Stretchable Redox-Active Semiconducting Polymers for High-Performance Organic Electrochemical Transistors

Wearable health monitoring technologies that can be integrated intimately with human bodies are of rapidly increasing interest in daily healthcare and self-diagnosis. Organic electrochemical transistor (OECT) is an emerging device platform for next-generation human-integrated bioelectronics owing to its high amplification and sensitivity to biological signals, and non-volatile signal retention for memory and computing applications. However, owing to the lack of stretchable redox-active semiconducting polymers, skin-like softness and stretchability have not yet been realized in high-performance OECTs for achieving seamless tissue-electronics interfaces. Here, we report two stretchable semiconductors for OECT devices, namely poly(2-(3,3′-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-[2,2′-bithiophen]-5)yl thiophene) (p(g2T-T)) and poly-[3,3′-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-2,2′-bithiophene] (p(gT2)), which give exceptional stretchability over 200% strain. In particular, the p(g2T-T) also shows great mechanical robustness for being able to sustain charge carrier mobility over 5000 stretching cycles, together with the OECT performance on par with the state of the art. Validated by the systematic structural characterization, sufficient chain alignment during stretching for stress dissipation is determined as the key enabler of high stretchability. By comparing the mechanical and electrical performance of different polymer semiconductors, the key design features for enabling the combination of high stretchability and high OECT performance are determined as non-linear backbone architecture, moderate side-chain density, and sufficiently high molecular weight. Using those highly stretchable polymer semiconductors, we fabricated an intrinsically stretchable OECT with the high normalized transconductance (~223 S cm^-1) and biaxial stretchability up to 100% strain. Furthermore, we demonstrated on-skin electrocardiogram (ECG) recording based on our stretchable OECT, and fabricated an intrinsically stretchable neuromorphic device with over 800 distinct memory states and good state retention capability (> 10⁴ s) with the high stretchability of up to 100% strain over 100 repeated cycles.