Dynamics of a mechanical monoleaflet heart valve prosthesis in the closing phase: Effect of squeeze film

An analysis of the dynamics of a mechanical monoleaflet heart valve prosthesis in the closing phase is presented. The backflow velocity of the fluid and the pressure distribution on the occluder during the closing phase were computed using a control volume approach in the unsteady state. Using moment equilibrium principles on the occluder motion and the squeeze film dynamics of the fluid between the occluder and the guiding strut at the instant of impact, the velocity of the occluder tip and the impact force between the occluder and the guiding struts were computed. The dynamics of fluid being squeezed between the occluder and the guiding struts are accounted for by Reynolds' equation. The effect of the fluid being squeezed between the occluder and the guiding strut was to reduce the velocity of the occluder tip at the instant of valve closure as well as to dampen the fluttering of the occluder before coming to rest in the fully closed position. The squeeze film fluid pressure changed rapidly from a high positive value (10 MPa) to a relatively large negative value (−15 MPa) in <1 msec. The results of this study may be extended for the analysis of cavitation inception and mechanical stresses on the formed elements and valve components, as well as to estimate the endurance limits of prosthetic valves.     

Visualizing the Histotripsy Process: Bubble Cloud–Cancer Cell Interactions in a Tissue-Mimicking Environment

Histotripsy is a non-invasive ultrasonic ablation method that uses cavitation to mechanically fractionate tissue into acellular debris. With a sufficient number of pulses, histotripsy can completely fractionate tissue into a liquid-appearing homogenate with no cellular structures. The location, shape and size of lesion formation closely match those of the cavitation cloud. Previous work has led to the hypothesis that the rapid expansion and collapse of histotripsy bubbles fractionate tissue by inducing large stress and strain on the tissue structures immediately adjacent to the bubbles. In the work described here, the histotripsy bulk tissue fractionation process is visualized at the cellular level for the first time using a custom-built 2-MHz transducer incorporated into a microscope stage. A layer of breast cancer cells were cultured within an optically transparent fibrin-based gel phantom to mimic cells inside a 3-D extracellular matrix. To test the hypothesis, the cellular response to single and multiple histotripsy pulses was investigated using high-speed optical imaging. Bubbles were always generated in the extracellular space, and significant cell displacement/deformation was observed for cells directly adjacent to the bubble during both bubble expansion and collapse. The largest displacements were observed during collapse for cells immediately adjacent to the bubble, with cells moving more than 150–300 μm in less than 100 μs. Cells often underwent multiple large deformations (>150% strain) over multiple pulses, resulting in the bisection of cells multiple times before complete removal. To provide theoretical support to the experimental observations, a numerical simulation was conducted using a single-bubble model, which indicated that histotripsy exerts the largest strains and cell displacements in the regions immediately adjacent to the bubble. The experimental and simulation results support our hypothesis, which helps to explain the formation of the sharp lesions formed in histotripsy therapy localized to the regions directly exposed to the bubbles.     

Ultrasound-mediated transdermal drug delivery: Mechanisms, scope, and emerging trends

The use of ultrasound for the delivery of drugs to, or through, the skin is commonly known as sonophoresis or phonophoresis. The use of therapeutic and high frequencies of ultrasound (≥ 0.7 MHz) for sonophoresis (HFS) dates back to as early as the 1950s, while low-frequency sonophoresis (LFS, 20-100 kHz) has only been investigated significantly during the past two decades. Although HFS and LFS are similar because they both utilize ultrasound to increase the skin penetration of permeants, the mechanisms associated with each physical enhancer are different. Specifically, the location of cavitation and the extent to which each process can increase skin permeability are quite dissimilar. Although the applications of both technologies are different, they each have strengths that could allow them to improve current methods of local, regional, and systemic drug delivery. In this review, we will discuss the mechanisms associated with both HFS and LFS, specifically concentrating on the key mechanistic differences between these two skin treatment methods. Background on the relevant physics associated with ultrasound transmitted through aqueous media will also be discussed, along with implications of these phenomena on sonophoresis. Finally, a thorough review of the literature is included, dating back to the first published reports of sonophoresis, including a discussion of emerging trends in the field.     

Ultrasound-mediated intracellular drug delivery using microbubbles and temperature-sensitive liposomes

A novel two-step protocol for intracellular drug delivery has been evaluated in vitro. As a first step TO-PRO-3 (a cell-impermeable dye that displays a strong fluorescence enhancement upon binding to nucleic acids) encapsulated in thermosensitive liposomes was released after heating to 42 °C. A second step consisted of ultrasound-mediated local permeabilization of cell membrane allowing TO-PRO-3 internalization observable as nuclear staining. Only the combination of two consecutive steps — heating and sonication in the presence of SonoVue microbubbles led to the model drug TO-PRO-3 release from the thermosensitive liposomes and its intracellular uptake. This protocol is potentially beneficial for the intracellular delivery of cell impermeable drugs that suffer from rapid clearance and/or degradation in blood and are not intrinsically taken up by cells.     

