These mandrel-free-fabricated coiled polymer fibers have spring indexes of: (C) 7.7 (D) 45.2 (E) 2.6 at the ends and 9.6 in the middle, and (F) where the spring index increases from 2.4 to 18.1. The spring index describes the ratio of a spring’s mean diameter to the diameter of its component fiber or wire. Springs with a small spring index are tightly wound and stiffer than those with a higher index, which are more loosely wound and more flexible, such as a Slinky toy. “High-spring-index yarns can be made much less expensively by this mandrel-free method,” Zhang said. Zhang said the fabrication method also enables the spring index to be varied along a muscle’s length.