Microbubbles coated with disaturated lipids and DSPE-PEG2000: phase behavior, collapse transitions, and permeability

Area of Science:

• Materials Science
• Biophysics
• Nanotechnology

Background:

• Ultrasound contrast agents (UCAs) utilize lipid-based shells, often phosphatidylcholine (PC) mixed with lipopolymers like distearoylphosphatidylethanolamine-polyethyleneglycol (DSPE-PEG2000), to stabilize microbubbles.
• Understanding the self-assembly and phase behavior of these monolayer coatings is crucial for optimizing UCA performance and stability.

Purpose of the Study:

• To investigate the impact of DSPE-PEG2000 species and PC chain length on microbubble monolayer coating properties.
• To characterize the phase behavior, collapse dynamics, shedding, air transport resistance, and dissolution rates of DSPE-PEG2000 coated microbubbles.
• To compare the behavior of DSPE-PEG2000 coatings with previously studied polyethyleneglycol 40 stearate (PEG40S) coatings.

Main Methods:

• Self-assembly of disaturated phosphatidylcholine (PC) and DSPE-PEG2000 at the air-water interface of microbubbles.
• Fluorescence microscopy to observe microbubble dissolution and film microstructure.
• Bright field microscopy to analyze surface contour and buckling phenomena.
• Modeling of microbubble dissolution data (radius vs. time) to determine air transport resistance.

Main Results:

• DSPE-PEG2000 coatings formed primarily condensed phase monolayers across various PC chain lengths (n=14-20), unlike PEG40S coatings which exhibited expanded or mixed phases.
• A novel surface buckling behavior was observed, attributed to slowed nucleation rates at condensed-condensed phase interfaces.
• Microbubbles with DSPE-PEG2000 coatings showed approximately 10x higher pre-exponential factors and persisted 4x longer than those with PEG40S coatings.
• Air transport resistance (R(shell)) was determined and modeled using a chain-length-dependent energy barrier model.

Conclusions:

• DSPE-PEG2000 promotes the formation of stable, condensed-phase monolayers, leading to enhanced microbubble stability and persistence.
• The condensed phase nature of DSPE-PEG2000 coatings significantly improves air transport resistance compared to expanded phase coatings.
• These findings highlight the potential of DSPE-PEG2000 as a superior component for advanced ultrasound contrast agents.