Enabling Negative Pressure Sensing Through Ferroelectret Device

We report a polypropylene (PP) based ferroelectret device with quasi-piezoelectricity features to measure negative pressures underwater. The device consists of a foam-structured middle layer with giant electric dipoles between two solid PP films. The quasi-piezoelectricity originates from the relative movement between two oppositely charged surfaces under externally applied pressures, which causes a redistribution of surface charges. In this configuration, negative pressure sensing is realized due to the pressure difference caused by the air cavities, which leads to an expansion of the foam and generates an output voltage proportional to the applied pressure. Finite element method (FEM) simulation and experimental validation are both carried out to confirm the mechano-electrical response under negative pressures. Further investigation reveals that the outputs of open-circuit voltage and short-circuit current follow the opposite behavior as that under positive pressures. The proposed tactile sensors can be potentially implemented in wide applications ranging from pneumatic robots to underwater assessment of invasive species.

I. Introduction

Flexible tactile sensors have attracted increasing attention due to their excellent adaptivity to arbitrarily curvilinear surfaces. A variety of emerging applications, such as structural health monitoring [1], tangible user interaction [2], and human–robot interactions [3], are envisioned by the continuing efforts made in this field toward high sensing performances with more accurate real-time tactile information acquisition, higher spatial-resolution pressure mapping, and instantaneous haptic profiles visualization [4]–[6]. Positive pressure (or compression) sensing has been the main focus of research and technological progress since it is the most typical type of pressure in direct human interactions. However, negative pressure (or partial vacuum) sensing is necessary for some applications, such as pipeline leakage detection [10] and monitoring of underwater species [7], [8]. Furthermore, enabling flexible pressure sensors capable of sensing negative pressures is imperative for realizing the ultimate goal of imitating or even augmenting the functionalities of the human sensory system [9].