Morphology modulation of artificial muscles by thermodynamic-twist coupling

Abstract

Human muscles can grow and change their length with body development; therefore, artificial muscles that modulate their morphology according to changing needs are needed. In this paper, we report a strategy to transform an artificial muscle into a new muscle with a different morphology by thermodynamic-twist coupling, and illustrated its structural evolution during actuation. The muscle length can be continuously modulated over a large temperature range, and actuation occurs by continuously changing the temperature. This strategy is applicable to different actuation modes, including tensile elongation, tensile contraction, and torsional rotation. This is realized by twist insertion into a fibre to produce torsional stress, and fibre annealing causes partial thermodynamic relaxation of the spiral molecular chains, which serves as internal tethering and inhibits fibre twist release, thus producing a self-supporting artificial muscle that actuates under heating. At a sufficiently high temperature, further relaxation of the spiral molecular chains occurs, resulting in a new muscle with a different length. A structural study provides an understanding of the thermodynamic-twist coupling. This work provides a new design strategy for intelligent materials.

High-Modulus Homochiral Torsional Oxide Ceramic Artificial Muscles

“…30,31 For example, twisting polymer yarns generates torque energy, and the yarn can form a coiled muscle when enough twists are inserted. 32,33 When the arti cial muscles are stimulated, the increased internal stress results in radial expansion of the yarn volume and axial contraction. 34,35 Due to the spiral structure, the axial shrinkage increases the yarn twist and generates more torque energy to balance the internal stress.…”

Tethering of twisted-fiber artificial muscles

“…75,81,[89][90][91][92][93][94] In the second section, we discuss the self-supported, twisted artificial muscle fibers obtained via the balanced geometrical design of the twisted fibers; these methods involve the use of self-plied yarns and tendril-like architectures. 82,89,90,95,96 In the third section, we describe the self-supported, twisted artificial muscles constructed using chemical bonds or non-covalent interactions, such as polymer infiltration and coating, 76,87,98,99 solvent immersion and evaporation, 70 thermal annealing, [100][101][102][103][104][105] and cross-linking of chemical bonds. 106,107 In the following sections, we summarize the abovementioned tethering methods that are adopted considering the actuation modes (rotation, contraction, and elongation) and actuation stimuli (thermal, electro-thermal, electrochemical, and water and organic moisture) of the actuators.…”

“…60,87,100,[112][113][114] Self-supported multimodal artificial muscles were created by accurately controlling the annealing temperature and the actuation temperature to modulate the internal stress state of the polymer chains in their amorphous phase. 105 Using this thermodynamic-twist coupling strategy (a strategy combining thermodynamics and mechanical twisting), reversible, irreversible, and partially reversible actuation modes can be realized with a wide range of actuation and annealing temperatures; these temperatures range from room temperature to the softening temperature of the material considered (Fig. 6c).…”

Perception-Actuation Integrated Artificial Muscle Fibers: From Structural Design to Applications

“…Artificial muscle fibers that provide large linear contractile strokes and high mechanical energy output are of great interest because of their potential applications in some smart structures and systems. − Materials for lightweight and flexible artificial muscle fibers are abundant, including but not limited to synthetic polymers, − natural polymers, − carbon nanomaterials, − and nickel–titanium alloys. − Artificial muscle fibers prepared from these materials in response to stimulation can generate a >50% reversible contractile stroke and a >2 J g –1 work capacity, − which are larger than the ∼20% contractile stroke and 0.04 J g –1 work capacity achieved by natural skeletal muscle . The development vision of artificial muscle fibers is not only to surpass natural muscle in terms of actuation performance but also to reproduce more similar functionalities of natural muscle, which is essential for their real-world application in some advanced and precise devices.…”