Spider dragline silk as torsional actuatordriven by humidity for applications asartificial muscle7 March 2019, by Thamarasee Jeewandara

In the study, Liu et al. used dragline silks from thepictured spider species Nephila eduli, Nephila pilipesand Argiope versicolor. Image credit: Spider ID(spiderid.com/pictures/?fwp_attributes=webs) Credit: Science Advances, doi: 10.1126/sciadv.aau9183

Spider silk is a self-assembling biopolymer withhydrogen bonds underlying its chemical structure,yet despite weak chemical bonding it outperformsmost materials relative to mechanical performance.The biopolymer is produced from the spider major ampullate gland and is an extraordinary fiber thatcan surpass most synthetic materials inmechanical toughness by balancing strength andextension/flexibility. Properties of spider draglinesilk include high thermal conductivity, peculiartorsion dynamics and the potential for exceptionalvibration propagation. To add more distinction tothe natural fiber, spider dragline silk display a giantshape-memory effect upon exposure to water; inan effect known as supercontraction. The uniqueand remarkable properties of spider dragline silkare attributed to its hierarchical structure andmorphology.

In a recent study, now published in ScienceAdvances, Dabiao Liu and co-workers at themultidisciplinary research fields of engineering,physics, molecular mechanics, biomedicalengineering and life sciences, report on the newfeature of humidity-induced torsional behavior of

spider silk. They demonstrated the impact of spiderdragline silk and possible structural origins of thetorsional response in the study with potential toengineer a "whole new class of materials".Understanding the structure-property relationship ofspider silk can benefit materials scientists byproviding an impression of the precise physicalnature of the biopolymer. New biomaterials basedon the significant mechanical properties of spidersilk can be engineered to translate the structure-property relationship of the material into practicalapplications.

Spider dragline silk material is sensitive to waterand can shrink up to fifty percent in length withradial swelling. Water can disrupt hydrogen bondsat high humidity to rearrange the nanocrystallinemolecules to lower energetic configurations,resulting in supercontraction. In applied sciencesand engineering, supercontraction can find originalapplications as artificial muscles or tensileactuators. For instance, spider silk from Nephilaclavipes and Ornithoctonus huwena can display areproducible shrink-stretch behavior due to waterand humidity, allowing cyclical weight lifting tooccur. Recent examples of such applicationsinclude engineered torsional artificial muscles withsynthetic polymers, carbon nanotubes and graphene-made fibers.

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Schematic diagram of the apparatus used to measuretorsional actuation of silks or other fibers driven by therelative humidity (RH). Credit: Science Advances, doi:10.1126/sciadv.aau9183

Although previous studies have investigatedtorsional properties of spider dragline silk, structuralorigin of its torsional behavior remains to beexplored in depth. In this work, Liu et al. observedthe unique behavior of spider dragline silk incomparison to control fibers such as Bombyx morisilk, Kevlar fiber and human hair. The scientistsdesigned the experiments to reveal the stepwisepersonal response of dragline silk to increasedhumidity. They conducted atomistic simulations ofthe two-component proteins MaSp1 and MaSp2 tounderstand the mechanism of structural twistbehavior at the level of the molecule. They thenproposed a possible relationship between theobserved twist deformation driven by humidity andthe molecular structure of dragline silk.

Liu et al. used dragline silks from Nephila pilipes,Nephila eduli and Argiope versicolor spider speciesby successfully replicating a previous method forsilk sample collection. They used an apparatus based on image processing to study humidity-driven torsional actuation of the thin fibers. In theexperimental setup, the scientists used a torsionpendulum made of a single fiber enclosed in ahumidity cabinet and recorded the motion of thependulum using a video camera while increasing or

decreasing the relative humidity (RH). Theydesigned two different protocols to understand theresponse of spider dragline silks to the changinghumidity; one protocol increased the RH stepwiseto maintain high values for a long period of time. Inthe second method, they cyclically changed the RHfrom 40 to 100 percent and returned to 40 percentfive times.

Left: SEM images of the fibers and the responses toenvironmental humidity stimulus. (A) B. mori silk (7.7 ±0.3 ?m in diameter). (B) Human hair (68.7 ± 2.5 ?m indiameter). (C) Kevlar fiber (10.7 ± 0.2 ?m in diameter).(D) Torsional responses of the representative fibers toenvironmental humidity: B. mori silk fiber (65.1 mm inlength), human hair (69.5 mm in length), and Kevlar fiber(86.9 mm in length). A negligible twist driven by humiditycan be seen in these fibers. Right: Torsional actuation ofspider dragline silks by increasing the RH from 40 to100%. (A) Torsional actuation of N. pilipes spiderdragline silk (121 mm in length, 3.1 ± 0.1 ?m indiameter). (B) Rotation speed (blue line) and angularacceleration (red line) of the torsional actuation of N.pilipes spider dragline silk. (C) Torsional actuation of A.versicolor spider dragline silk (87.9 mm in length, 6.7 ±0.1 ?m in diameter). (D) The rotation speed (blue line)and angular acceleration (red line) of A. versicolor spiderdragline silk. Inset shows the SEM images ofrepresentative silks. Credit: Science Advances, doi:10.1126/sciadv.aau9183

