In Situ Polymerized MXene/Polypyrrole/Hydroxyethyl Cellulose-Based Flexible Strain Sensor Enabled by Machine Learning for Handwriting Recognition

Abstract

Flexible strain sensors have significant progress in the fields of human–computer interaction, medical monitoring, and handwriting recognition, but they also face many challenges such as the capture of weak signals, comprehensive acquisition of the information, and accurate recognition. Flexible strain sensors can sense externally applied deformations, accurately measure human motion and physiological signals, and record signal characteristics of handwritten text. Herein, we prepare a sandwich-structured flexible strain sensor based on an MXene/polypyrrole/hydroxyethyl cellulose (MXene/PPy/HEC) conductive material and a PDMS flexible substrate. The sensor features a wide linear strain detection range (0–94%), high sensitivity (gauge factor 357.5), reliable repeatability (>1300 cycles), ultrafast response–recovery time (300 ms), and other excellent sensing properties. The MXene/PPy/HEC sensor can detect human physiological activities, exhibiting excellent performance in measuring external strain changes and real-time motion detection. In addition, the signals of English words, Arabic numerals, and Chinese characters handwritten by volunteers measured by the MXene/PPy/HEC sensor have unique characteristics. Through machine learning technology, different handwritten characters are successfully identified, and the recognition accuracy is higher than 96%. The results show that the MXene/PPy/HEC sensor has a significant impact in the fields of human motion detection, medical and health monitoring, and handwriting recognition.

Highly Stretchable, Highly Sensitive, and Antibacterial Electrospun Nanofiber Strain Sensors with Low Detection Limit and Stable CNT/MXene/CNT Sandwich Conductive Layers for Human Motion Detection

“…In addition, electrical stability of the CNT/MXene/CNT/TPU strain sensor is further characterized by the I – V curve under different stretching (Figure S6) and the Δ R / R 0 -strain curve at different stretching rates (Figure S7), which enable the sensor to accurately and timely transmit the signals captured in human motion detection to back-end processing equipment. Additionally, the performances of the CNT/MXene/CNT/TPU strain sensor and recent crack-based strain sensors are compared in terms of sensing range and sensitivity (Figure h), ,,− and the detailed parameters are shown in Table S1. It can be found that crack-based strain sensors are generally unable to combine a wide sensing range and high sensitivity.…”

“…0D nanoparticles (e.g., metal nanoparticles) have weak inter-particle forces and are at risk of peeling off during long-term use of the sensor . 1D nanomaterials [e.g., carbon nanotubes (CNTs) and metal nanowires] have a large aspect ratio, and the conductive network is better maintained during stretching, exhibiting a high stretching range and low sensitivity. , 2D nanomaterials (e.g., MXene and graphene) have a lamellar structure with a relatively low aspect ratio and cannot slide relative to each other between adjacent sheets, exhibiting high sensitivity and low stretch. , Therefore, it is difficult to achieve comprehensive detection of human motion using a single conductive material. Using different dimensional conductive fillers to form a uniform and continuous cooperative conductive network structure is a common strategy to improve the performance of conductive networks. , …”

“…19,20 2D nanomaterials (e.g., MXene and graphene) have a lamellar structure with a relatively low aspect ratio and cannot slide relative to each other between adjacent sheets, exhibiting high sensitivity and low stretch. 21,22 Therefore, it is difficult to achieve comprehensive detection of human motion using a single conductive material. Using different dimensional conductive fillers to form a uniform and continuous cooperative conductive network structure is a common strategy to improve the performance of conductive networks.…”

Flexible Sensors with a Sandwich Stack Structure of Few-Layer Ti3C2Fx/Polypyrrole Nanowires for Human Motion Detection and Healthcare Monitoring

“…Zhang et al reported a flexible sensor in situ polymerized Ti 3 C 2 F x /polypyrrole/hydroxyethyl cellulose. Polypyrrole (PPy) was introduced as the conductive material to enhance the sensor’s detection range …”

“…Polypyrrole (PPy) was introduced as the conductive material to enhance the sensor's detection range. 46 Herein, we successfully prepared the few-layer Ti 3 C 2 F x by HF/LiF etching, DMSO intercalation, ultrasonic exfoliation, and centrifugal separation. 47 Then, PPy nanowires were polymerized in situ via cetyltrimethylammonium bromide (CTAB) micelles as a soft template under the ice bath.…”