A supertough electro-tendon based on spider silk composites

Humanoid robotic hands, particularly those used as prosthetics, rely heavily on tendon-driven transmission systems for dexterity and simplicity. Traditional tendon materials, such as nylon, silicone rubber, and PET, are limited by their low toughness and high friction, leading to reduced durability and performance. Furthermore, the separate requirement for conductive wires for sensing systems creates complexity in the design of these robotic hands, especially for slender fingers. This necessitates a material that combines high toughness, conductivity, and stretchability—a combination not found in currently available materials. Metals, while highly conductive, lack the necessary toughness, and polymer-based conductors suffer from both low toughness and conductivity. This research aims to address this gap by developing a novel electro-tendon material that exhibits superior mechanical and electrical properties, enabling the creation of more robust and functional robotic hands.

The existing literature highlights the challenges of creating durable and functional tendon-driven robotic hands. Studies on the Okada Hand, Utah/MIT Hand, and DLR Hand illustrate the widespread use of tendon-driven systems, but also point to the limitations of conventional tendon materials. Previous research on polymer-based conductors shows low toughness (<100 MJ/m³) and conductivity (<100 S/cm), while traditional metals, though conductive, also lack sufficient toughness. Spider silk, known for its exceptional toughness, has been explored in other material applications but has not been effectively combined with conductivity for tendon use in robotics. This study builds on the existing understanding of material limitations and explores the potential of spider silk as a solution.

The electro-tendon was fabricated using *Nephila pilipes* spider dragline silk, single-walled carbon nanotubes (SWCNTs), and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). A layer of PEDOT:PSS was coated onto the spider silk to achieve high conductivity, leveraging its inherent flexibility and good dispersibility. SWCNTs were introduced to enhance the toughness of the silk. The process involved several steps: (1) preparing a nano-island structure on the spider silk through hydrophilization, AgNO3 treatment, and TCNQ immersion and annealing; (2) preparing the conductive spider silk composite by immersing the modified silk in PEDOT:PSS solution containing varying weight percentages of SWCNT and annealing; and (3) characterizing the morphology, mechanical (using an Instron mechanical tester), and electrical properties (using a Keithley 4200-SCS system). Dissipative particle dynamic (DPD) simulations were performed to understand the microscopic interaction between SWCNT and silk proteins. The electro-tendon was integrated into a 3D-printed robotic finger, and its durability was assessed through cyclic bending tests. A pressure feedback system was implemented to enable grasping functions, and the performance was evaluated by grasping different objects.

The fabricated spider silk composite electro-tendon achieved a remarkable toughness of 420 MJ/m³ and conductivity of 1077 S/cm, exceeding the properties of other flexible conductors. The material withstood more than 40,000 bending-stretching cycles without significant changes in conductivity. The wrinkled structure of the conductive layer, resulting from the intrinsic shrinkage of spider silk, ensured stable electrical signal transmission under strain. DPD simulations confirmed the crucial role of SWCNT in improving the mechanical properties of spider silk. The robotic finger incorporating the electro-tendon demonstrated superior durability, withstanding nearly 40,000 cycles of bending, compared to other materials. The finger also exhibited a substantial load-bearing capacity, lifting a weight comparable to a steel fiber based finger. The feedback system using the electro-tendon allowed precise grasping of objects, a function not achievable using traditional non-conductive tendons.

The results demonstrate the successful fabrication of a high-performance electro-tendon that surpasses the limitations of existing tendon materials for robotic hands. The combination of spider silk's toughness and the conductive properties of PEDOT:PSS and SWCNTs creates a material ideal for this application. The ability of the electro-tendon to simultaneously transmit both actuation and sensing signals simplifies the robotic hand design and enhances its functionality. The superior durability and load-bearing capacity contribute to more robust and reliable robotic hands. The successful grasping experiments highlight the practical implications of this novel material in creating more dexterous and responsive robotic systems.

This research successfully demonstrates the fabrication of a supertough, conductive electro-tendon based on spider silk composites. This novel material significantly improves the performance and durability of tendon-driven robotic hands, simplifying the design and enhancing grasping capabilities. Future research could explore the use of different spider silk types, optimization of SWCNT dispersion, and integration with more sophisticated sensing and control systems for advanced robotic applications beyond grasping.

The study primarily focused on the mechanical and electrical properties of the electro-tendon in a controlled laboratory setting. Further research is needed to evaluate the long-term performance and reliability of the material in real-world applications. The scalability and cost-effectiveness of the production process should also be considered for wider adoption. The current robotic hand design is relatively simple and further improvements could be made by incorporating more complex grasping strategies and advanced control algorithms.