Title: Supercoiling artificial muscles

Abstract

This talk describes a new mechanism for generating large contraction strains caused by swelling of pre-twisted fibres and yarns and is based on our recently published article. The swelling generates torsional strain energy that is released by converting twist to writhe and leading to the formation of loops or ‘supercoils’ along the fibre length. The same process occurs naturally in double stranded DNA and is partially responsible for the packing of the long DNA molecules into chromosomes. Supercoiling is a common everyday experience that occurs by adding twist to fibres, ropes, cables etc. and produces irritating tangles. However, our study is the first to demonstrate supercoiling without any addition of twist. The supercoiling muscles utilised helically oriented filaments embedded in a swellable matrix. Best results were obtained by using polyester sewing thread and crosslinked poly(acrylic acid) (PAA). Two sewing threads were embedded with PAA solution, plied together and heat treated to dry and crosslink the PAA using a diamine crosslinker. Immersion of the samples in acid and base solutions caused increasing swelling of the PAA matrix, which was resisted by the polyester filaments. The helical arrangement of the polyester filaments directed the swelling towards a partial untwist. However, if the sample ends were held to prevent rotation but still allow translation, then the swelling caused supercoiling with concomitant reduction in the end-to-end sample length. The amount of contraction strain was strongly influenced by the applied tension because the number of supercoil loops and their diameter depends on the applied stress. Samples that were over-twisted to form coils before crosslinking the PAA were able to generate ‘coiled coils’ on swelling. These samples showed an unusual combination of both high stroke (70%) and high work capacity (1 J/g) which exceeds the performance of natural muscle by more than 35 times.