(Digital Presentation) Harnessing Wide-Range, Highly Stable Pressure Sensitivity Via PEDOT-Cl Vapor Printed Textiles for Health Monitoring
Authors: Homayounfar S. Zohreh , Kiaghadi Ali , Ganesan Deepak , Andrew Trisha L. 
Published: November 2022

The advancement of wearable electromechanical sensors to detect biopotentials and body locomotion is critically important in evaluating human performance and improving off-site care applications. Electromechanical pressure sensors are defined as transducers that transform the mechanical deformations caused by applied pressure to a detectable electrical output. Depending on the transduction mechanism, pressure sensors fall within different categories including triboelectric, transistive, capacitive, piezoelectric, and piezoresistive sensors. Piezoresistive pressure sensors, in which the ultimate electrical output is resistance variations, are the most-widely used class due to the simplicity of read-out system in signal acquisition, simple working mechanism that allows a wide variety of materials to be used along with cost-effective fabrication process. However, their practicality is highly restricted by narrow range of detection (RoD), failure to sense both static and dynamic pressures simultaneously, and low stability against aging phenomena such as cycling abrasions, exposure to perspiration and washing. By taking advantage of vapor deposition of a p-doped conjugated polymer, poly(3,4-ethylenedioxythiophene):chloride (PEDOT-Cl), we introduce an ultra-stable pressure sensor that reveals high sensitivity in detecting real-time signals in such a wide range of pressures that has not been reported before (from heart beats to more than body weight). This one-step technique allowed us to tune the conductivity and subsequently sensitivity of the sensor accordingly. We observed that the sensor enjoys a multiscale working mechanism, in which we have the putative decrease in the thickness and therefore reduction in the air trapped in the free volume of the active layer in mili-centmeter scale, the percolation pathways in micro-milimeter scale, and piezoionic effect in nanometer scale which means that the redistribution of ions under the applied mechanical stress leads to the change in resistance. This multiscale sensitivity is the key to its broad RoD along with its ability to simultaneously detect subtle dynamic and static pressure in the presence of a base pressure. We developed two sensors: one with PEDOT-Cl coated cotton fabric and one with PEDOT-Cl coated cotton ball as the active layer. In fact, the ordered structure of fabric and the disordered structure of cotton ball play the role of a lattice for percolation; providing the structure with points of connectivity for charge carriers. As expected, the disordered nature of the cotton ball leads to a higher number of points of connectivity and therefore, lower range of variation and higher precision in data acquisition. While taking advantage of the presence of ions in our sensor, we protected the sensor against all the humidity-induced degradations entangled with ions and other aging processes via vapor deposition of Fluorinated hydrophobic moieties on all the sensor layers. With this protective coating, the sensor shows less than no change in resistance and sensitivity after staying in ~100% humidity for more than a week, and can stand more than 20 laundry cycles without any drop in signal quality. Also, it displays ultrastability with 99.21% over 70,000 bending cycles in ambient conditions, exceeding the durability cycles of sensors reported previously. The broad ability of this sensor was further confirmed by acquiring physiological signals and body motions such as heartbeats, respiration, Joint movements and step. All these properties, along with the low-cost and robust fabrication process, bears the testimony that this sensor will be uniquely placed in wearable health monitoring electronics for both diagnostic and treatment applications.
*Figure Caption: a) Schematic illustration of the sensor including the SEM image of the PEDOT-Cl coated layer, b) Physiological and body movements signals acquired via sensor, c) Sensitivity of the sensor over a wide range of pressures

Figure 1