1. Introduction
Recent interest exists in materials and fabrication techniques that
enable wearable electronics to be constructed with applications in daily
health monitoring, movement tracking, disease diagnosis, and intelligent
medicine. [1-5] Such technologies utilize the
capability of sensing materials to convert health-related physiological
signals into electrical forms of energy. Pressure sensors, to some
extent, play a pivotal role in intelligent wearable devices. A
significant challenge of conventional pressure sensors exposes the
limitations in narrow detection limit, low sensing performance, and
unsuitability of large-scale production. Suitable choices of sensing
materials, reasonable layout design and integration fashion have access
to establish flexible platforms, in some ways that overcome such
limitations. The exertion of sensors formed in compatibility and
flexibility could offer opportunities for directly monitoring and
long-term stable performance on the soft interfaces.
Effective efforts to the achievement of pressure sensors in high
performance involve electronic materials and mechanics principles. For
example, carbon materials are highly positive candidates for the
construction of pressure sensors, ranging from rigid activated carbon,
graphene, carbon black, to carbon nanotubes.[6-8]However, the binding forces of these materials must be further improved
for practical applications. Additionally, recent works demonstrated that
pressure sensors generated by the conversion of textiles or conductive
hydrogels usually exhibit excellent performance. In some cases, the
sensitivity of micro-pressure monitoring far exceeds the range
attainable to the detection,[9] thereby
photo-lithographically manufacturing microstructures (such as pyramids)
is more easily achieved than non-structured counterparts. Although the
construction of microstructures is intuitively accessible to the
enhancement of sensitivity, there are limitations to the production of
large areas and the cost of per unit area.[10] The
above limitations lead to a better understanding of the selection of
pressure-sensitive materials. Previously reported extensive exploration
of 2D materials in terms of transition metal carbides, carbonitrides,
and nitrides (MXenes) tends to accomplish sophisticated electronic
functions. The chemical composition of MXene is
Mn+1XnTx, where M, X,
and T represent the transition metal, C/N ratio, and surface functional
group, respectively, and n represents the number of surface functional
groups.[11] The practical possibilities of MXene
for the construction of multifunctional devices to advantages in good
electrical conductivity, photothermal conversion ability, rich surface
chemical properties, and large specific surface area are promising. By
comparison with conventional two-dimensional materials (such as GO),
MXene also shows better antibacterial properties, especially in terms of
gram-positive and gram-negative bacteria. However, because of the low
length-to-width ratio, directly assembling MXene nanosheets into the
desired macrostructure with the ability of monitoring in real time is
difficulty, thus limiting their applications.[12]Therefore, MXene displays excellent electrical and mechanical
properties, and can be formed in the formats of large-areas, the
suitable utility of flexible bio-integrated platforms.
Previous studies have confirmed that composites of polymers and MXene
have the potential for flexible sensing owing to good processability and
low cost. In such systems, polymers such as polyvinyl alcohol, polyvinyl
butyral, polydimethylsiloxane, polyvinylidene fluoride, and polyaniline
serve as the monomer with MXene for diverse functional
composites.[13-18] Dopamine (DA) is a natural,
non-toxic biomolecule that mimics the structure of the adhesion proteins
in mussels and seaweed. It has a large number of amine and catechol
functional groups, which are environmentally friendly and can reduce the
health risks when applied to human skin, making it very attractive for
flexible electronic products.[19] DA adhesively
polymerized on the target substrate in situ is well configured for the
protection of the circuit. In addition, DA polymerized into polydopamine
(PDA) leads to the spontaneous formation of hydrogen bonds with abundant
active functional groups.[20] Therefore, the
construction of a three-dimensional (3D) MXene/PDA composite film is
expected to provide flexible pressure sensors with excellent sensitivity
and biosafety.
Here, we report a portable and wearable MXene/polydopamine
(PDA)-composite-film-based pressure sensor, in which the key functional
constituent consists of the molecular structure of intercalated
spherical PDA, thereby contributing to the large-area fabrication
technique and the high-performance operation. Systematic studies of
formulations of composite films with or without polymerization
necessitate the layout design and integration fashion. The sensitivity
of the MXene/PDA-based pressure sensor is up to 138.8
kPa-1 when the pressure range is 0.18-6.20 kPa with
fast response and recovery speed (t1<100 ms; t2<50
ms). The sensor enables sensitive and accurate modes of precise
measurements of various health-related physiological signals in
real-time, involving wrist pulse, finger motions, vocalization and
facial expressions. The results demonstrate that MXene/polydopamine
(PDA)-composite-film-based pressure sensor serve as the basis of
portable and wearable platforms relevant to health monitoring and
prediction of disease diagnosis.