In this work we present an innovative structure for the realization of tactile transducers on flexible plastic substrates. The core of the device is a floating gate Organic Field Effect Transistor (OFET) biased through a control capacitor and with a sensing area directly connected to the floating gate. The floating gate dielectric has been realized by using a combination of two different ultrathin insulating materials (average thickness of 25 nm), composed by alumina (grown on a pre-deposited aluminum film that acts as the floating gate electrode) and Parylene C. Thanks to the high capacitance coupling the fabricated OTFTs can be operated at voltages as low as 1 V. A control capacitor is fabricated on the floating gate and used for setting the operational working point of the sensor.
In this way, if an additional electrical charge is somehow induced onto the sensing area fabricated on the floating gate, it leads to a charge separation in floating gate electrode, which, in turns, induces a modulation of the transistor threshold voltage. In order to achieve the sensitivity to pressure, a piezoelectric thin film, namely PVDF-TrFE, is transferred on the sensing area of the device. In this way, when pressure is applied on the PVDF-TrFE, the charges induced in the piezoelectric film, lead to a variation of OFET threshold voltage and a current variation can be detected at each pressure event. We will demonstrate that the fabricated devices are characterized by a reproducible response to applied forces within the range of 0-5 N with a resolution of 0.1 N.
Moreover, PVDF polymer is also characterized by pyro-electric properties, which make it suitable also for the realization of temperature sensors. This feature have been also exploited, and preliminary results on the fabrication and characterization of temperature sensors will be presented. The introduced approach represents a very simple and innovative solution for the realization of multimodal tactile sensing systems on highly flexible and possibly compliant substrates, which could be employed for a wide range of applications in the biomedical field and particularly suitable for the fabrication of artificial electronic skin.