Highly stretchable capacitive strain sensor enhanced with Barium titanate silicone elastomer composite

Abstract

Wearable electronics and soft robotics are emerging fields using soft and stretchable sensors for a variety of wearable applications. In this research, the performance of highly stretchable interdigitated capacitive (IDC) strain sensor was investigated, by fabricating sandwiched printed carbon black/Ecoflex interdigitated capacitive electrodes between stretchable Barium titanate (BTO)-silicone elastomer composite substrates. The different sizes of the BTO's nanoparticles like 100 nm or 200 nm BTO ceramic particles was dispersed uniformly in an Ecoflex™ 00-30, a silicone-based elastomer. The substrate of an IDC sensor with 100 nm particle size offers a higher relative permittivity and slightly higher change in capacitance with strain. It is demonstrated to be less reliable than an IDC sensor substrate made with 200 nm BTO nanoparticles. In this research, the influence of viscoelastic behaviour like stress relaxation, creep and strain rate dependent behaviour of BTO-Ecoflex composite substrate on the output signal of an IDC strain sensor was studied. The generalized Maxwell‐Wiechert (GMW) model was used to study the stress relaxation behaviour of BTO Ecoflex composite by varying the particle loading by 0, 10, 20, 30, and 40 wt% of 200 nm BTO ceramic particles embedded in an Ecoflex silicone‐based hyperelastic elastomer. Analysing the results shows that a pristine Ecoflex silicone elastomer is predominantly a hyperelastic material; the addition of BTO made the composite behave as a visco‐hyperelastic material. However, this stress relaxation behaviour of BTO-Ecoflex composite substrate was shown to have a negligible effect on the electrical sensing performance of the IDC large strain sensor, because the spacing between the interdigitated capacitive electrodes remains to be constant at a constant strain during the stress relaxation tests. The generalized Kelvin-Voigt (GKV) model was used to study the creep behaviour of the sensor’s substrate material, fabricated using silicone elastomer (Ecoflex 00-30) with barium titanate (200 nm BTO) filler. The results showed that the pristine Ecoflex silicone elastomer is predominately a hyperelastic material, which shows negligible creep, while the addition of BTO particles led to the composite exhibiting creep such that the composite behaves like a visco-hyperelastic material. Hence, this behaviour results in the creep affecting the electrical sensing performance of the capacitive strain sensors during static loading conditions, because the spacing between the interdigitated capacitive electrodes increases with increasing strain at constant load during creep tests. The creep tests’ information provides insights on the impact of composite composition on creep-resistance and output signal of the IDC strain sensor (capacitance). The non-linear elastic strain rate dependent behaviour of BTO-Ecoflex composite is studied by using Ogden model. The uniaxial tensile tests at strain rates of 5, 50, and 500 mm/min show a decrease in non-linearity of the stress-strain response of the composite, with an increase in filler loading; also, there is no strain rate effect on the performance of the IDC sensor fabricated with 40 wt% 200 nm BTO-Ecoflex composite substrate. The distribution and sedimentation of the BTO filler in Ecoflex is studied by performing scanning electronic microscopy (SEM) analysis on the cryo-fractured pristine Ecoflex and 10, 20 30 and 40 wt% of BTO-Ecoflex composites, where it was found that, due to the surplus BTO filler in the polymer, the distribution of the filler is more homogeneous in the Ecoflex polymer at higher filler loading levels like 40 wt% 200 nm BTO. The screen-printing technique was also successfully used to demonstrate the possibility of mass production of an IDC strain sensors, with 40 wt% 200 nm BTO-Ecoflex composite for large scale industrial production. An IDC sensor was therefore fabricated based on a 40 wt% 200 nm BTO-Ecoflex composite and mounted on an elastic elbow sleeve and equipped with electronics, and successfully functioned as a reliable and robust flexible sensor. This demonstrated an application to measure the bending angle of the elbow at slow and fast movement of hand. A linear relationship with respect to the elbow bending angle was observed between the IDC sensor output signal under 50% strain and the bending of the elbow of hand indicating its potential as a stretchable, flexible and wearable sensor.