Using scanning electron microscopy (SEM), thescientists first characterized the morphology andstructure of spider silks. They conducted screeningtests on three control fibers; B. mori silk, humanhair and Kevlar fiber. The experiments revealed thetorsional responses of the representative fibers toenvironmental humidity. They then observedhumidity-induced cyclic contractions/relaxations ofdragline silk from different spider species to

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understand torsional actuation driven by humidity indragline silk. After the tests, the surface of thedragline silk became rougher than at the initialstage. The spider dragline silk of N. pilipesachieved torsional deformation approximating2550/mm in one direction, a value greater than thatgenerated by carbon nanotube artificial muscles(2500/mm) powered by electricity. The value wasalso 1000 x greater than those reported for otheractuators based on shape-memory alloy andconducting polymers with twist deformation ability.For the A. versicolor dragline silk, the torsionalactuation started at 70 percent RH, this value waslower than that of N. pilipes dragline silk but stillcomparable to carbon nanotube muscles.

Torsional actuation of dragline silks to RH cyclicallychanging from ~40 to ~100%. (A) N. pilipes dragline silk(98 mm in length, 3.1 ± 0.1 ?m in diameter). (B) A.versicolor dragline silk (87.9 mm in length, 6.7 ± 0.1 ?min diameter). (C) N. edulis dragline silk (82 mm in length,2.8 ± 0.1 ?m in diameter). The horizontal dashed linesindicate the RH thresholds to trigger the twist. Thevertical dashed lines indicate the start and end of theinduced twist. Note that the rotation direction of clockwisedirection observed from top to bottom paddle is

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consistent for all silk samples. Credit: Science Advances,doi: 10.1126/sciadv.aau9183

Liu et al. then compared the results from thesecond protocol of cyclic humidity changes in whichthe spider dragline silk showed torsional responsesensitive to humidity, providing a method to controltwist deformation. As the number of RH cyclesincreased, the twist speed and angular accelerationof the dragline silk decreased, indicating thattorsional deformation was reaching a state ofsaturation. The scientists recorded that all silkselongated by approximately 5 to 10 percent aftereach test.

Since humidity-induced twist is a uniquecharacteristic of spider dragline silk, the scientistsinvestigated the molecular structure and themorphology of the material to reveal the underlyingmechanism of this behavior. They also analyzedthe specific secondary structures and hierarchicalstructural organization of the molecule. Liu et al.showed that the presence of proline in the MaSp2protein produced a more pronounced unidirectionaltwist at the scale of the single molecule. Thescientists therefore assumed that the striated linearproline ring orientation may have forced themolecule into a twisted pattern. Using molecularsimulation protocols at the protein level, theyexplained the observed glass transition behavior ofspider silk at high RH.

Mechanisms for humidity-induced torsion in dragline silkson a molecular level. (A) Representative angledisplacement curve for MaSp2, showing consistent andnegative angles traveling down the strands, whichcorresponds to clockwise twist. Inset shows molecularmodel of MaSp2. (B) Representative angle displacementcurve for MaSp1, showing alternating positive andnegative angles. Inset shows molecular model of MaSp1.(C) Hydrogen bond density scaled by the number ofthose residues present in the MaSp2 sequence. Prolineshows the lowest hydrogen bond density compared toother residues. (D) Hydrogen bonds (shown in blue)within a 3-Å radius around (i) glutamine (Gln), (ii) glycine(Gly), and (iii) proline (Pro). (E) Hydrogen bond densityscaled by end-to-end molecular length within a 3-Å radiusaround amino acids Glu, Gly, Ser, Tyr, and all aminoacids in sequences MaSp1 and MaSp2. (F) Hydrogenbonds shown in blue in (i) MaSp1 and (ii) MaSp2molecules. (G) Secondary structure content in MaSp1and MaSp2. (H) The location of proline residues (withproline rings shown in red) in MaSp2 depicts a striated,linear ring orientation. Zoomed panel shows dottedguiding lines representative of linear proline ringorientation. Credit: Science Advances, doi:10.1126/sciadv.aau9183

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In this way, Liu et al. showed that spider draglinesilk can generate a huge twist (up to 2550/mm forN. pilipes and 1270/mm for A. versicolor spiderdragline silks) under 70 percent RH. The scientistsshowed that torsional actuation of the materialcould be controlled simply by tuning the level of theRH. The observed power generated in dragline silkwas not passive but an active change of state inresponse to the driving force of humidity. Thehumidity-induced twist turned the dragline silk to actas a torsional actuator. These research findings willhave applications in the development of humidity-driven soft robots, novel sensors of precisehumidity, smart textiles or green energy devices.

More information: Dabiao Liu et al. Spiderdragline silk as torsional actuator driven byhumidity, Science Advances (2019). DOI:10.1126/sciadv.aau9183

Sinan Keten et al. Nanostructure and molecularmechanics of spider dragline silk proteinassemblies, Journal of The Royal Society Interface(2010). DOI: 10.1098/rsif.2010.0149

J. Foroughi et al. Torsional Carbon NanotubeArtificial Muscles, Science (2011). DOI:10.1126/science.1211220

© 2019 Science X NetworkAPA citation: Spider dragline silk as torsional actuator driven by humidity for applications as artificialmuscle (2019, March 7) retrieved 14 December 2021 from https://phys.org/news/2019-03-spider-dragline-silk-torsional-actuator.html

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