Spider silk mimics the action of human muscle tissue when exposed to water, a joint China-US research team has discovered, which could lead to new, low-energy materials for devices such as artificial limbs.
In a study published in Applied Physics Letters, a collaboration of scientists in China and the US used silk from the tiny Ornithoctonus Huwena spider to build water-activated artificial muscles.
“Spider silk is a natural biological material with high sensitivity to water, which inspires us to study about the interaction between spider silk and water,” said Zhu Hongwei, a professor at Tsinghua University.
“Ornithoctonus Huwena spider is a unique species as it can be bred artificially and it spins silk of nanoscale diameter.”
Besides the shrink-stretch ability of muscles, the way in which the motion is triggered—how the muscle is actuated—is a key part of its functionality, the study explained. The spider silk fibers, actuated by water droplets, showed impressive behavior in all the ways that matter to muscle performance, it added.
“In this work, we reveal the ‘shrink-stretch’ behavior of the O. Huwena spider silk fibers actuated by water, and successfully apply it on weight lifting,” said Zhu. “The whole process can cover a long distance with a fast speed and high efficiency, and further be rationalized through an analysis of the system’s mechanical energy.”
The research team looked at the actuation process in different scenarios, capturing the movement of the flexing fibers with high speed imaging. They actuated bare fibers on a flat surface (a microscope slide) and while dangling from a fixed point (held with tweezers) before adding a weight to the dangling configuration to test its lifting abilities.
Zhu and his group also investigated the microstructure of the proteins that make up the fibers, revealing the protein infrastructure that leads to its hydro-reflexive action. Electron microscopy gave a clear picture of the smooth inner threads that make up the fibrous structure, and a laser-based imaging technique, called Raman spectroscopy, revealed the precise conformation of the protein folding structures making up each layer. Specific molecular configurations of proteins that have a strong affinity for water and that rearrange in the presence of water are responsible for the spider silk’s actuation, the researchers found.
“Alpha-helices and beta-sheets are two types of secondary protein folding structures in spider silk proteins,” said Zhu. “Beta-sheets act as cross links between protein molecules, which are thought relevant to the tensile strength of spider silk. A-helices are polypeptide chains folded into a coiled structure, which are thought relevant to the extensibility and elasticity in spider silk protein.”
Returning the fiber back to its relaxed state requires only removing the water, which offers conservation along with its simplicity, the study said.
With some fine-tuning, there is also potential for designing the precise behavior of the shrink-stretch cycle. “In addition, as the falling water droplet can be collected and recycled, the lifting process is energy-saving and environmentally friendly,” said Zhu.
“This has provided the possibility that the spider silk can act as biomimetic muscle to fetch something with low energy cost. It can be further improved to complete staged shrink-stretch behavior by designing the silk fiber’s thickness and controlling droplet’s volume.”
Understanding this remarkable material offers new insight for developing any of a number of drivable, flexible devices in the future. “The interaction between matter and liquid may result in the structural changes of materials, which can be further applied to actuators, sensors and flexible devices,” such as artificial limbs, Zhu said.