Post-Formation Shrinkage and Stabilization of Microfluidic Bubbles in Lipid Solution

Abstract

Medical ultrasound imaging often employs ultrasound contrast agents (UCAs), injectable microbubbles stabilized by shells or membranes. In tissue, the compressible gas cores can strongly scatter acoustic signals, resonate, and emit harmonics. However, bubbles generated by conventional methods have nonuniform sizes, reducing the fraction that resonates with a given transducer. Microfluidic flow-focusing is an alternative production method which generates highly monodisperse bubbles with uniform constituents, enabling more-efficient contrast enhancement than current UCAs. Production size is tunable by adjusting gas pressure and solution flow rate, but solution effects on downstream stable size and lifetime have not been closely examined. This study therefore investigated several solution parameters, including the DSPC/DSPE-PEG2000 lipid ratio, concentration, viscosity, and preparation temperature to determine their effects on stabilization. It was found that bubble lifetime roughly correlated with stable size, which in turn was strongly influenced by primary-lipid-to-emulsifier ratio, analogous to its effects on conventional bubble yield and Langmuir-trough compressibility in existing studies. Raising DSPE-PEG2000 fraction in solution reduced bubble surface area in proportion to its reduction of lipid packing density at low compression in literature. In addition, the surface area was found to increase proportionately with lipid concentration above 2.1 mM. However, viscosities above or below 2.3-3.3 mPa·s seemed to reduce bubble size. Finally, lipid preparation at room temperature led to smaller bubbles compared to preparation near or above the primary lipid's phase transition point. Understanding these effects will further improve on postformation control over microfluidic bubble production, and facilitate size-tuning for optimal contrast enhancement.

The development of mechanically formed stable nanobubbles intended for sonoporation-mediated gene transfection

“…However, more than 1 mg/ml of phospholipid also tended to destabilize NBs, possibly due the over packing in the shells or single micelle formations that might adhere to the NBs. This observation is consistent with a previous report wherein the phospholipid concentration tended to affect the size and characteristics of NBs (Shih & Lee, 2016 ). Garg et al ( 2013 ) also showed that the phospholipid monolayer in shells of MBs was packed more tightly, leading to the increase of van der Waals interactions between phospholipids molecules.…”

Microbubble Agents: New Directions

“…Long-term size stability of microfluidically formed bubbles. Microfluidically formed lipid-coated bubbles are inherently unstable and prone to Ostwald ripening until they have dissolved to their stable size which is typically 2À3 times smaller than their initial on-chip bubble size (Talu et al 2008;Segers et al 2016b;Shih and Lee 2016). Once the bubbles have reached their final size, they are stable for days in a closed vial (Segers et al 2019).…”

“…On-chip stability of microfluidically formed bubbles. T a gg e d P Microfluidic monodisperse bubble formation requires lipid concentrations approximately 10 times higher than those in classic microbubble production methods to minimize on-chip bubble coalescence in the outlet of the flow-focusing device (Shih and Lee 2016;Segers et al 2017). Furthermore, a high molar concentration of PEGylated lipids with a long chain length is required to minimize coalescence which to date limits the freedom in the choice of lipid mixture for direct monodisperse bubble formation.…”

Freeze-Dried Therapeutic Microbubbles: Stability and Gas Exchange

“…Typically, for safe use in vivo , MBs should have a size no more than 10 μm in diameter. There are numerous preparation protocols for MBs that provide control over the size distributions that can be achieved. , Sonication and mechanical agitation are the conventional methods for producing MBs at a high concentration of ∼10 9 –10 10 MB/mL but have poor control over the MB size. , A flow-focused microfluidic (MF) technology has been successful for the production of monodisperse MBs with size control in a range of 2–50 μm. However, these MBs are susceptible to changes in size off-chip and are produced at relatively low concentrations (∼10 6 –10 8 MB/mL). − We have previously developed a rapid pressure drop on-chip method, which led to the production of MB populations with a size distribution in the range 0.5–3 μm as well as a “one-pot” MF production of liposome-loaded MBs at high concentrations (∼10 9 MB/mL). , …”