【Research Background】

Flexible pressure sensors with high sensitivity, fast response, easy integration, and low energy consumption are the keys to human-computer interaction and wearable electronic devices. These sensors can be integrated into non-invasive and continuous health monitoring equipment and motion detectors. In general, pressure sensors can be divided into the following four categories according to their sensing mechanism: resistance changes (piezoresistance) caused by pressure / deformation, capacitance, piezoelectricity and triboelectricity. Among them, the piezoresistive pressure sensor has received extensive attention because of its wide detection range, simple manufacturing, and convenient signal acquisition, and is of great significance to practical applications. Although the working mechanisms of these four sensors are different, they are all composed of flexible substrates, active materials and conductive electrodes. Active materials are the most important components in the structure of flexible pressure sensors and need to meet certain requirements, including mechanical flexibility, adjustable metal / semiconductor properties and porous structures. Although carbon-based nanomaterials such as graphene and carbon nanotubes (CNTs) have pressure properties, they have high costs and low productivity. Therefore, a new type of sensing active material is needed to realize a flexible piezoresistive pressure sensor.

Because of its two-dimensional structure, high conductivity, easy processing, and controllable synthesis process, MXenes is a promising electrode material for pressure sensors. Ti 3 C 2 T x is the most widely studied MXene, which has a wide range of uses, including energy storage, electromagnetic shielding, catalysis, photoelectricity, and sensors. Although the pure MXene and MXene mixtures described above achieve high strain coefficients over a large pressure range and fast response time, there are still three major challenges to be solved. First, pure MXene membranes and MXene-based composite membranes have limited elasticity; second, MXene‘s excellent conductivity makes the initial current value very high, resulting in a small change in the overall current, which limits the measurement coefficient of the piezoresistive sensor; The hydrophilic properties of MXene or inorganic MXene mixtures can cause significant changes in the initial resistance of MXene-based pressure sensors, which affects the stability of the device. In fact, the use of pressure sensors made of organic / inorganic composite materials to combine the elasticity of organic polymers with the conductivity of inorganic MXene sheets and combine it with an insulating polymer matrix can be considered as a solution to the above A wise strategy for the problem.

【Achievement Introduction】

Recently, Professor (Drexel University) Yury Gogotsi Jilin University professor Han Wei and Drexel University in internationally renowned academic journal ACS Applied Materials & Interfaces published an article on the subject is: Hydrophobic Polymer and Stable MXene-Pressure Sensors for Wearable Electronics Research In the paper, a hydrophobic organic / inorganic composite membrane was prepared by spin coating using natural elastic P (VDF-TrFE) and multilayer Ti 3 C 2 T x , and a stable piezoresistive pressure sensor was prepared using it. The pressure sensor exhibits assembled 817.4 kPa in the range of 0.072 to 0.74 kPa ^-1 high coefficient measurement, display 2213.68kPa in the range 0.74 to 3.083 kPa to ^-1 , having a fast response time of 16 ms and more than 99% Long-term stability. Its performance in the surrounding air has been maintained for more than 20 weeks, which is a challenge in sensing applications. This is the first demonstration of Ti 3 C 2 T x Long-term stability of MXene sensors. Its excellent stability is due to the fact that the multilayer MXene particles are mostly wrapped in an organic P (VDF-TrFE) polymer network, which protects MXene from oxidation. In order to verify the practicality of the assembled pressure sensor in the field of wearable electronics, the electromechanical characteristics for speech recognition and human activity detection were further studied. At the same time, a 10 × 10 integrated sensor array platform was built to demonstrate the pressure distribution function of the mapped space, record the motion occurring on the sensor array, and calculate the current change. This can be extended to speed measurement because knowing the path of movement and the time to write certain lines on the sensor array. By further optimizing the mechanical properties of the sensor and covering the road with a layer of low-power sensor arrays, it can even become a viable alternative to speed cameras and radar speedometers on highways. The co-first authors of this article are Dr. Li La and Fu Xiyao.

【Graphic introduction】

Figure 1. Preparation flow chart and physical characterization of Ti 3 C 2 T x @P (VDF-TrFE) composite membrane.

Figure 2.  Electromechanical performance of a sensor based on Ti 3 C 2 T x @P (VDF-TrFE) with a mass ratio of 1: 1.6.

Figure 3.  Speech recognition and human activity detection.

Figure 4. Test performance of a 10 × 10 pressure sensor array platformbased on a Ti 3 C 2 T x @P (VDF-TrFE) film witha mass ratio of 1: 1.6.

【Summary of this article】

     In this paper , a piezoresistive pressure sensor and sensor array are fabricated based on the hydrophobic inorganic / organic system structure of the mixed Ti 3 C 2 T x @P (VDF-TrFE) thin-film electrode. Due to the long-chain structure, viscoelasticity, insulation and hydrophobicity of PVDF-TrFE polymer, the resulting Ti 3 C 2 T x @P (VDF-TrFE) film exhibits good flexibility and adjustable initial resistance in air And stability. When the mass ratio of MXene to P (VDF-TrFE) is optimized to 1: 1.6, the sensor shows a higher strain coefficient, ranging from 0.072 to 0.74 kPa to 817.4kPa ^-1 , from 0.74 to 3.083kPa to 2213.68 kPa ^-1 , fast response time is 16 milliseconds, and can maintain more than 99% stable performance after 20 weeks in ambient air. The assembled pressure sensor is installed on the human body to monitor physiological signals, including speech recognition, muscle movement and real-time pulses. At the same time, a large area integration of 10 × 10 sensor arrays is designed to plot the spatial pressure distribution, record the speed of movement, and calculate the current change at the same time.

Literature link:

https://dx.doi.org/10.1021/acsami.0c00255 .

Source: MXene Frontier