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.