Biomedical subjects

Gang Pu

Publications and source records attributed to Gang Pu.

3 recordsLinked to original sources

PubMed2006

Collapse and shedding transitions in binary lipid monolayers coating microbubbles.

We report on a fluorescence microscopy study of the monolayer collapse and shedding behavior due to shell compression during the dissolution of air-filled, lipid-coated microbubbles in degassed media. The monolayer shell was comprised of saturated diacyl phosphatidylcholine (C12:0 to C22:0) and an emulsifier, poly(ethylene glycol)-40 stearate. The morphologies of monolayer collapse structures and shed particles were monitored as a function of phospholipid acyl chain length (n) and temperature. The two components formed a single miscible phase when the phospholipid was near or above its main phase transition temperature, and collapse occurred via suboptical particles to vesicles (both were shed) and tubes as chain length increased. Conversely, two-phase coexistence was observed when the lipid was below its main phase transition temperature. For these bubbles, a transition from primary collapse to secondary collapse was observed. Primary collapse was observed as a loss of expanded phase due to vesiculation. Secondary collapse involved the rapid propagation of monolayer folds and simultaneous deformation. For very rigid monolayers, we observed substantial surface buckling with simultaneous nucleation and growth of folds. The folds merged at a single point or region, providing a conduit for the entire excess lipid to shed in a single event, and the bubble smoothed and became more spherical. These results are discussed in the context of general binary phospholipid collapse behavior, microbubble dissolution behavior, medical applications, and the dissolution behavior of natural microbubbles.

Hot Temperature↗

PubMed2005

Effect of microstructure on molecular oxygen permeation through condensed phospholipid monolayers.

A method is presented that allows novel measurement of the effect of microstructure on the oxygen permeability of highly condensed, polycrystalline phospholipid monolayers. Oxygen permeability of the polycrystalline shell coating a stationary microbubble is measured directly using an apposing microelectrode in the induced transfer mode and modeling oxygen flux through the shell and intervening aqueous medium. Varying cooling rate through the phospholipid main phase transition permits control of shell microstructure by manipulation of crystalline domain size and shape. Domain boundary density, defined as the ratio of the mean domain perimeter to the mean domain area, of the microbubble shell is determined by fluorescence microscopy. Oxygen permeability was shown to increase linearly with domain boundary density at a constant phospholipid acyl chain length and, accordingly, was shown to decrease exponentially with increasing chain length at a constant domain boundary density. Modification of the energy barrier theory to account for microstructural effects, in terms of the domain boundary density, provides a general equation to model passive transport through polycrystalline monolayer films. Results from this method show promise in determining the gas transport kinetics of medical microbubbles and the gas exchange characteristics of biological monolayers.

Cell Membrane Permeability↗

PubMed2004

Surface phase behavior and microstructure of lipid/PEG-emulsifier monolayer-coated microbubbles.

Langmuir trough methods and fluorescence microscopy were combined to investigate the phase behavior and microstructure of monolayer shells coating micron-scale bubbles (microbubbles) typically used in biomedical applications. The monolayer shell consisted of a homologous series of saturated acyl chain phospholipids and an emulsifier containing a single hydrophobic stearate chain and polyethylene glycol (PEG) head group. PEG-emulsifier was fully miscible with expanded phase lipids and phase separated from condensed phase lipids. Phase coexistence was observed in the form of dark condensed phase lipid domains surrounded by a sea of bright, emulsifier-rich expanded phase. A rich assortment of condensed phase area fractions and domain morphologies, including networks and other novel structures, were observed in each batch of microbubbles. Network domains were reproduced in Langmuir monolayers under conditions of heating-cooling followed by compression-expansion, as well as in microbubble shells that underwent surface flow with slight compression. Domain size decreased with increased cooling rate through the phase transition temperature, and domain branching increased with lipid acyl chain length at high cooling rates. Squeeze-out of the emulsifier at a surface pressure near 35 mN/m was indicated by a plateau in Langmuir isotherms and directly visualized with fluorescence microscopy, although collapse of the solid lipid domains occurred at much higher surface pressures. Compression of the monolayer past the PEG-emulsifier squeeze-out surface pressure resulted in a dark shell composed entirely of lipid. Under certain conditions, the PEG-emulsifier was reincorporated upon subsequent expansion. Factors that affect shell formation and evolution, as well as implications for the rational design of microbubbles in medical applications, are discussed.

Lipids↗