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Piezoelectric Force Sensor

Double-Sided Conformable Piezoelectric Force Sensor with Enhanced Performance and Bending Correction




Flexible piezoelectric devices have gained considerable interest due to their potential for new applications, particularly in wearable technology. However, a significant challenge remains in measuring low forces on nonplanar and deformable surfaces. Indeed, conformability on complex surfaces induces bending stresses in the piezoelectric sensors, interfering with the measurement of compressive force. Yet such measurements can be valuable, especially in medical applications that involve assessing forces on soft tissues. This study presents an innovative highly sensitive conformable sensor based on a thin film of P(VDF-TrFE) copolymer.

The selection of the substrate is essential for ensuring the device's conformability, but it is also demonstrated that it can provide a substantial improvement in performance if its Young's modulus is lower than that of the active polymer. The effective piezoelectric charge coefficient of a sensor on TPU substrate is measured equal to −340 pC.N−1, representing a tenfold increase in the theoretical compression sensitivity of P(VDF-TrFE).

Additionally, a double-sided structure to eliminate the contribution of bending in the piezoelectric signal and tackle the challenge of conformability on complex surfaces is developed. Overall, the proposed device shows promising results for measuring low forces applied to soft biological tissues such as skin or heart valve leaflets.

This study involved the development of a conformable piezoelectric sensor capable of measuring low compressive forces on non-flat or deformable surfaces. It has been revealed that the conformability between the sensing device and the contact surface (i.e., depending on their shape and stiffness) substantially impacts the sensor response. Consequently, theoretical models together with experimental characterizations were thoroughly investigated to better address this issue.

First, the choice of substrate, which determines the sensor's conformability, proved to be crucial due to its mechanical interaction with the piezoelectric film. Bending stiffness was considered a relevant criterion for substrate selection, and two approaches were explored to achieve good conformability: low thickness or low Young's modulus of the substrate. Based on the Poisson effect, a substrate with high flexibility relative to the active layer was preferred over a thin thickness. It has been demonstrated that a remarkable tenfold increase in the compression sensing performance was obtained with a flexible TPU substrate as opposed to a rigid PI, reaching efficiencies close to those of their rigid ceramic counterparts such as PZT.

This finding opens up promising prospects for researching adequate substrate material as one of the key factors in optimizing the sensor response. A new analytical model was developed to estimate the effective piezoelectric coefficient in compression by considering the mechanical interaction between the active layer and the substrate. It is expected that the developed model could serve as a reference for substrate selection and device performance prediction.

Second, when the conformable sensor is subjected to a compressive force on a curved and/or deformable surface, a bending effect occurs that perturbs the sensing performance. Ideally, the output signal of the sensor should faithfully reflect the input compressive force. Hence, the bending signal must be eliminated to isolate the useful signal only. To achieve this, a double-sided structure was used to exploit the piezoelectric bimorph principle, wherein two opposite signals caused by bending are canceled out by averaging them. In practice, the bending effect, which depends on the bending direction and the sensor conformability, often predominates over the compression effect. Proper calibration and the application of corrective coefficients made it possible to successfully counterbalance the bending effect, which was essential for achieving an accurate output signal.

Finally, the challenges encountered in using the conformable device provide a foundation for future studies. The next logical step may involve studying the miniaturization of conformable sensors and their integration into piezoelectric arrays. This would enable high-precision pressure mapping on soft surfaces, opening up promising applications such as e-skin. It would also address the issue that a double-sided sensor subjected to several bending directions (e.g., if it is conformed to a wavy surface) does not function properly. Indeed, if an array were conformed to a wavy surface, locally each miniaturized sensor would likely be subjected to only a single bending direction.

Research on arrays introduces new challenges, such as crosstalk reduction, optimal addressing of a large number of electrodes, and the development of new interface electronics capable of simultaneously processing the piezoelectric signals of numerous bimorphs set in an array. Regarding the non-conformability of flat sensors on spherical surfaces, it would be valuable to study whether the double-sided sensor concept could be combined with structural design to improve conformability. As for future use of this sensor in medical applications, specific biocompatibility tests will need to be conducted on the device, as well as studies of sensor behavior and stability in an in vivo environment to ensure safety and reliability.

Overall, this successful proof-of-concept for a piezoelectric conformable sensor developed in this study has paved the way for further explorations in medical instrumentation. The proposed sensor allows measurement of low coaptation forces in the mitral valve helping cardiac surgeons to better understand the biomechanics of this complex apparatus, marking an important first milestone.

sensor technology, IoT sensors, motion detection, temperature sensor, pressure sensor, proximity sensor, infrared sensor, ultrasonic sensor, biometric sensor, image sensor, gyroscope, accelerometer, smart sensor, wireless sensor networks, gas sensor, humidity sensor, vibration sensor, light sensor, magnetic sensor, environmental sensor

#SensorTechnology, #SmartSensors, #IoTDevices, #MotionDetection, #TemperatureSensor, #PressureSensor, #ProximitySensor, #InfraredSensor, #UltrasonicSensor, #BiometricSensor, #ImageSensor, #Gyroscope, #Accelerometer, #WirelessSensors, #GasSensor, #HumiditySensor, #VibrationSensor, #LightSensor, #MagneticSensor, #EnvironmentalSensor

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