Conformable High Sensitivity Tactile Sensors for Electronic Skin Applications

Epidermal electronics is a new application field in which highly flexible devices can be transferred directly onto the skin and employed for the monitoring of different bio/physical parameters.<br/>In order to achieve such goal, it is necessary to develop a device architecture that can match the mechanical properties of the human skin, but also a procedure that allows such devices to be transferred in a reliable way, while preserving their performances. In this work we report on how tattoo-like electronic systems, can be easily fabricated on sub-micrometer thick plastic substrates and employed for the detection of different bio-mechanical parameters. The developed 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. Finally, at the top of this structure different kinds of electronic devices can be fabricated, such as sensors or electrodes for the monitoring of electro-physiological parameters. We have used such approach for the fabrication of high sensitivity tactile sensors realized using an Sub-micometer 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. It 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 deposited and poled on the sensing area of the device. In this way, when 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.<br/>Since the sensitivity of such architecture depends on the ratio between the sensing area and the channel area, reducing channel length can lead to a dramatic increase in the devise sensitivity. We have developed an up-scalable and easy process to reproducibly obtained transistors with channel lengths well below 1 um, which not only allowed to dramatically reduce the overall sensing system dimensions but also the final device sensitivity. The fabricated devices are capable to detect very small pressure, with an impressive sensitivity, and can detect forces within a range from 0.01 up to 1 N. and pressures below 200 Pa. Moreover, being PVDF 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.<br/>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.