Helical composite yarn actuators with a wide range of working temperature

Abstract

Actuators have been a significant field in recent years, owning to their wide applications, such as intelligent robots, prosthetic limbs for medical care, deformable textiles and energy harvesting systems. Different types of actuators are reviewed, including traditional electric actuator, flexible fluidic actuator, dielectric elastomer actuator, piezoelectric actuator, electrostrictive elastomer actuator, ionic actuator, shape memory alloy actuator and fiber-based coiled linear actuator. Traditional electric motor actuators and flexible fluidic actuator can achieve high strain, large energy density and high energy conversion efficiency, whereas the structures are so complicate and bulky that difficult to be miniatured. Dielectric elastomer actuators, piezoelectric actuators and electrostrictive elastomer actuators can realize high stress and response frequency, while the actuating voltages are too high to be applied in fields that require high safety. Ionic actuators can realize large strain, whereas the defects of low stress and stability cannot be neglected. Shape memory alloy actuators have significant advantage of high deformability, nevertheless the high hysteresis have restraint their efficient application. The fiber-based coiled linear actuators are particularly advantageous due to their good flexibility and actuating performance, e.g. high stress, strain and specific work, as well as low hysteresis and actuating voltage. However, the structures of most actuators have not been designed and controlled for performance, which were only determined by the selected materials rather than utilizing the proper mutual synergism of elements. Besides, the operation of actuators under low temperature have not been explored sufficiently. It can be seen that the infelxible structure design and limited range of working temperature have restraint the performance and application of actuators. Therefore, for widening the performance and application of current actuators, this study aims to fabricate novel helical composite yarn actuators (HCYAs), which possess simple/flexible/portable/programmable structure, low operating voltage, high strain, high stress, high energy density, fast response, long-time cyclability, wide working temperature range, low hysteresis, low cost and simple fabricating process. To achieve effective actuation among a wide range of working temperature for the HCYAs, the candidate materials in composite structure are supposed to possess different thermal expansion coefficient to each other for realizing significant anisotropy of composite yarn, resistance against extremely low and high temperature, as well as safety. Based on above consideration, polyimide (PI) and polydimethylsiloxane (PDMS) were selected from many fiber substrates and polymer matrixes to fabricate the composite yarn, owning to their temperature resistance, ductile physical property under extremely cold condition, different thermal expansion coefficient and biological safety. Influencing factors of fabricating composite yarn have been investigated for optimizing the morphology and property, e.g. concentration of coating solution, volume fraction of PDMS, coating method, filament number etc. Afterwards, thermally powered PI/PDMS HCYAs based on composite structure were fabricated by super-twisting process, while the morphology of HCYAs have been optimized from filament number, coil level, coil type and heat-setting temperature. The thermomechanical properties of HCYAs were then characterized in terms of isotonic, isometric and isothermal behaviors. In isotonic tests, the actuations can be influenced by heat-setting temperature, filament number, coiled level, load, volume fraction and low temperature condition. Lower heat-setting temperature and more filament number benefited higher volume fraction of PDMS matrix, thus further better actuating performance. Double-level coil HCYAs and single-double-mixed-level coil HCYAs could realize higher tensile actuation compared with the single-level coil HCYAs, owning to the level change during actuations. The typical 6*100f PI/PDMS HCYA can achieve tensile actuation of 20.7% under 1.2MPa among a wide temperature range from -50 °C to 160 °C, while high linearity (R2=0.99927), competitive specific work (158.9J/kg, 4 times of natural muscle) and low hysteresis (6.7%) can be realized. In isometric tests, filament number, coil level and extension rate showed most obvious influence on the actuation, i.e. increased load. More filament number enhanced the volume fraction of PDMS and further the effective actuation. Proper extension rate of sample in isometric tests prevented the compact touch of coils and sample fatigue, thus facilitating higher actuation. Typical 20% extended 6*100f PI/PDMS HCYA can realize nearly tripled stress (from 0.38 MPa to 1.07 MPa) among temperature change from 20 °C to 100 °C, with good cyclability and stability in long period time-delay experiments. An unusual thermally-hardening thermomechanical property was found in the isothermal tests. The relevant mechanism was analyzed through comparing the physical property of PI/PDMS HCYA, PET/PDMS HCYA and PET monofilament HCYA, as well as their components. The balance between diameter increase of spring-like anisotropic fiber (promote modulus) and molecular mobility increase in axial direction (reduce modulus) were verified as the dominant factor for the unusual behavior. This discovery paves a road to adjust the thermomechanical property of materials by designing composite structure rather than changing the materials themselves. Finally, electrothermally powered HCYAs were fabricated by adding conductive layer, thus the actuators can be triggered conveniently by joule heating. The process of electroless deposition of copper and silver has been optimized from solution concentration, processing temperature, processing time, filament number, yarn state etc. The resistance, strength and evenness are used to assess the qualities of conductive composite yarns and electrothermally powered HCYAs. Functional devices adopting electrothermally powered HCYAs were fabricated for practical applications, such as electrothermally powered artificial muscle for robotic arm, actuating strips and actuating fabrics for smart compressive stockings. The combination methods and weaving/braiding processes are developed through optimizing relevant parameters, including tension, filament/yarn number, end-fixing method, texture design etc.