Shock-wave-induced jetting of micron-size bubbles

Mechanisms of primary blast-induced traumatic brain injury: insights from shock-wave research

Traumatic brain injury caused by explosive or blast events is traditionally divided into four phases: primary, secondary, tertiary, and quaternary blast injury. These phases of blast-induced traumatic brain injury (bTBI) are biomechanically distinct and can be modeled in both in vivo and in vitro systems. The primary bTBI injury phase represents the response of brain tissue to the initial blast wave. Among the four phases of bTBI, there is a remarkable paucity of information about the cause of primary bTBI. On the other hand, 30 years of research on the medical application of shockwaves (SW) has given us insight into the mechanisms of tissue and cellular damage in bTBI, including both air-mediated and underwater SW sources. From a basic physics perspective, the typical blast wave consists of a lead SW followed by supersonic flow. The resultant tissue injury includes several features observed in bTBI, such as hemorrhage, edema, pseudoaneurysm formation, vasoconstriction, and induction of apoptosis. These are well-described pathological findings within the SW literature. Acoustic impedance mismatch, penetration of tissue by shock/bubble interaction, geometry of the skull, shear stress, tensile stress, and subsequent cavitation formation, are all important factors in determining the extent of SW-induced tissue and cellular injury. Herein we describe the requirements for the adequate experimental set-up when investigating blast-induced tissue and cellular injury; review SW physics, research, and the importance of engineering validation (visualization/pressure measurement/numerical simulation); and, based upon our findings of SW-induced injury, discuss the potential underlying mechanisms of primary bTBI.

Enhanced shock wave-assisted transformation of Escherichia coli

The objective of the study was to demonstrate that shock wave-induced transfer of DNA into bacteria can be increased by enhancing cavitation using dual-pulse (tandem) shock waves. Escherichia coli and plasmid were transferred to test vials. Competent cells were prepared at different concentrations of CaCl(2). Single pulses and tandem shock waves were compared as were three treatment temperatures: 0, 10 and 25 °C. Three delays (250, 500, 750 μs) between double pulses were tested. Characterization was achieved by using a plasmid that provided green fluorescent protein expression. At 0 °C double pulses generated at a delay of 750 μs significantly increased the number of fluorescent colonies compared with single pulses. In general, the lowest temperature enhanced the mean number of transformants compared with the two higher temperatures. A strong influence of the CaCl(2) concentration on the transformation efficiency was also found. The main conclusion is that gene transfer to target cells may be increased up to 50 times at 0 °C by enhancing cavitation using pairs of shock waves.

Contrast-enhanced and targeted ultrasound

Ultrasonic imaging is becoming the most popular medical imaging modality, owing to the low price per examination and its safety. However, blood is a poor scatterer of ultrasound waves at clinical diagnostic transmit frequencies. For perfusion imaging, markers have been designed to enhance the contrast in B-mode imaging. These so-called ultrasound contrast agents consist of microscopically small gas bubbles encapsulated in biodegradable shells. In this review, the physical principles of ultrasound contrast agent microbubble behavior and their adjustment for drug delivery including sonoporation are described. Furthermore, an outline of clinical imaging applications of contrast-enhanced ultrasound is given. It is a challenging task to quantify and predict which bubble phenomenon occurs under which acoustic condition, and how these phenomena may be utilized in ultrasonic imaging. Aided by high-speed photography, our improved understanding of encapsulated microbubble behavior will lead to more sophisticated detection and delivery techniques. More sophisticated methods use quantitative approaches to measure the amount and the time course of bolus or reperfusion curves, and have shown great promise in revealing effective tumor responses to anti-angiogenic drugs in humans before tumor shrinkage occurs. These are beginning to be accepted into clinical practice. In the long term, targeted microbubbles for molecular imaging and eventually for directed anti-tumor therapy are expected to be tested.

