Rational design of wearable strain sensors by employing dual-scale fillers and structural engineering

Abstract

To mimic human’s skin, wearable strain sensors that enable to detect multiple stimuli in the outside environment have been attracted a lot of attentions. One promising way to facilitate the wearable strain sensors processing is to utilise composites combing conductive fillers with deformable polymer matrix. This thesis firstly presents a brief introduction of the research advances of wearable strain sensors. Based on this review, a set of research questions and aims were formulated. The aims of this thesis are rational designs for wearable strain sensors with high sensitivity, high stretchability, low drift and multi-modality.

PDMS composites reinforced by a dual-scale carbon nanofibres network consisting of carbon nanofibres (CNFs) and short carbon fibres (SCFs) were fabricated to obtain a trade-off between sensitivity and stretchability compared to the composites with either CNFs or SCFs only. To further reduce the drift, a sandwich-like strain sensor by embedding 1D carbon nanofibres (CNFs) and 2D graphene nanoplates (GNPs) in a polydimethylsiloxane has been prepared to exhibit a high linearity up to 50% strain, a higher sensitivity and a much better stability in long-term cyclic tests. This can be attributed to the synergistic effect of 1D and 2D nano-carbons in creating a robust conductive work quantitatively analysed by synergy ratios. Moreover, this new highly flexible sensor enables to detect both large and subtle strain deformation of human motions as electronic skins. To achieve multi-modality sensing, wearable strain sensors made of hybrid aerogel of PEDOT:PSS and reduced graphene oxides (rGO) in a PDMS matrix has been presented, exhibiting positive and negative sensitivity to strain and temperature change. To differentiate the strain and temperature by using one single sensor, a new impedance method has also been employed by measuring impedance of this type of hybrid sensor at two different frequencies. Practical application demonstrations to detect various joint motions and the cyclic motion of a linear stage under different temperatures showed promising potentials in soft robotics.

In summary, the strategies by combining dual-scale fillers and structural engineering pave the way to realise rational design of wearable strain sensors for practical applications.