Printable and Stretchable Polymers and Liquid Metal Composites for Wearable Electronics

Printing has attracted significant attention as a key manufacturing technique due to its facile processing, low cost, and high customizability. In this context, solution-processable materials such as polymers are ideal candidates for printable electronics, as they readily form solutions or suspensions with solvents. This PhD thesis focuses on the fabrication of wearable devices using soft conducting materials enabled by printing techniques. First, fully printable organic electrochemical transistors were fabricated using a poly(vinyl alcohol) (PVA) hydrogel as the electrolyte and the conducting polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as the channel material. Stretchable silver electrodes provided a soft feature to drain/source, gate and interconnect . The maximum transconductance and on/off ratio were 1.04 ± 0.13 mS and 830, respectively. Owing to hydrogel network structure and differences in solvent composition, devices with different gel formulations exhibited distinct transient responses and long-term operational stability. The device was stable for 50 days and stretched up to 110% tensile strain, which makes it suitable for withstanding the mechanical deformation expected in wearable electronics. Moreover, since all printed components were mechanically deformable, the resulting transistors were fully stretchable, demonstrating their suitability for wearable electronic applications. In the second study, a printable multifunctional liquid-metal composite was developed as a stretchable electrode or interconnect for wearable devices. The polymer matrix of the composite was rationally designed and synthesized to ensure good printability, recyclability, and compatibility with both dry and underwater environments. The composite exhibited high stretchability up to 500% strain and high electrical conductivity (around 10⁵ S m⁻¹), while also being recyclable with a high recovery yield (around 95%) using green solvents. In addition, the conducting composite showed excellent durability in both dry and wet states, maintaining its stable resistance with cyclic strain ranging from 0 to 50% for over 16 h, in both ambient and aqueous environments and functioned effectively as a wearable sensor. The final study investigates the electrical anisotropy of PEDOT:PSS thin films. Electrical anisotropy is a promising strategy for improving device performance and energy efficiency, which is a critical consideration in wearable electronics. In-plane anisotropic PEDOT:PSS films were achieved by combining conventional spin coating with an ethylene glycol wet-stretch post-treatment. Analysis of apparent resistivity trends revealed a distinct minimum at approximately 25% strain, suggesting a condition of strain -induced structural reorganization. PEDOT:PSS films treated under these conditions displayed clear in -plane electrical anisotropy, with lower sheet resistance along the strain direction compared to the perpendicular direction . This approach provides a scalable route to introduce controlled electrical anisotropy into PEDOT:PSS thin films, offering potential benefits for a wide range of electronic and energy applications. This PhD work comprehensively investigated several soft material components for wearable electronic devices. Owing to the intrinsic mechanical softness of polymers, all major materials studied in this thesis were polymer-based. In the first work, the operational behavior of organic electrochemical transistors (OECTs) was systematically studied as a function of the chemical composition of printed poly(vinyl alcohol) (PVA)-based hydrogel electrolytes. In the second work, multifunctional liquid metal (LM) composites were developed through the rational design and synthesis of polymer matrices tailored for composite formation. In the final work, a wet-strain treatment was employed to induce electrical anisotropy in PEDOT:PSS thin films through polymer chain reorientation. Because this wet-strain process can be readily applied to printed films, future work will focus on extending this approach to printed PEDOT:PSS films and investigating their electromechanical characteristics under stretching.