This work reviews the recent progress of multifunctional conductive hydrogels from the aspects of classifications, properties and applications, and the current challenges and the future development strategies are discussed.
Flexible
wearable pressure sensors have attracted great interest
from researchers in recent years because of their important applications
in human–machine interaction, human behavior detection, medical
diagnosis, and other fields. At present, integrating multiple functions
such as pressure and temperature sensing and self-cleaning into a
single material remains a challenging task. Here, by in situ reduction
of graphene oxide (GO) grown on a sponge surface and deposition of
polypyrrole (PPy) nanoparticles, we have built a highly sensitive,
stable, and multifunctional rGO/PPy/poly(dimethylsiloxane) (PDMS)
polyurethane (PU) sponge (GPPS) sensor for the detection of pressure,
water level, and temperature. This multifunctional sensor shows excellent
pressure-sensing performance, ultrasensitive loading sensing of a
leaf (98 mg), and outstanding reproducibility over 5000 cycles. Due
to the stability of the superhydrophobic surface water contact angle
(WCA) = 153.3°, our sensor can work in an underwater environment,
which can sense water levels from 1 cm (∼98 Pa) to 40 cm and
also a variety of underwater behaviors (knock, ultrasonication, blow,
etc.) with high stability. In addition, the sensor can be integrated
into a circuit for the water level and pressure detection. The sensor
can also be used as a smart underwater-temperature sensor; it shows
a linear temperature coefficient of resistance (TCR) of 0.48% °C–1 in a temperature range of 35–80 °C. This
multifunctional sensor shows potential application prospects in wearable
electronic devices for sensing.
Wearable strain sensors have aroused increasing interest
in human
motion monitoring, even for the detection of small physiological signals
such as joint movement and pulse. Stable monitoring of underwater
human motion for a long time is still a notable challenge, as electronic
devices can lose their effectiveness in a wet environment. In this
study, a superhydrophobic and conductive knitted polyester fabric-based
strain sensor was fabricated via dip coating of graphene oxide and
polydimethylsiloxane micro/nanoparticles. The water contact angle
of the obtained sample was 156°, which was retained above 150°
under deformation (stretched to twice the original length or bent
to 80°). Additionally, the sample exhibited satisfactory mechanical
stability in terms of superhydrophobicity and conductivity after 300
abrasion cycles and 20 accelerated washing cycles. In terms of sensing
performance, the strain sensor showed a rapid and obvious response
to different deformations such as water vibration, underwater finger
bending, and droplet shock. With the good combination of superhydrophobicity
and conductivity, as well as the wearability and stretchability of
the knitted polyester fabric, this wireless strain sensor connected
with Bluetooth can allow for the remote monitoring of water sports,
e.g., swimming, and can raise an alert under drowning conditions.
Superhydrophobic
conductive materials have received a great amount
of interest due to their wide applications in oil–water separation,
electrically driven smart surface, electromagnetic shielding, and
body motion detection. Herein, a highly conductive superhydrophobic
cotton cloth is prepared by a facile method. A layer of polydopamine/reduced
graphene oxide (PDA/rGO) was first coated on the cotton fabric, and
then copper nanoparticles were in situ grown on the prepared surface.
After further modification with stearic acid (STA), the wettability
of the cotton surface changed from superhydrophilic to superhydrophobic
(water contact angle (WCA) = 153°). The electrical conductivity
of the PDA/rGO/Cu/STA cotton is as high as 6769 S·m–1, while the stearic acid effectively protects Cu NPs from oxidation.
As a result, the superhydrophobic PDA/rGO/Cu/STA cotton has shown
excellent electrical stability and can be used in detecting human
motions in both ambient and underwater conditions. The sensor can
recognize human motion from air into water and other underwater activities
(e.g., underwater bending, stretching, and ultrasound). This multifunctional
cotton device can be used as an ideal sensor for underwater intelligent
devices and provides a basis for further research.
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