Phase behavior of mixed lipid monolayers on perfluorocarbon nanoemulsions and its effect on acoustic contrast

Abstract

Lipid-stabilized nanoemulsions containing a volatile liquid perfluorocarbon core have been studied as ultrasound contrast agents owing to their ability to transform into high-contrast microbubbles when subjected to high intensity focused ultrasound (HIFU). However, while there have been several studies on the effect of acoustic parameters on contrast, the effect of the droplet’s stabilizing shell has not been studied as extensively. Inspired by previous studies showing lateral phase separation in microbubbles and vesicles, nanodroplets were formulated with a perfluorohexane core and a shell composed of varying amounts of saturated (DPPC) phospholipids, unsaturated (DOPC) phospholipids, and cholesterol, which were fractionated to obtain nanodroplets of mean diameter 300–400 nm and were stable over one week. When the DOPC content was increased to 40 mol%, ultrasound contrast increased by about one order of magnitude over DPPC-only droplets. Based on fluorescence microscopy results of lateral lipid phase separation on the droplet surface, the various combinations of DPPC, DOPC, and cholesterol were assigned to three regimes on the ternary phase diagram: solid-liquid ordered (low contrast), liquid ordered-liquid disordered (medium contrast), and solid-liquid disordered (high contrast). These regimes were confirmed by TEM analysis of nanoscale droplets. Droplets containing mixed lipid monolayers were also found to produce a significantly greater yield than single-component droplets. The discovery of the dependence of acoustic response on lipid phase separation will help to understand the formulation, behavior, and vaporization mechanism of acoustically-responsive nanoemulsions.

Adsorption process for phospholipids of different chain lengths at a fluorocarbon/water interface studied by Du Noüy ring and spinning drop

“…They are utilized for magnetic resonance imaging (MRI) as a contrast agent ( 19 F) to detect tissue artefacts such as venous thrombosis at an early stage [11] and various other applications in the field of MRI [12,13]. Moreover, Chattaraj et al [14] added PLs in various compositions to enhance the contrast and examine lipid layer behaviour. Fluor-containing compositions of drug or protein delivery systems were examined by Zhang et al [15], who took advantage of the fluorination effect of fluoroamphiphiles for a more efficient protein delivery.…”

Lipid Shell Composition Plays a Critical Role in the Stable Size Reduction of Perfluorocarbon Nanodroplets

“…We restricted this analysis to lipidshelled formulations because of their strong translational benefits: chiefly, their demonstrable biocompatibility and well-studied linker strategies for downstream functionalization. Previous research notes that the yield of and ultrasound signal produced by nanodroplets is increased by using a higher molar percentage of unsaturated phospholipids (Chattaraj, et al 2016). These same studies also describe that nanodroplets formed with 10% and 20% molar DSPE-mPEG2000 demonstrate a solid-liquid disordered phase existence, which is structurally similar to the formulations displaying the most ideal yields and imaging properties (Chattaraj, et al 2016).…”

“…Previous research notes that the yield of and ultrasound signal produced by nanodroplets is increased by using a higher molar percentage of unsaturated phospholipids (Chattaraj, et al 2016). These same studies also describe that nanodroplets formed with 10% and 20% molar DSPE-mPEG2000 demonstrate a solid-liquid disordered phase existence, which is structurally similar to the formulations displaying the most ideal yields and imaging properties (Chattaraj, et al 2016). Standard formulations for nanodroplets with acoustically-triggered vaporization have historically utilized no more than 10% molar PEGylated phospholipid (Dayton, et al 2006, Sheeran, et al 2011.…”

Nanoparticles Formed by Acoustic Destruction of Microbubbles and Their Utilization for Imaging and Effects on Therapy by High Intensity Focused Ultrasound

“…For example, in a separate study in our lab, we found that lateral lipid phase separation reduces the energy barrier to HIFU-induced vaporization of PFH nanodroplets. 52 Another possibility is that surfactant character of serum proteins are stabilizing the growing bubbles as they form. Similar results were obtained for imaging of particles in whole blood (Figure S13 ).…”

“…MBNP Imaging: Imaging of the MBNPs via simultaneous HIFU and CPS was performed in a manner similar to previous communications. 35 , 36 , 52 , 55 A High Intensity Focused Ultrasound transducer (Sonic Concepts H101, 64.0 mm Active Diameter × 63.2 mm Radius of Curvature) equipped with a coupling cone (Sonic Concepts C101) was filled with degassed and deionized water, submerged in a water tank, and connected to a 30 MHz Function/Arbitrary Waveform Generator (Agilent Technologies) via an AG Series Amplifier (T&C Power Conversion, Inc.), the latter operating at 100% amplifier output throughout the study. The waveforms were measured, and the peak pressure and acoustic intensities of these HIFU pulses were calculated to be approx.…”