Spider silk has been considered one of the strongest
materials under this weight, but researchers have recently discovered that
spider silk has another unusual property that may contribute to the development
of new artificial muscle or robotic drives.
The team found that this elastic fiber responded
very strongly to changes in humidity.
When the relative humidity in the air exceeds a
certain level, they suddenly contract and twist, generating enough power to
compete with other actuator materials being developed. An actuator is a device
that performs certain activities by moving, such as controlling a valve.
The findings, published in the journal Science
Progress, were presented by Professor Markus Buehler, Head of the Department of
Civil and Environmental Engineering at the Massachusetts Institute of
Technology, Anna Tarakanova, former postdoctoral fellow at the Massachusetts
Institute of Technology, Claire Hsu, undergraduate, and Dabiao Liu, associate
professor at Huazhong University of Science and Technology. Six people
co-authored.
Researchers have recently discovered a property of
spider silk called hyper contraction, in which elongated fibers contract
suddenly as humidity changes.
The new discovery is that this material not only
shrinks, but also twists at the same time, resulting in strong torsion.
“This is a new phenomenon,” Buehler said.
“We first discovered it by chance,” Liu said. “My
colleagues and I wanted to study the effect of humidity on spider silk.”
To do this, they hang a weight on the spider's silk
to make a pendulum and enclose it in a room that controls the relative humidity
of the room.
“When we increased the humidity, the pendulum began
to spin. This was unexpected. It really shocked me.”
The team also tested other materials, including
human hair, but did not find this twisting motion in other materials they
tried.
But Liu said that he immediately began to think,
this phenomenon "may be used for artificial muscles."
Buehler said: "This may be very interesting for
the robotics field." He thinks this is a new way to control certain
sensors or control devices.
“Controlling these movements by controlling humidity
is very accurate.”
Spider silk is known for its exceptional
strength-to-weight ratio, flexibility and toughness. Many research groups
around the world are working to replicate these properties in synthetic
versions of this protein fiber.
The purpose of this torsion is not clear from the
perspective of spiders. Researchers believe that this over-contraction may be
used to cope with moisture in the air to ensure that the spider's silk can be
tightened in the morning dew, protecting it from damage and maximizing the
vibration of the spider web while capturing prey.
"We haven't found any biological significance
in torsional movement," Buehler said.
But through a combination of laboratory experiments
and computer molecular models, they have been able to determine how the torsion
mechanism works. It is based on the folding of a protein building block called
pro line.
The study of this potential mechanism requires a
detailed molecular model, carried out by Tarakanova and Hsu.
“We tried to find molecular mechanisms for what our
collaborators found in the lab,” Hsu explained. “We actually discovered a
potential pro line-based mechanism.”
They show that with this particular pro line
structure, they are able to reproduce the torsional properties in the
simulation, without which there is no torsion.
"Spider silk is a protein fiber," Liu
explained.
"It consists of two major proteins, one is
MaSp1 and the other is MaSp2."
Pro line is essential for torsional reactions. It is
present in MaSp2. When water molecules interact with it, they destroy their
hydrogen bonds in an asymmetric way, leading to torsion.
The twisting is only done in one direction and it
occurs at a threshold of about 70% relative humidity.
"This protein has inherent rotational
symmetry," Buehler said. Through its torsional force, it makes "a
completely new material" possible.
Since this feature has been discovered, he suggests
that it may be replicated in synthetic materials.
“Maybe we can make a new polymer material to
replicate this behavior,” Buehler said.
"Silk's unique tendency to hyper contraction
and its response to external factors such as humidity,
“The unique super-shrinking tendency of silk and its
torsional behavior in response to external factors such as humidity can be used
to design responsive silk-based materials that can be precisely adjusted at the
nano scale.” University of Connecticut Assistant Professor Tarakanova said.
“There are a wide range of potential applications:
from humidity-driven software robots and sensors to smart textiles and green
energy generators.”
It is also possible that other natural materials can
exhibit this property, but if this is the case, this has not been noticed.
“This twisting movement can be found in other
materials we haven't studied yet,” Buehler said.
In addition to possible artificial muscles, this
discovery may also be used to develop accurate sensors for detecting humidity.
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