A supertough electro-tendon based on spider silk composites

Humanoid robotic hands rely on tendon-driven transmission systems to deliver actuator forces to joints with high dexterity and a compact form factor. Conventional tendon materials (nylon, silicone rubber, PET) show low toughness and high friction, reducing durability under repeated bending and stretch and complicating integration with electrical wiring because they are typically non-conductive. There is a clear materials gap for fibers that are simultaneously highly tough, conductive, and flexible for engineering applications such as robotic tendons. The study aims to create and demonstrate an electro-tendon that unifies mechanical strength and electrical conductivity, enabling a single fiber to transmit both force and electrical signals, and to validate its performance in a humanoid robotic finger/hand.

Prior robotic hands (e.g., Okada Hand, Utah/MIT Hand, DLR Hand) use tendon transmissions but face durability and integration challenges due to tendon materials. Polymer-based conductors typically have toughness <100 MJ/m³ and conductivity <100 S/cm, with PDMS-based conductors around 0.6–10 MJ/m³. Metals (Au, Al, Cu) have excellent conductivity but low toughness (~1–10 MJ/m³), making them unsuitable for flexible, durable tendons. Natural spider silk (Nephila pilipes dragline) is among the toughest natural fibers (~160 MJ/m³), outperforming Kevlar (~50 MJ/m³), suggesting a promising base for enhancement. Conducting polymer PEDOT:PSS can reach up to ~3000 S/cm after annealing and adheres well due to PSS hydrophilicity. SWCNTs provide exceptional mechanical properties (modulus ~1 TPa, tensile strength ~100 GPa) and can enhance conductivity via charge transfer from PEDOT to CNTs. Prior works have explored CNT-reinforced fibers and composites for conductivity and toughness, but materials that jointly achieve very high toughness and high conductivity with stretchability for tendon-like applications remain lacking.

Materials and spider silk collection: Nephila pilipes dragline silk was collected from spiders kept at ~25 °C and >65% humidity, fed live insects. Dragline diameter was 3–4 µm, smooth surface. Surface modification (nano-island seeds): Raw silk was rinsed in ethanol, plasma-treated (O2, 10 min) to introduce hydrophilic groups, immersed in 0.1 M AgNO3 in ethanol (5 min) to form –OAg seed sites, then exposed to 0.01 M TCNQ in ethanol via controlled drop-flow and annealed (100 °C, 5 min) to form a nano-island structure on the silk surface. Composite fabrication (conductive coating): PEDOT:PSS solutions containing SWCNT at 2.5, 5, 7.5, 10, and 12.5 wt% were prepared. Modified silk was coated via drop-flow (1 drop/5 s), then annealed at 80 °C for 5 min. Silk diameter increased to ~6–7 µm. A wrinkled conducting layer formed due to intrinsic silk supercontraction in water-containing PEDOT:PSS, stabilizing the conductive path under strain. Characterization: Morphology and cross-sections were examined by SEM (Zeiss SUPRA55). Cross-sections were prepared by embedding silk in UV-cured epoxy, freezing in liquid nitrogen, and cutting along fiber axis. Raman spectroscopy confirmed SWCNT (G band) presence. Mechanical properties (stress–strain, Young’s modulus, strength, toughness) were measured on an Instron tester (50 N load cell, 100 N grips); Young’s modulus from linear fit below 15% strain; toughness as area under stress–strain to failure. Electrical resistance was measured by two-probe using a Keithley 4200-SCS while axially stretching/compressing samples in a custom fixture. Conductivity stability was evaluated under cyclic strain (0–15%; also up to ~60% for maximum strain tests). DPD simulations: Coarse-grained DPD simulations (LAMMPS) modeled silk peptides as multiblock copolymers with hydrophobic “a” and hydrophilic “b” beads; water as hydrophilic “w” beads; SWCNT as hydrophobic “c” beads with parameters akin to “a”. Simulations ran under periodic boundary conditions in a 60×40×40 Å^3 box, equilibrated, then stretched along x at 5×10^−5 s^−1 under constant volume (NVE). Mechanical responses (toughness, Young’s modulus, strength) versus SWCNT wt% were obtained; due to coarse-graining and solvent, absolute stresses are approximate; focus was on trends and mechanisms (bridge effects between SWCNT and silk proteins). Robotic finger/hand assembly and tests: A PLA 3D-printed finger (human scale) was actuated by an electro-tendon (S-silk composite with 10 wt% SWCNT) routed internally and held by a silicone-based extensor. Finger kinematics (angle relative to vertical and tendon length change) were recorded during full bending (tendon pull 0 to 5.2 cm). Cyclic bending endurance tests compared electro-tendon with natural silk, nylon, carbon fiber, steel fiber, and PDMS fiber (all ~0.3 mm diameter). Lifting tests measured maximum weight a finger could lift with each tendon type (0.3 mm diameter). A pressure feedback system used a high-sensitivity pressure sensor (pyramidal microstructure; sensitivity ~24.8 kPa^−1; response <4 ms) mounted on the fingertip; the sensor was connected in series with a 100 kΩ reference resistor, with the electro-tendon serving as the signal transmission line. Control software stopped finger motion when measured pressure exceeded a programmed threshold. Grasping tasks included a balloon, a needle, and a cosmetic puff, with appropriate stop thresholds.

