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.