Page 188 - Read Online

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

with stability and fatigue resistance during long-term use. Furthermore, there is no obvious damage even
when the temperature is increased by 100 °C. Unlike previously reported liquid metal-based strain sensors
fabricated with the help of pre-patterning metal films [39,40] or those fabricated on extra substrates and then
fixed on joints [35,41] , the strain sensor manufacturing strategy proposed in this manuscript is undoubtedly
suitable for large-scale, low-cost, and feasible wearable electronics with a broader range of substrates. For
practical demonstration, we used this strain-sensing glove to monitor different finger gestures in real time,
effectively displaying even slight gesture changes during this period. Additionally, we manipulated the right
machine hand to achieve motions symmetrical to the left hand wearing the strain-sensing glove, potentially
offering an alternative to accomplish the task of human-machine collaboration.

EXPERIMENTAL
Preparation of the liquid metal slurry
In this study, we added 10 g liquid metals (EGaIn, Dongguan Huatai Metal Material Technology Co., Ltd.)
to 10 g absolute ethanol and sonicated the mixture for ten min (2 s on and 2 s off) with an ultrasonic cell
pulverizer (JY92-IIN, Shanghai Huxi Industry Co., Ltd.) to obtain a liquid metal particle dispersion. The
liquid metal particle dispersion was left standing in a room environment until obvious stratification
occurred. The upper layer of liquid shows a lighter color, and the lower layer is the precipitation composed
of the liquid metal particles we need. After we removed the upper layer of the light liquid mixture, 4 mL
polyvinyl alcohol (PVA, CAS:9002-89-5, Shanghai Macklin Biochemical Technology Co., Ltd.) aqueous
solution (5% w/v) was added to the container and stirred well until a homogeneous liquid metal slurry was
obtained.

Fabrication of strain-sensing gloves
Nitrile gloves (Shanghai AMMEX Corporation), commonly used in laboratories, were used as the main
body of the strain-sensing glove, acting as the substrate. To prepare a mask for scrape-coating, we first stuck
the double-sided adhesive tape (Crown 7972, Shenzhen Xinst Technology Co., Ltd.) on a thin PET film and
then used a laser (LPKF ProtoLaser U4) to cut out the desired patterns. Next, we tore off the protective film
of the double-sided adhesive tape and directly adhered the entire double-sided adhesive tape to nitrile
gloves. Thanks to pressure-sensitive adhesives, such masks exhibited good conformality with the substrate
material, which can avoid the leakage of the liquid metal slurry during scraping operation. After the liquid
metal slurry was uniformly scraped and coated on the mask, we placed the entire sample in a room
environment and allowed the solvent to evaporate entirely, naturally leading to the drying of the slurry. It is
worth noting that the liquid metal traces obtained after the solvent evaporates are insulating. However, the
inevitable stretching deformation on the glove during the process of tearing off the mask will activate the
conductivity of the liquid metal circuit. Subsequently, we encapsulated the liquid metal circuit with a
commercial silicone sealant (Kafuter K-705).

Characterization and test methods
We plated the unactivated liquid metal traces, the scratch-activated liquid metal traces, and the stretch-
activated liquid metal traces with gold for 60 s using a sputtering system (Quorum Q150T Plus) and then
used a scanning electron microscope (SEM) (Quanta™ 450 FEG, FEI Company) to observe and capture
surface morphology of these liquid metal traces. Tensile, compression, and fatigue tests performed on liquid
metal sensing circuits are realized using a sliding table driven by a stepping motor. In the heating test, we
used a hot stage to heat the sample to a specified temperature and kept it at that temperature for a certain
period. After the resistance became relatively stable, we measured the resistance of the sample. The
resistance of the liquid metal sensing circuit was measured using a multimeter test system (DAQ6510,
TEKTRONIX, INC.) in all characterization tests.

RESULTS AND DISCUSSION
Structures and fabrication of the strain-sensing glove
To achieve soft, stretchable, and durable strain sensors for monitoring finger gestures, we designed and