References
"The abrupt halt of a bumble bee's flight when it impacts the almost
invisible
threads of an orb web provides an elegant example of the
amazing
strength and toughness of spider silk. Spiders depend upon
these properties
for survival, yet the impressive performance of silk is not limited
solely to
tensile mechanics. Here, we show that silk also exhibits
powerful cyclic
contractions, allowing it to act as a high performance mimic
of biological
muscles. These contractions are actuated by changes in
humidity alone and
repeatedly generate work 50 times greater than the equivalent mass
of human
muscle. Although we demonstrate that this response is general
and occurs
weakly in diverse hydrophilic materials, the high modulus of spider silk is
such that it generates exceptional force. Furthermore,
because this effect
already operates at the level of single silk fibers, only 5
µm in diameter,
it can easily be scaled across the entire size range at which
biological
muscles operate. By contrast, the most successful synthetic
muscles developed
so far are driven by electric voltage, such that they cannot
scale easily
across large ranges in cross-sectional areas. The potential
applicability of
silk
muscles is further enhanced by our finding that silkworm fibers
also
exhibit cyclic contraction because they are already available
in commercial
quantities. The simplicity of using wet or dry air to drive
the biomimetic
silk
muscle fibers and the incredible power generated by silk offer unique
possibilities
in designing lightweight and compact actuators for robots
and
micro-machines, new sensors, and green energy production." (Agnarsson et al. 2009:1990)
"Spider dragline silk is a model biological polymer for biomimetic research due to its many desirable and unusual properties. 'Supercontraction' describes the dramatic shrinking of dragline silk fibers when wetted. In restrained silk fibers, supercontraction generates substantial stresses of 40–50 MPa above a critical humidity of ~70% relative humidity (RH). This stress may maintain tension in webs under the weight of rain or dew and could be used in industry for robotics, sensor technology, and other applications. Our own findings indicate that supercontraction can generate stress over a much broader range than previously reported, from 10 to 140 MPa. Here we show that this variation in supercontraction stress depends upon the rate at which the environment reaches the critical level of humidity causing supercontraction. Slow humidity increase, over several minutes, leads to relatively low supercontraction stress, while fast humidity increase, over a few seconds, typically results in higher supercontraction stress. Slowly supercontracted fibers take up less water and differ in thermostability from rapidly supercontracted fibers, as shown by thermogravimetric analysis. This suggests that spider silk achieves different molecular configurations depending upon the speed at which supercontraction occurs. Ultimately, rate-dependent supercontraction may provide a mechanism to tailor the properties of silk or biomimetic fibers for various applications." (Agnarsson et al. 2009:325)Journal article
Journal article
Journal article
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"Spider dragline silk is a model biological polymer for biomimetic research due to its many desirable and unusual properties. 'Supercontraction' describes the dramatic shrinking of dragline silk fibers when wetted. In restrained silk fibers, supercontraction generates substantial stresses of 40–50 MPa above a critical humidity of ~70% relative humidity (RH). This stress may maintain tension in webs under the weight of rain or dew and could be used in industry for robotics, sensor technology, and other applications. Our own findings indicate that supercontraction can generate stress over a much broader range than previously reported, from 10 to 140 MPa. Here we show that this variation in supercontraction stress depends upon the rate at which the environment reaches the critical level of humidity causing supercontraction. Slow humidity increase, over several minutes, leads to relatively low supercontraction stress, while fast humidity increase, over a few seconds, typically results in higher supercontraction stress. Slowly supercontracted fibers take up less water and differ in thermostability from rapidly supercontracted fibers, as shown by thermogravimetric analysis. This suggests that spider silk achieves different molecular configurations depending upon the speed at which supercontraction occurs. Ultimately, rate-dependent supercontraction may provide a mechanism to tailor the properties of silk or biomimetic fibers for various applications." (Agnarsson et al. 2009:325)
Journal article
Spider silk as a novel high performance biomimetic muscle driven by humidityJournal of Experimental BiologyJune 12, 2009
Journal article
Supercontraction forces in spider dragline silk depend on hydration rateZoologyMay 24, 2009
Spider silk used as artificial muscle
Journal article
How super is supercontraction? Persistent versus cyclic responses to humidity in spider dragline silkJournal of Experimental BiologyJune 12, 2009
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