Interaction dynamics of spatially separated cavitation bubbles in water

We present a high-speed photographic analysis of the interaction of cavitation bubbles generated in two spatially separated regions by femtosecond laser-induced optical breakdown in water. Depending on the relative energies of the femtosecond laser pulses and their spatial separation, different kinds of interactions, such as a flattening and deformation of the bubbles, asymmetric water flows, and jet formation were observed. The results presented have a strong impact on understanding and optimizing the cutting effect of modern femtosecond lasers with high repetition rates (>1 MHz).

Structure and dynamics of shock-induced nanobubble collapse in water

Shock-induced collapse of nanobubbles in water is investigated with molecular dynamics simulations based on a reactive force field. We observe a focused jet at the onset of bubble shrinkage and a secondary shock wave upon bubble collapse. The jet length scales linearly with the nanobubble radius, as observed in experiments on micron-to-millimeter size bubbles. Shock induces dramatic structural changes, including an ice-VII-like structural motif at a particle velocity of 1  km/s. The incipient ice VII formation and the calculated Hugoniot curve are in good agreement with experimental results.

Scaling law for bubbles induced by different external sources: theoretical and experimental study

The scaling relations for bubbles induced by different external sources are investigated based on a modified Rayleigh model and experimental observations. The equations derived from the modified Rayleigh model are presented to describe the collapse of bubbles induced by the different external sources such as electrical spark, laser, and underwater explosion. A scaling law is then formulated to establish the scaling relations between the different types of bubbles. The scaling law reveals the fact that the characteristic length scale factor differs from the characteristic time scale factor for the different types of bubbles. It is then validated by our experimental observations of the spark- and laser-generated bubbles as well as the bubbles induced by underwater explosions from previous published reports. With the present scaling law, studies on spark- or laser-generated bubbles as well as their applications (for example, in industrial or biomedical related applications) can benefit from the experiences and information built up over the years in underwater explosion bubbles. Conversely, it is possible to substitute a spark- or laser-generated bubble for an underwater explosion bubble in the study of a large-scale and complex physical problem.

Sonoporation by single-shot pulsed ultrasound with microbubbles adjacent to cells

In this article, membrane perforation of endothelial cells with attached microbubbles caused by exposure to single-shot short pulsed ultrasound is described, and the mechanisms of membrane damage and repair are discussed. Real-time optical observations of cell-bubble interaction during sonoporation and successive scanning electron microscope observations of the membrane damage with knowledge of bubble locations revealed production of micron-sized membrane perforations at the bubble locations. High-speed observations of the microbubbles visualized production of liquid microjets during nonuniform contraction of bubbles, indicating that the jets are responsible for cell membrane damage. The resealing process of sonoporated cells visualized using fluorescence microscopy suggested that Ca2+-independent and Ca2+-triggered resealing mechanisms were involved in the rapid resealing process. In an experimental condition in which almost all cells have one adjacent bubble, 25.4% of the cells were damaged by exposure to single-shot pulsed ultrasound, and 15.9% (approximately 60% of the damaged cells) were resealed within 5 s. These results demonstrate that single-shot pulsed ultrasound is sufficient to achieve sonoporation when microbubbles are attached to cells.

Numerical simulations of non-spherical bubble collapse

A high-order accurate shock- and interface-capturing scheme is used to simulate the collapse of a gas bubble in water. In order to better understand the damage caused by collapsing bubbles, the dynamics of the shock-induced and Rayleigh collapse of a bubble near a planar rigid surface and in a free field are analysed. Collapse times, bubble displacements, interfacial velocities and surface pressures are quantified as a function of the pressure ratio driving the collapse and of the initial bubble stand-off distance from the wall; these quantities are compared to the available theory and experiments and show good agreement with the data for both the bubble dynamics and the propagation of the shock emitted upon the collapse. Non-spherical collapse involves the formation of a re-entrant jet directed towards the wall or in the direction of propagation of the incoming shock. In shock-induced collapse, very high jet velocities can be achieved, and the finite time for shock propagation through the bubble may be non-negligible compared to the collapse time for the pressure ratios of interest. Several types of shock waves are generated during the collapse, including precursor and water-hammer shocks that arise from the re-entrant jet formation and its impact upon the distal side of the bubble, respectively. The water-hammer shock can generate very high pressures on the wall, far exceeding those from the incident shock. The potential damage to the neighbouring surface is quantified by measuring the wall pressure. The range of stand-off distances and the surface area for which amplification of the incident shock due to bubble collapse occurs is determined.

