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Untethered Miniature Soft Robots
Reference on achieving contactless manipulation of soft robots, detailing high level concepts and perspectives and technical skills of soft robots
Untethered Miniature Soft Robots: Materials, Fabrications, and Applications introduces the emerging field of miniature soft robots and summarizes the recent rapid development in the field to date, describing different types of functional materials to build miniature soft robots, such as silicone elastomer, carbon-based materials, hydrogels, liquid crystal polymer, flexible ferrofluid, and liquid metal, and covering the material properties, fabrication strategies, and functionalities in soft robots together with their underlying mechanisms.
The book discusses magnetically, thermally, optically, and chemically actuated soft robots in depth, explores the many specific applications of miniature soft robots in biomedical, environmental, and electrical fields and summarizes the development of miniature soft robots based on soft matter, fabrication strategies, locomotion principles, sensing and actuation mechanisms.
In closing, the text summarizes the opportunities and challenges faced by miniature soft robots, providing expert insight into the possible futures of this field.
Written by four highly qualified academics, Untethered Miniature Soft Robots covers sample topics such as:
- Soft elastomer-based robots with programmable magnetization profiles and untethered soft robots based on template-aiding
- Working mechanisms of carbon-based materials, covering light-induced expansion and shrinkage, and humidity-induced deformation
- Designing microscale building blocks, modular assembly of building blocks based on Denavit-Hartenberg (DH) matrix, and inverse and forward design of modular morphing systems
- Material designs of magnetic liquid crystal elastomers (LCE) systems, multiple-stimuli responsiveness of magnetic LCE systems, and adaptive locomotion of magnetic LCE-based robots
- Controllable deformation and motion behaviors, as well as applications of ferrofluids droplet robots (FDRs), including cargo capturing, object sorting, liquid pumping/mixing, and liquid skin.
Providing highly detailed and up-to-date coverage of the topic, Untethered Miniature Soft Robots serves as an invaluable and highly comprehensive reference for researchers working in this promising field across a variety of disciplines, including materials scientists, mechanical and electronics engineers, polymer chemists, and biochemists.
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Seitenzahl: 395
Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
1 Introduction to Untethered Miniature Soft Robots
1.1 Introduction
1.2 Working Mechanisms of Untethered Soft Robots
1.3 Fabrication Methods of Untethered Soft Robots
1.4 Applications of Miniature Soft Robots
1.5 Scope and Layout of the Book
Abbreviations
References
2 Silicone Elastomers‐Based Miniature Soft Robots
2.1 Introduction
2.2 Soft Elastomer‐Based Robots with Programmable Magnetization Profiles
2.3 Reprogrammable Soft Machines
2.4 Multi‐Stimuli Responsive Transformation of Magneto‐Elastomer Based Soft Structures
2.5 Multimodal and Bioinspired Locomotion Adopted for Elastomer‐Based Robot
2.6 Potential Biomedical Applications of Miniature Magnetic Soft Machines
2.7 Fluidic Pumping by the Magneto‐Elastomers
2.8 Other Potential Applications of the Magneto‐Elastomers
2.9 Summary
Abbreviations
References
3 Carbon‐Based Miniature Soft Robots with Rolled‐up Concept
3.1 Introduction
3.2 The Choices of Carbon‐Based Materials
3.3 The Working Mechanism of Carbon‐Based Materials
3.4 The Programming Shape Changes of Actuators and Their Applications
3.5 Summary
Abbreviations
References
4 Hydrogels‐Based Miniature Soft Robots
4.1 Introduction
4.2 Fabrication of Reconfigurable Hydrogel Micromachines
4.3 Modular Design of Reconfigurable Soft Robots Based on 4D Microscale Building Blocks
4.4 Application of Reconfigurable Soft Robots
4.5 Summary
Abbreviations
References
5 Liquid Crystal Network and Elastomer‐Based Miniature Soft Robots
5.1 Introduction
5.2 Stimuli‐Responsiveness Based on Programmed Director Field
5.3 Magnetic Liquid Crystal Elastomer Composites for Miniature Machines
5.4 Summary
Abbreviations
References
6 Flexible Ferrofluid as Soft Robotic Agents
6.1 Introduction
6.2 Description of Ferrofluids
6.3 Deformation Behaviors of FDRs
6.4 Applications of FDRs
6.5 Summary
Abbreviations
References
7 Conclusions and Future Prospects
7.1 Introduction
7.2 Other Functional Materials Used for Miniature Soft Robots
7.3 Multi‐Material Integration Strategies
7.4 Multifunctional Integration for Miniature Soft Robots with Perception Capabilities
7.5 Perspectives Toward Intelligent and Autonomous Soft Robots
Abbreviations
References
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 Working mechanisms of untethered soft robots. (a,b) Magnetic actu...
Figure. 1.2 Fabrication methods of untethered soft robots. (a,b) Molding met...
Figure. 1.3 Applications of miniature soft robots. (a) Magnetic soft robot a...
Chapter 2
Figure 2.1 Crosslinking and actuation of silicone elastomer‐based soft struc...
Figure 2.2 Template‐assisted magnetization process and magnetic response beh...
Figure 2.3 Schematic illustration of the morphological transformation of the...
Figure 2.4 Demonstration of the swelled and recovered magneto‐elastomer by a...
Figure 2.5 Measurement of the swelling ratios of the material in four kinds ...
Figure 2.6 The characterization of the magnetic hysteresis of the magneto‐el...
Figure 2.7 Using DIW to print silicon elastomers containing 20 vol % hard ma...
Figure 2.8 Miniature reconfigurable devices fabricated by bottom‐up assembly...
Figure 2.9 Schematic illustration of the process of voxel fabrication and ma...
Figure 2.10 SEM images and design of voxels with various geometries.
Figure 2.11 The magnetization of magnetic voxels. (a) A jig inside the VSM w...
Figure 2.12 Schematic illustration of the face‐bonding approach.
Figure 2.13 Schematic illustration of the edge‐bonding approach.
