A parylene-based waveguide has the physical characteristics necessary to enable it to emerge as the new standard in optical biointerfaces. Researchers at Carnegie Mellon University developed the highly flexible waveguide, which can additionally operate over a broad spectrum of light. The device answers the demand for miniaturized and flexible optical tools necessary for reliable ambulatory and on-demand imaging and manipulation of biological events within the body. Integrated photonic technology has mainly evolved around developing devices for optical communications, which has mainly relied on silicon photonics. A parylene photonic waveguide surrounded by neurons. Courtesy of Carnegie Mellon University College of Engineering. Silicon, while advantageous for myriad applications, is too rigid for soft tissue interactions. Silicon can damage tissue and lead to scarring due to the undulation of soft tissue against the device during respiration and other processes. To create an alternative material, Maysam Chamanzar, assistant professor of electrical and computer engineering at Carnegie Mellon University, and those in his lab, designed ultracompact optical waveguides by fabricating PDMS, a silicone-based organic polymer with a low refractive index, around a core of parylene-C, a polymer with a much higher refractive index. The contrast between those indices allows the waveguide to pipe light effectively, while the materials themselves remain pliant. The result is a platform that is flexible, can operate over a broad spectrum of light, and is just 10 µm thick. “We were using parylene-C as a biocompatible insulation coating for electrical implantable devices, when I noticed that this polymer is optically transparent. I became curious about its optical properties and did some basic measurements,” said Chamanzar said. “I found that parylene-C has exceptional optical properties. This was the onset of thinking about ‘parylene photonics’ as a new research direction.” Maysam Chamanzar, holding his team’s device. Courtesy of Carnegie Mellon University College of Engineering. The Chamanzar-led team crafted its device with neural stimulation in mind, to allow for targeted stimulation and monitoring of specific neurons within the brain. Crucial to this is the creation of 45° embedded micromirrors. While prior optical biointerfaces have stimulated a large swath of the brain beyond what could be measured, these micromirrors create a tight overlap between the volume being stimulated and the volume being recorded. These micromirrors also enable integration of external light sources with the parylene waveguides. “Optical packaging is an interesting problem to solve because the best solutions need to be practical. We were able to package our parylene photonic waveguides with discrete light sources using accessible packaging methods, to realize a compact device,” said Maya Lassiter, graduate research assistant. Chamanzar and his team are considering possible uses in wearable technologies. Parylene photonic devices placed on the skin could be used to conform to difficult areas of the body and measure pulse rate, oxygen saturation, blood flow, cancer biomarkers, and other biometrics. The research was published in Nature Microsystems & Nanoengineering (www.doi.org/10.1038/s41378-020-00186-2).
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