Design and optimization of nanostructured flexible sensors

Nowadays, there is a growing interest for flexible strain and pressure sensors due to the increasing demand in several applications fields like industrial, automotive, robotics, biomedical and many others. Conventional sensors have excellent sensitivities but show severe limitations due to their mechanical properties and poor flexibility, which can be overcome by using nanostructured materials. Nanostructured materials show unique and excellent properties with respect to their macroscopic counterparts. Their most important property is the high surface area to volume ratio, which offers interesting possibilities in several applications, as the improved sensitivity, electrical and mechanical performances in flexible sensors. Inside the nanoscale world, nanofibers by electrospinning are an emerging class in the sensing field, since they can be obtained by a simple, low cost and versatile process if compared to other types of nanostructures manufacturing methods. Especially, polymer-based nanofibers and composite nanomaterials, mostly based on carbon nanotubes (CNTs), have been largely employed as functional materials inside flexible piezoelectric and piezoresistive strain and pressure sensors, respectively. Poly(vinylidene fluoride) (PVDF) is, among all polymers, the most used one because of its piezoelectric properties that can be enhanced by nanoconfinement during the electrospinning nanofiber deposition. On the other hand, polymer/CNTs composite nanofibers exhibit a significant change in their resistivity with strain and therefore, provides a great potential to be used in various sensing applications. The aim of this thesis is the fabrication and characterization of two types of electrospun nanofibers mats: a piezoresistive one, made of CNTs/polymer nanocomposite, and a piezoelectric one, made of PVDF. Moreover, this thesis investigates the role of electrospinning process parameters on the final properties of functional NFs. Along with conventional parameters, rotating drum collectors with different diameters are analyzed to determine their effect on the final morphology, electrical performance and piezoelectric or piezoresistive response of the resulting functional nanofibers. Finally, it is also introduced the possibility of combing both piezoelectric and piezoresistive mechanisms in one single flexible sensing platform in order to allow the detection of static and highly dynamic signals.