MIT researchers have developed a stretchy optical fiber in which they injected multiple organic dyes (yellow, blue and green regions). In addition to lighting up, the dyes act as a strain sensor, allowing the researchers to quantify where and by how much the fiber has been stretched. Photo courtesy of the researchers.
MIT researchers have developed a stretchy optical fiber in which they injected multiple organic dyes (yellow, blue and green regions). In addition to lighting up, the dyes act as a strain sensor, allowing the researchers to quantify where and by how much the fiber has been stretched. Photo courtesy of the researchers.

Researchers from Massachusetts Institute of Technology (MIT) and Harvard Medical School have developed a biocompatible and highly stretchable optical fiber made from hydrogel – an elastic, rubbery material composed mostly of water. The fiber, which is as bendable as a rope of licorice, could serve as a long-lasting implant that would bend and twist with the body without breaking down. The team has published its results in a paper in Advanced Materials.

Using light to activate cells, and particularly neurons in the brain, is a highly active field known as optogenetics, in which researchers deliver short pulses of light from an LED source to targeted tissues through needle-like fibers.

"But the brain is like a bowl of Jell-O, whereas these fibers are like glass – very rigid – which can possibly damage brain tissues," says Xuanhe Zhao, an associate professor in MIT's Department of Mechanical Engineering. "If these fibers could match the flexibility and softness of the brain, they could provide long-term more effective stimulation and therapy."

Zhao's group at MIT, including graduate students Xinyue Liu and Hyunwoo Yuk, specializes in tuning the mechanical properties of hydrogels. These researchers have devised multiple recipes for making tough yet pliable hydrogels out of various biopolymers. The team has also come up with ways to bond hydrogels with various surfaces, such as metallic sensors and LEDs, to create stretchable electronics.

The researchers only thought about using hydrogels in optical fibers after conversations with the bio-optics group at Harvard Medical School, led by associate professor Seok-Hyun (Andy) Yun. Yun's group had previously fabricated an optical fiber from hydrogel material that successfully transmitted light through the fiber. However, the material broke apart when bent or slightly stretched. Zhao's hydrogels, in contrast, could stretch and bend like taffy. The two groups combined their efforts and looked for ways to incorporate Zhao's hydrogel into Yun's optical fiber design.

Yun's design consists of a core material encased in an outer cladding. To transmit the maximum amount of light through the core of the fiber, the core and the cladding should be made of materials with very different refractive indices, or degrees to which they can bend light.

"If these two things are too similar, whatever light source flows through the fiber will just fade away," Yuk explains. "In optical fibers, people want to have a much higher refractive index in the core versus the cladding, so that when light goes through the core, it bounces off the interface of the cladding and stays within the core."

Happily, they found that Zhao's hydrogel material was highly transparent and possessed a refractive index that made it ideal as a core material. When they tried to coat the hydrogel with a cladding polymer solution, however, the two materials tended to peel apart in response to any stretching or bending of the fiber.

To bond the two materials together, the researchers added conjugation chemicals to the cladding solution. When coated over the hydrogel core, these conjugation chemicals generate chemical links between the outer surfaces of both materials. "It clicks together the carboxyl groups in the cladding, and the amine groups in the core material, like molecular-level glue," Yuk says.

The researchers tested the optical fibers' ability to propagate light by shining a laser through fibers of various lengths. Each fiber transmitted light without significant attenuation, or fading. They also found that the fibers could be stretched over seven times their original length without breaking.

Once they had developed a highly flexible and robust optical fiber, made from a hydrogel material that was also biocompatible, the researchers began to play with the fiber's optical properties. They did this to see if they could design a fiber that was able to sense when and where it was being stretched.

They first loaded a fiber with red, green and blue organic dyes, placed at specific regions along the fiber's length. Next, they shone a laser through the fiber while stretching the region containing one of the dyes, and then measured the spectrum of light that made it all the way through the fiber. If stretching the region with the red dye, they noted the intensity of the red light.

They reasoned that this intensity relates directly to the amount of light absorbed by the red dye, as a result of that region being stretched. In other words, by measuring the amount of light at the far end of the fiber, the researchers can quantitatively determine where and by how much a fiber was stretched.

"When you stretch a certain portion of the fiber, the dimensions of that part of the fiber changes, along with the amount of light that region absorbs and scatters, so in this way the fiber can serve as a sensor of strain," Liu explains.

"This is like a multistrain sensor through a single fiber," Yuk adds. "So it can be an implantable or wearable strain gauge."

The researchers imagine that such stretchable, strain-sensing optical fibers could be implanted or fitted along the length of a patient's arm or leg, to monitor for signs of improving mobility. Zhao envisions the fibers may also serve as sensors, lighting up in response to signs of disease.

"We may be able to use optical fibers for long-term diagnostics, to optically monitor tumors or inflammation," he says. "The applications can be impactful."

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.