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Jin-Hoon Kim1 Seung-Rok Kim1 Jin-Woo Park1

1, Yonsei University, Seoul, , Korea (the Republic of)

Wearable or implantable biosensors for the monitoring of biosignals, such as body motion, body temperature, electrocardiogram (ECG), and electroencephalography (EEG) have become one of the most important elements of wearable electronic devices for healthcare application. These devices are attached to the skin or organs to detect various biosignals. Hence, the maintenance of good conformal contact between the sensor and skin is essential for obtaining precise biosignals because the human skin and organs are highly rough and dynamically moving. Various materials such as hydrogels, polyacrylate, polyurethane and silicone have been used to improve conformability by increasing the adhesion force between the biosensors. Among them, silicone-based polymers are one of the most suitable materials for biosensors due to its good biocompatibility, stretchability, transparency, and tunable adhesion forces. Since most biosignals are interpreted as electrical signals, solid conductive fillers such as conducting polymers, metal nanoparticles, metal nanowires, carbon nanotubes (CNT) have been integrated into silicone elastomers to make elastomers electrically conductive. In this study, we present a highly conformable, stretchable, and transparent electrode for application in epidermal electronics based on polydimethylsiloxane (PDMS) and Ag nanowire networks (AgNWs). With the addition of a small amount of a commercially available non-ionic surfactant, Triton X, PDMS became highly adhesive and mechanically compliant, which are key factors for the development of conformable and stretchable substrates. The Young`s modulus and elongation at break of a-PDMS were 40 kPa and over 400% respectively. Also, the adhesion force of the a-PDMS was seven times higher than the unmodified PDMS. The polar functional groups present in Triton X interacted with the Pt catalyst present in the PDMS curing agent, thereby hindering the crosslinking reaction of PDMS and modulating the mechanical properties of the polymer. Due to the strong interactions that occur between the polar functional groups of Triton X and AgNWs, AgNWs were effectively embedded in the adhesive PDMS (a-PDMS) matrix, and the highly enhanced conformability, mechanical stretchability, and transparency of the a-PDMS matrix were maintained in the resulting AgNW-embedded a-PDMS matrix. The AgNW-embedded a-PDMS showed stable stretchability compared with the AgNW-embedded unmodified PDMS electrodes. Finally, wearable strain and ECG sensors were fabricated from the AgNW-embedded a-PDMS. The a-PDMS-based strain and ECG sensors exhibited significantly improved sensing performances compared with those of the bare PDMS-based sensors because of the better stretchability and conformability to skin of the former sensors.

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