Browsing by Author "Zhong, Pei"

In this dissertation work, the aim was to garner better mechanistic understanding of how shock wave lithotripsy (SWL) breaks stones in order to guide design improvements to a modern electromagnetic (EM) shock wave lithotripter. To accomplish this goal, experimental studies were carefully designed to isolate mechanisms of fragmentation, and models for wave propagation, fragmentation, and stone motion were developed. In the initial study, a representative EM lithotripter was characterized and tested for in vitro stone comminution efficiency at a variety of field positions and doses using phantom kidney stones of variable hardness, and in different fluid mediums to isolate the contribution of cavitation. Through parametric analysis of the acoustic field measurements alongside comminution results, a logarithmic correlation was determined between average peak pressure incident on the stone surface and comminution efficiency. It was also noted that for a given stone type, the correlations converged to an average peak pressure threshold for fragmentation, independent of fluid medium in use. The correlation of average peak pressure to efficacy supports the rationale for the acoustic lens modifications, which were pursued to simultaneously enhance beam width and optimize the pulse profile of the lithotripter shock wave (LSW) via in situ pulse superposition for improved stone fragmentation by stress waves and cavitation, respectively. In parallel, a numerical model for wave propagation was used to investigate the variations of critical parameters with changes in lens geometry. A consensus was reached on a new lens design based on high-speed imaging and stone comminution experiments against the original lens at a fixed acoustic energy setting. The results have demonstrated that the new lens has improved efficacy away from the focus, where stones may move due to respiration, fragmentation, acoustic radiation forces, or voluntary patient movements. Using traditional theory of brittle fragmentation and newfound understanding of average peak pressure correlation to stone comminution, the entire set of stone comminution data for lens comparison was modeled using a Weibull-style distribution function. This model linked both the average peak pressure and shock wave dose to efficacy, including their respective threshold parameters, and demonstrated correlation of coefficients to cavitation activity. Subsequently, this model was used in prediction of stone comminution efficiency from mimicked respiratory motions in vitro, which compared favorably to actual simulated motion studies using both the new and original lenses. Under a variety of mimicked respiratory motions, the new lens produced statistically higher stone comminution efficiency than the original lens. These results were confirmed in vivo in a swine model, where the new lens produced statistically higher stone comminution after 1,000 and 2,000 shocks. Finally, a mechanistic investigation into the effects of cavitation with the original lens was conducted using an integrated, self-focusing annular ring transducer specially designed for tandem pulse lithotripsy. It was found that cavitation and stone comminution efficiency are progressively enhanced by tandem pulsing as source energies of both the primary LSW and trailing pressure pulse increase, which suggests cavitation and stress waves act synergistically enhance the efficacy in kidney stone fragmentation.

Tandem bubble interactions have been shown to produce jets that can be used to create membrane poration on single cells, and jet speed has been implicated as a critical parameter for tandem bubble-induced bioeffects. In this thesis, the dynamics of single and tandem bubbles in a microfluidic channel (25 × 800 μm in height and width) are investigated to access the effects of bubble size on tandem bubble interaction and resultant jet. Experimentally, the dynamics of bubble oscillation produced by laser irradiation of a gold dot (15 nm thick and 6 μm in diameter) coated on the glass substrate of the microfluidic channel are captured by a high-speed camera, from which the time history of bubble size and jet speed are determined. Numerically, the bubble dynamics are simulated using 3DynaFS-BEM (DYNAFLOW, INC.) based on a potential flow model solved by boundary element method (BEM). By adjusting the initial conditions in the BEM code, the dynamics of laser-generated single bubbles of different sizes were matched with experimental results. The model was subsequently used to simulate the tandem bubble interactions in anti-phase oscillation. The results show that jet shape and volume are predominately controlled by the maximum diameter of the first bubble (D1) while jet speed is linearly correlated with the maximum diameter of the second bubble (D2). In comparison, jet momentum and kinetic energy are more sensitive to variations in bubble size and increase more rapidly with both D1 and D2, especially at large bubble sizes.

