Cyclic jetting enables microbubble-mediated drug delivery

TL;DR

Ultrasound-driven single microbubbles induce drug uptake through cyclic microjets formed at mild ultrasound pressures via interfacial instability, elucidates the physics behind microbubble-mediated targeted drug delivery and provides the criteria for its effective and safe application.

Abstract

The pursuit of targeted therapies capable of overcoming biological barriers, including the tenacious blood-brain barrier, has spurred the investigation into stimuli-responsive microagents. This approach could improve therapeutic efficacy, reduce undesirable side effects, and open avenues for treating previously incurable diseases. Intravenously-administered ultrasound-responsive microbubbles are one of the most promising agents, having demonstrated potential in several clinical trials. However, the mechanism by which microbubbles enhance drug absorption remains unclear. Here, we reveal through unprecedented time-resolved side-view visualisations that single microbubbles, upon microsecond-long ultrasound driving, puncture the cell membrane and induce drug uptake via stable cyclic microjets. Our theoretical models successfully reproduce the observed bubble and cell dynamic responses. We find that cyclic jets arise from shape instabilities, warranting recognition as a novel class of jets in bubbles, distinct from classical inertial jets driven by pressure gradients. We also establish a threshold for bubble radial expansion beyond which microjets form and facilitate cellular permeation. Remarkably, these microjets occur at ultrasound pressures below 100kPa due to their unique formation mechanism. We show that the stress generated by microjetting surpasses all previously suggested mechanisms by at least an order of magnitude. In summary, this work elucidates the physics behind microbubble-mediated targeted drug delivery and provides criteria for its effective yet safe application.

Figures (11)

• Figure 1: Targeted drug delivery mediated by ultrasound-responsive microbubbles. (a) Schematic illustrating in-vivo extravascular drug delivery induced by the mechanical action exerted by an ultrasound-driven microbubble. (b) Schematic depicting the in-vitro test model employed to study the bubble-cell dynamics and the corresponding intracellular drug uptake with a side-view perspective. See Methods and Extended Data Fig. \ref{['fig:Setup']} for details about the experimental setup. (c-d) Response of a microbubble ($R_0 = 3µm$) to varying ultrasound pressure amplitudes and the corresponding model drug uptake. In (c), the applied ultrasound pressure induces spherical oscillations in the bubble, followed by asymmetric deformation ($p_{\rm a} = 60kPa, \ f = 1MHz$). However, this does not result in cell membrane poration and drug uptake. In (d), a higher ultrasound pressure causes the bubble to develop cyclic piercing microjets directed towards the cell ($p_{\rm a} = 160kPa, \ f = 1MHz$), resulting in cell membrane poration and drug uptake.
• Figure 1: Schematic of the experimental setup. (AOTF) Acousto-optic tunable filter, (BD) Beam dump, (BE) Beam expander, (DB) Dichroic beamsplitter, (EF) Emission filter, (L1-L4) Lens, (LS) Light source, (OL) Objective lens, (TC) Test chamber, (TL) Tube lens, (US) Ultrasound transducer, (XF) Excitation filter.
• Figure 2: Characterisation of bubble dynamics. In the legend, "low $p_{\rm a}$" and "high $p_{\rm a}$" refer to Figs. \ref{['fig:Fig1']}c and \ref{['fig:Fig1']}d, respectively. The red arrows at the top indicate when jets occur in the "high $p_{\rm a}$" case. See Methods for details about the theoretical models. (a) Experimental and theoretical radial motion of the microbubble. The uncertainty in the bubble radius measurement corresponds to half the pixel size (80nm). (b) Ultrasound pulse driving the microbubble. The pulse shape is recorded with a hydrophone. The pulse amplitude is inferred as the only fitting parameter from the corresponding radius-time curve in Fig. \ref{['fig:Fig2']}a. (c) Experimental and theoretical vertical position of the microbubble centroid. The yellow area represents the plastic substrate. The uncertainty in the bubble position measurement corresponds to half the pixel size (80nm). (d) Time evolution of the pressure gradient contributions from the ultrasound pulse and plastic substrate at the bubble position. The curves for the two ultrasound pressure cases are vertically offset to enhance visibility. (e) Evolution of the dimensionless impulse contributions from the ultrasound, the plastic substrate and the sum thereof over ultrasound cycles.
• Figure 2: Cyclic microjets generated by a microbubble ($R_0=3.0µm$) induce cell membrane poration, facilitating drug uptake ($p_{\rm a} = 175kPa, \ f = 1MHz$). Yet, in this instance, the bubble motion does not form a transendothelial tunnel, as the cell deformation recovers and the bubble returns to its initial position when the ultrasound pulse stops.
• Figure 3: Shape modes and jet formation of single microbubbles in contact with a PEG substrate. (a-f) Shape modes with angular wavenumbers $l$ ranging from 1 up to 6. The top panels display bright-field snapshots from two consecutive ultrasound cycles, at a generic time instant $t = t_i$ and one ultrasound period $T_{\rm US}=1µs$ later, at $t = t_i + T_{\rm US}$. These images illustrate the cyclic transition between a geometric pattern and its dual during the shape mode oscillation. Odd wavenumber shape mode patterns are self-dual. The red dashed sketches show the polyhedral representation of shape mode patterns. On bottom, the combination of spherical harmonics $Y_l^m$ representing the shape mode patterns. Red regions denote outward deformation, while blue regions indicate inward deformation. (g) Experimentally observed shape modes angular wavenumber $l$ as function of the equilibrium bubble radius. The uncertainty in the bubble radius measurement corresponds to half the pixel size (80nm). (h-k) Jets driven by shape modes with angular wavenumbers $l$ ranging from 1 up to 4. On top, bright field images depicting the jets generated by the shape pattern or by its dual. On bottom, bubble shape decomposition in breathing mode (spherical oscillation) and shape mode. Red regions denote outward deformation, while blue regions indicate inward deformation. Jets manifest during compression phases in the sunken regions of the shape mode.