Piezoresistive nanocomposite films for foot strike data monitoring

Abstract

It is important to understand the human gait especially for the patients under rehabilitation or in old age. Foot strike data is considered as the most important parameter to analyse gait. Piezoresponsive sensors are mostly used for this purpose. In this study, carbon nanofiber (CNF) based thermoplastic polyurethane (TPU) piezoresistive, nanocomposite films (15%, 20% and 25% w/w) were prepared, characterized and tested to study their suitability for such applications. Scanning electron microscope (SEM) studies on the cross section of nanocomposite films revealed that the CNF is well connected inside the TPU matrix. This well connected conducting network resulted in low bulk resistivity (98 ± 3.2 Ω-cm) and impedance (∼250 Ω). The compressive test results indicate more than threefold increase in strength due to CNF reinforcement. All the films showed excellent thickness recovery under cyclic loading while, 25% (w/w) CNF dispersed nanocomposite films’ showed more than 99% recovery after 100 cycles of cyclic compression. The samples also showed good dynamic mechanical stability from 1 to 10 Hz at body temperature (37 °C). The change in resistance under cyclic compression up to 70% strain was found to be linear with high correlation (R2 = 0.99). The gauge factor (up to 1.40 ± 0.03) shows little variation at different strain regions. A cyclic load setup was designed and fabricated to mimic the cyclic foot strike pattern of human gait. The nanocomposite films were tested at three different cyclic strain levels using the setup and the responses were monitored for 2 h using cathode ray oscilloscope (CRO). It has been observed that with the increase in CNF concentrations, the change in voltage goes up to 911.8 mV. The consistency of voltage response under cyclic compressive loading demonstrated in this work shows tremendous potential for these nanocomposite films to be successfully used for piezoresponsive applications of human gait monitoring.