Flexible High Sensitivity Tactile Sensors for Electronic Skin Applications

Nowadays, in an increasingly interconnected world, where the daily monitoring of various bio/physical parameters is growing rapidly, electronic devices and sensors are permeating everyday life. In this context, flexible and conformal materials and systems have allowed a dramatic growing of bio-integrated electronic systems for monitoring different parameters and, particularly, epidermal electronic is a new field of application in which electronic devices can be transferred directly onto the skin.<br/>To fulfill this aim, device architectures that can match the mechanical properties of the human skin have to be developed. Moreover, it is necessary to develop a procedure that allows to transfer onto the skin such devices in a reliable way, while preserving their performances.<br/>In this work we report a tattoo-like electronic systems, that can be easily fabricated on sub-micrometer thick plastic substrates and employed for the detection of several parameters. The proposed system has been fabricated on a plastic carrier, coated with a water-soluble material acting as sacrificial layer. At the top of such structure, an ultrathin film of Parylene C is deposited through a large area deposition process. This procedure is compatible with the fabrication of several kinds of electronic devices, such as sensors or electrodes for the monitoring of electro-physiological parameters.<br/>We have used such approach for the fabrication of high sensitivity transistor-based tactile sensors, employing a Sub-micrometer Channel Organic Charge Modulated Field Effect Transistor (SC-OCMFET). This architecture represents a versatile tool for the realization of a wide range of sensing applications, from bio/chemical to force/pressure sensing. The proposed device is based on a floating gate organic transistor, capable to be operated at low voltages thanks to an ultra-thin, hybrid dielectric. In order to achieve sensitivity to pressure, a piezoelectric thin film, namely PVDF, is coupled with the sensing area of the device. In this way, when a pressure is applied on the PVDF, the charges induced in the piezoelectric film led to a variation of transistor threshold voltage and a current variation can be detected as a result of the applied pressure. The signal of the piezoelectric material is thus locally amplified by the short-channel transistor. As the device sensitivity depends on the ratio between the sensing area and the channel area of the transistor, a reduction of the channel length can lead to a significant increase in the sensor sensitivity. We herein propose an easy and reproducible approach for the fabrication of sub-micrometer SC-OCMFETs that does not require expansive and time consuming high resolution techniques, and that allowed us to achieve very high sensitivity, if compared to the previously reported planar devices. Moreover, this approach, allows at the same time to dramatically reduce the area occupied by the amplifying transistors, which is very important when dealing with the fabrication of arrays and matrices of tactile sensors. The fabricated devices are capable to detect very small stimuli, with an impressive sensitivity, and can detect forces within a range from 0.01 up to 1 N and pressures below 100 Pa. Moreover, since the PVDF is also a pyroelectric material, temperature variations ranging from 10° up to 45 °C could be also detected. 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 the same device for the fabrication of multimodal tactile sensing systems. The highly flexibility of the developed structure, and the easiness of the employed process, make this solution very interesting for the fabrication of multimodal, highly compliant artificial skin.