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Page 2 of 12 Wu et al. Soft Sci 2023;3:35 https://dx.doi.org/10.20517/ss.2023.26

machine interaction [10-12] . Since wearable devices are mostly put on the limbs that are in motion from time to
time, the design and fabrication of strain sensors that can accurately detect motion activities is the key. To
date, various soft and stretchable strain sensors have been reported [13,14] , including resistive strain sensors
[15]
based on resistance changes of conductors upon deformation , capacitive strain sensors based on changes
of structural parameters of soft capacitors , and self-powered strain sensors based on triboelectric or
[16]
piezoelectric mechanisms [17,18] . Although self-powered strain sensors based on triboelectric or piezoelectric
nanogenerators can output a voltage signal without relying on an external power supply, such signals are
transient, with attenuation issues over time . Resistive and capacitive strain sensors can reflect the strain
[19]
state in real time, and the maintenance time of the signal depends on the maintenance of the strain state.
However, resistive strain sensors possess more straightforward device structures and designs, making them
highly suitable for the stable and continuous monitoring of strain states.

Developing flexible resistive strain sensors relies on the design of novel sensing materials, such as carbon
[20]
nanotubes (CNTs) , silver nanowires (Ag NWs) , liquid metals , and conductive hydrogels , which
[21]
[22]
[23]
can improve the sensitivity of flexible strain sensors and greatly broaden their manufacturing process.
Among them, liquid metals, one of the typical representatives of functional conductive materials, have
attracted great interest due to their excellent electrical conductivity and adaptability to various
deformations. Most reported liquid metals are associated with the gallium-indium eutectic alloy (EGaIn)
and the gallium-indium-tin eutectic alloy (EGaInSn). These alloys can maintain excellent metallic
conductivity while changing shapes fluidly under external force at room temperature (~25 °C). As a result,
they are considered as good conductive connections and ideal sensing units for deformation in flexible
electronics . Additionally, EGaIn and EGaInSn have almost negligible toxicity and low vapor pressure,
[24]
unlike toxic mercury , which allows liquid metals to be utilized in wearable devices with good
[25]
biocompatibility. To date, taking full advantage of arbitrarily deformable properties of liquid metals, various
strategies have been developed for fabricating liquid metal circuits and sensing units [26,27] , including
microfluidic injection , stamp printing , 3D printing , and so on. However, these methods are always
[28]
[30]
[29]
unsatisfactory and poor in applicability when fabricating wearable flexible strain sensors. The issues mainly
arise from the complicated and expensive pre-preparation work (preparing microfluidic channel for
injection and prototype for stamping printing) and low manufacturing efficiency (3D printing liquid metal
circuit line by line), resulting in higher costs and limiting their potential for further commercial production.

To avoid these complicated requirements for the process and fabrication of liquid metal circuits and
sensors, a special liquid metal slurry composed of micro/nano-particles (ranging from tens of microns to
hundreds of nanometers) has been reported [31-33] . This slurry can solve the typical processing issues caused
by surface tension from pure bulk liquid metals, making it easier to print or deposit on a wide range of
substrate materials [29,34,35] . However, some subsequent activation operations [36-38] and phase transformation
strategies are needed to break the isolation state among liquid metal particles to activate the overall
conductivity of liquid metal circuits. The question arises as to whether a general processing route can be
developed for such a liquid metal slurry, enabling its use in large-area flexible circuits and sensor arrays on
soft substrates.

In this study, we report a general processing strategy of the liquid metal slurry for flexible circuits and
sensors. The device demonstrated includes a resistive strain-sensing glove with liquid metal sensing units
conformally coated on finger parts, exhibiting excellent strain sensitivity and wearing comfort. The key
sensing circuits are realized by scraping the liquid metal slurry on the mask attached to a nitrile glove, and
the conductivity of the sensing circuit is activated during the process of peeling off the mask. The liquid
metal sensing circuit shows an excellent response to strains (ΔR/R ~35% when the strain is 100%), along
0