Sonoporation from jetting cavitation bubbles

Abstract

The fluid dynamic interaction of cavitation bubbles with adherent cells on a substrate is experimentally investigated. We find that the nonspherical collapse of bubbles near to the boundary is responsible for cell detachment. High-speed photography reveals that a wall bounded flow leads to the detachment of cells. Cells at the edge of the circular area of detachment are found to be permanently porated, whereas cells at some distance from the detachment area undergo viable cell membrane poration (sonoporation). The wall flow field leading to cell detachment is modeled with a self-similar solution for a wall jet, together with a kinetic ansatz of adhesive bond rupture. The self-similar solution for the delta-type wall jet compares very well with the full solution of the Navier-Stokes equation for a jet of finite thickness. Apart from annular sites of sonoporation we also find more homogenous patterns of molecule delivery with no cell detachment.

FIGURE 1

Sketch of the experimental setup. Shock waves in the water basin are generated with a piezoelectric transducer and are focused under an angle of 45° to the horizontal. The adhering HeLa cells on the petri dish are facing the shock-wave generator and are illuminated with a light guide. Imaging of the cell layer is done with a long-working-distance microscope connected to the high-speed framing system Brandaris 128.

FIGURE 2

(a) Sketch of the side view and top view of the bubble cluster. The incoming wave enters under an angle of 45°. Two bubble clusters are generated; one from the main wave, and a smaller one from the reflection at the inner surface of the petri dish. The side view (right) depicts areas A and C where bubble-collapse onto the adherent cells takes place. Area B is largely free of bubbles. (b) Large-scale fluorescence microscope picture of the substrate after the bubble collapse. Annular structures of molecular uptake (bright cells) are found in regions A and C, where region B is covered with a diffuse pattern. The white bar denotes a scale of 10 mm.

FIGURE 3

Closeup of typical cell detachment areas found in region A of Fig. 2. Viable porated cells are colored green (calcein uptake) and red cells are stained with ethidium bromide (permanent poration). The left image depicts the detachment and sonoporation from a single bubble, and the right image depicts two bubbles that have collapsed close to each other. The bar denotes 0.5 mm.

FIGURE 4

Closeup of typical cell detachment areas found in region B of Fig. 2. Viable porated cells are colored green (calcein uptake) and red cells are stained with ethidium bromide (permanent poration). Approximately 15% of the cells are permanently porated. The bar denotes 0.5 mm.

FIGURE 5

Three scanning electron microscopy (SEM) micrographs with increasing magnification from left to right. The white bar denotes a length of 20 μm. (a) Overview of a disk-shaped detachment site. (b) Magnified view of the grayed area in a showing rounded and piled-up cells at the border and flat cells further away. (c) The rounded cells at the border reveal structural damage on their membrane indicated with white arrows.

FIGURE 6

Detachment dynamics caused by the collapse of a single bubble close to a layer of adherent cells (shaded background). Here, selected frames from a framing sequence taken at 230,000 frames per second are depicted. The solid bar in the last frame denotes a scale of 1 mm. The numbers in the upper right of the frames indicate the elapsed time in microseconds after the start of the negative pressure. The bubble reaches maximum size at ∼t = 80 μs and shrinks to its smallest size at t = 154 μs. Then it reexpands as a corrugated toroidal bubble. From time t = 180 μs, the rim of the detached area becomes visible. The area continuous to expand even after the bubble has disintegrated.

FIGURE 7

Radius of the bubble (□) and the size of the detached area (○) as a function of time. Zero time denotes the instant when the negative pressure wave reaches the cells.

FIGURE 8

Streamlines for the similarity solution from Glauert of the wall jet for different values of the Re-number. The jet is flowing along the line x = 0 and impinging at the origin.

FIGURE 9

Sketch of the geometry to solve the Navier-Stokes equation in axisymmetry. The jet with a diameter of d_jet is released at a distance of h_jet from the rigid wall.

FIGURE 10

Streamlines for the full Navier-Stokes solution of the wall jet. The solid rectangle indicates the position of the incoming jet.

FIGURE 11

Profiles of the horizontal flow velocity for the full solution (solid lines) and the self-similar solution (dashed lines). For larger distances R_jet from the origin, both models show good agreement.

FIGURE 12

The wall shear stress as a function of the distance for the full solution (solid lines) and the self-similar Glauert solution. For distances larger than ∼3 R_jet from the origin, good agreement is obtained.

FIGURE 13

Radius of the detachment area x_c as a function of the maximum bubble radius R_max obtained from the numerical solution of Eq. 8 for τ_c (solid line) and through the approximation Eq. 10 (dotted line). The solid squares denote measurements.