Fundamentals of electrochemical doping in polythiophene polymers

Interfacing electronics and biology opens the need for materials having suitable electrical and mechanical properties, as transporting both ions and electrons and soft mechanical properties. Among all materials, polythiophene conjugates emerged for their higher biocompatibility, ionic as well as electronic conduction, and optical properties in the visible range. Remarkable results have been obtained interfacing polythiophenes-based transistors and actuators with mammalian cells, as well as plants, to a lower extent. However, the phenomena occurring at the interface with aqueous environment are yet to be characterized and understood. In particular, electrochemical doping/de-doping results in volume change, which can diminish device performance and reduce stability. However, the possibility to electrically address and modulate on-demand the volume can pave the way for a new class of devices, where both mechanical stimulation and mixed conduction play together. Here, we investigate the doping/de-doping behaviour of a new class of polythiophene based materials, conjugated to ethylene glycol side chain of different lengths. According to previous publications, our findings show that the electrochemical doping involves materials volume change in aqueous environment. Additionally, we demonstrate that the amount of volume change depends on the ionic strength of the solvent, thus further investigations are being carried out to elucidate the fundamental processes involved in the recorded large volume changes. The ionic intercalation is also influencing the mechanical properties of the doped as well as the de-doped material, as the mechanism is not fully reversible. We report also the mechanical characterization of the material family as a function of the ionic strength of the solvent. Our findings deterministically correlate the mechanical properties, the volume changes and the doping state of the material, laying the basis for the development of electrically addressable devices. This class of material and devices has a great potential interest for bioelectronic applications, as it accounts for ionic/electronic conduction and on-demand modification of stiffness, emerging as a smart platform for mammalian and plant cell stimulation and monitoring.