Silk’s Photonic Talents Brought to Light at FiO 2012

Natural silk's potential as an eco-friendly way to manipulate light for applications such as biosensors, lasers and photonic chips was presented by US and French researchers this week at the Optical Society's 96th annual meeting, Frontiers in Optics (FiO) 2012.

The golden orb web spider is common and, thanks to its big size (6 to 10 cm), its silk can be extracted easily. Courtesy of Michel Pézolet.

One of the strongest fibers in nature (the dragline used by spiders to form the structure of their webs is stronger, pound for pound, than steel), silk is biocompatible, biodegradable, and extremely hardy. Produced naturally by spiders and silkworms, it is a renewable resource. Added to these benefits is the more recent discovery that silk is a gifted manipulator of light, which can travel through silk almost as easily as it flows through glass fibers.

At presentations on Oct. 15, biomedical engineering professor Fiorenzo Omenetto of Tufts University in Boston and physicist Nolwenn Huby of CNRS Institut de Physiques de Rennes in France spoke on the optic and photonic applications of silk.

In his talk, “Silk-based Optics and Photonics,” Omenetto discussed his group's work fabricating concoctions of proteins that make use of silk's optical properties for implantable sensors and other biology-technology interfaces. His team works with the biopolymer silk fibroin extracted from silkworm cocoons and is pursuing it as a way to integrate optics with living tissue.

In its efforts to exploit silk’s optical benefits, Omenetto's group is developing silk-based materials that look like plastic but that retain the optical properties of pristine silk. One advantage of these materials is that they can degrade and be reabsorbed by the body. A sensor or tag made of silk protein could be implanted — at the site of a fractured bone, for example, to monitor healing — and merely left to dissolve. Once its purpose was served, harmlessly fade away.

Omenetto is investigating a range of questions, from fundamental to commercial, that go beyond implantable optics. His team recently won an INSPIRE grant from the National Science Foundation to create electronic components that are compostable. He has developed and tested a blue laser made from silk fiber-doped materials that not only is biodegradable, but that also uses less power to induce lasing than the acrylic materials that are commonly used. He is also exploring the possibilities of using silk to integrate a technological component with living tissue.

"We’re thinking of how to scale up [production], how to interface with current technology," he said. He hopes some of the more "gadgetlike" fruits of his labor will be commercially available within the next five to 10 years.

A pristine silk fiber integrated into a photonic chip. The fiber connects three disks that can hold light. During the experiment, light is injected into one of the disks and propagates along the silk to the other two. Courtesy of Nolwenn Huby.

Silk-doped composites were the subject of Omenetto's talk at FiO, but the optical merits of pristine silk were the subject of Huby’s ("Optical Propagation and Integration of Pristine Major Ampullate Spider's Silk Fibers"). Her team is experimenting with pure spider silk as a relatively inexpensive and ecologically friendly way to manipulate light within photonic chips.

Huby talked about her team's use of pristine, natural spider silk to guide light through photonic chips — technology that could give birth to silk-based biosensors and medical imaging devices for use in the body.

Huby's silk is collected by a group of molecular spectroscopy experts led by Michel Pézolet of Laval University in Quebec City, then integrated into circuits at her team's lab at Rennes. To her knowledge, this is the first time the optical properties of pristine silk have been exploited.

By integrating real spider silk into a microchip, the researchers found that silk could not only propagate light, but also direct, or "couple," it to selected parts of the chip. Huby hopes this work can be used to create biosensors that could detect the presence of a molecule or the activity of a protein: As light waves passed through a blood sample, for example, the molecules would change the properties of the light wave in an observable way.

A close-up view of a silk fiber taken with scanning electron microscope. Optics and photonics applications of silk were the subject of two papers presented at Frontiers in Optics 2012. Courtesy of Nolwenn Huby.

The next step for her team, Huby said, is to see what biologists and medical professionals need and then create devices they can use. Besides making biosensors, spider silk could also provide a light source for taking pictures inside the body. Natural silk is only 5 µm in diameter, less than a tenth the width of a human hair. A pristine fiber of spider silk that is so thin and strong could carry light into the body through a very small opening, providing less invasive ways to perform internal imaging or even chemical diagnoses using spectroscopy, which is the analysis of matter based on its interaction with light.

FiO 2012 is being held together with Laser Science XXVIII, the annual meeting of the American Physical Society Division of Laser Science.