Abstract:

Objective Flexible strain sensor shows broad application prospects in human-computer interaction, electronic skin, intelligent wearables and other aspects. Many researchers have spared no effort to study the materials for the improvement of sensing performance. However, sufficient attention to the applicability of the sensors is still lacking in terms of the sensing parameters such as sensor's size, applied strain, tensile rate. In this study, the polyurethane thread coated with polypyrrole (PPy/PU) filament via in-situ polymerization was used as strain sensor, and the sensor length, strain, and tensile rate were investigated to conclude a suitable parameter for the monitoring of the index finger bending.

Method Because the group of organic molecular is responsive to the infrared light and each group exhibits their unique vibration forms, Fourier transform infrared (FT-IR) spectrometer was used to identify the materials. FT-IR (BRUKER Vertex 70) was used to characterize the groups of the PPy/PU filament with wavenumber from 4 500 to 400 cm^-1. Scanning electron microscope (JEOL JSM-7800F) based on secondary electrons imaging was used to observe the surface morphologies of PU filament and PPy/PU filament. Also, electronic universal material testing machine (Instron 5976) and electrochemical workstation (CHI-660e) were combined to investigate the resistance variation and sensing performance of the PPy/PU filament.

Results The FT-IR characteristic peaks for PU filament were detected, which showed that they almost disappeared after the in-situ polymerization of PPy, indicating the favorable covering of the conductive layer. The FE-SEM images also demonstrated the full deposition of PPy on the PU fibers. The prepared PPy/PU filament exhibited a resistance of 268.9 Ω per centimeter, indicated two linear response region including 0-63% strain and 118%-243% strain with Gauge Factor values of 1.82 and 43.3 respectively, and revealed a short response time (200 ms) for 10% strain. Stretched to various strains, PPy/PU filament with different lengths demonstrated that although different initial spacing (i.e. the spacing between the upper and lower fixtures before stretching) may result in different extensions for the same strain, the changes in relative resistance (ΔR/R₀) were basically on the same magnitude order, namely, the change in ΔR/R₀ was determined by tensile strain rather than tensile length. It was also found that although the same strain required more stretching length for the longer samples, their variation of ΔR/R₀ was actually smaller, possibly because the longer samples would disperse more force, leading to less changes in conductive channels and less damage to the material. As a result, the longer samples with length of 6 cm exhibited lower increase of ΔR/R₀ after 100 cyclic stretching, indicating better stability. However, the low variation of ΔR/R₀ during stretching is adverse to signal analysis. As for the monitoring application, sensing materials need to have significant signal changes and relatively stable peak value of ΔR/R₀, and its length also needs to match the size of the monitored joints. Therefore, PPy/PU filament with functional length of 1 cm was selected for monitoring of index finger bending, which generated evident signals (one signal peak with one finger bending). Besides, similar signals for multiple bending indicated repeatable monitoring performance of this PPy/PU sensor.

Conclusion This study provides a new viewpoint to the applicability of the sensor materials. The sensing performance is not only determined by the micro-properties (such as doping level, crystallinity, conductivity, and so on) of the materials, but also closely related to its macroscopic elements. Thus, the sensor size should be taken into account in order to avoid unstable or unclear signals. As the PPy/PU exhibits great sensing performance and possesses favorable flexibility inherited from the PU filament, PPy/PU filament is of enormous application potential in the wearable electronics field.

Key words: conductive thread, strain sensor, polypyrrole, polyurethane, sensing material, applicability