Stretchable Strain Sensors Made of Multi-wall Carbon Nanotube-based Composite
With the advancement of the living standard worldwide, an increasing number of people pay more attention to their health. Consequently, the industry of wearable devices for health monitoring has been experiencing rapid development. Stretchable sensors are the core components for wearable devices. However, further research is still needed to improve performance and reduce cost. In addition, stretchable sensors are currently always employed in other emerging industries, such as robotics, smart textiles, electronic skins and so forth. These fields not only require the sensors to possess excellent sensing performance but also need multi-functionality for practical applications. In this study, two types of stretchable sensors based on MWCNTs/PDMS composite were fabricated and characterized. At first, the mechanism of the conductivity of the CNTs-based composite was theoretically investigated through numerical simulation. The shape of CNTs has a significant influence on the percolation threshold for conductivity, but the electrical conductivity of CNTs merely influences the electrical conductivity of the composite. With regards to the piezoresistivity of the composite, not only does the property of CNTs play an essential role, but also the Poisson’s ratio of the polymer matrix. This work gives some suggestion to select elastic polymer and CNTs. The suitable polymer needs to have lower Poisson’s ratio and CNT should have large aspect ratio. In addition, the volume fraction of CNTs embedded in the polymer is suggested to be slightly more than the percolation threshold. Following these, the composite sensor can acquire better performance. Then, the core material for the stretchable sensor, the MWCNTs/PDMS composite, was made. The common issue in the fabrication process is the agglomeration of MWCNTs in the matrix. This was resolved by using pentane as a solvent. It is attributed to the low boiling point of the pentane, which significantly shortens the evaporation period in the fabrication process. Thirdly, the sandwich-like sensor based on MWCNTs/PDMS composite was fabricated. Due to the encapsulation of PDMS, the stretchable sensor can be biocompatible and mechanically compliant with human bodies. Although the stretchable sensor performed not very well in the aspect of hysteresis, they exhibited excellent sensitivity, response time and durability. Moreover, the stretchable sensor successfully detects simple human motions, like wrist and finger bending. Finally, a stretchable strain sensor based on MWCNTs/PDMS composite with serpentine shape was devised and fabricated. Not only can this sensor distinguish tension strain from transverse and longitudinal direction, but also exhibit good linearity of response to tensile strain. In terms of sensitivity, hysteresis and response time, the stretchable strain sensor showed good performance. Even though this research successfully fabricated the MWCNTs/PDMS composite and corresponding stretchable sensors with good performance, the excellent multifunction of sensors still need to be developed in future work.