Researchers at the Massachusetts Institute of Technology (MIT) and Harvard Medical School have developed a biocompatible and highly stretchable optical fiber that could be implanted in a patient's body and use light to stimulate cells or sense signs of disease.

The fiber is made of hydrogel, an elastic, rubbery material composed mostly of water, and could bend and twist in a patient’s body without breaking down.

The MIT team behind this development is Xuanhe Zhao and his team of graduate students including Xinyue Liu and Hyunwoo Yuk have specialised in tuning the mechanical properties of hydrogels.

They worked on several recipes to make tough yet pliable hydrogels from biopolymers.

The team has also worked on making the hydrogels stick to various surfaces including metallic sensors and LEDs to produce stretchable electronics.

At first, the researchers only thought of exploring hydrogel’s use in optical fibres after conversing with Harvard Medical School’s bio-optics team led by Associate Professor Seok-Hyun (Andy) Yun.

Harvard’s team was already working on an optical fibre which would break when bent or slightly stretched. Both the teams have collaborated to bring the concept of hydrogel into the optical fibre design.

The optical fibre design from Yun consists of a core material enclosed in an outer cladding. For maximum amount of light to be transmitted through the core, the core and cladding should be made of materials with different refractive indices or the degrees at which they can bend light.

Hyunwoo Yuk said: “If these two things are too similar, whatever light source flows through the fiber will just fade away.

“In optical fibers, people want to have a much higher refractive index in the core, versus cladding, so that when light goes through the core, it bounces off the interface of the cladding and stays within the core.”

The team found that the refractive index of the hydrogel was ideal for core material, as it was highly transparent. But, there was an issue when the cladding polymer material was being applied.

The outer cladding would peel off when fibre was stretched or bent. The issue was solved by adding conjugation chemicals to the cladding solution.

During the testing phase, researchers found that the hydrogel fibres could propagate light without significant fading. They also found that the fibres could be stretched up to seven times their original length before breaking.

The researchers now focused on the sensing part of the fibre. They used red, green and blue organic dyes, at specific spots along the fibre’s length and shone a laser beam through the fibre and stretched one colour region. And noted the intensity of the red light.

The researchers say that by measuring the amount of light at the fibre, they can quantitatively measure the stretching of the fibre.

They say that such stretchable, strain-sensing optical fibres could be implanted along the length of a patient’s arm or leg and measure signs of improving mobility.

Tufts University biological engineering professor Fiorenzo Omenetto, who was not involved in the work said: “Hydrogel fibers are very interesting and provide a compelling direction for embedding light within the human body.  

“These efforts in optimizing and managing the physical and mechanical properties of fibers are necessary and important next steps that will enable practical applications of medical relevance.”

Image: Hydrogel-based optical fibres developed by teams from MIT and Harvard Medical School. Photo: Courtesy of Massachusetts Institute of Technology.