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...

3D printing via vat photopolymerization (VP) is a highly promising approach for fabricating magnetic soft millirobots (MSMRs) with accurate miniature 3D structures; however, magnetic filler materials added to resin either strongly interfere with the photon energy source or sediment too fast, resulting in the nonuniformity of the filler distribution or failed prints, which limits the application of VP. To this end, a circulating vat photopolymerization (CVP) platform that can print MSMRs with high uniformity, high particle loading, and strong magnetic response is presented. After extensive characterization of materials and 3D printed parts, it is found that SrFe12O19 is an ideal magnetic filler for CVP and can be printed with 30% particle loading and high uniformity. By using CVP, various tethered and untethered MSMRs are 3D printed monolithically and demonstrate the capability of reversible 3D‐to‐3D transformation and liquid droplet manipulation in 3D, an important task for in vitro diagnostics that are not shown with conventional MSMRs. A fully automated liquid droplet handling platform that manipulates droplets with MSMR is presented for detecting carbapenem antibiotic resistance in hazardous biosamples as a proof of concept, and the results agree with the benchmark.

In recent years, robotic vehicles have been deployed for planetary exploration purposes, surveying extraterrestrial terrains in search for possible signs of life and also to learn more about these unfamiliar worlds. While deploying robotic technologies for such space missions represents a significant milestone for humanity, these untethered robots may have limited dexterity and robustness due to them having rigid bodies. Therefore, there is potential to send untethered soft robots that are far more dexterous, robust, and functional than their rigid counterparts for planetary explorations in the future. Herein, some of the key advantages of deploying untethered soft robots for future planetary explorations are discussed. Also, some brief insights into how the environmental conditions on extraterrestrial terrains may affect the actuation, fabrication, and locomotion of soft robots are provided. While the emerging soft robotics technology is still relatively young, it is envisioned that these untethered soft machines have great potential to help create a paradigm shift for future planetary exploration.

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