Soft On-Demand—Electrochemical Modulation of Polythiophene Mechanical Properties for Smart Actuators

Interfacing electronics and biology opens the need for materials having suitable electrical and mechanical properties, as mixed ionic/electronic conductivity and softness. Among all materials, polythiophene conjugates emerged for their high 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 between the material and the aqueous environment, as water intercalation and oxygen reduction reactions, are raising interest for their impact on the device performances and stability. In particular, volumetric changes driven by water intake during electrochemical doping/de-doping emerged as a limitation for transistor performances, since the irreversible water intake results in lamellar order disruption and consequently reduced hole mobility.<br/>While this volumetric change represents a limitation for transistor devices, it opens to a new class of electrically addressable actuators, where both mechanical stimulation and mixed conduction play together. Since the electrochemical doping/de-doping is accompanied by water intake in the thin film structure, we expect the mechanical properties to change accordingly. Here, we investigate the volumetric changes and their correlation with the mechanical properties of a new group of polythiophene based materials, conjugated to ethylene glycol (EG) side chain of different lengths. The EG-side chains account for a better stabilization of the water. According to previous publications, our findings show that the electrochemical doping drives volumetric changes in aqueous environment up to 250% vs initial volume. Additionally, we demonstrate that the amount of volume change depends on the ionic strength of the solvent and the side chain length. Moreover, the water intake determines changes in the mechanical properties, in particular softening the material upon volumetric expansion.<br/>Our findings deterministically correlate the mechanical properties, the volume changes and the doping state of the material, laying the basis for the development of low-voltage 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.