Figure 2.14 Demonstration of the fabrication of a 3D ring structure. (a) Sch...
Figure 2.15 Schematic diagram of a reconfigurable magnetoelastomer. (a) Reco...
Figure 2.16 Demonstration of the original shape (a), buckling transformation...
Figure 2.17 The changing of deformation amplitude of magneto‐elastomers (fiv...
Figure 2.18 Characterization of the impacts of geometry parameters and magne...
Figure 2.19 The morphological transformation of a magneto‐elastomer upon the...
Figure 2.20 The geometrical transformation of soft structures with different...
Figure 2.21 The geometric transformation of a square lattice. (a) Images of ...
Figure 2.22 The morphological transformation of cellular structures with dif...
Figure 2.23 Multimodal locomotion of magneto‐elastomer based soft robots. (a...
Figure 2.24 Continuous locomotion of soft robots in mixed solid–liquid and u...
Figure 2.25 Demonstration of magneto‐elastomer robots with bioinspired locom...
Figure 2.26 A miniature soft capsule and its potential biomedical applicatio...
Figure 2.27 A miniature soft anchoring machine and its potential biomedical ...
Figure 2.28 A peristaltic soft pump and its potential application. (a) Illus...
Figure 2.29 Cilia‐inspired magneto‐elastomer arrays. (a) Experimental setup ...
Figure 2.30 Dynamic fluidic behaviors induced by the morphological transform...
Figure 2.31 Efficient mixing of viscous fluid using the magneto‐elastomers. ...
Figure 2.32 Other functional demonstrations of magneto‐elastomers. (a) Recon...
Chapter 3
Figure 3.1 (a) The structures of graphene, graphite, CNT, diamond, amorphous...
Figure 3.2 (a) The schematic diagram of fabricating GNPs‐PDMS/PDMS actuator....
Figure 3.3 (a) The fabrication process of RGO‐TEMs‐PDMS/PDMS bilayer actuato...
Figure 3.4 (a) Schematic illustration of the fabrication process of SGA/PE b...
Figure 3.5 (a) Optical image and cross‐sectional SEM image of the rolled CNT...
Figure 3.6 (a) The schematic diagram and (b) optical images for the structur...
Figure 3.7 (a) Fabrication scheme for making SWCNTs‐PNIPAAm/LDPE bilayer act...
Figure 3.8 (a) The schematic diagram for the bending/unbending actuation of ...
Figure 3.9 The binding energy between a water molecule and graphene as well ...
Figure 3.10 (a) The schematic diagram of GO/MWCNT bilayer actuator.(b) T...
Figure 3.11 (a) The schematic diagram for laser reduction on one side of a G...
Figure 3.12 (a) The schematic diagram of the fabrication of structured GO fi...
Figure 3.13 (a) An orientable transporter fabricated by assembling 7 upright...
Figure 3.14 (a) The schematic diagram for laser scribing of programmable pho...
Figure 3.15 (a) GO fibers with region‐confined laser reduction.(b) The s...
Figure 3.16 (a) The schematic diagram of precise PPy patterning on a GO film...
Figure 3.17 (a) The schematic diagram of a smart GO/PPy gripper with a cross...
Figure 3.18 (a, b) The total process and (c) schematic diagram for wax print...
Figure 3.19 (a) The planar structure of the soft robot and the orientation o...
Figure 3.20 (a) The schematic diagram of the fabrication process and actuati...
Figure 3.21 (a) The schematic diagram of aligned LIG using the DLW method, t...
Chapter 4
Figure 4.1 Stimuli‐responsive hydrogels. (a) Thermo‐responsive hydrogels....
Figure 4.2 The two‐photon polymerization reaction adopted during the printin...
Figure 4.3 The SEM image of the printed pH‐responsive microball structure....
Figure 4.4 Schematic illustration of the 4D printing process with a DLW syst...
Figure 4.5 Schematic illustration of the micromechanical measurement.
Figure 4.6 Quantitative characterization of the mechanical property and the ...
Figure 4.7 The responsive behavior of the 4D printed microball structure. (a...
Figure 4.8 Structural design, simulation prediction, and experimental result...
Figure 4.9 Modulation of light field. (a) Schematic illustration of the comp...
Figure 4.10 Fabrication and characterization of pH‐triggered microtubes. (a)...
Figure 4.11 Segmented microtubule structures fabricated by dynamic holograms...
Figure 4.12 Chiral structures with torsional deformation capability fabricat...
Figure 4.13 pH‐responsive leaf structures prepared by reciprocating scanning...
Figure 4.14 Chiral distortion of leaf structures prepared by unidirectional ...
Figure 4.15 Schematic illustration of the transformations of traditional and...
Figure 4.16 Quantitative characterization of the curvatures of the deformed ...
Figure 4.17 Morphological transformations of articulated building blocks wit...
Figure 4.18 Schematic illustration and simulation study of the modular syste...
Figure 4.19 Morphing modular systems developed using the inverse and forward...
Figure 4.20 Demonstration of a microtransformer that morphed from a race car...
Figure 4.21 pH‐responsive microgripper for cell and cargo manipulation. (a) ...
Figure 4.22 Schematic illustration and experimental demonstration of pH‐resp...
Figure 4.23 Magnetic shape‐morphing microrobots used for target drug deliver...
Chapter 5
Figure 5.1 Actuation stimuli types of LCNs and LCEs. (a) Temperature‐respons...
Figure 5.2 Schematic illustration of the fabrication procedure for LCN micro...
Figure 5.3 Assembly of microscale LCE modules with heterogeneous director fi...
Figure 5.4 Alignment design and polarized optical microscope images of liqui...
Figure 5.5 Geometrical design, SEM and POM results of LCN microstructures wi...
Figure 5.6 LCN microstructures with 2D director field. (a–f) Director field ...
Figure 5.7 LCN microstructures with 3D director field. (a–f) Director field ...
Figure 5.8 3D‐to‐3D shape transformation of LCN microstructures with encoded...
