MMaterialsgateNEWS 2018/02/06

Silk fibers could be high-tech ‘natural metamaterials’

New research has demonstrated how the nano-architecture of a silkworm’s fiber causes “Anderson localization of light,” a discovery that could lead to various innovations and a better understanding of light transport and heat transfer.

The discovery also could help create synthetic materials and structures that realize the phenomenon, named after Nobel laureate Philip Anderson, whose theory describes how electrons can be brought to a complete halt in materials due to their “scattering and defects.” The new findings relate not to electrons, but to light transport.

Researchers demonstrated how the nano-architecture of the silk fibers is capable of light “confinement,” a trait that could provide a range of technological applications including innovations that harness light for new types of medical therapies and biosensing. This light-confinement effect in biological and natural tissue, which was unexpected, is made possible by the Anderson localization of light, said Young Kim, an associate professor in Purdue University’s Weldon School of Biomedical Engineering.

The new findings suggest silk fibers may represent “natural metamaterials” and “natural metastructures,” Kim said. (A YouTube video is available at

Various research groups have created synthetic “metamaterials” capable of the ultra-efficient control of light. However, metamaterials have limitations because they are often difficult to scale up for commercial production and pose other challenges. Because silk’s nano-architecture is “disordered” instead of meticulously designed periodic structures, the findings suggest a strategy to produce metamaterials that are less expensive to fabricate and manufacture and easier to scale up for industry.

“This is fascinating because realizing Anderson localization of light is extremely challenging, yet we now know that it can be achieved using irregular, disordered nanostructures to create highly packed nanomaterials for strong light scattering as a silkworm produces a silk fiber and spins a cocoon shell in nature,” Kim said.

The findings are detailed in a paper appearing on Wednesday (Jan. 31) in the journal Nature Communications. The paper’s lead author is Purdue postdoctoral research associate Seung Ho Choi.

“Our findings could open up new possibilities for metamaterials and metastructures,” said Kim, who is leading research to better understand the underlying reasons for silk’s white, silvery and lustrous reflection. “I know this is an oxymoron, but we are saying silk fibers represent ‘natural metamaterials’ and ‘natural metastructures.’”

The silk fibers are 10-20 microns in diameter and contain thousands of tiny nanofibrils, each around 100 nanometers wide. For perspective, a human hair is roughly 100 microns in diameter.

A silk fiber has numerous “scattering centers” inside. Anderson localization arises from this light scattering due to disorder in the nanostructure.

“Silk has many nanofibrils, which individually scatter light,” Kim said.

For the Anderson localization to occur, there must be both scattering and interference between scattered light waves. Densely packed irregular nanostructures cause light waves to interfere with each other, sometimes in destructive and sometimes in constructive ways. If constructive, the light is intensified.

“If waves are constructively interfering, this forms a very high energy inside the disordered media,” Choi said.

The small size and roughly parallel arrangement of the nanofibrils are conducive to the effect. The scattering power is maximized when there are many scattering centers and when their size is comparable to the wavelength of the light, both criteria found in the silk fibers.

Whereas commercial optical fibers must be specially engineered with a reflective coating, or cladding, to allow for the confinement of light, the silk fibers are able to achieve the feat naturally due to Anderson localization of light. The Anderson localization creates “modes” that make confinement of light possible without carefully engineered periodic structures. Instead, the same confinement is possible with disordered, more random designs.

“We found that most transmission of light disappears in most of the silk surface. However, counterintuitively, in a small area we found that the energy is confined, and this confined energy is transmitted through localized modes,” Kim said. “The localized mode is a unique pathway for energy flow.”

Although biological structures such as silk diffuse light, other natural materials with similar microstructures do not possess the localized, modes making Anderson localization of light possible.

“Such a difference makes silk particularly interesting for radiative heat transfer.” Kim said. The silk has a high emissivity for infrared light, meaning it readily radiates heat, or infrared radiation, while at the same time being a good reflector of solar light. Because the strong reflectivity from Anderson localization is combined with the high emissivity of the biomolecules in infrared radiation, silk radiates more heat than it absorbs, making it ideal for passive, or “self-cooling.”

