Tapered polymer fibers enhance light delivery for neuroscience research

Researchers have developed a reliable and reproducible way to fabricate tapered polymer optical fibers that can be used to deliver light to the brain. These fibers could be used in animal studies to help scientists better understand treatments and interventions for various neurological conditions.

The tapered fibers are optimized for neuroscience research techniques, such as optogenetic experiments and fiber photometry, which rely on the interaction between genetically modified neurons and visible light delivered to and/or collected from the brain.

“Unlike standard optical fibers, which are cylindrical, the tapered fibers we developed have a conical shape, which allows them to penetrate the tissue with more ease and to deliver light to larger volumes of the brain,” said research team member Marcello Meneghetti from the Neural Devices and Gas Photonics group at the Technical University of Denmark.

“Furthermore, making the fibers from soft, flexible polymers rather than stiff, sometimes brittle glass can reduce tissue inflammation over long periods of implantation.”

In the journal Optics Letters, the researchers describe how they designed and fabricated tapered polymer optical fibers for efficient light delivery in the brain. The tapers were crafted from optical fibers just 50 microns in diameter—approximately the width of a human hair.

“Our polymer tapers make it possible to modulate the behavior of and record activity from more neurons, thus allowing the study of larger brain circuits,” said Meneghetti. “This could produce deeper insights into how complex brain circuits function, how behaviors are controlled and how brain diseases or disorders might disrupt these circuits.”

Illuminating more neurons

Cylindrical optical fibers such as the ones used in telecommunications are very good at confining light, which means that light can enter the fiber and be delivered to the output with very little loss along the way. However, conical fibers allow light to leak from the sides of the fiber along the length of the tapered tip, which increases the volume that can be illuminated.

After observing the growing use of glass fiber tapers for efficient modulation and recording of neuronal activity, the researchers set out to adapt this technology to polymer optical fibers.

“Long-term inflammation and implant breakage are persistent challenges in silica- or silicon-based neurophotonics,” said Meneghetti. “The mechanical mismatch between implants and brain tissue triggers inflammation, and the brittleness of these materials leads to fractures at sub-millimeter scales. Using polymer-based fibers, which are over 10 times less stiff, significantly mitigates these issues.”

Optimizing fabrication

To create the new tapered polymer fibers, the researchers used numerical models to determine the ideal taper geometry and developed a chemical etching process to fabricate them. They tested various solvents and protocols to achieve the desired tips, verifying the geometry and ensuring surface integrity with scanning electron microscopy.

The researchers then used their tapered polymer fibers to illuminate slices of agarose gel, which is known to have optical properties similar to brain tissue. Compared to standard optical fibers with the same diameter and constituent material, the lateral spread of light was more than doubled using the tapered optical fibers.

“We hope that this research will lead to further advancements in the field while also paving the way for innovative devices,” said Meneghetti. “It might also be a useful stepping stone for anyone interested in these tapers for other applications such as sensing.”

Next, the researchers plan to demonstrate the new tapered fibers in an animal model to evaluate the functionality and ability to reduce inflammation.

In the future, combining the new fabrication process with post-processing techniques such as nanofabrication could lead to fully integrated devices that not only deliver and collect light but also detect electrical signals and sense temperature or chemical changes in the brain.

This could provide an even more comprehensive understanding of brain activity in both healthy and diseased states.