Phase and Dissolution Behavior of Lipid-Monolayer-Coated, Air-Filled Microbubbles: 
Effect of Lipid Hydrophobic Chain Length

Mark Borden (Ph.D. Chemical Engineering, U C Davis 2002)
Gang Pu (graduate student, Materials Science)
Gabe Runner (B.S. 2004, CPIMA SURE Program and U C Davis )
Esra Talu (graduate student, Materials Science)
Monica Lozano (graduate student, Materials Science)
Yuyi Shen (graduate student, Materials Science)

Recent Project Progress:
We investigated with fluorescence microscopy the phase behavior and morphology of monolayer shells composed of a homologous series of saturated phospholipids and an emulsifier on micron-scale bubbles used in biomedical applications.  At room temperature, we observed a homogenous shell on microbubbles of all sizes coated with short-chain lipids and emulsifier.  In contrast, coexistence of condensed phase lipid domains surrounded by an emulsifier-rich expanded region was found on larger microbubbles coated with long-chain lipids and emulsifier; microbubbles with a radius less then about 20 μm generally exhibited dark, fully compressed shells due to dissolution of the gas core.  A rich assortment of condensed phase area fractions and morphologies, including networked domains, were observed in each batch.  We performed Langmuir isotherms and fluorescence microscopy of Langmuir monolayers made of shell components under states of compression, compression/expansion cycles, and heating/cooling schedules in order to elucidate why such a variety of behaviors were observed and to gain insight into the formation and stabilization of microbubbles.  We also observed the effect of cooling rate and convection in the surrounding medium on condensed phase morphology of microbubble shells.  We then used all of the evidence to develop a possible chain of events in shell formation during microbubble formation and shell evolution.  We argue that the intrinsic phase behavior of the shell components in combination with the non-equilibrium and relatively uncontrolled conditions typical in microbubble formation results in a population of bubbles with heterogeneous shell composition and microstructure when observed within minutes to hours after formation.  Since a precise knowledge of surface architecture and properties should be important in medical applications,  there are important medical implications of our findings and advantages of using microbubbles to study monolayer phase behavior.

Project Publications:
“Dissolution Behavior of Lipid-Monolayer-Coated, Air-Filled Microbubbles:  Effect of Lipid Hydrophobic Chain Length”, Langmuir , Borden, M. A. and Longo, M. L., 2002, 18: 9225 - 9233.
“Oxygen Permeability of Fully Condensed Lipid Monolayers”, Borden, M. A., Longo, M. L., Journal of Physical Chemistry, 2004, 108(19); 6009-6016.
“Surface Phase Behavior and Morphology of Lipid/PEG Emulsifier Monolayer-Coated Microbubbles”, Borden, M. A., Pu, G., Runner, G., and Longo, M. L., Colloids and Surfaces, B, 2004, 35(3-4), 209-223.

Methods and Results:
 
Fluorescent micrographs of microbubbles cooled in vial to room temperature. (a) Shell contains PEG40S:NBD-PC; Shell contains PEG40S:NBD-PC:  (b) DiC12:0PC, (c) DiC14:0PC, (d & g) DiC16:0PC, (e & h) DiC18:0PC, (f & I) DiC20:0PC, (j & m) DiC22:0PC, (k, l, n & o) DiC24:0PC.  Scale bars represent 20 μm.


Fluorescent micrographs taken after heating 89% DiC18:0PC, 10% PEG40S, 1% NBD-PC Langmuir monolayer above main phase transition temperature followed by compression/expansion:  (A) just after spreading, Π = 20 mN/m; (B) heated to above 65 ˚C and cooled back to room temperature at about 1 ˚C/min, Π = 20 mN/m; (C) compressed to Π = 35 mN/m; (D) compressed to Π = 45 mN/m at room temperature; (E) expanded to Π = 30 mN/m at room temperature, bright spots denoted by arrow are surface-associated aggregates; (F) expanded to Π = 20 mN/m at room temperature, bright areas are clusters of surface-associated aggregates.  Scale bars represent 20 μm.