Pedro M. Reis, Ji-Sung Park, and Javier Sabater's KTH Solid Mechanics KEYNOTE seminar "The mechanics of physical knots: from shoelaces to surgical sutures"

Pedro Reis 2026_03_26.pdf (pdf 317 kB)

Abstract. Although most of us tie our shoelaces "wrongly," knots in ropes and filaments have served as functional structures for millennia, from sailing and climbing to sewing and surgery. Yet knowledge of physical knots remains largely empirical, and mechanics-based predictive models are lacking. Tight knots exhibit highly nonlinear, coupled behavior due to their intricate 3D geometry, large deformations, self-contact, friction, and polymer elasto-plasticity, making their modeling a challenging problem in mechanics. In this talk, we present recent work from our group combining precision experiments, FEM simulations, and analysis. We focus on surgical knots, which carry a particularly high risk of failure in clinical settings, including massive bleeding or unraveling of high-tension closures. Our results reveal a robust superlinear relationship among knot strength, applied pre-tension, and the number of throws, rationalized by a reduced-order model based on the capstan problem and Euler's elastica. We also examine overhand, figure-8, clove-hitch, and bowline knots, as well as knots in fluid-conveying biological tubes. Our findings have implications for clinical settings, surgical training, robotic-assisted surgery, and more mundane activities such as shoelace tying, sailing, and climbing.