Mechanical characterisation of lymph nodes and optimisation of needle-tissue interaction for endobronchial biopsy

Endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) is a minimally invasive biopsy technique used for lymph node sampling in lung cancer staging and for diagnosing granulomatous disease. The procedure’s performance is governed by the mechanics of needle–tissue interaction, specifically needle placement accuracy, insertion mechanics, and tissue acquisition efficiency. Despite its clinical utility, reported diagnostic yields remain limited to approximately 60–80%, primarily due to inadequate tissue capture with current needle geometries. Clinical audits, including those conducted by NHS Greater Glasgow & Clyde, confirm suboptimal diagnostic performance that is consistent across multiple institutions. These limitations contribute to repeat procedures, delayed diagnosis, and increased system-level resource utilisation. This thesis develops a combined experimental-computational framework to characterise and optimise needle insertion and tissue acquisition in lymphatic tissue. A constitutive characterisation methodology was first established to describe the nonlinear, anisotropic, quasi-incompressible, fibre-reinforced hyperelastic behaviour of lymph node tissue. Needle insertion mechanics were then quantified both experimentally and numerically across a range of controlled insertion velocities (13 mm/s, 9 mm/s, 3 mm/s, and 0.5 mm/s), and the force response was decomposed into insertion, cutting, and frictional components. Finally, parametric finite element simulations using the Coupled Eulerian–Lagrangian (CEL) formulation were employed to evaluate the influence of needle-tip geometry and cutting-plane configuration on tissue deformation, stress distribution, and sample capture efficiency. The validated soft tissue model accurately reproduces the mechanical response of lymphatic tissue under large deformations and dynamic loading. Results indicate a strong rate-dependent response, with increasing insertion velocity leading to higher peak insertion forces (5–8 N) and cutting forces (3.62–6.14 N), while reducing frictional forces (0.5–0.4 N) and the effective coefficient of friction (1.85–1.32), consistent with velocity-dependent interfacial behaviour. Furthermore, geometric optimisation of the needle tip significantly alters stress localisation and tissue failure mechanisms. Finite element analysis shows that a Franseen-type needle with three symmetric cutting planes enhances tissue engagement, reduces insertion forces by up to 35% compared with conventional EBUS-TBNA needle designs, and improves specimen length while reducing tissue deformation, thereby indicating improved sampling efficiency and mechanical performance