EDITOR’S CHOICE NEWS ORGANIC ELECTRONICS REPRODUCE SENSE OF TOUCH


A new tactile sensing device that simultaneously detects thermal and mechanical stimuli could find use in artificial skin systems


T


he human skin has millions of tactile receptors, which respond to


mechanical and thermal stimuli and can discriminate between the position, size, and duration of these stimuli. In the last decades, there has been a huge effort in developing novel technologies for reproducing the sense of touch. Ideally, artificial tactile systems should be


multimodal, that is, they should be able to respond simultaneously to mechanical and thermal stimuli. However, it is very challenging to integrate different types of sensors into one device that can be distributed over large and complex areas, while keeping fabrication costs low. In this respect, organic electronics- a new branch of electronics characterised by the use of conjugated polymers in the fabrication of electronic devices- represents a step forward. Conjugated polymers are organic macromolecules that can be dissolved into a solution and have the mechanical characteristics of plastics (that is, they are highly flexible and easy to transfer to any kind of surface). This means they can be deposited on large areas by using simple,


is based on that of a floating-gate organic transistor (Figure 1). The floating-gate electrode is partially covered with a combination of two ultrathin, hybrid dielectric layers in beige in Figure 1(a) which allow the device to be operated at very low voltages. The organic semiconductor- in blue in


Figure 1(a)—has been deposited over the source and drain electrodes—in yellow in Figure 1(a)—which are made of gold. There is another electrode, called the control gate, required for setting the working point of the device. The uncoated area of the floating gate is the sensing area, and it is the place where the transduction mechanism starts. To determine the sensitivity of the device,


cost-efficient technologies suitable for mass production, such as inkjet printing. Scientists at the Department of Electrical


and Electronic Engineering (DIEE) at the University of Cagliari, Italy have fabricated


a multimodal tactile transducer on a flexible plastic substrate using a low-voltage, organic, charge-modulated field-effect transistor (OCMFET). The architecture of the transducer


Figure 1:


Architecture of the transducer based on that of a floating-gate organic transistor


the scientists anchored a sensing layer directly exposed to the measurement environment- to the sensing area. This sensing layer can be chosen according to the specific external stimulus. To achieve sensitivity to pressure, a piezoelectric thin film—polyvinylidene fluoride (PVDF) was deposited on the sensing area of the device. When pressure is applied to the PVDF, the charges induced on the piezoelectric film lead to a perturbation in the charge that goes into the floating gate, thus modulating the concentration of carriers in the transistor channel. As a consequence, a current variation can be detected each time the film is pressed. DIEE www.unica.it


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