The wrinkling methods map the location of the forces indirectly and rely on deconvolving one of several possible solutions. But, says Chen, “because the posts move independently, if a post moves you know it's because the cell is pulling on it—there's really no other explanation. For the other methods you need to know where the adhesions are, or make other assumptions.” For example, the new method would spot the influence of an adhesion even if it lacked all known cell adhesion molecules.
Second, the posts can be varied in height. The tips of the posts all lie in one plane, but the bases of some can be raised to make shorter, stiffer posts in any pattern desired. This will allow the group to measure how the cell responds to differing resistance, without having to alter the hardness of the substrate's material, which might itself alter adhesion properties.
As an initial experiment, Tan and Chen have compared adhesion size with the magnitude of pulling force—an experiment that has yielded divergent conclusions in two recent studies. Using the posts, the size of adhesions correlates with magnitude of force for adhesions larger than 1 μm2, but some smaller adhesions generated much larger forces. “We see both types of reported relationships in the same cell,” says Tan. “That suggests both groups are right.”
The tops of the posts are coated with fibronectin using microprinting, thus keeping the cells restricted to the tops. This may be more physiological than it sounds. “The way cells seek out and find these posts may be similar to the way cells seek out collagen fibers in a loose network,” says Chen. Limited microprinting restricts the spreading of the cells. Such cells could not contract in response to serum, yet still responded to activated RhoA. ▪