Interaction between supersonic disintegrating liquid jets and their shock waves

We used ultrafast x radiography and developed a novel multiphase numerical simulation to reveal the origin and the unique dynamics of the liquid-jet-generated shock waves and their interactions with the jets. Liquid-jet-generated shock waves are transiently correlated to the structural evolution of the disintegrating jets. The multiphase simulation revealed that the aerodynamic interaction between the liquid jet and the shock waves results in an intriguing ambient gas distribution in the vicinity of the shock front, as validated by the ultrafast x-radiography measurements. The excellent agreement between the data and the simulation suggests the combined experimental and computational approach should find broader applications in predicting and understanding dynamics of highly transient multiphase flows.

Shock-induced collapse of a gas bubble in shockwave lithotripsy

The shock-induced collapse of a pre-existing nucleus near a solid surface in the focal region of a lithotripter is investigated. The entire flow field of the collapse of a single gas bubble subjected to a lithotripter pulse is simulated using a high-order accurate shock- and interface-capturing scheme, and the wall pressure is considered as an indication of potential damage. Results from the computations show the same qualitative behavior as that observed in experiments: a re-entrant jet forms in the direction of propagation of the pulse and penetrates the bubble during collapse, ultimately hitting the distal side and generating a water-hammer shock. As a result of the propagation of this wave, wall pressures on the order of 1 GPa may be achieved for bubbles collapsing close to the wall. The wall pressure decreases with initial stand-off distance and pulse width and increases with pulse amplitude. For the stand-off distances considered in the present work, the wall pressure due to bubble collapse is larger than that due to the incoming shockwave; the region over which this holds may extend to ten initial radii. The present results indicate that shock-induced collapse is a mechanism with high potential for damage in shockwave lithotripsy.

Dynamics of impacting a bubble by another pulsed-laser-induced bubble: jetting, fragmentation, and entanglement

We investigate experimentally the detailed dynamics of how an existing microbubble B1 is impacted and shattered by another nearby pulsed-laser-induced microbubble B2, and the backward interaction on B2 in a thin liquid layer. Mediated by the flow field, potential energy can be accumulated or lost through the alternate compression and expansion of the two bubbles. The symmetry breaking induced by the presence of the nearby counterbubble generates push-pull-type alternate forward and backward axial jetting on the compressed bubble associated with the elongated shape or even entrainment of the counterexpanding bubble into the jet-indented boundary. The strong penetrating axial jet through B1, and its interplay with the transverse jets by the flow field surrounding B1 in the first compression stage and the second expanding stage of B1 lead to a complicated fragmentation pattern of B1. Increasing the interbubble interaction by decreasing the interbubble distance causes B2 to become entangled with B1 through its entrainments into the backward axial jet-indented region of B2, in the expansion phase of B2. At the extreme of large laser energy for B2, the leftward reexpansion of B1 is suppressed. The strong shear flow field generates many tiny bubbles around the liquid-gas boundaries of the two axial jet-induced major daughter bubbles from B1. The detailed interaction behaviors over a broad range of the energy of B2, 0.14-0.55 microJ (corresponding to the maximum bubble expansion energy), and of the interbubble distance (170-500 microm) are presented and discussed.

