Fiber and Cannula

The ISL head fixture is supposed to hold a cannula for injections, a fiber for optical stimulation, and an electrode for EEG monitoring. In our conceptual design, we imagined a head fixture made out of plastic, as you can see here. This original fixture turns out to have several problems. The plastic body of the fixture will obstruct the surgeon's view of the skull hole, making vertical alignment difficult or impossible. The angled cannula deprives the surgeon of a vertical thread by which to hold the fixture during implantation. The proposed gold-pad grounding upon the fixture base might be covered or interfered with by the dental cement used subsequently to hold the fixture in place. To solve these problems, we propose the following, simpler head fixture.


The cannula is a C313GT or G313GFS from Plastics One, with the thread cut short. The fiber is a 330-μm outer diameter, 300-μm core silica fiber with polished base and tapered tip. The fiber is glued above a bare LED die in a 3.1-mm wide, 0.8-mm high package, which is in turn mounted on a 7-mm long 0.8-mm thick fiberglass printed circuit board. The circuit board provides a hole for the cannula. It provides footprints for isolating inductors that allow the L+ lead to be used as an RF antenna. It provides pads for two helical leads that carry L+ and L−.

The angle between the center-line of the fiber and the cannula will be roughly eight degrees, which is close to the half-angle of the fiber taper. Thus we expect the taper edge to be roughly vertical on the side closest to the cannula. When we insert a syringe down the cannula, its open end should face the fiber. By this means, whatever chemical it introduces into the brain tissue will move towards the fiber. The fiber tip will illuminate the treated tissue.

We provide the X+ electrode for monitoring local field potential with a 3-mm length of steel tube around the fiber. This tube does not extend to the base of the taper, but stops 0.5 mm short of the base, so as to allow access by the syringe to the illuminated tissue. The X- electrode will act as the EEG ground terminal. We propose a steel skull screw with a steel washer to extend its area of contact with the top of the skull.

The head fixture as shown consists of two pre-assembled parts. Part A is the fiber, LED, and printed circuit board. These we will assemble as one pice. Part B is the cannula. With the help of a mounting hole in the printed circuit board, the thread on the top of the cannula, and an adjustable assembly jig, we will align both pieces correctly. With the help of some tape and two stages of application, we will fill the gap between the fiber and cannula with epoxy for the stretch between the printed circuit board and the EEG electrode. Once glued together, parts A and B form the finished head fixture. This procedure allows for us to locate the fiber tip precisely with respect to the cannula center-line even if we encounter variations in the length of individual fibers and cannulas.

To implant the ISL head fixture, the surgeon holds it by the cannula threads and lowers it into a skull hole at least 1.6 mm in diameter. After setting the vertical position to their satisfaction, he places two skull-mounting screws nearby. He builds up dental cement around the screws and head fixture so as to bind the fixture to the screws and seal the hole in skull.