Figure 5.9 LCE microstructures based on 1D cubic voxels assembly and their t...
Figure 5.10 LCE microstructures based on the 2D assembly of cubic voxels and...
Figure 5.11 LCE microstructures based on the 3D assembly of cubic voxels and...
Figure 5.12 Illustration and experimental observation of the composite film ...
Figure 5.13 Reprogrammable LCE systems doped with Fe
3
O
4
nanoparticles. (a an...
Figure 5.14 Formation of bimorph material via the integration of LCEs and
ma
...
Figure 5.15 Dual‐responsive behaviors of the bimorph structures with three d...
Figure 5.16 Shape‐morphing behaviors of eight‐petal flower‐shape M‐PULCE act...
Figure 5.17 Local and sequential actuation of hand‐shape M‐PULCE actuator. S...
Figure 5.18 Magnetic field‐assisted multi‐level control and self‐healing of ...
Figure 5.19 Adaptive locomotion of a magnetic LCE‐based soft robots in air a...
Figure 5.20 Adaptability and reconfigurability of two kinds of magnetic LCE‐...
Figure 5.21 Untethered miniature 12‐legged robot made of bimorph material. (...
Chapter 6
Figure 6.1 a Transmission electron microscopy (TEM) image of the nanoparticl...
Figure 6.2 (a)–(c) The schematic diagram, optical images, and magnetic field...
Figure 6.3 (a) The schematic diagram of field‐induced droplet combination. (...
Figure 6.4 Overview of trans‐scale navigation and scale reconfiguration stra...
Figure 6.5 (a) The schematic diagram of the stretch mechanism of the FDR und...
Figure 6.6 (a) The schematic diagram for the splitting of centi‐FR into mill...
Figure 6.7 Conceptualization of the electromagnetic coils for generating con...
Figure 6.8 (a) Experimental setup of a 2D electromagnet array for generating...
Figure 6.9 (a) Snapshots of the dynamic motion of an FDR actuated in the dou...
Figure 6.10 (a) The deformability (a/b) of the FDR with increasing magnetic ...
Figure 6.11 (a) The schematic diagram of forming microstick, micropie, and m...
Figure 6.12 (a) FDRs exhibiting stretching and jumping behaviors under 1D os...
Figure 6.13 (a) FDRs performing rotating and tumbling motion patterns under ...
Figure 6.14 (a) The directional hurdling of FDR over successive obstacles un...
Figure 6.15 (a) The schematic diagram of a dynamic magnetic field. (b) The r...
Figure 6.16 The splitting numbers of FDR as a function of (a) the frequency ...
Figure 6.17 (a) The schematic diagram of the external applied magnetic field...
Figure 6.18 The deformation of an FDR as a function of the mixed viscosity....
Figure 6.19 (a) FDRs array created by field‐induced splitting for different ...
Figure 6.20 (a) The optical images of FDRs on different substrates at 0 and ...
Figure 6.21 (a) The schematic diagram of a ring‐shaped FDR for capturing and...
Figure 6.22 (a) The schematic diagram of transforming a traditional pill int...
Figure 6.23 (a) Vertical collectives combined with microfluidic chips to ach...
Figure 6.24 (a) The schematic diagram for 2D nonreciprocal motion of single ...
Figure 6.25 (a) Concept of the programmable fluidic mixing device. (b) The s...
Figure 6.26 The liquid skin transforms silicone sheet into (a) walking spide...
Figure 6.27 Preparation and application of ferrofluid with phase change abil...
Figure 6.28 Demonstration of the manipulation of objects of different shapes...
Chapter 7
Figure 7.1 Shape‐memory materials and biohybrid materials. (a) 4D‐printed SM...
Figure 7.2 Multi‐material integration. (a) Fabrication of a spider‐like musc...
Figure 7.3 Soft robots with perception capabilities. Soft robotic grippers f...
Figure 7.4 Comparison and summary of smart materials for untethered miniatur...
Figure 7.5 Perspective for future soft robotic systems.
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Untethered Miniature Soft Robots
Materials, Fabrications, and Applications
Li Zhang, Jiachen Zhang, Neng Xia, and Yue Dong
Authors
Prof. Li Zhang
The Chinese University of Hong Kong
William M.W. Mong Engineering Building
Shatin, N.T., Hong Kong SAR
China
Dr. Jiachen Zhang
City University of Hong Kong
Yeung Kin Man Academic Building
Kowloon, Hong Kong SAR
China
Dr. Neng Xia
The Chinese University of Hong Kong
William M.W. Mong Engineering Building
Shatin, N.T., Hong Kong SAR
China
Dr. Yue Dong
Guangdong Provincial Key Laboratory of Intelligent Morphing Mechanisms and Adaptive Robotics
School of Mechanical Engineering and Automation
Harbin Institute of Technology Shenzhen (HITSZ)
China
Cover Images: © Li Zhang, The Chinese University of Hong Kong
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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Print ISBN: 978‐3‐527‐35177‐0
ePDF ISBN: 978‐3‐527‐84089‐2
ePub ISBN: 978‐3‐527‐84090‐8
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Preface
Soft robots with large deformation capabilities have received extensive attention in both academia and industry. Owing to the outstanding maneuverability and environmental adaptability, they are expected to perform complex tasks in tortuous or unstructured spaces and present great application prospects in industrial monitoring, space exploration, biomedical fields, and many others. However, with the increasingly complex application scenarios and the demand for in vivo medical treatment, the development of miniaturized soft robots shows great potential. Due to the limited onboard space, conventional components and control methods are difficult to adapt to these miniaturized soft robots. The development of smart materials, especially stimuli‐responsive materials, provides a powerful solution for the construction of miniaturized soft robots, which derives the topic of this book: Untethered Miniature Soft Robots: Materials, Fabrications, and Applications.