An introduction to bubble dynamics

The creation, understanding and control of mesoscopic chemical objects are at the core of many areas of chemistry and physics. Their preparation depends on a variety of nucleation and aggregation processes and they are exploitable for practical purposes, including fabrication techniques. At the opposite end, are cavitation phenomena. They also originate from a nucleation event, but result in the formation of a bubble. In recent years, bubbles have ceased to be a problem and have become increasingly more attractive for medical and industrial applications. Our understanding of bubble dynamics has steadily increased and we believe that it is timely to attempt to summarize in simple terms the knowledge accumulated in this area for a chemical audience. In this introduction, we focus on the nature of bubble formation, evolution and collapse. We discuss the macroscopic models that were developed at the beginning of the 20th century and accompany them with the results of more detailed molecular dynamics simulations.

Interaction of lithotripter shockwaves with single inertial cavitation bubbles

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.

Interaction between shock wave and single inertial bubbles near an elastic boundary

The interaction of laser-generated single inertial bubbles (collapse time = 121 mus) near a silicon rubber membrane with a shock wave (55 MPa in peak pressure and 1.7 mus in compressive pulse duration) is investigated. The interaction leads to directional, forced asymmetric collapse of the bubble with microjet formation toward the surface. Maximum jet penetration into the membrane is produced during the bubble collapse phase with optimal shock wave arrival time and stand-off distance. Such interaction may provide a unique acoustic means for in vivo microinjection, applicable to targeted delivery of macromolecules and gene vectors to biological tissues.

Controlled multibubble surface cavitation

Heterogeneous bubble nucleation at surfaces has been notorious because of its irreproducibility. Here controlled multibubble surface cavitation is achieved by using a hydrophobic surface patterned with microcavities. The expansion of the nuclei in the microcavities is triggered by a fast lowering of the liquid pressure. The procedure allows us to control and fix the bubble distance within the bubble cluster. We observe a perfect quantitative reproducibility of the cavitation events where the inner bubbles in the two-dimensional cluster are shielded by the outer ones, reflected by their later expansion and their delayed collapse. Apart from the final bubble collapse phase (when jetting flows directed towards the cluster's center develop), the bubble dynamics can be quantitatively described by an extended Rayleigh-Plesset equation, taking pressure modification through the surrounding bubbles into account.

Bubbling in unbounded coflowing liquids

An investigation of the stability of low density and viscosity fluid jets and spouts in unbounded coflowing liquids is presented. A full parametrical analysis from low to high Weber and Reynolds numbers shows that the presence of any fluid of finite density and viscosity inside the hollow jet elicits a transition from an absolute to a convective instability at a finite value of the Weber number, for any value of the Reynolds number. Below that critical value of the Weber number, the absolute character of the instability leads to local breakup, and consequently to local bubbling. Experimental data support our model.

High-speed photography during ultrasound illustrates potential therapeutic applications of microbubbles

Ultrasound contrast agents consist of microscopically small encapsulated bubbles that oscillate upon insonification. To enhance diagnostic ultrasound imaging techniques and to explore therapeutic applications, these medical microbubbles have been studied with the aid of high-speed photography. We filmed medical microbubbles at higher frame rates than the ultrasonic frequency transmitted. Microbubbles with thin lipid shells have been observed to act as microsyringes during one single ultrasonic cycle. This jetting phenomenon presumably causes sonoporation. Furthermore, we observed that the gas content can be forced out of albumin-encapsulated microbubbles. These free bubbles have been observed to jet, too. It is concluded that microbubbles might act as a vehicle to carry a drug in gas phase to a region of interest, where it has to be released by diagnostic ultrasound. This opens up a whole new area of potential applications of diagnostic ultrasound related to targeted imaging and therapeutic delivery of drugs such as nitric oxide.

