Optical epoxy

The LAPS chip was connected to the tip of the multifunctional fiber with silver paint (SPI supplies, Inc.). The exposed edges of the LAPS chip were then covered with medical epoxy (McMaster-Carr). At the backend of the fiber, the electrode was exposed from the fiber side by removing the polymer cladding with a surgical scalpel under a microscope and connected to a copper wire using silver paint. The free end of the copper wire was soldered to a pin connector. An optical ferrule 6.5-mm-long, with a 2.5-mm-outer-diameter and 1-mm-inner-diameter was connected to the fiber with optical epoxy (Thorlabs) and cured overnight. The ferrule end was polished with a Thorlabs fiber polishing kit to achieve optimal optical coupling (Fig 4a).

To optically couple as-drawn fibers to a light source, 9- to 10-cm-long fibers were inserted into a 10.5-mm-long, 2.5-mm-diameter, 231-μm bore size ceramic ferrule (Thorlabs) and affixed with optical epoxy (Thorlabs). The ferrule edge was then polished using a Thorlabs fiber polishing kit. Fiber was then threaded through ~7.5 cm of microrenathane tubing (BrainTree Scientific) to provide structural stability for tunneling. The proximal ~6.75 cm of the tubing closest to the ferrule was opacified with liquid electrical tape (Starbrite) to reduce non-specific activation of CCK-expressing cells in the skin (Fig. 5h). The final length of the device was ~9.25 cm including the ferrule; ~1.5 cm of the device could be illuminated and ~0.75 cm of fiber extended beyond the tubing. The average power recorded from the device tip was measured using a photodetector (S140C, 250–1100 nm, 500 mW; Thorlabs) attached to a power meter (PM100D, Thorlabs). Average power output (optical intensity) at the end of the PC/PMMA fiber with a 5-V, 40-Hz, 532-nm laser input was 1.07 mW mm^–2 before implantation.