High-precision acoustic measurements of the nonlinear dilatational elasticity of phospholipid coated monodisperse microbubbles†,Soft Matter

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High-precision acoustic measurements of the nonlinear dilatational elasticity of phospholipid coated monodisperse microbubbles†
Soft Matter ( IF 2.9 ) Pub Date : 2018-10-16 00:00:00 , DOI: 10.1039/c8sm00918j
Tim Segers _{1,

2,

3,

4,

5} , Emmanuel Gaud _{1,

2,

3} , Michel Versluis _{4,

5,

6,

7,

8} , Peter Frinking _{1,

2,

3}

Affiliation

The acoustic response of phospholipid-coated microbubble ultrasound contrast agents (UCA) is dramatically affected by their stabilizing shell. The interfacial shell elasticity increases the resonance frequency, the shell viscosity increases damping, and the nonlinear shell viscoelasticity increases the generation of harmonic echoes that are routinely used in contrast-enhanced ultrasound imaging. To date, the surface area-dependent interfacial properties of the phospholipid coating have never been measured due to the extremely short time scales of the MHz frequencies at which the microscopic bubbles are driven. Here, we present high-precision acoustic measurements of the dilatational nonlinear viscoelastic shell properties of phospholipid-coated microbubbles. These highly accurate measurements are now accessible for the first time by tuning the surface dilatation, that is, the lipid packing density, of well-controlled monodisperse bubble suspensions through the ambient pressure. Upon compression, the shell elasticity of bubbles coated with DPPC and DPPE-PEG5000 was found to increase up to an elasticity of 0.6 N m⁻¹ after which the monolayer collapses and the elasticity vanishes. During bubble expansion, the elasticity drops monotonically in two stages, first to an elasticity of 0.35 N m⁻¹, and then more rapidly to zero. Integration of the elasticity vs. surface area curves showed that, indeed, a phospholipid-coated microbubble is in a tensionless state upon compression, and that it reaches the interfacial tension of the surrounding medium upon expansion. The measurements presented in this work reveal the detailed features of the nonlinear dilatational shell behavior of micron-sized lipid-coated bubbles.

中文翻译：

磷脂涂层单分散微泡的非线性膨胀弹性的高精度声学测量†

磷脂涂层的微泡超声造影剂（UCA）的声学响应受其稳定壳的影响很大。界面壳的弹性增加了共振频率，壳的黏度增加了阻尼，非线性的壳粘弹性增加了谐波回波的产生，而谐波回波通常在对比增强型超声成像中使用。迄今为止，由于驱动微小气泡的MHz频率的极短时间尺度，所以从未测量过磷脂涂层的表面积相关的界面性质。在这里，我们提出了磷脂包覆的微泡的膨胀非线性粘弹性壳性能的高精度声学测量。现在，这些高精确度的测量值首次可以通过在环境压力下调节良好控制的单分散气泡悬浮液的表面膨胀（即脂质堆积密度）来首次获得。压缩后，发现涂有DPPC和DPPE-PEG5000的气泡的壳弹性增加到0.6 N m^-1之后，单层塌陷，弹性消失。在气泡膨胀期间，弹性分为两个阶段单调下降，首先下降到0.35 N m^-1的弹性，然后更快地下降到零。弹性对表面积曲线的积分表明，确实，磷脂包裹的微泡在压缩时处于无张力状态，并且在膨胀时达到周围介质的界面张力。这项工作中提出的测量结果揭示了微米级脂质包裹的气泡的非线性膨胀壳行为的详细特征。

更新日期：2018-10-16

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