One of the main issues when dealing with the reproduction of the sense of touch is the capability of fabricating, possibly with similar fabrication processes, different kinds of sensing devices on the same system. In this work we introduce a novel approach for the fabrication, using a single device, of a bimodal sensing element that it is able to detect at the same time both mechanical stimuli and temperature variations. The approach is very simple, and can be employed for fabricating devices on highly flexible, and possibly compliant, substrates that can be easily transferred on unconventional, no necessarily planar substrates, for artificial skin applications.
The core of the sensing system is a low voltage charge modulated Field Effect Transistor in which a floating gate is fabricated on a flexible substrate and partially coated with a combination of two ultrathin insulating materials, namely Al2O3 and Parylene C, with a nominal thickness of 6 nm and 30 nm respectively. Thanks to the very high capacitance coupling such devices can be operated at very low voltages. On the top of the coated area source and drain electrodes are fabricated and the organic semiconductor is deposited in order to have the final OFETs structure. Moreover, a third electrode called control gate is also fabricated and used for setting the working point of the device. In all the reported cases TIPS-Pentacene was used as organic active layer allowing to achieve mobilities up to 0.4 cm2/Vs with remarkably small leakage currents (50/100 pA) and Ion/Ioff around 104.
Finally, on the uncoated area of the floating gate (sensing area), a thin film of a piezo(pyro)-electric polymer, namely PVDF-TrFE, is deposited from liquid phase by spin coating and also by inkjet printing, and subsequently poled. In this way, the charges induced on the PVDF-TrFE film by and external mechanical and/or thermal stimulus will induce a charge separation in the floating gate of the sensing structure, thus leading to a variation of the output current of the OFETs.
We will demonstrate that such devices can be employed for detecting forces up to 2 N with a resolution of 0.05 N, and are able to detect dynamic stimuli at a frequency up to 100 Hz. Moreover, at the same time, they are also capable to detect temperature variations ranging from 10° up to 45 °C. We will also demonstrate that the sensitivity of this structure can be tuned by properly changing the layout of the sensing area of device.
Interestingly, since the responses of the device to the two different physical stimuli are characterized by marked differences in sensitivity and response time, it is possible to employ these devices for the fabrication of multimodal tactile sensing systems.