3D printing of conducting polymers

Conducting polymers, possessing intrinsic electrical conductivity, are attractive materials for energy storage, flexible electronics, and bioelectronics due to their unique properties, including stability and biocompatibility. However, traditional fabrication techniques like inkjet printing, screen printing, electrochemical patterning, and lithography suffer from limitations in resolution, dimensionality, cost, and complexity. These limitations hinder rapid innovation and broader applications. Three-dimensional (3D) printing offers a superior alternative, enabling the fabrication of microscale structures in a programmable, facile, and flexible manner. While 3D printing of other materials (metals, hydrogels, etc.) has advanced significantly, 3D printing of conducting polymers has lagged due to the limited printability of existing inks. This research aims to address this challenge by developing a high-performance 3D printable ink based on PEDOT:PSS, a widely used conducting polymer, and demonstrating its application in creating complex, high-resolution conducting polymer devices.

The existing literature extensively covers the applications of conducting polymers in various fields but highlights the limitations of traditional fabrication methods. Inkjet printing, while offering some advantages, is constrained by resolution and typically produces only 2D patterns. Screen printing and other methods share similar limitations. The review also notes the recent advancements in 3D printing of other materials, emphasizing the potential of this technology for fabricating complex structures not readily achievable with traditional techniques. The lack of suitable 3D printable conducting polymer inks is identified as a major bottleneck. This sets the stage for the current study, which focuses on developing a novel ink and demonstrating its use in creating advanced devices.

The researchers developed a paste-like conducting polymer ink using a commercially available PEDOT:PSS aqueous solution. The process involved cryogenic freezing of the solution followed by lyophilization to isolate PEDOT:PSS nanofibrils. These nanofibrils were then re-dispersed in a water-DMSO mixture to create a concentrated suspension with desirable rheological properties for 3D printing. The concentration of PEDOT:PSS nanofibrils was carefully optimized to balance printability and prevent nozzle clogging. Small-angle X-ray scattering (SAXS) and rheological measurements characterized the microscopic and macroscopic properties of the ink at different concentrations. The 3D printing was performed using a custom-designed Cartesian gantry system with various nozzle sizes (200-30µm). After printing, the structures were dry-annealed at 130°C to improve conductivity. The resulting 3D-printed structures could be further converted into hydrogels through swelling in a wet environment. The researchers characterized the electrical conductivity, flexibility, mechanical properties (Young's modulus via nanoindentation), and electrochemical properties (cyclic voltammetry and electrochemical impedance spectroscopy) of the resulting structures. Multi-material 3D printing was demonstrated by integrating the conducting polymer ink with a PDMS ink. Finally, the fabrication of a soft neural probe for in vivo single-unit recording was demonstrated and its performance evaluated in mice.

The developed PEDOT:PSS-based ink showed superior 3D printability, allowing for the fabrication of high-resolution (down to 30 µm) and high-aspect ratio (over 20 layers) structures, including overhanging features. The dry-annealed 3D-printed conducting polymers achieved high electrical conductivity (over 155 S cm⁻¹), comparable to high-performance conducting polymers reported in the literature. The hydrogel form exhibited a Young's modulus of around 1.1 MPa, significantly lower than the dry state, indicating softness suitable for biointegration. The 3D-printed structures demonstrated excellent flexibility and retained high conductivity even after repeated bending cycles. Electrochemical impedance spectroscopy (EIS) revealed comparable ionic and electronic conductivity in the hydrogels. Cyclic voltammetry demonstrated a high charge storage capability with remarkable electrochemical stability. Multi-material 3D printing using the conducting polymer ink and PDMS allowed for the rapid fabrication of a high-density flexible electronic circuit capable of operating an LED. Furthermore, a soft neural probe with nine channels (30 µm diameter), suitable for in vivo neural signal recording, was successfully fabricated and implanted in mice. The probe successfully recorded continuous neural activities (local field potentials and action potentials) for extended periods, demonstrating the viability of this 3D printing approach for bioelectronic applications.

The findings address the limitations of existing methods for fabricating conducting polymer structures by providing a high-performance 3D printable ink. The superior printability, high conductivity, flexibility, and biocompatibility of the resulting structures open up new possibilities for flexible electronics, wearable devices, and bioelectronics. The demonstrated ability to create complex, high-resolution devices in a fast and streamlined manner significantly advances the field. The successful in vivo recording using the 3D-printed neural probe showcases the potential of this technology for biomedical applications. The relatively high conductivity of both the dry and hydrogel forms opens up potential applications requiring both high conductivity and soft biocompatible interfaces.

This study successfully developed a high-performance 3D printable conducting polymer ink based on PEDOT:PSS, resolving a critical challenge in the field. The resulting ink enabled the fabrication of high-resolution, high-aspect-ratio structures and devices with excellent conductivity and flexibility. The successful creation of functional devices, including a soft neural probe for in vivo recording, demonstrates the broad applicability of this technique. Future research could explore different conducting polymers, optimize ink formulations for further improved properties, and investigate the long-term stability and biocompatibility of the 3D-printed structures in more complex in vivo models.

The study primarily focused on the PEDOT:PSS conducting polymer. The generalizability of the findings to other conducting polymers needs further investigation. The long-term biocompatibility and stability of the hydrogel form in vivo require more extensive testing. While the neural probe demonstrated functionality, larger-scale studies with more animals are needed to establish statistical significance and assess variability.