Background: High intensity focused ultrasound (HIFU) is an emerging non-invasive treatment modality for localized treatment of cancers. While current clinical strategies employ HIFU exclusively for thermal ablation of the target sites, biological responses associated with both thermal and mechanical damage from focused ultrasound have not been thoroughly investigated. In particular, endogenous danger signals from HIFU-damaged tumor cells may trigger the activation of dendritic cells. This response may play a critical role in a HIFU-elicited anti-tumor immune response which can be harnessed for more effective treatment.

The interaction of a lithotripter shock wave (LSW) with laser-generated single vapor bubbles in water is investigated using high-speed photography and pressure measurement via a fiber-optic probe hydrophone. The interaction leads to nonspherical collapse of the bubble with secondary shock wave emission and microjet formation along the LSW propagation direction. The maximum pressure amplification is produced during the collapse phase of the bubble oscillation when the compressive pulse duration of the LSW matches with the forced collapse time of the bubble.

The dynamic interaction of a shockwave (modelled as a pressure pulse) with an initially spherically oscillating bubble is investigated. Upon the shockwave impact, the bubble deforms non-spherically and the flow field surrounding the bubble is determined with potential flow theory using the boundary-element method (BEM). The primary advantage of this method is its computational efficiency. The simulation process is repeated until the two opposite sides of the bubble surface collide with each other (i.e. the formation of a jet along the shockwave propagation direction). The collapse time of the bubble, its shape and the velocity of the jet are calculated. Moreover, the impact pressure is estimated based on water-hammer pressure theory. The Kelvin impulse, kinetic energy and bubble displacement (all at the moment of jet impact) are also determined. Overall, the simulated results compare favourably with experimental observations of lithotripter shockwave interaction with single bubbles (using laser-induced bubbles at various oscillation stages). The simulations confirm the experimental observation that the most intense collapse, with the highest jet velocity and impact pressure, occurs for bubbles with intermediate size during the contraction phase when the collapse time of the bubble is approximately equal to the compressive pulse duration of the shock wave. Under this condition, the maximum amount of energy of the incident shockwave is transferred to the collapsing bubble. Further, the effect of the bubble contents (ideal gas with different initial pressures) and the initial conditions of the bubble (initially oscillating vs. non-oscillating) on the dynamics of the shockwave-bubble interaction are discussed.

The interaction of laser-generated tandem microbubble (maximum diameter about 50 μm) with single (rat mammary carcinoma) cells is investigated in a 25-μm liquid layer. Anti-phase and coupled oscillation of the tandem microbubble leads to the formation of alternating, directional microjets (with max. microstreaming velocity of 10 m/s) and vortices (max. vorticity of 350,000 s−1) in opposite directions. Localized and directional membrane poration (200 nm to 2 μm in pore size) can be produced by the tandem microbubble in an orientation and proximity dependent manner, which is absence from a single oscillating microbubble of comparable size and at the same stand-off distance.

A simple method for preparing artificial kidney stones with varying physical properties is described. BegoStone was prepared with a powder to water ratio (by weight) ranging from 15:3 to 15:6. The acoustic properties of the phantoms were characterized by using an ultrasound transmission technique, from which the corresponding mechanical properties were calculated based on elastic wave theory. The measured parameters for BegoStone phantoms of different water contents are: longitudinal wave speed (3148 – 4159 m/s), transverse wave speed (1813 – 2319 m/s), density (1563 – 1995 kg/m3), longitudinal acoustic impedance (4.92 – 8.30 Kg/m2*s), transverse acoustic impedance (2.83 – 4.63 Kg/m2*s), Young’s modulus (12.9 – 27.4 GPa), bulk modulus (8.6 – 20.2 GPa), and shear modulus (5.1 – 10.7 GPa), which cover the range of corresponding properties reported in natural kidney stones. In addition, diametral compression tests were carried out to determine tensile failure strength of the stone phantoms. BegoStone phantoms with varying water content at preparation have tensile failure strength from 6.9 – 16.3 MPa when tested dry and 3.2 – 7.1 MPa when tested in water-soaked condition. Overall, it is demonstrated that this new BegoStone preparation method can be used to fabricate artificial stones with physical properties matched with those of natural kidney stones of various chemical compositions.