To deliver light to the brain safely and efficiently, a research team at the Technical University of Denmark designed tapered optical fibers made with soft, biocompatible polymer. The tapered fibers are optimized for light-based neural stimulation techniques like optogenetics. They could be used in studies to develop treatments and interventions for a range of neurological conditions. The tapered fibers have a conical shape that allows light to leak from the sides of the fiber along the length of the tapered tip. The conical design increases the volume that can be illuminated, enabling homogeneous light delivery to a large volume as well as spatially resolved illumination, while being minimally invasive. “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,” professor Marcello Meneghetti said. Researchers developed tapered polymer optical fibers that are optimized for delivering light to the brain. A scanning electron microscopy image of one of the tapers is shown here. Courtesy of Marcello Meneghetti/Technical University of Denmark. Until now, tapers for light-based neural applications have been used only with optical fibers made with silica. The stiffness of the silica material can cause damage to tissue when the fiber is used for prolonged periods of time. “Making the fibers from soft, flexible polymers rather than stiff, sometimes brittle glass can reduce tissue inflammation over long periods of implantation,” Meneghetti said. “Long-term inflammation and implant breakage are persistent challenges in silica- or silicon-based neurophotonics,” he said. “The mechanical mismatch between implants and brain tissue triggers inflammation, and the brittleness of these materials leads to fractures at sub-millimeter scales.” After observing the growing use of glass fiber tapers for efficient modulation and recording of neuronal activity, the researchers decided to adapt the technology to optical fibers made with polymer to mitigate the possibility of breakage and inflammation. According to Meneghetti, polymer-based fiber are more than 10 times less stiff than glass-based fibers. The researchers developed a reliable, reproducible way to fabricate tapered polymer optical fibers for sending light to the brain. They used numerical models to determine the optimal materials and best taper geometry for the fiber. They developed a chemical etching process to fabricate the fibers and fabricated two types of polymer optical fibers using thermal fiber drawing. They made the tapers with fibers that were 50 μm in diameter. To achieve the shape intended for the fiber tips, the researchers tested various solvents and protocols. Using scanning electron microscopy, they verified the accuracy of the taper geometry and the surface integrity. They also investigated the effect of different parameters on the etching process and on the quality of the tapers that were produced. The researchers tested the polymer fibers by using them to illuminate slices of agarose gel, which is known to have optical properties similar to brain tissue. The tapered optical fibers were found to more than double the lateral spread of light, compared to standard optical fibers with the same diameter and constituent material. The researchers optimized the tapered fibers to produce a large illumination volume for neuroscience research techniques, such as optogenetic experiments and fiber photometry, that rely on the interaction between genetically modified neurons and visible light delivered to and/or collected from the brain. “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,” Meneghetti said. “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.” Next, the researchers plan to demonstrate the tapered fibers in an animal model to evaluate the functionality of the fibers and the ability of polymer-based optical fibers to reduce the chance of inflammation. In the future, the new fabrication process for the fibers could be combined with post-processing techniques such as nanofabrication. This 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. Such a device could provide a more comprehensive understanding of brain activity in both healthy and diseased states. “We hope that this research will lead to further advancements in the field, while also paving the way for innovative devices,” Meneghetti said. “It might also be a useful stepping stone for anyone interested in these tapers for other applications such as sensing.” The research was published in Optics Letters (www.doi.org/10.1364/OL.546470).