Auxetics and the Physics of Venous Stenting

The founding of Auxetics in 2019, a start-up developing a radically different venous stent, could have only happened through the amalgamation of physics and medicine. It started with a venous stenting occurrence observed by three interventional radiologists operating at the Department of Interventional Radiology at Oregon Health and Science University in Portland, OR: Ramsey Al-Hakim, MD, John Kaufman, MD, and Khashayar Farsad, MD, PhD. They had noticed that after venous stenting, stenosis would sometimes occur in an untreated adjacent vein. At heart, Al-Hakim was an engineer; he’d gotten his biomedical engineering degree at Georgia Tech before becoming an interventional radiologist and engaging in medical research at OHSU. The phenomenon piqued his interest. “I read everything I could find about venous biomechanics, which was not much,” he says.

He decided to do some experimentation on benchtop, in vivo, and computational models to see what mechanical effects could be causing stent adjacent stenosis (SAS), and he realized that adjacent vessel deformation was due to the Poisson effect. Named after a French mathematician operating in the early 19th century, the Poisson ratio describes the way the thickness of materials changes in response to elongation. Think of a rubber band; it contracts perpendicularly when it is elongated, and most materials do. In that case, the rubber band has a high Poisson ratio. Veins aren’t very elastic to begin with, and become even stiffer when diseased, particularly in the case of post-thrombotic syndrome, so they exhibit less perpendicular contraction in response to elongating stress, that is, they foreshorten in response to dilation by stenting. This causes the opposite to happen in the adjacent unstented segment, which experiences a decrease in diameter in response to elongating stress.