In this thesis, a self-focusing multi-spark (SFMS) shock wave generator is developed to provide flexibility in controlling the beam size and shape in an electrohydraulic shock wave lithotripter. Such a device will allow us to better distribute the shock wave energy to match the anatomic features in the urinary collecting system or respiration movement of the stone to improve stone fragmentation efficiency while reducing tissue injury. In this study, we present the design, fabrication and evaluation of the multi-pin titanium electrodes by 3D printing, integration of the SFMS shock wave generator, acoustic field characterization based on hydrophone measurements, and stone fragmentation tests using stone phantoms confined within a polyurethane rubber holder of elliptical shape. The effects of pin number on pressure output and electrode degradation are evaluated in order to produce a consistent pressure waveform with increased electrode lifespan.

Experiments were conducted using two transducer configurations: case 1 (axisymmetric) – with all transducers connected, and case 2 (non-axisymmetric) - with transducers on the two side sections disconnected. A fiber optic probe hydrophone was mounted on a 3D computer controlled translational stage to perform acoustic field characterization. Stone fragmentation test was conducted with stone phantoms placed inside a stone holder made of soft tissue mimicking material polyurethane rubber to evaluate the stone comminution efficiency. To assess the effect of multi-pin design on electrode damage and output pressure variations, transducer lifespan experiments were performed. Specifically, individual transducers used in the SFMS, but with different pin numbers (1, 10, and 45) were fired up to 2000 shocks.

The FOPH measurement results show that the SFMS can generate an axisymmetric focal zone with the -6 dB focal width of 16 mm, or a non-axisymmetric focal zone with the -6 dB focal width elongated to 28 mm in the side-section direction, while in the perpendicular direction the -6 dB focal width is 16 mm, accompanying a peak positive pressure of 39.0±1.8 MPa at an input electric pulse energy of 600 J. Such a focal zone has stone comminution efficiency of 37.68±5.11% and 69.58±8.29% for stone fragment smaller than 2.0 mm and 2.8 mm, respectively, after 150 pulses. After more than 2000 pulses, the pressure output drops only by 2%, and lifespan of the transducer (defined by a peak output pressure drop less than 10%) is expected to exceed 6000 pulses. Altogether, we have demonstrated that the SFMS can generate an elongated non-axisymmetric focal zone with higher stone comminution efficiency, and has significantly increased electrode lifespan. The SFMS shock wave generator may provide a flexible and versatile design to achieve accurate, stable, and safe lithotripsy for kidney stone treatment.