Interaction and fragmentation of pulsed laser induced microbubbles in a narrow gap

We investigate the interaction dynamics of an existing stable microbubble B1 and another laser induced nearby expanding microbubble B2 in a thin ink sheet between two glass slices. The fast expanding B2 causes anistropic compression of B1 with a forward penetrating jet. In the subsequent expansion stage of B1, the gas associated with jet protrusion to the opposite edge of B1 and the nonuniform surrounding flow field induce necking with transverse inward jetting from the side lobes, which further interact with the axial jet and lead to the final fragmentation into smaller bubbles. At small interbubble distance, the backward interaction from B1 first leads to the pointed pole of the expanding B2 and then a backward jetting during its collapsing. The strong interaction can merge the two bubbles with complicated asymmetric intermediated patterns.

Effects of radial shock waves on membrane permeability and viability of chondrocytes and structure of articular cartilage in equine cartilage explants

OBJECTIVE

To investigate in vitro effects of radial shock waves on membrane permeability, viability, and structure of chondrocytes and articular cartilage.

SAMPLE POPULATION

Cartilage explants obtained from the third metacarpal and metatarsal bones of 6 horses.

PROCEDURE

Equine cartilage was subjected to radial shock waves and then maintained as explants in culture for 48 hours. Treatment groups consisted of a negative control group; application of 500, 2,000, and 4,000 impulses by use of a convex handpiece (group A); and application of 500, 2,000, and 4,000 impulses by use of a concave handpiece (group B). Effects on explant structure were evaluated by use of environmental scanning electron microscopy (ESEM). Membrane permeability was determined by release of lactate dehydrogenase (LDH). Chondrocyte viability was assessed by use of vital cell staining. Comparisons of LDH activity and nonviable cell percentages were performed by ANOVA.

RESULTS

Cell membrane permeability increased significantly after application of 2,000 and 4,000 impulses in groups A and B. A significant decrease in cell viability was observed for application of 4,000 impulses in explants of group A. There was no detectable damage to integrity of cartilage explants observed in any treatment group by use of ESEM.

CONCLUSIONS AND CLINICAL RELEVANCE

Radial shock waves do not appear to structurally damage articular cartilage but do impact chondrocyte viability and membrane permeability. Caution should be exercised when extremely high periarticular pulse doses are used until additional studies can determine the long-term outcome of these effects and appropriate periarticular treatment regimens can be validated.

Shock wave interaction with laser-generated single bubbles

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.

Ultrasound-induced encapsulated microbubble phenomena

When encapsulated microbubbles are subjected to high-amplitude ultrasound, the following phenomena have been reported: oscillation, translation, coalescence, fragmentation, sonic cracking and jetting. In this paper, we explain these phenomena, based on theories that were validated for relatively big, free (not encapsulated) gas bubbles. These theories are compared with high-speed optical observations of insonified contrast agent microbubbles. Furthermore, the potential clinical applications of the bubble-ultrasound interaction are explored. We conclude that most of the results obtained are consistent with free gas bubble theory. Similar to cavitation theory, the number of fragments after bubble fission is in agreement with the dominant spherical harmonic oscillation mode. Remarkable are our observations of jetting through contrast agent microbubbles. The pressure at the tip of a jet is high enough to penetrate any human cell. Hence, liquid jets may act as remote-controlled microsyringes, delivering a drug to a region-of-interest. Encapsulated microbubbles have (potential) clinical applications in both diagnostics and therapeutics.

Perfectly monodisperse microbubbling by capillary flow focusing: an alternate physical description and universal scaling

In a recent work [Phys. Rev. Lett. 87, 274501 (2001)], a method to produce monodisperse microbubbles was described. The physics of the phenomenon was explained in terms of the absolute instabilities of a gas microjet formed when a liquid stream which surrounds a coflowing gas stream is forced through a small orifice. Now, a much more consistent physical picture to describe the phenomenon which corrects prior assumptions is presented. Consequently, a much simpler and universal scaling law for the microbubble size is finally obtained which involves the orifice diameter and the gas/liquid flow rates ratio only. All data shown in prior works, together with newly obtained data sets, have been analyzed anew. These are in remarkable agreement with the here proposed scaling law.