• The spider silk-based electro-tendon achieves toughness of 420 MJ/m³ (with 10 wt% SWCNT) and conductivity of 1,077 S/cm (at 7.5 wt% SWCNT), outperforming other flexible conductors in combined metrics.
• Conductivity and toughness increase with SWCNT content, saturating near ~12.5 wt%; maximum strain remains ~60% with only ~5% conductivity change at this high strain; conductivity remains nearly unchanged under repeated 0–15% strains.
• Wrinkled conducting layer formed via silk supercontraction maintains a stable conductive path during stretching/compression.
• DPD simulations reveal SWCNT–silk interactions and bridging enhance strength and toughness; simulated trends for toughness, Young’s modulus, and strength versus SWCNT wt% agree with experiments.
• Robotic finger performance: resting angle ~8°; maximum bending ~73° with 5.2 cm tendon pull taking ~1.5 s; electro-tendon resistance remained nearly unchanged across bending angles.
• Endurance: the electro-tendon-enabled finger with toughness 420 MJ/m³ withstood ~40,000 full bending cycles, nearly double that of natural spider silk (~190 MJ/m³); nylon and carbon fibers endured fewer cycles; steel and PDMS fibers failed to complete full bending due to low toughness.
• Load-lifting: with 0.3 mm diameter fibers, the electro-tendon could lift ~7.6 kg, comparable to steel fiber and higher than nylon, commercial carbon fiber, natural spider silk, and PDMS fibers.
• Pressure-feedback grasping: sensor-to-software signaling via the electro-tendon enabled grasping without damage. Example mappings: 0 Pa → 0°, 113 Pa → 19°, 327 Pa → 32°, 749 Pa → 43°. Balloon (4.8 cm diameter) grasped with stop threshold 170 Pa; measured 178 Pa. Nylon (insulating) tendon failed due to lack of feedback. Needle grasp stopped at ~0.015 N (>0.012 N threshold); puff grasp stopped at ~430 Pa (>400 Pa threshold).

The work addresses the materials gap in tendon-driven robotics by integrating mechanical robustness and electrical conductivity into a single fiber. By combining spider silk’s intrinsic toughness with SWCNT reinforcement and a PEDOT:PSS conductive coating, the electro-tendon simultaneously transmits actuation force and electrical feedback signals. The wrinkled conductive layer maintains electrical pathways under cyclic deformation, enabling reliable feedback during motion. Simulation and experiments consistently show that SWCNT introduce bridging interactions that enhance mechanical performance without sacrificing extensibility. In robotic demonstrations, the electro-tendon simplifies system architecture by eliminating separate wiring for sensors, enabling compact, human-sized fingers with responsive grasp control. The material’s endurance over tens of thousands of cycles and high load capacity suggest suitability for practical robotic manipulation where durability and gentle handling are required. Compared with conventional polymer conductors and metals, the composite achieves a superior toughness–conductivity combination, making it relevant beyond robotics for flexible electronics where both robustness and conductivity are critical.

A spider silk/SWCNT/PEDOT:PSS composite electro-tendon was developed that combines very high toughness (420 MJ/m³) with high conductivity (1,077 S/cm) and stable performance under repeated bending and stretching. The material enables humanoid robotic fingers to both actuate and carry feedback signals through a single fiber, simplifying design and allowing delicate, feedback-controlled grasping of diverse objects. Potential applications include robust interconnects for flexible electronics, anti-static durable textiles, and tough cables in engineering systems. Potential future research directions include scaling fabrication and uniformity control of coatings, improving SWCNT dispersion and loading strategies, long-term environmental stability testing (humidity, temperature, wear), integration into multi-fingered hands and full prostheses, and exploring bio-inspired architectures or alternative nanofillers to further optimize the toughness–conductivity balance.

• SWCNT dispersion in water was poor, limiting additional mechanical gains beyond ~10–12.5 wt% loading despite increased CNT content.
• DPD simulations, due to coarse-graining and inclusion of solvent, yield approximate absolute stress values and elevated ultimate strains; results were interpreted qualitatively for trends and mechanisms rather than exact values.
• Robotic demonstrations focused on basic grasping tasks and specific test conditions; broader operational environments and long-term aging effects were not reported.