Researchers Create Safe Optical Fiber for Delivering Light into the Body

An Electrical Engineer and a Biomaterials Engineer in Penn State’s Materials Research Institute have combined their expertise to create a flexible, biodegradable optical fiber to provide light into the body for medical applications.

A view of light delivery through a citrate-based optical fiber (Credit: D. Shan/C. Zhang/Penn State)

The ability to supply light into the body is vital for drug activation, diagnosis of disease, laser surgery, optical imaging and in the experimental field of optogenetics, in which light is used to control the function of neurons in the brain. Until now, delivering light into the body was hard and usually required the implantation of an optical fiber made of glass.

The problem is that visible light can only penetrate to a certain depth, maybe hundreds of microns. 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.

Jian Yang, Professor of Biomedical Engineering, Pennsylvania State University

Presently, people are using glass fiber to send light into the biological tissue at depth, but glass is fragile and is not biodegradable. It can break and hurt the tissue if implanted.

Researchers are starting to look toward supple polymer fibers as a solution. Yang earlier invented a polymer based on citrate, a naturally occurring chief ingredient in metabolism, which he developed as a universal platform for biomedical applications, such as scaffolds for tissue engineering, biodegradable bone screws for bone fixation and nanoparticles for delivering time-released therapeutic drugs.

Currently, Yang is partnering with Zhiwen Liu, Penn State Professor of Electrical Engineering, on using Yang’s citrate-based polymer to develop a step-index optical fiber for light delivery inside the body.

A step-index fiber has a core material that conveys the light and a cladding that protects the core and ensures light does not escape. Yang’s lab creates and tests the polymer and then takes it to Liu’s lab to convert it into a fiber. Once the fiber is analyzed and modified, Yang’s lab implants the fiber in biological tissue for testing.

The present work demonstrates the first citrate-based flexible biodegradable polymeric step-index fiber.

Dingying Shan, a Ph.D. Student in Yang’s group and Co-first Author on a recent paper in the journal Biomaterials

“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, a recent Ph.D. Graduate in Liu’s group.

As the core and cladding have identical mechanical features, the optical fiber can bend and stretch without the layers pulling apart, as can occur with dissimilar materials. The two materials will also biodegrade at comparable rates in the body, without damage.

“We believe this new type of biodegradable, biocompatible and low-loss step-index optical fiber can facilitate organ-scale light delivery and collection. And that it will become an enabling tool for diverse biomedical applications where light delivery, imaging or sensing are desired,” Shan said.

“This new type of fiber creates a transparent window for peeking into a turbid tissue, and can enable new opportunities for imaging,” Liu said. As an initial step, the team first measured light propagation features of the fiber and then applied this information to showcase image transmission through the 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. In addition to sensing and imaging, we can add therapeutic chemicals, drugs or biological molecules for disease treatment.

Jian Yang, Professor of Biomedical Engineering, Pennsylvania State University

In addition to Senior Authors Jian Yang and Zhiwen Liu, other authors on the paper “Flexible, biodegradable citrate-based polymeric step-index optical fiber” are former Graduate Student Surge Kalaba and current Ph.D. student Gloria Kim in Yang’s group, and Nikhil Mehta, a Ph.D. graduate in Liu’s group.

This research received support from the National Institutes of Health.

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