Sharpening of optic fibers for atraumatic implantation into brain structures.

M. Pasierski1,2, Składowska A1., Kublik. E1.
1) Department of Neurophysiology, Nencki Institute of Experimental Biology, Warsaw, Poland
2) Second Faculty of Medicine, Medical University of Warsaw, Warsaw, Poland

Increasing popularity of in vivo experiments using optogenetical methods has created a need for means of atraumatic implantations of optical fibers. Typically, thick (~200-400 m ) optical fibers are used to deliver strong light beams activating brain structures. Due to their size and blunt tips they cause irreversible damage to the tissue. Atraumatic implantation stands for inserting optical fiber into brain structures in a way that harms as few cells as possible ensuring better neuronal response after light stimulation. In the presented paper we describe methods of sharpening optical fibers so they could serve this goal, in ways that do not decrease significantly the power of light transmitted through them and doesn't alter electrode recordings taken after implantation.
The first of the described methods involves creating a rigid gelatin tip at the end of the fiber. The fiber itself, or with an electrode array attached to it, is embedded in the mixture while heated and then pulled in order to form a sharp arrowhead. After cooling the end is hard but soluble. Such arrohead can be used for guiding the blunt fiber through the tissue during implantation, so as to decrease the damage; then it is dissolved and does not influence data collection after on. If necessary (e.g. for implantation to very deep structures, the tip may be coated in paraffin to prevent premature dissolution.
The second method involves tapering of a fiber itself. It can be done by pulling heated fiber. While glass require very high temperature, plastic fibers (POF) can be pulled at around 100 oC – temperature easy to obtain and control e.g. with hot air soldering station. Tip of a glass fiber can be tapered by chemical reaction with hydrogen fluoride (HF) which results in etching the fiber ending. A fiber is placed perpendicularly over the surface of HF solution. The gradient of HF vapor over the surface generates conditions for the sharpening of the fiber. Fibers with sharp tips not only cause less damage to the tissue; properly designed taper should allow for optimal dissipation of light beam within brain structures of different shapes. With chemical etching we can control the shape (length and slope) of a fiber tip by manipulation of the time of the etching, HF concentration and also by covering fibers with calcium gluconate gel which inhibits the reaction.

[Supported by Polish National Science Centre grant 2013/08/W/NZ4/00691]