"For all naturally occurring fibres, fibrin fibres are the ones you can stretch the furthest before they break," said Martin Guthold of Wake Forest University. "This was a stunning revelation because people hypothesized that these fibres stretched but broke much easier. In some cases, fibrin fibres had the ability to be stretched more than six times their length before they broke."

The fibres, which are roughly 100 nm in diameter, form from the soluble glycoprotein fibrinogen. This conversion is triggered by the release of specific chemical signals in the body when a blood vessel is damaged. The protein factor XIIIa induces covalent crosslinks in the fibrin fibres so that they form a branched network that makes up the body of the clot.

To study the fibres, Guthold and colleagues used the tip of an atomic force microscope to apply a force and a fluorescence microscope to image the process. They tested fibres that spanned a 12 µm gap in the substrate, looking at fibrin that had formed both in the presence of XIIIa and in its absence (i.e. fibrin without cross-linking).

The crosslinked fibrin fibres extended 332%, or 4.32 times their original length. The uncrosslinked fibres, on the other hand, extended 226%. The best-performing fibres underwent a strain of more than 500%. According to the researchers, these extensibilities are the largest of any protein fibre.

The crosslinked fibres could undergo 180% strain and still return to their original length whereas the uncrosslinked fibrin showed an elastic limit of 120%.

The researchers say that this effect of crosslinking is unusual – biological fibres such as collagen, spider silk and keratin generally become less extensible and stiffer when crosslinked. In the case of fibrin, the increased elasticity on crosslinking indicates that the crosslinks are directional, along the fibre axis.

"The fibrin fibres need to stop the flow of blood, so there is a lot of mechanical stress on those fibres," said Guthold. "Our discovery of these mechanical properties of individual fibrin fibres shows that these fibres likely endow blood clots with important physiological properties. They make blood clots very elastic and very stretchable."

While the individual fibres had an extensibility of more than 330%, networks of fibrin fibres have extensibilities of 100– 200%. The scientists say that this is extraordinary, since networks have two mechanisms for extension – alignment of the fibres and then stretching of the fibres. They believe their data suggests that clot rupture arises from the rupture of branch points rather than of individual fibres, as was previously assumed.

"This new information also helps us to understand how tough it is to remove a clot that is preventing blood flow to a person's heart or brain, causing a heart attack or stroke," said Roy Hantgan of Wake Forest University.

The team is now in contact with a company that uses an ultrasonic device to break up blood clots. The company would like to know the mechanical properties of the fibrin so that it can determine how much force to apply to rupture clots.

The researchers reported their work in Science.