“You may have heard that silk underwear can keep you cooler in summertime and warmer in winter,” Kim said. “We have learned the basic mechanism behind this observation.”

The work is led by researchers in Purdue’s Weldon School of Biomedical Engineering; the Department of Agricultural Biology of the National Institute of Agricultural Sciences in South Korea; and the Materials and Manufacturing Directorate of the U.S. Air Force Research Laboratory. A complete list of co-authors is available in the abstract.

“Our findings could open up largely unexplored opportunities for engineering, energy, and biomedical areas,” Kim said. “However, while direct applications could be possible, we really want to learn from silk to help develop material synthesis and design processes in the future.”

Source: Purdue University – 31.01.2018.

Investigated and edited by:

Dr.-Ing. Christoph Konetschny, Inhaber und Gründer von Materialsgate
Büro für Material- und Technologieberatung
The investigation and editing of this document was performed with best care and attention.
For the accuracy, validity, availability and applicability of the given information, we take no liability.
Please discuss the suitability concerning your specific application with the experts of the named company or organization.

You want additional material or technology investigations concerning this subject?

Materialsgate is leading in material consulting and material investigation.
Feel free to use our established consulting services

MMore on this topic

Reconstituted silk can be several times stronger than the natural fiber and made in different forms.

When it comes to concocting the complex mix of molecules that makes up fibers of natural silk, nature beats human engineering hands down. Despite efforts to synthesize the material, artificial varieties still cannot match the natural fiber’s strength. But by starting with silk produced by silkworms, breaking it down chemically, and then reassembling it, engineers have found they can make a material that is more than twice as stiff as its natural counterpart and can be shaped into complex structures such as meshes and lattices. The new material is dubbed regenerated silk fiber (RSF) and could find a host of applications in commercial and biomedical settings, the researchers say. The findings... more read more

Consumers want fuel-efficient vehicles and high-performance sporting goods, municipalities want weather-resistant bridges, and manufacturers want more efficient ways to make reliable cars and aircraft.

What's needed are new lightweight, energy-saving composites that won't crack or break even after prolonged exposure to environmental or structural stress. To help make that possible, researchers working at the National Institute of Standards and Technology (NIST) have developed a way to embed a nanoscale damage-sensing probe into a lightweight composite made of epoxy and silk. The probe, known as a mechanophore, could speed up product testing and potentially reduce the amount of time and materials needed for the development of many kinds of new composites. The NIST team created their probe from a dye known as rhodamine spirolactam (RS), which changes from a dark state to a light... more read more

The silk of the Ornithoctonus Huwena spider demonstrates impressive weight-lifting abilities with efficient, water-driven actuation

Our muscles are amazing structures. With the trigger of a thought, muscle filaments slide past each other and bundles of contracting fibers pull on the bones moving our bodies. The triggered stretching behavior of muscle is inherently based in geometry, characterized by a decrease in length and increase in volume (or vice versa) in response to a change in the local environment, such as humidity or heat. Variations of this dynamic geometry appear elsewhere in nature, exhibiting a variety of mechanisms and structures and inspiring development in artificial muscle technology. Spider silk, specifically Ornithoctonus Huwena spider silk, now offers the newest such inspiration thanks to research... more read more

An international scientific team in which UPM researchers are involved has developed a bioinspired method that for the first time will allow researchers to spin artificial silk fibers as spiders do and to efficiently produce kilometers of silk

Researchers from the Centre for Biomedical Technology at Universidad Politécnica de Madrid were involved in this research project that was recently published in the journal Nature Chemical Biology. The results of the study show the first procedure to produce artificial spider silk by imitating the natural procedure of spiders. This imitation was obtained by developing recombinant proteins with the same water solubility that the natural silk and a spinning system based on, as it is occurs in the spider glands, aqueous solutions, stress generated during the spinning and pH reduction. The production of fibers that equal or improve the excellent properties of spider silk is a breakthrough... more read more

MaterialsgateNEWSLETTER

Partner of the Week

Search in MaterialsgateNEWS

Books and products

MaterialsgateFAIR:
LET YOURSELF BE INSPIRED