All-Fabric Piezoionic Sensor for Simultaneous Sensing of Static and Dynamic Pressures
Authors: Homayounfar S. Zohreh , Kiaghadi Ali , Ganesan Deepak , Andrew Trisha L. 
Published: July 2021

The development of flexible and textile-based wearable pressure sensors has provided the opportunity of continuous and real time measurement of human physiological and biomechanical signals during daily activities. Pressure sensors are transducers that convert an exerted compression stress into a detectable electrical signal. Different transduction mechanisms have been introduced so far including triboelectricity, transistivity, capacitance, piezoelectricity, and piezoresistivity. Piezoresistive pressure sensors are the most widely used type due to the simplicity of their structure, and the wide range of materials that can be selected along with low-cost fabrication methods, and easy read-out system required for signal extraction.

A vast majority of piezoresistive sensors developed so far are on-skin sensors developed to detect subtle pressures (1 Pa-10 kPa) for touchpads and electronic skin applications. However, to sense physiological signals such as pulse, respiration, and phonation the sensor range of detection must fall within medium range of 10 kPa to 100 kPa. As expected, for larger-scale human motion detection such as sleep posture and footwear evaluation, the sensor must be able to sense compression stresses larger than 100 kPa. This wide range of detection required by the piezoresistive pressure sensors is one of the important challenges in designing these sensors.

Many of the piezoresistive sensors function based on employing the composite of conductive additives in an elastomer as an active layer. The functionality and sensitivity of these sensors are highly limited by the poor bulk mechanical properties of the elastomer in addition to unbreathability and the complications arising from the skin-sensor interface. Textile-based sensors overcome the issues regarding the elastomer sensors to a good extend. These sensors are mainly developed through coating fibers by conductive inks or intrinsically conductive polymers (ICPs). However, these sensors suffer from major drawbacks. First, the high conductivity of the conductive coatings leads to shortening in signals upon the application of a small amount of pressure. These sensors can respond either to static or dynamic pressures and once being pressed by a pressure, completely lose their sensitivity to further pressure exertions which resembles a connection/disconnection mode of performance. Second, the sensors need to be used in tight-fitting clothing to be able to capture signals which makes it quite uncomfortable and hinders the widespread adoption of the device in society.

Here, we introduce an all-fabric piezoionic pressure sensor, called “PressION”, that successfully overcomes the mentioned drawbacks. We gained significant wide range of pressure detection (several kPa to larger than 100 kPa) through tunning the conductivity of the active layer by coating the fabric with an ion-conductive polysiloxane polymer. This optimal conductivity of the fabric makes the sensor capbale of simultaneously responding to both static and dynamic pressures. To overcome challenge of tight-fitting clothing, we took advantage of the fact that even with loose-fitting garments, there are still some parts of the garment which are pressed due to the exerted pressure by the body limbs over the torso or against an external surface such as bed/chair or even a subtle pressure exerted by a blanket over the body. The success of PressION was confirmed by data acquisition from different body locations (Fig1a-h). All these characteristics makes PressION an ideal candidate to be embedded in daily garments for future biomedical and psychological studies.

-Figure Caption-

A variety of physiological signals can be extracted by gently placing PressION on different locations of the body: (a) artery pulse from the face; (b) phonation from throat; (c) joint motion at the elbow or knee; (d) heartbeat from the chest while lying face down on a bed; (e) steps from the sensor used as an insole in footwear; and respiration from the sensor placed on the (f) back, (g) side or (h) front of the body.

Figure 1