Modification and redesign of existing pipettes on Thingiverse to allow dispensing using commercial 200ul pipette tips. I have yet to calibrate it and measure its accuracy, but this will come down to the setting and fit of the lower gear. My dispensations of liquids have been very consistent so far.
The idea was to move from getting suction from a balloon membrane to instead using a plunger and a tube as I had difficulties with retaining the siphon pressure.
The things you need to get it to work besides the prints are the following:
3 o-rings (3 x 7 mm) in EPDM, Nitrile or other suitable material
PEZ dispenser spring or similar
Commercial tips, such as from Eppendorf
To print, use the highest possible setting for the plunger, gears and the pipette tube. I used 100% infill with 2 shells. Use 15% infill for the main body.
Prior to assembly, I suggest you have sand paper ready and try to assemble the plunger with o-rings and pass them through the pipette tube to make sure you have a good fit. Have some water handy and test its fit and suction like you would with a syringe. I suggest you lubricate the o-rings to make this work better. When you are happy with the fit and function, with as little friction as possible while retaining the siphon effect, disassemble and go on for the final assembly.
To assemble, put the pipette tube into the main body and push it in all the way so it protrudes slightly at the top opening. On the plunger, thread the spring. Now the more tricky part. Insert the gears into the top cavity and thread the tip of the plunger into the top opening and through the centre of the gears. With the tip of the plunger protruding through the lower gear, use your fingers or tweezers to slide over the three o-rings. Lubricate them in advance as well as the opening of the tube as this may facilitate the process. The tip of the plunger has two recesses. The o-rings should be fitted so that the middle ring is locked in place by a top and bottom o-ring in their recesses. When in place, insert the tip of the plunger into the pipette tube and start rotating the gears so that the plunger is inserted further into the pipette and until the top gear can no longer be turned. Test the spring function and the siphon effect and calibrate as needed.
I have further modified mine to use a stronger spring as the PEZ one was a bit weak and I will likely redesign the plunger to accomodate a round spring soon, unless anyone beats me to it!
I hope this is useful to any DIY biologist or molecular gastronomist out there =)