The increasing demand for advanced robotic systems capable of performing intricate tasks, particularly in delicate applications, has spurred the development of soft robotics. Soft robots offer advantages in terms of safety and compatibility compared to traditional rigid robots. A key challenge in soft robotics is the development of compliant actuators that possess properties such as flexibility, softness, durability, and material compatibility. Ionic soft actuators, a type of artificial muscle, are promising candidates due to their ability to operate based on the movement of ions within an electrolyte membrane, producing bending motion under an applied electric field. Previous research has explored various electrode materials, including carbon nanotubes, graphene, and their composites, to improve actuation performance, focusing on enhancing capacitance and conductivity to achieve high displacement under low voltages. However, challenges remain in achieving the necessary combination of properties for practical applications, such as long-term durability and high actuation speed. This research aims to address these limitations by developing a novel ionic soft actuator utilizing covalent triazine frameworks (CTFs). CTFs are chosen due to their unique properties: high nitrogen content, electronic conjugation, high surface area, and chemical stability under various conditions, all of which are expected to contribute to enhanced actuation performance.

Ionic soft actuators are a type of artificial muscle operating based on the movement of ions within an electrolyte membrane. The application of an electric field causes ion migration, resulting in volume changes and bending. Many studies have investigated different electrode materials for these actuators to achieve high charge storage even at ultralow voltages. Materials such as carbon nanotubes, graphene, and graphdiyne have been explored, showing promising results in terms of bending deformation and blocking force. Research has also focused on improving charge transfer by incorporating conductive networks with high-capacitance materials. While heteroatom-doped porous carbon electrodes enhance actuation performance by increasing surface area and capacitance, limitations exist in the thermal stability of materials during carbonization, leading to compromised conductivity and reduced actuation speed. The authors highlight the advantages of covalent triazine frameworks (CTFs) over other porous carbon structures for overcoming such limitations. CTFs, with their inherent chemical stability and extended conjugation, offer superior performance potential for ionic soft actuators.

The research involved the synthesis of novel conductive CTFs from a polymer of intrinsic microporosity (PIM-1) using an ionothermal method. The PIM-1 precursor was synthesized according to a published procedure. CTF synthesis was performed at different temperatures (400, 500, and 600 °C) using anhydrous ZnCl2 as both a catalyst and solvent. The resulting CTFs (designated TP4, TP5, and TP6) were characterized using various techniques to determine their structural and electrochemical properties. These techniques included ¹H NMR, BET surface area analysis, SEM, HRTEM, N2 adsorption-desorption isotherms, CO2 adsorption, FT-IR spectroscopy, Raman spectroscopy, PXRD, XPS, and elemental analysis. Electrochemical characterization was performed using cyclic voltammetry (CV) in aqueous (KOH, H2SO4) and non-aqueous (EMIM-BF4/acetonitrile) electrolytes to assess specific capacitance. Ionic soft actuators were fabricated by drop-casting electrode solutions (prepared by combining CTFs with PEDOT-PSS) onto both sides of Nafion/EMIM-BF4 electrolyte membranes. Actuator performance was evaluated by measuring bending displacement under various square and sinusoidal input voltages and frequencies using a laser displacement sensor. The phase delay was also determined. A soft robotic touch finger array was constructed using ten TP6PP actuators to demonstrate the practical application of the actuator in playing an electronic piano.

The synthesized PIM-1 based CTFs (TP4, TP5, TP6) exhibited high surface areas (920, 1071, and 1192 m² g⁻¹, respectively), micro- and mesoporous structures, and increased nitrogen content compared to the PIM-1 precursor. The highest surface area and nitrogen content were observed for TP6 (synthesized at 600°C). Electrochemical analysis showed high specific capacitances for TP6, reaching 522, 337, and 467 F g⁻¹ in aqueous H2SO4, KOH, and non-aqueous EMIM-BF4/acetonitrile electrolytes, respectively. The CTF-based actuators (TP4PP, TP5PP, TP6PP) demonstrated significantly improved actuation performance compared to a pure PEDOT-PSS actuator (PP). TP6PP showed the best performance with a peak-to-peak displacement of 17.0 mm under ±0.5 V square wave input and 13.5 mm under ±0.5 V sine wave input, representing 3.1 and 3.4 times improvement over PP, respectively. The TP6PP actuator exhibited a linear relationship between bending displacement and input voltage (0.1-1.0 V), high cyclic stability (99% after 15,000 cycles), and significantly reduced phase delay compared to PP. The mechanism of enhanced performance is attributed to the increased surface charge and charge conduction paths due to the interaction between CTFs and PEDOT-PSS, evidenced by FT-IR and conductivity measurements. Finally, a ten-finger array based on TP6PP actuators successfully played a simple tune on a touchscreen, demonstrating the feasibility of this technology for practical soft robotic applications.

The results demonstrate the successful development of a high-performance ionic soft actuator using CTFs derived from PIM-1. The superior actuation performance of the CTF-based actuators, particularly TP6PP, is attributed to the unique combination of high surface area, abundant heteroatoms, and the extended conjugated network in the CTF structure. The significant enhancement in bending displacement and response speed over existing actuators is a major contribution. The linear relationship between displacement and input voltage signifies the potential for precise control, crucial for robotic applications. The demonstration of a functional soft robotic touch finger array playing a musical instrument highlights the actuator's suitability for practical use in delicate tasks requiring soft touch. These findings offer a significant advancement in soft robotics by addressing the longstanding challenge of creating highly responsive and durable soft actuators for complex applications.

This research successfully synthesized highly porous and conductive CTFs from PIM-1, resulting in a novel soft actuator with exceptional performance. The TP6PP actuator exhibited record-breaking bending displacement under ultralow voltages, along with enhanced stability and speed. The successful demonstration of a soft robotic touch finger array playing a piano underscores the technology's potential for various soft robotics applications. Future research could explore optimization of CTF synthesis, integration with different electrolyte systems, and the development of more sophisticated robotic systems incorporating these advanced actuators.

While the study demonstrates excellent performance, there are limitations. The fabrication process for the actuator is currently a laboratory-based method and may need further optimization for mass production. The long-term durability test was limited to 15,000 cycles, and further testing over a longer duration would provide more comprehensive data. The current application example (playing a piano) is relatively simple and further research is needed to explore the capabilities of this technology in more complex and demanding applications. The cost-effectiveness of the materials and fabrication process needs further assessment for large-scale implementation.