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6619996 
Journal Article 
Strain-induced crystallization to prolong the lifetime of pneumatic artificial muscles 
Kojima, A; Okui, M; Hisamichi, I; Tadami, N; Tsuji, T; Nakamura, T; , 
2019 
SPIE-INT SOC OPTICAL ENGINEERING 
BELLINGHAM 
WOS:000484718200023 
Various types of soft actuators have been developed for application in wearable movement-assist devices or soft robots. The authors have developed a straight-fiber-reinforced pneumatic rubber artificial muscle (SF-ARM). The SF-ARM is composed of rubber that is reinforced with fibers aligned only in the axial direction. When air pressure is applied to the SF-ARM, the reinforced fibers limit the rubber expansion to the radial direction so that the muscle contracts in the axial direction. The SF-ARM contracts by 38% at maximum, and this contraction rate exceeds the contraction rate of the McKibben artificial muscle. However, the SF-ARM is not well-suited for practical use because the strain on the SF-ARM while it is actuated is large which can cause fatigue failure of the rubber.This study focuses on suppressing the growth of cracks using strain-induced crystallization of the natural rubber, to prolong the lifetime of the SF-ARM. Natural rubbers form a crystalline layer in the direction perpendicular to the direction of stretching. This crystal layer effectively suppresses the growth of cracks in the SF-ARM when under strain. Deliberately developing a crystal layer should extend the lifetime of the SF-ARM. First, this study confirmed the formation of a crystal layer under extension of natural rubber (NR) and styrene butadiene rubber (SBR) using wide-angle X-ray diffraction measurements. Next, the strain concentration near the crack was analyzed using finite element method simulations. Finally, fatigue-life tests were conducted with SF-ARMs made of NR and SBR. 
MartinPalma, RJ; Knez, M; Lakhtakia, A; 
978-1-5106-2585-3 
Conference on Bioinspiration, Biomimetics, and Bioreplication IX, held at SPIE Smart Structures + Nondestructive Evaluation 
Denver, CO