Abstract:

Objective Artificial muscle actuators are highly desired for applications such as soft robotics, smart wearables and smart textiles due to their inherent flexibility and actuation properties similar to those of skeletalmuscle-like systems. The single component and poor structural stability of fiber-based actuators make it difficult to achieve industrial applications. As a result, yarn-based actuators with low yarn anisotropy and good mechanical properties owing to twisting have been developed. Among them, pneumatic actuators and electromagnetic motors provide low accuracy and bulky equipment, temperature-sensitive actuators prepared from shape memory alloys have high hysteresis under high temperature stimulation, and some moisture-sensitive actuators have low sensitivity, poor cycle recovery effectiveness, complex preparation processes and harsh experimental conditions. Therefore, there is a need for a yarn-based actuator with simple preparation process, stable structure, good cycle recovery effectiveness and promising possibility for industrialization.
Method Inspired by the skeletalmuscle-like systems, the research reported in this paper used hydrophilic viscose filaments and low melting point hydrophobic polyester filaments to prepare the yarn-based actuators by merging, doubling and low temperature thermal annealing. The actuation function of the actuator is achieved by using the asymmetric response of the two materials under external wet stimulating conditions. Specifically, under wet stimulation, the prepared yarn-based actuator using hydrophilic viscose filaments as the actuating source undergoes wet swelling untwisting and transmits torsional potential energy. During wetness reduction, the low melting point polyester filament acts as a spring frame and the fixed torsional deformation is reduced. The yarn-based actuator structure was characterized and the factors influencing the synergistic response of the viscose and polyester filaments to the actuation performance and cycle recovery effectiveness under wet stimulation were analyzed.
Results The results show that the yarn-based actuator has a hierarchical, alternating skeletalmuscle-like structure of viscose and polyester filament bundles (Fig. 2). During the preparation of the yarn, when the yarn was twisted to a critical level, the actuation performance increased with yarn twist, yarn density and viscose content (Fig. 3, Fig. 4 and Fig. 6), and the actuation cycle recovery effectiveness decreased with the increase of the yarn linear density and the viscose content (Fig. 5 and Fig. 7). The maximum contraction strain and maximum contraction stress within a single cycle reached 83.15% as shown in Fig. 6(a) and 9.61 cN as indicated in Fig. 6(b), respectively, and the maximum contraction strain and maximum contraction strain recovery rate >90% within 30 consecutive cycles with no significant fatigue loss during actuation and recovery (Fig.5 and Fig.7), confirming the advantages of the skeletalmuscle-like structure and demonstrating its stable actuation behavior. Depending on the relationship between the range of twist, the twist density, and the viscose content, yarn actuator parameters can be determined (Fig. 8). Sample 9 with 33 tex yarn density and 90% viscose content were twisted to 1 000 turn/m and 2 000 turn/m respectively, resulting in moisture-sensitive flexible robotic arms and smart textiles, such as the medical wound-healing material and humidity-regulating clothing(Fig. 9).
Conclusion The use of this type of yarn-based actuator with a skeletalmuscle-like structure improved the low sensitivity, poor cycle recovery effectiveness and harsh experimental conditions of the moisture-sensitive actuators. The characterization of macroscopic fiber and yarn as well as internal potential energy conversion illustrates the actuation mechanism of the actuation performance and reversion effectiveness, and this work serves as a theoretical benchmark for further enhancing yarn-based smart material properties. Additionally, the trial-preparation fabric actuators in the lab offer opportunities for higher-hierarchical deformation, multifunctional applications, and large-scale production of yarn-based actuators. The development of applications in cutting-edge industrial sectors such as smart textiles and pharmaceutical materials will be the main emphasis of the following phase, which will open up new possibilities and bring yarn-based actuators closer to everyday life.

Key words: viscose filament, polyester filament, yarn-based actuator, moisture-sensitive, skeletalmuscle-like structure, actuation property, cyclic strain recovery effect, smart textile