Nanocomposite-based flexible tactile sensors

The use of smart devices and applications that require more interaction between human and machine has increased significantly in many areas since flexible electronic devices starting to find more place in the traditional electronics world. Tactile sensors, which are an important part of flexible electronics with the function of converting an external mechanical effect into electrical signals, are used especially in the biomedical field to diagnose and treat diseases by monitoring electrophysiological signals and to increase the functionality of prosthetic devices. Besides, tactile sensors are the most fundemental component in robotic systems developed for commercial or educational purposes and in the development of next–generation touch screens as well as in various applications where artificial intelligence is integrated. For this reason, the developments and improvements have a very important place as they directly affect the efficiency and performance of devices and applications that have the potential to use flexible tactile sensors. The variety of materials used in the production of flexible tactile sensors, different production methods and working mechanisms of the sensors offer a wide range of research opportunities. In this thesis, polymer nanocomposites consisting of polymer and conductive fillers were used in the sensing layers of sensors that are fabricated on flexible substrates. Polyvinyl alcohol (PVA) and sodium alginate (SA) were preferred as polymers, especially because they are biocompatible and easy to use. Carbon black (CB), carbon nanotubes (CNTs), graphene nanoplatelets (GnPs) and silver nanoparticles (Ag NPs) were used as conductive fillers. Different nanocomposites were produced so that each nanocomposite consists of a polymer and a filler, the nanocomposites were characterized and used to produce nanocomposites–based sensors with different sensor designs. The obtained flexible tactile sensors are based on PVA–CB, PVA–CNT, PVA–GnP, PVA–Ag NP, SA–CB and SA–CNT nanocomposites. In addition to tactile sensors produced by screen–printing on paper, melt–blown and cotton fabrics, there are also sensors produced without using any substrate. Produced sensors were tested via electromechanical measurements under different pressure values. PVA–fCNT, PVA–GnP and PVA–Ag NP nanocomposites–based sensors fabricated with the same structural design have shown that the phase angle change versus applied force can be used in touch sensing. Also, the PVA–Ag NP nanocomposite–based sensor mounted on a glove was tested in an application consisting of holding and lifting a full bottle, with measurements taken over an impedance analyzer circuit. The concept of determining the point where force is applied and the magnitude of the force are demonstrated using the SA–CB nanocomposite–based sensor. In addition, the frequency responses of the sensors produced without any substrate were examined in certain frequency ranges and their equivalent circuits were created. The obtained data show that polymer nanocomposite–based flexible tactile sensors perform the sensing function effectively. In addition, taking impedance measurements for the analysis of sensors in different designs gave the opportunity to evaluate both the capacitive and piezoresistive properties of the sensor at the same time. It has been shown that sensors in different frequency ranges can have different operating mechanisms and equivalent circuits can be determined accordingly. It has been demonstrated that the change in resistance and capacitance can be used for touch sensing, as is usually the case, and depending on the design, the phase angle change can be used for the same purpose.