This pipette is based on http://www.thingiverse.com/thing:64977 but with a few modifications.
You can see the pipette in action here: https://www.youtube.com/watch?v=nqb27um8KTk
One important modification was to remove the threaded bolt from the plunger, and redesign the plunger so that a long (75mm) M4 bolt could be inserted through the length of the plunger replacing the original thread (see the detailed picture of the plunger). The thumb gears have also been modified to accept the M4 rod. The top part was made a lot bulkier so it could more easily be printed. Also, all models were rotated around the X axis and set for printing.
Although all parts fit nicely together, I have been struggling a bit to get it to work. The bottom part was not fully airtight (and it really has to) but that could easily be solved by painting the membrane chamber with a good few coats of ABS juice. Also fitting the straw with a bit of ABS juice where the straw enters seals off any possibility of air escaping along the straw.
The membrane (I used lab gloves) also easily splits and replacing the membrane is a bit of work. In my last reinstall I added an extra membrane which seems to work very well, even with actual 200 microliter tips! (which fit snuggly on top of straws, who would've guessed ...) I'll be testing its consistency (will it be below 10 microliter???) in a real lab later this week.
Print all parts (thumb wheel twice). I printed on a printrbot/Wallace++ clone with a layer thickness of 0.3mm.
Screw a M4 75mm long bolt all the way through the plunger
Continue with directions of the original design at http://www.thingiverse.com/thing:64977