Fully Printed Flexible Pressure Sensor with a Spontaneously Formed Porous Conductive Architecture

The flexible pressure sensor is highly desired for next-generation wearable electronics and intelligent robots [1]. Recently, piezoresistive pressure sensors have attracted many interests due to their ability in both dynamic and static force detection, simple device structure, and easy signal processing [2]. Constructing a porous conductive network is a promising approach to realizing high-performance pressure sensors. Previously, researchers use a commercial sponge or a sacrificial template (sugar, salt, or polymer particles) to construct a porous conductive network, in which the conductive materials such as carbon nanotube (CNT), carbon black (CB) are either dispersed in the polymer matrix or decorated on a porous wall [3, 4]. However, these methods have the challenge in patterning and scalable fabrication, while removing templates is time-consuming, and the thickness of the fabricated sensors is usually on the millimeter scale.<br/><br/>In this study, we will present a fully printed flexible pressure sensor using a novel polydimethylsiloxane/carbon black (PDMS/CB) composite that could spontaneously form a porous conductive architecture [5]. The printable ink was created by simply mixing the PDMS, CB, and deep eutectic solvent (DES). It could be directly printed on a flexible substrate using stencil printing. After a pre-annealing at 75 °C to cure PDMS, and a final-annealing at 140 °C to remove the DES, a 3D porous conductive structure was formed in the printed composite film. Our experiments revealed that the DES induces phase segregation between the PDMS and CB and serves as a liquid template to form the porous structure during the annealing process. A printed pressure sensor with a sensing layer as thin as 200 µm exhibited a high pressure sensitivity of 0.014 kPa^-1 and a wide working range up to 100 kPa, as well as excellent reliability and stability. We have successfully demonstrated our printed pressure sensor in several potential practical applications. The sensor attached to the finger could detect the touch forces and the hand gesture, which has the potential for developing smart gloves. Furthermore, the sensor also can be applied in the robotic tactile device, to monitor the grasping process of a robot gripper. In addition, different from other reported “cut-and-paste” methods, a high-resolution flexible pressure sensor array could be directly fabricated by printing.<br/><br/>Compared with the previously reported flexible piezoresistive pressure sensors, the new composite ink has significant advantages: i) prepared by simple mixing; ii) facile for the scalable and patternable stencil printing; iii) spontaneously form a porous conductive network without any complex post-treatment. This work provided a straightforward solution to realize a high-performance flexible pressure sensor. We anticipate its practical applications in wearable devices, human-machine interfaces, and robot tactile feedback systems.<br/><br/>[1] B. Shih et al., <i>Sci. Robot.</i> <b>2020</b>, 5, eaaz9239.<br/>[2] S. R. A. Ruth et al., <i>Adv. Funct. Mater.</i> <b>2020</b>, 30, 2003491.<br/>[3] Y. Song et al., <i>Small</i> <b>2017</b>, 13, 1702091.<br/>[4] H.-B. Yao et al., <i>Adv. Mater.</i><b> 2013</b>, 25, 6692.<br/>[5] Y.-F. Wang et al., <i>Adv. Mater. Technol.</i> <b>2021</b>, 2100731.