超声联合微泡阻断兔正常肝血流的再研究

目的 探讨采用改进声学参数的超声治疗仪激励循环微泡空化,延长正常兔肝血流阻断时间的可行性.方法 改进型脉冲式超声空化治疗仪,治疗头频率831 kHz,峰值负压4.3 MPa,平均声强1.25 W/cm2.9只新西兰大白兔麻醉后开腹暴露肝,经耳缘静脉缓慢推注微泡的同时予以超声辐照肝5min;其中6只分别于治疗前、治疗后即刻、30 min、60 min及48 h对治疗靶区行超声造影,视觉观察并用声学密度方法测量各时间点峰值强度(PI)变化;其余3只用于治疗后即刻病理学检查.结果 治疗前肝实质造影增强显著并且均匀,PI为(-51.88±4.26)dB,治疗后即刻靶区PI降至(- 62.53±4.83)dB,视觉观察血流灌注几乎完全消失,30～60 min后肝血流灌注呈缓慢恢复,48 h后几乎恢复正常,PI为(- 52.00±4.60)dB.治疗后即刻、30 min、60 min肝实质平均PI低于治疗前及治疗后48 h,差异均有统计学意义(P＜0.05),治疗前与治疗后48 h差异无统计学意义(P＞0.05).光镜下可见肝细胞弥漫性混浊肿胀,挤压肝窦及窦状间隙;门静脉周围出血、血肿形成.结论 超声联合微泡可以阻断正常兔肝的血流灌注达1h,其阻断机制可能主要是肝细胞肿胀.     

An efficient treatment strategy for histotripsy by removing cavitation memory

Cavitation memory effects occur when remnants of cavitation bubbles (nuclei) persist in the host medium and act as seeds for subsequent events. In pulsed cavitational ultrasound therapy, or histotripsy, this effect may cause cavitation to repeatedly occur at these seeded locations within a target volume, producing inhomogeneous tissue fractionation or requiring an excess number of pulses to completely homogenize the target volume. We hypothesized that by removing the cavitation memory, i.e., the persistent nuclei, the cavitation bubbles could be induced at random locations in response to each pulse; therefore, complete disruption of a tissue volume may be achieved with fewer pulses. To test the hypothesis, the cavitation memory was passively removed by increasing the intervals between successive pulses, ?t, from 2, 10, 20, 50 and 100, to 200 ms. Histotripsy treatments were performed in red blood cell tissue phantoms and ex vivo livers using 1-MHz ultrasound pulses of 10 cycles at P−/P+ pressure of 21/59 MPa. The phantom study allowed for direct visualization of the cavitation patterns and the lesion development process in real time using high-speed photography; the ex vivo tissue study provided validation of the memory effect in real tissues. Results of the phantom study showed an exponential decrease in the correlation coefficient between cavitation patterns in successive pulses from 0.5 ± 0.1 to 0.1 ± 0.1 as ?t increased from 2–200 ms; correspondingly, the lesion was completely fractionated with significantly fewer pulses for longer ?ts. In the tissue study, given the same number of therapy pulses, complete and homogeneous tissue fractionation with well-defined lesion boundaries was achieved only for ?t ≥ 100 ms. These results indicated that the removal of the cavitation memory resulted in more efficient treatments and homogeneous lesions.     

Characterization of the Dynamic Activities of a Population of Microbubbles Driven by Pulsed Ultrasound Exposure in Sonoporation

Ultrasound-driven microbubble activities have been exploited to transiently disrupt the cell membrane (sonoporation) for non-viral intracellular drug delivery and gene transfection both in vivo and in vitro. In this study, we investigated the dynamic behaviors of a population of microbubbles exposed to pulsed ultrasound and their impact on adherent cells in terms of intracellular delivery and cell viability. By systematically analyzing the bubble activities at time scales relevant to pulsed ultrasound exposure, we identified two quantification parameters that categorize the diverse bubble activities subjected to various ultrasound conditions into three characteristic behaviors: stable cavitation/aggregation (type I), growth/coalescence and translation (type II) and localized inertial cavitation/collapse (type III). Correlation of the bubble activities with sonoporation outcome suggested that type III behavior resulted in intracellular delivery, whereas type II behavior caused the death of a large number of cells. These results provide useful insights for rational selection of ultrasound parameters to optimize outcomes of sonoporation and other applications that exploit the use of ultrasound-driven bubble activities.     

