Mechanics of hierarchical twisted and coiled polymer artificial muscles: Decoupling force from kinematic limits

TL;DR

This paper introduces a hierarchical helical structure for twisted and coiled polymer artificial muscles, enabling decoupling of force from actuation stroke and significantly enhancing their performance through a novel thermo-mechanical model.

Contribution

It presents a new hierarchical design and a coupled thermo-mechanical model that predicts improved force generation without sacrificing actuation stroke in TCP artificial muscles.

Findings

Hierarchical topology amplifies isometric actuation stress.

Optimal bundle complexity balances load-sharing and geometric confinement.

Volumetric energy density is scale-invariant, allowing linear force scaling.

Abstract

Thermally actuated twisted and coiled polymer (TCP) artificial muscles exhibit exceptional specific work capacities but are limited by an inherent competition between load-bearing capacity and actuation stroke. To address this limitation, we investigate a hierarchical helical structure designed to decouple force generation from kinematic limits. We propose a coupled thermo-mechanical model incorporating inter-filamentary contact mechanics and geometric nonlinearities to predict the assembly's equilibrium response. The results indicate that this hierarchical topology significantly amplifies isometric actuation stress compared to monofilament baselines, while maintaining a biological-like contraction stroke of approximately 22%. A critical topological threshold governed by the balance between cooperative load-sharing and geometric confinement is identified. Beyond an optimal bundle complexity, the geometric jamming dominates, as excessive inter-filamentary friction hinders actuation. Furthermore, we elucidate a stiffness-stroke synergy in homochiral configurations, where high helical angles amplify the thermal untwisting torque to overcome increased structural rigidity. Crucially, the volumetric energy density exhibits scale invariance regarding the hierarchical radius, implying that absolute force output can be linearly scaled through geometric upsizing without compromising efficiency. These findings provide a mechanics-based rationale for the structural programming, demonstrating that soft actuator performance limits are dictated by topological order rather than intrinsic material properties.