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Page 2 of 12 Peng et al. Soft Sci 2023;3:36 https://dx.doi.org/10.20517/ss.2023.28

INTRODUCTION
Soft and stretchable electronics that can conformably deform with nonplanar and dynamically moving
surfaces hold great promise in various fields, such as wearable devices, soft robotics, and implantable
devices . Stretchable conductors with high conductivity are the basic building blocks for high-
[1-7]
[8]
performance stretchable electronic devices . Among various conductive materials designed for stretchable
[8,9]
conductors , metals are quite attractive due to their merits of high electric conductivity, high thermal
conductivity, and good stability. However, conventional solid metals can not withstand large deformation
since they are rigid and brittle. To endow metal conductors with stretchability, conductive elastomeric
composites with metal particle fillers [10,11] and rational structure design (e.g., serpentine circuits, mesh
[12]
structures, microcracks) have been frequently employed. However, the preparation of structure-based
stretchable conductors involved complicated and costly metal deposition and microfabrication processes
[8]
such as photolithography . Although stretchable metal-elastomer composites show the advantages of ease
of processing, printability, and high durability , a high concentration of rigid metallic particles is required
[13]
to form percolated conducting pathways . This could lead to the degradation of the mechanical properties
[14]
[15]
of the composite by increasing its stiffness or tensile modulus . In addition, since solid metals can not
conformally deform with the elastomer, the separation of the metal particles poses a challenge to
[16]
maintaining the conductance stability of the composite when stretched .
Gallium-based liquid metals (LMs) have emerged as attractive conductive fillers for stretchable conductors
due to their excellent metal conductivity, inherent stretchability/deformability, self-healing ability, and
neglected toxicity [17-24] . Unlike solid metal fillers, the fluidic LM particles can be deformed along the
[25]
stretched matrix and maintain conductance stability . However, most reported LM-elastomer (LME)
composites are initially insulators and require an additional sintering process to realize electrical
conductivity by rupturing the LM particles for electric connection [16,26-28] . To date, various post-sintering
methods have been reported, such as mechanical pressing [16,29] , freezing , and laser activation . However, a
[31]
[30]
high volume of LM addition (~50%) is required for sintering strategies, thus greatly increasing the
[26]
composite density and leading to material waste . Besides, new conductive pathways could also be
activated during long-term use, which may induce the short-circuit issue . Moreover, mechanical sintering
[32]
may induce structure damage in delicate circuits, which is also not applicable to high-resolution circuits
with microscale line widths . Therefore, new strategies without post-sintering are highly needed for the
[33]
fabrication of initially conductive LME composites.

In this article, we report a post-sintering-free method to create initially conductive LME composites by
magnetic manipulation. This was achieved by dispersing LM ferrofluid into the elastomer. We show that
continuous conductive pathways can be formed by applying the magnetic field to induce the magnetic
aggregation and interconnection of the LM ferrofluid particles. This sintering-free method also offers
several distinct advantages: (i) simple and low-cost preparation in an equipment-free manner; (ii)
noncontact activation of the conductive pathways without damaging the circuits; (iii) being compatible with
microscale circuits; (iv) enabling the composite with magnetic responsiveness; (v) without high loading of
LM. In addition, the resulting LME composite shows high stretchability (~650% strain) and high
electromechanical stability. We also demonstrated the applications of stretchable and responsive LME
conductors in stretchable circuits, flexible magnetic switches, and soft hydrogel actuators.

EXPERIMENTAL
Chemicals and materials
The gallium and indium were purchased from Shanghai Macklin Biochemical Co., Ltd, Shanghai, China.
The eutectic gallium-indium alloy (EGaIn) was prepared by heating the gallium and indium with a mass