Systematic Investigation on the Mechanisms for Water Responsive Actuation Using Commercial Sewing Threads

Abstract

Water-responsive materials alter their volumes in response to relative humidity changes. Recent progress in water-responsive materials demonstrates that this novel category of materials can perform better than natural muscles and conventional actuators by utilizing sustainable and renewable water energy. Textile fibers make excellent candidates for viable water-responsive actuators due to their scalability. However, the lack of a fundamental understanding of water responsiveness impedes their real-world applications. In this research, we selected seven different types of commercially available sewing threads: natural cellulose (cotton, flax). natural protein (wool, silk). regenerated cellulose (rayon), and synthetic (nylon, polyester) to systematically investigate their water-responsive behavior. All the sewing threads, without any chemical nor physical modification, except for polyester, repeatedly lifted a weight in response to humidity changes and generated higher specific energy density (15–210 J/kg) than mammalian muscles (8 J/kg). While the maximum water sorption by a sewing thread does not directly correlate to its mechanical actuation, the disruption of intermolecular hydrogen bonding between polymer chains by sorbed water seems critical for WR tensile actuation. In addition, the twisting of fibers to the right degree could direct the WR actuation in the desired direction. Our research with common threads sheds light on the water-responsive mechanisms and provides design guidelines for a large-scale water-responsive actuator applicable in various engineering systems, including energy harvesting, soft robotics, and protective gears that can help expand the applicability of natural fibers.