Blood clots are one way in which the body heals itself after injuries on even the tiniest level. The process is fast, reliable and goes on every minute of the day without our being aware of it. Now, a team led by MIT assistant professor of materials science and engineering Alfredo Alexander-Katz is studying blood clots as a new model for producing self-healing materials.
Blood clotting seems simple. You cut yourself and your body stops the bleeding and closes off the wound. However, that simple sentence covers an incredibly complex biological mechanism involving all manner of chemical and mechanical processes that still aren’t fully understood.
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Clotting or coagulation uses a squad of molecules present in the tissues and bloodstream. Many of these molecules flow through the veins and arteries like little medical kits waiting until needed. If a blood vessel is injured or a cell dies, this sets off a cascade of events designed to halt bleeding and protect the wound from infection. The MIT team focused their attention on just one part of the very involved process: how clotting uses blood flow to plug a wound.
What they discovered was the opposite of what one would expect. Usually when a fluid is flowing, it keeps it from solidifying. It’s the principle behind cement mixers and Slurpee machines. By keeping the cement or frozen drink stirring, crystals are prevented from forming that would turn the liquid into a solid mass. With blood, it's exactly the opposite. The faster the flow, the faster a clot forms. According to Alexander-Katz, “Part of it is chemistry, and part of it is mechanical, which has to do with the flow itself.”
The process that the team studied involves platelets and a biopolymer molecule called Willebrand factor (vWF). Platelets are nucleus-free blood cells that have a number of functions in clotting. In this case, they act as building blocks for a blood clot. vWF is a long-chain molecule that floats in the bloodstream, coiled up like a roll of adhesive tape. As blood flow increases, such as during an injury, the flow causes the vWF to stretch out.
When coiled up the vWF just rolls by, but when stretched, the exposed sticky surfaces start to catch hold of the platelets and entangle them. The faster the flow, the more molecules uncoil. Other cells get caught up and a plug is formed within seconds. This, again, sounds simple, but clotting is a process that needs careful control if the vessel isn't to end up completely clogged. For that reason, there are also “molecular scissors” that cut up the plug as it forms. As the blood flow increases, so does the clotting until the scissors can’t keep up. When the flow decreases, the clot starts to dissolve as the scissors get to work.
The upshot of all this is a new model for a self-repairing material. Such materials have been around since before World War II, but they've been relatively simple in principle. Some, such as self-sealing fuel tanks, use a layer that expands to fill punctures. Others rely on capillaries filled with resins that spread and harden to repair damage. However, blood clots are dynamic. They form under certain conditions and dissolve when those conditions no longer pertain. More important, they aren't a simple sealing compound, but a construction – one that forms with remarkable speed and is reversible.
These properties make clots very interesting to engineers. By imitating the blood clot mechanism, the MIT team believes that it could find a wide range of applications. Not only could it be used in self-repairing materials, but it could be used in self-assembling ones as well. Indeed, the ability to control the process by controlling the rate of flow would make it ideal for everything from inks to self-healing tires.
The findings of the team have been published in the online journal Nature Communications.
In the video below, Alexander-Katz describes the process.