Ultrasound (US) neuromodulation has the unique advantage to noninvasively manipulate neural activity in deep brain with high spatial resolution and flexibility in beam steering. In recent years, there is a growing interest in producing accurate and cell-type specific US neuromodulation using sonogenetics, in which the targeted cells/neurons are genetically modified via overexpression of mechanosensitive (MS) ion channels that can be activated by US. This emerging technique has spurred extensive research activities with the hope for potential clinical treatment of neurological disorders, such as Parkinson’s disease, epilepsy and depression. It has been proposed by mainstream journals that the neuromodulating effects of US are associated with changes in membrane potential due to US-induced cell/neuron membrane deformation and the activation of MS ion channels, in which the latter mechanism is given greater prominence in sonogenetics. In this process, US exerts its mechanical effects in different forms including acoustic radiation force (ARF, associated with momentum transfer from the US wave field to the medium), acoustic streaming (displacement of fluid), and cavitation (generation of bubbles within the tissue). Despite much efforts in the field, the physical mechanism by which US is converted into an effective energy form to elicit neuromodulation remains poorly understood, and there is little consensus about optimal US parameters required to evoke a sonogenetic response with minimal adverse effect. Understanding how US interacts with cell membrane with specific US parameters/configurations will be important to optimize this technology. A significant barrier to advancing the sonogenetics is the lack of technologies and experimental systems to capture and dissect the dynamic interaction of ultrasound with target cells and the resultant cell membrane deformation (or strain) and its correlation to MS channel dynamics at the single cell level. To resolve these technical challenges, we have developed a novel vertically deployed surface acoustic wave (VD-SAW) transducer platform that can be readily integrated with a fluorescence microscope for simultaneous observation and monitoring of the interaction of US waves with target cells, the mechanical strain and stress in cell membrane, and the resultant bioeffects at the single cell level. In Chapter 1, we introduced the background of US neuromodulation and sonogenetics, followed by a description of the major challenges in this field and the key questions we’re going to address in this dissertation. In Chapter 2, we investigated the activation of Piezo1, one of the few eukaryotic channels known to be responsible to US, by monitoring the intracellular calcium response. We observed that Piezo1 activation is highly determined by shear stress amplitude and pulse length (PL) of the stimulation. Under the same acoustic energy, we identified an optimal PL that leads to maximum cell deformation, and Piezo1 activation rate with minimal injury. Our results suggested the optimal PL is related to the viscoelastic response of cell membrane and the gating dynamics of Piezo1 which has not been considered in previous sonogenetics studies. In Chapter 3, we further constructed a 3D cell culture model in collagen hydrogel to better mimic the realistic cell culture condition. The hydrogel ruled out the involvement of acoustic streaming and thus facilitated the investigation of the role of ARF in sonogenetics which is a more practical form of US energy in vivo. The VD-SAW array integrated with 3D cell culture model was translated to a confocal fluorescence microscope for acquiring the initiation of intracellular calcium response and the cell membrane deformation in 3D. With the system upgrade, we found ARF is more efficient than acoustic streaming for activating Piezo1 channel. We also observed that the Piezo2, a homolog to Piezo1, requires higher power of US than Piezo1 for activation. Interestingly, we found Piezo2 is sensitive to membrane compression, while Piezo1 is sensitive to membrane tension. The observed differences of mechanical sensitivities and activating schemes between Piezo1 and Piezo2 were consistent with previous evidence in cell mechanotransduction studies using patch clamp. We further combined numerical modeling and 3D confocal imaging with digital volume correlation to analyze cell membrane stress under insonification. We established the relationship between US-induced mechanical effect and cellular bioeffects (Ca2+ signaling via MS channel activation) by a key parameter: the total strain energy, which take accounts of magnitude of stress, volume of cell under stress, and sonification time. Our results suggested that the stress distribution and total strain energy induced by US were strongly correlated to the directions of ARF. We’ve shown that the total strain energy could highly recapitulate the effectiveness of ARF on Piezo1 activation. Overall, we have developed a miniatured, highly compatible and controllable VD-SAW transducer for studying sonogenetics at the single cell level. Our preliminary results provide new insights into the mechanisms of ultrasonic activation of Piezo. By virtue of its dimensions, compatibility, and targetability, the VD-SAW transducer can be readily applied for studying the mechanisms and key parameters underlying the activation of other MS channels by US in various types of cell/neuron. We also expect that VD-SAW can be translated to ex vivo (e.g. brain slice) or in vivo application with the advancement of fabrication and proper compensation of the skull-induced US attenuation.

The primary goal of this study is to verify the applicability of a miniature wet-coupling device specifically designed for shock wave lithotripsy (SWL) and its effect on improving the stone comminution efficiency. Despite the technical and functional improvements implemented in the recent years, modern shock wave lithotripters have failed to re-achieve the treatment outcome of their ancestor, the original Dornier HM3. The defects of the dry-coupling approach used in contemporary lithotripters have been considered as one of the key factors that contribute to their reduced stone fragmentation efficiency. Therefore, this study aims at addressing this drawback by designing and developing a new coupling device to substitute the contemporary dry-coupling device.

The idea of the miniature wet-coupling design is to combine the strengths of the water-bath coupling and the dry coupling to produce a high-quality coupling while still ensuring great user convenience. In this study, the wet-coupling device was designed for a contemporary electromagnetic (EM) lithotripter with an acoustical lens, consisting of a patient-interface component, a water-filled bellow, and a connecting unit in between. Through 3D printing of customized molds and downstream mold casting with polyurethane rubber, these components were built individually and then assembled together. To verify the performance of this wet-coupling device, in vitro stone fragmentation tests and coupling quality tests were performed with two coupling setups on the same EM lithotripter and a PVC torso model to mimic a clinical SWL treatment. Soft cylindrical BegoStone phantoms (10 mm × 10 mm, D × H) were treated by shock waves delivered at a pulse repetition frequency (PRF) = 1.5 Hz and a power output set of E2.5. The coupling interface condition and subsequent changes during the shock wave treatment were captured after every 500 shocks.