In recent years, advanced polymer materials and their fabrication technologies have developed rapidly. Tough materials such as silicon‐based elastomers and double‐network hydrogels facilitate the construction of robust robotic structures. Widely available stimuli‐responsive materials, such as magnetic materials, liquid crystal elastomers, responsive hydrogels, and shape‐memory polymers, enable miniaturized robots to operate under various external stimuli (e.g. magnetic fields, light fields, and chemical stimuli). The application of biocompatible and biodegradable materials makes it possible for robots to evade immune system attacks or carry therapeutic cells to perform precision medical tasks. Self‐healing polymers based on dynamic networks or non‐covalent bonding enable soft robots to repair themselves autonomously after damage and are widely used to develop modular robotic systems. Meanwhile, advances in additive manufacturing techniques (such as two‐photon polymerization and direct ink writing) and assembly approaches could satisfy the fabrication of 3D robotic structures on a range of scales from micron to centimeter scale, and even integrating different materials in 3D space. Therefore, a large number of achievements in materials and manufacturing have greatly enriched the design space of miniaturized soft robots and enhanced their performance.
This encouraging prospect has inspired a great deal of fundamental research and application exploration of miniaturized soft robotic systems. Existing soft robotic systems have achieved remarkable progress in independent control, selective manipulation, multimodal locomotion, and precise prediction due to sophisticated regulation and modeling techniques. It provides powerful tools for many application scenarios in the field of biomedicine, such as micro‐manipulation, targeted drug delivery, and biological scaffolds, and is expected to improve the quality of life of patients. In addition, combined with flexible electronics technology, miniaturized flexible robots play an important role in applications including environmental monitoring and sensing.
The goal of this book is to convey the latest progress of untethered miniaturized soft robotic systems, focusing on the materials, preparation, and applications related to soft robotic systems. It is suitable for students, engineers, and scientists who are engaged in the research of soft robotics and soft materials. The authors will systematically introduce the research progress of miniaturized soft robots as well as the opportunities and challenges in the future, hoping to inspire more excellent ideas from researchers and enhance general readers' interest in miniature soft robots.
This book consists of seven chapters. The first chapter mainly introduces various soft robots developed in recent years. Chapter 2 presents the recent research outcome of silicone elastomer‐based untethered miniature robots. Chapter 3 introduces botanical‐inspired strategies for constructing multi‐stimuli‐responsive robots using carbon‐based materials. Chapter 4 discusses 3D‐printed environmental‐responsive hydrogels for constructing programmable miniature robots. Chapter 5 is devoted to the application of liquid crystal networks and elastomers to fabricate monolithic or bimorph‐material‐based robots with programmable shape‐morphing and self‐adaptation capabilities. Chapter 6 presents ferrofluid droplets used to form miniature soft robots. Finally, Chapter 7 summarizes the recent key progress made by researchers on the investigation of miniature soft robots and discusses the prospects and challenges toward intelligent and autonomous soft robots.
The book is written with the kind help of many organizations and individuals. The authors would like to acknowledge support from the Hong Kong Research Grants Council with project Nos. RFS2122‐4S03 (RGC Research Fellow Scheme), R4015‐21 (RGC Research Impact Fund), E‐CUHK401/20 (EU/RGC Research and Innovation Cooperation Co‐funding Mechanism by RGC), 14300621 (RGC GRF), 14301122 (RGC GRF); the HKSAR Innovation and Technology Commission (ITC) with project No. MRP/036/18X (ITC Midstream Research Programme for Universities); the Croucher Foundation with project No. CAS20403; the Chinese University of Hong Kong (CUHK) internal grants; the SIAT‐CUHK Joint Laboratory of Robotics and Intelligent Systems; the Multiscale Medical Robotics Center (MRC) at the Hong Kong Science Park; the CUHK Chow Yuk Ho Technology Centre for Innovative Medicine; and the CUHK T Stone Robotics Institute.
We are very grateful to Prof. Bradley Nelson from ETH Zurich, Switzerland; Prof. Joseph Sung Jao‐yiu, Dean of Lee Kong Chian School of Medicine and Senior Vice President (Health and Life Sciences), at the Nanyang Technological University; Prof. Yangsheng Xu, President of CUHK (Shenzhen); Prof. Carmel Majidi, at the Carnegie Mellon University; Prof. Liu Wang, at the University of Science and Technology of China; Prof. Philip Chiu Wai Yan and Prof. Samuel Au Kwok Wai, Co‐Directors of MRC, the other collaborators, as well as the colleagues from the Faculty of Engineering and Faculty of Medicine at CUHK, for their long‐term guidance and/or generous support to our group's research. We appreciate all the lab ex‐ and current members, including Dr. Tiantian Xu, Dr. Xiaohui Yan, Dr. Jiangfan Yu, Dr. Yabin Zhang, Dr. Ben Wang, Dr. Tony Chan Kai Fung, Dr. Lidong Yang, Dr. Mengmeng Sun, Dr. Dongdong Jin, Dr. Xingzhou Du, Dr. Fengtong Ji, Jialin Jiang, and many other researchers, for contributing their excellent research work to this book. We would also like to thank Jiaqi Zhu, Yanfei Cao, Xurui Liu, Xinyu Hou, and Bonan Sun in our group for their assistance in drawing figures and proofreading. The editorial and production staff of Wiley are gratefully acknowledged for the kind arrangement in writing this book that enables its publication. Thanks to the great and fascinating efforts and discoveries numerous researchers keep pushing the forefront of untethered miniature soft robots, and we sincerely hope the prospect of intelligent soft robots presented in this book can come into reality in the near future.
Finally, we would like to use this chance to send our best greeting for celebrating the 60th anniversary of the Chinese University of Hong Kong in 2023.
