Investigation of the Mechanical Reliability of a Velostat-based Flexible Pressure Sensor Anis Fatema, Ivin Kuriakose, Deeksha Devendra, Aftab M. Hussain * , Member, IEEE FleCS Lab, Center for VLSI and Embedded Systems Technology (CVEST), International Institute of Information Technology, Hyderabad, India * Email: aftab.hussain@iiit.ac.in Abstract—The technological advancements in healthcare mon- itoring devices, automation, consumer electronics, and soft robotics have resulted in extensive research in flexible pressure, force, and tactile sensors. Piezoresistive sensors are the most widely used flexible pressure sensors due to their low-cost fabrication, high flexibility and simple data-acquisition circuits. In this paper, we report the bending response of a velostat- based flexible pressure sensor by examining its reliability when subjected to repeated mechanical stress. The observed deviation in output voltage was 0.95% for 15 mm, 0.95% for 20 mm, 0.97% for 25 mm, and 2.2% for 30 mm bending radii, for 150 bending cycles, with respect to the flat position. We present a two-parameter (a, b) calibration for the pressure sensor with a fixed bias resistance in the readout circuit. This model can be used to further minimize the deviation due to bending cycles. The results obtained from the experimental research have shown a practical possibility of implementing velostat-based sensors for both static and dynamic flexible systems. Index Terms—Reliability; piezoresistive; pressure sensor; me- chanical stress; velostat I. I NTRODUCTION Pressure sensors are commonly used for tracking and eval- uating pressure in various applications which include but are not limited to aerospace [1], [2], automobiles [3], [4], biomedical [5]–[8], consumer and portable electronics [9]– [13], environmental monitoring [14], [15], industrial [16], [17], robotics [18], [19] and wearable electronics [20], [21]. Based on the sensing principle employed, pressure sensors can be categorised into capacitive [22], optical [23], piezoelectric [24], [25] and piezoresistive pressure sensors [26]–[28]. Capacitive pressure sensors use a thin diaphragm as a sensing element. They are most frequently selected for their better tolerance to temperature drift, high scalibility for minia- turization, low-power consumption, large dynamic range and better sensitivity [22]. However, they require complex readout circuitry and are better suited for measuring low pressures [29]. Optical pressure sensors have great attributes such as high sensitivity and outstanding immunity to interference. However, their bulky system configurations hinder their ap- plications in many areas where flexibility is a requirement [30]. Piezoelectric sensor systems have advantage of dual energy flow, i.e, they can be used as a sensing element and also an energy generating element. They are generally employed for measuring high dynamic pressures. However, these sensors require sophisticated fabrication tools and a complex electronic interface. Piezoresistive pressure sensors use different types of materials and structures that can be used in large array of applications. They have become a dominant category in pressure sensing because of their facile fabrication, low-cost, simple signal processing circuitry and standard data acquisition process [26]. Flexible piezoresistive materials have demonstrated greater advantages in easy deployment, which is of particular importance for biomedical, soft robotics and human-machine interacting systems [31]. This is because the flexibility of any system can be determined by the material used and the thickness in the direction normal to the axis of flexing [32]. With the increase in demand for flexible pressure sensors, a piezoresistive material named velostat has been extensively ex- plored due to its flexibility which is well suited for mechatron- ics and biomedical applications [33]. It is an elastic polymer impregnated with carbon black to enable electrical conductiv- ity. It is lightweight, heat sealable, flexible, and foldable. It can be used in various applications due to its low cost and easy handling process. Hopkins et. al used a velostat-based pressure sensor for in socket pressure sensing. The system demonstrated utility for assessing contact and movement patterns within a prosthetic socket, potentially useful for improvement of socket fit, in a low cost, low profile and adaptable format [34]. Athavale et. al custom designed a 4×2 configuration velostat- based sensor array to assess electrode contact pressure during in-vivo recordings in the gut. The designed array showed better response and was more repeatable than a flexiforce A201 sensor [35]. In [33], [36], the sensitivity, repeatability and hysteresis have been investigated when the sensor is placed on a flat surface. In our previous work [26], we have reported different static and dynamic characteristics of the sensor in a4×4 flexible pressure sensor array when placed on a flat surface. In this work, we present a systematic study to understand the influence of repeated mechanical stress on the character- istics of a velostat-based resistive pressure sensor. We present a mathematical model to parameterize the reliability of a piezoresistive pressure sensor. We have studied the effect on the response of the sensor after being subjected to 150 bending cycles for up to 16 hours. This will help us understand whether the sensor can be used on curved surfaces like tubes, wearable hand gloves, textiles, human skin etc. and will enable researchers to use these sensors with the awareness of its advantages and limitations in different applications. 978-1-6654-4273-2/22/$31.00 ©2022 IEEE 2022 IEEE International Conference on Flexible and Printable Sensors and Systems (FLEPS) | 978-1-6654-4273-2/22/$31.00 ©2022 IEEE | DOI: 10.1109/FLEPS53764.2022.9781575