Abstract

Abstract: The determination of the critical puncture force required to pierce a soft material is key to improve surgical technology (e.g. robotic surgery), manufacturing (e.g. food processing) and in-situ material characterization. This seminar discusses a mechanical theory able to calculate the critical depth 𝑑𝑐 and force 𝐹𝑐 required to insert a cylindrical needle with a spherical tip into a hyper elastic material. Needle insertion occurs as a mechanical instability, by which the needle-specimen system snaps between the ‘indentation’ configuration and the ‘penetration’ one, which then becomes energetically favoured. The model considers quasi-static indentation, thus neglects rate-dependent behaviour in the material. It also considers frictionless contact, and the cut material is assumed to be incompressible. Both 𝑑𝑐 and 𝐹𝑐 are functions of the toughness and shear modulus of the cut material, and of the radius of the needle. The scaling relations obtained from the model are then compared against experiments, giving good agreement. To account for frictional and adhesive contact between needle and specimen, the model is extended to correlate interfacial properties with the evolution of the puncture force with penetration depth. The extended model is again compared against experiments to provide validation, and shows that the penetration of tough and soft materials is controlled by friction, while for brittle and stiff materials adhesion prevails. Finally, we analyse the role of volumetric compressibility in puncture, and discover that tough and soft materials develop higher puncture resistance when compressible (lower bulk modulus).

Mattia Bacca obtained a PhD in Structural Engineering in 2013, at the at the University of Trento, which awarded his thesis as ‘Best PhD Thesis’ that year. Continuing his studies, he joined the University of California, Santa Barbara (USA) as a Postdoctoral Fellow, prior to joining the University of British Columbia (Canada), in 2017, as a faculty member. He is Assistant Professor in Mechanical Engineering, and member of the Biomedical School, and the Institute of Applied Mathematics. During his career as a faculty, he received the Early Career Award from the Human Frontiers in Science Program, who is funding part of his research. His research is devoted to understanding the biological world through the use of mechanics via the development of mathematical and computational models.