Cross-posted to Biocurious a blog about biology through the eyes of physicists.

The function and dysfunction of blood clots are often directly related to their mechanical properties: clots stop blood from flowing through wounds but can also break away (embolize) and block blood vessels causing stroke. Strength and plasticity are both important for ensuring the former is more common than the latter and so people have been studying the mechanics of clots for over 50 years. 

Despite this history, or perhaps because of it, new discoveries are being made all the time. Take the recent observations reported by Liu et al. last year in Science (abstract is free). They used a combined fluorescence and atomic force microscope (not unlike the one I've been working with recently!) to stretch single fibrin fibers--the ones that make up the protein mesh of blood clots shown in green in the image above--to see how far they could stretch. They found that some fibers could stretch up to 5 times their relaxed length before breaking! Check out the movies at Martin Guthold's site.