Tungsten needles

MATERIALS

Potassium nitrite (KNO₂) solution, saturated, prepared in water

Alternatively, use 1 N NaOH.

Clips, alligator

Dish, porcelain

Ethanol burner

Forceps, fine

Pasteur pipettes with customized opening diameters

Prepare the pipettes such that the opening of the resulting capillary pipette will hold the sharpened tungsten needles.

Power cable

Stereomicroscope (e.g., Stemi SV11; Zeiss)

Transformer, alternating current, capable of generating 4-8 V and up to 5 A

Tripod

Wire, platinum

Wire, tungsten, 0.1- to 0.2-mm diameter

1. Clamp a 3- to 4-cm piece of tungsten wire in an alligator clip fixed to a tripod. Clamp a 3- to 4-cm piece of platinum wire in another alligator clip fixed to a tripod.
2. Submerge the tip of the platinum wire in a porcelain dish filled with the saturated KNO₂ solution (or 1 N NaOH).
3. Connect the power cables of the alligator clips to the transformer.
4. While monitoring under a stereomicroscope, dip the tip of the tungsten wire repeatedly in the KNO₂ solution to obtain a 2- to 3-mm-long conus with the desired sharpness.
5. Using fine forceps, place the sharpened tungsten needle into the opening of the capillary pipette.
6. Using an ethanol burner, melt the glass around the needle, fixing it into place.
7. Wash the tungsten needle extensively in water to remove any residual salt crystals.

Blunt or damaged needles can be resharpened by flame-polishing using an ethanol burner. Although needle tips are less smooth after flame-polishing (compared with the procedure above), flame-polishing is acceptable as a fast and convenient method during embryo dissection.