Transparent, Patternable and Stretchable Conducting Polymer Solid Electrode for Dielectric Elastomer Actuators

The growing interest in virtual and augmented reality has raised the importance of haptic devices to provide realistic sensations to the users. Dielectric elastomer actuators (DEA) are one of the promising candidates for haptics due to their lightweight, stretchability and conformability to human skin. To achieve a large actuation strain, DEA requires a compliant electrode with low modulus and high electrical conductivity. While carbon grease has been commonly used as a compliant electrode, there has been a lack of extensive research in the development of a compliant solid electrode that can attain a similar actuation as its non-solid counterpart. In this study, we present a transparent, stretchable, and patternable solid electrode composed of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) diacrylate (P123DA). The unique combination of PEDOT:PSS and P123DA allows for tunable electrical and mechanical properties by varying the P123DA to PEDOT:PSS ratio, which is essential for optimizing the actuation performance. The study investigated the performance of solid electrodes with different P123DA to PEDOT:PSS weight ratios of 1, 10 and 20 (denoted as R = 1, 10, 20), revealing that increasing P123DA content improves mechanical compliance but reduces electrical conductivity. The optimal ratio of P123DA to PEDOT:PSS (R = 10) achieves excellent performance, providing actuation comparable to non-solid electrodes like carbon grease, while maintaining high optical transparency over 95%.
The electrical conductivity of the P123DA/PEDOT:PSS decreased from 198 S/cm for pristine PEDOT:PSS to 77.9, 1.43, and 0.08 S/cm for R = 1, 10, and 20, respectively. Morphological analysis using atomic force microscopy (AFM) revealed that as the ratio of P123DA/PEDOT:PSS increased from R = 1 to 10, fiber formation was evidently increased with the highest fiber density. In contrast, further increasing the ratio to R = 20 led to a reduction in fiber density, due to insufficient PEDOT:PSS content to form additional fibers with P123DA. In addition to these morphological changes, the Derjagun-Muller-Toporov (DMT) modulus measurements showed that increasing the ratios of P123DA to PEDOT:PSS from R =1 to 20 resulted in a decrease in moduli from 226 to 10.6 MPa, which is advantageous for achieving a greater actuation performance. Combining the electrical and mechanical properties of the P123DA/PEDOT:PSS electrodes, it was observed that actuation performance increased as the ratio increased from R = 1 to 10, primarily due to the reduction in modulus. However, at R = 20, actuation performance decreased compared to R = 10, as the lower electrical conductivity resulted in a slower response to the applied electric field. This underscores the importance of tuning the electrical and mechanical properties of DEA electrodes to optimize the actuation capabilities.
In summary, this work developed a transparent, stretchable, and patternable solid P123DA/PEDOT:PSS electrodes for DEAs. By exploring various ratios of P123DA to PEDOT:PSS, this study emphasizes the importance of tailoring the mechanical and electrical properties based on the specific requirements of different applications. For static applications, where larger actuation is desired, the modulus of the electrode material is critical, allowing for compromises in electrical conductivity to achieve lower modulus. Conversely, in dynamic applications that demand rapid responses, such as vibrotactile haptics and flying robots, electrical conductivity becomes a dominant factor affecting actuation performance. This study provides valuable insights into the effect of electrode properties in determining the actuation performance of DEAs.