as prospective aerial vehicles that can significantly enhance the efficiency of search-and-rescue missions, [16][17][18] perform dangerous operations in hazardous environments, [18] and function as miniature nodes in sensor networks. [18] Because miniature robots are expected to operate in various types of environments, it is essential for them to possess effective locomotive gaits to navigate well in such terrains. [19,20] This will be especially true if the robots have to negotiate across highly unstructured environments such as within the human body or at disaster sites. [5,10,21,22] Of the miniature robots, the soft robots are significantly more promising than their rigid counterparts in developing dexterous gaits because their degrees of freedom and adaptability are considerably higher. [23][24][25] By having the ability to generate a series of time-varying shapes, miniature soft robots have shown to be able to perform various types of locomotion, which allow them to negotiate across complicated obstacles in their environments. [19,26] In addition to using such locomotion for navigation purposes, these gaits could also be beneficial for studying the locomotion of various small organisms. [4,13] In this report, we review the locomotion producible by miniature soft robots. The scope of this report is therefore different from reviews and commentaries that focus on the actuation, [3] applications, [2,21] or design [27] of miniature soft robots. It is also significantly different from reviews that focus on the applications of miniature robots, [1,10,11] and others that review the general principles [24,[28][29][30][31][32][33][34][35] or locomotion [34,36,37] of macro-scale soft robots. To facilitate our discussion, here we categorize the locomotion of the miniature soft robots into terrestrial, aquatic, and aerial locomotion (Figure 1). Under each category, we will highlight their key advancements, as well as their open challenges and future outlook. Except for the aerial robots that are in the centimeter-scale, our discussions will focus on soft robots that are in the micro/millimeter length scales. It is noteworthy that the discussions in this report will be restricted to synthetic materials. For miniature bio-robots based on natural materials, interested readers may refer to other reviews on biomolecular, biohybrid actuation, or microorganism-driven systems. [38][39][40][41][42][43] The report is organized as follows: Section 2 provides a brief discussion about the general actuation principles of miniature soft robots. This is followed by Sections 3-5 in which we discuss the advancements of miniature soft robots in their Miniature soft robots are mobile devices, which are made of smart materials that can be actuated by external stimuli to realize their desired functionalities. Here, the key advancements and challenges of the locomotion producible by miniature soft robots in micro-to centimeter length scales are highlighted. It is highly desirable to endow these small machines with dexterous locomotive gaits a...

Magnetic miniature robots (MMRs) are small‐scale, untethered actuators which can be controlled by magnetic fields. As these actuators can non‐invasively access highly confined and enclosed spaces; they have great potential to revolutionize numerous applications in robotics, materials science, and biomedicine. While the creation of MMRs with six‐degrees‐of‐freedom (six‐DOF) represents a major advancement for this class of actuators, these robots are not widely adopted due to two critical limitations: i) under precise orientation control, these MMRs have slow sixth‐DOF angular velocities (4 degree s−1) and it is difficult to apply desired magnetic forces on them; ii) such MMRs cannot perform soft‐bodied functionalities. Here a fabrication method that can magnetize optimal MMRs to produce 51–297‐fold larger sixth‐DOF torque than existing small‐scale, magnetic actuators is introduced. A universal actuation method that is applicable for rigid and soft MMRs with six‐DOF is also proposed. Under precise orientation control, the optimal MMRs can execute full six‐DOF motions reliably and achieve sixth‐DOF angular velocities of 173 degree s−1. The soft MMRs can display unprecedented functionalities; the six‐DOF jellyfish‐like robot can swim across barriers impassable by existing similar devices and the six‐DOF gripper is 20‐folds quicker than its five‐DOF predecessor in completing a complicated, small‐scale assembly.

Magnetic miniature robots (MMRs) are mobile actuators that can exploit their size to noninvasively access highly confined, enclosed spaces. By leveraging on such unique abilities, MMRs have great prospects to transform robotics, biomedicine, and materials science. As having high dexterity is critical for MMRs to enable their targeted applications, existing MMRs have developed numerous soft‐bodied gaits to locomote in various environments. However, there exist two critical limitations that have severely restricted their dexterity: 1) MMRs capable of multimodal soft‐bodied locomotion have only demonstrated five‐degrees‐of‐freedom (five‐DOF) motions because the sixth‐DOF rotation about their net magnetic moment axis is uncontrollable; 2) six‐DOF MMRs have only realized one mode of soft‐bodied, swimming locomotion. Herein, a six‐DOF MMR is proposed that can execute seven modes of soft‐bodied locomotion and perform 3D pick‐and‐place operations. By optimizing its harmonic magnetization profile, the MMR can produce 1.41–63.9‐fold larger sixth‐DOF torque than existing MMRs with similar profiles, without compromising their traditional five‐DOF actuation capabilities. The proposed MMR demonstrates unprecedented dexterity: it can jump through narrow slots to reach higher grounds; use precise orientation control to roll, two‐anchor crawl, and swim across tight openings with strict shape constraints; and perform undulating crawling across three different planes in convoluted channels. An interactive preprint version of the article can be found at: https://www.authorea.com/doi/full/10.22541/au.164087652.25227465.

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