Hong Kong SAR, ChinaMay 2023
Li Zhang
(On behalf of all authors)The Chinese Universityof Hong Kong
1Introduction to Untethered Miniature Soft Robots
1.1 Introduction
Miniature soft robots with inherent compliance could exhibit dynamic interaction with the real world (1, 2). These controllable microdevices attracted growing attention because of their promises in a wide spectrum of applications, e.g. biomimetic study, environmental monitoring, precision medicine, as well as minimally invasive surgery (3, 4). With a proper design of miniaturized structure and selection of material, the actuation and locomotion of miniature soft robots in tortuous and unstructured environments such as artificial vascular networks and animal tissues have been verified (5–8). Advanced control techniques of physical fields allow us to elaborately tune the transformation of the robots, subsequently resulting in the generation of a variety of locomotion modes inspired by their natural counterparts, e.g. earthworm, inchworm, midge larvae, starfish larvae, bacteria, as well as jellyfish (9–14).
Various functional polymers have been introduced to endow robotic structures with intelligent properties, including self‐healing property (15–17), degradability or stimuli‐responsive deformation (18–21). Due to the limited onboard space, conventional control units or driven parts can be hardly integrated into the miniature soft robots. In this respect, diverse stimuli‐responsive materials (e.g. liquid crystal elastomers [LCE], shape memory polymers [SMP], and hydrogels) have been adopted for the construction of micromachines, so that they can present controllable deformation under external stimuli. In addition, to ensure the service life of soft robots, such as avoiding the influence of cracks caused by sharp parts in the physical environment or fatigue damages induced by multiple cycles of large deformations, self‐healing polymers based on noncovalent interaction mechanisms or dynamic covalent networks have been proposed. Biomedical application is one of the most important development directions of miniature soft robots. The in vivo environments usually require the biodegradability or biocompatibility of robotic materials such as gelatin methacryloyl (GelMA) as well as zwitterionic materials (22, 23). For instance, miniature soft machines have been applied for targeted cell delivery. The machines that load cells should be biocompatible to facilitate the adhesion and growth of therapeutic cells. Moreover, for robotic structures that can be hardly retrieved, they should be biodegradable to avoid the adverse effect of long‐term accumulation. It is worth noting that sophisticated application scenarios may bring high requirements for the adaptability and versatility of a robotic system, which are often difficult to meet by robotic structures made of single‐component materials difficult to meet. Therefore, the seamless integration of multiple functional modules or material compositions in the robotic system becomes pivotal, albeit a challenge (24, 25).
1.2 Working Mechanisms of Untethered Soft Robots
1.2.1 Magnetic Actuation
Soft robots actuated by external magnetic field have been intensively investigated due to their operation capability in large and enclosed workspaces, e.g. human body, and their great potential in minimally invasive surgery (26). Magnetic field forces and torques could be used for the actuation of magnetic small‐scale robots based on the interaction between the magnetic properties of the robot and the externally exerted magnetic fields (Figure 1.1a,b) (36, 37). Assuming that there is no current in the workspace, according to Maxwell's equations, it can be derived that the static magnetic field satisfies Eq. (1.1)(38):
where B = [Bx, By, Bz] represents the applied magnetic field, whose gradient matrix is traceless and symmetric.
Figure 1.1 Working mechanisms of untethered soft robots. (a,b) Magnetic actuation (27, 28). (a) A hexapod soft structure with programmable ferromagnetic domains was fabricated by direct ink writing (DIW) printing technique. The 2D–3D morphological transformation is realized under the action of magnetic torque.
Source: Kim et al. (27)/Reproduced from Springer Nature.
(b) A microneedle robot with a magnetic base navigates over obstacles and penetrates the wall of the small intestine under the control of permanent magnets.
Source: Zhang et al. (28)/John Wiley &Sons.
(c,d) Acoustic actuation (11, 29). (c) Ultrasonic‐powered microrobots inspired by starfish larvae. Under ultrasonic stimulation, the cilia array in the robot body produces a swinging motion, which in turn induces complex vortices flows.
Source: Dillinger et al. (11)/Reproduced from Springer Nature/CC BY 4.0.
(d) Ultrasound‐actuated structures with arrays of bubbles. Under ultrasonic stimulation, the bubbles resonate and meanwhile form a thrust force acting on the surface of the structure.
Source: Adapted from Qiu et al. (29).
(e,f) Light actuation (9, 30). (e) The photo‐responsive liquid crystal elastomer () generates oscillatory behavior under the stimulation of two mutually perpendicular laser beams, as well as the photothermal distribution on the surface of the LCE structure.
Source: Deng et al. (30)/Reproduced from John Wiley & Sons, Inc./CC BY 4.0.
(f) LCE‐based soft robot driven by dynamic light field. The beam distribution in the space is adjusted in real time through a digital micromirror device (DMD), and then the deformation control of the soft robot is realized.
Source: Palagi et al. (9)/Reproduced from Springer Nature.
(g) Propulsion driven by Marangoni effect. A rove beetle‐inspired hydrogel rotor achieves efficient rotation and energy transfer by consuming hexafluoroisopropanol (HFIP fuel).
Source: Wu et al. (31)/Springer Nature/CC BY 4.0/Public domain.
(h) Thermal actuation. The self‐propelled rolling of a robot with a bilayer structure (ferroelectric polyvinylidene fluoride (PVDF) and polydopamine‐modified reduced graphene oxide‐carbon nanotube ()) was realized via a mechano‐thermal feedback loop.
Source: Wang et al. (32)/Springer Nature/CC BY 4.0/Public domain.
(i) Deformation driven by humidity stimuli.
Source: Adapted from Dong et al. (33).
(j) Chemical actuation.
Source: Hu et al. (34)/John Wiley & Sons.
(k) Biohybrid actuation.
Source: Wang et al. (35)/American Association for the Advancement of Science.
