Here is a conundrum: Suppose you want to measure growth rates of bacterial cultures in an aerobic environment, on LB, in 96-well plate format. So, you buy some plates from your favorite supplier, dispense some LB into the wells with a multichannel pipetter, inoculate from whatever your source is, and pop it into your plate reader and start the program. You come in the next day and copy the data from Excel into a civilized piece of software. Profit!
Now, what do you do if your plate reader was installed upside down? Your media will spill out and get all over the expensive bits inside the machine, and your PI will become sad and despondent. Now, you’re pretty clever, and so you might ask, “What if we put the plate in upside down too?” And that would perhaps solve the problem. But what if the instrument randomly changes its orientation as the experiment proceeds? And what if the instrument is permanently in free fall? What then, kemosabe?
This is basically the situation aboard the International Space Station, aboard which we are going to do a growth assay with a microplate reader. One would suppose that NASA has a thing for this, and one would be correct. The problem is that the thing is not ready yet, and the experiment is going ahead without the thing. So, here is your mission, should you choose to accept it : Design a cup that doesn’t spill when it is open and upside down.
Jenna, David and I have been working hard on this problem for about six months, and in these posts I will describe how we finally solved it.
Designing experiments for space is tricky. It is surprising how many things we do are entirely dependent on gravity. Even something as simple as transferring a sample from one tube to another depends on gravity. For example, gravity keeps the liquid from flying out of the tube and hitting the ceiling when you open the tube. Gravity causes the liquid to bunch together at the bottom of the tube where you can get at it, instead of sticking in random blobs all over the inner surface. It causes bubbles in the liquid to rise and pop. Simple things like having liquid sitting quietly in a 96-well plate are not so simple once there isn’t any gravity to make the liquid stay where you put it.
Here are some of the solutions I’ve considered. We’ve tested some of them, and others I kept to myself so that David and Jenna wouldn’t tie me to a chair and leave me in the room with minus eighties.
Idea 1 : Seal the plates with a gas-permeable membrane. This failed because the gas-permeable membranes turn crispy and fall off when you freeze them.
Idea 2 : 3D print an array of Klein bottles!
Idea 3 : Seal the plates with a standard heat-applied membrane, and hope we find plenty of BSL-1 bugs with robust anaerobic metabolism. Unfortunately, it turned out that there were very few bugs that were both BSL-1 and grew anaerobically under these conditions.
Idea 4 : Weld pieces of laser cut acrylic together into a 96-well plate with tiny micron-size vent holes. This… kinda worked. Refinements could maybe make it work pretty well. Unfortunately, my test plates kept leaking, and the welding compound is probably too stinky to pass NASA’s strict off-gas testing.
Idea 5 : Cast a plate out of PDMS with 96 1ml bubbles carefully suspended in an 8×12 array. Fill the bubbles with hypodermic needles. Laser cut micron-size vent holes, and rely on the strong hydrophobicity of PDMS to prevent liquid from escaping through the ventilation holes.
Some of you may be wondering, “What the heck is PDMS, and why did you try messing around with it in the first place?” The International Space Station maintains very strict controls on volatile compounds that evaporate from things they bring aboard, which rules out a lot of useful things, like pretty much every glue you can think of. PDMS happens to be food safe. In fact, it is an ingredient in many cosmetics and foods, including McDonald’s Chicken McNuggets. Now, the idea of eating silicone elastomer (the same material used in the front shock absorber on my mountain bike) may not sound appetizing, but it there is pretty good evidence that it is at least not demonstrably harmful.
Unfortunately, the plates still leak if you squeeze or bend them, and playing around with sharps full of bacteria is no fun at all.
Idea 6 : Combine ideas 3 and 4, and make an acrylic plate with a PDMS gasket. Sadly, it turned out to be pretty difficult to make the gasket flat enough to seal.
Idea 7 : I went back to thinking about 3D printing Klein bottles again.
Idea 8 : Acoustic trapping! OK, not really.
Tune in tomorrow to read about lucky Idea Number Nine, which is what we’re actually going to do! For now, a poem from my childhood :
Three jolly sailors from Blaydon-on-Tyne
They went to sea in a bottle by Klein.
Since the sea was entirely inside the hull
The scenery seen was exceedingly dull.
— The Space Child’s Mother Goose, Frederick Winsor & Marian Parry