CTF-based soft touch actuator for playing electronic piano

The study addresses the challenge of achieving large, fast, and controllable bending deformation in electro-ionic soft actuators under ultra-low voltages for delicate soft robotic tasks. Conventional ionic actuators often lack high displacement and exhibit irregular responses at low voltages due to limited capacitance, conductivity, and ion transport. The authors propose employing covalent triazine frameworks (CTFs) derived from PIM-1 to create nitrogen/oxygen-rich, conjugated, hierarchically porous electrodes blended with PEDOT-PSS. The hypothesis is that CTFs’ high surface area, extended π-conjugation, and heteroatom content will increase charge storage, enhance conductivity, speed ion transport, and thereby improve bending performance, stability, and controllability at ultralow voltages, enabling applications like soft touch on fragile interfaces.

The paper surveys electro-ionic actuators and related electrode materials: CNTs, graphene, CNT-graphene hybrids, and graphdiyne have achieved high capacitance and notable displacement (e.g., graphdiyne 237 F g−1, 16 mm at 2.5 V). Nitrogen-rich 3D nanostructures reached 325 F g−1 and 6 mm at 0.5 V. MXene-based actuators showed high bending under 1.0 V AC. Conductive meshes (e.g., graphene mesh with N-doped graphene) enhanced performance by >600%. However, many porous carbons from MOFs/COFs lose long-range conjugation upon carbonization (>400 °C), limiting charge delocalization and dynamic response (phase delay). CTFs are highlighted as robust, nitrogen-rich, conjugated porous frameworks with high surface area and stability used in catalysis, gas storage, batteries, and supercapacitors (e.g., 383 F g−1 reported). The authors identify a gap: CTFs have not been applied to soft actuators, and PIM-1 as a precursor could yield CTFs with permanent microporosity and enriched heteroatom content to overcome prior limitations.

Materials and synthesis: PIM-1 was synthesized via nucleophilic substitution between tetrafluoroterephthalonitrile and 5,5,6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane in DMF with K2CO3 at 65 °C (72 h), followed by precipitation, solvent exchange, washing, and vacuum drying (yield 66.5%). CTFs (TP4, TP5, TP6) were prepared ionothermally by mixing PIM-1 (100 mg) with anhydrous ZnCl2 (5 equiv.), vacuum drying (100 °C, 8 h), flame-sealing, and heating to 400/500/600 °C (1 °C/min) for 48 h. Post-treatments included refluxing in water (8 h), acid washing (1 M HCl overnight), thorough rinsing to neutrality, acetone wash, and vacuum drying. A 700 °C sample (TP7) was also synthesized to assess thermal limits. Characterization: Morphology by SEM and HRTEM; porosity by N2 adsorption-desorption at 77 K (BET surface area) and Ar adsorption at 87 K (microporosity); CO2 adsorption at 273 K up to 1 bar; FT-IR, Raman, powder XRD, solid-state 1H/13C CP-MAS NMR; XPS (survey and high-resolution N 1s and C 1s); EELS elemental mapping; TGA/DTG under N2. Electrochemistry: CV of CTF powders (three-electrode cell; Ag/AgCl reference, Pt counter) in 1 M H2SO4, 1 M KOH, and 0.5 M EMIM-BF4/ACN, scan rates 1–10 mV s−1, potential window −0.5 to +0.5 V. Powder electrodes were fabricated by mixing CTF (80 wt%), acetylene black (10 wt%), and PTFE (10 wt%) and loading onto glassy carbon. Actuator fabrication: Nafion in DMAc (0.05 g/mL) was mixed with EMIM-BF4 (0.03 g/mL), cast to form an 80 µm ionic-liquid-loaded membrane. Electrodes were prepared by dispersing CTFs in DMSO (1 mL, sonication) and mixing with PEDOT-PSS to 0.35 mg mL−1 CTF concentration, stirring overnight. Equal volumes were coated on both sides of the Nafion/EMIM-BF4 membrane and dried (70 °C, 1 h), yielding actuators TP4PP, TP5PP, TP6PP (thickness 115 ± 5 µm). A control PP actuator used PEDOT-PSS only. Electrode thicknesses were 17–20 µm. Electrical/mechanical: Four-probe conductivity of electrodes; tensile testing of membranes/electrodes; blocking force via a 50 mN load cell. Actuation testing: Tip displacement recorded with a laser sensor under AC (sine and square, ±0.1 to ±1.0 V, 0.1–10 Hz) and DC (±0.5 V) in air. Phase delay Δφ calculated as 2πfΔt by comparing input voltage and output displacement. Durability tested up to 15,000 cycles at ±1.0 V, 1 Hz. Mechanistic probing: FT-IR shifts of PEDOT-PSS C–O and CTF C=N bands; electrode conductivity changes with CTF addition; CV of full actuators (two-electrode cell) to estimate areal capacitance; EIS in Supplementary Information. Demonstration: A 10-actuator TP6PP array arranged with electrical connections and a custom hardware keyboard to individually address actuators, mounted over a smartphone touch screen running a piano app, used to play “Happy Birthday.”

