Flexible Sensor based on Three Dimensional Polymer Nanocomposites

Flexible strain sensor, an electronic device that is sensitive to the applied strain, has the advantage of ordinary one for excellent monitoring of mechanical force, at the same time processes a good flexibility and deformation capacity, thus have become an ideal material for wearable electronics due to their simple structure and facile fabrication process. Traditional strain sensor due to the rigid conducting basement is incompressible and tough resulting in a mechanical mismatch that limits the functionality of these devices for wearable electronics. To overcome these challenges, an ideal strategy to achieve fully flexible sensor is to enable the conducting basement themselves to be deformable, which possess the ability to maintain good electrical function under deformation. Polymer nanocomposites filled with carbon nanotubes (CNTs) have attracted much attention to satisfy the material requirements for flexible sensors. Much progress has been made in this field with varying degree of success, however, if the material was effectively used as reinforcement, proper dispersion of CNTs into the polymer matrix had to be guaranteed. In addition, there was a concern that dispersed CNTs had a tendency to re-agglomerate in the polymer matrix during the processing of nanocomposites, especially for those using thermosetting and rubbery matrices which generally required a curing process under a raising temperature. In this context, CNTs with three-dimensional (3-D) structures, such as foam, aerogel, sponge, were developed in recent years to overcome these problems, furthermore, 3-D structure endow CNTs more space to deform under strain.

In this view, researchers from Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, developed a new fabrication for flexible polymer-based nanocomposites reinforced by CNT foam conducting basement and explore their electrical properties and piezoresistive property. In their approach, CNT foam with 3-D structure was fabricated via CVD process by using cotton as a template to solve the problems arising from the dispersion and possible reagglomeration of nanofillers in a polymer matrix. The results showed that by using cotton as a template, CNTs were grown on the carbonized cotton fiber, forming hierarchical structures with excellent electrical conductivity, structure stability, hydrophobic and absorption performance, thus providing a convenient way to prepare nanocomposites by monomer self-diffusion. For polymer matrix, polydimethylsiloxane (PDMS), a kind of silicone-based elastomers, was chosen as the elastomer matrix on the basis of high elasticity and flexibility over a broad range of strains. In nanocomposites, the introduction of CNTs onto the carbonized cotton fiber significantly enhances the piezoresistive performance of nanocomposites with high reliability and sensitivity. The CNT foam nanocomposite possessed a wide sensing range in the compression strain of 0%-80%, a high sensitivity of 10⁴ at 60% strain.

In summary, a facile method to prepare CNT foam with excellent electrical conductivity, structure stability, hydrophobic and absorption performance was developed, and this method solved the tough issue of CNT dispersion. The novel 3-D structure of CNT foam was able to deform under compression, endowing nanocomposites a wide response range and high sensitivity for external mechanical load.