Anomalous Stiffening of a Conjugated Polymer During Electrochemical Oxidation

Abstract The mechanical mismatch between semiconductors and biological tissues can be a challenge for the development of conformal bioelectronics. Organic mixed ionic‐electronic conductors (OMIECs) such as conjugated polymers with oligoether side chains are promising materials due to their low stiffness, which may minimize adverse immune reactions and thus promote biocompatibility. However, significant volume changes during electrochemical cycling—driven by ion and water ingression and expulsion—can lead to drastic changes in stiffness, complicating device‐tissue mechanical matching across redox states. Here, the electromechanical response of a thienothiophene‐based conjugated polymer with triethylene glycol side chains is investigated. Electrochemical nanoindentation and atomic force microscopy reveal a modest and reversible increase in elastic modulus at room temperature from ≈70 to more than 120 MPa upon electrochemical oxidation. This unusual mechanical stability is attributed to a reversible increase in π‐stacking that compensates for swelling‐induced softening. These findings demonstrate that it is feasible to design OMIEC materials with stable mechanical properties across redox states, opening new possibilities for compliant and tissue‐matched bioelectronic interfaces that remain mechanically invariant during operation.