Acoustic droplet vaporization is initiated by superharmonic focusing

Acoustically sensitive emulsion droplets composed of a liquid perfluorocarbon have the potential to be a highly efficient system for local drug delivery, embolotherapy, or for tumor imaging. The physical mechanisms underlying the acoustic activation of these phase-change emulsions into a bubbly dispersion, termed acoustic droplet vaporization, have not been well understood. The droplets have a very high activation threshold; its frequency dependence does not comply with homogeneous nucleation theory and localized nucleation spots have been observed. Here we show that acoustic droplet vaporization is initiated by a combination of two phenomena: highly nonlinear distortion of the acoustic wave before it hits the droplet and focusing of the distorted wave by the droplet itself. At high excitation pressures, nonlinear distortion causes significant superharmonics with wavelengths of the order of the droplet size. These superharmonics strongly contribute to the focusing effect; therefore, the proposed mechanism also explains the observed pressure thresholding effect. Our interpretation is validated with experimental data captured with an ultrahigh-speed camera on the positions of the nucleation spots, where we find excellent agreement with the theoretical prediction. Moreover, the presented mechanism explains the hitherto counterintuitive dependence of the nucleation threshold on the ultrasound frequency. The physical insight allows for the optimization of acoustic droplet vaporization for therapeutic applications, in particular with respect to the acoustic pressures required for activation, thereby minimizing the negative bioeffects associated with the use of high-intensity ultrasound

The Use of Preoperative Nutritional Interventions to Protect Against Hepatic Ischemia-Reperfusion Injury

Preoperative fasting was introduced in the 19th century to reduce the risk of aspiration pneumonia while patients were under general anesthesia. During the last decades, the value of preoperative fasting has been questioned, and more liberal guidelines have been proposed, such as the use of preoperative carbohydrate-rich drinks. Here we review both old and new evidence supporting the view that fasting slightly longer than overnight is beneficial for an entirely different purpose: protection against certain types of stress, such as ischemia-reperfusion injury. We provide a framework to explain these benefits as well as future applications and alternatives that could be used to induce the protection afforded by nutritional interventions. Liver Transpl 15:1183-1191, 2009. (C) 2009 AASLD

Glucose Supplementation Does Not Interfere With Fasting-Induced Protection Against Renal Ischemia/Reperfusion Injury in Mice

Background. Preoperative fasting induces robust protection against renal ischemia/reperfusion (I/R) injury in mice but is considered overcautious and possibly detrimental for postoperative recovery in humans. Furthermore, fasting seems to conflict with reported benefits of preoperative nutritional enhancement with carbohydrate-rich drinks. Here, we investigated whether preoperative ingestion of a glucose solution interferes with fasting-induced protection against renal I/R injury. Methods. Mice were randomized into the following groups: fasted for 3 days with access to water (fasted) or a glucose solution (fasted + glc) and fed ad libitum with water (fed) or a glucose solution (fed + glc). After induction of bilateral renal I/R injury, all animals had free access to food and water. Calorie intake, body weight, insulin sensitivity, kidney function, and animal survival were determined. Results. Fed + glc mice had a comparable daily calorie intake as fed mice, but 50% of those calories were obtained from the glucose solution. Fasted + glc mice had a daily calorie intake of approximately 75% of the intake of both fed groups. This largely prevented the substantial body weight loss seen in fasted animals. Preoperative insulin sensitivity was significantly improved in fasted + glc mice versus fed mice. After I/R injury, kidney function and animal survival were superior in both fasted groups. Conclusions. The benefits of fasting and preoperative nutritional enhancement with carbohydrates are not mutually exclusive and may be a clinically feasible regimen to protect against renal I/R injury