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, organic mixed ionic-electronic conductors (OMIEC), and in particular polythiophene conjugates, emerged for their higher biocompatibility, ionic as well as electronic conduction, and optical properties in the visible range. Electrochemical doping/dedoping is key for modulating OMIEC’s conductivity, charge storage and volume, enabling high performing bioelectronic devices such as recording and stimulating electrodes, transistors-based sensors and actuators. Remarkable results have been obtained interfacing polythiophenes-based transistors and actuators with mammalian cells as well as plants. However, electrochemical doping has not been explored to the same extent for modulating the mechanical properties of OMIECs on demand and for drug delivery applications.

We first investigated the doping/de-doping behaviour of a representative glycolated polythiophene (p(g3T2)). According to previous publications, our findings show that the electrochemical doping involves volumetric change in aqueous environment. We report a qualitative and quantitative study on how the mechanical properties change in-situ during electrochemical doping and de-doping for p(g3T2). Its Young’s Modulus changes from 69 MPa in the dry state to less than 10 MPa in hydrated state and then further decreases down to 0.4 MPa when electrochemically doped, representing the largest modulation reported for an OMIEC so far. Additionally, we demonstrate that the amount of volumetric change, hence the viscoelastic changes, depend on the ionic strength of the solvent and on the polymer’s side chain design.

Then, we harnessed the volumetric exchange for controlled drug delivery. By reversibly expanding up to 300% during doping, p(g3T2) forms pores in the nm-size range resulting in a conducting hydrogel. P(g3T2)-coated 3D carbon sponges enable temporally patterned loading and release of molecules, spanning molecular weights of 800-6000 Da, from simple dyes up to the hormone insulin. Molecules are loaded as a combination of electrostatic interactions with the charged polymer backbone and physical entrapment in the porous matrix.

Overall, 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 bioelectronics’ 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.