Ultrasonically Actuated Medical Needle - Non-Linear Effects and Applications

Medical needles serve a central role in everyday healthcare, exemplified by the 16 billion injections administered globally every year. The needles typically feature a very sharp structure at their tip (lancet), which facilitates the penetration through the skin and tissue to achieve a medical purpose. When used in combination with a syringe, hypodermic needles enable e.g. injection of substances into the patient's body, or to extract cells from the target. Despite their extensive use in medicine, the design of medical needles has remained largely unchanged throughout recent decades and limitations related to pain, delivery of entities, precision and spatial localization still remain. The primary aim of this Thesis was to study how ultrasound can be used to bring new functionalities to a standard medical needle in vision to address major issues still present in some medical applications. The solution consisted in coupling a Langevin transducer (f ~ 33 kHz) to a 21G hypodermic needle (outer diameter = 0.81 mm; length = 80 mm) through an aluminum waveguide to produce flexural standing waves within the needle structure. The experiments focused on exploring the influence of the needle vibration on matter, when the ultrasonically actuated needle was operated in different types of media, such as water, tissue-like phantom gels and ex vivo soft tissue. Numerical simulations were also conducted to deepen the understanding of some relevant non-linear ultrasound phenomena involved in the experiments, namely acoustic radiation force, acoustic streaming and cavitation, generated by the oscillating needle. The results showed that the actuation of a medical needle contributes to generating a force field on the medium surrounding the needle tip, which can be exploited to manipulate soft matter or solid objects. When used to perform ultrasound-enhanced fine-needle aspiration biopsy, the ultrasonic oscillation of the needle allowed to increase the mass of the collected sample by 3–6x in different ex vivo bovine tissue types, as compared to when standard fine-needle aspiration biopsies were retrieved. Experimentally and numerically, we were able to demonstrate generation of acoustic streaming emanating outwards from the needle tip, which allowed to translate microparticles inside water, and helped to promote the translation of nanoparticles and liquids inside tissue-like porous structures. Formation of transient cavitation events was also captured with high-speed imaging in water and soft tissue, exhibiting a strong threshold behavior dependent on the total acoustic power employed. The findings presented in this Thesis advance the field of oscillating medical needles, showing a great potential in various medical applications including biopsy, drug or gene delivery and tissue ablation.