Synergistic Ablation of Liver Tissue and Liver Cancer Cells with High-Intensity Focused Ultrasound and Ethanol

We investigated the combined effect of ethanol and high-intensity focused ultrasound (HIFU), first, on heating and cavitation bubble activity in tissue-mimicking phantoms and porcine liver tissues and, second, on the viability of HepG2 liver cancer cells. Phantoms or porcine tissues were injected with ethanol and then subjected to HIFU at acoustic power ranging from 1.2 to 20.5 W (HIFU levels 1–7). Cavitation events and the temperature around the focal zone were measured with a passive cavitation detector and embedded type K thermocouples, respectively. HepG2 cells were subjected to 4% ethanol solution in growth medium (v/v) just before the cells were exposed to HIFU at 2.7, 8.7 or 12.0 W for 30 s. Cell viability was measured 2, 24 and 72 h post-treatment. The results indicate that ethanol and HIFU have a synergistic effect on liver cancer ablation as manifested by greater temperature rise and lesion volume in liver tissues and reduced viability of liver cancer cells. This effect is likely caused by reduction of the cavitation threshold in the presence of ethanol and the increased rate of ethanol diffusion through the cell membrane caused by HIFU-induced streaming, sonoporation and heating.     

Albumin Acts Like Transforming Growth Factor β1 in Microbubble-Based Drug Delivery

Unlike lipid-shelled microbubbles (MBs), albumin-shelled microbubbles (MBs) have not been reported to be actively targeted to cells without the assistance of antibodies. Recent studies indicate that the albumin molecule is similar to transforming growth factor β (TGF-β) both structurally and functionally. The TGF-β superfamily is important during early tumor outgrowth, with an elevated TGF-β being tumor suppressive; at later stages, this switches to malignant conversion and progression, including breast cancer. TGF-β receptors I and II play crucial roles in both the binding and endocytosis of albumin. However, until now, no specific albumin receptor has been found. On the basis of the above-mentioned information, we hypothesized that non-antibody-conjugated albumin-shelled MBs can be used to deliver drugs to breast cancer cells. We also studied the possible roles of TGF-β₁ and radiation force in the behavior of cells and albumin-shelled MBs. The results indicate that albumin-shelled MBs loaded with paclitaxel (PTX) induce breast cancer cell apoptosis without the specific targeting produced by an antibody. Applying either an acoustic radiation force or cavitation alone to cells with PTX-loaded albumin MBs increased the apoptosis rate to 23.2% and 26.3% (p < 0.05), respectively. We also found that albumin-shelled MBs can enter MDA-MB-231 breast cancer cells and remain there for at least 24 h, even in the presence of PTX loading. Confocal micrographs revealed that 70.5% of the breast cancer cells took up albumin-shelled MBs spontaneously after 1 d of incubation. Applying an acoustic radiation force further increased the percentage to 91.9% in our experiments. However, this process could be blocked by TGF-β₁, even with subsequent exposure to the radiation force. From these results, we conclude that TGF-β₁ receptors are involved in the endocytotic process by which albumin-shelled MBs enter breast cancer cells. The acoustic radiation force increases the contact rate between albumin-shelled MBs and tumor cells. Combining a radiation force and cavitation yields an apoptosis rate of 31.3%. This in vitro study found that non-antibody-conjugated albumin-shelled MBs provide a useful method of drug delivery. Further in vivo studies of the roles of albumin MBs and TGF-β in different stages of cancer are necessary.     

载IR780/PFH纳米粒双模态成像及增效高强度聚焦超声消融的体外实验研究

摘 要 目的 制备一种近红外荧光多功能纳米粒,评估其体外超声、光声成像能力以及协同高强度聚焦超声（HIFU）消融牛肝的效果,通过空化检测探讨其增效HIFU的机制。方法 采用超声薄膜法制备载 IR780和全氟己烷（PFH）的脂质体纳米粒(IR780-CLs),使用透射电镜观察纳米粒的结构特征,多角度粒度分析及Zeta电位仪检测纳米粒的粒径和电位,紫外分光光度计检测IR780包封率,观察其体外超声、光声成像效果,通过离体牛肝实验评估IR780-CLs体外增效HIFU的作用,并通过被动空化检测,取宽带噪声信号幅度的均方根值（RMS）随时间的变化曲线反应实时空化信号。结果 制备的IR780-CLs纳米粒呈大小均匀的球形,平均电位(34.5 ± 3.2) mV,粒径(246.5 ± 12.4) nm,IR780包封率为88%,IR780-CLs纳米粒具有良好的体外超声、光声成像效果,纳米粒光声信号强度与浓度间存在良好的线性关系,体外牛肝实验表明其具有显著增效HIFU的作用,被动空化检测进一步验证了IR780-CLs组空化信号RMS值高于PBS组。结论 本实验成功制备多功能纳米粒IR780-CLs,具有良好的超声、光声成像能力,并能显著提高HIFU消融牛肝的效率,空化效应为增效HIFU的主要机制。