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Peng et al. Soft Sci 2023;3:36 https://dx.doi.org/10.20517/ss.2023.28 Page 5 of 12

The mechanism of conductive pathway formation in the magnetic LME is shown in Figure 1C. Initially, the
LM ferrofluid microparticles are evenly distributed in the elastomer precursor solution. Under the magnetic
field, the LM ferrofluid particles aggregate to connect to form continuous conductive networks at the
composite bottom. We then cured the elastomer to form an initially conductive LME that can maintain
high conductance stability when stretched.

The magnetic LME composites also show printability, allowing us to print various functional electronic
components, such as micro-circuit chips [Figure 1D], patterned stretchable circuits [Figure 1E], and
multilayer circuits. Furthermore, owing to the excellent thermal conductivity and thermal stability of the
LM ferrofluid, the conductive LME composite can be embedded in temperature-responsive hydrogel
actuators to achieve wireless induction heating [Figure 1F].

Preparation and characterization of conductive LME composite
The preparation of the conductive LME composite is schematically depicted in Figure 2A, and the details
are provided in the Experimental Section and Supplementary Table 1. In a typical operation, we dispersed
the LM ferrofluid to the Ecoflex 0030 part A by mechanical stirring. We observed the solution changed from
colorless to opaque grey-black during the stirring. This is due to the formation of LM ferrofluid particles
under the shear force. After mixing with the Ecoflex 0030 part B, we applied the magnetic field to the
solution to attract LM ferrofluid microparticles to the composite bottom [Figure 2B]. The magnetic
aggregation can increase the local concentration of LM ferrofluid particles and reduce the distance between
the particles. Therefore, the LM ferrofluid particles tend to connect to form continuous conductive
networks.

We also found the LM ferrofluid cannot maintain its ellipsoidal shape compared to the pure LM [Figure 2C
and Supplementary Figure 3A]. To further explore the properties of the LM ferrofluid, we performed the
contact angle and surface tension tests [Supplementary Figure 3B]. The results show that the contact angle
and the surface tension of the LM ferrofluid were lower than those of the pure LM [Figure 2D]. We also
compared the particle distribution in the composite before and after applying the magnetic field. For the
composite without applying the magnetic field, the LM ferrofluid microparticles are uniformly dispersed in
the elastomer matrix [Supplementary Figure 4]. For the one with magnetic aggregation, the composite
exhibits the Janus structure with high stretchability [Figure 2E and F]: the upper insulative layer was in light
grey with a low concentration of LM particles, while the lower conductive layer was grey and black with
highly condensed magnetic LM particles.

Under the magnetic field, the LM ferrofluid microparticles aggregated at the bottom of the elastomer
matrix. Since LM ferrofluid particles are almost incompressible, the magnetic force induces the LM
ferrofluid droplets to merge by breaking the oxide layer of the LM [Supplementary Figure 5]. The SEM
image shows the distribution of LM ferrofluid particles is increased from the top to the bottom, and the
particles coalesce into a continuous conductive pathway at the bottom [Figure 2G]. We also used 3D micro-
CT tomography to study the microstructure of the composite. We found that the denser LM ferrofluid
droplets were aggregated at the bottom while the low-density elastomer matrix was concentrated at the top
[Figure 2H]. SEM and corresponding EDS analysis further show that Cu@Fe microparticles were wrapped
by LM in the composite [Figure 2I and J]. Our proposed magnetic aggregation method can activate the
conductive filler networks before the elastomer curing without the post-sintering operation. Therefore, this
approach can overcome the limitations of mechanical sintering on the hardness of the matrix, as the soft
elastomer matrix (Ecoflex) cannot provide strong mechanical support for sintering.