Bio-inspired, moisture-powered hybrid carbon nanotube yarn muscles

Artificial muscles that transform input electrical, thermal, or chemical energy to the mechanical energy of tensile contraction, torsional rotation, or bending have attracted enormous interest1,2,3,4. Natural phenomena, such as the opening of pine cones5 and seed dispersal6 in response to changes in humidity, have inspired the development of self-powered hygromorph artificial muscles capable of generating useful mechanical7,8,9,10,11,12,13,14,15 and electrical energy16,17,18,19,20. Since most of these moisture driven artificial muscles produce bending8,9,10,11,12,13,14,15,17,18,19 or torsional15,19 rotation, they are difficult to upscale by configuring multiple actuators to work in parallel. Sahin’s group recently developed a hygromorph tensile actuator16 that could be upscaled, and demonstrated the use of this actuator for evaporation-driven engines and mechano-electrical energy generators. However, the realized gravimetric work capacity (0.017 kJ kg−1)16 was much smaller than demonstrated during contraction for thermally powered hybrid carbon nanotube (CNT) yarn muscles (1.36 kJ kg−1)3 and twisted polymer fibre muscles (2.48 kJ kg−1)4. To develop hygromorph tensile artificial muscle that provide large work capacity, we employed the technique of biscrolling to generate twisted yarn structures that combine both coiled and wrinkled structural features. Previous hybrid yarn tensile artificial muscles have been constructed by biscrolling a guest, or by such methods as infiltrating a paraffin wax guest into a coiled yarn3. Changing the volume of the guest by heating the coiled yarn generates large tensile contractions for yarns made purely by yarn twisting, so that the handedness of yarn twist and coiling are identical3. Given that actuation of a coiled hybrid yarn muscle depends on volume-change-driven dimensional changes, which produce muscle length contraction, yarn diameter increase, and yarn untwist, we sought to enhance muscle performance by introducing a wrinkled structure that was bio-inspired by Bacillus spores. It is well known that the wrinkled coat of the spore can enable a large volume change of 12% by unfolding of the wrinkled structure when water is absorbed17,21. Here, we developed a bio-inspired hybrid yarn artificial muscle (HYAM) with a coiled and wrinkled structure by highly twisting a CNT sheet stack that incorporated a hydrophilic poly(diallyldimethylammonium chloride) (PDDA) guest. Changes to the morphology of the PDDA/CNT HYAM can be driven by absorbing water or by a change in the ambient relative humidity (RH). The water-driven HYAM provides a large tensile stroke (up to 78%), a large gravimetric work capacity (2.17 kJ kg−1) and high volumetric work capacities (1.8 MJ m−3).