Tapering the tips of fiber optic cannulas can be useful for delivering light or collecting light (fiber photometry) to larger volumes of neural tissue than the typically used flat face tips. A tapered fiber tip will be a tip that is not flat, but of some angled or (typically) conical shape. These tips can be fabricated with pipette puller systems, polishing, or chemically etching the glass quartz.
Here we describe how to get a conical shape using chemical etching with Hydrofluoric Acid (HF).
From Wikipedia: “Hydrogen fluoride gas is an acute poison that may immediately and permanently damage lungs and the corneas of the eyes. Aqueous hydrofluoric acid is a contact-poison with the potential for deep, initially painless burns and ensuing tissue death. By interfering with body calcium metabolism, the concentrated acid may also cause systemic toxicity and eventual cardiac arrest and fatality.”
This is the acid that is used in mafia films to dissolve bodies. Do not store in glass containers. Neutralize with magnesium sulfate. Do not use latex gloves, use only neoprene or nitrile. Keep calcium gluconate gel around the product at all times.
If your institution does not have its own HF guidelines, Harvard offers a great one
- Find an appropriate clear container (e.g., polyethylene of Teflon®) to hold an appropriate volume of HF acid (the volume depends on the number of probes your are tapering, their length, the number of times the HF has been used, etc.). Under a fume hood, fill with the HF acid.
- Cover the top of the HF acid with a thin (~1 mm) layer of mineral oil. This prevents vapor etching and etching at the surface of the HF.
- Measure the length of the optic cannula you will ultimately want (fiber plus ferrule). Add an additional length that will be cut off later (say 3 cm). Cuts can be made with scissors, razors, or whatever. We will etch one end and polish the other later.
Hold all fibers you wish to etch with something such that all to-be-etched ends are at the same height. We use blue painters tape (it doesn’t leave residue). Place the tape face up on a table. Put the fibers in the middle of the tape such that the ends all align and fold the tape over. Hold the tape with helping hands.
- Carefully lower the tips of the optic fibers into the solution until the tips are about 2 mm below the HF. Wait 85 minutes. Raise above and allow to completely dry.
- Remove and place in a container of water for 5 minutes (‘dirty’ solution). Raise above and allow to completely dry. Rinse in Acetone for 5 minutes (‘dirty’ solution). Raise above and allow to completely dry. Rinse under tap water for 1 minute (clean).
- The dirty solutions can be appropriately stored and used multiple times. Contact your Environmental Health and Safety department for disposal methods.
- It can be difficult to remove the coating of shorter optics. Therefore you can optionally soak in clean Acetone overnight.
- Our resources page lists several general optic fabrication techniques to attach the etched glass to a ferrule and polish. This includes the great Jove video.
There are a variety of additional resources this is loosely based off of, including Patrick Lambelet, Abdeljalil Sayah, Michael Pfeffer, Claude Philipona, and Fabienne Marquis-Weible, “Chemically etched fiber tips for near-field optical microscopy: a process for smoother tips,” Appl. Opt. 37, 7289-7292 (1998)
ONE Core acknowledgement
Please acknowledge the ONE Core facility in your publications. An appropriate wording would be:
“The Optogenetics and Neural Engineering (ONE) Core at the University of Colorado School of Medicine provided engineering support for this research. The ONE Core is part of the NeuroTechnology Center, funded in part by the School of Medicine and by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health under award number P30NS048154.”