• CTF synthesis and structure: TP4/TP5/TP6 prepared at 400/500/600 °C retained hierarchical porosity and conjugated frameworks; BET surface areas: 920, 1071, 1192 m² g−1 (TP4, TP5, TP6). TP7 (700 °C) degraded: surface area reduced to 404 m² g−1. Elemental N increased from PIM-1 (~6%) to TP4 9.30%, TP5 11.40%, TP6 10.40%. CO2 uptake at 273 K, 1 bar: TP4 133 mg g−1, TP5 151 mg g−1, TP6 152 mg g−1 (vs PIM-1 111.4 mg g−1), up to 36% higher. Ar sorption (TP6) confirmed micropores centered at ~1.1 nm.
• Spectroscopy and crystallinity: FT-IR showed disappearance of PIM-1 nitrile at 2241 cm−1; CTFs displayed C=C/C=N stretches (1622–1490 cm−1). Raman D/G bands with IG/ID increasing from 1.47 (TP4) to 1.58 (TP6), indicating larger condensed frameworks. XRD indicated semicrystalline structures. XPS N 1s deconvolution showed increased graphitic N from TP4 to TP6.
• Electrochemistry (powders): Rectangular CVs in aqueous and ionic liquid electrolytes (−0.5 to +0.5 V). TP6 exhibited highest specific capacitances at 0.01 V s−1: 522 F g−1 (1 M H2SO4), 337 F g−1 (1 M KOH), 467 F g−1 (0.5 M EMIM-BF4/ACN). CVs remained rectangular up to 10 mV s−1.
• Electrochemistry (actuators): Areal capacitance at 1 mV s−1: PP 10 F cm−2, TP4PP 36 F cm−2, TP5PP 73 F cm−2, TP6PP 114 F cm−2 (>11× PP). Electrodes’ conductivity increased: PP 5.88 S cm−1; TP4PP 13.32 S cm−1; TP5PP 34.71 S cm−1; TP6PP 60.88 S cm−1.
• Actuation performance: Under ±0.5 V, 0.1 Hz square wave, TP6PP achieved 17.0 mm peak-to-peak displacement (3.1× PP’s 5.66 mm). Under sine wave, TP6PP reached 13.5 mm (3.4× PP’s 4.0 mm). Displacement scaled linearly with voltage (±0.1 to ±1.0 V), R² = 0.9794. At 1.0 V sine, across 0.1–10 Hz, TP6PP maintained the highest displacement; all decreased with frequency.
• Phase delay: TP6PP showed phase delays 4.1× (at 0.1 Hz) and 3.5× (at 5.01 Hz) lower than PP, indicating faster charge/discharge.
• DC actuation: At 0.5 V DC, TP6PP reached 11.92 mm maximum displacement (no back-relaxation), ~1.4× higher than AC at same amplitude; ~3.2× PP at all times. Rise time to maximum <10 s.
• Durability: TP6PP maintained ~99% displacement after 15,000 cycles at ±1.0 V, 1 Hz in air.
• Blocking force: Under 2.0 V DC, TP6PP generated blocking force equal to 24× its weight and ~100% higher than PP; force can be further improved by design/material changes (noted by authors).
• Mechanism support: FT-IR shifts (PEDOT C–O from 1260 to 1265 cm−1; CTF C=N from ~1590–1495 to 1595–1516 cm−1) and conductivity gains support enhanced charge transfer paths and altered PEDOT-PSS configuration via CTFs’ electron donor/acceptor behavior, increasing areal capacitance.
• Demonstration: A 10-actuator TP6PP array reliably played a smartphone piano app (Happy Birthday), validating gentle, controlled soft touch on fragile screens.

The findings validate that integrating PIM-1-derived CTFs with PEDOT-PSS addresses key limitations of ionic soft actuators at ultralow voltages. The hierarchical porosity and extended conjugation of CTFs increase accessible surface area and charge delocalization, while nitrogen/oxygen functionalities elevate surface polarity and capacitance. Consequently, actuators exhibit substantially higher bending displacement at ±0.5–1.0 V, faster response (reduced phase delay), and excellent durability without back-relaxation, which are critical for precise control in soft robotics. Linear displacement-voltage behavior simplifies control system design and indicates efficient, loss-minimized charge transport through the conjugated CTF/PEDOT network. The successful smartphone piano demonstration underscores practical soft-touch capability on fragile interfaces, indicating applicability to delicate manipulation tasks (e.g., HMIs, biomedical device operation).

The study introduces metal-free, PIM-1-derived covalent triazine framework electrodes blended with PEDOT-PSS to realize high-performance electro-ionic soft actuators operating at ultralow voltages. TP6-based actuators delivered record bending displacements (17.0 mm at ±0.5 V square; 13.5 mm at ±0.5 V sine), markedly reduced phase delay, linear voltage-displacement characteristics, and 99% durability over 15,000 cycles, with no back-relaxation under DC. Mechanistic analyses attribute performance to CTFs’ hierarchical porosity, high capacitance, and enhanced electronic pathways altering PEDOT-PSS charge distribution. A 10-actuator array successfully performed soft touch on a smartphone to play a piano app, demonstrating practical relevance. Future work could increase blocking force and functionality by adjusting actuator thickness, employing stiffer ionomer membranes, or incorporating additional conductive/heavy fillers (e.g., metal nanowires) on CTFs, while maintaining the metal-free advantages and stability of CTFs.

• Processing constraints: Ionothermal synthesis is limited to ≤600 °C; higher temperatures (e.g., 700 °C) degrade CTF backbones and reduce surface area.
• Dispersion challenge: CTFs are difficult to disperse in many common solvents; DMSO was required to achieve homogeneous mixing with PEDOT-PSS.
• Mechanical trade-off: Adding CTFs increased electrode stiffness, though electrochemical benefits outweighed reduced compliance.
• Force output: While improved, blocking force remains a target for further enhancement (e.g., thicker actuators, stiffer membranes, or added conductive fillers).
• Evaluation scope: Performance was demonstrated primarily in air and on touchscreen tasks; broader environmental and long-term operational testing in varied conditions were not reported.