The magnetic force ( f) and torque (τ) applied to a magnetic device that are induced by a nonuniform field and the misalignment of directions of magnetic field and magnetization, respectively, can be calculated as Eq. (1.2) and Eq. (1.3):
The programmable parameters of magnetic field include the direction, amplitude, and gradient. Thanks to the high temporal resolution control and the capability of deep tissue penetration, multimodal locomotion including biomimetic modes on various terrains, sophisticated functionalities, and shape‐morphing behaviors could be achieved by the magnetic machines. For instance, a variety of agile movements are developed for magnetic robots with fruitful inspirations from multilegged animals, zebrafish larvae, midge larvae, scallops, and jellyfish (1039–42). Moreover, advanced functionalities including self‐adaptation, shape memory, logic circuits, and mechanical tunability are achieved by the integration of magnetic control properties with structural designs and intelligent materials. Encoded with heterogeneous 3D magnetization profiles, magnetic robots could exhibit multiple complex deformations, e.g. 2D‐to‐3D and 3D‐to‐3D structural changes, upon magnetic stimulation. The arrangement of heterogeneous magnetization inside robotic structures is extensively studied, such as with the assistance of photolithography, modular assembly using bonding agents or dynamic covalent bonds, template‐assisted magnetic programming, 3D printing techniques, and laser heating (16, 24, 27,43–47). In addition, the magnetothermal effect that could remotely generate heat using high‐frequency magnetic fields is also developed for the activation of magnetic soft robots.
1.2.2 Light Actuation
Light is another commonly adopted actuation source for untethered miniature soft robots with the advantages of high spatial and temporal resolutions, enabling the precise and selective control (48). Sophisticated optical equipment including photomasks, optical choppers, and lenses have been developed for the precise light actuation (Figure 1.1e,f). For instance, a single material component of LCE in cilia shape developed by Li et al. could present diverse complex deformation behaviors including photophobic, phototropic motions, bending, and twisting using tunable light source (49). The photo‐responsive properties of soft robots could adapt to a wide spectrum or selective wavelength based on the optical absorptive and chemical properties of the material components. Photothermal effect and photochemical reactions are widely adopted as the working mechanisms for the synthetic light‐responsive actuators. For example, through photonic–thermal energy conversion or photoisomerization of azobenzene derivatives, the ordering change in liquid crystal networks is often activated. A variety of other mechanisms including water desorption, change of surface tension, hydrophobicity, and magnetic properties, inequivalent thermal strain, and shape memory effect are also developed for the actuation of light‐responsive materials (33, 50, 51). For photo‐responsive actuators, a two‐layer structure design including an active layer and a passive layer has been widely adopted. The bending deformation of the bimorph actuators can be calculated according to the Timoshenko theory (50):
ρ represents the curvature radius. hi and Ei (i = 1, 2) are the thickness and elastic modulus of the materials, respectively. Lei (i = 1, 2) represent the expansion or shrinkage of the materials.
1.2.3 Acoustic Actuation
Acoustic actuation owns a wide range of applications in miniature robots and biomedical fields due to its good environmental adaptability and deep penetration into biological tissues (52, 53). Acoustic radiation force and bubble assistance vibration have been developed for acoustic‐actuated microrobots (Figure 1.1c,d).
The acoustic radiation force (FA) applied to microrobots and the resonance frequency of a microbubble (fre) can be obtained by Eqs. (1.7) and (1.8), respectively (54).
Ω is the surface of the microrobot. σ is the stress applied to the microrobot's surface and 〈σ〉 represents the time averaging value of σ. ρ and n refer to fluid density and the normal direction of the microrobot surface, respectively. v1 is the vibration velocity. v2 is the streaming velocity. R represents the bubble's radius and L represents the cavity's length. P0 and κ are the hydrostatic liquid pressure and adiabatic index, respectively.
Mechanical resonance generated by integrated oscillatory units, e.g. sharp structures or bubbles in robot body, is needed to efficiently transform acoustic energy into mechanical energy and produce propulsion. Activated by acoustic energy, diverse locomotion modes including rotation, sliding movement, vertical climbing, starfish larvae‐like motions, and pull‐type motions have been achieved for microrobots (11, 53, 55). A large thrust force can be generated via bubble assistance vibration in liquid environment even upon a low‐amplitude acoustic field, while this type of actuation would be limited by the long‐term stability of bubble. In comparison, the vibration of flexible structures is free from the long‐term stability problem, but requires the input of high acoustic energy. One of the important properties of acoustic actuation is selective activation based on the design of robotic structures with well‐separated resonance frequencies. To generate precise acoustic fields, strategies including time‐lapse Fourier synthetic harmonics, acoustic holography methods, and phased‐array acoustic waves have been proposed (56, 57).
1.2.4 Thermal Actuation
Thermo‐responsive soft materials can be actuated by the change of environmental temperature in liquids or air (Figure 1.1h). However, at the small‐scale size, the direct conversion of thermal energy to robots is relatively difficult. Strategies including electrical heating, electromagnetic heating, and photothermal conversion have been developed for miniature robots made of temperature‐responsive materials, e.g. SMP, LCE, and poly(N‐isopropylacrylamide) (PNIPAAm) hydrogel (58). To generate heterogeneous deformation or execute target functions with better output performance, diverse methods have been proposed including the integration of materials with distinct thermal response behaviors, anisotropic alignment or patterning of thermal‐responsive units, the optimization of robotic structures, and the pre‐treatment of the thermal‐responsive robots (59, 60). For instance, soft robots that could exhibit self‐oscillating behaviors upon temperature gradient generated by a hot plate were developed by Wang et al. and Dong et al. (32, 33). Through the optimization of robotic structures, the thermal‐responsive robots could achieve continuous crawling and rolling locomotion on the hot plate.
1.2.5 Chemical Actuation
Stimuli‐responsive soft materials can also be actuated by other environmental stimuli including Marangoni effect (Figure 1.1g), humidity, chemical fuels, and ionic strength (54). Chemical reactions that could generate bubbles have been widely adopted to propel miniature robots. The volumetric swelling or deswelling behaviors of water‐absorbing materials have been exploited for hygroscopic robots to achieve programmable shape‐morphing and movements (Figure 1.1i,j). Made of smart materials, soft robots could also exhibit responsive behaviors to the variations in pH, ionic strength, selective DNA sequences, surface tension gradient, and the presence of solvent or solvent vapor (61, 62). Due to the advantages of ease in downscaling and wireless control, these robots could present high mobility and execute tasks in hard‐to‐reach environments. Nevertheless, for some robots that rely on chemical reactions, the continuous supply of chemical fuels is challenging in confined spaces and the fuels may induce adverse effects to the real environment.
