All‐Hydrogel‐Based Organic Electrochemical Transistors for Implantable Physiological Signal Monitoring

All‐hydrogel based n‐type depletion‐mode OECTs is developed using a semiconductor hydrogel formed via ionic‐liquid‐mediated phase separation, combining efficient ion‐electron transport, tissue‐like softness, and long‐term biocompatibility. The device can monitor EOG and ECG on human skin and record real‐time ECG subcutaneously in rats, providing a versatile platform for multimodal physiological sensing and implantable bioelectronics. Abstract Organic electrochemical transistors (OECTs) are an advanced technology for interfacing with biology, capable of efficiently transducing ionic currents into amplified electronic signals at low operating voltages. However, their clinical potential is critically undermined by the mechanical mismatch between conventional device materials and soft tissues. This fundamental limitation is resolved by developing a versatile all‐hydrogel‐based OECT, which is made possible by a novel n‐type depletion‐mode semiconductor hydrogel. Fabricated via an ionic liquid‐mediated phase separation of poly(benzodifurandione)/polyacrylamide, the material uniquely combines efficient ion‐electron coupled transport with tissue‐like softness. This fully compliant architecture delivers a high transconductance of 43 mS, remains stable over 1500 bending cycles, and shows excellent biocompatibility. The devices enable real‐time monitoring of human electrocardiography/electrooculogram (ECG/EOG) signals on the skin and long‐term subcutaneous recording of nociceptive/ECG signals in rats. This study provides a robust materials and device strategy, establishing a versatile platform for multimodal physiological signal monitoring, pain assessment, cardiovascular health evaluation, and implantable precision bioelectronic therapy. All-hydrogel based n-type depletion-mode OECTs is developed using a semiconductor hydrogel formed via ionic-liquid-mediated phase separation, combining efficient ion-electron transport, tissue-like softness, and long-term biocompatibility. The device can monitor EOG and ECG on human skin and record real-time ECG subcutaneously in rats, providing a versatile platform for multimodal physiological sensing and implantable bioelectronics. Abstract Organic electrochemical transistors (OECTs) are an advanced technology for interfacing with biology, capable of efficiently transducing ionic currents into amplified electronic signals at low operating voltages. However, their clinical potential is critically undermined by the mechanical mismatch between conventional device materials and soft tissues. This fundamental limitation is resolved by developing a versatile all-hydrogel-based OECT, which is made possible by a novel n-type depletion-mode semiconductor hydrogel. Fabricated via an ionic liquid-mediated phase separation of poly(benzodifurandione)/polyacrylamide, the material uniquely combines efficient ion-electron coupled transport with tissue-like softness. This fully compliant architecture delivers a high transconductance of 43 mS, remains stable over 1500 bending cycles, and shows excellent biocompatibility. The devices enable real-time monitoring of human electrocardiography/electrooculogram (ECG/EOG) signals on the skin and long-term subcutaneous recording of nociceptive/ECG signals in rats. This study provides a robust materials and device strategy, establishing a versatile platform for multimodal physiological signal monitoring, pain assessment, cardiovascular health evaluation, and implantable precision bioelectronic therapy. Advanced Science, EarlyView.