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May 04, 2012

Weaving Blood Vessels

Technology Review - Thin off-white threads of human cellular material spiral around the spindle of a machine that is braiding them into a sturdy rope. It sounds macabre, but the inspiration for the material, made by San Diego–based Cytograft Tissue Engineering, is health, not horror: the biological strands could be used to weave blood vessel patches and grafts that a patient's body would readily accept for wound repair. The process is faster and could be more cost-effective than other methods of producing biological tissue replacements.


Clean crochet: A specialist weaves a blood vessel graft from human threads on a sterile tubular loom.
Cytograft





The company developed the "human textile" idea from earlier work using sheets of biological material to reconstruct blood vessels. Basically, researchers grow human skin cells in a culture flask under conditions that encourage the cells to lay down a sheet of what is known as extracellular matrix—a structural material produced by animal cells that makes up our connective tissue. Cytograft can harvest these sheets from the culture flasks and then roll them into tubes that become replacement blood vessels. Blood vessels produced in this manner are still being tested—but they have performed well, with no signs of rejection, in a few patients in Europe and South America.

The rolling process, however, is expensive and time-consuming, in part because cells must be used to fuse the tube together so that it is sturdy enough for transplantation. Slicing the sheets into thin ribbons that can be spooled into threads makes it possible to use automated weaving and braiding machines to create three-dimensional structures that do not require fusing.

Biological braids: A machine braids together 48 threads of human extracellular material.
Cytograft



Cytograft's implants contain no cells. Though the company's earlier implants were made of extracellular matrix produced from a patient's own cells, its researchers can now harvest the material from cells unrelated to the person receiving the graft and remove the "donor" cells completely. "We don't need the cells," says L'Heureux. "The cells can come from the patients after implantation."

Without any foreign cells to alert a patient's immune system, the company could produce blood vessels ahead of time for use in any patient. Such replacement vessels would be less expensive and more readily accessible than what's available today. "One of Cytograft's biggest advantages will be off-the-shelf availability," says Breuer.

The company is also working on a technique in which the cell-produced sheets are processed into particles instead of threads. The biological bits can then be molded together, says L'Heureux, giving tissue engineers two advantages. Molding the particles together leaves a complex network of channels behind—exactly what tissues engineers will need in order to produce, eventually, something like a liver, pancreas, or kidney. With most other technology, there is "no guarantee that the channels will be maintained," says L'Heureux. The particles could also be injected, he says, which could add volume to tissues for cosmetic or reconstructive purposes.

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