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].