Page 189 - Read Online

Page 4 of 12 Wu et al. Soft Sci 2023;3:35 https://dx.doi.org/10.20517/ss.2023.26

fabricated a strain-sensing glove, as shown in Figure 1A. The strain-sensing glove is associated with a three-
level structure, including the whole strain-sensing glove, the structure of the strain sensor on a single finger,
and components of the liquid metal sensing unit. The SEM image shows the surface morphology of liquid
metal sensing circuits, which are formed by interconnected liquid metal particles [Figure 1A]. In this strain-
sensing glove, the nitrile glove serves as the substrate of the wearable strain sensor, and the liquid metal is
fabricated into circuits and sensing units due to its intrinsic stretchability and ideal conductivity at the metal
level, and commercial silicone sealant encapsulates the main deformation parts of liquid metal circuits to
protect sensing function. Different from the flexible circuit formed by the bulk liquid metal in previous
reports [40,42,43] , the strain-sensing circuit in the glove is formed by numerous liquid metal particles stacked
together, and the interconnections among liquid metal particles result in good conductivity of the whole
[44]
circuit when used as macroscopical strain sensors . As shown in Figure 1B, the strain-sensing glove
exhibits great flexibility and deformability as a general wearable device, which can withstand different
mechanical deformations of twisting, rolling, and stretching.

The fabrication process of the strain-sensing glove is straightforward and low-cost, which is very suitable for
commercial mass production, and the entire manufacturing procedure is shown in Figure 1C. First, we used
a laser to cut out desired sensing circuit patterns on a mask with pressure-sensitive adhesives. Notably, the
pressure-sensitive adhesive is essential for the subsequent scraping operation because it can prevent the
liquid metal slurry from leaking into unwanted areas. As previously reported [34,36,45] , some natural properties
of liquid metals (such as strong fluidity, huge surface tension, etc.) can pose hurdles for processing ideal
continuous circuits. However, methods that prepare bulk liquid metals into forms of micro/nano-particles
can effectively bypass these intrinsic constraints of liquid metal physical properties on their patterning
process [33,46,47] . In this study, we prepared a slurry consisting of liquid metal particles by sonicating bulk
liquid metals in absolute ethanol. To prevent circuit cracks after drying, a PVA solution was added to the
]

liquid metal slurry, which served as a binder [Supplementary Figure 1 [35,41] . Then, we scrape-coated the
prepared liquid metal slurry on the mask attached to the nitrile glove. After the solvent evaporated
completely, we peeled off the mask to obtain an activated conductive liquid metal sensing circuit. Finally,
commercial silicone sealant was used as the encapsulation layer for the sensing circuits, which exhibited
good ductility to match the nitrile glove substrate after curing. The solvents used in this manufacturing
procedure are absolute ethanol and deionized water, causing no harm to the human body and the
environment. Therefore, there are no strict requirements on the operating environment, indicating that this
strategy for manufacturing a strain-sensing glove has great potential for daily use and mass production.

Activation mechanism of liquid metal circuits
As mentioned above, liquid metal circuits obtained by scraping liquid metal slurries are non-conductive
even after the solvent has completely evaporated, which obviously cannot meet the sensing requirements.
Therefore, some activation operations are necessary to obtain conductive liquid metal patterns, and the
mechanism of mechanically activating the liquid metal circuit to restore its conductivity is shown in
Figure 2A. The isolated state of the liquid metal particles is destroyed by external mechanical forces (such as
scratching and stretching operations we used in this study), resulting in the interconnections among
numerous liquid metal particles, which is manifested as the recovery of the electrical conductivity of liquid
metal circuits macroscopically. Notably, the activation effect is permanent and irreversible for restoring the
conductivity; that is, once the external force activates the conductivity of circuits, the conductivity of the
liquid metal traces will always exist. The optical image of the unactivated traces and the SEM image of
corresponding surface morphology are shown in Figure 2B, where the liquid metal particles composing the