The wet-coupling setup has successfully maintained a steady and uniform coupling interface with minimal coupling defects. In contrast, the dry-coupling setup resulted in an initial bubble formation and a subsequent bubble aggregation within the coupling medium that impeded the energy delivery. The wet-coupling setup resulted in a significantly-improved comminution efficiency (41.5% vs. 27.3%) after 2000 shocks. Based on the modular design, the wet-coupling device can be easily modified for other dry-coupling lithotripters and serve as a promising optimization for clinical SWL.

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Nano Pulse Lithotripter (NPL) is a new technology in intracorporeal lithotripsy. It utilizes a high-voltage spark discharge of about 30-nanosecond duration, released at the tip of a flexible probe under endoscopic guidance, to break up kidney stones into fine powders for spontaneous discharge. Several primary damage patterns have been observed during the NPL treatment of hard and soft BegoStones: crater formation near the probe tip, crack development from the distal wall of the stone, and crack initiation in the form of radial and ring-shape circumferential fracture in the proximal surface of stones of small sizes. Compared to the traditional intracorporeal lithotripsy technologies (laser and EHL), NPL has been shown to comminute kidney stones with higher efficiency, especially with hard stones, although its working mechanisms are largely unclear. Multiple potential contributory factors have been proposed: direct dielectric breakdown in the stone material near the NPL probe tip, shock wave induced by the spark discharge in the fluid, and cavitation and resultant asymmetric collapse of bubbles. Various experiments have been carried out to correlate each of the proposed mechanisms with the damage patterns observed. Comparison between micro-CT images of the damage initiation sites and COMSOL simulation of the stress field in the stone indicate that the observed cracks are most likely to be produced by the locally intensified tensile stresses associated with the surface acoustic waves (SAW) generated by the incidence of the spark-generated, spherically divergent shock wave on the proximal surface of the stone, and/or their interactions with bulk acoustic waves (P or S) upon reflection from the stone boundaries. Dielectric breakdown is found to potentially contribute to crater formation, especially for soft BegoStones. However, the contribution of cavitation to stone fragmentation in NPL appears to be minimal.

I investigate the generation and propagation characteristics of leaky Rayleigh waves (LRWs) by a spherical shock wave incident on a glass-water boundary both experimentally and numerically. The maximum tensile stress produced on the solid boundary was attributed to the dynamic interaction between the LRWs and an evanescent wave generated concomitantly along the boundary. The resultant tensile stress field drives the initiation of pre-existing microcracks and their subsequent extension along a circular trajectory, confirmative with the direction of the principal stress on the boundary. We further demonstrated that this unique ring-like fracture, prevalent in damage produced by high-speed impact, can be best described by the Tuler-Butcher criterion for dynamic brittle failure, and the orientation of the ring fracture extension into the solid also follows closely the trajectory of the local maximum tensile stress distribution.

