[tools] Custom micropipettes

Wikipedia article on air-displacement micropipettes: https://en.wikipedia.org/wiki/Air_displacement_pipette

Video del desarmado y ensamblaje de una micropipeta: https://www.youtube.com/watch?v=Ezo2KEa7Uvo

Contents:

Comment index:

Motivation

Re-purposing manual micro-pipettes has some trade-offs:

  • It is hard to know for sure the fully extended position and the position of the first stop. Some range is lost because of this.
  • The adapters are rather large (wide), and only two pipettes fit, leaving no room for other tools. We would like to have all 4 pipettes, without wasting so much space.

Keywords

Pneumatic rod seal. O-ring.

General tasks

  • Phase 1:
    • Research air-displacement mechanisms, propose possible implementations.
    • Identify sources of non-ideality in the mechanism, and estimate corrections due to, for example: mechanism backlash, inner dead volume, liquid column weight, capillarity, and surface tension interactions.
    • Produce documentation.
  • Phase 2:
    • Produce 3D models of the mechanism.
    • Define corrections quantitatively, and validate them through simulations.
    • Produce documentation.
  • Phase 3:
    • Produce schematics for manufacturing.
    • Produce documentation.

Research tasks

DIY inspiration:

Identified design principles

  • Use a precise air-displacement mechanism: high accuracy, high precision, low backslash.
  • Keep the chamber volume low, in order to maximise the pressure-delta generated by the mechanism.
  • Correct for fluid physics stuff (pressure from the column of the liquid being aspirated; pressure from capillary/surface-tension interactions between the liquid and the pipette tip; ...).
  • Are there others?

Ask for help

In the crudest sense, micropipettes are just syringes with fancy engineering for sub-microliter precision.

On the theory side, we need help learning what "fancy" means.

  • ¿What are the important design principles?

For example, I imagine we need to:

  • Use a precise air-displacement mechanism.
  • Keep the chamber volume low, in order to maximize the pressure-delta generated by the mechanism.
  • Correct for fluid physics stuff (pressure from the column of the liquid being aspirated; pressure from capillary/surface-tension interactions between the liquid and the pipette tip; ...).
  • Are there others?

On the implementation side, we need help in three areas:

  • Choosing a precise air-displacement mechanism, or ways of precisely changing the volume of a small chamber.
    • What precise "air dosing" mechanisms exist?
    • Which parts do they use? and how they are named?
    • Here are other (not necessarily good) ideas:
      • A simple syringe like in this paper: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6187506/)
      • Milled steel piston + chamber with o-ring.
      • Milled steel "hole" + piston with o-ring (syringe-like).
      • Diaphragm pumps, or other kinds of pumps.
      • Changing the volume by heating/cooling the air in the chamber.
  • Choosing materials for the parts (piston, seal, housing).
    • ¿Are there self-lubricating, chemically-resistant materials for piston/o-ring pairs?
  • Being able to manufacture the parts precisely enough.
    • ¿Can we mill the parts with our CNC machines? ¿What precision is required?
    • ¿Could we use a tempered steel rod and common o-rings?
    • ¿Is PTFE an option?
    • ¿Are there any off-the-shelf, cheap parts that are incredibly well machined for some reason?

Parts of a pipette

As found here: https://en.wikipedia.org/wiki/Air_displacement_pipette

image

Probing for tips

See: https://gitlab.com/pipettin-bot/pipettin-grbl/-/issues/80#note_1188204337

Tip ejection

Can be achieved with a "U" shaped part fixed to the machine's frame.

No need for a tip ejection button.

Edited by naikymen