Small universal mechanical module driven by a liquid metal droplet

A Controllable Cargo Delivery Vehicle Driven by Electrically Actuated Galinstan Droplets

As an emerging functional material, liquid metal has attracted extensive attention due to its unique physical/chemical properties. Particularly, the combination of the intrinsic fluidity with rapidly stimuli-responsiveness to electrical field endows it potential as soft actuators to be employed in soft robots. Herein, we developed a small controllable vehicle driven by electrically actuated Galinstan droplets. A series of experiments were carried out to evaluate the vehicle's performance, including straight translational locomotion at various speeds and rotational motion from different starting angles. And then, the vehicle's excellent mobility is further demonstrated through its ability to follow complex trajectories. More importantly, by redesigning the vehicle's frame, it can be adapted for multiple functions, such as cargo transportation and loading/unloading tasks. The present finding is envisaged to have the potential to expand current research on soft robot and further advance the development of micro-factory.

The continuous actuation of liquid metal with a 3D-printed electrowetting device

The ability of liquid metals (LMs) to recover from repeated stretching and deformation is a particularly attractive attribute for soft bioelectronics. In addition to their high electrical and thermal conductivity, LMs can be actuated, potentially enabling highly durable electro-mechanical and microfluidics systems for applications such as cooling, drug delivery, or reconfigurable electronics. In particular, continuous electrowetting (CEW) phenomena can actuate liquid metal at relatively low voltage and affordable power requirements for wearable systems (~ < 10 V, ~ 10 - 100 µW) by inducing a surface tension gradient across the LM. However, sustaining LM actuation remains challenging due to factors such as electrolyte depletion, polarity changes in multi-electrode systems, and limitations related to LM composition. Here, we demonstrate LM actuation in a circular conduit for prolonged durations of at least nine hours. We enabled sustained actuation by sequentially applying short, direct current (DC) pulses through a multi-electrode system based on the dynamics of LM actuation. As a proof of concept, we also demonstrated the ability of LM to transport electrically conducting, non-conducting, and magnetic materials within a microchannel and show the liquid metal actuation system can be potentially miniaturized to the size of a wearable device. We envision that with further miniaturization of the device architectures, our CEW platform can enable future integration of low-voltage electro-mechanical systems into a broad range of wearable form factors.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s44258-025-00052-8.

Bioinspired Liquid Metal Based Soft Humanoid Robots

The pursuit of constructing humanoid robots to replicate the anatomical structures and capabilities of human beings has been a long-standing significant undertaking and especially garnered tremendous attention in recent years. However, despite the progress made over recent decades, humanoid robots have predominantly been confined to those rigid metallic structures, which however starkly contrast with the inherent flexibility observed in biological systems. To better innovate this area, the present work systematically explores the value and potential of liquid metals and their derivatives in facilitating a crucial transition towards soft humanoid robots. Through a comprehensive interpretation of bionics, an overview of liquid metals' multifaceted roles as essential components in constructing advanced humanoid robots-functioning as soft actuators, sensors, power sources, logical devices, circuit systems, and even transformable skeletal structures-is presented. It is conceived that the integration of these components with flexible structures, facilitated by the unique properties of liquid metals, can create unexpected versatile functionalities and behaviors to better fulfill human needs. Finally, a revolution in humanoid robots is envisioned, transitioning from metallic frameworks to hybrid soft-rigid structures resembling that of biological tissues. This study is expected to provide fundamental guidance for the coming research, thereby advancing the area.

Principles and methods of liquid metal actuators

As a promising material, liquid metals (LMs) have gained considerable interest in the field of soft robotics due to their ability to move as designed routines or change their shape dramatically under external stimuli. Inspired by the science fiction film Terminator, tremendous efforts have been devoted to liquid robots with high compliance and intelligence. How to manipulate LM droplets is crucial to achieving this goal. Accordingly, this review is dedicated to presenting the principles driving LMs and summarizing the potential methods to develop LM actuators of high maneuverability. Moreover, the recent progress of LM robots based on these methods is overviewed. The challenges and prospects of implementing autonomous robots have been proposed.

Magnetic Liquid Metal Droplet Robot with Multifunction and High Output Force in Milli-Newton

Magnetically actuated miniature robots have immeasurable potential in lab-on-a-chip and biomedical due to their ability to navigate in constrained space. However, current soft robots made by elastomers have limited functionalities and are prevented from very narrow environments such as channel much smaller than their size because of their non- or limited deformability. In this study, a soft and multifunctional robot based on liquid metal (magnetic liquid-metal droplet robot [MLDR]) with high output force is reported. It is fabricated by engulfing iron particles into a Galinstan droplet. By changing the shape and motion of permanent magnets, the MLDR can be reshaped and moved. The MLDR can also be split in batches and merged efficiently. It shows good softness and flexibility when navigating freely in a narrow channel, and thus can pass through a confined space smaller than its size easily. Furthermore, the MLDR can also push and spread the accumulated liquid in a desired path, and manipulate the motions of small objects well. Benefiting from the solidification-like phenomenon, an MLDR can output milli-Newton-level force much higher than the output force of ferrofluid droplet robots in micro-Newton level. The demonstrated capabilities of the MLDR are promising for the applications in lab-on-a-chip or biomedical devices.

