1. Introduction
Robotics and especially, soft robotics, a sub-branch of robotics, have recently attracted broad interest in the scientific community.[1,2] This interest is fueled by the flexible and deformable nature of the materials used, and (for some) that they can be deformed upon a specific trigger, i.e., temperature, pH, magnetic field, and electric field, and thus, feature distinct advantages for their application in cargo transportation, drug delivery, microsurgery and so forth.[3-6] For example, soft robots have been introduced, which can navigate and pass-through narrow gaps that are smaller in size than the initial robot shape via (triggered) deformation (triggered shape transformation).[7-12] In addition, the suppleness of the soft robots can mitigate damage to an object or body in contact with the robot.
Evolution and adaption to everchanging environments have brought about a large variety of soft-bodied living things, which offer abundant inspiration to the design and construction of soft robotics.[13] For instance, mollusks (invertebrates) can stretch, curl up, and travel by relying on a complex mixture of biophysical and chemical signals originating from the surrounding (environmentally triggered transformations). For example, leeches, which belong to the phylum mollusks, can perform multimodal movements, including reconfigurable deformation, swimming, inchworm-like movement, and peristalsis, according to different environmental triggers, i.e., temperature and chemical composition of the water.[14] Inspired by the phylum mollusk, various soft robots have been developed, that can perform reconfigurable and self-adaptable actuation in different environments.[15-18] Therein, droplet-based soft robots have attracted great attention due to their excellent mechanical properties stemming from their liquid nature, including the high softness and extreme deformability.[19] Plenty of droplet-based soft robots have be developed in recent years, such as water droplet robots,[20-23] ferrofluid droplet robots,[24,25] oil droplet robots,[26] and liquid metal droplet robots[27-35] for a variety of engineering and biomedical applications, such as drug delivery, cargo transportation, and mixing of chemicals in lab-on-a-chip applications.[24,32,36,37] Furthermore, fundamental studies on droplet-based soft robots have provided striking insights into the physics and hydrodynamics of droplets spanning a wide range of spatial scale (macro to microscale).[38,39] Although the deformability and mobility of the droplet-based robots have been advantageous for ample applications, most of the droplet-based robots offer only low electrical conductivity, limiting their range of applications substantially.
In this regard, room temperature liquid metals, metals and alloys that are in the liquid state at or near room temperature, have garnered extensive attention from the scientific community as well as industry due to their extraordinary combination of physical properties. [40-44] Several metals and alloys are known and in use for experiments and industrial applications. Broadly known are alloys based on Bi, such as the ternary alloy Field’s metal and quaternary alloy Wood’s metal, Pb-based alloys, and gallium and its alloys. Especially, the Ga-based liquid metals, that are gallium, the eutectic mixture of Ga and In (EGaIn), the eutectic mixture of Ga, In and Sn at a composition of Ga ∼68.5 wt%, In ∼21.5 wt%, Sn ∼10 wt% (Galinstan), and the Ga-Sn mixture have been investigated due to the low melting point and strong supercooling.[44-48] Furthermore, the gallium-based liquid metals feature (values for Galinstan) high electrical (0.34·105 S/cm) and thermal conductivity (≈ 25 W/m·K), high surface tension (≈ 600 mN/m), and low toxicity while exhibiting a low viscosity of around 2.5 cP at room temperature.[3,42] These properties and the ability to tune the physical properties, i.e., surface/interfacial tension, thermal conductivity, and rheological properties (viz., viscosity), as well as the ability to actuate and deform the liquid metals via ample methods, such as light, pH, chemical environment, and magnetic field, render them intriguing candidates for various applications, including high-precision manipulation, flexible electronics and soft robotics to drug delivery systems.[2,49-52] For example, Xu et al.[53] reported a kind of magnetic liquid droplet robot that can move due to a magnetic gradient field and this robot is able to perform cargo transfer and  vessel cleaning. Sun et al.[28] proposed a liquid metal-based robot that can jump in order to avoid obstacles, climb steep slopes, and rotate its body to the desired posture, and they anticipate application of this robot in targeted drug delivery. Liu et al.[54] developed a magnetic liquid metal droplet, which can be stretched both horizontally and vertically, for electrode connections. Wang et al.[55] proposed a ferrofluid comprising Ga and iron particles, and realized with it magnetic manipulation of non-magnetic objects via thermal switchable on-demand grasp and release enabled by the phase transition of the magnetic liquid metal composites. Although the deformability and mobility of the droplet-based robots have been advantageous for ample applications, it is still a challenge to fix complex droplet shapes against the intrinsic droplet shrinking towards a spherical and low interfacial energy state. Furthermore, most of the droplet-based robots offer only low electrical conductivity and the potential applications in transient electronics and circuit welding are rarely explored.
In this work, we designed and developed a liquid metal droplet-based soft robot with high electrical conductivity and excellent shape transformation ability based on magnetic actuation and infrared light triggered shape encoding (Figure 1a-1c). The soft robot comprises of liquid metal and carbonyl iron (iron pentacarbonyl) as magnetic particles. The thus obtained composite is used as a reconfigurable conductor in a circuit and serves as a switch, which can be controlled remotely. Under an external magnetic field, the soft robots, i.e., magnetic liquid metal composite, can perform multimodal movement, including reversible telescopic deformation, bending, and on-demand locomotion (Figure 1, Figure S1, Movie S1). In addition, the liquid metal droplet robots can also perform reversible coalescence and splitting by using specific magnetic fields. The telescopic and bending deformation of the liquid metal droplet robot can be exploited to realize selective interconnection and disconnection of electrodes in a complex circuit. Programmable shape encoding of the liquid metal robots and the phase transition of the robot between liquid and solid states can be utilized to fix a desired shape via cooling and localized infrared light irradiation. Furthermore, damaged circuits can be repaired by remote actuation, controllable coalescence, and on-demand circuit welding. In addition, the liquid metal can be fixed reversibly in a shape by cooling below the solidification temperature. Finally, the liquid metal composite utilized in this robot can be reclaimed and recycled. Therefore, this magnetic field responsive liquid metal-based soft robots are attractive devices as they expand the application scenarios of droplet-based soft robots toward on-demand circuit welding and transient and recyclable electronics.