Polymers and papers, which exhibit piezoelectricity, find a wide range of applications in the industry. Ever since the discovery of PVDF, piezo polymers and papers have been widely used for sensor and actuator design. The direct piezoelectric effect has been used for sensor design, whereas the inverse piezoelectric effect has been applied for actuator design. Piezo polymers and papers have the advantages of mechanical flexibility, lower fabrication cost and faster processing over commonly used piezoelectric materials, such as PZT, BaTiO₃. In addition, many polymer and paper materials are considered biocompatible and can be used in bio applications. In the last 20 years, heterostructural materials, such as polymer composites and hybrid paper, have received a lot of attention since they combine the flexibility of polymer or paper, and excellent pyroelectric and piezoelectric properties of ceramics. This paper gives an overview of piezoelectric polymers and papers based on their operating principle. Main categories of piezoelectric polymers and papers are discussed with a focus on their materials and fabrication techniques. Applications of piezoelectric polymers and papers in different areas are also presented.

In this paper, Lead Zirconate-Titanate (PZT)/Polydimethylsiloxane (PDMS) based composite is fabricated and characterized for its potential application as a flexible piezoelectric material. Thin films of the composite are prepared by dispersing different volume percentage of PZT particles in cross-linked PDMS matrix. Corona poling of the films is performed to increase the piezoelectric charge constant. X-ray diffraction results indicate that poling aligns the randomly oriented ferroelectric domains in the PZT particles along the poling axis. Piezoelectric voltage response and piezoelectric charge constant values before and after the poling show the improvement in piezoelectricity due to poling. The piezoelectric charge constant, d33 and magnitude of relative permittivity of the films increase and flexibility of the films decrease with the increase of PZT particle volume percentage in the composite. Results show that for 28 v% PZT composite poled film a d33 of 78.33 pC/N was achieved with a Youngs modulus of around 10 MPa and magnitude of relative permittivity of 10. The film demonstrates effectiveness for tactile sensing by generating charges across the film proportional to the finger pressure applied on them. The easy fabrication process makes the composite a promising candidate to build flexible low cost tactile sensing devices.

Printed electronics employing flexible substrate offers prospective features for various applications such as, tactile sensing, energy harvesting, wearable electronics and acoustic wave sensors. In this work, an acoustic FPW (flexural plate wave) sensor is printed on thin and flexible PZT-PDMS (lead zirconate titanate-poly dimethyl siloxane) composite film with silver ink. The prototype FPW resonator has a resonant frequency of 22.65 MHz with an attenuation of -1.552 dBm. Gravimetric mass sensitivity of the sensor was measured by applying PDMS layers in between the input and output interdigital transducers (IDTs). The mass sensitivity was measured to be -7.8 cm ² /g. The sensor is highly responsive to VOCs (volatile organic compounds) with PDMS as a sensing layer. Gas sensitivities with acetic acid and toluene concentrations were measured to be 0.66 and 160.63 kHz/ppm, respectively. The limit of detection for acetic acid and toluene were 10.9 and 0.03 ppm, respectively. Further, the sensor shows good repeatability and response time with both the VOCs. The thermal stability and high piezoelectric charge coefficient of PZT-PDMS composite compared to other piezoelectric composite and polymer substrates are advantageous for the flexible printed sensor. The reported FPW gas sensor shows potential for low concentration measurement of VOCs.

Temperature must be accounted for in order to provide accurate measurements in acoustic wave resonator based gas sensors. We present a flexible printed flexural plate wave (FPW) sensor for CO ₂ monitoring employing temperature compensation. The FPW sensor is based on a differential interdigitated transducers (IDTs) structure consisting of an input IDT and two identical output IDTs with same distance from the input IDT. Among the two outputs of the differential structure, one is sensitive to the target gas CO ₂ and the other one is insensitive to the target gas. The difference between the two output signals’ resonant frequencies is considered as the sensor’s differential response. Experiments at 25 ^∘ C and 45 ^∘ C show that the sensor’s differential response changes linearly with CO ₂ concentration over 20–3000 ppm range. The differential responses at both temperatures are very similar showing the ability of the sensor’s structure to compensate for the temperature change. Sensitivity and limit of detection for CO ₂ sensing are measured to be 543 Hz ppm ⁻¹ and 5.6 ppm, respectively. Further, the sensor shows good repeatability and low hysteresis. Sensor’s responses are mostly measured by measuring the scattering parameter, forward transmission (S ₂₁ ) between each output IDT and input IDT of the sensor in frequency domain. In the last section, a prototype time domain measurement system is demonstrated to show the feasibility of the time domain measurements with the sensors. The sensor with the potential to be printed with roll-to-roll printing can be a good candidate for CO ₂ monitoring where temperature varies.