Scientists Find Band Gaps in Spider Silk

Band gaps in spider silk have been found by Rice University Scientists in Singapore and Europe.

Image Credit: Hellen Grig | Shutterstock.com

The scientists state that their new findings could lead to the development of unique materials that can control heat and sound in the same way as semiconducting circuits control electrons.

The study explores the structure of spider silk at a microscopic level, finding new characteristics in its transmission of quasiparticles of sound and phonons, and the details have been published in Nature Materials.

The study is the first to reveal that phonon band gaps can be found in spider silk. Just like the fundamental property of semiconducting materials electronic band gap, which permits and blocks the passing of electrons, phonon band gaps block phonon waves of particular frequency levels.

According to the scientists, their study is the first finding of a “hypersonic phononic band gap in a biological material.”

Though scientists are yet to discover how spiders use of this characteristic, the implications of the property on materials are clear, says Rice Engineering Dean and material scientist, Edwin Thomas.

The co-author of the study notes that other polymers can be developed by imitating the crystalline microstructure of the spider silk. As heat uses phonons to propagate through solids, the imitation of the microstructure can lead to novel thermal or sound insulations and dynamic, tunable metamaterials such as phonon waveguides.

Phonons are mechanical waves, and if a material has regions of different elastic modulus and density, then the waves sense that and do what waves do: They scatter. The details of the scattering depend on the arrangement and mechanical couplings of the different regions within the material that they’re scattering from.

Edwin Thomas, Engineering Dean, Rice University

Spiders use vibrations and locate their food or detect defects in their web by reading and sending them. Scientists belive that the spider’s web is capable of transmitting sound of various ranges that the spider can understand However, the scientists also found that sound can be dampened by silk, to an extent.

“(Spider) silk has a lot of different, interesting microstructures, and our group found we could control the position of the band gap by changing the strain in the silk fiber,” Thomas said. “There’s a range of frequencies that are not allowed to propagate. If you broadcast sound at a particular frequency, it won’t go into the material.”

In 2005, Thomas collaborated with material scientist George Fytas, from the University of Crete and the Institute of Electronic Structure and Laser Foundation for Research and Technology-Hellas, Greece, to draft the characteristics of hypersonic phononic crystals. The study involved the measurement of phonon propagation, and led to the discovery of band gaps in synthetic polymer crystals that are arranged in definite intervals.

Phononic crystals give you the ability to manipulate sound waves, and if you get sound small enough and at high enough frequencies, you’re talking about heat. Being able to make heat flow this way and not that way, or make it so it can’t flow at all, means you’re turning a material into a thermal insulator that wasn’t one before.

Edwin Thomas, Engineering Dean, Rice University

The researchers decided to thoroughly explore a spider’s dragline silk. Dragline is used as a lifeline when the spider is suspended in air, and to construct the spokes and outer rim of the web. The acoustic characteristics of silk are topics of recent research, despite thousands of years of study on the material.

The structure of silk is hierarchical and consists of proteins folded into sheets and leading to the formation of crystals. Soft amorphous chains connect the hard protein crystals, stated Thomas. When the connecting chains are stretched or relaxed, the change in the mechanical coupling between the protein crystals lead to change in the acoustic characteristics.

Brillouin light scattering tests were conducted on silks under different stress levels by Fytas’ team in the Max Planck Institute for Polymer Research located in Mainz, Germany.

That was George’s genius, with Brillouin scattering, you use light to create phonons as well as absorb them from the sample. BLS allows you to see how the phonons move around inside any object, depending on the temperature and the material’s microstructure.

Edwin Thomas, Engineering Dean, Rice University

The researchers observed an increase of 31% in the bandwidth frequency and a reduction of 15% in the phonon velocity when the silk was “super contracted.” A reduction in the bandwidth, by 33% and a 27% increase in the velocity was observed when the silk was strained. A band gap of 14.8 gigahertz and width of about 5.2 gigahertz was observed in the uncontracted silk.

The researchers also observed a “unique region of negative group velocity.” Under these conditions there was a backward movement in the phase velocity, despite a forward movement of phonon waves, said Thomas. They believe that the reaction may enable the focusing of hypersonic phonons.

Right now, we don’t know how to do any of this in other macromolecular fiber materials. There’s been a fair amount of investigation on synthetic polymers like nylon, but nobody’s ever found a band gap.

Edwin Thomas, Engineering Dean, Rice University

Dirk Schneider of ebeam Technologies, Bern, Switzerland, and Nikolaos Gomopoulos of the Swiss Federal Institute of Technology in Lausanne, both formerly of the Max Planck Institute; Cheong Koh of DSO National Laboratories, Singapore; Periklis Papadopoulos of the Planck Institute and the University of Ioannina, Greece; and Friedrich Kremer of the Institute of Experimental Physics at the University of Leipzig, Germany, co-authored the paper. Fytas is University of Crete professor and has an appointment at the Planck Institute. Thomas is a professor of nanoengineering and materials science and of chemical and biomolecular engineering and the William and Stephanie Sick Dean of Rice’s George R. Brown School of Engineering.

The study has been supported by the European Research Council, the Deutsch Forschungsgemeinschaft (German Research Foundation), the Sonderforschungsbereich/Transregio (Collaborative Research Center) and The Aristeia Alliance of the Mediterranean Institute for Scientific Research.

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