Here are Step-By-Step Instructions for making the applicator:
1. Heat one end of the 250 microliter microcap in an open flame until it narrows down to an opening about 0.5 mm in diameter. Use a diamond pencil to scratch and break the microcap about 0.8 cm from this end that you have just heated.
2. Use the diamond pencil to cut 8 pieces of the fused silica tubing about 10 cm long.
3. Collect the 8 pieces together and fit them through the long (unflamed) piece of the microcap.
4. Mix up a small amount of Epoxy, line up all of the silica tubes flush and apply Epoxy close to the open ends, but not so close that the Epoxy will block the open ends. This takes some practice!
5. Now, before the Epoxy cures you want to fit the 0.8 cm flamed piece of the microcap over the ends of the silica tubes and push the tubes in as far as they will go.
6. Once this first application of Epoxy cures, we make a new batch and apply it to the silica tubes further back and then slide the longer piece of microcap down to meet the shorter piece.
To exchange the solution around a cell as quickly as possible you need to apply pressure to the solution reservoir. This figure shows the effect of increasing the pressure on the activation and deactivation of whole-cell current evoked by 160 µM kainate in a freshly dissociated spinal cord neuron:
This figre shows the circuit we use to drive the valves with TTL pulses from the computer:
