robotic gripper with variable stiffness design (Reproduced with permission.^{[ 202 ]} Copyright 2017, Mary Ann Liebert). Scale bar is 40 mm. b) Printed LM‐based perceptive soft gripper with embodied intelligence (Reproduced with permission.^{[ 205 ]} Copyright 2022, Mary Ann Liebert). c) A soft robotic gripper with embedded sensors for vital signal sensing (Reproduced with permission.^{[ 203 ]} Copyright 2021, American Association for the Advancement of Science). d) A LCE gripper with embedded LM as both heaters and strain sensors (Reproduced with permission.^{[ 206 ]} Copyright 2021, American Chemical Society). e) A reconfigurable micro‐gripper that can be reprogrammed by UV exposure (Reproduced with permission.^{[ 216 ]} Copyright 2018, Springer Nature). Scale bar is 5 mm. f) An IR light‐triggered soft gripper with capability of lifting 146 times self‐weight (Reproduced with permission.^{[ 209 ]} Copyright 2022, American Chemical Society). g) A thermal pneumatic soft gripper enabled by liquid‐vapor phase transition (Reproduced with permission.^{[ 214 ]} Copyright 2021, Mary Ann Liebert).

Figure 10

Biomedical Applications of Soft Thermal Actuators. a) Schematic of an unactuated and actuated soft robot with pacing electrode and epicardial sensors inserted into a live mouse. b) Image of an actuated soft robot on a living beating mouse heart.^{[ 75 ]} Scale bar is 5 mm. (a and b: reproduced with permission.^{[ 75 ]} Copyright 2024, Springer Nature) c) Image of facial rehabilitation device in action (Reproduced with permission.^{[ 220 ]} Copyright 2017, IEEE) d) Schematic of flexible braille interactive device (Reproduced with permission.^{[ 221 ]} Copyright 2022, American Chemical Society. e) Schematic of a thermal gripper in varying temperatures (Reproduced with permission.^{[ 222 ]} Copyright 2015, American Chemical Society) f) Schematic of layer‐by‐layer Plasmonic AdPatch (Reproduced with permission.^{[ 223 ]} Copyright 2024, American Chemical Society).

Figure 11

Interaction and Immersion Applications of Soft Thermal Actuators. a) Photograph of Reliebo with an outline where the SMA–soft pneumatic actuator is located.^{[ 238 ]} b) Schematic of how Reliebo interacts with people. (a and b: Reproduced with permission.^{[ 238 ]} Copyright 2024, Association for Computing Machinery) c) Photograph of a thermally responsive variable stiffness social robot in use. (Reproduced with permission.^{[ 239 ]} Copyright 2021, IEEE) d) Schematics of an on‐skin thermo‐haptic device in cooling and heating mode with locations of P‐type and N‐type thermoelectric pellets.^{[ 240 ]} e) Photograph of the on‐skin thermo‐haptic device highlighting the patterned Cu electrode embedded into the Ecoflex elastomer (d and e: Reproduced with permission.^{[ 240 ]} Copyright 2020, WILEY‐VCH). f) Images of ThermAirGlove with thermo‐haptic actuators in use, resting, and the corresponding IR image when glove is filled with cold air.^{[ 241 ]} g) TAGlove application in virtual scene where user is grasping a foam ball (f and g: Reproduced with permission.^{[ 241 ]} Copyright 2020, WILEY‐VCH) h) An artificial flower enabled by knitted SMA patterns showing movements blooming (Reproduced with permission.^{[ 242 ]} Copyright 2017, WILEY‐VCH) i) Blooming motion of a color shifting flower composed of anisotropic soft actuators (Reproduced with permission.^{[ 243 ]} Copyright 2018, WILEY‐VCH).