1.2.6 Biohybrid Actuation
Chemical energy could be efficiently transformed into mechanical work by biological microorganisms and cells. Due to the limitations of materials and fabrication techniques, it remains challenging to develop artificial robots with comparable control and actuation performance at microorganism and cell scales. Soft robotic systems that integrate flexible materials and biological components exhibit the capability of generating high power density and high output force for small‐scale robotic actuation and promise practical application prospects in organ‐on‐a‐chip, tissue engineering, drug delivery, minimally invasive surgery, and cell manipulation (Figure 1.1k) (63–66). For instance, different substrates, e.g. elastomers and hydrogels, could be integrated with modified muscle cells which could respond to chemical fuels such as ATP, electrical stimulation, or light via optogenetic modification. Diverse functions including sensing, pumping, or artificial muscles could be executed via the contractile locomotion of these cells. In addition, organisms such as sperm cells, invertebrates, zebrafish, and Escherichia coli integrated with control components could enable the achievement of steerable movements of these biohybrid machines along targeted trajectories. Moreover, biological units in these artificial micromachines could carry functional dopants such as drugs and cells for therapeutic use to execute biomedical tasks.
1.3 Fabrication Methods of Untethered Soft Robots
1.3.1 Molding
Molding is a commonly used fabrication method that cures polymeric precursors to fabricate robots with specific shapes in molds (Figure 1.2a,b). For complex 3D structures, molds formed by 3D printing techniques (such as two‐photon polymerization [TPP] or fused deposition modeling [FDM]) are usually used for molding or casting processes. On the one hand, molds made of sacrificial materials such as PVA materials could be used. After the curing of the polymeric precursors, the mold is removed, and the formed 3D structure is retained. On the other hand, the mold can also be used as a part of the robotic structure. For example, magnetically responsive mechanical metamaterials are fabricated by injecting magnetorheological fluids or elastomers into 3D‐printed lattice structures (72, 73). When using the molding method to form robotic structures, it is usually necessary to generate internal heterogeneous orientations in polymers to fulfill the deformation or actuation requirements, especially for structures based on magnetic composites and liquid crystal materials. When polymeric precursors doped with hard magnetic or soft magnetic particles are molded, a uniform external magnetic field can be applied during the preparation process to control the orientation of the magnetic particles. In addition, after the curing of hard magnetic soft materials, the soft structure can be deformed into a targeted shape with the assistance of template and magnetized by applying a strong magnetic field under the deformed state, which results in a nonuniform magnetization profile formed in the fabricated soft materials. Compared with other fabrication techniques, e.g. stereolithography (SLA) and DIW, the molding method does not impart high requirements for material properties including optical and rheological properties. Thus, the molding method exhibits procedural simplicity and wide applicability, and is most commonly adopted for the preparation of robotic prototypes.
Figure. 1.2 Fabrication methods of untethered soft robots. (a,b) Molding method.
Source: Gu et al. and Lum et al. (67, 68)/Springer Nature/CC BY 4.0/Public domain/Pnas.
(c–e) Additive manufacturing.
Source: Adapted from Kim et al., Xu et al., and Li et al. (27, 45, 69)/Springer Nature/American Association for the Advancement of Science.
(f–h) Semiconductor and microelectronic techniques.
Source: Bandari et al., Kim et al., and Miskin et al. (61, 70, 71)/Reproduced from Springer Nature.
(i,j) Modular assembly based on bonding agents.
Source: Zhang et al. and Dong et al. (24, 25)/American Association for the Advancement of Science (AAAS)/© The Authors, some rights reserved; exclusive licensee AAAS. Distributed under a CC BY‐NC 4.0 license http://creativecommons.org/licenses/by-nc/4.0/.
1.3.2 3D Printing Techniques
The progress in the development of miniature soft robots with more sophisticated structures and functions has been accelerated by the advances in diverse 3D printing techniques. Classified based on their working mechanisms, widely used 3D printing techniques for the fabrication of miniature soft robots include material extrusion, light‐based 3D printing, and material jetting (Figure 1.2c–e) (74).
Extrusion‐based 3D printing techniques usually deposit uncured material through a nozzle via mechanical force. 3D structures are printed by stacking consecutive layers of 2D patterns with the assistance of motile extrusion nozzles or a three‐axis motion platform. Fused filament fabrication (FFF) and DIW are two powerful extrusion 3D printing techniques. For the FFF method, a heater near the nozzle is used to liquefy thermoplastic polymers, which would be solidified again after extruding out the nozzle with an ambient temperature lower than the glass transition temperature. For the DIW method, with the assistance of piston, screw, or air pressure, pseudoplastic polymers that exhibit reversible rheological behavior are extruded out of the nozzle. With such a flexible filament‐by‐filament printing strategy, complex structures made of diverse materials such as elastomers, hydrogels, and shape memory polymers could be fabricated. Compared with the printing speed of FFF (40–480 mm s−1), DIW presents a relatively slow speed (∼10 mm s−1). Nevertheless, due to a large variety of printable materials and multi‐material ability, DIW is more attractive for broad application prospects in robotics (75–77). A customized DIW printing system developed by Zhao et al. exhibits superior capability of in situ magnetization programming which is achieved by controlling the magnetizations of hard magnetic particles upon the localized magnetic field generated by coils near the nozzle (27). Complicated structures with desired magnetization distributions can be prepared by tuning the printing direction to achieve functional shape transformation under magnetic stimulation.
