Art of pulling glass

Pulling micropipettes or microelectrodes is a science and an art, requiring some finesse. Here we discuss the five major factors which can affect the shape of a pulled glass micropipette or microelectrode.

The PUL-1000 is a microprocessor controlled, four-stage, horizontal puller for making glass micropipettes or microelectrodes. Here we will look at factors that affect the pulling of glass. 

As the temperature of glass increases, the glass transitions gradually from a hard and relatively brittle solid state into a soft and viscous state. To form glass into certain shapes using a puller, heat is applied through a filament. Many factors affect the heat transferred from a filament to a glass capillary.

  1. The filament holders heat up when current passes through the filament, which can get even hotter as the puller is used continuously. To reduce the heat residue built-up, allow time for the colder ambient air to cool the components. 

  2. The convection of air in the ambient environment (both inside and outside of the cover chamber) affects the heat transfer from the filament to the glass. So, room temperature and humidity affect the glass transition. 

  3. The heat radiating from the filament  plays the most important role in the glass transition. The distance between the surface of the glass and the surface of the filament affect the amount of heat transferred. It is important that the glass is placed at the center of the filament. All facets of glass should be heated evenly. 

  4. The filament is made of a platinum/iridium alloy, one of the most inert substances in the heating material market. Yet, it slowly oxidizes in air’s oxygen, causing a reduction of mass during use. It will eventually burn out. As the mass changes, the temperature also changes constantly. 

  5. Different glass capillaries with different outside diameters (OD) vary in their glass softening points. The glass composition and variations affect the softening points. There is also some variation in the same glass capillary products taken from different lots. 

You may need to make some fine adjusts of the pulling sequence parameters based on the glass capillary product you use and your laboratory environmental conditions. As a rule of thumb, the parameters (Heat, Force, Distance, and Delay) can be altered to achieve the desired shapes and dimensions of pre-pulled pipettes.

  • Increase heat for longer tapers and decrease it for shorter tapers.
  • Increase the force for smaller tips and longer tapers or decrease it for larger tips and shorter tapers.
  • Increase the distance for smaller tips and decrease it for larger tips.
  • Increase the delay for shorter tapers or decrease it for longer tapers.

Here’s a bonus tip. To make a large tip > 5 µm, break off the final tip. Use a breaking back technique using blades or tweezers, or drag the tip on a Kimwipe. A second process such as beveling and/or fire polishing may also be needed. 

Remember residual filament heat, environmental conditions, the position of the glass relative to the filament, the age of the filament and the variations in glass all affect the shape of the pulled micropipette. Be patient and make adjustments to your program as necessary.

Parameter  Increase (↑)  Decrease (↓) 
Heat  Longer Taper Shorter Taper 
Force  Smaller Tips, Longer Taper Larger Tips, Shorter Taper
Distance  Smaller Tips Larger Tips
Delay  Shorter Taper Longer Taper

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