Stronger vacuum artificial muscles by their cross-sectional perimeter modulations

This is the authors' abstract. We don't add key points for this paper.

Increasing the contraction force of vacuum artificial muscles (VAMs) without enlarging their cross-sectional area remains challenging. Here, we demonstrate that increasing the cross-sectional perimeter enhances contraction force at a fixed area. A quasistatic output force model based on virtual work was developed and experimentally validated using circular, regular hexagonal, regular pentagonal, square, and equilateral triangular VAMs of equal cross-sectional area. The model and experiments yielded the same force ranking: triangular, square, pentagonal, hexagonal, and circular, which was consistent with the ranking of their cross-sectional perimeters. After subtracting the common pressure force, the axial force generated by side-skin tension per unit effective perimeter was nearly identical across the five VAMs, indicating approximate proportionality between this force and effective perimeter. Based on this relationship, bioinspired multicell VAMs were developed to increase the total effective perimeter without increasing the total area. A six-cell VAM composed of triangular cells generated twice the contraction force of an equal-area single-cell hexagonal VAM. This advantage persisted as the axial gap between adjacent internal frames varied, showing that the perimeter-based force-enhancement rule is not limited to a specific axial structural configuration. These results establish a simple, general geometric rule for enhancing VAM contraction force without increasing cross-sectional area.