Development of Intrinsically Stretchable Transistor Arrays Utilizing Liquid Metal for Health Monitoring Systems

The distribution of strain and stress inevitably deforms devices, leading to significant performance degradation. Therefore, there is increasing attention on the field of intrinsically stretchable electronics, which can overcome mechanical mismatches caused by lateral strain. In particular, with the aging population, it is essential for electronic devices to be attached to the skin and capable of quantitatively processing electrophysiological signals, which are very small values emitted from the human body, to enable professional and systematic real-time health monitoring and healthcare at home. Human sensory information such as auditory, tactile, and visual data are converted into electrophysiological signals and transmitted to the brain, which processes these complex signals simultaneously. Developing electronic devices composed of intrinsically stretchable materials that can seamlessly integrate with human skin is crucial to achieve this. The stretchable transistor array proposed in this study consists of output devices and electronic circuits based on stretchable materials, providing opportunities for human-machine interaction.<br/>Liquid metal can exist in a liquid state at room temperature due to its low melting point and can exist as very small particles (spherical). This allows it to have inherent stretchability and high electrical conductivity due to its metallic nature (Particle size: 2-3 μm, Stretchability: 300%, Electrical conductivity: 3.4 x 10^6 S/cm). Pure liquid metal is challenging to coat as a film on hydrophobic substrates due to its high surface tension. Previous studies have mixed liquid metal with polymers or other components to form a film, which degrades the electrode performance. To address this, our study disperses liquid metal particles in two hydrophilic solvents with different boiling points and coats them on the substrate based on the Marangoni flow principle (Boiling point of DI water: 100°C, Boiling point of ethyl acetate: 77.1°C). The most critical step in liquid metal patterning through photolithography is the etching process. To break the bond between the thin (~5 nm) metal oxide layer surrounding the liquid metal particles and the hydrophobic substrate, wet etching with SC-2 (hydrochloric acid + hydrogen peroxide) solution is necessary. This is because gallium, which constitutes most of the liquid metal, reacts with hydrogen peroxide and hydrochloric acid, enabling etching (Resolution: &lt;5 μm).<br/>There is a concern that the performance of stretchable transistors may degrade under high strain or repetitive stretching. Therefore, utilizing elastic stiff islands of materials with selectively variable cross-linking density significantly enhances the device's reliability and durability (PDMS modulus: 1 MPa, modulus of elastic material (SEBS): ~20 MPa). Transistors using s-CNT as the channel and SEBS H1052 as the dielectric layer are expected to show improved performance compared to previously reported stretchable transistors. <br/>The fabricated stretchable transistor arrays are highly stretchable and attachable to the skin. These arrays can receive very small electrophysiological signals and perform complex tasks such as signal processing. This study is significant as it is the first to research electrodes and transistor arrays patterned through photolithography composed entirely of liquid metal, achieving intrinsically stretchable electronic devices.