Liquid metal droplet motion transferred from an alkaline solution by a robot arm

The excellent motion performance of gallium-based liquid metals (LMs) upon the application of a modest electric field has provided a new opportunity for the development of autonomous soft robots. However, the locomotion of LMs often appears in an alkaline solution, which hampers the application under other different conditions. In this work, a novel robot arm is designed to transfer the motion of the LM from an alkaline solution in a synchronous drive mode. The liquid metal droplet (LMD) at the bottom of the robot arm is actuated using a DC voltage to provide the driving force for the system. By introducing an end effector at the center of the robot arm, the synchronous motion of the system is replicated and can be applied to different situations. The theoretical understanding of continuous electrowetting (CEW) at the LM interface is explained, and then the motion performance of the robot arm against the function of the applied voltage and driving direction is investigated. Moreover, several applications using this robot arm, such as pattern drawing, cargo transportation, and drug concentration detection, are demonstrated. The presented robot arm has the potential to observably expand the application fields of the LM.

Desktop-level small automatic guided vehicle driven by a liquid metal droplet

Gallium-based liquid metals (LMs) are a new type of intelligent material, and their ability to move under the action of an electric field provides new opportunities for the design of small flexible vehicles. However, due to the extremely high fluidity of LMs and the poor automatic control ability of LM vehicles, it's still a huge challenge to control the movement of LMs flexibly and accurately. Therefore, in this paper, a small traction vehicle is designed by putting the flexible LM in rigid armor to make the movement more controllable. Moreover, a desktop-level small automatic guided vehicle (sAGV) system is built by using an external control circuit to follow a predetermined trajectory. Firstly, the basic characteristics of the vehicles driven by a LM droplet are simulated and analyzed. Then the effects of different factors on the movement velocity of the vehicles are measured by experiment. Finally, as a preliminary application test, the sAGV system is used to control the vehicles following a specific trajectory and realize the targeted transportation of cargos. The sAGV system designed in this paper can realize the automatic and precise control of the movement of the small vehicle. The current findings will inspire the further construction of complex small operating systems and the realization of accurate control.

Microscale impeller pump for recirculating flow in organs-on-chip and microreactors

Fluid flow is an integral part of microfluidic and organ-on-chip technology, ideally providing biomimetic fluid, cell, and nutrient exchange as well as physiological or pathological shear stress. Currently, many of the pumps that actively perfuse fluid at biomimetic flow rates are incompatible with use inside cell culture incubators, require many tubing connections, or are too large to run many devices in a confined space. To address these issues, we developed a user-friendly impeller pump that uses a 3D-printed device and impeller to recirculate fluid and cells on-chip. Impeller rotation was driven by a rotating magnetic field generated by magnets mounted on a computer fan; this pump platform required no tubing connections and could accommodate up to 36 devices at once in a standard cell culture incubator. A computational model was used to predict shear stress, velocity, and changes in pressure throughout the device. The impeller pump generated biomimetic fluid velocities (50-6400 μm s-1) controllable by tuning channel and inlet dimensions and the rotational speed of the impeller, which were comparable to the order of magnitude of the velocities predicted by the computational model. Predicted shear stress was in the physiological range throughout the microchannel and over the majority of the impeller. The impeller pump successfully recirculated primary murine splenocytes for 1 h and Jurkat T cells for 24 h with no impact on cell viability, showing the impeller pump's feasibility for white blood cell recirculation on-chip. In the future, we envision that this pump will be integrated into single- or multi-tissue platforms to study communication between organs.

DC electric field-driven heartbeat phenomenon of gallium-based liquid metal on a floating electrode

The heart beating phenomenon of room temperature liquid metal (LM) mercury has attracted much attention in the past years, but its research and application are limited because of the low vapor pressure and high toxicity. Here, a fundamental scientific finding is reported that the non-toxic eutectic gallium indium (EGaIn) alloy droplets beat periodically at a certain frequency based on a floating electrode under the stimulation of the direct current (DC) field. The essential characteristics of heart beating are the displacement and the projected area change of the LM droplet. The mechanism of this phenomenon is the self-regulation of interfacial tension caused by chemical oxidation, chemical corrosion, and continuous electrowetting. In this article, a series of experiments are also carried out to examine the effects of different factors on the heartbeat, such as voltage, the volume of the droplet, the droplet immersion depth, the electrolyte solution concentration, the distance of electrodes, and the type of floating electrode. Finally, the heartbeat state and application boundary of the LM droplet under different conditions are summarized by imitating the human life process. The periodic changes of the LM droplet under an external DC electric field provide a new method to simulate the beating of the heart artificially, and can be applied to the research of organ chip fluid pumping in the future.