van Elburg Benjamin, Lassus Anne, Cherkaoui Samir, Lajoinie Guillaume, Versluis Michel, Segers Tim
Physics of Fluids Group, TechMed Centre, University of Twente, P.O. Box 217, Enschede, 7500 AE, the Netherlands.
Bracco Suisse S.A., Route de la Galaise 31, Geneva, 1228, Switzerland.
J Colloid Interface Sci. 2025 May;685:449-457. doi: 10.1016/j.jcis.2025.01.114. Epub 2025 Jan 16.
Monodisperse phospholipid-coated microbubbles, with a size and resonance frequency tuned to the ultrasound driving frequency, have strong potential to enhance sensitivity, efficiency, and control in emerging diagnostic and therapeutic applications involving bubbles and ultrasound. A key requirement is that they retain their gas volume and shell material during physiologic pressure changes and withstand the overpressure during intravenous injection. The shell typically comprises a mixture of a phospholipid (e.g., DSPC) mixed with a PEGylated phospholipid (e.g., DPPE-PEG5000). We hypothesize that (i) lipid-coated microbubbles destabilize when shell buckling occurs under pressurization, (ii) the overpressure at which buckling occurs (buckling pressure) is linked to the molar fraction of PEGylated lipid in the shell, and (iii) PEGylated lipid can be selectively expelled from the shell by fluidizing it at elevated temperatures.
The buckling pressure was measured using ultrasound attenuation spectroscopy while the ambient pressure was varied. When the ambient pressure increased, the microbubble resonance frequency dropped sharply due to shell buckling and the associated loss of elasticity. The buckling pressure P was obtained for monodisperse microbubbles formed by microfluidic flow-focusing, with DPPE-PEG5000 mixed with DSPC at molar fractions from 1.5% to 10%. Additionally, P was quantified for microbubbles containing 10 mol% PEG after heating at temperatures ranging from 40C to 70C. The molar PEG content of the microbubbles was analyzed using high-performance liquid chromatography.
Quasi-static compression of a microbubble above its buckling pressure leads to its destabilization. Lowering the PEG molar fraction from 10 to 1.5% increased the buckling pressure from 3 kPa to 27 kPa. Similarly, heating the 10 mol% bubble suspension at 60C for one hour raised the buckling pressure by 20 kPa, due to the selective loss of PEGylated lipid from the shell, without affecting the monodispersity of the bubbles. The higher buckling pressure significantly improved microbubble stability, allowing them to withstand pressurization cycles of up to 45 kPa, nearly three times the systolic blood pressure in vivo.
单分散磷脂包被的微泡，其大小和共振频率与超声驱动频率相匹配，在涉及气泡和超声的新兴诊断和治疗应用中，具有增强灵敏度、效率和可控性的强大潜力。一个关键要求是，它们在生理压力变化期间保持其气体体积和外壳材料，并能承受静脉注射时的超压。外壳通常由磷脂（如二硬脂酰磷脂酰胆碱）与聚乙二醇化磷脂（如二棕榈酰磷脂酰乙醇胺-聚乙二醇5000）的混合物组成。我们假设：（i）当在加压下外壳发生屈曲时，脂质包被的微泡会失稳；（ii）发生屈曲时的超压（屈曲压力）与外壳中聚乙二醇化脂质的摩尔分数有关；（iii）通过在升高的温度下使其流化，聚乙二醇化脂质可从外壳中选择性地排出。
使用超声衰减光谱法在改变环境压力的同时测量屈曲压力。当环境压力增加时，由于外壳屈曲以及相关的弹性丧失，微泡共振频率急剧下降。对于通过微流控流动聚焦形成的单分散微泡，获得了屈曲压力P，其中二棕榈酰磷脂酰乙醇胺-聚乙二醇5000与二硬脂酰磷脂酰胆碱以1.5%至10%的摩尔分数混合。此外，对在40℃至70℃温度范围内加热后含有10摩尔%聚乙二醇的微泡的屈曲压力P进行了量化。使用高效液相色谱法分析微泡的聚乙二醇摩尔含量。
微泡在其屈曲压力以上的准静态压缩会导致其失稳。将聚乙二醇摩尔分数从10%降至1.5%，使屈曲压力从3千帕增加到27千帕。同样，将10摩尔%的气泡悬浮液在60℃加热一小时，由于外壳中聚乙二醇化脂质的选择性损失，屈曲压力提高了20千帕，而不影响气泡的单分散性。更高的屈曲压力显著提高了微泡的稳定性，使其能够承受高达45千帕的加压循环，几乎是体内收缩压的三倍。
