Bonus mechanobiology

Last post for PHYL3001- for real this time! This lecture was pretty much just about two methods of measuring the traction force, as well as some clarification on mechanomemory. Since I've already discussed mechanomemory here, this will just be a very short post that explains how we measure traction force.

The two methods of measuring traction force are the micropillar/microneedle technique, as well as traction force microscopy (TFM).

Micropillar/Microneedle

Instead of creating a flat hydrogel, you can create a hydrogel with lots of little micropillars (or "microneedles"). When cells pull on the micropillars, they bend. Micropillars have a spring constant, which depends on several other factors, such as the height of the pillars and the material used. The force generated by the cell can be calculated by multiplying the spring constant of the micropillars by the displacement.

If you are using the micropillar method, care should be taken to ensure that the pillars are not too long or too far apart. If pillars are too long, then they are at risk of "lateral collapse," which basically means they can tilt even if force isn't applied to them (imagine a tall cylinder made out of jelly- a short cylinder would keep its shape but a long one would easily bend and break). If pillars are too far apart, there may be a "sagging effect" where the gel in between pillars touches the cell.

The micropillar method is cheaper than the TFM method that I'm about to introduce, and requires less specialised equipment. However, it can only measure forces in the x and y directions, and the discontinuity of the hydrogel (i.e. spaces between micropillars) means that there will also be discontinuity in your results.

Traction Force Microscopy (TFM)

To do TFM, you need a special hydrogel with fluorescent microbeads embedded. When cells pull on the gel, they pull the beads as well. When the cells are removed by using trypsin, the beads return back to their original place. The location of the beads (before and after trypsin) can be determined by using a confocal microscope, and the distance that the beads move can be measured in order to determine the amount of force.

TFM has an advantage over the micropillar method in that it can measure forces in three dimensions (x, y and z), as confocal microscopes can take pictures at different depths. Unfortunately, specialised equipment such as confocal microscopes and microbeads can be expensive, and so not every lab will be able to use this technique.