Intrinsically stretchable organic electrochemical transistor passivated by photopatternable thiol-ene crosslinked PDMS

Abstract

Recently, there is an increasing demand for a personalized medical system that transmits personal biomedical data to professionals. Here, the bioelectronic devices detect biomedical data in real time through biocompatible sensors. Bioelectronic devices is helpful in diagnosing diseases in early stages, monitoring patients with chronic diseases, and collecting disease-related data. Among many bioelectronic devices, the organic electrochemical transistors (OECTs) are widely used for medical purposes because they can have high transconductance due to injection of ions and high stretchability by applying stretchable substrates. Currently, OECTs are used for measuring electrophysiological signals such as electrocardiogram (ECG), electroencephalogram (EEG), electromyogram (EMG). Especially, intrinsically stretchable OECTs, which are made of a material with low Young's modulus, have high compatibility with soft human skins or tissues, and can measure accurate signals through conformal contact with human body. To fabricate an intrinsically stretchable OECT, soft and stretchable substrates and passivation layers are required. The most commonly used material is polydimethylsiloxane (PDMS), which has high stretchability (~160%) and chemical stability. Especially for passivation layer, patterning of PDMS is essential because the passivation layer must be open at the sensing area and the contact pad. Generally, the most popular technique for the PDMS patterning is replica molding (REM). REM is a PDMS patterning method by pouring PDMS into a mold with a specific shape. However, REM requires complex photolithographic process for mold fabrication and skillful skills in demolding process. In this work, in order to solve this problem effectively, we introduced a new photopatternable PDMS, which is “photopatternable thiol-ene crosslinked PDMS (TC-PDMS)”, and innovative patterning method, which is “oxygen inhibition photolithography (OIP)”. Like conventional PDMS, TC-PDMS had high stretchability (~140%) and low Young's modulus (~2.9 MPa). In addition, it was possible to make patterns of 100 µm size through OIP. Through the new photopatternable PDMS, which is TC-PDMS, and patterning method, which is OIP, we constructed the PDMS patterning system and applied this system to the passivation layer of intrinsically stretchable OECT. The fabricated stretchable OECT showed low leakage current (~20 µA) due to the TC-PDMS passivation layer, and high transconductance (0.258 mS) at high strain (80%). Finally, this intrinsically stretchable OECT was applied to stretchable ECG sensor, and the stable ECG signals were measured from human body.