Construction of Twisted/Coiled Poly(lactic acid) Fiber-Based Artificial Muscles and Stable Actuating Mechanism

Exploring artificial muscles based on renewable, biodegradable, and biocompatible polymer fibers is essential to broaden their applications in biomedicine and bionic fields. In this work, by constructing a poly(l-lactic acid) (PLLA)/poly(d-lactic acid) (PDLA) stereo-complex, PLA-based as-spun fibers were prepared by melt-spinning, while stretching orientation and annealing processes were further employed to improve the molecular anisotropy of the fibers (PLLA/D-S-A). On this basis, PLLA/D-S-A fiber-based artificial muscles were fabricated by twisting, coiling, and two-step heat setting. With the increase of twisting density/coiling diameter and decrease of merging strand numbers, the spring index as well as the actuating contraction strain of the artificial muscle increased. After initial training, the cyclic shrinkage–recovery process of the artificial muscle was stable, and the contraction strain remained at about 15.30% under 8 V electric stimulus by applying a load with 50 times the artificial muscle weight, exhibiting a high actuating cyclic stability. The high orientation degree in both the crystalline and amorphous regions of the PLLA/D-S-A fiber ensured a high axial negative expansibility under external stimuli, resulting in high actuating contraction strain for artificial muscles. Meanwhile, the perfect oriented crystalline structures endowed fibers with high thermal stability, and the crystallinity, contents of amorphous phase and mesophase, and the crystalline size changed slightly at the actuation temperature, which was conducive to achieving high circulatory stability for artificial muscles. Such PLA-based artificial muscles display promising applications as soft actuators in tissue engineering, healthcare, and so on.