MEchanics GAthering -MEGA- Seminar: Talk 1 - Why do surgeons sleep better with plasticity in their knots? Talk 2 - Snap buckling of bi-stable beams under magnetic actuation


Event details

Date 02.12.2021 16:1517:30  
Speaker Paul Johanns & Arefeh Abbasi (fleXLab, EPFL)
Location Online
Category Conferences - Seminars
Event Language English
Talk 1: Why do surgeons sleep better with plasticity in their knots? by Paul Johanns (fleXLab, EPFL)

Abstract Surgery, rooted etymologically in “hand work”, is a craftsmanship; high-quality suturing requires optimal manual skills reached only after years of experience. Knots are a suture’s weakest link and their failure can be disastrous. While monofilaments (versus braided ones) have the advantage of lower infection risks, they are more challenging to ensure mechanical safety. Preventing failure of surgical knots in monofilaments requires for the applied tension to lie within a precise range; under-tension yields insufficient plastic deformation, with the elastic energy stored in bending serving as an unravelling driver, while filament rupture can result from over-tension. We perform an investigation on the operational and safety limits of surgical knots, highlighting the previously overlooked but crucial effect of plastic deformation. We analyze the sutures produced by an experienced surgeon and in a model system to characterize the relevant range of applied tensions and geometric features, through mechanical testing and X-ray tomographic imaging. Our experiments combined with FEM simulations enable us to rationalize the primary ingredients dictating the mechanical performance of surgical knots.

Supported by the Fonds National de la Recherche, Luxembourg 12439430.

Bio Paul Johanns got his B.Sc. and M.Sc. both in Mechanical Engineering from ETH Zurich. He performed his Master Thesis at Caltech working on tensegrity-inspired structures for impact protection. Here, he developed a strong interest for the mechanics of slender structures before joining EPFL’s fleXLab as a Ph.D. student in 2018. His research focuses on the fundamental understanding of the topological influence on the mechanics of complex knots.

Talk 2: Snap buckling of bi-stable beams under magnetic actuation, by Arefeh Abbasi (fleXLab, EPFL)

Abstract We investigate the mechanics of bi-stable, hard-magnetic, elastic beams under magnetic actuation, combining precision model experiments, 3D finite element modeling (FEM), and a reduced-order centerline-based theoretical model. In the experiments, a beam is fabricated using a hard magneto-rheological elastomer (hard-MRE) magnetized such that the beam contains two segments with antiparallel magnetization along the beam centerline but otherwise identical mechanical properties. The beam is initially pre-loaded into a curved, bi-stable configuration by fixing its end-to-end shortening. Then, under the application of an external uniform magnetic field, the beam can be made to snap, reversibly, between its two stable states. First, we experimentally characterize the critical field strength for the onset of snapping for different levels of end-to-end shortening. We perform 3D FEM simulations, which adopt an exiting continuum theory for hard-MREs. In the FEM, the RIKS method is used to analyze high-order deformation modes and the corresponding energy states of the beam during snapping. In parallel, a reduced-order centerline-based beam theory is developed to rationalize the observed magneto-elastic response. The validity of the theory and simulations is established by their excellent quantitative agreement with experiments. Finally, we consider the case of combined mechanical (point indentation) and magnetic loading. We examine how the applied field affects the bi-stability and quantify the maximum load-bearing capacity of the beam under indentation. Our work provides a set of predictive tools for the rational design of one-dimensional, bi-stable, magneto-elastic structural elements, which could potentially form the basis of a novel class of functional mechanisms.

Bio Arefeh Abbasi got his B.Sc. and M.Sc. both in Mechanical Engineering from Shiraz University, Iran. She developed a strong interest for the mechanics of slender structures before joining EPFL’s fleXLab as a Ph.D. student in 2019. Her research focuses on the snap buckling of magneto-active structures for designing functional devices.


Practical information

  • General public
  • Free


  • MEGA.Seminar Organizing Committee


Solids Structures Fluids