Penn State's Biodegradable Optical Fiber Delivers Light Safely Into Human Body

By Jof Enriquez,
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Engineers at Pennsylvania State University (Penn State) have fabricated a new biodegradable optical fiber made of citrate-based polymers that can be implanted safely in the body to image deep tissue or deliver drugs.

Advances in photonics have paved the way for many biomedical applications, such as laser surgery, optical imaging, drug delivery, and optogenetics – which involves the manipulation of light to control brain cells to treat neurological disorders. The clinical utility of these procedures has, so far, been limited by the fact that light carried by silicon optical fibers is unable to penetrate deeper into body tissue.

"The problem is that visible light can only penetrate to a certain depth, maybe hundreds of microns," said Jian Yang, professor of biomedical engineering, Penn State. "Near infrared light might be able to penetrate a few millimeters to a centimeter, but that is not enough to see what is going on."

Currently, most optical fibers inserted or implanted in the human body are made of either plastic or silica glass, which are readily available and easy to work with as photonic materials, but are not inherently biocompatible with tissue. In addition, silicon optical fibers are generally considered rigid and brittle, and not amenable to bending and shifting motions once implanted.

To address this challenge, researchers are experimenting with more biocompatible materials, such as hydrogels and organic thin films. Penn State's solution is a new type of optical fiber made of a polymer based on citrate, an acid compound that is a key intermediate in metabolism.

Prof. Yang had invented the citrate-based polymer earlier as a material for biodegradable bone screws, scaffolds for tissue engineering, and nanoparticles for drug delivery. For this new research, published in Biomaterials, Yang and Zhiwen Liu, Penn State professor of electrical engineering, spearheaded the work in creating the first citrate-based flexible biodegradable polymeric step-index fiber.

"Because the material is nontoxic and biodegradable, the citrate-based fiber could be left inside the body for long periods without the need for a second surgery to remove it," Yang said. "In addition to sensing and imaging, we can add therapeutic chemicals, drugs or biological molecules for disease treatment."

A typical step-index fiber has a core wrapped in a protective cladding. The core has its own refractive index, while the cladding has a different index so that it can prevent light from leaking away and redirect it to the core. While their refractive indices need to be different, the engineers needed to control the magnitude of that difference to achieve the desired effect. 

"The use of the citrate-based polymers enables ultrafine tuning of refractive index differences between the core and the cladding layers," added co-first author Chenji Zhang.

The core and the cladding materials have very similar mechanical properties, so the whole fiber structure remains intact as it bends and shifts inside the body. Moreover, the two layers biodegrade at similar rates.

Co-first author, Dingying Shan, added that "this new type of biodegradable, biocompatible and low-loss step-index optical fiber can facilitate organ-scale light delivery and collection" and "will become an enabling tool for diverse biomedical applications where light delivery, imaging or sensing are desired."