On March 1, 2019, the research paper entitled as “Spider dragline silk as torsional actuator driven by humidity” was published by the research group of Liu Dabiao of the Department of Mechanics subordinated to the School of Civil Engineering and Mechanics on Science Advances, a sub-magazine of Science issued by American Association for the Advancement of Science (AAAS). Dr. Liu Dabiao is the first author and one of the joint corresponding authors of the paper, and Professor He Yuming guided such research and postgraduate Zheng Shimin and undergraduates Yu Miao and Yu Longteng participated in such research. The collaborators of the paper also include Professor Markus J. Buehler (joint corresponding), Dr. Anna Tarakanova and Claire C. Hsu of Massachusetts Institute of Technology (MIT), Professor D. J. Dunstan of the Queen Mary University of London and Professor Liu Jie of Hubei University.

The spider dragline silk is a biological protein fiber secreted by the gland of the spider, so great attention is paid to it by the scholars at home and abroad due to its excellent mechanical properties, elaborate hierarchical structure and potential prospect. The spider will give rise to silk of different properties and functions out of the needs of survival or due to stimulation from the outside world during its life circle. The silk secreted by the major ampullate gland, called dragline silk, has the most superior mechanical properties and is famous as “bio-steel”. It is known from previous research that the spider dragline silk is sensitive to water and is featured by “supercontraction”. When relative humidity reaches certain level, it can shrink for about 50% in the direction of length. The property of “supercontraction” of spider dragline silk realizes its huge potential value in the fields such as artificial muscle or stretching actuator. However, how the humidity will affect the torsional deformation of spider dragline silk is unknown yet.

According to the research results of such team, when the relative humidity reaches about 70%, the spider dragline silk starts giving rise to torsional deformation above 300 degrees per millimeter. Torsional deformation of the spider dragline silk can be controlled precisely by the research staff through regulation of relative humidity. This feature is not identified in fibers such as B. mori silk, human hair and Kevlar fiber, etc. Afterwards, the physical mechanism of such phenomenon is elaborated based on the molecular structure of spider dragline silk. Such silk is mainly made up of the MaSp1 and MaSp2 proteins, and the latter contains substantial proline rings which are crucial to the twisting reaction. When water molecules interact with the proline they disrupt its hydrogen bonds in an asymmetrical way that causes the torsional deformation. This discovery can inspire design of a novel torsional actuator or artificial muscle and is conductive to developing new sensors, smart textile or green energy equipment.

“This is a fantastic discovery because the torsion measured in spider dragline silk is huge, a full circle every millimeter or so of length,” says Professor Pupa Gilbert, a famous biophysicist at the University of Wisconsin at Madison, who was not involved in this work. Gilbert adds, “This is like a rope that twists and untwists itself depending on air humidity. The molecular mechanism leading to this outstanding performance can be harnessed to build humidity-driven soft robots or smart fabrics.”

Such work is appraised by Professor Huang Yonggang (a well-known solid mechanician and an academician of NAE) of Northwestern University that “have used silk’s known high sensitivity to humidity and demonstrated that it can also be used in an interesting way to create very precise torsional actuators. Using silk as a torsional actuator is a novel concept that could find applications in a variety of fields from electronics to biomedicine, for example, hygroscopic artificial muscles and humidity sensors. ” It is believed by Professor Huang Yonggang that “What is particularly noteworthy about this work is that it combines molecular modeling, experimental validation, and a deep understanding by which elementary changes in chemical bonding scale up into the macroscopic phenomena. This is very significant from a fundamental science point of view, and also exciting for applications.”

The work is supported by the National Natural Science Foundation of China, the Young Elite Scientist Sponsorship Program by CAST, the Natural Science Foundation of Hubei Province, MSCA-IF, and the Fundamental Research Funds for the Central Universities (HUST), etc.