Zero Order

Simple misunderstandings

Posted in Uncategorized by Amy Ross on July 30, 2010

In many ways,  lab techniques have a component of oral tradition. Sure, you can follow a protocol or an instruction manual, but it’s always better to have someone explain, or better yet demonstrate, a procedure. But as information gets passed from one person to another, like the children’s game Telephone, some of the details get lost.

I wanted to calculate the mesh size of my hydrogels, and in order to do that, I needed several measurements, including the volume when fully swollen. Our  hydrogel samples are fairly small (I typically do discs 2 mm thick and 6mm wide), so it’s hard to get an accurate measure by using calipers. A previous grad student had measured volume by employing Archimedes’ principle: the buoyant force on an immersed object (causing the apparent decrease in weight when compared to weighing in air) is equal to the weight of liquid it has displaced.  He weighed hydrogels in air and in butanol. This will give the weight of the displaced butanol, which can be divided by the density to obtain the volume. Why weigh in butanol?  The hydrogels need to be weighed in a liquid they can’t absorb. Water or anything aqueous could be absorbed and skew the results. So instead, the hydrogels were weighed in an organic liquid.

So I tried this, using the other grad student’s equations,  and I kept getting highly inconsistent and sometimes impossible  results.  I did six groups of polyacrylamide hydrogels, twice, and still couldn’t figure out the problem.  I put it aside to work on my experiments with cells. Meanwhile, one of our summer undergrads, Megan, needed to find mesh size for her PEGDA hydrogels.  I gave her the equations and showed her what I was doing, and she kept getting odd results as well. It turns out the way I was weighing the hydrogels was wrong.

To employ Archimedes’ principle, the object being weighed has to be suspended in the liquid. I weighed my hydrogels by dropping them in a (tared) vial of butanol. The density of the hydrogel was greater than the density of the butanol, so naturally, they sank to the bottom. If the hydrogel is sitting on the bottom, there is no buoyant force acting on it.


Figuring out how to suspend the hydrogel was another challenge.  The gross physical structure of a hydrogel is like Jell-O: it can easily be mushed or torn.  Megan rigged up what looks like a miniature version of  an Easter egg coloring dipper, and she found it worked pretty well. I’m going to steal it from her when she goes back to school 🙂


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