The primary goal of this dissertation was to assess the feasibility of transforming an electromagnetic (EM) shock wave lithotripter with an acoustic lens as its focusing device from the original axisymmetric pressure distribution to a non-axisymmetric steerable acoustic field. This work was motivated by the desire to better match the distribution of effective acoustic pressure and pulse energy with the trajectory and anatomical features around renal and ureteral calculi during clinical shock wave lithotripsy (SWL). The acoustic field transformation was accomplished by the design of a fan-shaped acoustic barrier (mask) placed on top of the lithotripter acoustic lens to selectively reduce the source aperture along the direction of the barrier axis, therefore effectively broadening the beam width (BW) of the lithotripter field in this preferred direction. Moreover, the geometry of the original lens (L₁) was modified so that the acoustic focus of the new lens (L₂) at high output voltages (necessitated by the incorporation of the mask) is closely aligned with the lithotripter focus. The mask was further driven by a motor-controlled gear system to rotate around the lithotripter axis, generating a steerable and non-axisymmetric acoustic field. In this dissertation project, a linear acoustic model was first used for parametric studies to assess the effects of mask geometry (opening angle and thickness) on beam elongation and peak pressure reduction. Based on this analysis, two mask geometries (L₂+M₈₀₂₅ and L₂+M₉₀₃₀) were selected for modest and maximum beam elongation within the acceptable output range of the shock wave source. The acoustic and cavitation fields of the new lens with masks, as well as the corresponding field produced by the original lens, were characterized using fiber optical probe hydrophone measurements and stereoscopic high-speed imaging. Different output voltage settings were used for each lens configuration (i.e., 14 kV for L₁, 15.8 kV for L₂+M₈₀₂₅, and 17 kV L₂+M₉₀₃₀) to produce equivalent acoustic pulse energy of 45 mJ in all setups, measured in the lithotripter focal plane. Under this condition, L₂+M₈₀₂₅ and L₂+M₉₀₃₀ generate lower peak pressure (38.2 and 36.8 MPa) with a significantly broadened BW_y (11.4 and 14.3 mm) along the y-axis (head-to-toe direction of the patient), which is aligned with the mask axis, compared to the high peak pressure (44.1 MPa) and moderate BW (7.5 mm) of L₁. It is worth noting that L₂+M₈₀₂₅ and L₂+M₉₀₃₀ produce a BW_x (7.6 and 7.5 mm) in the orthogonal direction to the mask axis, which is also comparable to L₁. Similarly, the beam width of the cavitation field was broadened from 8.1 to 12.2 mm for L₂+M₈₀₂₅, and from 10.9 to 17.9 mm for L₂+M₉₀₃₀, compared to the range of 8.8 to 9.4 mm measured from L₁. In comparison, L₂+M₈₀₂₅ produces a denser and narrower bubble cloud along the y-axis than L₂+M₉₀₃₀. In vitro stone comminution (SC) tests in a tube holder (Diameter = 14 mm) have demonstrated that L₂+M₈₀₂₅ and L₂+M₉₀₃₀ are more effective at off-axis positions and during simulated respiratory motion along the elongated beam direction. The results of SC also confirmed the correlation between SC and the average peak pressure, p_+(avg), and effective acoustic pulse energy, E_eft, delivered to the stone, as shown in previous studies. Furthermore, a ureter model was developed and used to assess the performance of L₂+M₉₀₃₀, which has the maximally elongated BW under various static and simulated respiratory motion conditions. The results suggest that L₂+M₉₀₃₀ can produce significantly better SC than L₁ when the elongated beam is effectively aligned with the stone/fragments in the ureter or with their motion trajectory during the course of SWL treatment. Altogether, the results of this dissertation work have demonstrated in vitro that a non-axisymmetric and steerable acoustic field can significantly enhance stone comminution under clinically relevant SWL conditions. Future work is warranted to optimize the mask design and steering protocol to maximize the benefit of such an adaptable and versatile design to improve the performance and safety of clinical EM lithotripters.

