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

ratio of 3:1 (75 g gallium and 25 g indium). Two elastomers of Ecoflex (Ecoflex 0030, Smooth-on, USA) and
PDMS (Sylgard 184, Dow-Corning, USA) were used in this work. N-Isopropylacrylamide monomer
[NIPAM, 98%, stabilized with hydroquinone methyl ether (MEHQ)] was purchased from Aladdin
(Shanghai, China). Dimethylaminoethyl methacrylate (DMAEMA, 98.5%, stabilized with MEHQ) was
supplied by TCI (Shanghai, China). Hydrolyzed poly(vinyl alcohol) (PVA124, 99%, MW ~105 kDa),
poly(sodium-p-styrene sulfonate) (PSS, 98%, MW ~100 kDa), and ammonium persulfate (APS, 98%) were
purchased from Sinopharm Chemical Reagent Beijing Co. Tetramethylethylenediamine (TEMED, 98%) and
7-hydroxy-4-methyl coumarin (98%) were supplied by J&K (Beijing, China).

Preparation of the LME composite
To prepare the LM ferrofluid, 2.8 g of Cu@Fe particles, 1 mL of EGaIn (~6.8 g), and 4.0 mL of the
hydrochloric (HCl) solution were added into a clean beaker and stirred at 600 rpm to disperse Cu@Fe
particles into EGaIn. To prepare the composite, 9.0 g of LM ferrofluid and 3.0 g of Ecoflex 0030 A were
added into a beaker. Then, an electric stirrer equipped with a 4.0 mm diameter plastic stirring paddle was
used to mix two parts at 500 rpm for 8 min. Then, 3.0 g Ecoflex 0030 B was added, and another 2 min
mixing was required. Next, the mixture was poured into a plastic petri dish with a diameter of 6 mm and
was degassed in a vacuum chamber for 5 min to remove air bubbles. Finally, the plastic petri dish was
placed above the magnet for 5 min. The matrix magnet consists of 25 small cube magnets (5 mm × 5 mm ×
5 mm) with N poles and S poles alternately arranged into a 5 × 5 square shape. Finally, the composite
solution was cured at 60 °C for 30 min.

Preparation of the hydrogel actuator
The bilayer hydrogels were prepared in two simple stages. Free radical polymerization was used to create the
initial layer of the PNIPAM/PVA hydrogel. Specifically, the NIPAM was added to the PVA solution
(5 wt%), followed by adding the poly(ethylene glycol) diacrylate crosslinker with stirring for 30 min. Then,
the initiator APS solution (30 mg/mL) and TEMED were added. The solution was quickly pumped into the
glass cell with a rubber spacer (0.5 mm in thickness). To prepare the PNIPAM/PVA semi-IPN hydrogel,
polymerization was carried out at 4 °C for 6 h. The second layer of the PDMAEMA/PSS hydrogel was
synthesized by injecting DMAEMA pre-gel solution consisting of polymerizable monomer DMAEMA, PSS
aqueous solution (10 wt%), and APS into the glass cell with a spacer thickness of 1.0 mm. The curing of the
second layer was conducted at 20 °C for 6 h, accompanied by the reactive solution penetrating the surface of
the first PNIPAM/PVA layer to create a strong adhesion between the two layers. To make it easier to peel
off the composite layer, the upper glass substrate was covered with a hydrophobic PDMS membrane.

Characterization
The resistance of the LME composite was measured by a digital multimeter (Keithley DMM6500). The
surface temperature of the hydrogel actuator was captured by a near-infrared camera (FLIR ONE Pro).
Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) mapping
characterizations were taken using the Phenom XL. The micro-CT photos were taken using the ZEISS-
Metrotom micro-CT tomography system.

RESULTS AND DISCUSSION
Preparation and mechanism of conductive LME without sintering
We reported a universal synthetic method of magnetic aggregation to create highly conductive LME
without post-sintering. Unlike conventional LME, which uses pure LM particles as the conductive filler and
requires the sintering to form conductive pathways [Figure 1A], we use the LM ferrofluid as the filler for
magnetic manipulation of the LM particles to achieve electric conductivity. The LM ferrofluid was prepared
by mixing Cu@Fe microparticles (~10 μm in diameter) with EGaIn in the HCl solution [Figure 1B and
Supplementary Table 1]. The SEM and EDS images of the Cu@Fe microparticles are shown in