Light‐based 3D printing is a kind of technique using focused laser light or patterned ultraviolet to solidify photopolymerizable polymer resins. Printing methods including digital light processing (DLP), computed axial lithography (), continuous liquid interface production (CLIP), and direct laser writing (DLW) have been developed. During the DLP printing, a supporting plate would move according to the given commands to generate a sequential pattern on each layer whose resolution could reach 7 μm. Compared with voxel‐based printing methods such as selective laser sintering (SLS) and DLW, the printing speed of DLP is relatively high (40–400 mm h−1) and DLP shows good compatibility with diverse photocurable composites including polymeric precursors doped with micro−/nano magnetic particles or conductive fillers (78–80). For example, during the selective curing of the photopolymer resin with patterned ultraviolet, electromagnetic coils or a permanent magnet are adopted in the printing system to tune the alignment or magnetization of magnetic particles, leading to the formation of soft‐magnetic and hard‐magnetic soft robotic structures. By initiating two‐photon or multi‐photon polymerization with femtosecond laser pulses, the DLW method could achieve a printing resolution of 100 nm, promising great potential in the microscopic fabrications of robots. Nevertheless, the printing speed (∼2 mm h−1) and allowable printing dimensions (∼2.2 × 2.2 × 0.25 mm3) of the DLW method are usually limited (81).
1.3.3 Semiconductor and Microelectronic Techniques
Semiconductor processing technology offers a powerful platform for the etching and deposition of diverse materials (Figure 1.2f–h). Due to the advantages of easy integration with electronic components and mass fabrication, they have been widely used for the development of flexible electronic devices. Compressive buckling and self‐rolling strategies have been developed to form complex 3D structures (61,82–84). The control of internal stress in the processed materials enables the formation of robotic structures with hierarchical 3D layouts integrated with electronic components. As shown in Figure 1.2f, 3D microfliers that carry electronic components including near‐field communication (NFC) chips and silicon nanofilm transistors are developed by Kim et al. By applying metal hard mask and oxygen plasma reactive ion etching, patterned SMP films were obtained, followed by depositing a multilayer of Ti/Mg/Ti/SiO2(70). There are hydroxyl groups generated on the surface of the ozone‐treated SiO2 layer, which could ensure strong covalent bonding between a pre‐strained silicone elastomer substrate and the treated SMP film at specific locations. By using transfer printing technique, electronic components encapsulated by the polyimide can be integrated with the SMP film. A morphological transformation of the film from 2D structure to 3D wind‐dispersed seeds shape structure was performed via the release of pre‐strain and mechanical buckling process. These flying devices released in the air can present controllable falling speeds and execute tasks such as environmental monitoring via the loaded wireless electronic devices. In addition, a flexible motile microsystem was proposed by Bandari et al. shown in Figure 1.2(61). A wireless energy transfer module in the microsystem facilitates the control of the propulsion direction of the microsystem and the power supplier of the integrated electronic modules. After the depositing of multiple layers of polymers on a substrate, self‐rolling operation activated by the removal of sacrificial layer was adopted for the generation of tubular shape structures at the edges of the proposed microsystem. In the micro‐robotic system, there are heating wires and a square coil capable of heating the catalytic engines to tune the production of oxygen and wirelessly transfer energy. Recently, Miskin et al. developed a new class of electrochemical‐actuated microrobot systems that are compatible with silicon processing technology, which allows the formation of thousands of microrobots in a chip via an integrated process (71). With the photovoltaics integrated into the robotic structure, the legs of the microrobots would deform upon laser stimulation, leading to the locomotion of the micro‐robotic system.
1.3.4 Modular Assembly Based on Bonding Agents
The bottom‐up assembly strategy allows a facile integration of heterogeneous components in a robotic system by using bonding agents including tapes, glues, and uncured materials (Figure 1.2i,j). For instance, uncured elastomers were adopted by Zhang et al. to connect heterogeneous micro‐components in small‐scale magnetic soft machines and develop micro‐robotic systems with arbitrary material compositions and 3D magnetization profiles (Figure 1.2i) (24). With the assistance of TPP‐printed micro‐molds, edge connection or surface connection between magnetic modules and skeleton modules could be formed by the bonding agents. Microrobotic systems developed by this bottom‐up assembly strategy exhibit great promises for diverse biomedical applications including anchoring machines, soft capsules, and peristaltic pumps. In addition, Dong et al. developed multifunctional and multimodule magnetic soft robots using adhesive sticker network (Figure 1.2j) (25). The adhesive sticker allows the transfer of hard magnetic particles with different 3D magnetization patterns to the substrate and the easy integration of diverse functional modules including photosensitive modules, oil absorption modules, and electronic components.
1.4 Applications of Miniature Soft Robots
1.4.1 Biomedical Application
Recent advances in miniature robotic techniques, e.g. medical guidewire and catheter, greatly promote the practical applications of minimally invasive surgery with shortened recovery times and reduced surgical risks (85–87). The catheter‐based interventions have been widely adopted for the cardiovascular disease treatment (88, 89). Nevertheless, further advancements in catheter technology require them to perform more functional and even challenging tasks in addition to delivering catheters to designated locations, such as the capability of monitoring diverse physiological environments, delivering therapeutic cells or drugs, performing thermal or electrical stimulation to soft tissue, embolization, and clot removal. To fulfill these requirements, the catheter structures need to integrate with various functional components, e.g. soft sensors and manipulator tools. However, the introduction of functional components would impede the miniaturization of catheter structures, which is important for the application of medical catheters in narrow spaces. A feasible solution for the trade‐off between miniaturization and functionalization is provided by semiconductor processing technology. A hollow microcatheter with a diameter of 0.1 mm carrying magnetic sensor and electrically actuated gripper was developed by Rivkin et al. via the self‐rolling method (90). The fabricated microcatheter exhibited the locomotion capability in a thin curved channel (only 0.2 mm diameter), and the fluid delivery function in the stomach and esophagus of mouse. The end of the catheter is integrated with a conductive polymer film actuated microgripper, which could capture 0.1 mm particles in a tube. In addition, the magnetic sensor based on the anisotropic magnetoresistive effect in the microcatheter could execute in vivo localization and navigation with a resolution of 0.1 mm. Han et al. proposed the integration strategy of multilayer configurations of soft electronic arrays and actuators on commercialized endocardial balloon catheters (89