The primary goal of this dissertation research is to develop an experimental system and associated techniques that can be used to investigate the bioeffects produced by cavitation bubbles at the single cell level. Such information has been lacking due to the randomness and complexity in cavitation inception and subsequent bubble-bubble interaction generated by an acoustic field typically used in therapeutic ultrasound applications. Connection between cavitation activities and bioeffects produced in cells nearby presents another challenge that has not been resolved satisfactorily. In this work, we developed a laser-based system for generating tandem bubbles with a maximum diameter about 50 µm (i.e., on the scale of a single cell) in a microfluidic channel of 25 µm in height and 800 µm in width. We further developed techniques for micropatterning of individual gold dots (15 nm thick and 6 µm in diameter) used for bubble generation, which are precisely aligned at various stand-off distances (SD) from individual islands (32 x32 µm2) coated with fibronectin used for cell adhesion. The dynamics of tandem bubble interaction with resultant jet formation, microstreaming and vortex flow in the microfluidic channel were captured by high-speed imaging and particle image velocimetry (PIV). The deformation of the target cell was recorded by high-speed imaging as well (using a second camera) immediately after the tandem bubble interaction and assessment of membrane strain was aided with 2 µm sized polystyrene beads attached to the cell membrane. Membrane poration was characterized by uptake of fluorescent propodium iodide (PI) into the target cell, from which the normalized maximum pore size was estimated. Using this experimental system, we have observed the complete process of bubble-bubble interaction with resultant jetting flow, cell deformation, and localized pinpoint membrane rupture with progressive diffusion of macromolecules into the target cell. Furthermore, we observed a clear SD dependence in the treatment outcome produced by the tandem bubbles. At short SD of 10 µm, all treated cells underwent necrosis with high yet unsaturated level of PI uptake, indicating that the cell could not reseal the poration site. At intermediate SD of 20 ~ 30 µm, 58% to 80% of the cells were observed to have repairable membrane poration with low to medium but saturated level of PI uptake. At long SD of 40 µm, no detectable PI uptake was observed, corresponding to no membrane compromise. Within the repairable membrane poration group, the sub-population of cells that eventually survived without apoptosis increased from about 9% at SD of 20 µm with strong adhesion to about 70% at SD of 30 µm with no adhesion at the leading edge facing the jetting flow. The maximum PI uptake, pore size, and membrane strain estimated could vary by more than an order of magnitude, which is similar to the magnitude of variations in pore size (0.2 ~ 2 µm) produced by tandem bubbles observed by SEM. The large principal strain (> 500%) with associated high strain-rate (> 106*s-1) produced by the tandem bubbles provide a unique tool to examine the bioeffects of cavitation at the single cell level and potentially a diverse range of applications to be explored.

Nano pulse lithotripsy (NPL) is a novel medical technology to fragment urinary calculi through electrical discharge. Compared with traditional shock wave lithotripters utilizing focused shock waves, NPL is ideal for investigating the stress field induced by surface acoustic waves (SAWs), such as the leaky Raleigh waves (LRWs), on the fluid-solid boundary. A pioneering study has recently demonstrated the generation of LRWs induced by the spherically divergent shock wave at the borosilicate glass-water boundary in NPL treatment. The resultant tensile stress field was found to play an important role in the initiation of cracks and the formation of ring-like fracture on the glass surfaces. This prior work motivates us to investigate the effect of SAWs on surface flaws that can be artificially and controllably created on the glass surface by microindentations to mimic the surface erosion induced by cavitation. In this study, we used a microhardness tester with a Vickers indenter and applied a load of 1.0 kg with a dwell time of 10 s to produce indentations at various radial distances (1.0 mm, 1.5 mm, 2.0 mm and 2.5 mm) on the borosilicate glass samples (50 x 50 x 3.3 mm in LxWxH). Each indentation creates a pyramid shaped impression with an average diagonal length of 55 m and penetration depth of 7.8 m. At a standoff distance of 1.5 mm, the shock waves generated by the NPL probe were applied to the glass surface until the glass was broken. After each shock impact, the crack initiation and extension around each indentation site were recorded, from which the speed of crack development represented by the arc length per shock was calculated. Through these experiments, we have made the following important observations. First, the artificially induced surface flaws made by the microindentation can well control the location of the crack initiation since the presence of surface flaws will significantly weaken the glass surface. Under the effect of the maximum tensile stress (σ_(T,max)) generated by NPL shock wave impact, the cracks extending from the indentation impression site are predominantly aligned perpendicular to the direction of LRWs (which is also the direction of σ_(T,max)). Second, compared with the number of shocks required to initiate the ring-like fracture on the original (untreated) glass surface, fewer shock waves are needed to initiate the ring-like crack formation and extension from the impression site presumably due to the higher stress concentration built up at the tip of the surface flaws during NPL, which can greatly reduce the tensile stress or stress integral required to initiate a crack. The average speed of ring-like fracture formation at a radial distance about 1.5 mm is 0.26 mm per shock on the glass without microindentations. By contrast, the average speed of crack extension at the same radial distance with microindentations can increase to 0.34 mm per shock. Furthermore, the speed of crack extension varies with the radial distance of the microindentation from the NPL probe axis, largely following the variation of the local tensile stress integral generated by the LRWs. Altogether, these findings suggest potential synergy between shock wave-induced LRWs and surface flaws (e.g., produced by cavitation erosion pitting during shock wave lithotripsy) that may lead to improved stone comminution, which warrants future investigations.